1

Code division Multiple Access

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 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 Advantages:

all terminals can use the same frequency, no planning needed huge code space (e.g. 232) compared to frequency space interferences (e.g. white noise) is not coded forward error correction and encryption can be easily integrated

2

Code division Multiple Access - 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“

3

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

10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1

01 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

Ad

Ak

As

4

CDMA on signal level II

signal A

data B

key B

keysequence B

signal B

As + Bs

data key

1 0 0

00 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1

11 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

Bd

Bk

Bs

As

5

CDMA on signal level III

Ak

(As + Bs) * Ak

integratoroutput

comparatoroutput

As + Bs

data A

1 0 1

1 0 1 Ad

6

CDMA on signal level IV

integratoroutput

comparatoroutput

Bk

(As + Bs) * Bk

As + Bs

data B

1 0 0

1 0 0 Bd

7

CDMA on signal level V

comparatoroutput

wrong key K

integratoroutput

(As + Bs) * K

As + Bs

(0) (0) ?

8

SAMA - Spread Aloha Multiple Access

Aloha has only a very low efficiency, CDMA needs complex receivers to be able to

receive different senders with individual codes at the same time Idea: use spread spectrum with only one single code (chipping sequence) for

spreading for all senders accessing according to aloha

1sender A0sender B

0

1

t

narrowband

send for a shorter periodwith higher power

spread the signal e.g. using the chipping sequence 110101 („CDMA without CD“)

Problem: find a chipping sequence with good characteristics

1

1

collision

9

SAMA

Here, the chipping sequence used is

[1 1 0 1 0 1]

Very important:- All the signals are sent with the same chipping sequence

- BUT the chipping phase differs slightly.How do we separate the signals?- One receiver is synchronized to signal A and the other to signal B.

• THROUGHPUT IS THE SAME AS CLASSICAL ALOHA• BUT IT IS MORE ROBUST BECAUSE OF SPREAD SPECTRUM

10

Comparison SDMA/TDMA/FDMA/CDMA

Approach SDMA TDMA FDMA CDMA

Idea segment space intocells/sectors

segment sendingtime into disjointtime-slots, demanddriven or fixedpatterns

segment thefrequency band intodisjoint sub-bands

spread the spectrumusing orthogonal codes

Terminals only one terminal canbe active in onecell/one sector

all terminals areactive for shortperiods of time onthe same frequency

every terminal has itsown frequency,uninterrupted

all terminals can be activeat the same place at thesame moment,uninterrupted

Signalseparation

cell structure, directedantennas

synchronization inthe time domain

filtering in thefrequency domain

code plus specialreceivers

Advantages very simple, increasescapacity per km²

established, fullydigital, flexible

simple, established,robust

flexible, less frequencyplanning needed, softhandover

Dis-advantages

inflexible, antennastypically fixed

guard spaceneeded (multipathpropagation),synchronizationdifficult

inflexible,frequencies are ascarce resource

complex receivers, needsmore complicated powercontrol for senders

Comment only in combinationwith TDMA, FDMA orCDMA useful

standard in fixednetworks, togetherwith FDMA/SDMAused in manymobile networks

typically combinedwith TDMA(frequency hoppingpatterns) and SDMA(frequency reuse)

still faces some problems,higher complexity,lowered expectations; willbe integrated withTDMA/FDMA

11

Telecommunications Systems

year

Su

bsc

rib

ers

[mill

ion

]

0

200

400

600

800

1000

1200

1400

1600

1996 1997 1998 1999 2000 2001 2002 2003 2004

approx. 1.7 bn

GSM total

TDMA total

CDMA total

PDC total

Analogue total

W-CDMA

Total wireless

Prediction (1998)

Mobile Phone Subscribers world-wide

12

Development of mobile telecommunication systems

1G 2G 3G2.5G

IS-95cdmaOne

IS-136TDMAD-AMPS

GSM

PDC

GPRS

IMT-DSUTRA FDD / W-CDMA

EDGE

IMT-TCUTRA TDD / TD-CDMA

cdma2000 1X

1X EV-DV(3X)

AMPSNMT

IMT-SCIS-136HSUWC-136

IMT-TCTD-SCDMA

CT0/1

CT2IMT-FTDECT

CD

MA

TD

MA

FD

MA

IMT-MCcdma2000 1X EV-DO

HSDPA

13

GSM

GSM formerly: Groupe Spéciale Mobile (founded 1982) now: Global System for Mobile Communication Pan-European standard (ETSI, European Telecommunications

Standardisation Institute) simultaneous introduction of essential services in three phases

(1991, 1994, 1996) by the European telecommunication administrations (Germany: D1 and D2) seamless roaming within Europe possible

today many providers all over the world use GSM (more than 200 countries in Asia, Africa, Europe, Australia, America)

more than 1.2 billion subscribers in more than 630 networks more than 75% of all digital mobile phones use GSM (74% total) over 200 million SMS per month in Germany, > 550 billion/year

worldwide(> 10% of the revenues for many operators)[be aware: these are only rough numbers…]

14

Additional features of GSM

GSM-Rail19 available channels exclusively for the

railway data and voiceEmergency calls with ACKVoice group call serviceVoice broadcast serviceControl of railway switches, gates and

signals

15

Performance characteristics of GSM (wrt. analog sys.)

Communication mobile, wireless communication; support for voice and data services

Total mobility international access, chip-card enables use of access points of

different providers Worldwide connectivity

one number, the network handles localization High capacity

better frequency efficiency, smaller cells, more customers per cell High transmission quality

high audio quality and reliability for wireless, uninterrupted phone calls at higher speeds (e.g., from cars, trains)

Security functions access control, authentication via chip-card and PIN

16

Disadvantages of GSM

There is no perfect system!! no end-to-end encryption of user data no full ISDN bandwidth of 64 kbit/s to the user, no transparent B-

channel

reduced concentration while driving electromagnetic radiation

abuse of private data possible roaming profiles accessible

high complexity of the system several incompatibilities within the GSM standards

17

GSM: Mobile Services

GSM offers several types of connections

voice connections, data connections, short message service multi-service options (combination of basic services)

Three service domains Bearer Services Telematic Services Supplementary Services

GSM-PLMNtransit

network(PSTN, ISDN)

source/destination

networkTE TE

bearer services

tele services

R, S (U, S, R)Um

MT

MS

18

Bearer Services

Telecommunication services to transfer data between access points

Specification of services up to the terminal interface (OSI layers 1-3)

Different data rates for voice and data (original standard) data service (circuit switched)

synchronous: 2.4, 4.8 or 9.6 kbit/s asynchronous: 300 - 1200 bit/s

data service (packet switched) synchronous: 2.4, 4.8 or 9.6 kbit/s asynchronous: 300 - 9600 bit/s

Today: data rates of approx. 50 kbit/s possible – will be covered later!

19

Bearer Services

Bearer services permit transparent and non-transparent, synchronous and asynchronous data services

Transparent Bearer services only use the functions of physical layer for data transmission.Forward error correction is possibleData rates : 2.4, 4.8 & 9.6 kbps

20

Non-Transparent Bearer Services

Use layers 2 & 3 to introduce error correction and flow control

Special selective-reject mechanisms to trigger re-transmission of erroneous data.

Bit error date is < 10-7

21

Tele Services I

Telecommunication services that enable voice communication via mobile phones

All these basic services have to obey cellular functions, security measurements etc.

Offered services mobile telephony

primary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz

Emergency numbercommon number throughout Europe (112); mandatory for all service providers; free of charge; connection with the highest priority (preemption of other connections possible)

Multinumberingseveral ISDN phone numbers per user possible

22

Tele Services II

Additional services Non-Voice-Teleservices

group 3 fax voice mailbox (implemented in the fixed network

supporting the mobile terminals) electronic mail (MHS, Message Handling System,

implemented in the fixed network) ...

Short Message Service (SMS)alphanumeric data transmission to/from the mobile terminal (160 characters) using the signaling channel, thus allowing simultaneous use of basic services and SMS(almost ignored in the beginning now the most successful add-on!)

23

Supplementary services

Services in addition to the basic services, cannot be offered stand-alone

Similar to ISDN services besides lower bandwidth due to the radio link

May differ between different service providers, countries and protocol versions

Important services identification: forwarding of caller number suppression of number forwarding automatic call-back conferencing with up to 7 participants locking of the mobile terminal (incoming or outgoing calls) ...

24

Architecture of the GSM system

GSM is a PLMN (Public Land Mobile Network) several providers setup mobile networks following

the GSM standard within each country components

MS (mobile station) BS (base station) MSC (mobile switching center) LR (location register)

subsystems RSS (radio subsystem): covers all radio aspects NSS (network and switching subsystem): call forwarding,

handover, switching OSS (operation subsystem): management of the network

25

GSM: overview

fixed network

BSC

BSC

MSC MSC

GMSC

OMC, EIR, AUC

VLR

HLR

NSSwith OSS

RSS

VLR

26

Nomenclature

OMC : Operations and Maintenance centre

MSC : Gateway MSC

EIR : Equipment Identity Register

AUC : Authentication Centre

PDN : Public Data Network

27

GSM: elements and interfaces

NSS

MS MS

BTS

BSC

GMSC

IWF

OMC

BTS

BSC

MSC MSC

Abis

Um

EIR

HLR

VLR VLR

A

BSS

PDN

ISDN, PSTN

RSS

radio cell

radio cell

MS

AUCOSS

signaling

O

28

GSM: system architecture

Um

Abis

ABSS

radiosubsystem

MS MS

BTSBSC

BTS

BTSBSC

BTS

network and switching subsystem

MSC

MSC

fixedpartner networks

IWF

ISDNPSTN

PSPDNCSPDN

SS

7

EIR

HLR

VLR

ISDNPSTN

29

System architecture: radio subsystem

Um

Abis

A

BSS

radiosubsystem

network and switchingsubsystem

MS MS

BTSBSC MSC

BTS

BTSBSC

BTSMSC

30

System architecture: network and switching subsystem

networksubsystem

MSC

MSC

fixed partnernetworks

IWF

ISDNPSTN

PSPDNCSPDN

SS

7

EIR

HLR

VLR

ISDNPSTN

ComponentsMSC (Mobile Services Switching Center):IWF (Interworking Functions)

ISDN (Integrated Services Digital Network)PSTN (Public Switched Telephone Network)PSPDN (Packet Switched Public Data Net.)CSPDN (Circuit Switched Public Data Net.)

DatabasesHLR (Home Location Register)VLR (Visitor Location Register)EIR (Equipment Identity Register)

31

Radio Subsystem

Consists of mobile stations and Base station subsystems

A Interface : Between RSS and NSSO interface between RSS & OSS – uses

X25 standard for data transfer with OMC

32

BSS

GSM network conprises many BSSs each controlled by a base station controllerPerforms all radio functionsCoding-decoding of voiceData rate adoptionContains a BSC and several BTSs.

33

BTS

Comprises radio equipment (antennas, signal processing entities, amplifiers etc)

Can form a radio cell or several cells using sectored antennas.

Um interface is used to connect a BTS to Mobile station.

Um interface contains all the mechanisms for wireless transmission namely TDMA, FDMA, CDMA etc.

34

Base Station Controller

Manages BTSsAllocates frequenciesManages handover from BTS to another

BTS within the same BSS.Performs paging functionsMultiplexes the radio channel onto a

fixed network.

35

Mobile Station : MS

Consists of user independent hardware and software Subscriber Identification Module : SIM

MS can be identified via a IMEI (International Mobile equipment identity)

SIM can be used to personalize user preferences

IMEI can be used for device specific features such as theft protection etc.

36

Mobile Station : MS

SIM card has identifiers and tables to store: Card type Serial no. List of subscriber services Personal Identity No.(PIN) Pin Unblocking Key(PUK) Authentication key International Mobile Subscriber Identity (IMSI)

37

Mobile Station : MS

Dynamic Information Stored on MS Cipher key Kc Location information (Temporary Mobile Subscriber

Identity) Location Area Identification