Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

Introduction 217high speed as well [13]. Moreover, the DVB-H standard has been specified, which canbe considered as an extension of DVB-T for hand-held devices with high mobility.The common feature of the current wireless standards that offer high data rates is the useof multi-carrier transmission, i.e. OFDM [8, 15–19, 21–24]. In addition, these standardsemploy adaptive technologies by using several transmission modes, i.e. allowing differentcombinations of channel coding and modulation together with power control.A simple adaptive strategy was introduced in DAB using multi-carrier differential QPSKmodulation (and also in GSM using single-carrier GMSK modulation) with several puncturedconvolutional code rates. By applying a simple combination of source and channelcoding, the primary goal was to protect the most important audio/speech message partwith the most robust FEC scheme and to transmit the less important source coded dataeven without FEC. This technique allows one to receive the highest quality sound/speechin most reception conditions and an acceptable quality in the worst reception area, whereit should be noticed that in the case of analogue transmission no signal would be received.DVB-T and DVB-H employ different concatenated FEC coding rates with high ordermodulation up to 64-QAM and different numbers of sub-carriers and guard times. Herethe objective was also to provide different video quality versus distance and different cellplanningflexibility, i.e. a countrywide single-frequency network or a regional network,for instance, using the so-called ‘taboo’ channels (free channels that cannot be used foranalogue transmission due to the high amount of co-channel interference).In WCDMA/UMTS, besides using different FEC coding rates, a variable spreadingfactor (VSF) with adaptive power control is introduced. As in GSM, the combination ofFEC with source coding is also exploited. The variable spreading code allows one to havea good tradeoff between coverage, single-cell/multi-cell environments, and mobility. Forhigh coverage areas with a high delay spread, large spreading factors can be applied andfor low coverage areas with a low delay spread the smallest spreading factor can be used.LTE provides an adaptive resource allocation by flexible coding and modulation (upto 64-QAM) with channel-dependent scheduling. HARQ with incremental redundancy isapplied which triggers the rate matching procedure.In IEEE 802.11a, HIPERMAN, and IEEE 802.16x standards/WiMAX, a solution basedon the combination of multi-carrier transmission with high order modulation (up to 64-QAM) with adaptive FEC (variable rate coding) and adaptive power control is adopted.For each user, according to its required data rate and channel conditions the best combinationof FEC, modulation scheme, and the number of time slots is allocated. The mainobjective is to offer the best tradeoff between data rate and coverage, where the mobilityis not of big importance at the first stages. These standards also allow different guardtimes adapted to different cell coverage.Offering a tradeoff between coverage, data rate, and mobility with a generic air interfacearchitecture is the primary goal of the next-generation wireless systems. Users having nomobility and the lowest coverage distance (pico cells) with an ideal channel conditionwill be able to receive the highest data rate, while, on the other hand, the subscriber withthe highest mobility conditions and highest coverage area (macro cells) will be able toreceive the necessary data rate in order to establish the required communication link.The aim of this chapter is to examine in detail the different application fields of multicarriertransmission for multi-user environments. This chapter gives an overview withimportant technical parameters and highlights the strategy behind the choices.