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  1. 1. Discuss the way in which 3rd generation mobile communications standards have evolved from 2nd generation technology. Specifically focus on the physical layer of the various systems. Has it been an evolution or a revolution, in terms of transmitter and receiver techniques? Introduction common standard throughout Western Europe, GSM users are able use the same phone With the recent development of voice over IP, throughout the continent. In contrast countries and a mass of alternative communication such as the US have not been as organised in methods, telecommunications firms are their standards take-up, hence GSM, beginning to experience a decline in voice CDMAone (CDMA based system) and IS-136 service revenues. This has seen recent (TDMA based system) have all come into use, promotion of SMS (Short Message Service) hence disabling the roaming capability which and other data based services. However it is is provided in Europe. still estimated that only 10% (see [1p2]) of revenue is added though data services. In order Despite these standards, 2G networks are only to maintain revenue levels the able to reach 14.4Kbps [5p104], which is telecommunications industry has sought to sufficient for voice telephony, SMS and little introduce new multimedia data services, Nokia else. When the ITU (International for one, claims that by 2005 50% of operator Telecommunications Union) met in 2000 to income will be data-service based. Existing 2G draft up a final plan for the harmonization of (second-generation) communications 3G (see [6]) all three standards had to be infrastructure is unable to support such high accounted for so as to prevent the initial data-rate services, and thus this has resulted in complexities present in 2G from reoccurring in what is termed the ‘3G’ (third-generation) of the new standard. Of the three-mentioned telecommunication services, which is generally standards in use: GSM, CDMA, and TDMA, aimed at supporting data intensive multimedia exist some commonalties. GSM for instance services (see [2,3,4p.1]) which 2G was unable to uses TDMA as its radio transmission sustain. technology. Thus the underlying technologies left to account for were GSM/TDMA and The transition from 2G to 3G effectively gave CDMA. the communications sector a chance to mainstream the protocols to be employed, in The ITU devised IMT-2000 (International the hope of allowing global roaming (amongst Mobile Telecommunications - 2000), which is other services), which proved unrealisable in a set of standards aimed at harmonizing the deployment of 2G. However as worldwide 3G systems; consequently allowing considerable costs have already been outlaid in for global roaming. In accounting for all the the existing 2G networks, providers have existing standards and commercially vested sought a method of evolution rather then interest (see [7]), IMT-2000 allowed for the revolution, it being the cheaper alternative in following: most cases. Thus what follows is a discussion  WCDMA on how the existing 2G platforms have evolved  CDMA2000 to take on the 3G specifications, particular  TD-CDMA/TD-SCDMA importance is dedicated to how transmitter and  DECT receiver techniques have adapted where  UWC-136 needed, with the majority of scenarios not requiring any radial change in technique. Having spent countless millions in creating 2G networks, providers were naturally unwilling The evolution of 2G to 3G – a physical layer to scrap existing infrastructure in order to perspective deploy 3G. Hence evolutionary paths were selected in going from GSM/TDMA and The majority of cellular services currently CDMA standards to those outlined by deployed are 2G (digital services). Depending IMT-2000. Evolutionary paths have been on global location, the underlying technology defined variably throughout literature (for supporting the network will vary. For instance contrasting perceptions see [3,8p.6, 9fig1]), the European market relies on GSM (Global however the model shown in Figure 1 draws System for Mobile communications), being a 1
  2. 2. on work featured in [3, 9fig 1], by showing relevant parts GSM TD-SCDMA EDGE GPRS WCDMA TDMA (Is-136) CDMA2000 CDMAone CDMAone (IS-95A) (IS-95B) 2G 2.5G 3G Figure 1 Part of the evolutionary track present between 2G to 3G, for a more complete reference refer to [8fig1] of the evolutionary chain. Noticeably there based data networks [10p.353]. GPRS is able to appears a 2.5G, which is aspires to introduce attain speeds between 50Kbps-115Kbps packet switched services to existing 2G [10p.352, 11], however this issue seems open to infrastructures. debate circulating about the implementation used. 2.5G Standards GSM is a TDMA based technology, however 2G networks functioning at 9.6Kbps GSM is inefficient in many aspects. Some (14.4Kbps for data) are typically circuit- GSM time slots go unused, this allows for switched [10p1]. In a voice-orientated system GPRS by dedicating these slots to GPRS’s this makes a lot of sense, as duplex packet-switched services, eventually launching communications are often taking place with the packets across the same air interface. This uplink and downlink being used at implies that no new network resources need to approximately the same rate. Furthermore be allocated, no extra bandwidth is required, voice telephony is usually only associated with and hence the existing network resources are two parties, hence it makes sense to dedicate a made use of more efficiently. In terms of circuit. In 3G, circuit-switched networks will transmitter and receiver techniques new play a minor role in bearing network traffic. algorithmic approaches are required, four of The majority of traffic will be carried over these techniques are discussed in [12], packet-switched infrastructure. Which implies however these revolve about software that the data will be divided in much the same implementations. way as IP addressing works for TCP, before it GPRS users inserted into the is transmitted. This has many implications, for same voice slot one; providers must get accustomed to charging per-packet instead of by connection time. The 2.5G aids providers in preparing them for the transition to 3G and hence in using a full-blown packet switched network. Voice user slots GPRS: In moving from a purely circuit- Figure 2 8 GSM TDM slots which combine to switched network to a packet-switched form a channel, each voice channel is shared network GPRS (General Packet Radio across multiple GPRS users and users are Services) is often an initial step. GPRS is given more slots based on their data complementary to GSM and hence enables the requirements. existing GSM circuit-switched technology to remain in tact, while allowing services such as GPRS are packet-switched; due to negligible SMS to migrate to a packet-switched interface, end-to-end connection establishment time’s which is likely to operate using X.25 or IP, 2
  3. 3. users are virtually ‘online’ from the point of modulation technique (between GMSK and 8- switching on their mobile receiver. These users PSK) to attain 1 of 10 optimal data rates are allocated a small part of the time that ([10pp..361-363,14]) which are based on 10 coding would be allowed for a voice circuit and this schemes. As in the case of GPRS, EDGE also behaves as a master channel (see Figure 2). hosts both packet-switched and circuit- Additional time slots are made available based switched information, with the ultimate drive on user data requirements. This works well for being toward full packet-switched networks. bursty data such as that found in accessing the Once again transmitter and receiver changes Internet/intranet, as data intense operations are have to occur in the case of EDGE, to account dispersed among periods of inactivity. Another for the additional modulation techniques, approach involves using multiple channels in however the majority of changes will occur in the bid of extending the data rate possible (see software and not hardware, with the pre- [13]). However both techniques involve using existing infrastructure being used as the empty TDMA slots and hence the existing foundation. transmission techniques hold, yet slight receiver software and antenna changes CDMAone (IS-95B): CDMAone is the second (adaptive antenna’s discussed later) may be stage of IS-95A, which is based on the CDMA required. (Code Division Multiple Access) system. CDMA techniques use different receiver and EDGE: the EDGE (Enhanced Data Rates for transmitter techniques in contrast to GSM GSM Evolution) system is designed as an systems. Principally CDMA users are upgrade to GPRS in bringing it closer to 3G distinguished based on a pseudo random code standards1, in fact providers that were unable that is unique to each mobile receiver. IS-95A to secure 3G spectrum licenses have seen systems are restricted to low data rates, to EDGE as a relatively low cost alternative, overcome this, IS-95B was developed, the which is able to increase their networks data primary difference, beside the shift from rate capabilities and take them one step closer circuit to packet based switching, is the to IMT-2000. These higher rates are achieved assigning of multiple codes to each user (8 through using what has been termed ‘higher- codes). Resulting in multiple channels being level’ [10p.361] modulation schemes. These allocated per mobile receiver simply based on schemes adapt the data modulation scheme to the chipping margins available to a network. the channel quality hence ensuring optimal This can result in data rates of up to 115Kbps transmission rates at all time. It has been (8*14.4Kbps) however 61 Kbps [5] seems to specified that 384 Kbps [8] is the maximal rate be the rate attained in practice. Another attainable (in which case the technology significant change is associated with the choice qualifies for 3G standards), however the rate of carriers; IS-95A systems used the 1.25MHz averages out at about 150 Kbps. band, whereas IS-95B makes use of the 850 MHz and 1900 MHz bands [5]. In terms of EDGE technology achieves these rates by transmitter, nothing changes however Rake using a pulse-shaped 8-PSK (Phase Shift receiver techniques used will need some Keying) modulation scheme, which works to software/hardware upgrade to cope with increase the gross data bit rate leaving the air multiple PN sequences, however the interface. This differs form the traditionally underlying decoding remains fixed. CDMAone used GMSK (Gaussian Minimum-Shift (in both its 95A and 95B types) behaves as the Keying) scheme in a number of important basis for one of the most important 3G ways: GMSK is known to hold a constant standards WCDMA and hence this makes envelope, whereas pulse-shaped 8-PSK is able CDMA systems more cost effective to build to vary in both amplitude and phase [15p.78]2. upon when contrasted to GSM systems. Incorporated into EDGE is the pre-existent GMSK modulation scheme used by the 3G Networks underlying GSM network, and depending on the channel, EDGE is able to alter its 3G networks offer greater total speeds then those offered by 2G and 2.5G, yet this is often 1 According to IMT-2000 standards (anything balanced by spectrum licensing and greater then 144Kbps) EDGE is listed as being infrastructure costs. One of the aims of 3G a 3G solution, however in contrast [5p.105, solution providers is to phase out the circuit- 10p.361] lists it as being as 2.5G. switching with packet-switching; this change 2 For a more detailed comparison and the will begin in the data realm and work its way specifics of the transition to pulse-shaped 8- into the voice domain. Nevertheless IMT-2000 PSK the reader is directed to [15] encompasses both types of communications. 3
  4. 4. Frequency 1 1 - 18 WCDMA: Wideband CDMA (WCDMA) is reuse factor one 3G solution favoured by GSM/GPRS Power control systems. Capabilities of the WCDMA allow: frequency 1500 Hz 2 Hz or lower wider transmission bandwidth; higher chipping rates; multiplexed multi-rate services (allowing Modulation QPSK GMSK the operation of multiple tasks simultaneously); fast power control in the 5MHz bandwidth Frequency gives diversity with Frequency hopping downlink [16p.85] (downlink being the most diversity Rake receiver used). Time slot based Load-based packet WCDMA promises an optimal data rate of Packet data scheduling with scheduling GPRS 2Mbps over a direct spread downlink. The extension in bandwidth is possible based on Downlink Supported for Not supported by the transmit improving downlink standard, but can be higher chipping rates (3.84 Mcps [16p.85, 17]), diversity capacity applied thus a 5 MHz bandwidth becomes possible3. The increase in bandwidth results in less fading across the radio transmission channel, Table 1 WCDMA versus GSM, adapted from resulting in higher SNR values at the receiver. [17] What makes WCDMA a possibility for evolution is that it is able to encompass both WCDMA IS-95 TDM (time division multiplexing) and FDM Carrier spacing 5MHz 1.25 MHz (frequency division multiplexing), this implies Chip rate 3.84 Mcps 1.2288 Mcps portability between both GSM/GPRS setups, 1500 Hz, both Uplink: 800 Hz, Power control which are based on TDM/TDMA technologies, frequency uplink and downlink: slow and CDMA setups which are FDM/FDMA downlink power control setups. However you will note from Figure 1 Modulation QPSK Uplink: OQPSK, that traditional CDMA systems have not been downlink: QPSK routed to WCDMA, this is based on WCDMA Base station Not needed Yes, typically being asynchronous, which is contrary to 2G synchronization obtained via GPS CDMA technology that works off a Efficient radio Yes, provides Not needed for synchronized architecture. Synchronisation resource required quality of speech only management implies that the receiver tracks the phase and algorithms service networks frequency of the received signal, thus narrow band CDMA systems are coherent. Packet data Load-based packet transmitted as short Packet data scheduling circuit switched The use of higher chipping rates, permits a calls larger range multi-path diversity [17], which as Supported for later shown can be exploited by Rake receiver Downlink transmit Not supported by improving techniques in terms of reconstructing the diversity the standard downlink capacity transmitted signal.4 With this in mind its possible to see why WCDMA has been built with adaptive antenna arrays and antenna Table 2 WCDMA versus IS-95, otherwise diversity in mind [18p.72]. known are traditional narrowband CDMA, adapted from [17] Tables 1 and 2 give comparison between existing 2G systems and WCDMA. CDMA2000: There is not a great deal of difference between CDMA2000 and WCDMA GSM WCDMA, despite this CDMA2000 has been Carrier 5MHz 200 kHz marked as the upgrade to IS-95 systems. This spacing has manly been due to CDMA2000 being a synchronised technology and hence suitable 3 5MHz restriction in order to maintain channel for upgrading IS-95 systems which are separation without high power-level likewise synchronised. Another issue is that differences between adjacent carriers, see CDMA2000 was designed with existing IS-95 [16p.84] systems in mind and hence all effort has been 4 There are also changes in the physical frame made to allow simple evolution from either structure of the uplink and downlink, amongst IS-95A or IS-95B (Table 3 gives some detail other things for further reading consult [18p.74] as to the adaptability available). Other differences between WCDMA and 4
  5. 5. CDMA2000 include differing: frame length, CDMA systems, yet once operating at high downlink pilot structure, spectrum use, data rates it becomes even more of an issue. chipping rates (WCDMA can attain a possible Traditionally time diversity is circumnavigated 10% gain on CDMA with regards to chipping though the use of Rake receivers, which utilise rates), and critically the selection of ‘fingers’ to attain time and space diversity, synchronization. Despite all these differences these improve correlation measure at the CDMA2000 remains a CDMA technique and receiver. These methods have not changed in hence much of the existing receiver and the slightest for WCDMA/CDMA2000, [21] transmitter mechanism will remain fixed. suggests that additional complexity has been added, with as many as 32 fingers being required to attain a data rates of 2Mbps in 3G Transmitter and Receiver techniques CDMA scenarios The same paper [21] makes it clear that 3G CDMA based systems using Having looked at part of the 2G to 3G Rake receivers and high chip rates, will allow migratory tracks, it becomes obvious that there for a possible 2Mbps data rate across the is no great revolution taking place in terms of existent channel. receiver and transmitter techniques Channel model: the environment that signals must propagate through results in both long- term and short-term fading. Short-term fading resulting in signal multipath. Long-term fading however has spawned techniques such as frequency hopping, which continues to be used in 3G systems. The scenario for GSM evolution has also been a slightly evolutionary, but by far not revolutionary. GPRS systems will continue to use GSM’s TDMA core utilising the vacant time slots to its advantage. This can simply be done in software; the existing base stations remain unchanged. Receiver technique holds constant except in the case of GPRS, it must Table 3 Physical layer characteristics of IS-95 now utilise adaptive antennas. Adaptive and CDMA2000, taken from [19table 3] antennas are able to direct their beam for selectivity, which may be required depending For instance CDMA transmitters as proposed on the implementation. by [20p.730] take on the task of encoding speech, then interleaving the result followed by In contrast EDGE technology takes complexity applying a PN sequence and a modulation back into the transmission, by varying the scheme prior to transmission. In making the modulation method to 8-PSK, however the move to WCDMA or to CDMA2000 the main transmission process remains fixed as does the structure of CDMA holds. All that changes are reception procedure. Once again as in the case parameters associated with the accepted of WCDMA and CDMA2000 high data rates frequencies etc. However WCDMA and have posed problems in term of spectral CDMA2000 are far from revolutionary efficiency. techniques, they are simply evolved methods and hence they require slightly evolved This has caused many providers to take up transmission and reception techniques. You research surrounding time-space diversity will observe that no new modulation schemes techniques, [22] presents a solid argument as were put into use for WCDMA or to why antenna arrays, as a source of diversity, CDMA2000, (eg. In the case of CDMA, QPSK will be required at some point in 3G has been around for a long time implying the transmissions5. Other options suggested by same for its receivers and transmitters). [22] include BLAST (owned by Bell Labs). An interesting concept for a receiver is also put However what is obvious is that higher speeds forth by [23], where by a re-configurable will introduce complexities into the system, the hardware based receiver is able to receive both main concern being high data rates and receiver complexity versus cost. For instance 5 time dispersion is an issue of concern for all Relates closely to attaining spectral efficiency in order to attain high data rates 5
  6. 6. CDMA and GSM and ultimately their 3G by- 2002 products WCDMA and CDMA2000, this actually works towards meeting IMT-2000 [6] IMT Website (Visited on 2/10/02) specifications for global roaming. Conclusion [6] “IMT-2000 Global Standard, International” (Visited on 22/10/02) Having looked at some of the evolutionary http://www.mobilecomms- tracks between 2G and 3G technologies, its possible to see that not much has really changed. The majority of improvement has [7] “What is IMT-2000” (Visited on 2/10/02). been in terms of using spectrum in a more efficient way, resulting in data rate project/What_is_IMT2000-2.pdf improvements. Due to the backward compatibility of 3G systems the majority of 3G [8] Dixit, Sudhir; Guo, Yile; Antoniou, Zoe. infrastructures will be able to use the same “Resource Management and Quality of Service transmission and reception techniques in Third-Generation Wireless Networks”. employed by their 2G origins, with slight pp.125-133. IEEE Communications Magazine, modifications where needed in order to cope Feb 2001. with the increased data rate. [9] Dahlgren, Fredrik.“Future Mobile Phones - Thus there appears to be no great revolution in Complex Design Challenges from an the receiver or transmitter techniques Embedded Systems Perspective” (Visited on employed. However there is suggestion that to 22/10/02). Ericsson Mobile Communications optimise the capability of 3G, space-time diversity techniques (mainly in terms of noteDahlgren.pdf antenna arrays) will need to be employed, and this generally implies a revolution in the [10] Garg, K. Vijay. “Wireless Network transmitter and receiver techniques in use, Evolution: 2G to 3G”.Prentice Hall 2002 however this area is still open to research. [11] “GPRS”. (Visited 22/10/02),28989 3,sid9_gci213689,00.html References [12] Lin, P.; Lin, Y.B.; “Channel allocation for GPRS”. IEEE Transactions on Vehicular [1] “Opening a new road to 3G evolution” Technology, v 50, n 2, March 2001, pp. (Visited on 10/10/02). Nokia - White Paper 375-387 ons/files/white_papers/ [13] “How GPRS Works - Radio interface” (Visited on 22/10/02). [2] “Guest Editorial: QoS and resource allocation in the 3rd-generation wireless cture.html networks”. p.115 IEEE Communications Magazine, Feb 2001. [14] “Enhanced Data rates for GSM Evolution (EDGE)” (Visited on 22/10/02) [3] “What is 3G?” (Visited 21/10/02). [15] Mashhour, Ashkan “Understanding [4] “Supporting 3G services with a solid offset 8-PSK modulation for GSM EDGE” platform” (Visited 9/10/02) Ericsson CDMA Microwave Journal, v 42 n 10, Oct, 1999, pp. 78-92 CPPArticle.PDF [16] Ojanpera, Tero; Prasad, Ramjee; [5] De Vriendt, J.; Laine, P.; Lerouge, C.; “Overview of air interface multiple access for Xiaofeng Xu. “Mobile network evolution: a IMT-2000/UMTS”. IEEE Communications revolution on the move”. pp. 104 –111. IEEE Magazine, v 36, n 9, Sep, 1998, pp. 82-95 Communications Magazine, v 40 n 4, April 6
  7. 7. [17] Harri Holma, Antti Toskala, "WCDMA for UMTS: Radio Access for Third Generation Mobile Communications," John Wiley & Sons 2001 [18] Dahlman, Erik; Gudmundson, Bjorn; Nilsson, Mats; Skold, Johan. “UMTS/IMT-2000 based on wideband CDMA”. IEEE Communications Magazine, v 36, n 9, Sep, 1998, pp. 70-80 [19] Douglas N. Knisely, Sarath Kumar, Subhasis Laha, Sanjiv Nanda, “Evolution of Wireless Data Services: IS-95 to cdma2000,” IEEE Communication Magazine, v 36 n 10, pp.140-149, Oct, 1998. [20] Proakis, J.G; Masoud, S. “Communication Systems Engineering”-2nd edition. Prentice Hall 2002. [21] Aldis, J.; Barton, S.K. “On the feasibility of a 2-Mb/s bearer service in a future cellular radio system using code-division multiple access”. IEEE Transactions on Vehicular Technology, v 48 n 5, Sept. 1999. pp.1392 – 1403. [22] Lozano, A; Farrokhi, F. R.; Valenzuela, R. A. “Lifting the Limits on High-Speed Wireless Data Access Using Antenna Arrays”, IEEE Communications Magazine, Sept 2001. pp.156-162 [23] Minnis, B; Moore, P. “A re-configurable receiver architecture for 3G Mobiles”. IEEE Radio Frequency Integrated Circuits Symposium 2002.pp.187-190 7