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

6Additional Techniquesfor Capacity and FlexibilityEnhancement6.1 IntroductionAs shown in Chapter 1, wireless channels suffer from attenuation due to the destructiveaddition of multi-path propagation paths and interference. Severe attenuation makes itdifficult for the receiver to detect the transmitted signal unless some additional, lessattenuatedreplicas of the transmitted signal are provided. This principle is called diversityand is the most important factor in achieving reliable communications. Examples ofdiversity techniques are:– Time diversity. Time interleaving in combination with channel coding or spreadingprovides replicas of the transmitted signal in the form of redundancy in the temporaldomain to the receiver.– Frequency diversity. The signal transmitted on different frequencies induces differentstructures in the multi-path environment. Replicas of the transmitted signal are providedto the receiver in the form of redundancy in the frequency domain. The bestexamples of how to exploit the frequency diversity are the technique of multi-carrierspread spectrum and coding in the frequency direction.– Spatial diversity. Spatially separated antennas provide replicas of the transmittedsignal to the receiver in the form of redundancy in the spatial domain. This can beprovided with no penalty in spectral efficiency.Exploiting all forms of diversity in future systems (e.g. B3G, 4G) will ensure the highestperformance in terms of capacity and spectral efficiency.Furthermore, the future generation of broadband mobile/fixed wireless systems will aimto support a wide range of services and bit rates. The transmission rate may vary fromvoice to very high rate multi-media services requiring data rates up to 1 Gbit/s. Communicationchannels may change in terms of their grade of mobility, cellular infrastructure,required symmetrical or asymmetrical transmission capacity, and whether they are indooror outdoor.Multi-Carrier and Spread Spectrum Systems Second Edition K. Fazel and S. Kaiser© 2008 John Wiley & Sons, Ltd