principles of digital mobile communication systems · principles of digital mobile communication...

Principles of Digital Mobile Communication Systems

- The GSM System

by Petri Jarske Contents:

Principles of Cellular Mobile Communications Systems 2 The mobile radio environment 8 The GSM System 20 Basic Architecture 20 Architecture Evolution 24 Transmission inside GSM 31 The Radio Interface 38 Principles of Signalling 61 Radio Resource Management 70 Mobility & Security Management 82 Communication Management 89 Network Management 93 Evolution of the GSM System 101 CDMA Systems Intro 107

Principles of Cellular Communications Systems A typical mobile communication environment can be described with the following assumptions: • The communication network operators and service providers

want to provide mobile communications services to a large number of customers.

• The customers are distributed over a (possibly) large

geographical area. • The customers want to be able to access the services while

moving around in the service area (the degree of mobility may vary, depending on the system, from 0 to 250 km/h).

• The operators can use certain limited band of radio

frequencies for the wireless part of the communication. Now, the problem in the exponentially growing markets of mobile communications is: • How can the capacity of the communications system be

increased (and increased, and increased, and ···) ? In this context capacity = number of customers receiving services with satisfactory quality of service

2

Straightforward solution: • The same frequencies used over the whole geographical

area. • For increasing capacity, use more efficient source coding

and modulation (compression, efficient modulation, TDMA, CDMA, etc.)

• No need to worry about the location of the customer. Cellular solution: • Divide the geographical area into small subareas (cells), and

assign each cell enough frequency resource to serve the customers in this area (see figure below, real cells are not that regular in shape and size).

• The same frequency resource may be used in many cells provided that they are separated by enough distance. Capacity can be increased simply by making the cells smaller and smaller...

Are there other advantages, in addition to the higher capacity? The cellular solution also introduces new problems. What could these be? Think before turning the page.

3

Other advantages: • Well, at least lower transmitter powers −> less interference

to other users of the same system, less interference to other systems, also longer battery life

• Flexible coverage: small cells for densely populated areas, large cells for rural areas.

Problems to be considered: • In order to provide service anytime, the network has to know

the location of each customer to some accuracy (location management) at least when a call is coming to the mobile unit.

• To provide continuous service even when the customer is moving, handover procedures are needed. That is, the service connection is passed to a new base station every time the quality of the existing connection gets too low.

There are two extreme alternatives to handle the previous problems. (1) The location of the customer is not known prior to the call but a paging message is sent to the whole network when a call arrives, or (2) the location of the customer is kept in a central database with the precision of one cell. Many of the existing solutions, such as GSM, are something between these extremes because the signalling load can be minimized that way. • Also, the network becomes more complicated and expensive

when smaller cells are introduced. The cost issues are not much emphasised in this text but they are very important for the operator, and also to the customer.

4

Some further “tricks” to improve system efficiency in general (not only capacity): • transmitter power control: When the transmitted power level

of each transmitter is kept to the minimum required for satisfactory quality, the interference caused to other cells sharing the same resource is also minimized. This way, the cells sharing the same radio resource can be built closer to each other, and capacity increases (compared to the non-controlled case).

• frequency hopping: The interference degrading the transmission quality is not equally distributed to all radio channels. By changing the channel frequency periodically for each user, the quality can be made approximately equal for everybody. This way many users can be served with satisfactory quality, rather than serving a few customers with good quality, and leaving some without service.

• discontinuous transmission: In speech communication, the active speech covers only about 25···40% of time. During the silent periods, it is sufficient to transmit only very little information, for example, one or two frames per second. This again reduces the average transmitted power, and reduces the interference caused to other users. The cost, however, is increased complexity because voice activity detection (VAD) has to be implemented.

• mobile assisted handover: This reduces the network complexity by giving the responsibility of monitoring the signals from neighboring cells to the mobile terminal. These measurements are needed for the handover decision, and would, otherwise, require constant message exchange between neighboring cells.

5

Overview of mobile services Service provision of a particular user depends on: • contents of the subscription held by the user • capabilities of the serving network • capabilities of the user equipment Examples Services in GSM: • Speech − probably the most important also in the future • Circuit switched data − currently up to 38,4 kbits/s

commercially available, or more • Packet data – available • Short messages − point-to-point & broadcast • Supplementary services − call forwarding, barring, etc. • Multimedia messages Services in IEEE802.11 wireless LANs: • Packet data, up to 11 Mbits/s in 2.4GHz ISM frequencies,

54 Mbits/s available • Packet data, up to 54 Mbits/s in 5-6 GHz frequencies

available • IP based core network, operator not necessary • No special speech channel, voice over IP (VoIP)

6

Presumed evolution of Wireless Cellular Communication:

ETSI: GSM evolution toWCDMA (UMTS)

ANSI: US-CDMA evolution to cdma2000

ARIB: selection of 3G technology forJapan etc.

3GPP

ITU-T: IMT-2000

3G

3GPP: 3rd Generation Partnership Project is a co-operation project between the standardisation bodies mentioned above. Global 3G did not happen. There is also 3GPP2 for U.S. and some other areas.

7

The mobile radio environment General description Radio propagation mechanisms are strongly affected by the wavelengths used, and the environment (natural or human-made). Buildings are wave scatterers. The sizes of buildings are typically many wavelengths of the used frequency, creating reflected waves at that frequency. Typically, the antenna height of a mobile unit is much lower than the average height of houses. Given the conditions above, and the propagation frequency clearly above 30 MHz, the environment forms a multipath propagation medium. The base-to-mobile link is usually less than 25 km, so the radio horizon need not be considered. Actually, earth's curvature reduces interference from distant sources. For large cell designs (radius 6.5 ··· 13 km) the height of the base station antenna is usually 30 ··· 90 m. The height of a mobile unit antenna is about 2 ··· 3 m. The base station antenna is usually clear of its surroundings, whereas the mobile-unit antenna is embedded in them.

Base station

antenna

8

From this description of the environment, we might imagine that the mobile site will receive many reflected waves and (possibly) one direct wave. We can assume that the reflected waves received at the mobile site come from different angles equally distributed throughout 360°. If the direct signal is strong compared to reflected signals, the received signal level can be described with Rician statistical model. If the direct signal is weak (or non-existent), the received signal level can be described with Rayleigh statistical model. Path loss and fading In free space, signal attenuates 6 dB / octave (of distance). That is, if the distance from the transmitter is doubled, the free space path loss will be 6 dB more. The signal strength r(x) or r(t) can be, for modelling purposes, separated into two parts called long-term fading m(t) or m(x), and short-term fading r0(t) or r0(x) as r(t) = m(t)·r0(t) or r(x) = m(x)·r0(x) The long term fading is the envelope of the fading signal, or local mean.

∫∫+

−

+

−

==Lx

Lx

Lx

Lx

drmL

drL

xm ξξξξξ )()(21)(

21)(ˆ 0

9

When the length L is properly chosen, this becomes

∫+

−

=Lx

Lx

drL

xmxm ξξ )(21)()(ˆ 0

The long-term signal fading m(x) is mainly caused by terrain configuration and the built environment between the base station and the mobile unit. Terrain configurations can be classified, for example, as • Open area • Flat terrain • Hilly terrain • Mountain area and the human made environment as • Rural area • Suburban area • Urban area Short-term fading is mainly caused by multipath reflections of a transmitted wave by local scatterers such as buildings or natural obstacles.

10

Classification of channels In a dispersive medium, there are two kinds of spread: Doppler spread F and multipath spread δ. Doppler spread F is spreading in frequency, and multipath spread δ is spreading in time. In a strict sense, all media are dispersive. We can classify a medium's characteristics based on the signal duration T and the signal bandwidth W. Nondispersive channels A nondispersive but fading channel is created if

WTF 1and1

<<<< δ

In many practical systems, the values of T and W can be chosen so that the channel can be considered nondispersive. Time-dispersive channels

WT 1and >>>> δδ but T

F 1<<

11

Frequency-dispersive channels

TFWF 1and >>>>

but

W1

<<δ

Guess what is doubly-dispersive channel.

12

Delay spread The mean delay time Td of a channel can be calculated as

∫∞

⋅=0

)( dttetTd

and the delay spread ∆ as

2

0

22 )( dTdttet −⋅=∆ ∫∞

where e(t) is the impulse response of the channel.

Typical values for the delay spread are:

Type of environment Delay spread ∆ In-building < 0.1 µs Open area < 0.2 µs

Suburban area 0.5 µs Urban area 3 µs

13

Prediction of propagation loss As we saw earlier, the local mean (long-term fading) of the received signal level can be obtained by averaging a suitable spatial length over a piece of raw data. The choice of suitable L is essential for obtaining a good estimate of the local mean. In practice, L in the range 20λ···40λ is acceptable. 36···50 samples in an interval of 40 wavelengths is adequate for obtaining the local means. The measurements are usually recorded while the mobile units are travelling along a road (street). The recorded signals from the mobile paths have to be converted to radio path.

14

Models for path loss Note: path loss model is only for path loss prediction and not for multipath fading. Assume that the characteristics of a rough earth surface are random in nature and that the radius of curvature of the surface irregularities is large compared to the wavelength of the incident wave. Then the received signal can be represented by a scattered field Es which can be approximated by combining the direct wave and the reflected wave.

Es = (1 + avej∆ψ)E The reflection coefficient is av and ∆ψ is the phase difference between the direct and reflected wave. The phase difference can be expressed as

dd ∆⋅=∆⋅=∆λπβψ 2

where β is the wave number and ∆d is the difference between the two radio path lengths. E is the direct wave received at the mobile antenna.

15

According to the free-space propagation path loss, the received power from a direct wave is

2

0 4 ⎟⎟⎠

⎞⎜⎜⎝

⎛=

dPP tr π

λ

In the mobile radio environment, the incident angle is usually small, and therefore, the reflection coefficient is approximately av= −1 and ∆ψ<<1. The received power of the scattered field becomes

( )22

22

4sincos1

4ψ

πλψψ

πλ

∆⎟⎟⎠

⎞⎜⎜⎝

⎛≈∆−∆−⎟⎟

⎠

⎞⎜⎜⎝

⎛=

dPj

dPP ttr

For d >> h1+h2 we can approximate

dhh

λπψ 214

≈∆

This gives

2

221 ⎟

⎠⎞

⎜⎝⎛≈

dhhPP tr

This is an imperfect formula since it does not involve wavelength. It indicates two correct facts • the equation shows a path loss of 40 dB/dec which has been

verified from the experimental data to be roughly true • the equation shows a 6 dB/oct rule for an antenna height gain at the

base station, i.e. doubling the antenna height at the base gains 6 dB which also seems to be roughly true within certain limits

but there are two weak points, too • the wavelength term is missing but the measured data show that the

path loss is a function of frequency • the equation shows a 6 dB/oct rule also for an antenna height gain

at the mobile unit which is not true in practise

16

An area-to-area path loss prediction model An area-to-area prediction is sometimes used to predict path loss over a general flat terrain without knowing the particular terrain configuration. The area-to-area path loss prediction requires two parameters: (1) the power at the reference (1-mile) point of interception Pr0 and (2) a path loss slope γ. The field strength of the received signal Pr can be expressed as

000

0 αγ n

rr ff

rrPP

−−

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=

or in dB

000

0 loglog αγ +⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟⎟

⎠

⎞⎜⎜⎝

⎛−=

ffn

rrPP rr

where r is in miles or kilometers and r0 equals 1 mile or 1.6 km. γ is expressed as γth power in the linear formula, and γ dB/dec in the dB formula. α0 is an adjustment factor. This is a general formula that can be used for different frequency ranges above 30 MHz. The assumed default conditions are: frequency f0 = 900 MHz base station antenna heigth = 30.48 m (100 ft) base station power at the antenna = 10 watts base station antenna gain = 6 dB above dipole gain mobile unit antenna height = 3 m (10 ft) mobile unit antenna gain = 0 dB above dipole gain

17

The adjustment factor is used for different conditions as follows: α0 = α1α2α3α4α5 (or α0 = α1+α2+α3+α4+α5 in dB) where

2

1 m48.30(m)height antennastation base new

⎟⎠⎞

⎜⎝⎛=α

ν

α ⎟⎠⎞

⎜⎝⎛=

m3(m)height antennastation mobile new

2

W10powerer transmittnew

3 =α

4dipole /2 respect togain with antennastation base new

4λα =

α5 = antenna gain correction factor at the mobile unit The parameters γ and Pr0 are found from empirical data:

Terrain Pr0 (mW) Pr0 (dBm) γ γ (dB/dec) free space 10−4.5 −45 2 20 open area 10−4.9 −49 4.35 43.5 suburban 10−6.17 −61.7 3.84 38.4

Philadelphia 10−7 −70 3.68 36.8 Newark 10−6.4 −64 4.31 43.1 Tokyo 10−8.4 −84 3.05 30.5

The values of n and v is also found from empirical data. In suburban or open area with frequencies < 450 MHz n=20 dB/dec. In urban areas with >450 MHz frequencies n=30 dB/dec is recommended.

⎩⎨⎧

<>

=3m height antennaunit mobile newfor 13m height antennaunit mobile newfor 2

ν

18

The model of Okumura et al. (From M. Hata, "Empirical Formula for Propagation Loss in Land Mobile Radio Services", IEEE Trans. Vehicular Tech., VT-29, No. 3, August 1980.) The standard formula for propagation loss is Lp (dB) = 69.55 + 26.16 log fc − 13.82 log hb − a(hm) + (44.9 − 6.55 log hb) log R where fc is the used frequency 150···1500 MHz, hb is the base station antenna height 30···200 m, R is distance 1···20 km, hm is the mobile antenna height, and a(hm) is a correction factor for hm given by

MHz400 city, largefor MHz400 city, largefor

city small-mediumfor

97.4)75.11(log2.310.1)54.1(log29.8

)8.0log56.1()7.0log1.1()(

2

2

≥≤

⎪⎩

⎪⎨

⎧

−−

−−−=

c

c

m

m

cmc

m

ff

hh

fhfha

In suburban areas the loss is Lps = Lp{urban area} − 2 (log (fc/28))2 − 5.4 and in open areas Lpo = Lp{urban area} − 4.78 (log fc)2 + 18.33 log fc − 40.94 Street orientation channel effect The signal strength received from a street in line with the base station is about 10 dB higher than the signal from a street perpendicular to the base. This phenomenon diminishes at about 8 km distance. Note, that the previous description gave only examples of how radio path loss is modelled in mobile communication systems. It is not a complete list, and the exact numbers are not relevant for this course.

19

The GSM System In the following text, we will concentrate mainly on the GSM system. Basic Architecture The GSM system, as originally specified in 1991, has a hierarcical architecture, typical for 2nd generation cellular systems:

OSS

BTS

BSC

BTS

BTS

BSC

BTS

TRAU

TRAU

MSCVLR

HLRACEIR

PSTNISDN

SMSCVMS

BTS = base transceiver station BSC = base station controller TRAU = transcoder & rate adapter unit MSC = mobile (services) switching centre VLR = visitor location register AC = authentication centre HLR = home location register OSS = operation sub-system including network management (NMS) SMSC = short message service center VMS = voice message system EIR = equipment identity register

20

GSM Network Elements: The Mobile Station (MS) MS = ME + SIM Mobile Equipment (ME): generic radio and processing functions to access the network, human interface and/or interface to other terminal equipment. Subscriber Identity Module (SIM): a smart card containing all the subscriber related information, confidentiality related information. Something to think about: What advantages follow from making the ME and SIM separate entities? The Base Station Subsystem (BSS) BSS = BSC + BTS + TRAU Base Station Controller (BSC) is in charge of the radio interface management, allocation and release of radio channels, handover management (up to some tens of BTS’s). Base Transceiver Station (BTS): radio transmission and reception from antennas to the radio interface specific signal processing, handling 1 ··· 10 radio carriers at a time. Transcoder & Rate Adapter Unit (TRAU): GSM-specific speech encoding and decoding, bit rate adaptation.

21

The Network & Switching Subsystem (NSS) NSS = MSC + VLR + HLR + AC + EIR Mobile services Switching Center (MSC): performs the basic switching function, coordinates the set-up of calls to and from GSM users, manages communications between GSM and other telecommunications networks. Visitor Location Registers (VLR): database storing temporarily subscription data for those subscribers currently located in the service area of the corresponding MSC, holds data of their current location area. Home Location Register (HLR): database holding subscriber information relevant to the provision of telecommunications services, some information related to the current location of the subscriber (mainly under which MSC/VLR the user can be found). Authentication Centre (AC): database maintaining security related information of the subscriptions. Equipment Identity Register (EIR): database maintaining security related information of the mobile equipment (separate from subscriptions).

22

The Operation Sub-System (OSS) Operation Sub-System (OSS): (1) network operation enabling the operator to observe system load, blocking rates, handovers, etc. and providing means to modify network configuration, (2) equipment maintenance aiming at detecting, locating and correcting faults, (3) subscription management for registering new subscriptions, modifying and removing subscriptions, as well as billing information. Tasks (1) & (2) are major part of the Network Management System (NMS). Task (3) is more service management, not directly related to network status. Value Add Services The services offered in the basic GSM network are similar to those available in a sophisticated PSTN network. Mobility is the main feature differentiating the basic GSM system from fixed telephony systems. On top of this, the first services adding value to the GSM network, have been Short Message Services (SMS) and Voice Messaging System (VMS). Especially, the success of SMS has been surprisingly good. More value add services have been, and will be built as the capabilities of the GSM network improve over time. Intelligent Network (IN) features are added to the GSM networks, in order to enable tailored services to different customer groups, or individual subscribers.

23

Architecture Evolution The GSM specification is evolving constantly. Some major development lines are reviewed here, from architecture point of view. The functionality of these new features will be discussed more, later on. Step 1: Higher data rates The basic GSM offers circuit switched data transfer services with rates up to 9.6 kbits/s which is not sufficient for many services. Higher data rates are possible by changing channel coding, and using several physical channels (time slots) for a high rate connection. High Speed Circuit Switched Data (HSCSD) is an implementation of this concept in GSM. HSCSD will be discussed in more detail later. For the GSM architecture, as presented on page 20, HSCSD does not introduce any visible changes in the block diagram (so let's not redraw it here). It does, however, require HW and SW changes in most of the network elements shown in the architecture block diagram.

PSTNISDNPDN

BTS BSC/TRAU MSC

. . .. . .IW F

HSCSD principle

24

Step 2: Packet data Wired data networks have typically used packet data transfer. In order to connect smoothly to these networks, and to use the radio resource more efficiently, General Packet Radio Service (GPRS) has been specified to GSM. From architecture point of view, in addition to HW & SW changes in the existing network elements, GPRS also introduces new network elements called Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN).

OSS

BTS

BSC

BTS

BTS

BSC

BTS

TRAU

TRAU

MSCVLR

HLRACEIR

PSTNISDN

SMSCVMS

GGSNSGSN IPIP

networksIP

25

Step 3: Higher data rates (again) When we want to go beyond HSCSD in data rates with minimal changes in the frame structure and protocols, it is necessary to change the modulation used in the physical layer to represent the transmitted bit stream. In the GSM case, this is done by packing 3 bits per symbol on the physical layer, instead of 1 bit per symbol of basic GSM. This is called Enhanced Data rates in GSM Environment (EDGE). For the architecture presented on previous page, EDGE does not introduce any visible changes in the block diagram (so let's not redraw it here). It does, however, require HW and SW changes in most of the basic network elements shown in the architecture block diagram. Step 4: Completely new radio interface For the 3rd generation (3G) cellular networks, the core of the network, at least in Europe and Japan, will be based on GSM. The air interface, however, will be based on CDMA technology which is completely different from basic and enhanced GSM. For controlling the CDMA radio network, similar network elements are needed, as in GSM but different terminology is used in order to draw distinction between 2G and 3G systems. Instead of BTS, we have Base Station (BS), and instead of BSC, we have Radio Network Controller (RNC), in the 3G network. Of course, the detailed functionality of these elements is also different from GSM.

26

OSS

BTS

BSC

BTS

BTS

BSC

BTS

TRAU

TRAU

MSCVLR

HLRACEIR

PSTNISDN

SMSCVMS

GGSNSGSN IPIP

networksIP

BS

RNC

BS

IWU

The 2G and 3G radio interfaces will co-exist for a long period of time. Also other radio interfaces, such as DECT or wireless LANs, may utilise the same core network.

27

Step 5: All-IP

IP NETWORK

BTS

BSC

BTS

TRAU

MSCVLR

HLRACEIR

PSTNISDN

SMSCVMS

GGSNSGSN IPnetworks

BS

RNC

BS

IWU

It is expected that eventually GSM, and 3G networks will evolve into all-IP architecture. Majority of the traffic will use packet transfer. IP will support mobility management, and quality of service (QoS) features.

28

In the following, we will concentrate on the basic GSM functionality, and will revisit HSCSD, GPRS, and EDGE later on. Architecture & GSM Functional Planes Functionally, the GSM system can be divided into five planes:

Transmission: provides the means to carry user information (speech or data) on all segments along the communication path, and to carry signalling messages between entities. Radio Resource Management (RR): establishes and releases stable connections between mobile stations and an MSC, and maintain them despite user movements. The RR functions are mainly performed by the MS and the BSC.

29

Mobility Management (MM): functions are handled by the MS (or SIM actually), the HLR/AuC, and the MSC/VLR. These include also management of security functions. Communication Management (CM): is setting up calls between users, maintaining and releasing them. In addition to call control, it includes supplementary services management, and short message management. Operation, Administration & Maintenance (OAM): enables the operator to monitor and control the system. The following figure tries to illustrate the relationship between the network elements and functional domains:

GMSC = Gateway MSC, a switching centre which is able to find the corresponding HLR based on the called number. GMSC and MSC/HLR may be physically one unit.

30

Transmission inside GSM On the network side, the GSM system is designed to be compatible with ISDN where the transmission rates are multiples of 64 kbit/s. On the air interface, however, the net bit rate per channel is less than 16 kbit/s. For adapting the different rates, the Transcoder / Rate Adaptor Unit (TRAU) has been introduced. For speech, TRAU includes the speech codecs. TRAU belongs functionally to the BTS but its actual location is not strictly specified.

Transmission of speech and data is next briefly described in the • radio interface • BTS - TRAU interface • interface between TRAU and point of interconnect with

other networks (IWF)

31

Speech on the radio interface Speech processing for transmission over the air interface includes the following functions: • Speech coding • Error protection (codec specific) • Error detection (CRC) • Bad Frame Handling (substitution) • Voice Activity Detection / Discontinuous Transmission

(VAD/DTX) • Manufacturer specific audio features

- noise cancelling - spectrum equalization - echo cancellation

For spectrum efficiency, as low bit rate as possible on the radio path (but with acceptable quality, of course), is required. Speech coding takes care of this. In the first phase of GSM spec, a “full rate” speech channel was defined, with provision of “half rate” in the second phase. Why? Today, the GSM standard includes the following codecs: • Full rate (FR), 13 kbit/s RPE-LTP • Half rate (HR), 5.65 kbit/s VSELP • Enhanced full rate (EFR), 12.2 kbit/s ACELP • Adaptive Multi Rate (AMR), ACELP

12.2, 10.2, 7.95, 7.4, 6.7, 5.9, 5.15, 4.75 kbit/s • AMR wideband codec

32

As an example, we can take a brief look at the original full rate codec. The full rate speech encoder, compressing 64 kbit/s −> 13 kbit/s, is a so called RPE-LTP (regular pulse excitation - long term prediction) encoder. Speech is encoded in blocks of 20 ms, that is, 160 samples having 8 bits each (in A-law representation) are encoded into 260 bits as illustrated in the figure below. Since this is not a speech processing course, we will not go into details of the speech codec.

Decoder basically the previous stuff in reverse order: Up-sampling − LTP filter − LPC filter − de-emphasis filter

33

The transmitted parameters, after speech encoding, are NOT equal in importance. Therefore, they are divided into 3 classes of importance, each protected against transmission errors in a different manner. This will be described later. Discontinuous transmission When the user is speaking, speech is encoded at the normal rate 13 kbit/s (260 bits / 20 ms). Otherwise a bit rate around 500 bit/s (260 bits / 480 ms) is used which is sufficient to encode the background noise. The background noise is regenerated to the listener. Why? Discontinuous transmission (DTX) requires voice activity detection (VAD). What are the advantages of discontinuous transmission?

34

Speech on the BTS-TRAU interface If TRAU is physically distant from BTS, the 13 kbit/s stream is carried to the TRAU over standard digital links making use of 16 kbit/s circuits. The 20 ms frame synchronization cannot be derived from the 13 kbit/s flow. Therefore, some auxiliary information is added. This also includes information for speech/data, full/half rate and bad frame indication. Total 316 bits / 20 ms. Speech on the TRAU-IWF interface On a 64 kbit/s link, the standard G.711 speech transmission is used with A-law coding.

35

Data in the basic GSM system Several connection types are provided. Why? Basic division is into “T” and “NT” modes. In “T” (or transparent) mode, the error correction is entirely done by a forward error correction (FEC) mechanism. In “NT” (or non-transparent) mode, an additional scheme is used where information is repeated when it has not been correctly received by the other end. The “T” mode connection types of the basic GSM are summarized in the following table:

User rate Intermediate rate

Channel type Residual error rate

9600 bit/s 12 kbit/s full rate 0.3 % 4800 bit/s 6 kbit/s full rate

half rate 0.01 % 0.3 %

2400 bit/s or less

3.6 kbit/s full rate half rate

0.001 % 0.01 %

(residual error rates for typical urban conditions with frequency hopping) For “NT” mode, the Radio Link Protocol (RLP) is added which is basically a link protocol of repetition-when-needed type.

36

The following table summarises all basic GSM data connection types:

Type QoS two-way delay TCH/F9.6 T low 330 ms

TCH/F9.6 NT high > 330 ms TCH/F4.8 T medium 330 ms TCH/F2.4 T medium 200 ms TCH/H4.8 T low 600 ms

TCH/H4.8 NT high > 600 ms TCH/H2.4 T medium 600 ms

(don’t take the quality estimation too literally - note differences) The “T” mode of transmission is derived from the ISDN specifications (but we will not discuss these much in this course). In the “NT” approach, the transmission is considered as a packet data flow (although the offered service, end-to-end, is a circuit service).

37

The Radio Interface The radio interface, in addition to the fact that the users move, is the source of many difficult problems that need to be solved in the GSM system (just as in any mobile communication system). The radio interface needs to be specified in very detail, in order to achieve full compatibility mobile stations and networks of different manufacturers. Spectral efficiency of a cellular system is one of the key economic factors. The multiple access scheme used in GSM is a combination of TDMA and FDMA. FDMA is mainly used to share spectrum between neigboring cells. The basic time division is into 8 time slots but the actual time division scheme is more complicated, as we will soon see. Logical channels The basic division between logical channels is:

Traffic channels / Control channels.

38

The main task of the communication system is to transport user information. For the speech and different types of data communications, the radio interface accommodates bi-directional connections. For these purposes, Traffic CHannels (TCH) are assigned to the user. Full rate traffic channels may be denoted by TCH/F, and half rate channels by TCH/H. All the other logical channel types can be regarded as control or signalling channels. One exception to the previous statements is the transfer of point-to-point short messages, which is implemented in a similar way as signalling. When a mobile station is connected to the network (whether or not there is a user communication in progress), signalling messages are exchanged between the mobile station and other network elements. For signalling in connection with a call, two possibilities are offered: Each assigned traffic channel comes with an associated low rate signalling channel called Slow Associated Control CHannel (SACCH). This bi-directional channel is capable of carrying about 2 messages / second, with a transmission delay of about 0,5 second.

39

The other alternative is (surprise!) Fast Associated Control CHannel (FACCH) which is actually not a separate logical channel but uses the traffic channel (TCH) by replacing a user data frame with a signalling frame when necessary. A signalling frame is marked with one bit called stealing flag. Signalling connection is often necessary also when there is no call in progress (supplementary services management, short messages, location updating, etc.) For this purpose, a Stand-alone Dedicated Control CHannel (SDCCH) is set up. Sometimes, this is also referred to as TCH/8 since its characteristics are very close to the traffic channel but it uses only 1/8 of the capacity of a full rate traffic channel. TCH/8 also has an SACCH associated with it. For spectrum efficiency, traffic channels are allocated to users only when needed (in PSTN you always have the connection to the network). Therefore, we can distinguish between dedicated mode and idle mode of the mobile system. A mobile station is in dedicated mode when it has a TCH assigned to it. In idle mode (but power on), the mobile station is far from idle. It must continuously listen to one base station, and also monitor up to 6 other base stations.

40

Before a mobile station can communicate with a base station, it must become and stay synchronised with it. For this purpose, two logical channels are broadcast from each base station: the Frequency Correction CHannel (FCCH), and the Synchronisation CHannel (SCH). General information concerning each cell (identity, which network it belongs to, which frequencies are used, etc.) is broadcast regularly on the Broadcast Control CHannel (BCCH). After the mobile station has synchronised itself with the base, it can access the network through the Random Access CHannel (RACH). Paging messages are sent on the Paging CHannel (PCH) and messages indicating the allocated channel on Access Grant CHannel (AGCH). Because these are similar and never used simultaneously, they can be treated together as PAGCH. Cell broadcast short messages are broadcast on the Cell Broadcast CHannel (CBCH). This requires about 80 bytes every 2 seconds. The common channels FCCH, SCH, BCCH, PAGCH as well as the CBCH are downlink (from base to mobile) only. The RACH is uplink (from mobile to base) only. The other channels, called dedicated channels (TCHs ans SACCHs), are bi-directional.

41

The multiple access scheme The radio interface of GSM uses a combination of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) with slow frequency hopping. The basic unit of transmission on the radio path is a sequence of about 156 modulated bits called burst. They are sent in time and frequency windows called slots. The center frequencies of the slots are placed 200 kHz apart within the frequency band reserved for GSM, and the duration of one slot is 15/26 ms ≈ 0.577 ms. All slots in a cell are aligned in time. This is illustrated in the following figure.

42

The time axis is divided into 8 distinct slots, numbered 0···7. The information of certain logical channels is mapped to certain time slot number. For example, if the shadowed burst in the previous figure belongs to certain logical channel, the next time we can find information belonging to the same logical channel is at least 8∗15/26 ms later. The frame structure is as follows: • 8 consecutive time slots form a TDMA Frame. • 26 or 51 TDMA Frames form a Multiframe. • 51 or 26 Multiframes form a Superframe. • 2048 Superframes form a Hyperframe. The length of a hyperframe is 3 hours 28 minutes 53,76 sec. At the base station, the transmitted and received bursts are synchronized such that the received burst arrives 3∗15/26 ms after the burst with the same time slot number is transmitted. Transmission 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2

Reception 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

So, this is the base station viewpoint. The figure looks similar for a mobile station very close to the base station. The purpose of this arrangement is to avoid simultaneous transmission and reception in the mobile station. For a mobile station several kilometers away from the base station, the propagation delays have to be considered (30 km distance => 200 µs round trip delay). This is compensated in the mobile station by transmitting the bursts earlier. The timing is adjusted with the timing advance parameter.

43

This timing arrangement also has an impact on the future development of the GSM system. Think about, for example, increasing the user data rates without changing the air interface totally. In the following description, each rectangle denotes one slot with certain time slot number. The slots with other time slot numbers are not shown. So, adjacent slots in the figures are separated by 8∗15/26 ms. The following figures try to show how different logical channels are grouped on respective time sequences: TCH/F + SACCH T T T T T T T T T T T T S T T T T T T T T T T T T - ...0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 0 TCH/H + SACCH T T T T T T T T T T T T S T T T T T T T T T T T T S ...0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 0 Here the slots denoted with bold and italic characters belong to two different logical channels.

44

TCH/8 + SACCH (8 channels grouped) T1 T1 T1 T1 T2 T2 T2 T2 T3 T3 T3 T3 T4 T4 T4 T4 T5 T5 T5 T5 T6 T6 T6 T6 T7 T7

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 T7 T7 T8 T8 T8 T8 S1 S1 S1 S1 S2 S2 S2 S2 S3 S3 S3 S3 S4 S4 S4 S4 - - -26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 T1 T1 T1 T1 T2 T2 T2 T2 T3 T3 T3 T3 T4 T4 T4 T4 T5 T5 T5 T5 T6 T6 T6 T6 T7

51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 T7 T7 T7 T8 T8 T8 T8 S5 S5 S5 S5 S6 S6 S6 S6 S7 S7 S7 S7 S8 S8 S8 S8 T - - ...76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 0 TCH/8 + SACCH (4 channels grouped, with common ch.)

T1 T1 T1 T1

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 T2 T2 T2 T2 T3 T3 T3 T3 T4 T4 T4 T4 S1 S1 S1 S1 S2 S2 S2 S2 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

T1 T1 T1

51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 T1 T2 T2 T2 T2 T3 T3 T3 T3 T4 T4 T4 T4 S3 S3 S3 S3 S4 S4 S4 S4 ...76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 0

45

The empty slots in the previous figure are used for common channels. FCCH + SCH F S F S F S 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

F S F S 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 BCCH + PAGCH/3

B B B B P P P P P P P P P P P P 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 OK, maybe it is not necessary to show all possible channel combinations.

46

Some examples of possible cell configurations follow (here TN = timeslot number). A small capacity cell with a single transmitter/receiver:

TN0: FCCH, SCH, BCCH, PAGCH/3, RACH/H; 4x(TCH/8+SACCH) TN1···7: one TCH/F+SACCH each.

A medium capacity cell with 4 transmitters/receivers:

one TN0: FCCH, SCH, BCCH, PAGCH/F, RACH/F; 2x8x(TCH/8+SACCH) 29x(TCH/F+SACCH)

A large capacity cell with 12 transmitters/receivers:

one TN0: FCCH, SCH, BCCH, PAGCH/F, RACH/F one TN2, TN4, and TN6: BCCH, PAGCH/F, RACH/F 5x8x(TCH/8+SACCH) 87x(TCH/F+SACCH)

47

The frequency band The so called primary band of GSM includes two 25 Mhz subbands.

Other bands: Extension to 33 MHz with 882−890 MHz and 927−935 MHz GSM1800 bands 1710−1785 MHz and 1805−1880 MHz In the US GSM1900 The carriers spacing is 200 kHz

The border frequencies are usually not used, which limits the number of frequencies to 122 in the 25 MHz band. There may be additional national limitations.

48

Frequency hopping The radio interface of GSM uses slow frequency hopping. Each burst is transmitted with one frequency, in GSM.

This provides at least two advantages: Frequency diversity: Mobile radio transmission is subject to severe multipath fading, but different frequencies fade independently. For example, when a mobile is standing still or moving very slowly, the signal may fade for several burst periods, and the connection may be lost. If different frequency is used for each burst, consecutive frames are probably not lost, and the connection quality may be acceptable. Interferer diversity: Cells using same frequencies interfere each other less if their hopping sequences are independent. Less interference means better re-use of the radio resource (cells sharing the same resource may be closer to each other), and thus, better spectrum efficiency.

49

Frequency hopping is not used on common channels (FCCH, SCH, BCCH, PAGCH, RACH and CBCH). The downlink common channels all use the same frequency. Also, signal on the frequency of the common channels is transmitted continuously, even if no information is to be transmitted, because mobile stations in neighboring cells continuously measure the signal level from the base stations. When there is not information to be transmitted, dummy frames are used. Hopping Sequences With or without frequency hopping, always the uplink frequency = downlink frequency + 45 MHz. For a set of n frequencies 64 x n different hopping sequences can be built, in GSM. They are described by two parameters, the Mobile Allocation Index Offset (MAIO, n different values), and Hopping Sequence Number (HSN, 64 different values). Two channels having the same HSN but different MAIO never use the same frequency at the same time. On the other hand, two channels having the same MAIO but different HSN interfere with the probability 1/n. The sequences are pseudo-random, except for the one with HSN = 0 which uses the frequencies in increasing order. Channels in one cell usually have the same HSN but different MAIO. Adjacent cells use different set of frequencies. Distant cells using the same frequency sets should use different HSN to minimise interference.

50

From source data to radio waves As an example, let us look at speech.

Speech Speech

Digitizing and source coding

Source decoding

Channel coding

Channel decoding

Interleaving

De-interleaving

Ciphering

Deciphering

Burst formatting

Burst decoding

Modulation

Demodulation

Note that in the source (speech) codec, encoding is more complicated than decoding. On the other hand, in the channel codec, decoding is much more complicated than encoding. Also, demodulation (including equalisation, synchronisation, etc.) is computationally intensive.

51

The following blocks are common to all transmission modes. • channel coding introduces redundancy into the data flow by

adding information calculated from the actual data, in order to allow correction, or at least, detection of transmission errors.

• interleaving mixes the bits of several code words such that consecutive bits are spread over several bursts. This is done because transmission errors often occur in bursts such that many consecutive bits (sometimes hundreds) are lost, and on the other hand, channel codecs perform better on uncorrelated errors.

• ciphering modifies the contents of the burst by performing an x-or -operation between a pseudo-random bit sequence and 114 bits of a normal burst. De-ciphering is done exactly the same way. The pseudo-random sequence is derived from the burst number, and a session key with a simple but confidential algorithm.

• burst formatting adds some tail bits at the ends, and a training sequence in the middle of the burst, in order to help synchronisation and equalisation of the received signal.

• modulation transforms the binary signal into an analog waveform which is mixed into the selected frequency and the selected timeslot.

The receiver end is more or less logically the reverse operations in reverse order.

52

Channel coding provides protection against bit errors in the transmission channel. Error correction is mainly done with the convolutional codes, and block (parity) codes are for detecting remaining errors. In common channels, a so called Fire code is used which is capable of correcting errors occurring in groups. The following figure (next page) summarises basic GSM channel coding schemes.

Some explanation to the figure:

In each box, the last line indicates the chapter of GSM spec. 05.03 defining the function. In the case of RACH, P0 = 8 and P1 = 18; in the case of SCH, CSCH, CTSBCH-SB and CTSARCH, P0 = 25 and P1 = 39. In the case of data TCHs, N0, N1 and n depend on the type of data TCH.

Interfaces:

1) information bits (d);

2) information + parity + tail bits (u);

3) coded bits (c);

4) interleaved bits (e).

53

speech frame112 bits

3.2


3.1

message184 bits

4.1.1

data frameN0 bits3.n.1

messageP0 bits

4.6, 4.7, 5.3.2

RLC blockQ0 bits5.1.n.1


3.1

interface1

interface2

TCH/HS(half rate

speech TCH)

TCH/FS(full rate

speech TCH)

SACCH, FACCH,BCCH, CBCH, PCH

AGCH, SDCCHdata TCHs

PRACH

RACH,SCH

cyclic code+ tail

in: 260 bitsout: 267 bits

3.1.1

cyclic code+ tail


3.2.1

Fire code+tail


4.1.2

+tailin: N0 bits

out: N1 bits3.n.2

cyclic code+ tail

in: P0 bitsout: P1 bits

4.6, 4.7, 5.3.2

cyclic code+ tail

in: Q0 bitsout: Q1 bits

5.1.n.2

cyclic code+ repetitionin: 244 bits

out: 260 bits3.1.1

interface3

interface4

TCH/F2.4 others

TCH/FS, TCH/EFSTCH/F2.4, FACCH

others

encryption unit

diagonal interleaving+ stealing flags

in: 456 bitsout: 4 blocks

diagonally interleavedto depth 19, starting

on consecutive bursts3.n.4

reordering and partitioning+stealing flagin: 456 bits

out: 8 blocks3.1.3, 4.1.4, 4.3.4

block rectangularinterleavingin: 8 blocksout: pairs of

blocks4.1.4

block diagonalinterleavingin: 8 blocksout: pairs of

blocks3.1.3, 4.3.4

reordering and partitioning+stealing flagin: 228 bits

out: 4 blocks3.2.3

block diagonalinterleavingin: 4 blocksout: pairs of

blocks3.2.3

convolutionalcode

k=7, 2 classesin: 121 bitsout: 228 bits

3.2.2

convolutionalcode

k=5, 2 classesin: 267 bitsout: 456 bits

3.1.2

convolutionalcode

k=5, rate 1/2in: 228 bits

out: 456 bits4.1.3

convolutionalcode

k=5, rate rin: N1 bits

out: 456 bits3.n.3

convolutionalcode

k=5, rate rin: P1

out: P2 bits4.6, 4.7, 5.3.2

convolutionalcode

k=5, rate rin: Q1 bits

out: 456 bits5.1.n.3

PDTCH(1-4),PBCCH, PAGCH,

PPCH, PNCH, PTCCH/D

reordering and partitioning+code identifier

in: 456 bitsout: 8 blocks

4.1.4

interface0

TCH/EFS(Enhanced full

rate speech TCH)

CS-1 others

CS-4others

PTCCH/U

CTSAGCH, CTSPCHCTSBCH-SB,CTSARCH

54

Bad frame substitution In speech channels, an important matter affecting speech quality is the bad frame substitution. After error correction, even in good conditions, several % of the speech frames may still be errorneous. In the method originally proposed in the GSM specification, a lost frame is substituted by the previous frame. If several speech frames are lost, they are substituted by attenuated versions of the previous good frame. Each good frame is reproduced with full amplitude regardless of the condition of neighboring frames. This kind of approach causes strange sound effects in poor channel conditions. One can imagine a situation where the system manages to get a correct speech frame through only occasionally. The correct frame is reproduced with full amplitude, and the missing frames after it are replaced by attenuated versions of the previous correct frame. This can be heard as some kind of ringing. Later, this strategy has been improved, and audio signal processing is developed to improve the sound quality in poor channel conditions.

55

Bursts Normal burst Tail

3

Information

58

Training sequence

26

Information

58

Tail 3

Access burst Tail

3

Training sequence

26

Information

36

Tail 3

Synchronisation burst Tail

3

Information

39

Training sequence

64

Information

39

Tail 3

Frequency correction burst

All zeros

148 Some notes: • When modulated, the frequency correction burst produces

almost pure sine wave signal. • Training sequences are pseudo-random sequences with

narrow autocorrelation function. • Adjacent base stations use different training sequences. • The mobile station has to switch off its transmitter between

bursts. Is this a problem?

56

Modulation The modulation chosen in GSM is Gaussian Minimum Shift Keying. This is a quadrature phase modulation scheme where the phase )(tφ of the signal ))(cos()( 0 tttE φω += is changed according to the input data. One can think that the function describing the phase change is a ramp filtered by a low-pass filter whose impulse response is a gaussian pulse. The filtering spreads the phase change over 3 bit periods. Later when higher data rates (up to almost 400 kbits/s) are introduced to GSM, 8-PSK modulation is adopted. This will be described later. In GMSK of GSM, the modulating symbol rate is 1/T = 1 625/6 ksymb/s (i.e. approximately 270.833 ksymb/s), which corresponds to 1 625/6 kbit/s (i.e. 270.833 kbit/s). Before the first bit of the bursts enters the modulator, the modulator has an internal state as if a modulating bit stream consisting of consecutive ones (di = 1) had entered the differential encoder. Also after the last bit of the time slot, the modulator has an internal state as if a modulating bit stream consisting of consecutive ones (di = 1) had continued to enter the differential encoder. Each data value di is differentially encoded. The output of the differential encoder is: })1,0{(ˆ

1 ∈⊕= − iiii dddd

where ⊕ denotes modulo 2 addition.

57

The modulating data value αi input to the modulator is: α αi i id= − ∈ − +1 2 1 1( { , })

The modulating data values αi excite a linear filter with impulse response defined by:

g t h t rect tT

( ) ( ) *= ⎛⎝⎜

⎞⎠⎟

where the function rect(x) is defined by:

rect tT T

for t T⎛⎝⎜

⎞⎠⎟

= <1

2

rect tT

otherwise⎛⎝⎜

⎞⎠⎟

= 0

and * means convolution. h(t) is defined by:

TTt

thδπ

δ⋅

⎟⎟⎠

⎞⎜⎜⎝

⎛ −

=)2(

2exp

)(22

2

where δπ

= =ln( )

.2

20 3

BTand BT

where B is the 3 dB bandwidth of the filter with impulse response h(t), and T is the duration of one input data bit.

58

The phase of the modulated signal is:

ϕ α π( ') ( )

'

t h gii

t iT

= ∑ ∫−∞

−

u du

where the modulating index h is 1/2 (maximum phase change in radians is π/2 per data interval). The time reference t' = 0 is the start of the active part of the burst. This is also the start of the bit period of bit number 0 (the first tail bit). The modulated RF carrier, except for start and stop of the TDMA burst may therefore be expressed as:

))'('2cos(2)'( 00 ϕϕπ ++⋅= ttfTEtx c

where Ec is the energy per modulating bit, f0 is the centre frequency and ϕ0 is a random phase and is constant during one burst.

59

The average spectrum of the GMSK modulated signal is relatively narrow band but 100 kHz away from the center frequency the spectrum has dropped only about 10 dB.

Is this a problem?

60

Principles of Signalling Many functions performed by such a complex network as GSM are distributed over several distant machines, and information exchange (signalling) is needed to coordinate these functions. Signalling is required between: • MS and BTS • BTS and BSC • BSC and MSC • MSC and point of entry to external network • NSS entities (MSC/VLR, GMSC, HLR/AuC, EIR) • OSS and NSS entities + BSC Signalling information is organised into messages. Linking The link protocols in GSM have very similar functionality but they are not the same for all interfaces. The main protocols are summarised in the following table:

Interface Link protocol Comment MS - BTS LAPDm GSM specific BTS - BSC LAPD from ISDN BSC - MSC MTP, level 2 ITU-T SS7

MSC/VLR/HLR protocol

61

Signalling messages are sent over 64 kbit/s circuits, except for the radio interface. On the radio interface, SACCH and FACCH are used, as described earlier. The signalling message information on the link layer is structured into frames. LAPD and MTP-2 have the frame structure of HDLC which is briefly described here. HDLC frames start and end with an 8-bit flag pattern 0 1 1 1 1 1 1 0 ···frame··· 0 1 1 1 1 1 1 0

In the actual data, after each sequence of 5 consecutive “1”s, an extra “0” has to be inserted (irrespective of what is the following bit) in order to avoid flag patterns inside the data. At the receiving end, the extra zeros have to be removed. There are, for example, commercial chips to do this automatically. One flag can be used to indicate both the end of one frame and the start of the next one. With the flag mechanism, the frame contents may be of variable length, without frame length indication. There is, however, a maximum length (272 octets) defined in the SS/ protocols which is sufficient to cover most signalling needs.

62

In LAPDm (radio interface), the use of flags is not necessary. Why? A LAPDm frame has a maximum length of 21 (SACCH) or 23 (TCH) octets. The missing two octets on SACCH are used for timing advance and power control information. Upper layer messages have to be segmented to these fixed lengths. A “more” bit enables the receiver to reconstruct the original message. A length indication is included in every frame, and unused frames are filled with special fill bytes.

63

For error detection, both LAPD and MTP-2 use the HDLC scheme of adding 16 redundancy bits to each frame. The generator polynomial, in this case (using the same notation as earlier), is g(D) = D16+D12+D5+1 LAPDm uses the error correction and detection mentioned in the radio interface chapter. Error detection serves two purposes. If errors are detected, repetition of the frame can be asked for. The second purpose is to monitor the link quality. The link is declared out of order when the error rate exceeds some given treshold (e.g. frame error rate > 4∗10−3). Filling frames are transmitted if nothing else needs to be transmitted, in order to make error counting reliable. Concerning error correction, there are two modes in all three protocols: • non-acknowledged mode, in which frames are transmitted

only once regardless of the error detection status. • acknowledged mode, in which errorneous frames are

repeated. Why, or in what kind of situation, would one want to use non-acknowledged mode if acknowledged mode is available? Think about it before turning page.

64

An example of messages where non-acknowledged mode is more appropriate than acknowledged, are the measurement messages sent by the mobile station. It is better for the base station to get a new up-to-date report rather than a repeated old report, in case a report is lost. Acknowledgement and repetition is based on cyclic frame numbering. In LAPD and LAPDm, acknowledgement os done by the receiver transmitting the number of the next expected frame to the sender. In MTP-2, correctly received frames are acknowledged by sending back the number of the latest correctly received frame. In LAPD, a window mechanism is used where the window size defines how many frames can be sent but not yet acknowledged. There is a maximum for the number of repetitions. The acknowledged mode transmission must be set up with a simple handshaking procedure. The link layer offers the possibility of multiplexing independent flows on the same channel. For example, on the radio interface, signalling and short messages are transmitted on the same channel. The two flows are distinguished with a link identifier (SAPI). However, short messages are not transmitted on the FACCH which makes the short message transmission rather slow,

65

roughly 80 bytes or 600 bits per second. Upper layers reduce this rate further. RLP is another HDLC-like protocol which concerns the transfer of user data (not signalling info). Networking The link protocols enable the exchange of frames between two entities directly interconnected through some physical medium. There are, however, functions which involve entities not directly interconnected, such as MS <−> MSC. Routing is an essential function of networking. This can be done with either datagrams or virtual circuits. Both are used in GSM. Another issue in networking is the possibility to have several independent connections in parallel between entities. From the mobile station point of view, both of the previous issues are handled by the Protocol Discriminator (PD). It gives the functional partition of the messages, but because of the GSM architecture, this partition also corresponds to an entity on the network side (see table on next page).

66

Initial assignment:

Mobile originating call establishment:

68

Networking in the NSS In all cases in the NSS, the SS7 signalling network standards (ITU-T) are used. Two levels of networking: • National networking is based on MTP3 (message transfer part, level

3). • Interconnection of national networks is based on SCCP

(signalling connection control part). Also, two levels of addressing: • MTP address, Signalling Point Code (SPC) • Global title on top of SPC The global title may be a PSTN number, data number, or GSM IMSI. The MTP address in the national network is derived from this. The following figure tries to illustrate this.

In the previous figure, A derives the SPC for GA from the global title, GA derives the SPC for GB, and GB derives the SPC for B.

69

Radio Resource Management In contrast to fixed telecommunication network, in mobile communication network the access resources are allocated to the user only on demand and for the duration of the call. Another cellular system specific feature is the fact that the connection is maintained despite the movements of the user. These are the main functions of radio resource management. Most of the functions in the RR plane relate to the management of transmission between MS and anchor MSC. (Anchor MSC is the single MSC that takes care of the management functions during the whole call. If the MS moves to another MSC area during the call, some of the duties are shared between this relay MSC and the anchor MSC.)

70

Access The MS accesses the network by sending a message on the random access channel (RACH). The network answers by sending and initial assignment message on the paging and access grant channel (PAGCH). This contains the description of the allocated channel. Access on the RACH is not regulated. Two MS’s may send access requests simultaneously. Most of the complexity of the access procedure comes from fixing this problem. Paging and discontinuous reception When an incoming call (from the MS point of view) arrives, the MSC/VLR requests the BSS to perform paging in some of the cells of the BSS. The BSC is in charge of managing the PAGCH. For the sake of power consumption, the downlink common control channel can be divided into several paging sub-channels. The MS listens only to one sub-channel, and sleeps otherwise. The subscribers are assigned to paging sub-channels based on the last digits of their international mobile subscriber identity (IMSI).

71

Transmission mode management Transmission modes

TCH/8 TCH/F TCH/H signalling only signalling only

speech data 3.6 kbit/s data 6 kbit/s

data 12 kbit/s T data 12 kbit/s NT

signalling only speech

data 3.6 kbit/s data 6 kbit/s T

data 6 kbit/s NT

The transmission mode is chosen by the MSC, depending on the end-to-end service. The BSC is in charge of choosing the channel, and coordinating the different machines, including the MS. The connection is always started as “signalling only”.

72

Cipher mode management The connection is always started in non-ciphered mode because ciphering requires a user specific key and the network has to know the identity of the subscriber before it can be used. As in transmission mode management, the MSC makes the decision (upon request from the subscriber) about the transition to ciphered mode. The MSC has to provide the ciphering parameters to all network elements concerned. Discontinuous transmission During speech connection, the user data does not always contain meaningful information (speech silences). Discontinuous transmission may be used to minimise transmission on the radio path, in this case. Discontinuous transmission may be applied independently to each direction. Again, the decision comes from the MSC.

73

Handover preparation Handover may be necessary (or of benefit) for at least three reasons: • when the MS leaves the radio coverage area of the cell in

charge (“rescue handover”), • the overall interference level would be lower if the MS

would be in contact to another cell (“confinement handover”), or

• the cell in charge becomes congested but some of the nearby cells are not (“traffic handover”).

Depending on the purpose of the handover, the criteria for making the decision about handover may differ. The main criterion for rescue handover is the quality of transmission in the current connection. This is indicated by error rate, received signal level, and propagation delay. For confinement handover, the uplink and downlink transmission quality should be known for neighboring cells in case the MS would be in connection with that cell. In practise, only downlink signal levels are measured. The decision process for traffic handover requires information on the load of each BTS, known by the MSC’s and BSC’s. The algorithms for the handover decision are not defined in the GSM specifications.

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Measurements In order to make efficient handovers, measurements should be done as often as possible. The minimum rate of reporting, in GSM, is once per second. The mobile station must report measurements for the serving cell, and up to 6 neighbour cells. The reports are carried on the SACCH which is capable of carrying about 260 bit/s, which is enough for reporting twice per second when the SACCH is not used for other purposes in parallel. Because of the TDMA scheme, the MS has a chance to measure the neighbour cells during the interval between uplink transmission and downlink reception. Each BTS has to transmit continuously, in every burst period, on one frequency, at constant power level. A list of frequencies to be measured is sent to the mobile stations. Each base station has a “color code” included in its transmission. This is included in the measurement report in order to specify which cell was actually measured (the MS may be able to hear several cells using the same beacon frequency. In GSM, the mobile station acquires synchronisation with all cells on which it reports measurements. That is, in addition to FCCH, also SCH is decoded.

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Power control The advantages of power control are (at least) reduced power consumption in the terminal equipment which leads to longer battery life, and reduced interference to other users of the network. In GSM, both uplink and downlink power control may be applied independently. The range for uplink power control depends on the MS power class, but anyway, it is between 20 and 30 dB, with steps of 2 dB. For example, for a class 2 GSM mobile station, the possible transmitter power levels are 13, 15, 17, ···, 39 dBm (20 mW ··· 8W). The mobile station power classes are summarised here.

Class GSM900 DCS1800 1 20W 1W 2 8W 0.25W 3 5W - 4 2W - 5 0.8W -

The transmission power is adjusted in steps of 2 dB (not more), and not more often than 60 ms. So, if an MS is commanded to change it power level 10 dB, it will be adjusted in 5 steps. The initial power level used in access, is fixed for each cell. This level is broadcast on the BCCH.

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Timing advance The transmission and reception of bursts at the base station must be synchronized, as described earlier. Therefore, the MS must compensate for the propagation delays by advancing its transmission 0 ··· 233 µs which is enough to handle cells of radius up to 35 km.

The access burst is short because the timing advance is not known before access.

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Radio channel management The channel configuration of a cell is the list of channels defined to be used in the cell. The channel configuration may change in time. Access Channels The capacity requirement for access channels (RACH, PAGCH) varies depending on traffic load. There are five possible access channel capacities:

CCCH capacity

(TACH equiv.)

number of MS groups

RACH burst rate

(bursts / sec.)

PAGCH message rate (messages / sec.)

½ 1 114.7 12.7 1 1 216.7 38.2 2 2 433.4 76.5 3 3 650.0 114.7 4 4 866.7 152.9

The access channel configuration is broadcast in the BCCH messages. The MS transmits access rerquests and listens to PAGCH corresponding to its own group only. Traffic Channels TACH/F can be exchanged to 8 TACH/8’s (and vice versa).

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Dedicated channel allocation Dedicated channels, as seen from the network side, are either allocated to a mobile station, or in a pool of idle channels. The choice of the allocated channel is the responsibility of the BSC. Channels need to be allocated at: • initial channel assignment (e.g. call setup or location update) • subsequent assignment (e.g. change from TCH/8 to TCH/F) • handover The aim of channel management is to maximise the total traffic which can be served with the given resources. For example, for setting up a call, a TCH/F will be required (eventually) but a TCH/8 would be sufficient until the actual conversation begins. Different strategies can be used here ranging from allocation of TCH/F at initial assignment to allocation of TCH/F only after the called party has answered.

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The radio resource management procedures are described here briefly: Access and Initial Assignment

MS channel request (on RACH)

BTS

MS immediate assignment

(on PAGCH) BTS

MS

“initial message”

(on TACH)

BTS

Reasons: • location updating • answer to paging • user’s request (e.g. outgoing call) MS will repeat unanswered request after a random interval. This is controlled by two broadcast parameters: average time between repetitions, and maximum number of retransmissions. In severe overload situations, user’s may be randomly blocked. Only after the initial message, the network knows the identity of the MS. The MS classmark is sent here including MS revision level, RF power capability, encryption algorithm, frequency capability, short message capability.

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Paging The paging procedure is very straightforward. The MSC sends a paging message to the BSC’s of the location area, the BSC’s send a paging command to the BTS’s concerned, and BTS sends paging messages on PAGCH. For simplicity, the rest of the RR management procedures are only listed here: Transmission mode (change) management Cipher mode management Discontinuous transmission mode management Handover execution Call re-establishment RR-session release Load management SACCH procedures -radio transmission control (power&timing, downlink) (measurements, uplink) -general information Frequency redefinition General information broadcasting (BCCH) -cell selection information -information for idle mode functions -information needed for access -cell identity

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Mobility & Security Management Mobility allowed for the subscribers and its automatic management is a fundamental service in a cellular network. It is also the source of many problems to be solved. Location management GSM has been designed to enable international coverage. The services the user can access when she moves are determined by her subscription, and coverage limitations. The international GSM network is divided into PLMN’s (Public Land Mobile Network) each limited in coverage within the borders of one country. Countries may have more than one PLMN’s whose coverage areas overlap −> competition. In order to allow roaming (moving from one PLMN area to another), the PLMN’s must communicate with each other. A GSM user has a subscription relationship with a single PLMN which we can call the home PLMN. GSM phase 1 specifications treat all PLMN’s other than the home PLMN on the same basis for selection. Access to them may be allowed or not depending on other conditions (such as agreements between operators). Access to PLMN’s of other countries is possible if the subscription allows it. The GSM specification also allows national roaming but this is not commonly implemented.

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Mainly for paging purposes, the PLMN area is divided into location areas. Each location area is managed by one MSC/VLR.

In order to obtain normal service, the subscriber must be registered in the location area of the cell. The registration state is changed in a location update procedure which is initiated by the MS. The identity of the last location area is stored on the SIM (even if the MS is switched off). PLMN selection If the serving PLMN can no longer provide normal service, the MS will search for the whole spectrum to find which PLMN’s cover the location. The selection of PLMN may be manual or automatic. At switch-on, the home PLMN is searched first. In manual mode, a list of found PLMN’s is displayed, and the user chooses one from the list. In automatic mode, the MS will make the

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selection automatically based on a list of preferred PLMN’s stored on the SIM. Cell selection The selection of the serving cell is mainly based on transmission quality which is measured as the signal level received by the MS. The criterion that is (or may be) used in cell selection can be described as follows. C1 = A − max(B,0) (all values in dB) where A = (average received level) − p1and B = p2 - (MS maximum RF power) The parameter p1 is the minimum of the received level with which the cell can be accessed. This may be in the range −110 ··· −48 dBm. The parameter p2 is the maximum tx power allowed for an MS in the cell. These parameters are broadcast by the cell. In cell selection, only cells with positive C1 are considered. The candidate cells are ordered according to C1.

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The criterion C1 determines two things: • the coverage limit of each individual cell (note: this may be

different for different MS’s) • the boundary between two adjacent cells (see figure)

These borders are not fixed but change over time depending on weather, traffic conditions etc. Near the border between cells, the MS might have to change back and forth between the two cells if the decision is strictly based on which one has better C1. This is prevented with another broadcast parameter called cell reselect hysteresis. In the new candidate cell, C1 must be c.r.h. higher than the C1 of the serving cell before changing to the new cell. In this case, the borderline where handover occurs depends on the direction of movement (figure).

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So, when the level received from another cell becomes considerably higher than the level of the serving cell, the MS will start listening to the new cell. If the new cell is in another location area, the MS initiates a location update procedure. If there is a call in progress, handover is executed. Location management architecture The HLR is basically an intelligent database used to store some location information, and subscription related information. The VLR is a database where subscriber information is temporarily stored for those subscribers currently registered in the MSC area. Location update procedures Location update is done, naturally, when the MS moves from one location area to another, but also periodically. The period may vary from 6 minutes to 24 hours. The MS initiates the location update procedure (see figure).

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If HLR needs to be contacted, the new MSC needs to know the identity of the subscriber in order to know which HLR to contact. Subscribers are identified by IMSI (International Mobile Subscriber Identity) which is as follows Mobile Country Code (3 digits)

Mobile Network Code (2)

Mobile Subscriber Identification Number (max. 10 digits)

On the air interface, an alias called TMSI (Temporary Mobile Subscriber Identity) is used whenever possible. IMSI attach and detach procedures In order to avoid unsuccesful trials to route calls to an MS which is swithed off, IMSI attach and IMSI detach mechanism has been introduced. These are very similar procedures to location update but usually HLR is not concerned. When the MS is switched off, IMSI detach procedure will inform the corresponding MSC/VLR that it is no use trying to route calls to this user now. If the power is switched on while the MS is in the same location area, an IMSI attach procedure is executed. Otherwise, location update will be done.

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Security management In wireless communication, generally, the security issues are especially important. Radio transmissions are easy to listen, and unregistered use of the network may cause severe economical losses. The security functions of GSM serve two goals: • protect the network against unauthorised access • protect the privacy of the user The first goal is achieved by authentication. The second goal is achieved by ciphering the user traffic and signalling, as well as using a temporary identity on the air interface. Authentication is started by sending a random number to the MS. The MS (or actually the SIM) calculates a response using the random number and a secret key stored on the SIM using a confidential algorithm. If the response matches the value calculated on the network side, the authentication is OK. The algorithm may be operator dependent. Ciphering is done by generating a ciphering sequence based on the current frame number and another secret key. The transmitted data is then x-or’ed with the ciphering sequence. At the receiving end, the same operation is repeated which deciphers the data. The encryption keys are also generated locally in a similar way as the authentication response is calculated.

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Communication Management GSM is basically an access network for general telecommunication systems, such as PSTN or ISDN. The communication management procedures of GSM are simplified and somewhat adapted copies of those specified for ISDN. Management functions • Attributes of communication: directory number of the called

party, forwarding conditions, etc. • Setting up the transmission path: MSC analyses the called

number and requested service in order to choose the external network where the call will be routed.

• Routing of mobile terminated calls: some explanation

follows shortly. • Management of alternate services: alternate speech and data,

multiple calls, etc. • Transmission of DTMF tones • Release of the call • Supplementary services: call forwarding, barring, etc. • Short message communication

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Routing of a mobile terminating call is started by first asking the necessary information from the HLR of the called party. The HLR, where the query should be addressed, can be determined from the first digits of the called number.

country code

national destination

code

subscriber number

e.g.

+44 385 (UK Vodafone GSM number) +358 50 (Radiolinja GSM number)

The first digits of the subscriber number identify the HLR of the user. Note that the subscriber number, and IMSI are two different numbers. The HLR record contains sufficient information for finding the MSC where the user is currently visiting. Who pays what in mobile terminating calls? The total cost of a call depends (obviously) on the location of the GSM subscriber (long distance / local). Also, probably the calling party would like to know in advance how much he will be charged. Furhermore, it is assumed that GSM subscribers do not want their location to be known.

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Therefore, the charge to the calling party is independent of the actual location of the called party. The called party will pay for the rest of the expenses. Examples follow:

situation charging call within one country

from PSTN to home PLMNthe calling party pays for the call to PSTN operator,

PSTN operator may compensate to PLMN

operator (if agreed) call to a GSM phone whose home PLMN is in the same

country but the user is roaming in another PLMN

the calling party pays the same as above, the called

party will pay for the international part to the home PLMN operator

call to a GSM phone whose home PLMN is in another country, and the GSM user

is roaming in a third country

the calling party pays for an international mobile call to the home PLMN country, the called party pays the

same way as above Note that the GSM user gets her bills only from her home PLMN operator. The different operators involved will deal with the compensations between themselves. Communication management procedures are (in normal conditions) rather straightforward message exchanges. An example of a mobile originated call establishment was presented earlier (page 67).

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Short messages For a short message communication, a virtual circuit is not established but the messages are delivered similarly to signalling messages. A short message communication is limited to one message (but of course, applications using short messages may combine several messages). The transmission of a message is relayed by a Short Message Service Centre (SM-SC).

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Network Management Because of the complexity of modern telecommunication networks, and because of the need for cost-effectiveness, it is impossible to maintain and run the network without an independent computer network dedicated for this purpose. The general tasks of network management are: • service & subscriber management • mobile station management • maintenance • system engineering and operation. These will be briefly described here.

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Service & subscriber management This task involves the subscription administration, as well as billing and accounting. Access to the GSM services is possible only for subscribers known by the network. From the network point of view, the subscription is materialized in the SIM, and the corresponding entries in HLR and the authentication centre (AuC). Naturally, means are needed to create, upgrade, and delete subscription data. A commercial structure which is becoming more and more common is the concept of service providers.

Network operator

HLR info

charge info

Service provider

subscription

billing subscription

billing

☺

☺

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As mentioned earlier, the subscriber receives a single bill from the operator or service provider with which the customer hold her subscription. The services used in other networks are paid by the operator of the home PLMN of the user, and naturally, the home PLMN operator will charge the user.

Home PLMN

charge info +invoice

Visited PLMN

HLR info charge info

Service provider

(if any)

subscription

billing

access to service

☺

Maintenance Maintenance includes the techniques aiming at minimising the loss of service quality when a failure occurs, as well as measures to minimise failure occurrences. There are, naturally, numerous sources of failure. Even though electronic components are very reliable, there are tens of

95

thousands of them in a network, and some more or less serious failures will probably occur daily. In principle, all network elements and events should be monitored for possible failures. Regular and automatic testing should also be performed. The users are a valuable source of information, as well. To minimise the loss of service quality, it is useful to be able to do some “first aid” repair automatically, or at least remotely. For example, a faulty base station can be barred, and the neighboring base stations reconfigured for the actual repair to take place. Mobile station management All mobile stations must be type approved. When type approved, the mobile station receives a Type Approval Code (TAC) which is part of the International Mobile Equipment Identity (IMEI) number.

TAC FAC serial number reserved

FAC = final assembly code to identify the final assembly plant. Serial numbers are allocated to the manufacturers. Operators are notified of the valid IMEIs they can expect on their network. The network may ask for the IMEI of a mobile station, for example, if it detects a problem which may be caused by the mobile station.

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Lists of IMEIs are stored in the Equipment Identity Register (EIR). The information stored in the EIR can be operator dependent. However, for example, the control of stolen mobile stations is effective only when operators have common IMEI checking policy. The GSM operators use three levels (or lists) for the status of IMEI’s: • The white list includes the ranges of IMEI’s allocated to

type approved mobile equipment. • The black list includes mobile equipment that need to be

barred because they are stolen, or because of severe malfunctions.

• The grey list includes the IMEI’s of suspicious cases. System engineering A network in operation is composed of actual machines. The operator must choose how many of each machine to order, with which capacity, where to install them, etc. Also, the traffic does not remain constant. Installing from the start a large enough capacity for long term traffic is not cost effective. Therefore, the system engineering needs to be refined while in operation. The goal of cellular planning is to choose the cell sites and cell parameters (frequency allocation, capacity, power, etc.) in

97

order to provide economically continuous coverage, and support the required traffic density. Cellular planning is a cost optimisation problem with some constraints, such as: • range: The transmitted power is limited to 20W in vehicle-

mounted, and 2W in handheld GSM phones. The propagation loss of the signal is a complicated process but usually the received average signal level is modelled to be proportional to d−α where d is the distance and α is a constant in the range 3 < α < 4 depending on the environment. Because of multipath propagation, the actual signal level varies a lot around this average.

• interference: In cellular systems, the main sources of

interference to a particular user are the other users of the same system. In addition to range, interference is another factor affecting the cell size, especially in dense traffic areas.

In addition to these factors, cost efficiency is affected by: • handover criteria • power control • discontinuous transmission • frequency hopping In addition to cellular planning, the designer has to deside the cell capacity in terms of number of traffic, control and access channels.

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The following table summarises the parameters which system engineering must manage to optimise the network configuration for a given quality of service to an expected number of users.

Area Parameters

Cell planning frequencies

beacon frequencies hopping sequences

power control parameters handover parameters

cell selection parameters BSIC

Dimensioning

# of common channels # of traffic channels

location areas periodic location updating

Load control overload control parameters

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Network operation This is the major task of network management. CCITT (or ITU-T) has developed a Telecommunication Management Network (TMN) concept which aims at providing basis for integrated management networks. TMN distinquishes three main categories of functions: • The operation system functions, which are the management

applications running on the operating staffs’ workstations. • The mediation functions are introduced for concentration

purposes, and for adaptation of the generic operation system functions to specific machines.

• The data communication functions, which are the transmission means used to link the operation system functions, the mediation functions, the traffic handling equipment, and the workstations. These can be point-to-point lines, or X.25 or SS7 network.

The GSM specifications include a basis for so called Q3 protocol, which is a protocol for network management between operation system functions and mediation functions. Most of the other protocols are left for the operators (or manufacturers) to specify.

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Evolution of the GSM System After the introduction of basic GSM specification and service, numerous new features and capabilities have been specified. The GSM system specification has been developed in phases, and the work is still going on. In the following, we will study the main new features specified since the basic GSM introduction. High Speed Circuit Switched Data A straightforward way, in principle, to increase the data rate in GSM data transmission is to use several time slots out of each TDMA frame for one data connection. However, from terminal design (and cost) point of view, the matter is not so straightforward. A major part of the cost and manufacturing problems comes from the RF and IF parts of the terminal. With the existing specification (one slot per connection), the MS does not have to transmit and receive at the same time which simplifies the design.

0MS RX

MS TX

Monitor

1 2 3 4 5 6 7 0 1

0 2 3 4 5 65 6 7 1

Now, if in the previous figure, there is enough time between the TX/RX activities (shaded areas), the terminal can be implemented using one frequency synthesizer (it takes some time for the synthesizer to change from one frequency to another).

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If two time slots per TDMA frame is used for one connection, which doubles the data rate, the timing is as follows.

MS RX

MS TX

Monitor

0 1 2 3 4 5 6 7 0 1

0 2 3 4 5 65 6 7 1

In this case, the TX/RX activity periods are not overlapping but more efficient technology is needed to implement this with a single synthesizer. This makes the terminal more complicated and expensive. Also, power consumption is higher. If more time slots are used in order to obtain higher data rate, the TX/RX activity periods necessarily overlap, and more frequency synthesizers (2 or 3) are needed for the implementation. MS RX

MS TX

Monitor

0 1 2 3 4 5 6 7 0 1

0 2 3 4 5 65 6 7 1

3 slots: 4th slot: 5th slot: The previous figure shows 3, 4, and 5 slots/frame configuration. Eight time slots would mean continuous transmission as a full duplex FDMA system. Monitoring neighboring base stations would require an independent receiver, and the terminal would be much more expensive than one slot terminals. Also, power consumption would be much higher.

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For some applications, the uplink/downlink division is useful to be asymmetric. Usually, the downlink is required to have more data rate. The asymmetric cases illustrated below (2+1 & 3+1 slots) can be implemented without need to transmit and receive at the same time in the terminal device. MS RX

MS TX

Monitor

0 1 2 3 4 5 6 7 0 1

0 2 3 4 5 65 6 7

MS RX

MS TX

Monitor

0 1 2 3 4 5 6 7 0 1

2 3 4 5 65 6 7 1

The examples above were based on the assumption that one user gets several consecutive time slots. From the operator point of view, it would be more efficient to be able to allocate time slots more freely. For example, one user might get time slots 1, 3, and 6 in case of 3-time-slot operation. Without any assumptions about the allocated time slots, the mobile terminal should be capable of full-duplex operation with independent monitoring of neighboring base stations. In this case the terminal equipment would be almost as expensive as an 8-slot terminal. The multi-slot systems have required changes in several aspects of the specifications such as ciphering, frequency hopping, and generally radio resource management functions.

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General Packet Radio Service (GPRS) The GSM system was not originally designed for packet data transfer but many data applications are bursty and it is more efficient if the radio resource is reserved only when something needs to be sent. GPRS features include: • True packet radio system - sharing the network and air

interface resources • Volume based charging • TCP/IP (Internet & Intranet) interworking, and SMS over

GPRS, (and X.25 interworking) • Peak data rate from 9.05 kbps to 171.2 kbps • Protocols designed for evolution of radio

EDGE - new GSM modulation Migration into 3rd Generation

The following figure shows (once more) the GPRS reference model.

BTS BSC

MSC

SGSN GGSN Internet

PSTN

GPRS Backbone IP Network

GPRS Core

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The new elements introduced in GPRS are the serving GPRS support node (SGSN) and the gateway GPRS support node (GGSN). SGSN's tasks include: authentication & authorization, GTP tunneling to GGSN, ciphering & compression, mobility management, session management, interaction with HLR, MSC/VLR, charging & statistics, as well as NMS interfaces. GGSN's tasks include: interfacing to external data networks (resembles a data network router), encapsulating data packets in GTP and forwarding them to right SGSN, routing mobile originated packets to right destination, filtering end user traffic, as well as collecting charging and statistical information of data network usage. After logging to GPRS network, the radio resource is not dedicated to a particular user, but users can request channel capacity with a simple and fast procedure. New modulation, Higher data rate - EDGE In the GMSK modulation of basic GSM, the modulating symbol rate is about 271 ksymbols/s, and 1 bit/symbol is transmitted. With 8-PSK modulation and keeping the symbol rate, we can transmit 3 bits/symbol, and increase the data rate correspondingly.

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The 3 bits are mapped to a 8-PSK constellation using Gray-code as shown in the following figure.

(0,0,1)

(1,0,1)

(d3i, d3i+1, d3i+2)=(0,0,0)

(0,1,0)(0,1,1)

(1,1,1)

(1,1,0)(1,0,0)

I

Q

The 8-PSK symbols are continuously rotated by 8

3π radians per symbol before pulse shaping.

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CDMA Systems Intro In digital systems, there are three basic multiple access schemes: frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). Quite many systems are combinations of FDMA/TDMA (such as GSM or US TDMA), or FDMA/CDMA (such as US CDMA). In theory, it does not matter whether the spectrum is divided into frequencies, time slots, or codes. However, in practical systems, especially, mobile cellular communication, we find that some schemes are better suited in certain communication media than others. CDMA systems are commonly based on spread spectrum technologies which have originally been developed for military communication purposes. In military applications, the advantages are the facts that spread spectrum signals are difficult (for those who do not know the codes) to • detect • receive and decode • jam

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In mobile cellular communications, additional advantages of spread spectrum technology are: • s.s. signal is less sensitive to frequency selective multipath

fading because of the large bandwidth which gives natural frequency diversity

• careful frequency planning of cells is not required because

all cells may use the same frequency band • handovers can be made more reliably when the frequency

does not change, only code is changed (soft handover) • network capacity (number of users) does not have any strict

upper limit, additional users are seen as increased interference level

• utilisation of voice activity cycles increases capacity directly • instead of an equalizer, a correlator can be used in the

receiver • no guard times or guard bands needed in CDMA On the other hand, in order to get maximum capacity, the transmitter powers should be accurately controlled, especially in the uplink (MS to BS) direction. Otherwise, an MS near the BS may block another MS which is far away from the BS.

108

Spectrum spreading can be accomplished by direct sequencing or frequency hopping. Direct sequence method: Each information bit is symbolized by a large number of coded bits called chips. For example, if an information bit rate R = 10 kbits/s is used and it needs a information bandwidth B = 10 kHz, and if each bit is coded by 100 chips then the chip rate is 1 Mchips/s which needs a DS bandwidth Bss= 1 MHz. The spectrum spreading is measured by the processing gain (PG, in dB)

⎟⎠⎞

⎜⎝⎛=

BBPG SSlog10

The PG of our example is 20 dB. Frequency hopping method: A frequency hopping receiver would equip N frequency channels for an active call in order to hop over those N frequencies in some determined hopping pattern. If the information channel width is 10 kHz and there are N=100 channels to hop, the FH bandwidth Bss=1 MHz. The processing gain is again 20 dB. The hopping can be either fast, which makes two or more hops for each symbol, or slow, which puts two or more symbols for each hop. The transmission data rate is the symbol rate. GSM, for example, uses slow frequency hopping.

109

The basic DS technique is illustrated in the following figure.

Let x(t) be a BPSK data stream (a sequence of +1's and −1's). Then S(t) = x(t) cos(2πf0t) The spreading sequence G(t) also is BPSK (G(t) = ±1). Then St(t) = x(t) G(t) cos(2πf0t)

110

At the receiving end, St(t − T) is received after T seconds of propagation delay. An estimate of the delay T̂ is obtained with a correlator. Then S(t − T) = x(t − T) G(t − T) G(t − T̂ ) cos[2πf0(t − T)] If the delay T is estimated correctly, and since G(t) = ±1 G(t − T) G(t − T̂ ) = 1 Then S(t − T) = x(t − T) cos[2πf0(t − T)] and the data is recovered by modulating with the carrier. The spreading sequences G(t) are usually pseudonoise (PN) sequences, which can be generated, for example, with simple sequence generators.

Different sequence generators (even with same number of shift registers) produce different length sequences. The maximum length of a sequence from a generator with N registers is L = 2N − 1.

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The spreading sequences should have a few special properties • correlation between two different spreading codes F and G

∑ +⋅=n

FG mnGnFm )()()(φ

should be small for all values of m. • autocorrelation of a spreading code G

∑ +⋅=n

GG mnGnGm )()()(φ

should be small for all values of m except m = 0. The design of a large number of such codes is a challence. Reduction of interference by a DS signal Example: narrow-band interference

112

An example of cellular CDMA systems is IS-95 (or US-CDMA) specified by the Telecommunications Industry Association (TIA). In the U.S. the operators have a choice of implementing the US-TDMA (IS-54), US-CDMA (IS-95), or any other system within their frequency allocation. IS-54 and IS-95 have been defined such that they can coexist with the analog AMPS system, and dual-mode terminals are used until the digital services become more common. The following figure shows the IS-95 network reference model.

Immediately, one can see that this is very much like GSM, at this level. Naturally, there are differences if we go to details. One major difference is the air interface which is, of course, based on CDMA technology instead of TDMA.

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The frequency spectrum for cellular systems in the U.S. is allocated (in an auction) to network operators in slices of 1.25 Mhz. An operator might get, for example, a set of ten 1.25 Mhz channels in each direction. A channel in the IS-95 system occupies (almost) the whole 1.25 Mhz band. Reverse (MS to BS) and forward (BS to MS) channels have different configuration. A reverse CDMA channel is composed of access channels and traffic channels. All traffic and access channels share the same spectrum using direct sequence CDMA. The spectrum spreading is done in several phases. First, a 64-ary Walsh modulator is used, that is, each 6 bits in the sequence selects a row in a 64x64 Walsh matrix to be transmitted further. Each traffic channel is identified by a distinct user long code sequence. Signals from different MSs are distinguished by length 242 − 1 PN sequence with user address dependent time offset. Following the long PN code, the signals are further spread in quadrature by two short (215 − 1) PN codes.

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The following figure illustrates the reverse traffic channel operations from data to continuous waves.

The forward channel (BS to MS) is different in characteristics, and also the transmission method of IS-95 forward channel is different from reverse channel. In this case, the short PN codes (I and Q) are used for separating transmissions from different BSs. Length 64 Walsh functions are used for separating signals within a cell. The following figure illustrates the forward channel

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The forward channel contains traffic channels, synchronisation channel, paging channel, and a pilot signal. -------- Introduction to 3rd generation systems utilising CDMA will be given in a separate presentation.

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principles of digital mobile communication systems · principles of digital mobile communication...

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