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. 2 32 ) compared to frequency space interferences (e.g. white noise) is not coded forward error correction and encryption can be easily integrated
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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“
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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
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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
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CDMA on signal level III
Ak
(As + Bs) * Ak
integratoroutput
comparatoroutput
As + Bs
data A
1 0 1
1 0 1 Ad
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CDMA on signal level IV
integratoroutput
comparatoroutput
Bk
(As + Bs) * Bk
As + Bs
data B
1 0 0
1 0 0 Bd
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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
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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
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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
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
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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
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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…]
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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
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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
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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
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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
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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)
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!
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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
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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
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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
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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!)
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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) ...
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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
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GSM: overview
fixed network
BSC
BSC
MSC MSC
GMSC
OMC, EIR, AUC
VLR
HLR
NSSwith OSS
RSS
VLR
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Nomenclature
OMC : Operations and Maintenance centre
MSC : Gateway MSC
EIR : Equipment Identity Register
AUC : Authentication Centre
PDN : Public Data Network
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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
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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
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System architecture: radio subsystem
Um
Abis
A
BSS
radiosubsystem
network and switchingsubsystem
MS MS
BTSBSC MSC
BTS
BTSBSC
BTSMSC
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System architecture: network and switching subsystem
ISDN (Integrated Services Digital Network)PSTN (Public Switched Telephone Network)PSPDN (Packet Switched Public Data Net.)CSPDN (Circuit Switched Public Data Net.)
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