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Basics of Wireless and Mobile Communications

Wireless Transmission Frequencies Signals Antenna Signal propagation Multiplexing Modulation Spread spectrum Cellular systems

Media Access Schemes Motivation SDMA, FDMA, TDMA, CDMA Comparison

Basic Functions in Mobile Systems Location management Handover Roaming

Cellular Communication Systems 2 Andreas Mitschele-Thiel, Jens Mückenheim October 2013

References

Jochen Schiller: Mobile Communications (German and English), 2nd edition, Addison-

Wesley, 2003 (most of the material covered in this chapter is based on the book)

Holma, Toskala: WCDMA for UMTS. 3rd edition, Wiley, 2004


Mobile Communication Systems – the Issues:

What does it require? Provide telecommunition services

voice (conversation, messaging) data (fax, SMS/MMS, internet) video (conversation, streaming, broadcast)

anywhere coverage anytime ubiquitous connectivity, reachability wireless without cord/wire mobile in motion, on the move (terrestrial) secure integrity, identity, privacy, authenticity,

non-repudiation reliable guaranteed quality of service


Frequencies for communication (spectrum)

VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency

Frequency and wave length:

λ = c / f wave length λ, speed of light c ≅ 300 x 106 m/s, frequency f

1 Mm 300 Hz

10 km 30 kHz

100 m 3 MHz

1 m 300 MHz

10 mm 30 GHz

100 µm 3 THz

1 µm 300 THz

visible light VLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmission coax cable twisted pair

GSM, DECT, UMTS, WLAN


Electromagnetic Spectrum

100 MHz: UKW Radio, VHF TV 400 MHz: UHF TV 450 MHz: C-Netz 900 MHz: GSM900 1800 MHz: GSM1800 1900 MHz: DECT 2000 MHz: UMTS (3G) 2400 MHz: WLAN, Bluetooth 2450 MHz: Mikrowellenherd 3500 MHz: WiMax

o


Frequencies for mobile communication

VHF-/UHF-ranges for mobile radio simple, small antennas good propagation characteristics (limited reflections, small path loss,

penetration of walls) typically used for radio & TV (terrestrial+satellite) broadcast,

wireless telecommunication (cordless/mobile phone)

SHF and higher for directed radio links, satellite communications small antenna, strong focus larger bandwidth available no penetration of walls

Mobile systems and wireless LANs use frequencies in UHF to SHF spectrum

systems planned up to EHF limitations due to absorption by water and oxygen molecules (resonance

frequencies) weather dependent fading, signal loss caused by heavy rainfall etc.


Frequencies and regulations

ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Examples of assigned frequency bands (in MHz):

Europe USA Japan

Cellular Phones (licensed)

GSM 450-457, 479-486/460-467,489-496, 890-915/935-960, 1710-1785/1805-1880 UMTS (FDD) 1920-1980, 2110-2190 UMTS (TDD) 1900-1920, 2020-2025

AMPS, TDMA, CDMA 824-849, 869-894 TDMA, CDMA, GSM 1850-1910, 1930-1990

PDC 810-826, 940-956, 1429-1465, 1477-1513

Cordless Phones (un-licensed)

CT1+ 885-887, 930-932 CT2 864-868 DECT 1880-1900

PACS 1850-1910, 1930-1990 PACS-UB 1910-1930

PHS 1895-1918 JCT 254-380

Wireless LANs (un-licensed)

IEEE 802.11 b 2400-2483 802.11a/HIPERLAN 2 5150-5350, 5470-5725

902-928 IEEE 802.11 2400-2483 5150-5350, 5725-5825

IEEE 802.11 2471-2497 5150-5250

Others RF-Control 27, 128, 418, 433, 868

RF-Control 315, 915

RF-Control 426, 868

WiMax (IEEE 802.16, licensed)

2.3GHz, 2.5GHz and 3.5GHz



Abbreviations: AMPS Advanced Mobile Phone

System CDMA Code Division Multiple

Access CT Cordless Telephone DECT Digital Enhanced

Cordless Telecommunications

GSM Global System for Mobile Communications

HIPERLAN High-Performance LAN

IEEE Institute of Electrical and Electronics Engineers

JCT Japanese Cordless Telephone

NMT Nordic Mobile Telephone PACS Personal Access

Communications System PACS-UB PACS- Unlicensed

Band PDC Pacific Digital Cellular PHS Personal Handyphone

System TDMA Time Division Multiple

Access WiMAX Worldwide

Interoperability for Microwave Access

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UMTS Frequency Bands (FDD mode only)

Operating Band

Frequency Band

UL Frequencies UE transmit

(MHz)

DL Frequencies UE receive

(MHz)

Typically used in region ...

I 2100 1920 - 1980 2110 - 2170 EU, Asia

II 1900 1850 - 1910 1930 - 1990 America

III 1800 1710 - 1785 1805 - 1880 EU (future use)

IV 1700 1710 - 1755 2110 - 2155 Japan

V 850 824 - 849 869 - 894 America, Australia, Brazil

VI 800 830 - 840 875 - 885 Japan

VII 2600 2500 - 2570 2620 - 2690 „Extension Band“

VIII 900 880 - 915 925 - 960 EU (future use)

IX 1800 1749.9 - 1784.9 1844.9 - 1879.9 Japan

X 1700 1710 - 1770 2110 - 2170 America/US

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UMTS Frequency Bands (FDD mode only), Germany

Operator Uplink (MHz) Downlink (MHz) Carriers Auction Price

Vodafone 1920,3 – 1930,2 2110,3 – 2120,2 2x10 MHz 16,47 Mrd. DM (8,42 Mrd. €)

Currently spare

1930,2 – 1940,1 2120,2 – 2130,1 2x10 MHz

16,45 Mrd. DM Group 3G

(Marke Quam) E-Plus 1940,1 – 1950,0 2130,1 – 2140,0 2x10 MHz

16,42 Mrd. DM (8,39

Mrd. €)

Currently spare

1950,0 – 1959,9 2140,0 – 2149,9 2x10 MHz

(16,37 Mrd. DM Mobilcom; returned)

O2 1959,9 – 1969,8 2149,9 – 2159,8 2x10 MHz

16,52 Mrd. DM (8,45 Mrd. €)

T-Mobile 1969,8 – 1979,7 2159,8 – 2169,7 2x10 MHz

16,58 Mrd. DM (8,48 Mrd. €)

In 2000, the UMTS frequency bands were auctioned in Germany. 6 operators won 10 MHz each, for total 50 B€

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Basic Lower Layer Model for Wireless Transmission Transmit direction Receive direction Data link layer – media access – fragmentation – reassembly

– frame error protection – frame error detection

– multiplexing – demultiplex Physical layer – encryption – decryption

– coding, forward error protection

Digital Signal

Processing – decoding, bit error correction

– interleaving – deinterleaving – modulation – demodulation

– D/A conversion, signal generation

– A/D conversion; (signal equalization)

– transmit – receive

Wireless Channel (path loss)

– Intersymbol-Interference (distortion of own signal) – Intercell-Interference (multiple users) – Intracell-Interference (multiple users) –Thermal Noise


Signals in general

physical representation of data function of time and location signal parameters: parameters representing the value of data classification

continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values

signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift ϕ sine wave as special periodic signal for a carrier:

s(t) = At sin(2 π ft t + ϕt)

amplitude

frequency phase shift


Composed signals transferred into frequency domain using Fourier transformation

Digital signals need infinite frequencies for perfect transmission modulation with a carrier frequency for transmission (analog signal!)

Signal representations

f [Hz]

A [V]

ϕ

I= M cos ϕ

Q = M sin ϕ

ϕ

A [V]

t[s]

amplitude (time domain)

frequency spectrum (frequency domain)

phase state diagram (amplitude M and phase ϕ in polar coordinates)


Fourier representation of periodic signals

)2cos()2sin(21)(

11nftbnftactg

nn

nn ππ ∑∑

∞

=

∞

=

++=

1

0

1

0 t t

ideal periodic signal real composition (based on harmonics)

Every periodic signal g(t) can be constructed by


Signal propagation

Propagation in free space always like light (straight line, line of sight) Receiving power proportional to

1/d² (ideal), 1/dα (α=3...4 realistically) (d = distance between sender and receiver)

Receiving power additionally influenced by

fading (frequency dependent) shadowing reflection at large obstacles scattering at small obstacles diffraction at edges

reflection scattering diffraction shadowing


Radio Propagation: Received Power due to Pathloss

1m 10m 100m Ideal line-of sight (d-2): 1 1:100 1:10000

Realistic propagation 1 1:3000 to 1:10 Mio to (d-3.5…4): 1:10000 1:100 Mio 35-40

dB 35-40

dB


Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

Time dispersion: signal is dispersed over time interference with “neighbor” symbols, Inter Symbol Interference (ISI)

The signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the different parts

Multipath propagation

signal at sender signal at receiver

Delayed signal rec’d via longer path

Signal received by direct path


Effects of mobility – Fading

Channel characteristics change over time and location signal paths change different delay variations of different signal parts (frequencies) different phases of signal parts

quick changes in the power received (short-term fading or fast fading)

Additional changes in distance to sender obstacles further away

slow changes in the average power received (long-term fading or slow fading)

short-term fading

long-term fading

t

power


Fast Fading

simulation showing time and frequency dependency of Rayleigh fading (model for urban environments)

V = 110km/h 900MHz


Signal propagation ranges

distance

sender

transmission

detection

interference

Transmission range communication possible low error rate

Detection range detection of the signal

possible no communication

possible Interference range

signal may not be detected

signal adds to the background noise


Interference


Carrier to Interference Ratio (CIR, C/I)

(Uplink Situation)

Ratio of Carrier-to-Interference power at the receiver

The minimum required CIR depends on the system and the signal processing potential of the receiver technology

Typical in GSM: C/I=15dB (Factor 32)

NICCIRj +

=∑


Range limited systems (lack of coverage)

Mobile stations located far away from BS (at cell border or even beyond the coverage zone)

C at the receiver is too low, because the path loss between sender and receiver is too high

C/I is too low

No signal reception possible


Interference limited systems (lack of capacity)

Mobile station is within coverage zone C is sufficient, but too much

interference I at the receiver

C/I is too low

No more resources / capacity left


Information Theory: Channel Capacity (1)

Bandwidth limited Additive White Gaussian Noise (AWGN) channel

Gaussian codebooks Single transmit antenna Single receive antenna (SISO)

Shannon (1950):

Channel Capacity <= Maximum mutual information between sink and source

Signal-to-noise ratio SNR

o


Information Theory: Channel Capacity (2)

For S/N >>1 (high signal-to-noise ratio), approximate

Observation: Bandwidth and S/N are reciproke to each other This means:

With low bandwidth very high data rate is possible provided S/N is high enough Example: higher order modulation schemes

With high noise (low S/N) data communication is possible if bandwidth is large Example: spread spectrum

Shannon channel capacity has been seen as a “unreachable” theoretical limit, for a long time. However:

Turbo coding (1993) pushes practical systems up to 0.5 dB to Shannon channel bandwidth

o


Link Capacity for Various Rate-Controlled Technologies

The link capacity of current systems is quickly approaching the Shannon limit (within a factor of two). Future improvements in spectral efficiency will focus on intelligent antenna techniques and/or coordination

between base stations.

Link performance of OFDM & 3G systems are similar and approaching the (physical) Shannon bound

-15 -10 -5 0 5 10 15 20 0

1

2

3

4

5

6

required SNR (dB)

achi

evab

le ra

te (b

ps/H

z)

Shannon bound Shannon bound with 3dB margin

(3GPP2) EV-DO (IEEE) 802.16

(3GPP) HSDPA

o


Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission

Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna

Real antennas always have directive effects (vertically and/or horizontally)

Radiation pattern: measurement of radiation around an antenna

Antennas: isotropic radiator

z y

x

z

y x ideal isotropic radiator


Antennas: simple dipoles

Real antennas are not isotropic radiators but, e.g. dipoles with lengths λ/4 on car roofs or λ/2 as Hertzian dipole

shape of antenna proportional to wavelength

Example: Radiation pattern of a simple Hertzian dipole

Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power)

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

simple dipole

λ/4 λ/2


Antennas: directed and sectorized

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

top view, 3 sector

x

z

top view, 6 sector

x

z

Often used for microwave connections (narrow directed beam) or base stations for cellular networks (sectorized cells)

directed antenna

sectorized antenna


Antenna

downtilt

3-sectorized


Real world propagation examples


Antennas: diversity

Grouping of 2 or more antennas multi-element antenna arrays

Antenna diversity

switched diversity, selection diversity receiver chooses antenna with largest output

diversity combining combine output power to produce gain cophasing needed to avoid cancellation

+

λ/4 λ/2 λ/4

ground plane

λ/2 λ/2

+

λ/2


Goal: multiple use of a shared medium Multiplexing in 4 dimensions

space (si) time (t) frequency (f) code (c)

Multiple use is possible, if resource (channel) is different in at least one dimension

Important: guard spaces needed!

s2

s3

s1

Multiplexing

f

t

c

k2 k3 k4 k5 k6 k1

f

t

c

f

t

c

channels ki


Frequency multiplex

Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages:

no dynamic coordination needed applicable to analog signals

Disadvantages: waste of bandwidth

if the traffic is distributed unevenly

inflexible guard space

k2 k3 k4 k5 k6 k1

f

t

c


f

t

c

k2 k3 k4 k5 k6 k1

Time multiplex

A channel gets the whole spectrum for a certain amount of time Advantages: only one carrier in the

medium at any time throughput high even

for many users

Disadvantages: precise synchronization

needed


f

Time and frequency multiplex

Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM (frequency hopping) Advantages:

some (weak) protection against tapping

protection against frequency selective interference

but: precise coordination required

t

c

k2 k3 k4 k5 k6 k1


Code multiplex

Each channel has a unique code All channels use the same spectrum at the same time Advantages:

bandwidth efficient no coordination and synchronization

necessary good protection against interference and

tapping Disadvantages:

complex receivers (signal regeneration) Implemented using spread spectrum technology

k2 k3 k4 k5 k6 k1

f

t

c


Cellular systems: Space Division Multiplex

Cell structure implements space division multiplex: base station covers a certain transmission area (cell)

Mobile stations communicate only via the base station

Advantages of cell structures: higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area, etc. locally

Disadvantages:

fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells

Cell sizes vary from 10s of meters in urban areas to many km in rural areas (e.g.

maximum of 35 km radius in GSM)


Cellular systems: Frequency planning I Frequency reuse only with a certain distance between the base stations Typical (hexagon) model: reuse-3 cluster: reuse-7 cluster:

Other regular pattern: reuse-19 the frequency reuse pattern determines the experienced CIR Fixed frequency assignment:

certain frequencies are assigned to a certain cell problem: different traffic load in different cells

Dynamic frequency assignment: base station chooses frequencies depending on the frequencies already used in

neighbor cells more capacity in cells with more traffic assignment can also be based on interference measurements

f4 f5

f1 f3

f2

f6

f7

f4 f5

f1 f3

f2

f6

f7

f4 f5

f1 f3

f2

f6

f7 f2

f1 f3

f2

f1 f3

f2

f1 f3


Cellular systems: frequency planning II

f1 f2

f3 f2

f1

f1

f2

f3 f2

f3 f1

f2 f1

f3 f3

f3 f3

f3

f4 f5

f1 f3

f2

f6

f7

f3 f2

f4 f5

f1 f3

f5 f6

f7 f2

f2

f1 f1 f1 f2 f3

f2 f3

f2 f3 h1

h2 h3 g1

g2 g3

h1 h2 h3 g1

g2 g3

g1 g2 g3

3 cell cluster

7 cell cluster

3 cell cluster with 3 sector antennas


Spread spectrum technology:

Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code

⇒ protection against narrow band interference

Side effects: coexistence of several signals without dynamic coordination tap-proof

Alternatives:

Direct Sequence (UMTS) Frequency Hopping (slow FH: GSM, fast FH: Bluetooth)

detection at receiver

interference spread signal

signal (despreaded)

spread interference

f f

power power


Effects of spreading and interference

dP/df

f

i) narrow band signal

dP/df

f

ii) spreaded signal (broadband signal)

sender

dP/df

f

iii) addition of interference

dP/df

f

iv) despreaded signal

receiver f

v) application of bandpass filter

user signal broadband interference narrowband interference

dP/df


Spreading and frequency selective fading

frequency

channel quality

1 2 3

4

5 6

narrow band signal

guard space

2 2

2 2

2

frequency

channel quality

1

spread spectrum

narrowband interference without spread spectrum

spread spectrum to limit narrowband interference


DSSS (Direct Sequence Spread Spectrum) I

XOR of the signal with pseudo-random number (chipping sequence) many chips per bit (e.g., 128) result in higher bandwidth of the signal

Advantages

reduces frequency selective fading

in cellular networks base stations can use the

same frequency range several base stations can

detect and recover the signal soft handover

Disadvantages

precise power control needed

user data

chipping sequence

resulting signal

0 1

0 1 1 0 1 0 1 0 1 0 0 1 1 1

XOR

0 1 1 0 0 1 0 1 1 0 1 0 0 1

=

tb

tc

tb: bit period tc: chip period


DSSS (Direct Sequence Spread Spectrum) II

X user data

chipping sequence

modulator

radio carrier

spread spectrum signal

transmit signal

transmitter

demodulator

received signal

radio carrier

X

chipping sequence

lowpass filtered signal

receiver

integrator

products

decision data

sampled sums

correlator

o


Modulation

“The shaping of a (baseband) signal to convey information”. Basic schemes

Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

Digital modulation

digital data is translated into an analog signal (baseband) ASK, FSK, PSK differences in spectral efficiency, power efficiency, robustness

Motivation for modulation

smaller antennas (e.g., λ/4) medium characteristics Frequency Division Multiplexing spectrum availability


Modulation and demodulation

synchronization decision

digital data analog

demodulation

radio carrier

analog baseband signal

101101001 radio receiver

digital modulation

digital data analog

modulation

radio carrier

analog baseband signal

101101001 radio transmitter

o


Digital modulation

Modulation of digital signals known as Shift Keying

Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference

Frequency Shift Keying (FSK):

needs larger bandwidth

Phase Shift Keying (PSK): more complex robust against interference

1 0 1

t

1 0 1

t

1 0 1

t


Advanced Frequency Shift Keying

bandwidth needed for FSK depends on the distance between the carrier frequencies

Idea: special pre-computation avoids sudden phase shifts MSK (Minimum Shift Keying)

MSK technique: bit stream is separated into even and odd bits, the duration of each bit is

doubled depending on the bit values (even, odd) the higher or lower frequency,

original or inverted is chosen the frequency of one carrier is twice the frequency of the other, eliminating

abrupt phase changes

even higher bandwidth efficiency using a Gaussian low-pass filter GMSK (Gaussian MSK), used for GSM and DECT


Example of MSK

data

even bits

odd bits

1 1 1 1 0 0 0

t

low frequency

high frequency

MSK signal

bit

even 0 1 0 1

odd 0 0 1 1

signal h l l h value - - + +

h: high frequency l: low frequency +: original signal -: inverted signal

No phase shifts!

Transformation scheme


Advanced Phase Shift Keying

BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral efficiency robust, used e.g. in satellite systems

QPSK (Quadrature Phase Shift Keying):

2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to BPSK more complex used in UMTS and EDGE (8-PSK) often also transmission of relative, not absolute phase shift:

DQPSK - Differential QPSK (IS-136, PHS) Puls filtering of baseband to avoid sudden phase shifts => reduce bandwidth of modulated signal

Q

I 0 1

Q

I

11

01

10

00


Quadrature Amplitude Modulation

Quadrature Amplitude Modulation (QAM) combines amplitude and phase modulation it is possible to code n bits using one symbol 2n discrete levels: e.g. 16-QAM, 64-QAM

n=2: 4-QAM identical to QPSK bit error rate increases with n, but less errors compared to comparable

PSK schemes

Example: 16-QAM (1 symbol = 16 levels = 4 bits) Symbols 0011 and 0001 have the same phase, but different amplitude 0000 and 1000 have different phase, but same amplitude also: 64-QAM (1 symbol = 64 levels = 6 bits) QAM is used in

UMTS HSDPA (16-QAM) UMTS LTE (64-QAM) standard 9600 bit/s modems

0000

0001

0011

1000

Q

I

0010

Media Access Schemes (Recap from Advanced Mobile Communication Networks course)

Motivation limits of CSMA/CD hidden and exposed terminals near-far problem

TDD vs. FDD TDMA

Aloha, slotted Aloha Demand Assigned Multiple Access (DAMA)

CDMA theory and practice Comparison


Media Access: Motivation

The problem: multiple users compete for a common, shared resource (medium) Can we apply media access methods from fixed networks?

Example CSMA/CD

Carrier Sense Multiple Access with Collision Detection (IEEE 802.3) send as soon as the medium is free (carrier sensing – CS) listen to the medium, if a collision occurs stop transmission and jam

(collision detection – CD) Problems in wireless networks

signal strength decreases (at least) proportional to the square of the distance

the sender would apply CS and CD, but the collisions happen at the receiver

it might be the case that a sender cannot “hear” the collision, i.e., CD does not work

furthermore, CS might not work if, e.g., a terminal is “hidden”


Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium -> CS fails collision at B: A cannot detect the collision -> CD fails A is “hidden” for C

Exposed terminals

B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not

necessary C is “exposed” to B

Motivation - hidden and exposed terminals

B A C

B A C


Terminals A and B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns out A’s signal C cannot receive A

Severe problem for CDMA-networks – precise power control needed!

Motivation - near and far terminals

A B C


Access methods SDMA/FDMA/TDMA

SDMA (Space Division Multiple Access) segment space into sectors, use directed antennas cell structure

FDMA (Frequency Division Multiple Access) assign a certain frequency to a transmission channel between a sender

and a receiver permanent (e.g., radio broadcast), slow hopping (e.g. GSM), fast

hopping (FHSS, Frequency Hopping Spread Spectrum) TDMA (Time Division Multiple Access)

assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time

The multiplexing schemes presented previously are now used to control medium access!


Communication link types

Each terminal needs an uplink and a downlink Types of communication links: Simplex

unidirectional link transmission

Half Duplex Bi-directional (but not simultaneous)

Duplex

simultaneous bi-directional link transmission, two types: Frequency division duplexing (FDD) Time division duplexing (TDD)


Duplex modes

Frequency Division Duplex (FDD)

Separate frequency bands for up- and downlink

+ separation of uplink and downlink interference

- no support for asymmetric traffic

Examples: UMTS, GSM, IS-95, AMPS

Fd

Fu

Td Tu

Td Tu

Time Division Duplex (TDD)

Separation of up- and downlink traffic on time axis

+ support for asymmetric traffic

- mix of uplink and downlink interference on single band

Examples: DECT, UMTS (TDD)


FDD/FDMA - general scheme, example GSM

f

t

124

1

124

1

20

200 kHz

890.2

935.2

915

960


TDD/TDMA - general scheme, example DECT

1 2 3 11 12 1 2 3 11 12

t downlink uplink

417 µs


Mechanism random, distributed (no central arbiter), time-multiplex Slotted Aloha additionally uses time-slots, sending must always start at

slot boundaries Aloha

Slotted Aloha

Aloha/slotted aloha

sender A

sender B

sender C

collision

sender A

sender B

sender C

collision

t

t


DAMA - Demand Assigned Multiple Access

Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution for packet arrival and packet length)

Reservation can increase efficiency to 80%

a sender reserves a future time-slot sending within this reserved time-slot is possible without collision reservation also causes higher delays typical scheme for satellite links application to packet data, e.g. in GPRS and UMTS

Examples for reservation algorithms:

Explicit Reservation (Reservation-ALOHA) Implicit Reservation (PRMA) Reservation-TDMA


Access method DAMA: Explicit Reservation

Explicit Reservation (Reservation Aloha): Two modes:

ALOHA mode for reservation: competition for small reservation slots, collisions possible

reserved mode for data transmission within successful reserved slots (no collisions possible)

synchronisation: it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time

Aloha reserved Aloha reserved Aloha reserved Aloha

collision

t


Access method CDMA

CDMA (Code Division Multiple Access) all terminals send on the same frequency probably at the same time and

can use the whole bandwidth of the transmission channel each sender has a unique random number, the sender XORs the signal with

this random number the receiver can “tune” into this signal if it knows the pseudo random

number, tuning is done via a correlation function Advantages:

all terminals can use the same frequency, less planning needed huge code space (e.g. 232) compared to frequency space interference (e.g. white noise) is not coded forward error correction and encryption can be easily integrated

Disadvantages:

higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal)

all signals should have the same strength at a receiver (power control)


CDMA Principle

Code 0

Code 1

Code 2

Σ

data 0

data 1

data 2

Code 0

Code 1

Code 2

data 0

data 1

data 2

sender (base station) receiver (terminal)

Transmission via air interface


CDMA by example

Source 2

Source 1

data stream A & B

Code 2

Code 1

spreading

Source 2 spread

Source 1 spread

spreaded signal


CDMA by example

Sum of Sources Spread

+

overlay of signals

Sum of Sources Spread + Noise

transmission and distortion (noise and interference)

Despread Source 2

Despread Source 1

decoding and despreading


CDMA in theory

Sender A sends Ad = 1, key Ak = 010011 (assign: „0“= -1, „1“= +1) sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)

Sender B sends Bd = 0, key Bk = 110101 (assign: „0“= -1, „1“= +1) sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

Both signals superimpose in space interference neglected (noise etc.) As + Bs = (-2, 0, 0, -2, +2, 0)

Receiver wants to receive signal from sender A apply key Ak bitwise (inner product)

Ae = (-2, 0, 0, -2, +2, 0) • Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6 result greater than 0, therefore, original bit was „1“

receiving B

Be = (-2, 0, 0, -2, +2, 0) • Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. „0“


CDMA on signal level I

data A

key A

signal A

data ⊕ key

key sequence A

Real systems use much longer keys resulting in a larger distance between single code words in code space

1 0 1

1 0 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1 0 1 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

Ad

Ak

As


CDMA on signal level II

signal A

data B

key B key

sequence B

signal B

As + Bs

data ⊕ key

1 0 0

0 0 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1 1 1 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

Bd

Bk

Bs

As

1 0 -1


CDMA on signal level III

Ak

(As + Bs) * Ak

integrator output

comparator output

As + Bs

data A

1 0 1

1 0 1 Ad

1 0 -1

1

-1

1 0 -1


CDMA on signal level IV

integrator output

comparator output

Bk

(As + Bs) * Bk

As + Bs

data B

1 0 0

1 0 0 Bd

1 0 -1 1

-1 1 0 -1


comparator output

CDMA on signal level V

wrong key K

integrator output

(As + Bs) * K

As + Bs

(0) (0) ?

Assumptions orthogonality of keys neglectance of noise no differences in signal level => precise power control

1 0 -1 1

-1

1 0 -1


Comparison SDMA/TDMA/FDMA/CDMA

Approach SDMA TDMA FDMA CDMA Idea segment space into

cells/sectors segment sending time into disjoint time-slots, demand driven or fixed patterns

segment the frequency band into disjoint sub-bands

spread the spectrum using orthogonal codes

Terminals only one terminal can be active in one cell/one sector

all terminals are active for short periods of time on the same frequency

every terminal has its own frequency, uninterrupted

all terminals can be active at the same place at the same moment, uninterrupted

Signal separation

cell structure, directed antennas

synchronization in the time domain

filtering in the frequency domain

code plus special receivers

Advantages very simple, increases capacity per km²

established, fully digital, flexible

simple, established, robust

flexible, less frequency planning needed, soft handover

Dis-advantages

inflexible, antennas typically fixed

guard space needed (multipath propagation), synchronization difficult

inflexible, frequencies are a scarce resource

complex receivers, needs more complicated power control for senders

Comment only in combination with TDMA, FDMA or CDMA useful

standard in fixed networks, together with FDMA/SDMA used in many mobile networks

typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse)

still faces some problems, higher complexity, lowered expectations; will be integrated with TDMA/FDMA

Basic Functions in Mobile Systems

Location management Handover Roaming Authentication (see later)


Location Management

The problem: locate a mobile user from the network side (mobile-terminated call)

Two extreme solutions:

Mobile registers with each visited cell

(e.g. direct call to the hotel room to reach a person) – signaling traffic to register mobile when cell is changed – network has to maintain location information about each mobile + low signaling load to page mobile (i.e. in one cell only)

Page mobile using a network- or worldwide broadcast message

(e.g. broadcast on TV or radio to contact a person) – heavy signaling load to page the mobile (i.e. in all cells) + no signaling traffic while mobile is idle


RA

RA

RA RA

RA

RA RA

RA

RA

Location Update

Location Update

Location Update

Location Update

Location Update

Location Management

The issue: Compromise between minimizing the area where

to search for a mobile minimizing the number of

location updates

Solution 1: Large paging area

Solution 2: Small paging area

Paging Signalling Cost

Paging Area Update Signalling Cost

TOTAL Signalling Cost

∑ ∑ +

=


Handover

The problem: Change the cell while communicating

Reasons for handover: Quality of radio link

deteriorates Communication in other cell

requires less radio resources Supported radius is

exceeded (e.g. Timing advance in GSM)

Overload in current cell Maintenance

Link

qua

lity

Link to cell 1 Link to cell 2 time

cell 1

cell 2

Handover margin (avoid ping-pong effect)

cell 1 cell 2


Roaming

The problem: Use a network not subscribed to

Roaming agreement needed between network operators to exchange information concerning: Authentication Authorisation Accounting

Examples of roaming agreements: Use networks abroad Use of T-Mobile network by O2 (E2) subscribers in area with no O2 coverage

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