Adaptive and diversity techniques for wireless digital communications over fading channels

Demand for mobile and personal communications is growing at a rapid pace, both in terms of the number of potential users and the introduction of new high-speed services. Meeting this demand is challenging since wireless communications axe subject to three major constraints: a complex and harsh fading channel, a scarce usable radio spectrum, and limitations on the power and size of hand-held terminals. Therefore, effective spectral and power efficient fading mitigation techniques are required. In this thesis we discuss two particular techniques to overcome fading: diversity combining and adaptive transmission. We first present a new approach for the performance evaluation of digital communications over fading channels. This approach, which is based on alternate representations of classic functions arising in the error probability analysis such as the Gaussian Q-function and the Marcum Q-function, leads to a unified analytical framework to determine the exact error rate of digital communications with multichannel reception. The unified approach gives new analytical expressions for average bit error rate under very general channel conditions, in addition to simplifying previously-known results both analytically and computationally. We next present methods of adapting to random channel variations in order to increase the spectral efficiency of wireless systems. As a first step we determine the theoretical spectral efficiency limits of wireless communication systems subject to the underlying severe multipath fading. We then propose an adaptive M-QAM modulation technique and analyze its performance over Nakagami-m fading channels. 0We also propose an adaptive modulation technique which combines voice and data, and study its performance. Finally, we consider adaptive modulation in cellular systems, and compute the resulting area spectral efficiency, defined as the maximum bit/sec/Hz/Km2, which can be supported in the system.