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

1FundamentalsThis chapter describes the fundamentals of today’s wireless communications. First adetailed description of the radio channel and its modeling is presented, followed by theintroduction of the principle of OFDM multi-carrier transmission. In addition, a generaloverview of the spread spectrum technique, especially DS-CDMA, is given and examplesof potential applications for OFDM and DS-CDMA are analyzed. This introduction isessential for a better understanding of the idea behind the combination of OFDM withthe spread spectrum technique, which is briefly introduced in the last part of this chapter.1.1 Radio Channel CharacteristicsUnderstanding the characteristics of the communications medium is crucial for the appropriateselection of transmission system architecture, dimensioning of its components, andoptimizing system parameters. Especially mobile radio channels are considered to be themost difficult channels, since they suffer from many imperfections like multi-path fading,interference, Doppler shift, and shadowing. The choice of system components is totallydifferent if, for instance, multi-path propagation with long echoes dominates the radiopropagation.Therefore, an accurate channel model describing the behavior of radio wave propagationin different environments such as mobile/fixed and indoor/outdoor is needed. This mayallow one, through simulations, to estimate and validate the performance of a giventransmission scheme in its several design phases.1.1.1 Understanding Radio ChannelsIn mobile radio channels (see Figure 1-1), the transmitted signal suffers from differenteffects, which are characterized as follows.Multi-path propagation occurs as a consequence of reflections, scattering, and diffractionof the transmitted electromagnetic wave at natural and man-made objects. Thus, atthe receiver antenna, a multitude of waves arrives from many different directions withdifferent delays, attenuations, and phases. The superposition of these waves results inamplitude and phase variations of the composite received signal.Multi-Carrier and Spread Spectrum Systems Second Edition K. Fazel and S. Kaiser© 2008 John Wiley & Sons, Ltd