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Dm1 e

  1. 1. DM1 DM1 DIGITAL MOBILE TELEPHONYRadio School DM1 Digital Mobile TelephonyMobile Telephone Generation 2 (G2) Modulator Detector Channel coder Channel decoder Speech coder Speech decoder RCUCore Unit Radio Systems and Technology 1
  2. 2. DM1 DIGITAL MOBILE TELEPHONYEricsson Radio Systems 2000 2
  3. 3. DM1 DIGITAL MOBILE TELEPHONYDigital Mobile Telephone DM1Mobile Telephone Generation 2 (G2)Contents Page Contents Page1 Overview 4 3 D-AMPS, original system 79 3.1 Overview 792 GSM, original system 8 3.2 System background 812.1 Background to GSM 8 3.3 Radio specification 832.1.1 System specification, introduction 8 3.4 Speech coding 912.1.2 System technology development 11 3.5 Comparison of the GSM and D-AMPS systems 1012.1.3 System options for GSM 182.2 Overview of the radio system 27 4 PDC, generation 1 1062.2.1 Introduction 272.2.2 TDMA structure of traffic channels 29 5 Cordless Telephone 1082.2.3 Structure of data bursts in a TDMA time slot 30 5.1 Overview 1082.2.4 Multiframe with SACCH 33 5.2 DECT 1122.2.5 Duplex arrangement 34 5.3 PHS 1172.2.6 Diversity against fast fading 352.2.7 Background to choice of radio 6 Further development of NMT 119 system parameters 38 6.1 Shut-down of NMT 900 1192.3 Detailed systems description 39 6.2 Modernization of NMT 1192.3.1 Introduction 392.3.2 Signalling, TDMA structure 41 7 Further development of GSM2.3.3 Channel coding and interleaving 52 and D-AMPS 1212.3.4 Radio modem 59 7.1 Improved speech coding 1212.3.5 Channel equalization 66 7.2 Digital signal channel for D-AMPS 1242.4 Radio performance 70 7.3 Adaptation to data transmission 1292.5 The fixed network 712.5.1 The speech path 71 8 Cell structures 1482.5.2 Switching and control 74 8.1 Additional frequency bands 148 8.2 Need for hierarchical cell structures 148 8.3 Land mobile satellite communication 150 Appendix. Follow-up questions 156 3
  4. 4. DM1 DIGITAL MOBILE TELEPHONY1. Overview The first generation of mobile telephone (“G1”) comprised of systems based on analog speech transmission. One example is NMT, which is overviewed in module G1. The second generation of mobile telephone (“G2”) or first gene- ration of digital mobile telephone, is based on digital transmission of speech. One of the main reasons for the introduction of digital speech was improved frequency economy through reduced radio bandwidth per speech channel and/or reduced cluster size. Examples of G2 systems are GSM (Europe), D- AMPS (USA) and PDC (Japan). GSM, D-AMPS and PDC are discussed in sections 2 to 4. The dominating G2 system is GSM with 60% of the market and 230 milj. users at the end of 1999. The cellular systems above have wide-area coverage, with nearly full coverage of large regions. Another type of G2 system, with local service areas, is cordless telephone. See section 5. The main cordless telephone sys- tem in Europe is DECT (Digital European Cordless Telephone - new name Digital Enhanced Cordless Telecommunications). In Japan, PHS (Personal Handy Phone System), is used extensively in areas with high traffic density. GSM and to some extent D-AMPS have been exported to different parts of the world. The G1 and G2 generations were originally optimized for speech trans- mission - data transmission was a secondary service, used to a very small extent. The third generation of mobile telephone (“G3”) or second generation of digi- tal mobile telephone is optimized for a mixture of different services such as speech, data and video, incl. multi media. This is covered in module DM2. User data rates up to 2 Mb/s shall be accomodated. The dominating transmisssion mode will be based on Internet (packet oriented, IP protocol). Due to the large penetration of G2 systems there has been a strong motivation to modernize them to partially accomodate the new services which will be handled by the G3 systems. See section 7. The major development in this respect is the introduction of EGPRS (EDGE-based GPRS) in networks based on GSM and D-AMPS. For good radio connections (high C/N and C/I values) user rates up to 384 kb/s are possible. (384 kb/s is the lower data rate specified for the European G3 alternative UMTS). Even before GPRS and EDGE there has been a gradual evoluation of the G2 systems, i.e. introduction of improved speech codecs and low-bandwidth data services. One example is the SMS (Short Message Service). D-AMPS was originally an add-on to Analog AMPS, sharing the same radio channels and relying on A-AMPS for the signalling and control functions to set up a traffic channel. Later D-AMPS became self sufficient by adding a digital control channel. See section 7.2. A major difference between the G1 and G2 generations is that FDMA is used for G1 and TDMA (combined with FDMA) for G2. The G3 systems are mainly based on CDMA for multiple access to the radio medium, even if CDMA is often complemented by TDMA and FDMA. One G2 system (IS-95) is based on CDMA. For that reason it is included in module DM2, which covers the G3 systems. A summary of the main characteristics of the different generations of cellular systems is given in figure 1.1. 4
  5. 5. DM1 DIGITAL MOBILE TELEPHONY Generation of cellular systems G1 G2 G3- G3 G3 + (G4) Generation of 1 2 digital cellular System examples NMT GSM, PDC GPRS+ WCDMA HIPERLAN/2 AMPS D-AMPS, cdma One EDGE TDCDMA WLAN TACS DECT, PHS MCCDMA WATM Maximum user 9.6 kb/s 384 kb/s 2 Mb/s 20 Mb/s data rate Analog Digital Internet Multimedia High speed Dominating Service speech speech Data Internet data Speech IP-telephony using IP Multiple Access FDMA TDMA TDMA CDMA OFDM (CDMA) (TDMA) (CDMA) Duplex arrangement FDD FDD, TDD FDD, TDD FDD, TDD FDD,TDD Introduced 1982 1992 2000 2002 2002 In module G1 DM1 DM1 DM2 DM2Figure 1.1 The systems, mentioned above, are terrestrial cellular networks, which do not cover areas with very low user density due to economic considerations. Also, there are limited roaming possibilities between cellular systems in different regions. Therefore mobile satellite networks with word wide or regional coverage are established. See section 8.3. The satellite services are considera- bly more expensive than terrestrial cellular systems due to the difficult link budget, but anyway a considerable market is foreseen, mainly combined with and as gap-fillers to the terrestrial systems (using dual-mode or triple mode terminals). The first worldwide land-mobile satellite network IRIDIUM started commercial service late 1998, and before year 2001 additional systems are established. However, there are indications that the market will be less than foreseen. Due to the rapid expansion of the mobile telephone market, large improvements are necessary with respect to frequency economy and geographic availability. One important system feature is hierarchical cell structures, incl. good hand-over capabilities between different hierarchical layers. The highest layer is satellite cells, the lowest layer is indoor pico cells. Cell structures are discussed in section 8. The G3 systerns have maximum user data rates up to 2 Mb/s in pico cells (indoors) and possibly also in micro cells (hot spot outdoors). However there will be a need for even higher data rates - up to 10 times higher. Systems with this capability are generally wireless extensions of high speed LANs (Local 5
  6. 6. DM1 DIGITAL MOBILE TELEPHONYArea Networks), see column G3+ in figure 1.1. They could also form part ofthe next generation (G4) of mobile telephone, which is not yet clearly defined.However, the general concept of G4 is a closely integrated cluster of severalsystems in a hierarchical structure. The highest layer (satellite networks)would. have world-wide coverage but small bandwidth capabilities, perhaps50 kb/s. The lowest layer contain G3+ systerns with around 10 Mb/s maxi-mum user data rate but these systerns could only provide coverage indoorsand of very small outdoor areas. This layer might cover around I % of a re-gion. The middle layers contains the G3 and the G3- systerns, G3- providingsubstantial coverage of a region, and G3 metropolitan areas with high trafficdensity, which motivates the use of small cells. The hierarchical structure issketched in figure 1.. G4. Hierarchical concept Maximum user data rate 20 Mb/s G3+ Broadcast LMDS satellites (downlink) 2 Mb/s 384 kb/s G3 (BRAN) G3- 150 kb/s (UMTS) (GSM+) (DAMPS+) (DECT+) 50-100 kb/s Mobile Bluetooth satellite systems Coverage MobilityFigure 1.2Other key concepts of G4 are the use of packet/IP based transmission for alltypes of services and the interaction between intelligent networks and termin-als (software-controlled). The terminals are continuously connected to the op-timum system within the hierarchical structure, considering the coverage situa-tion and its current need for bandwidth. Store-and-forward capabilities make itpossible to transfer a large amount of data, when the terminal moves throughthe coverage area of a G3+ system. Outside of metropolitan areas, the maxi-mum data rate would be quite limited, which still could be useful, bycompressing the source data rates and keeping only the essential information. 6
  7. 7. DM1 DIGITAL MOBILE TELEPHONYAdditional systems could be added to this structure. The combination of largecoverage and broadband could be provided in the outward direction by exten-sion of satellite-based digital TV and radio. In the other direction G3 or G3-systerns could be used (asymmetric service). Other related systerns couldpoint-to-multipoint fixed networks (MDS) and moderate-rate, short-rangenetworks (i.e. Bluetooth). 7
  8. 8. DM1 DIGITAL MOBILE TELEPHONY2 GSM, original system2.1 Background to GSM2.1.1 System specification, introduction The initiative for a digital mobile telephone system came from the Scandina- vian Telecommunication Administrations, which submitted in1981 a joint proposal to CEPT for the specification of a pan-European mobile telephone system, conceivably to be based on digital transmission. The reason for proposing serious consideration of a digital transmission system was based on the findings of studies conducted by a Scandinavian working group. In 1982, CEPT appointed the GSM group (Groupe Special Mobile), whose members consisted of representatives from a number of countries in Western Europe, to investigate the idea. Following system studies coordinated by the GSM group, a decision was ta- ken in 1985 to draw up a goal specification for a digital system. The general criteria stipulated were that the new system should provide at least the same speech quality and spectrum efficiency as the existing analog mobile telephone systems. Another requirement was that the estimated cost of the fully developed system, when in mass production, should be lower than that of the existing analog ones. In addition, the system must be able to interface with the ISDN on the fixed side, even if some services requiring wide bandwidth might not be available due to the frequency shortage. Also, a number of GSM specific services were required, see figure 2.1a. GSM Services GSM Services = GSM Specific Services + ISDN Services GSM Specific services: PAN-European roaming Authentication (fraud control) Ciphering (speech, data, signalling information) User confidentiality (Ciphered subscriber number on radio path) Figure 2.1a At that time, simulations and experiments of digital-speech transmission systems based on FDMA had progressed far enough to predict with considerable certainty that a new system based on digital transmission would be able to offer higher performance than existing analog systems. However, it seemed likely that further development work could result in alternative forms of multiple access to FDMA with improved system performance. Due to technical uncertainties it was not yet possible to recommend any other multiple-access arrangement. The main unknown factor was if it would be 8
  9. 9. DM1 DIGITAL MOBILE TELEPHONYpossible to suppress strong intersymbol interference caused by time dispersionin wideband radio transmission. The GSM group therefore decided that anevaluation should be made of systems based on other types of multiple accessthan FDMA.Nine R & D groups in Western Europe designed test systems, which wereevaluated in Paris in autumn 1986 by means of laboratory evaluations,employing fading simulators and field tests. It was very much on the basis ofthese comparative tests that the GSM group recommended in spring 1987 thata joint pan-European mobile telephone system should be developed, based ondigital speech transmission and Narrowband TDMA (NTDMA). The systemwould be called GSM. This was followed by a Memorandum of Understan-ding signed by 13 countries, under which they agreed to introduce GSM byJuly 1991.Key features of the outlines specification of 1987 were TDMA with 8 timeslots in a time frame of 4.6 ms, an advanced version of a RELP speech coderwith a data rate of 13 kb/s, convolution coding for error correction, andGMSK modulation with 200 kHz channel spacing.A comprehensive specification, drawn up by a consolidated GSM group (thepermanent nucleus), was ready by the end of 1988. The extensivedocumentation covered not only the different radio subsystems but also thenetwork services to be offered and interfaces to the fixed network. However, agreat deal of work still remained on the fine details of the design, and this wasmade the responsibility of the European Telecommunication Standards Insti-tute (ETSI). The work on developing the GSM as a commercial product pro-ved to require considerably more resources than had been foreseen. Inconsequence, the project overran the original time plan by about a year. Thefirst large scale introduction of GSM was in Germany in 1992-93, where thecapacity of the existing analog mobile telephone system had inadequate trafficcapacity and high costs for subscribers.The growth in the number of GSM subscribers during the same period wasslower in the Nordic countries. The main explanation for this was that theNMT network had still not reached its capacity limit, and many mobiletelephone subscribers held the opinion that the service offered by NMT, withwide coverage in Scandinavia, was adequate and relatively low priced.However, a sharp upturn in the number of GSM subscribers came in thebeginning of 1994. In middle 95 the Swedish frequency administrationauthority decided that part of the frequency band used by NMT 900 should begiven over GSM. As more and more users prefer GSM, the NMT 900 servicewill be shut down in a few years time. See section 6. GSM 900 will then haveaccess to a 2x25 MHz wide spectrum. 9
  10. 10. DM1 DIGITAL MOBILE TELEPHONY Frequency allocation in western Europe for the 900 MHz cellular systems Mobile to base 890 899 915 MHz Analog systems GSM Base to mobile 935 944 960 MHz Analog systems GSMFigure 2.1bSystems of the GSM type are also used at 1800 MHz in Europe and at 1900MHz in the US (the PCS band).When the system was introduced, its full name was changed to Global Systemfor Mobile Communications, which meant that the abbreviation GSM couldstill be used. 10
  11. 11. DM1 DIGITAL MOBILE TELEPHONY2.1.2 System technology development General Development of digital mobile system 1. Military systems Motivated by requirements on secure enciphering Otherwise marginal transmission performance 2. FDMA GMSK gives reasonable compromize between implementation complexity and frequency economy Channel coding necessary to counteract fast fading Low-rate speech codecs 3. TDMA Implementation advantages MAHO Certain flexibility in demand assignment of bandwidth 4. DS-CDMA (FH-CDMA) (interference-limited systems) Interference averaging of cochannel interference Bandwidth on demand (fast dynamic allocation) Improved frequency diversity (bandwidth expansion) 5. OFDM for systems with very high user data rates (simplified channel equalization) low bandwidth expansion (2 and 3 channel-limited systems, i.e. system capacity limited by number of radio channels per cell) 1. Circuit switching 2. DSI, Statistical Multiplex, Asynchronous TDMA 3. Packet transmission Figure 2.2a The origin of digital speech transmission in mobile systems was military and police applications, where digital speech was motivated by the need for very secure enciphering. One example from Sweden is given the block diagram in figure 2.2b. In other respects, these early systems had very marginal performance in comparation with the corresponding analog systems. Available technology made it extremely difficult to transmit digital speech within the channel width specified for the corresponding analog systems. An extensive development of digital radio transmission technology was therefore necessary before digital G2 systems could compete with the analog systems with respect to system cost, speech quality and spectrum. 11
  12. 12. DM1 DIGITAL MOBILE TELEPHONYThe introduction of digital speach was originally motivated byrequirements for very secure encryptionSEMIDIGITAL MOBILE RADIO SYSTEM (also including normal analog speech)From the 1980-1985 period Traditional analogSpeech FM-radio (TX) (TX) (TX) Transmitter 10 10 0-6 kHz kb/s kb/s Speech Enciphering Baseband codec unit Modem (Rx) (Rx) (Rx) ReceiverSpeech 25 kHz db 0 Adjacent channel Problem: Marginal speech quality at 10 kb/s (adaptive delta modulation) Marginal receiver sensitivity (data modem for 10 kb/s that complies with 70 dB adjacent selectivity) 15 kHz High complexity, using VLSI technologies of the 1980s -70 f Interference on Transmitter spectrum adjacent channel Figure 2.2b FDMA Development of the fundamental technology for digital cellular systems started in Sweden at the beginning of around 1980, and some years later in other western European countries. Initially, the Swedish studies focused on the simplest system configuration based on FDMA (see Figure 2.3). The principal subsystems were speech coding, channel coding and radio modem. 12
  13. 13. DM1 DIGITAL MOBILE TELEPHONY Digital mobile telephone system (FDMA). Data Radio transmitter Radio Speech Encryp- Channel modem Power Speech encoder tion unit encoding 16 kb/s (T) Amplifier 10 kb/s (Speech Key (Channel codec) Key codec) Radio receiver Speech Speech Radio Front- Decryp- Channel decoder tion unit decoding modem end (R) DataFigure 2.3The main finding of the work was that digital radio transmission eventuallycould provide better speech quality and higher spectrum efficiency. Theconclusion was based on the results of a combination of computersimulations, laboratory tests using Rayleigh fading simulators and field tests.The improvement in spectrum efficiency compared with analog mobiletelephone systems was largely due to major advances in speech coding andchannel coding. The Swedish FDMA system incorporated an early RELP-type16-kb/s speech coder with a permissible bit error rate of 1%, and channelcoding optimized to suppress the effect of fading dips caused by the fast fa-ding due to multi path propagation. Additional facilities to deal with fadingwere soft channel decoding and interleaving (see section 2.3.3). Thesemeasures to counter fading yielded a significant reduction of the requiredprotection ratio. The GMSK modem also contributed to the good spectrumefficiency through its combination of moderate protection ratio and fairlynarrow modulation spectrum.The difference in spectrum efficiency between analog and digital transmissionis shown in Figure 2.4, which compares three cellular system alternatives: a) Analog speech transmission and FM modulation b) Digital speech transmission without channel coding c) Digital speech transmission with FEC channel codingThe alternatives b and c use 16 kb/s speech coders.In all three cases it is assumed that a duplex band of 2 x 10 MHz isavailable. In alternative b), the input data rate to the modulator is 16 kb/s,which corresponds to a necessary channel separation of 15 kHz. Inalternative c), the data rate, going through the channel coder, increases from16 to 27 kb/s, which requires a channel spacing of 25 kHz – in other words,the network in this case has available 400 two-way speech channels. Theoverall spectrum efficiency is also determined by the required reuse distancebetween co-channel cells (cluster size), which, in turn, depends on the localmean of the protection ratio, KI over the fast fading. 13
  14. 14. DM1 DIGITAL MOBILE TELEPHONYA typical requirement for analog cellular systems (without diversity) isKI = 18 dB. Results from lab tests indicate that KI = 20 dB is required inoption b) and KI = 13 dB in option c). The improvement going from b) toc) can be explained by considerable diversity gain from channel coding.The protection ratio is the required power ratio between the wanted signal Cand the co-channel interference I needed for adequate transmission quality,e.g.KI = (C/I)min Comparison of spectrum efficiency Analog system Digital system (companded FM) with without channel coding Data rate, speech encoder - 16 kb/s 16 kb/s System data rate - 16kb/s 27kb/s Channel spacing 25 kHz 15 kH/z 25 kH/z Protection ratio 18 dB 20 dB 13 dB (local mean) Cluster size 3x7 3x9 3x3 Spectrum efficiency: Channels per MHz per cell 1.9 2.4 4.4 Traffic per cell for 12.4 e 17.2 e 35.1 e 10MHz system C and I subject to Rayleigh fading 120° sector antennas. Each base station serves 3 cellsFigure 2.4Besides by KI, the required normalized reuse distance, D/d (D: reusedistance, d: cell radius), is also determined by the distance-dependence ofthe global propagation attenuation (propagation exponent), the structure ofthe shadow fading (variance of the log-normal distributions and thecorrelation between the fading in C and I), and the required area availability(the proportion of the area of a cell in which the local mean of C/I exceedsthe protection ratio). The required cluster size is determined by D/d. Thecluster size is the number of cells with different channel allocations that isrequired to enable co-channel cells to be adequately separated. The clustersize of 3 x 3, shown in figure 2.5, is often used for the GSM. (The localmean of the protection ratio for GSM is 9-10 dB). Each base station siteserves three cells. 14
  15. 15. DM1 DIGITAL MOBILE TELEPHONYCell structure with clusters Cluster size 3 x 3 4 7 4 7 5 8 1 5 8 1 5 8 2 6 9 2 6 9 2 4 7 3 4 7 3 4 7D 5 1 8 1 5 8 1 9 6 2 6 9 2 6 9 3 4 7 3 4 7 3 d 1 1 D: reuse distance d: cell radiusFigure 2.5Simulations based on typical propagation characteristics gave the relationshipshown in Figure 2.6. The same characteristics between the protection ratio(local mean) and the geographic availability applies also to other types ofmultiple access. The figure gives a rough indication of the needed protectionratio and thus the cluster size for several MA alternatives. Previously 90 %geographic availabily was considered marginally acceptable. In the future,higher availability would be required, which tends to increase the necessarycluster size. 15
  16. 16. DM1 DIGITAL MOBILE TELEPHONYProbability distribution for (C/I < KI) for different cluster sizesAvailability Probability C/I < KI 1.0 20% 0.8 Cluster 1 3 9 size 12 0.6 50% 0.5 0.4 21 27 80% 0.2 90% 90% 0.1 availability KI=(C/I)min 100% 0.0 local average DS-CDMA FH-CDMA 20 30 dB -10 10 GSM NMT KI : protection ratio Figure 2.6 The figure shows, for instance, that for 90% availability (C/I > KI over 90% of the cell) and KI = 18 dB (local mean), a cluster size of 21 is required. (Each site serves three ells.) Accordingly, the total number of radio channels available to the system must be distributed over 21 cells. (See also module S4). The number of traffic channels available per cell is thus derived from the clus- ter size, the channel spacing (per traffic channel) and the total frequency band available. The average number of speech channels per cell that can be serviced during busy hour is less than the number of radio channels, otherwise too much traffic is lost due to traffic overload (blocking) during traffic peaks. If a loss system (Erlang B) with 2% permissible blocking is assumed, the carried traffic per cell will be that shown in Figure 2.4. (See also module S3). As is evident from Figure 2.4, systems using digital speech transmission with FEC channel coding might achieve a spectrum efficiency three times higher than the cellular systems of the first generation (G1). Although channel coding implies an increased input data rate to the modulator – in other words, wider channel spacing than in a corresponding system without channel coding – this is more than compensated for by the considerable reduction in the required protection ratio. A significant improvement in the overall spectrum efficiency is thus obtained. 16
  17. 17. DM1 DIGITAL MOBILE TELEPHONYTDMAThe use of digital transmission means that other forms of multiple access canbe used besides FDMA. The most readily available option is TDMA, possiblycombined with time duplex (TDD: time division duplex). This offers furtheradvantages in terms of system performance and cost savings. A summary ofthe advantages of TDMA is presented infigure 2.7. At TDMA, Time DivisionMultiple Access, each radio channel is time-shared between several transmis-sion channels. See figure 2.22 (basic frame) and 2.26. Advantages of TDMA (+ TDD) Fewer radio units and simpler antenna filters at the base No duplex filter needed at terminals Mobile Assisted Handover (MAHO) Wide radio channels reduce requirements on frequency stability and selectivity TDM instead of FDM replaces analog high-Q filters with digital VLSIFigure 2.7The possibility of listening or transmitting in other frequency or time slots inidle periods during each frame affords important system benefits. Suchperiods can be used for system signalling, preparing for handover and ifantenna diversity is used at the terminals, to select and connect a suitableantenna to the receiver input before the reception time slot occurs.An important facility is Mobile Assisted Hand Over (MAHO), i.e. the informa-tion needed by the system control to determine when hand-over shall takeplace comes both from the terminal and the base. The mobile must measureC/I and C/N for signals from adjoining cells and transmit this information tothe base.TDMA also allows a terminal to transmit and receive in different time slots(time duplex). This eliminates relatively expensive and bulky duplex filters atthe terminals.These advantages often outweigh the disadvantages of TDMA. Thedrawbacks are listed in figure 2.8. 17
  18. 18. DM1 DIGITAL MOBILE TELEPHONY Drawbacks of TDMA Higher transmitter peak power level for a given mean power level (determines range) Wide modulation bandwidth can result in intersymbol interference due to multipath propagation (need for adaptive channel equalization) Greater equipment complexity (requires advanced VLSI with low power consumption) Increased channel spacing which reduces flexibility of frequency planning Figure System options for GSM The choice of multiple-access arrangement for GSM was largely based on the results of the evaluations made in Paris in late 1986 and early 1987. The majority of the test systems were based on TDMA. The main contenders for GSM were Narrowband TDMA (NTDMA) and Wideband TDMA (WTDMA). Several versions of Narrowband TDMA were evaluated by the GSM group. One French proposal (SHF-900) combined TDMA with low-rate channel coding supported by frequency hopping. The proposal (MAX 2) from the Swedish Telecom Administration was for 8-PSK and as little as four time slots in the TDMA frame. This gave such a narrow modulation bandwidth that channel equalization would only have been necessary for very difficult propagation conditions. However, the TDMA option specified by the GSM group corresponded most closely to the experimental system based on NTDMA developed by Ericsson (DMS 900). The main competitor was WTDMA, (CD900) which, in terms of performance, for the most part was on a par with the best NTDMA alternatives. An interesting finding of the Paris tests was that several systems achieved roughly the same spectrum efficiency (see figure 2.9). The differences among the systems with regard to the radio bandwidth per speech channel were offset by different protection-ratio requirements. As described earlier, different protection ratios result in different cluster sizes. Roughly speaking, if a system can cope with half the channel width per speech channel, an increase in the cluster size by a factor of two can be allowed without any impact on the total spectrum efficiency. The CD 900 system (SEL, Germany), based on wideband TDMA, incorporated very powerful, low-rate channel coding, which increased the required bandwidth but resulted in a much lower protection ratio than narrowband TDMA. At the other end, MAX 2, was designed for the narrowest possible channel width per speech channel, which resulted in a fairly high protectionratio. 18
  19. 19. DM1 DIGITAL MOBILE TELEPHONY Spectrum efficiency for different mobile telephone systems Equivalent bandwidth per speech channel Curves corresponding to constant spectrum ef ficiency CD 900 100 a) Existing analog systems b) 2 x a SHF 900 50 US-(A-AMPS) DMS 90 25 x UK-TACS GSM MAX 2 12,5 Protection 5 10 15 20 25 ratio Cluster size N=3 N=9 N=27 (for 90% area availability) Figure 2.9 Several of the experimental systems achieved better spectrum efficiency than the analog mobile telephone systems. This was one of the criteria stipulated at the outset that the digital systems would have to meet. The evaluation also seemed to show that digital systems can provide better speech quality also during fast fading.Main features of DMS-90 (NTDMA, ERA proposal) A block diagram of the ERA’s test system is shown in Figure 2.10, and the system’s multiple-access structure in figure 2.11. The channel coding was supported by interleaving and frequency hopping. This gave a considerable diversity gain – i.e. low protection ratio – even for portable terminals (quasistationary propagation channel). The interleaver splitted up a 384-bit block from the channel coder into four sub blocks of 96 bits, which were distributed among four time slots. (Each time slot could accommodate 2 x 96 user bits, i.e. contained blocks from two of the 384 bit blocks). The time dis- persion of the propagation channel was handled by the adaptive equalizer, which also could give multi-path diversity if the propagation channel had fairly large time dispersion. The impulse response of the radio channel was determined with the help of a training sequence at each time slot of the TDMA frame. The training sequence is used also for frame synchronization, see figure 2.22. 19
  20. 20. DM1 DIGITAL MOBILE TELEPHONYNarrowband TDMA DMS-90 Speech Channel coder Radio Pulsed trans- encoder Interleaving modulator mitter stage Frequency- hopping synthesizer Quality Speech Deinterleaving Adaptive A/D Demo- HF decoder Channel decoder equalizer dulator IF Binary signal Correlator T raining sequence Figure 2.10 To improve the performance of the channel decoder, the quality (estimated ber) of each bit to the decoder is an additional input to the decoder. This procedure is called soft decoding. DMS -90. Channel coding, interleaving and frequency hopping 32 ms From 16 kbps 256 256 speech coder 24 kbps 384 384 From (12.8) RS channel coder From TDMA 192 192 192 192 192 192 340 kbps interleaver (interleaving depth 4) From 192 192 f1 f1 frequency hopper 192 192 f2 f2 192 f3 192 f4 Figure 2.11 20
  21. 21. DM1 DIGITAL MOBILE TELEPHONYSince the initial development phase and the Paris evaluations, furtherdevelopment of the GSM radio transmission system has resulted insubstantially better performance than DMS 90. The final GSM specificationand implementation gave a protection ratio of KI < 10 dB even for portableterminals, when frequency hopping is used. The improvement is mainlyachieved through further refinement of the channel coding. Further rapidadvances in speech coding have also made it possible to reduce the data ratefrom the speech coder with more or less the same speech quality. In 1994 ahalf-rate speech coder was standardized, further improving the spectrumefficiency by a factor of two. (Some people have expressed the opinion that,instead of reducing the data rate, the advances made in speech coding couldbetter be used to improve the speech quality.)The relationship between C/I or C/N on the one hand and subjective speechquality on the other differs between analog and digital transmission (see figure2.12). (TACS is the British analog mobile telephone system.) Speech quality in GSM and TACS Speech quality TACS Acceptable speech GSM quality (K I )GSM (K I )TACS C 10 20 30 40 dB I KI: protection ratioFigure 2.12With digital transmission using FEC channel coding, the speech quality is al-most constant down to a threshold that corresponds to the error-correction li-mit of the channel decoder. If the input signal to the receiver falls below thislevel, the error-correction fails and speech quality rapidly degrades. If thequality of the input signal to the receiver is high, analog mobile telephonesystems are superior, since the speech coder causes some quality degradationeven if the C/I and C/N are high enough for no transmission errors to occur.Main features of CD-900 (WTDMA, German-French proposal)One of the demonstration systems evaluated in Paris was the CD-900WTDMA system, which was developed by a consortium led by the Germanorganization SEL. The system concept was based on an earlier military 21
  22. 22. DM1 DIGITAL MOBILE TELEPHONYproject – AUTOTEL. The technical performance and spectrum efficiency ofthe CD-900 system were on a par with the best NTDMA systems. Because thepublished information is limited, to gain a general idea of the system we needto examine the combined available information on Autotel and CD-900.The main system characteristic of wideband TDMA was a very wide modula-tion bandwidth through a combination of many time slots per TDMA frameand a substantial bandwidth expansion through low-rate channel coding. Thechannel coding is based on near-orthogonal codes, i.e. optimum soft decodingcan be based on matched filters implemented by a correlation procedure. Theprinciple is shown in figure 2.13, which applies to AUTOTEL. A group offour information bits is coded into 16 chips.In the CD-900 system, five information bits are coded into 32 chips, and anadditional sixth bit is transmitted via the polarity of the chip sequence. Agroup of 32 chips can be considered to form a symbol in an alphabet of size26 = 64. If the QAM arrangement is included, the size of the symbol alphabetis 128. Correlator-based matched receiver (CD-900) 4 bits 16 chips Correlator (Block code: 16, 4) ∑ Filter matched to binary sequenceFigure 2.13The powerful channel coding produces a high coding gain, i.e. a substantialreduction in the required C/I with respect to co-channel interference. A furtherreduction in the required protection ratio in a rapid fading situation is obtainedfrom a considerable gain from frequency diversity. The reason is the wide mo-dulation bandwidth which, for most propagation conditions, is much greaterthan the correlation bandwidth of the propagation channel. The combinedcoding and diversity gains enable the local mean of the protection ratio to bebrought down as low as (C/I)min ≈ 4dB. This means that a cluster size of threeis adequate. 22
  23. 23. DM1 DIGITAL MOBILE TELEPHONY Wideband TDMA (CD-900). Modem diagram. I channel 6-bit D/A 12-bit block ≈900 MHz Baseband Transmitter + amplifier modulator π/2 D/A Multipath 6-bit propagation Q channel Linear amplifier I channel N 6-bit C A/D 12-bit block Baseband ≈900 MHz Icoch demodulator HF & IF π/2 A/D 6-bit Q channelFigure 2.14To achieve reasonable spectrum efficiency despite the large bandwidth expan-sion due to channel coding, the WTDMA system uses linear modulation(QAM), i.e. a linear transmitter amplifier has to be used (see figure 2.14). Eachcoded radio symbol carries 12 information bits, 6 bits on each of the I and Qchannels. To obtain a sufficient modulation bandwidth both for accuratemeasurement of the impulse response (for setting of the channel equalizer)and for a high frequency diversity gain, a TDMA arrangement with as manyas 63 time slots was used (60 traffic channels and three channels for signal-ling). The time slots were used jointly by the three sector cells belonging toeach base-station site. Thus, each cell was allocated an average of 20 trafficchannels. (Cluster size of 3 means that all base-station sites can use the sameradio channels. This simplifies the cell-frequency planning).Advanced digital signal processing is used for the channel equalization (seefigures 2.15 and 2.16). Each data burst, which comprises a number of blocks(radio symbols) each carrying 12 information bits, starts with a synchroniza-tion/training sequence. The wide modulation bandwidth allows accuratemeasurement of the impulse response of the propagation channel. This infor-mation is used by the advanced channel equalizer, which in an optimum wayadds together the signal power from the different propagation paths. Thechannel equalization is placed before the symbol detector. It is a filter matchedto the impulse function h(t) of the radio channel. The filter convolves thereceived burst (excluding the training sequence) with h(T-t). This gives opti-mum, coherent addition of the signals from propagation paths with differentdelays, thereby eliminating intersymbol interference and at the same timeachieving frequency (or multi-path) diversity.After this initial signal processing, detection takes place by determining fromwhich of the 32 matched filters the highest absolute value is obtained at thesampling instant. (The symbol alphabet comprises 32 near-orthogonalsymbols.) In addition, the polarity of the output signal from the selected filteris measured. 23
  24. 24. DM1 DIGITAL MOBILE TELEPHONY Wideband TDMA (CD-900). Demodulator with channel equalization Training sequence (sync word) Sync correlator Channel impulse response h(t) Sequence R Memory inverter h(T-t) Matched Symbol Decision Data word filter correlator circuit 6 bits Suppresses (1 of 64) per word time dispersion Training sequence (sync word) (Data words comprising 6 bits/32 chips R: receiver front-end Figure 2.15 WIDEBAND TDMA (CD-900). Modem based on digital signal processing 6 bits Code Generates generator bipolar D/A waveform 32 chip sequence ± polarity Sync. word Sync. Corre- signal Decision circuit lator • Select input with h (t)6 bits highest absolute value. Polarity? Inversion C N A/D • Code conversion t =>-t from 64 chips to 6 bits h (T-τ) s(τ) h(T-τ)dτ Icoch Convolution s (t) Time alignment of signals Filter matched to with different propagation 32 different code delays. Eliminates time sequences Filter matched to dispersion and gives large radio channel diversity gain against impulse response Rayleigh fading Figure 2.16 24
  25. 25. DM1 DIGITAL MOBILE TELEPHONYExamples of the output signal from the sync correlator (i.e. the measuredimpulse response from the radio channel) have been published (seefigure 2.17). AUTOTEL Typical output signal from sync correlator A Same as A Different propagation paths 2µsFigure 2.17As mentioned above, the wide modulation bandwidth (B) in combination withthe advanced signal processing to handle the time dispersion due to multi-pathresulted in very efficient frequency diversity that suppressed most of the fastfading. See figure 2.18. 25
  26. 26. DM1 DIGITAL MOBILE TELEPHONYReceived signal power f=900 MHz(db) Sample separation: 30 cm B=25 kHz20 No suppression of Rayleigh fading10 0 narrowband-10-20 0 100 200 300 400 Sample number(db) B=6 MHz20 Frequency diversity nearly eliminates fast fading10 0 wideband-10-20 0 100 200 300 400 Sample number B: width of radio channel Figure 2.18 This WTDMA system is of considerable general interest as it has several char- acteristics of the G3 systems based on DSCDMA. Considerable bandwidth expansion is used, which with CDMA terminology gives processing gain, reducing the requirement on protection ratio. The protection ratio is further lowered through the frequency diversity. The result is that a cluster size of three is sufficient. (DS-CDMA develops these concepts further so that the cluster size can be reduced to one.) The overall performance of the testbeds based on NTDMA and WTDMA were very similar. So why was NTDMA chosen instead of WTDMA? A comparison between NTDMA and WTDMA based on the finding of the Pa- ris evaluations indicates comparable spectrum efficiency and speech quality. The choice therefore had to be made on the basis of other system and implementation characteristics. An advantage of WTDMA in areas with high traffic density is the large number of speech channels per carrier. This reduces the cost of the base sta- tion equipment for cells which must handle a large number of traffic channels. 26
  27. 27. DM1 DIGITAL MOBILE TELEPHONY Also all base-station sites can use the same radio channels. This facilitates frequency planning of the individual networks. However, the wide channel spacing imposes considerable limitations on the gradual transfer of frequencies from the analog to the digital mobile telephone system. Moreover, the cost per speech channel is high for small base station sites that are located outside high-density urban areas and only have a traffic volume for a small number of radio trunks. Some additional drawbacks with WTDMA are also shown in figure 2.19. In its assessment, the GSM group attached great importance to the apparently greater technical risk inherent in WTDMA. Not only were highly linear trans- mitter amplifiers an unproven technology in mobile radio applications but the wide modulation bandwidth also imposed a heavy demand on high-speed di- gital signal processing for channel equalization and detection. Why was WTDMA rejected? • Inflexible frequency planning • Too complex base stations in rural locations • High peak output power at portable terminals • More complex, high-speed digital signal processing (technical risk, increased power drain) • Linear modulation (technical risk with linear transmitter amplifiers) Figure 2.192.2 Overview of the radio subsystem2.2.1 Introduction Section 2.2 gives a first overview of the radio transmission after that a connection has been established. Some of the features are covered in more detail later, and also the signalling procedure to set up a call. A simplified transmission block diagram is given in figure 2.20. The Viterbi procedure in connection with convolutional channel coding is described in module DT12. 27
  28. 28. DM1 DIGITAL MOBILE TELEPHONY GMS Simplified Block Diagram (excl. ciphering) Speech Channel Radio Coding Coding Speech Error Burst- Modulator coder protection formatting Transmitter data Speech Viterbi Viterbi decoder decoder equalizer Receiver quality inter- leavingFigure 2.20The most important transmission specification items (“air interface”) are shownin figure 2.21. The frequency band comprises 2 x 25 MHz in a duplex arr-angement with 124 duplex channels with 200 kHz channel spacing. Thischannel spacing allows a system data rate of about 270 kb/s with GMSK mo-dulation and modest adjacent channel selectivity requirement (9 dB). 270 kb/scorresponds to a symbol length of 3.7 ms. An overview of GMSK is given insection 2.3.4. Speech coding is discussed in section 3.4. Radio transmission specification for GSM Frequency band: 890 - 915 MHz (uplink) (frequency duplex) 935 - 960 MHz (downlink) Channel spacing: 200 kHz Modulation: GMSK System data rate: 271 kb/s TDMA Frame: 4.6 ms Time slots: 8 x 0.58 ms Data rate (full-rate traffic channel): 22 kb/s Speech coder: Regular Pulse Exited LPC-LTP 13 kb/s Diversity: Channel coding Interleaving Frequency hopping Channel equalizationFigure 2.21 28
  29. 29. DM1 DIGITAL MOBILE TELEPHONY In the original design, full-rate speech coders having a data rate of 13 kb/s are used. However, at a later stage it will be possible to use also half-rate speech coders, by accomodating both full-rate and half-rate traffic channels. As well as speech, data transmission at different speeds can also take place over full-rate or half-rate traffic channels (see section 2.2.3). During a call, the terminals and base need to exchange large amounts of infor- mation, especially to prepare for the next hand-over to another cell. Sometimes it may also be necessary to switch to a different radio channel in the same cell for the connection, e.g. if the first channel is experiencing strong interference. The Associated Control Channels are used for this signalling, either the Slow Associated Control Channel (SACCH) or the Fast Associated Control Channel (FACCH). The FACCH is used during the actual channel-switching phase, during which a large amount of information needs to be transferred. The SACCH is used, for instance, for transferring from terminal to base measurements of the received signal levels from nearby cells, needed for MAHO. In the outward direction, in- formation is sent on the current transmitter power to be used by the terminal. The SACCH has several additional functions as mentioned below. To indicate that a 57 bit sequence is used for signalling (FACCH) an associated one bit “stealing flag” is set.2.2.2 TDMA structure for traffic channels Each radio channel (carrier) is divided by a TDMA arrangement into 8 channels used for user traffic and system signalling. The Slow Associated Control Channel (SACCH) is implemented by multiplexing each physical channel (a certain time slot in a sequence of TDMA frames) between two virtual channels within a multiframe of 26 basic TDMA frames. Most of the time slots are used for traffic but, in some of the basic TDMA frames, the eight time slots are used instead for the SACCH. Each SACCH frame are associated with 8 traffic channels (one time slot per traffic channel). A multiframe has a time of 120 ms (see figure 2.22). 29
  30. 30. DM1 DIGITAL MOBILE TELEPHONY Multiframe Full rate 26 frames of which 120 ms 24 used for traffic TF 0 TF 1 TF 2 TF 10 TF 11 SF TF 12 TF 13 ... TF 22 TF 23 Idle Half rate 26 frames of which 120 ms 2 x 12 used for traffic TF A0 TF B0 TF A1 TF A5 TF B5 SF TF A6 TF B6 ... TF TF SF A11 B11 TF = Traffic frame Basic Frame SF = Signalling frame 1 frame = 120/26 = 4.615 ms GS = Guard slot On = Transmitter switch on 0 1 2 3 4 5 6 7 Off = Transmitter switch off T= Tail bits Time slot Flag Flag GS GS T DATA Sync DATA T On 3 57 bits 1 26 bits 1 57 bits 3 Off 0.031 0.546 ms ms 1 time slot = 4.615/8 = 0.577 ms Gross data rate for full-rate physical channel: 114/0.004615 = 24.7 kb/s Traffic channel: 24 x 114 = 2.736 bits are sent during 120 ms. Therefore, data rate = 2.736/0.12 = 22.8 kb/s SACCH: 114 114 bits are sent during 120 ms. Therefore, data rate = ≈ 1kb / s 0 .12 Figure 2.22 If the traffic consists of full-rate channels (8 traffic channels per carrier), 24 frames of the multiframe are used for user traffic and one frame for signalling. One frame is not used. (This frame is used by the terminals to read the base identity (“BSIC”) of carriers from other cells.) If instead half-rate channels are used (16 traffic channels per carrier), each time slot in a TDNA frame is time multiplexed between two traffic channels, which occupy the time slot alternately each during 12 frames of a multi-frame. The remaining two basic TDMA frames are used for two SACCH signalling channels. Each SACCH is associated with 8 traffic channels.2.2.3 Structure of data bursts in a TDMA time slot A time slot of length 0.577 ms is used as follows (see figure 2.22). Guard slot, timing advance To prevent data bursts from different terminals overlapping in the input to the base-station receivers, a guard slot with a time of 31µs has been introduced. 30
  31. 31. DM1 DIGITAL MOBILE TELEPHONYThis is needed, above all, to cope with variations in the two-way propagationtime to terminals at different distances from the base. The guard slotcorresponds to a two-way propagation path of about 4.5 km.This is considerably less than the maximum specified range (cell radius 35km). Therefore, to prevent data bursts from different terminals overlapping inthe input to the base-station receivers, the base-station instructs the terminalsto insert a suitable delay between received and transmitted data bursts. Seefigure 2.23. Time alignment MS1 MS2 BS Base station transmits MS1 MS2 3 timesl. MS1 receives and transmits RX TX MS2 receives and transmits τ RX 2.5 timesl. TX τ Base station receives τ : propagation delay MS1 MS2Figure 2.23The delay is adjusted such that a transmitted burst from the terminal reachesthe base-station receiver at the right instant relative to the time-slot structure.The closer a terminal is to the base station, the greater will be the delayinserted. Thus, regardless of how far the terminal is away from the base, thebursts arriving to the base receiver will always arrive roughly in the middle ofthe intended time slot. The measurements made to determine the timingadvance can also be used to calculate the distance between the terminals andthe base. (During the first contact from a terminal to the base the terminal hasnot yet been instructed about the suitable timing advance. Therefore signallingbursts with much larger guard times must be used, see figure 2.43).The timing advance values might be used as one of the parametersdetermining when hand over between cells shall take place. It could also beused for add-on position systems, which will be an important added value ser-vice.On/Off switching of transmitters. Tail bits.The transmitter pulsecorresponding to a data burst must have rounded start and end. If not,additional widening of the spectrum relative to the basic modulation spectrumof GMSK will occur. A small part of the guard slot is used for this rounding ofthe transmitter pulse.To facilitate channel equalization, each burst starts and ends with three bits(0,0,0). The channel equalizer has to cope with time dispersion up to four 31
  32. 32. DM1 DIGITAL MOBILE TELEPHONYsymbol intervals. The three bits in the beginning and end of the burst ensurethat channel equalization of the information bits can start and end in a knownstate. The three bits at the end had the same function as the tail bits used atconvolution coding of finite length sequencies. See module DT12.Synchronization and training sequenceThe impulse response of the radio channel can change drastically during aframe of 4.6 ms. This means that for each time slot, the receiver must carryout bit synchronization and set the channel equalizer. The impulse responsecan sometimes change even during a burst (if the terminal velocity is veryhigh and especially when moving up to 1800 MHz). Therefore, if the settingof the channel equalizer was optimized with respect to the impulse response atthe beginning of the burst, the equalization may be sub optimum for the lastpart of the burst. This results in increased ber. The degradation becomes largerfor large width of the doppler spectrum (depends on the terminal speed andthe radio frequency). See figure 2.24. ber GSM900 10-1 250 km/h 5 4 10-2 3 2 1 0 km/h 10-3 0 20 40 60 80 100 Bit position (distance to training sequence)Figure 2.24To avoid the complication of having to adapt the channel equalizer tovariations in the impulse response of the propagation channel during a timeslot, short slots are used and, in addition, the training sequence is placed in themiddle of the burst. The setting of the channel equalizer is based on a knownbit sequence of 26 bits, which is also used for the bit synchronization. Thissequence can also be used to equalize the first part of the data burst, since thereceived data burst is stored in a buffer before channel equalization anddetection are initiated. 32
  33. 33. DM1 DIGITAL MOBILE TELEPHONY The 16 bits in the middle of the sequence have good cyclic correlation char- acteristics. To keep the good correlation characteristics up to the maximum specified time dispersion, the first 5 bits of the basic 16 bit sequence are repeated at the end, and the last 5 bits also placed at the beginning. See figure 2.25. The equalizer is further discussed in section 2.3.5. GSM. Training (synchronization) sequence Length of synchronization sequence is 26 bits (several different sequencies used in system) The good correlation characteristics apply to the mid 16 bits This 16 bit word has perfect cyclic correlation characteristics The 5 end bits on each side are taken from the other side of the 16 bit word. The perfect correlation is preserved over ± 5 bits time shift (which corresponds to the maximum width of the impulse response) 5 16 5 Figure 2.25 To reduce the risk for synchronizing to a distant strong cochannel carrier, 8 different training sequencies (“color codes”) are used see figure 7.18b. Transmission of user information. Fast Associated Control Channel Each data burst comprises two user sequences of 57 bits each. A flag bit is associated with each 57-bit sequence and this denotes whether the sequence contains normal speech information or if the sequence is instead being used for system signalling (FACCH). A short break in the speech transmission will hardly be noticeable, since the speech coder fills out the slot with information taken from the previous speech frame.2.2.4 Multiframe with SACCH During a multiframe of 120 ms, 24 or 12 bursts will be allocated to a traffic channel (corresponding, respectively, to full-rate and half-rate traffic channels). In the former case, the average interval between bursts will be 5 ms, i.e. 200 bursts a second. The gross data rate (including channel coding) for a full-rate channel will be 22.8 kb/s (200 x 114), and for a half-rate channel 11.4 kb/s. In addition to the above, a Slow Associate Control Channel (SACCH) is also included. A full-rate traffic channel shall carry the signal from a 13 kb/s speech coder. Channel coding increases the data rate from 13 to 22.8 kb/s. For each speech frame of 20 ms, 260 bits will be output from the speech coder and 456 from the channel coder. A 456 block is divided into 8 blocks of 57 bits, which are interleaved over time slots within 8 consecutive traffic TDMA frames (signalling frames and empty frames are skipped). Each burst carries informa- 33
  34. 34. DM1 DIGITAL MOBILE TELEPHONY tion from two adjacent 20 ms sequences from the channel coder. (See figure 2.45b.)2.2.5 Duplex arrangement, MAHO, Frequency Hopping The two time slots corresponding to a two-way traffic channel are mutually displaced in time (see figure 2.26). The figure corresponds to the case in which a base station not using frequency hopping has been allocated four carriers, each of which carries eight physical channels in a TDMA frame. The mutual displacement of the time slots for the outward and inward directions corresponds to a quasi time-duplex arrangement (even if FDD is used by the system, some TDD advantages is obtained). No duplex filter is required in the terminals, therefore. Instead there is a fast T/R switch, which alternately connects the transmitter and receiver to the antenna. A terminal receiver also has time during a TDMA frame to measure the carrier level of a signal from one of the six nearby cells (see figure 2.27). This is part of the information needed for MAHO, see figure 2.28. This procedure is further discussed in the next section on system signalling. To provide enough time for receive, transmit and listening during each TDMA frame, the terminal’s frequency synthesizer must be able to change frequency fast enough. This has influenced the specification of the number of slots per TDMA frame. An option is also frequency hopping, in which the utilized duplex channel is changed for each TDMA frame. To avoid collisions a coordinated hopping pattern must be used within each cell. The base informs the terminals about the hopping pattern on the Broadcast Control Channel and on the SACCH. The frequency hopping arrangements are discussed in more detail in module DM2 in connection with FH-CDMA. Physical channels of a base station with four radio channels (carriers). Total of 4 x 8 = 32 physical channels accessible 0 1 2 3 Inward 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 f1A direction 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 f2 A 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 f3 A 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 f4 A 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 f1BOutward direction 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 f2 B 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 f3 B 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 f4B 0 1 2 3 Displacement f1A + f1B form a duplex channel with 8 time slots ≈ 1/3 TDMA frame Figure 2.26 34
  35. 35. DM1 DIGITAL MOBILE TELEPHONY TDMA, time duplex, frequency hopping and listening-in by terminal Rx c0 0 1 2 3 4 5 6 7 Inward c1 Rx direction 0 1 2 3 4 5 6 7 c2 (serving cell) Tx c0 I 0 1 2 3 4 5 6 7 Outward I Tx c1 d, e direction I 0 1 2 3 4 5 6 7 c2 (serving cell) Listening d0 Outward Listening direction e0 (adjacent cells) c = Radio channels allocated to serving cell c 0 + c 0 comprise a duplex pair d, e I = Radio channels allocated to adjacent cells Tx = Transmitting interval Rx = Receiving interval Figure 2.27 Mobile Assisted Hand Over (MAHO) The terminal measures: Signal strength and BER on used channel Signal strength from neighbour cells Averaging over many field strenght measurements to get local average over fast fading Check that signal comes from neighbour cell This information is transferred to base station Figure Diversity against fast fading Instead of antenna diversity at the terminals, a combination of channel coding, interleaving and coordinated frequency hopping is used to obtain large diversity gain in respect to the multi-path fading. In addition, the modulation bandwidth is so large that additional frequency diversity (or multi-path diversity) is obtained in connection with the channel equalization. A necessary condition is that the propagation channel has fairly large time dispersion. Together, these give such high diversity and coding gains that the required protection ratio (the local mean over the fast fading) will typically be 9 - 10 dB. This is compatible with a cluster size of 3 x 3. Interleaving a full-rate traffic channel, means that the 456 bits in a 20-ms speech frame are split up into 57 bits sequencies, which are spread out over 8 35
  36. 36. DM1 DIGITAL MOBILE TELEPHONY TDMA frames, that is over 40 ms (see section 2.3.3). If the duration of a fa- ding dip is not more than a few milliseconds, typically only one time slot (one TDMA frame) is affected. The deinterleaver will then change the error burst to a relatively random error sequence spread over 8 code words. Thus, 1/8 of the bits in each code word will be subject to a ber of about 50%. (It is assumed that the interval between fading dips generally is more than 8 frames.) The FEC is so powerful that nearly perfect error correction is possible. Greater interleaving depth cannot be used, as it would give rise to excessive transmis- sion delay. Because the interleaving depth is only four for a half-rate channel, the coding gain will be somewhat lower. Fading dips that are longer than the channel coding with interleaving can cope with, occur over quasi-stationary propagation paths – something that affects portable terminals in particular. In this case, a fading dip could affect several consecutive TDMA frames, which drastically reduces the effect of interleaving. The situation can be much improved through frequency hopping, whereby each physical channel is switched between different radio channels which can be chosen, for instance, from a 4-group. For each TDMA frame, the carrier frequency is changed. The size of a typical frequency hop is usually large enough to give nearly uncorrelated fast fading in the different frequency slots. Another advantage of frequency hopping is that averaging occurs in respect of co-channel interference from different cells. Frequency hopping is specified only as an option. Without frequency hopping, several dB higher protection ratio would be needed for portable terminals. A complication is that frequency hopping can not be used for the main signalling radio channel - the Broadcast Carrier, see section 2.3.2. This must be on a fixed frequency, known by the terminals. In some system implementations, a larger cluster size is therefore used for the Broadcast Carrier than for the frequency hopping traffic channels.2.2.7 Background to the choice of primary radio parameters The symbol rate over the radio channel and the primary TDMA structure are a compromise between acceptable transmission performance over the worst specified propagation channel and implementation/cost limitations. In gene- ral, the factors summarized in figure 2.29a should be considered when determining a suitable TDMA format. The specific air-interface requirements that must be complied with at GSM are summarized in figure 2.29b (The equalizing window is the width of the delay interval that the equalizer can cope width. It is centered over the major part of the impulse response of the propagation channel, see figure 2.31). This leads to the conclusions in figure 2.30a and b. 36
  37. 37. DM1 DIGITAL MOBILE TELEPHONY The TDMA structure is determined by: Time for measurements on nearby cells (Mobile Assisted Hand Over - MAHO) Time duplex advantages TIme for frequency switching (Frequency synthesizer 1 ms) Low TDMA overhead (long bursts, narrow guard slots) Low transmission delay due to TDMA formating (small frame length - high system data rate) Large range for given transmitter peak power (portable terminal, moderate slots per frame) Moderate equalizer complexity (Equalizing window covering a small number of symbols) (Short burst length - smaller than correlation time of channel)Figure 2.29a Overall system requirements for GSM Micro diversity against fast fading Equalizer window at least 16 µs Fixed equalizer setting during data burst - Reasonable equalizer complexity limits equalizing range to 4 radio symbols, i.e. minimum symbol length 4 µs (250 kbaud). Impulse response stationary over 0.25 ms (for high speed trains) With training sequence in middle of burst, maximum burst length 0.5 ms. Length of TDMA frame shall permit different frequencies for transmit, receive and listen (MAHO). - Switching time of frequency synthesizer assumed to 1 ms. Minimum length of TDMA frame 4 ms. Maximum transmission delay due to TDMA formatting and interleaving 40 ms - Total transmission delay should be less than 85 ms. Interleaving depth must be less than 10.Figure 2.29b 37