Cellular NetworksWhy use cellular networks? What mobile radio services where provided before cellular?•Use multiple low-power transmitters (100 W or less), because the range of such transmitter is small, an area divided into cells

1.each served by its own antenna

2.Served by base station consisting of transmitter, receiver, and control unit

3.Band of frequencies allocated

4.Cells set up such that antennas of all neighbors are equidistant

(hexagonal pattern)

Frequency Reuse1. Adjacent cells assigned different frequencies to avoid

interference or crosstalk2. Objective is to reuse frequency in nearby cells Minimum distance between co-channel cells? (minimum

distance between centers of cells that use the same band of frequencies(co-channels))

Number of frequencies assigned to each cell? (10 to 50) Transmission power control needed (to allow

communication within a cell using a given frequency while limiting the power at that frequency that escapes the cells into the adjacent ones. )

Reuse factor? (N, number of cells in the repetition pattern)

Ways to increase the capacity

Add new channels (if such exists…)Frequency borrowing from adjacent cells

(dynamically or permanent)Cell splitting –cells in areas of high usage

can be split into smaller cellsCell sectoring –cells are divided into a

number of wedge-shaped sectors, each with their own set of channels

Microcells–radius less than 1 km. Used at crowded streets, inside buildings, along highways,…

Cellular System Overview

Cellular Systems Terms

Base Station (BS) –includes an antenna, a controller, and a number of receivers

Mobile Telecommunications Switching Office MTSO (or Mobile Switching Center MSC) –connects calls between mobile units

Two types of channels available between mobile unit and BS

1.Control channels –used to exchange information having to do with setting up and maintaining calls

2.Traffic channels –carry voice or data connection between users

Typical call Monitor for strongest signal Request for connection Paging Call accepted Ongoing call Handover (handoff) or termination Also: call blocking, call drop, call to fixed phone network

Mobile Radio Propagation EffectsTake into consideration in design:1.Signal strength2.Must be strong enough (signal quality at

the receiver)3.Must not be too strong (co-channel

interference)4.Fading5.Cell size6.Carrier freq., antenna positions, …7.Differ between urban, suburban and open

areas

Handover

Also called Handoff (mainly in US)Network initiated or mobile-assistedPerformance metrics that may be used in

decision:1.Cell blocking probability (new call being

blocked)2.Call dropping probability (termination of call

due to handoff)3.Call completion probability (admitted call is

not dropped before it terminates)4.Probability of unsuccessful handover

(handoff is executed while the reception conditions are inadequate)

5: Handover blocking probability (handoff cannot be successfully completed)

6: Handover probability (handoff occurs before call termination)

7: Rate of handover (number of handovers per unit time)

8: Interruption duration (the interruption of time during the handoff in which the mobile is not connected to either BS )

9: Handover delay (the mobile moves from one point to another where the other handoff should occur but it delays)

Handover Strategies Used to Determine Instant of Handover

Relative signal strengthBecause signal strength fluctuates due to the

multipath effects, even with power averaging, this approach can lead to a ping pong effect in which a mobile unit is repeatedly passed back and forth between two base stations.

Relative signal strength with threshold

Handoff occurs if 1. Signal at a current BS is sufficiently weak (less then a

predefined threshold)2. The other signal is stronger of the two.3. If the signal at a current BS is sufficiently strong,

handoff is unnecessary.4. There are three threshold schemes (th1,th2 & th3)5. For th1, no need for handoff, for th2 handoff will occur

at L2, for th3, handoff will occur at L4. this reduces the quality of communication and probability of call dropping increased.

6. It is because of the interference between powers of to base stations.

Relative signal strength with hysteresis

1. The power of the BS 2 is sufficiently stronger, and the power of the BS 1 weakens gradually is the case of perfect handoff (by a margin H).

2. In this case handoff occurs at L3.3. This scheme prevents ping pong effect, because when

handoff occurs, the effect of margin H reversed.4. Hysteresis (H) is known as relay hysteresis. 5. Handoff from BS 1 to BS 2 occurs when signal strength

reaches or exceeds H and vice versa.

Relative signal strength with hysteresis and threshold

Handoff occurs when,1. Current signal level drops below the threshold2. Target BS is stronger then the current one by a

hysteresis margin H.3. In our case it is at th3

Prediction techniquesThe handoff decision is based on the expected future

value of the received signal strength.

Power ControlWhy is it desirable to include dynamic

power control in a cellular system?1. Received power must be sufficiently above the

background noise for effective communication2. Desirable to minimize power in the transmitted signal

from the mobile Co-channel interference Health concerns Battery power

In spread spectrum systems using CDMA it is crucial for the decoding to equalize the received power level from all mobile units at the BS

Types of Power Control Open-loop power control1. Depends solely on mobile unit with no feed back from

BS, and is used in some SS (spread spectrum) systems2. Uses the pilot signal3. Not as accurate as closed-loop, but can react quicker to

fluctuations in signal strength (important in CDMA)4. Pilot allows the MS to acquire the timing of the forward

(BS to MS)CDMA channel and provides the phase reference for demodulation.

5. MS monitors the received power level of pilot and sets the transmitted power in the reverse (Ms to BS) channel inversely proportional to it.

6. That’s why CDMA mobile sets get less battery power then GMS mobile sets.

Closed-loop power control1. Adjusts signal strength in reverse channel (MS to BS)

based on metric of performance2. Which means BS makes the power adjustment

decisions, received SNR, received BER, received signal power level.

3. BS makes power adjustment decision and communicates to mobile on control channel

4. Closed loop power control is used to adjust a power in the forward channel.

5. Which means that power for the GSM mobile sets are adjusted by the BS. So because of dual power control they take more battery consumption then CDMA mobile sets.

Example: GSM uses closed-loop with 8 power classes in BS (2.5-320 W) and 5 in mobile station (0.8-20 W)

Traffic Engineering“To size the network to be able to handle some expected level of load”

In practice, not feasible to have capacity handle all possible load

For L subscribers and capacity of N simultaneous users:

1.L <N–non blocking system2.L> N–blocking system

Blocking System Performance Questions (Assignment)

Probability that call request is blocked?

What capacity is needed to achieve a certain upper bound on probability of blocking?

What is the average delay?What capacity is needed to achieve a certain average delay?

Traffic IntensityLoad presented to a system:

A=λhλ= mean rate of calls (requests) attempted per unit time

h =mean holding time per successful call

A= traffic intensity [erlang]

Factors that Determine the Nature of the Traffic Model

Manner in which blocked calls are handled1. Lost calls delayed (LCD) –blocked calls put in a queue

awaiting a free channel2. Blocked calls rejected and dropped3. Lost calls cleared (LCC) –user waits a random time

interval before another attempt4. Lost calls held (LCH) –user repeatedly attempts calling Number of traffic sources, L1. Whether number of users is assumed to be finite or

infinite2. Infinite L easier to analyze (reasonable assumption

when L is at least 5 to 10 times the capacity)

First-Generation Analog (examples)

Advanced Mobile Phone Service (AMPS) In North America, two 25-MHz bands allocated to AMPS

(to/from base station), below 900 MHz Each band split in two to encourage competition Channel spacing 30 kHz, which allows 416 channels per

operator from which 21 channels for control and 395 channels to carry calls (Total 832 channels, uplink(MS to BTS) & downlink(BTS to MS))..See table 10.4 page 318 book.

Nordic Mobile Telephone System (NMT) Mainly in Scandinavia, but later spread Two systems: 450 MHz and 900 MHz Channel spacing 12.5 kHz, 1999 channels (NMT900)

Differences Between First and Second Generation Systems

Digital traffic channels in second generationEncryption –all second generation systems

provide encryption to prevent eavesdropping (To listen secretly)

Error detection and correction in second-generation digital traffic

Channel access –second-generation systems allow channels to be dynamically shared by a number of users

Higher data rates

Mobile Wireless TDMA Design Considerations

Number of logical channels: (number of time slots in TDMA frame)

Maximum cell radius (R): 35 Km to give a sufficiently high traffic level in rural areas

Frequency: Region around 900 MHz.Maximum vehicle speed (Vm): 250 Km/hr or 69.4 m/s to accommodate mobile units in high speed trains

Maximum coding delay: Approximately 20 ms to avoid unduly to delays within fixed networks, which may involve satellite links

Maximum delay spread (Δm): The difference in the propagation delay among different multipath signals arriving at the same antenna.

Bandwidth: Not to exceed 200 KHz corresponding to 25 KHz/channel

Global System for Mobile Communications, GSM

Developed to provide a common 2G technology for Europe

Now mainly in Europe and Pacific Asia but also in North America.

Standardized functional elements (equipment from different manufacturers can be combined)

Today probably the most popular standard for new implementations

Subscriber Identity Module (SIM) concept of great importance

Mobile Station

Mobile station communicates across air interface with base station transceiver in same cell as mobile unit

Mobile equipment (ME) –physical terminal, such as a telephone or PCS

ME includes radio transceiver, digital signal processors and subscriber identity module (SIM)

GSM subscriber units are generic until SIM is inserted

SIMs roam, not necessarily the subscriber devices (only the device can be used for emergency services without SIM)

Base Station Subsystem (BSS)

BSS consists of base station controller and one or more base transceiver stations (BTS)

Each BTS defines a single cellIncludes radio antenna, radio transceiver

and a link to a base station controller (BSC)

BSC reserves radio frequencies, manages handover of mobile unit from one cell to another within BSS, and controls paging

Network Subsystem (NS)

NS provides link between cellular network and public switched telecommunications networks

Controls handovers between cells in different BSSs

Authenticates users and validates accounts

Enables worldwide roaming of mobile users

Central element of NS is the mobile switching center (MSC)

Mobile Switching Center (MSC) supported by 4 databases

Home location register (HLR) database –stores information about each subscriber that belongs to it

Visitor location register (VLR) database –maintains information about subscribers currently physically in the region

Authentication center database (AuC) –used for Authentication activities, holds encryption keys (note: no encryption in PSTN end) Equipment identity register database (EIR) –keeps

track of the type of equipment at the mobile station. Blocking stolen mobile phones, un-approved phones, …

Radio Link

2 x 25MHz allocatedCombines FDMA and TDMA125 channels full duplex, spaced 200kHz, data rate 271 kbps

TDMA Format Time Slot Fields

Trail bits–allow synchronization of transmissions from mobile units located at different locations from base station

Encrypted bits–encrypted data in blocks by conventional encryption of 114 plaintext bits into 114 cipher text bits. Encrypted bits are then placed in two 57 bits fields in the time slot

Stealing bit-indicates whether block contains data or is "stolen“ for urgent control signaling

Training sequence–used to adapt parameters of receiver to the current path propagation characteristics and to select the strongest signal in case of multi path propagation.

Timing sequence is also known as bit pattern that differs for different adjacent cells.

Guard bits–used to avoid overlapping with other bursts due to different path delays

GSM Frame Format

8 slot TDMA frames are typically organized into 26-frame multi frame. One of the frame in multi frame is used for control signaling and the other is currently unused, leaving 24 frame for data traffic.

So each traffic channel receives one slot/frame and 24 frames/120ms multi frame, then the resulting data rate is:

The GSM specification also allows half rate traffic channels, with two traffic channels occupying one time slot in 12 of the 26 frames. With the use of half rate speech coders, this effectively doubles the capacity of the system.

There is also a 51 frame used for control traffic.

Frame structure is the division of defined length of digital information into different fields (information parts). A GSM frame is 4.615 m sec and it is composed of 8 time slots (numbered 0 through 7). During voice communication, one user is typically assigned to each time slot within a frame. The GSM system also combines frames to form Multi frames.

Multi frames are frames that are grouped or linked together to perform specific functions. Multi frames on the GSM system use established schedules for specific purposes, such as coordinating with frequency hopping patterns. Multi frames used in the GSM system include the 26 traffic multi frame, 51 control multi frame, super frame, and hyper frame.

Traffic Multi frame Structures - The 26 traffic multi frame structure is used to send information on the traffic channel. The 26 traffic multi frame structure is used to combine user data (traffic), slow control signaling (SACCH), and idle time period. The idle time period allows a mobile device to perform other necessary operations such as monitoring the radio signal strength level of a beacon channel from other cells. The time interval of a 26 frame traffic multi frame is 6 blocks of speech coder data (120 m sec).

Control Multi frame Structures - The 51 control multi frame structure is used to send information on the control channel. The 51 frame control multi frame is sub divided into logical channels that include the frequency correction burst, the synchronization burst, the broadcast channel (BCCH), the paging and access grant channel (PAGCH), and the stand-alone dedicated control channel (SDCCH). The PAGCH is logically sub divided into PCH and AGCH.

Super frame - A super frame is a multi frame sequence that combines the period of a 51 multi frame with 26 multi frames (6.12 seconds). The use of the super frame time period allows all mobile devices to scan all the different time frame types at least once.

Hyper frame - A hyper frame is a multi frame sequence that is composed of 2048 super frames, and is the largest time interval in the GSM system (3 hours, 28 minutes, 53 seconds). Every time slot during a hyper frame has a sequential number (represented by an 11 bit counter) that is composed of a frame number and a time slot number. This counter allows the hyper frame to synchronize frequency hopping sequence, encryption processes for voice privacy of subscribers' conversations.

Speech and Data EncodingSpeech coding

1.RPE-LPE, regular pulse excited -linear predictive coding (Data from the previous samples are used to predict the current sample) (each sample is then encoded to consist of bits representing the coefficients of the linear combination of previous samples plus an encoded form of the difference between the predicted and actual sample)

1. Bits divided into 3 classes with different protection

2. 22.8 kbps resulting data rate3. spread into different time slots,

interleaving (interleaving is a way to arrange data in a non-contiguous (referring to two or more information's of the system which are not connected) way to increase performance, e.g ,, In error-correction coding, particularly within data transmission, disk storage, and computer memory.)

4. Gaussian minimum shift keying is used

Data encoding

1.Error correcting code2.Interleaving3.9.6, 4.8 or 2.4 kbps

Freq. Hopping and Delay Equalization

Frequency hopping1. Slow frequency hopping, hop for each TDMA frame2. Suppresses multipath fading and co-channel

interference Delay equalization1. Several users with different distance share same TDMA

frame2. Base station provides control signal, mobile unit adjusts

timing3. Tail and guard bits provide margin

Functions Provided by Protocols Protocols above the link layer of the GSM

signaling protocol architecture provide specific functions:

1.Radio resource management ( controls the setup, maintenance, termination of radio channels including handoff)

2.Mobility management (Manages the location, updating registration procedures, as well as security authentications)

3.Connection management (Handles the setup, maintenance and the terminations of calls (connection between end users))

4. Mobile application part (MAP) (handles Most of the signalling between different entites in the fixed part pf the network such as HLR, VLR)

5. BTS management (Performs various management and administrative functions at the BTS under the control of the BSC)