02 Tm2100eu02tm 0001 Transmission Principles

Transmission Principles Siemens

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Contents

1 GSM Network Structure 3

2 Duplex Transmission & Multiple Access 17

3 GSM - Fixed Network Transmission 29

4 GSM Air Interface 35

Transmission Principles

Siemens Transmission Principles

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1 GSM Network Structure


GSM Network Structure

Fig. 1


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GSM: The Network Structure

The international GSM service area covers all countries in which there is a GSMnetwork.

Networks provisioned by an operator on a national level for public mobilecommunication are called Public Land Mobile Networks PLMN. PLMNs builttogether with public fixed networks, i.e. "conventional" PSTN (Public SwitchedTelephone Network) or ISDN (Integrated Services Digital Network) networks thetelecommunication infrastructure of a country.

A Public Land Mobile Network is divided into mobile and fixed network components.They are connected via air interfaces.

Fixed Network Components of the PLMN

The fixed network components of a GSM-PLMN consist of:

Base Station Subsystem BSS: The BSS is the fixed network part of the PLMNradio access (Radio SubSystem RSS). It realizes the radio transmission via theradio interface. Several fixed radio station, so-called Base Stations BS are co-ordinated by one control unit.

Network Switching Subsystem NSS: The NSS forms the interface between theradio subsystem and the public fixed networks (PSTN, ISDN, PDN). It executes allsignaling functions for setting up connections from and to mobile subscribers. It issimilar to the exchanges of fixed network communication systems, but itfurthermore fulfils important mobile communication specific functions, e.g. keepingtrack of the users / mobile stations location.

Mobile components of the PLMN

The Mobile Stations MSs are regarded as mobile part of the PLMN. The air or radiointerface represents the connection between the MS and the PLMN fixed networkcomponents BSS and NSS. The organization of the radio interface is decisive foradvantages and disadvantages of different mobile systems.


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Mobile

terminal device

BSSBase Station

Subsystem

NSSNetwork Switching

Subsystem

control/switching of

mobile services

BSSBase Station

Subsystem

BSSBase Station

Subsystem

PLMNPublic Land Mobile Network

PSTNPublic Switched

Telephone Network

ISDNIntegrated Services

Digital Network

PDNPublic Data

Network

MSMobile

Station

Mobile

components

Fixed network

components

UmAir Interface

Fixed

network

GSM Network Structure: Concept

Fig. 2


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Mobile Components

Mobile components are the Mobile Stations MS which transmit the users speech anddata to the PLMN. The Mobile Station MS consist of:

ME: Mobile Equipment,

SIM: Subscriber Identification Module,

The MS is not necessarily the termination point for the users data transmission. ATerminal Equipment TE, e.g. laptop, fax machine,... can be connected to the MS forfinal data handling.

The Mobile Station MS

An important difference between fixed network communications and mobilecommunications is the separation of equipment and subscriber identity. It is possiblefor the mobile subscriber to use various mobile terminal equipment with a personalidentity by means of the SIM card, which includes his subscriber identity. The mobilestation is defined as: MS = ME + SIM.

The SIM card is allocated and activated by the provider upon completion of thecontract. It is realized by means of a chip which contains a variety of permanent andtemporary information for the subscriber (e.g. personal telephone register) and abouthim/her. Along with the personal (secret) ID numbers (IMSI - International MobileSubscriber Identity, TMSI - Temporary Mobile Subscriber Identity) these storedinformation are for example algorithms and keys for ciphering the transmission.

The PIN (Personal Identity Number) is important for the subscriber; it must beentered by the mobile subscriber before the start of the conversation in order toprevent fraud by unauthorized intruders. As a rule, calls cannot be made without aSIM card in the ME and without the PIN being entered. Emergency calls are anexception.


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SIMSubscriber Identification Module

MS = ME + SIM

Mobile Components

SIM card: the heart of MS

Different equipments, one SIM (one bill) Security: PIN (exception: emergency call) Chip with subscriber identification,

security algorithms,

personal phone book,...

Fig. 3


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The Cellular Network

The breakthrough in mobile communications with regards to subscriber numbers andcapacity was made possible by the introduction of the cellular radio system. Thecellular communication system was tested in various countries during the 1970s.

Cellular networks of the first generation were introduced, e.g.:

1979 in the USA: AMPS (Advanced Mobile Phone Service)

1981 in Scandinavia: NMT (Nordic Mobile Telephone)

1985 in Germany: C-450 (Siemens)

1985 in Great Britain: TACS (Total Access Communications System)

The successive digital systems of the second generation, and therefore GSMsystems, are structured as cellular communication systems in the same way as theanalogue systems.

Principle of the Cellular Communication System

PLMNs operating on a national level are divided by location into servicing areas, so-called cells, in which a Base Transceiver Station BTS supplies the mobile subscribersof the area concerned. The cells represent the smallest service area in the PLMNnetwork.

A variety of cells ensures service of the total PLMN service area. The cells aretheoretically arranged in a so-called honeycomb pattern. Adaptations to thepopulation/ traffic density and the topography of the service area lead to a moreirregular pattern.

The service areas of the individual cells partially overlap. In order to avoidinterference of different subscribers in surrounding cells the cell structure isorganized according to the principle of cellular systems, frequency re-use. Thenarrow available frequency range is divided into individual frequencies (channels).Only some of these channels are used in a certain cell, the remaining channels areused in the adjacent cells. The same frequency is used again in cells which aresufficiently far apart from each other to avoid interchannel interference. This meansthat any area can be covered and thus an enormous increase in network capacitycan be achieved with a small supply of channel frequencies.


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The Cellular

Network

Principle: Many cells (BTS)

Full coverage

Partial overlap of cells

Distribution of frequency resources

Only a few frequencies per cell

Frequency re-use

Solution:

cell,

radio cell

r = cell radius(cell parameter)

Principle:

~ 4 r

channels

u, v, w

channels

x,y,z

r

channels

x,y,zco-channel interference zone

= cluster area

re-use distance

for HF channel frequency

re-use distancefor

HF channel frequency

Fig. 4


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Cluster

A certain minimum distance must be maintained between cells using the samefrequencies in order to prevent interference or at least keep it to a bare minimum.This minimum distance, the so-called frequency re-use distance, depends on theconcrete network planning and corresponds to approximately 4 times the cell radius.On this principle, the available channels can be divided e.g. into 7 parts anddistributed over the PLMN area in such a way that each cell contains one of these 7sets of frequency channels. The minimum area in which the whole range of HFchannels is used is described as a cluster. Planning a concrete network implies thatthe population/traffic density, the topography of the area to be supplied, etc. must betaken into account. This network planning is an extremely difficult process; there isspecial network planning software for this purpose.


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Frequency re-use distance: avoid inter-channel interferences

Cluster: smallest domain within which all frequency resource is used

(GSM900: typ. 7/9 cells)

Network planning: difficult

The Cellular Network / Principles of Network Planning

Fig. 5


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The GSM Cell

The higher the traffic density, the smaller the cell area since a limited number of HFchannels can only cope with a limited traffic volume. This can be carried out via areduction of the cell radius or by dividing the cells into sectors.

Cell Size / Hierarchical Cellular Structures HCS

The size and shape of the cell depend on:

The range of the MS radio contact (MS output peak power); topography (e.g.mountains, buildings, vegetation etc) and climate play a role here.

Traffic density

The maximum radius of a cell broadcast channel is 35 km in the GSM900 system, 8km in the GSM1800 system. The possibility of setting up "extended range cells" witha radius of up to 100 km has been integrated into GSM Phase 2+ for GSM900systems. This should allow coverage of sparsely populated areas and especiallycoastal regions. The extended cell concept results in a reduced capacity.

Transmit power is limited for higher traffic densities in order to achieve a high degreeof re-use of frequencies over smaller cells: The size of clusters is inverselyproportional to the capacity of the radio system.

A Hierarchical Cell Concept (Rec. 05.22) is planned for towns, with an extremely highdensity of mobile subscribers.

Macro-Cell: The "normal" cells are called Macro Cells. They have ranges fromapproximately one km to several (extended cell concept: 100 km).

Micro Cell: Cells for the support of restricted areas with very high mobile userdensity, e.g. shopping malls, railway and subway stations, airport terminals. Theirradius ranges from some 100 meters to approximately 1 km.

Pico Cell: Cells for the support of indoor applications, e.g. offices. Their rangeshould be several 10m.

Velocity dependent Handover are necessary in the Hierarchical Cellular Structures.

Cell Coverage

Omni Cells: The BTS is equipped with omni-directional antennae and serves a360 angle.

Sector Cells: The BTS supplies the cells with directional antennae. The cell shapeis a circular segment. Sectors of e.g. 180 or 120 are covered.


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Cell Size and Coverage

Maximum cell size

GSM90035 km

(100 km)

8 kmGSM1800

Cell coverage

360

180180

cell 1

cell 2

120120

cell 1

cell 2

cell 3

120

omni cell

180

sector cells

120

sector cells

(extended cell)

Hierarchical Cellular Concept:

Macro cells: min. 500 m

Micro cells: some 100 m

Pico cells: some 10 mspeed-dependent allocation

Fig. 6


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Roaming / Location Registration / Handover

Roaming

A further innovation of the cellular system was so called Roaming. This means that asubscriber can move freely within the PLMN and remain reachable on a singlepersonal telephone number anywhere in this area. With GSM this concept of roamingcan be expanded to the international area (international roaming). A subscriberwhose home PLMN has a roaming agreement with other countries' GSM-PLMNs canalso be reached in these PLMNs (Visited PLMN - VPLMN) without dialing thecorresponding VPLMNs code; calls can also be made from that VPLMN. Aprerequisite is of course that subscribers authorization for international roaming.

Location Registration / Location Update / Location Area

The subscriber has to be located in the respective cellular network. A procedureknown as Location Registration or Location Update Procedure LUP carries outthis function. It is important that the subscriber's temporary location area is recorded /registered with this procedure when the subscriber's mobile station is switched onand checked in, to forward calls to him. The temporary Location Area LA is the areain which the MS can move freely without having to carry out a location update. As arule, the location area consists of a multiple cells and is configured by the operatoraccording to the traffic or population density.

Handover

In cellular networks, it is not necessary for the subscriber to have his call interruptedwhen changing from one cell's service area to the area of a surrounding cell, as longas the cell areas overlap. This overlapping should be guaranteed with good planning.If the MS can receive better supply from another cell than the one currently in useduring a call, the MS connection will be diverted to the relevant cell. This proceduredesigned for system quality maintenance ideally takes place without the user beingable to notice and is known as handover.


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Roaming, Location Update

& Handover

BS

BS

Location Update: Location Area: most precise location information

stored in the network

Location Registration: initial registration

Location Update: update of registration

MS

Handover

Fig. 7


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2 Duplex Transmission & Multiple Access


Duplex Transmission

& Multiple Access

FDD TDD

UL DL

Duplex

transmission

Multiple

Access

FDMA

TDMA CDMA

Fig. 8


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Duplex Transmission and Multiplex Procedure

In a cell for access to a network two different principles have to be co-ordinated: Theway of co-ordinating UL and DL, i.e. the Duplex Transmission, and the way ofenabling the simultaneous access of several user to the same Base Station, i.e. themultiple access principle.

Duplex Transmission: FDD & TDD

Modern cellular mobile radio systems of the first (1G) and second generation (2G)enable full duplex transmission. Simultaneous communication on both sides, i.e.(virtually) simultaneous transmission and reception is thus possible.

The transmission directions are designated as Uplink UL (MS to BTS) and DownlinkDL (BTS to MS).

There are two duplex transmission principles:

Frequency Division Duplex FDD: Transmission and reception take place indifferent frequency ranges. The distance between the Uplink UL and Downlink DLfrequency range is designated as duplex distance.

Time Division Duplex TDD: Transmission and reception take place in the samefrequency band. Uplink UL and Downlink DL transmission take place at differenttimes. There is fast switching between UL and DL transmission, so that the userhas the impression of simultaneous transmission and reception.


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receive

transmit receive

transmit

transmit

transmitreceive

receiveMS

BS

UL ULDL DL

time t

T

frequency f

Duplex distance

UL / DLseparated by

frequency !

Same

frequency

UL / DLseparated by

time!

FDDFrequency

Division Duplex

Uplink UL

Downlink DL

Base Station BS Mobile Station MS

TDDTime

Division

Duplex

Fig. 9


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Multiplex Access: FDMA, TDMA and CDMA

Several subscribers in one cell must be able to use the frequency range available formobile communications together. Thus there must be procedures for regulatingsimultaneous access of different subscribers without disturbances. There are threedifferent general procedures, partially in combination, which are used for co-ordinating the frequency resources:

FDMA - Frequency Division Multiple Access

TDMA - Time Division Multiple Access

CDMA - Code Division Multiple Access

FDMA - Frequency Division Multiple Access

FDMA is a multiple access principle used widely in the first (analogue) generation 1Gof mobile communications. It is however also used in the second (digital) generation2G of mobile communications, usually in combination with TDMA and in the thirdgeneration 3G together with CDMA.

The available frequency reserves are divided into channels of the same bandwidthfor FDMA. A certain frequency uplink and downlink is made available to an individualsubscriber. Simultaneous calls and information transmissions of various subscribersthus take place on different frequencies. The transmitter and receiver must have acommon knowledge about the channel frequencies to use.


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FDMAFrequency Division

Multiple Access

Multiplex Access

TDMATime Division

Multiple Access

CDMACode Division

Multiple Access

Co-ordination

of limited frequency resources

for different subscribers

Fig. 10


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TDMA - Time Division Multiple Access

The allocation of the available frequency range is made with respect to time forTDMA. A frequency band is not permanently available to one mobile station; it isused by several different mobile stations. Time is therefore split into individual timeslots. The individual mobile stations are assigned the frequency range for theduration of a TDMA time slot in a periodically exclusive manner.

A certain number of subscribers can use a certain frequency range virtuallysimultaneously with TDMA. The message information of a subscriber is taken apartand transmitted piece by piece to the corresponding time slots. The informationcarrying HF transmission in an individual time slot designated as a "burst".

CDMA - Code Division Multiple Access

In CDMA systems the users of one cell are not separated by frequency or time.Different to FDMA or TDMA simultaneously they take place in the same frequencyrange. The users are separated by unique Codes. The Base Station and MobileStation must have common knowledge of the Codes used. The information of asingle user is spread up from a narrowband signal to a wideband signal using a high-frequency code (high so-called "chiprate"). This spread information is transmitted viaradio interface. After receiving the information, it is de-spread using the same code toregenerate the original information.

The Codes in principal have orthogonal properties.


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frequency f

time t

power

TS 1

TS 2

TS 3

TDMA

frequency f

time t

power

1 2 3

FDMA

frequency f

time t

power

1

2

3

CDMA

Multiple

method

BS & MS share

knowledge about

FDMA

TDMA

CDMA

Frequency

Time

PN code

P

P P

Multiple Access methods

Fig. 11


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Transmission via GSM Radio Interface Um

A combination of FDMA and TDMA is used for GSM. The GSM physical channels aredefined by a pair of frequency bands (for UL and DL) and a Time Slot TS.

FDMA in GSM

In the GSM system, a band width of 200 kHz is defined for one frequency band.These HF channel widths are perfectly suited to the demands for speechtransmission.

Allocation to (E-) GSM900, GSM-R, GSM1800 and GSM1900 is as follows:

GSM900: (880) 890 - 915 MHz; 925 (935) - 960 MHz; 124 (174) channel pairs ;with a duplex distance of 45 MHz

GSM-R: 876 - 880 MHz; 921 - 925 MHz; 19 channel pairs; with a duplex distanceof 45 MHz

GSM1800:1710 - 1785 MHz; 1805 - 1880 MHz; 374 channel pairs; with a duplexdistance of 95 MHz

GSM1900: 1850 - 1910 MHz; 1930 - 1990 MHz; common use along with otherstandards (e.g. IS-95; D-AMPS); with a duplex distance of 80 MHz

In GSM for DL the higher and for UL the lower frequency range is used in general.

Remark: In co-ordination with the frequency plan regulation, there is a 200 kHzprotective band inserted between the lower limit frequency and the first carrier ofevery sub-band, i.e. the corresponding channels are not used. This protective bandknown as the "guard band" is an accepted, virtually "unavoidable loss" for preventinginterference between different applications in the totally filled frequency range.


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FDMA in GSMGSM900 / 1800 Frequency Allocation

C - Radio Frequency Channel (RFC)200 kHz

UPLINK (UL) DOWNLINK (DL)

Guard band

(880) 890 MHz

1710 MHz

915 MHz

1785 MHz

(925) 935 MHz

1805 MHz

960 MHz GSM900

1880 MHz GSM1800

Duplex distance 45 MHz resp. 95 MHz

25 (35) MHz

75 MHz

25 (35) MHz

75 MHz

Transmit bandof the Base Station

C

124

(174)

374

C

124'

(174')

374'

C

1

C

2

C

3

C

1'

C

2'

C

3'

Transmit bandof the Mobile Station

Fig. 12


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TDMA in GSM

Each of the 200 kHz frequency bands is further sub-divided by TDMA into 8 so calledTime Slots TS. This produces 8 physical channels within one frequency band. InGSM a physical channel is thus defined by a determined frequency channel UplinkUL and Downlink DL and a determined time slot TS

In the GSM system, up to 8 (with half-rate transmission even 16) calls can betransmitted "simultaneously" on one frequency band.

A sequence of 8 time slots TS in one radio channel is referred to as a TDMA frame. ATDMA frame has a duration of 4.615 ms, an individual time slot a duration of approx.0.577 ms. The users data are transmitted virtually "piece by piece" on one specifictime slot every TDMA frame.


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GSM:combined

FDMA/TDMA

TDMA

frame

FDMA

time

frequency200 kHz

0

1

3

2

4

5

7

6

1

0

1TS = 577 s

1 TDMA frame =8 TS = 4.615 ms

1TS = 577 s

1 TDMA frame =8 TS = 4.615 ms

Fig. 13


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3 GSM - Fixed Network Transmission

PCMPulse Code

Modulation

speech band 1

speech band 3

speech band 2common line

Multi-

plexerband

3 2 1

1 0 1 1

0 0 1 1

A/D conversion

1 1 0 0

GSM - fixed network transmission


Fig. 14


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PCM30: Transmission in GSM fixed network part

Information (conversations, data, signaling) is exclusively transmitted digitally viaPCM30 lines in the GSM-PLMNs fixed network part.

Pulse Code Modulation - PCM

Sampling values of a speech information are transmitted using binary code words(digitally) in PCM.

Due to the digital structure of the message, the PCM signals are less susceptible tointerference than analogue signals. Regenerators reconstruct the original digitalsignal at the receiving end. Analogue signals, on the other hand, can only beamplified (including noise peaks).

Amongst other things, during Pulse Code Modulation (PCM) an analogue oscillationis converted into a digital signal. A PCM signal can be transmitted alone or beembedded in a TDMA frame with other PCM signals (multiplexing).

The conversion of an analogue telephone signal into a digital signal is carried out inthree steps:

1. Band limitation: A bandpass filter restricts the incoming signal to the audiblefrequencies, i.e. to 300 to 3400 Hz.

2. Sampling: Sampling values are taken at fixed intervals from the limited telephonesignal. The sampling frequency must be greater than twice the highest frequencywithin the analogue signal (Shannon Theorem). Internationally specified: 8000 Hz.

3. 8-bit coding: Every amplitude value of the sampled (Pulse Amplitude Modulated -PAM) signal is transformed into an 8-bit word. The 8-bit word enables the analoguesignal to be represented in 256 quantization intervals.

Since the transmission of an 8-bit word requires only a portion of the sampling

interval (125 s) of the analogue signal, the 8-bit information is temporallymultiplexed (TDMA-procedure). 8 bits are transmitted in each time slot.

Using PCM30 transmission systems, a total of 30 digital user values can betransmitted in the time frame of the sampling period of an analogue value, i.e. in 125

s.


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1. Band limitation

(300-3400 Hz)

2. Sampling (8000 Hz)

3. 8-bit coding

Generation of a PCM Signal

transmission of the coded

sample value of signal 1

coded sample value

signal 2

time slot

0 1 0 0 1 1 0 1

signal 1

Fig. 15


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PCM30

PCM30 transmission systems use digital transmission lines or radio relay. A PCM30frame consists of 32 time multiplexed time slots.

The 32 time slots can contain pulse code modulated message information (speech,data) or signaling information in the form of 8-bit words.

The total bit rate of a PCM30 line is 2048 kbit/s

Time slot 0: alternately frame identification word and service word (alarms)

Time slots 1-15 and 17-31: calls or data

Time slot 16: signaling channel

The pulse frames are transmitted in a direct sequence.


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PCM30: TDMA Principle

telephone channels 1 - 15 telephone channels 17 - 31

frame alignment/

service word channelsignaling channel

time

slot

PCM30PCM30

pulse frame pulse frame pulse frame

Fig. 16


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4 GSM Air Interface

GSM Air Interface

Advantage:

mobility

Single cell systems Cellular mobile communication systems

Limits:

1st generation 2nd generation incl. satellite roaming

cell national GSM service area unlimited

GSM (Ph1/2) (GSM Ph2+)


Fig. 17


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Radio Interface: Advantages, Problems and Solutions

The air or radio interface, i.e. the connection between the MS and fixed networkcomponents, represents the fundamental difference to a fixed networktelecommunication system. The radio interface has its specific advantages, but alsoshows problems and disadvantages inherent to mobile communications.

Advantage: Mobility

The main advantage of mobile communications is the unrestricted mobility which canbe achieved only via a radio interface. Mobility was extremely restricted, especially inthe early years of mobile communications (one-cell systems). Mobility only reachedas far as the radio coverage between the MS and the transmission/receivinginstallations would allow. These limits were stretched significantly by cellular mobilecommunication networks of the first generation (since the early 1980s). Nationalborders and the degree of area coverage of a PLMN within a country formed theborders. In the GSM system, national borders no longer represented restrictions tomobility owing to inter-national roaming. It is still the case that nation-wideconnectivity is only offered around urban areas and along main traffic routes in largeareas of central Europe. Unlimited world-wide mobility is possible in co-operationbetween GSM and MSS such as Iridium, Globalstar and ICO.

Problems & Solutions on the Radio Interface

Cost Aspect: Problem - The need to built up a new network architecture withthousands of BTS. But: Compared with the costs for a fixed network ISDN / PSTNinfrastructure, a GSM PLMN is comparable cheap, because there is no need formillions of lines into every private household.

Capacity: The capacity of transmission via radio interface is a great problem inmobile communications. Optimized usage of radio resources reducing the cellsizes, introducing sector cells and introducing the Hierarchical Cellular Structureswith Macro, Micro and Pico Cells solves this problem.

Data Rate: GSM (Phase 1/2) offers a maximum 9.6 kbit/s, compared to the 64kbit/s of ISDN. Introduction of HSCSD, GPRS and EDGE enhances the GSM datarates significantly.

Security Aspect: The radio interface can be intercepted with comparatively littletechnical expenditure. 1G could be intercepted without any problem, while thedigital transmission of the second generation offers protective measures againstinterception; the transmission is coded.

Health Aspect: The mobile radio frequencies lie near the resonance frequency ofwater (2.45 GHz). In order to keep thermal exposure to the mobile radio user aslow as possible there are maximum power limitations for mobile phones, 2 W forGSM900 and 1 W for GSM1800.


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The Air Interface Um:Problems of radio transmission and possible solutions

Cost Aspect:

Capacity:

Data Transmission Rate:

Security Aspect:

Health Aspect:

Construction of mobile

communication network

cheaper than terrestrial network

GSM900 / E-GSM: 124 / 174 frequency bands

GSM1800: 374 frequency bands

increasing subscriber numbers, data transmission

Resource optimization / protection !!!

GSM Ph1/2: 9.6 kbit/s

Ph2+: HSCSD, GPRS, EDGE > 100 kbit/s

Eavesdropping easy!

GSM offers encryption

H2O resonance frequency (2.45 GHz)

Thermal load

Pmax

= 2 / 1 W (GSM900/1800)

Fig. 18


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Problems of Physical Transmission

Screening: If there are hindrances between transmitter and receiver, the signalswill weaken. A connection can thus become problematic or impossible. In GSMthere is therefore the possibility of regulation of the transmitting power (PowerControl - PC) from mobile and base stations over several orders of magnitude.

Multipath Propagation: Multipath propagation through reflection and dispersionof radio waves leads to phase-shifted reception of signals of different paths. Theinterference can distort, amplify or erase the signal. An attempt to compensate fornegative effects of multipath propagation is given by power control, frequencyhopping, two antenna receivers for the base station (antenna diversity) andredundancy of the transmitted information.

Distance MS - BTS: The distance between MS and BTS has proved to beproblematic in several ways. The receive power sinks with increasing distancebetween transmitter and receiver theoretically with the square of the distance.Various physical effects such as atmospheric attenuation (weather-dependent)reduce the receive power even more. This attenuation depends on the frequencyand increases with increasing frequency in mobile radio relevant frequencyranges. The distance furthermore causes a reception de-lay, which may lead tointerference between neighboring time slots in TDMA. GSM responds to this delayby means of a regulation of the transmission time (Timing Advance TA). GSM900cells (GSM Phase 1/2) are limited to maximum 35 km, GSM1800 cells tomaximum 8 km radius as a result of the distance-related problems. There is thepossibility in GSM Phase 2+ to realize "Extended Range Cells" with a maximumradius of 100 km for GSM900.

MS Speed: Moving mobile stations can cause transmission distortions due toDoppler effect. A compensation for this effect up to a maximum speed of 250 km/h(130 km/h), for GSM-R a more powerful compensation for speeds of up to 450km/h was deloped.

Interference with external systems: The receive quality can also be disturbed byelectromagnetic waves from outside systems (e.g. car ignition, generators, PCs).A compensation is being tried out by means of the mechanisms described undermultipath propagation.


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Radio Transmission: Physical Disturbances

Mobility

Screening

Multipath propagation

Distance MS-BS

MS speed

External system interferencetransmitted signal

received

signals

signal to

antenna

Digital systems offer manyerror recognition and

correction mechanisms( redundancy)

signal attenuation (Power Control PC) interference (PC, f-hopping, diversity, regeneration) power loss (f-dep.); delay (PC, TA, cell size) Doppler effect (corrections) quality loss (PC, f-hopping, regeneration)

Fig. 19


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Frequency Resources: Optimized Utilization

In order to be able to keep up with the increasing demands on mobilecommunications despite the limited resources of the radio interface differentapproaches are being pursued.

Additional Frequency Ranges: The simplest way to cope with the growingdemand for mobile communications is to expand the available frequency range.This approach was pursued with E-GSM and GSM1800. Any further futureexpansion would be problematic as other frequency ranges are already reservedfor other applications.

Speech Compression: Speech compression in GSM allows a reduction of voiceinformation from 64 kbit/s to 13 kbit/s in the so-called Full Rate FR speech and to5.6 kbit/s with the Half Rate HR speech. HR speech thus leads to a considerableincrease in capacity. Central aspects of HR speech are described in the GSM Rec.06.02, 06.20 - 22, 06.41 and 06.42.

Cell Size Reduction/Coverage: The most important measure for increasing thecapacity of GSM networks lies in a reduction of the cell size. The resources of aradio cell are available to a small geographical area through the reduction of thecell radius or through the limitation of the cell coverage (sector cell). By doing so,the density of mobile communication subscribers and consequently the systemcapacity can be considerably increased. By halving the cell radius, its capacity isincreased by a factor of four. Nevertheless the size of a (normal = macro) cell cannot be reduced indiscriminately. Hierarchical Cell Concepts (Rec. 05.22) withmacro, micro and pico cells are significantly enhancing efficiency.

OACSU (Off Air Call Set Up): Traffic channels are allocated only after a success-full call setup, that is after the called subscriber (delayed allocation). The OACSUprocedure thus serves to improve the frequency efficiency; it can be used foroverload handling.

Tariffs: Introduction of day- & night time tariffs can help to level down peak loads.

Discontinuous Transmission DTX: For a conversation, this will mean that justspeech phases are transmitted. Background noise, or so called comfort noise istransmitted with a greatly reduced bitrate (500 bit/s instead of 13 kbit/s as withspeech phase) in phases in which a subscriber is silent. The other subscribershould thus not worry that connection has been broken off. In order to makediscontinuous transmission possible, the presence of "useful" information fortransmission must be determined by means of Voice Activity Detection VAD. DTXaspects are included in GSM-Rec.06.31 and 06.41, VAD aspects in Rec. 06.32and 06.42.


TM2100EU02TM_000141

Frequency Resources: Expansion / Optimized Utilization

GSM900: 2 x 25 MHz

Extension of frequency range:

E-GSM: 2 x 35 MHz

GSM1800 2 x 75 MHz+

Fixed network: 64 kbit/s

Speech compression:

FR:

13 kbit/s

Digital speech information

HR:5.6

kbit/s

Half Ratespeech

Full Ratespeech

Cell size

reduction:

(Radius reductionand sectorization)

35 / 8 km 500 m

omnicell

180 / 120

sector cell

OACSU (Off Air Call Set Up)

Time Balance / Tariffs

DTX (Discontinuous Transmission) / VAD (Voice Activity Detection)

Fig. 20


TM2100EU02TM_000142

Advantages of Digital Transmission

Digital transmission has many advantages over analog transmission:

Network Capacity: The capacity of mobile communication networks can beconsiderably increased by the possibility of compressing digitalized speechinformation. The disadvantage of speech compression is a loss of information(reduction of speech quality).

Service Offer: Digital data transmission simplifies the transmission of signalinginformation. This makes the introduction of a wide, quickly growing range ofservices possible in GSM beyond pure speech or data transmission.

Cost Aspect: Digital equipment is less expensive to manufacture owing to betterpossibilities for use in highly integrated microelectronics. Purchase costs as wellas operation and maintenance costs are thus less expensive and have allowedGSM's breakthrough onto the mass market.

Miniaturization: Microelectronics used for digital information transmission allowsa relatively simple reduction of the hardware (in comparison to analogtransmission), especially of the mobile stations. Mobile phones have been usedwith GSM since the start; their weight has been reduced from over 500 g to some50g within a couple of years.

Security Aspect: Digital information can be ciphered much more easily thananalog information. Transmission via radio interface is protected from fraud andunauthorized interception in GSM by the ciphering the digital user data (speech,data) and signaling data.


TM2100EU02TM_000143

ENCRYPTION

MODULE

Input data

(plain text)

Output data

(coded text)

Code

sequence

Advantages of Digital Information Transmission

Network capacity speech compression

Service offer signaling

Cost aspect manufacture, operation, maintenance

Miniaturization microelectronics

Security aspect easily coded

Transmission quality regenerability

Fig. 21


TM2100EU02TM_000144

Transmission Quality: Signal transmission via radio interface leads to consider-able distortions and weakening of the transmitted signals. Digital signals arefundamentally less susceptible to interference than analog signals and are bettersuited to regeneration. Analog speech connections become increasingly worsewith increasing distance from the transmitter until they eventually disconnect.Digital transmissions on the other hand maintain a constant good quality over along distance and then disconnect almost suddenly.


TM2100EU02TM_000145

S / N

signal

quality

distance to transmitter r

analog signal

digital signal

Quality of Digital & Analog Signal Transmission

Fig. 22


TM2100EU02TM_000146

Reliable Transmission via Um: Channel Coding

Various measures are taken in GSM to protect transmissions via radio interface frominterference, distortions and loss of information. These measures are taken by meansof channel coding.

The transmission is protected in such a way that a certain number of transmissionerrors can be corrected by the error correction procedure, the so-called ForwardError Correction (FEC). By means of FEC the Bit Error Rates (BER) of the radiointerface transmission are reduced to a rate of 10-5 to 10-6 from an unacceptablevalue of 10-3 to 10-1. Redundancy is added to the information to be transmitted inorder to al-low recognition and correction of transmission errors.

Channel coding of information on the transmit side comprises three steps:

1. Adding of parity check bits and fill bits

2. Error protection (redundancy) with convolutional coding

3. Spreading by time: interleaving

The same steps are carried out in reverse order at the receiving side.

The added parity check bits serve to recognize incorrigible errors on the receivingside. The parity check bits are of special use in speech transmission. If incorrigibleerrors are indicated, the corresponding speech information is rejected and an attemptis made to interpolate the information from the preceding speech information.

Convolutional coding serves to create redundancy. The original information (speech,data, signaling) is coded along with the parity bits. Important information runs throughmathematical algorithms, where redundancy is added and the arrangement of theinformation is changed.

Interleaving serves to temporally spread information. Information is collected up to adetermined number of bits and is spread by time. The interweaving of the redundantinformation has the effect that information loss due to frequent short disturbances canbe compensated by means of temporal spreading of the information.


TM2100EU02TM_000147

Reliable Transmission via Um:

Channel Coding

Addition of:

parity

and filler

bits

transmission side

Convo-

lutional

coding

redundancy

Inter-

leaving

temporalspreading

Parity

check

Convo-

lutional

decoding

De-inter-

leaving

reception side

Um

Fig. 23


TM2100EU02TM_000148

Speech Coding: FR, HR and EFR

Speech transmission is of central importance in GSM. Speech information is handledespecially by the radio interface for secure and resource-preserving transmission.Speech information is compressed and then redundancy is added (channel coding).There are three different speech codecs available in GSM for compression of speechinformation: the Full Rate (FR) Speech Codec was specified for GSM Phase 1, i.e.from the start, in Phase 2 the Half Rate (HR) Speech Codec and in Phase2+ theEnhanced Full Rate (EFR) Speech Codec were added.

Full Rate FR and Enhanced Full Rate EFR Speech Codecs compress speechinformation from 64 kbit/s - used in digital line connected telephone networks such asISDN - to 13 kbit/s respectively 12.2 kbit/s. So 13 kbit/s / 12.2 kbit/s are the net datarate for speech transmission via the radio interface. The gross data rate after addingredundancy in channel coding is 22.8 kbit/s with FR and EFR.

Half Rate HR Speech Codec compresses speech information from 64 kbit/s to 5.6kbit/s. The gross data rate after adding redundancy is 11.4 kbit/s. The connectionsof two Half Rate speech using subscribers can be realized in one physical channeltogether, with a gross data rate of 22.8 kbit/s.

Models for speech generation are generally used for speech coding. Periodically re-turning elements of speech are identified as phonemata; redundancy is removedfrom the speech information. Even the attributes of hearing, especially the spectralcovering effect, are taken into account in different ways.

More efficient speech recognition mechanisms are of use for the HR introduced inGSM Phase 2 and EFR introduced in Phase 2+. The HR codec delivers a somewhatlower speech quality in comparison to the FR codec if transmission is undisturbed. Itis more robust against radio specific disturbances owing to the relatively strong errorprotection. The EFR codec offers a significant increase in quality in comparison to theFR codec. It sounds more natural and "smoother" according to subjective test results.


TM2100EU02TM_000149

Speech Coding: FR, HR, EFR

Speech coding models of speech and hearing Removal of redundant information (periodic)

Transmission of central speech information

Reduction of speech information: 64 kbit/s 13 / 5.6 kbit/s (net data rate)

Full Rate (FR) CodecGSM Ph1;

13 kbit/s

Redundancy (channel coding)

9.8 kbit/s

Enhanced Full Rate (EFR) CodecGSM Ph2+;

12.2 kbit/s

Redundancy (channel coding)

10.6 kbit/s

Gross data rate via Um: 22.8 kbit/s

Half Rate (HR)Codec; GSM Ph2;

5.6 kbit/s

Redundancy

5.8 kbit/s

Gross data rate via Um: 11.4 kbit/s

HR & EFR: improved, acoustically optimized

speech coding

HR, FR almost the

same quality

Fig. 24


TM2100EU02TM_000150

Transmission PrinciplesGSM Network StructureDuplex Transmission & Multiple AccessGSM - Fixed Network TransmissionGSM Air Interface

02 Tm2100eu02tm 0001 Transmission Principles

Documents

mobile equipment

fixed network communications

mobile subscribers

radio transmission

fixed radio station

users mobile stations

public fixed networks

plmnthe mobile stations