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SUMMARYFollowing a unified analytical framework, the bit error rate (BER) of several coherent and non-coherent binary modulation schemes is derived for a switched diversity system. The two variants of switched combining that have been investigated are switch and stay combining and switch and examine combining. For channel modelling, at first a simple slow flat fading channel is assumed, where the amplitude attenuation obeys the Rayleigh distribution. Later the BER calculations are repeated for cascaded Rayleigh fading channel case. Rayleigh fading is the most popular model for electromagnetic signal propagation in wireless media when both or either of the transmitter/receiver is fixed. On the other hand, when both the transmitter and the receiver are mobile, a cascaded (or double) Rayleigh fading model is better suited. The applicability of these two models, namely simple and cascaded Rayleigh model, has been indicated by several theoretical studies and their suitability is established by various field measurements. In our paper, simple closed-form BER expressions as a function of switching threshold have been found and optimum switching thresholds have been computed for both these models as well as for both types of diversity combining described earlier. The results presented in this paper can be very useful for communication system designers to analyze link quality of switched diversity assisted systems in various wireless environments.

For mitigating the deleterious effects caused by time-varying multipath fading, most of the modern digital wireless systems employ some sort of diversity combining. Combining of the different diversity branches, however, may be performed either before demodulation (predetection combining), or after it (postdetection combining). It has been shown earlier that postdetection schemes outperform their predetection counterparts for Rayleigh and less-severe-than Rayleigh (e.g. Rician)-fading channels. In this study, the authors consider Hoyt fading that characterises wireless environments experiencing moresevere-than Rayleigh fading, and compare error performance of postdetection combining with the results available for predetection combining. In particular, two of the combining variants namely selection combining and switched combining are investigated, and symbol error probability of a non-coherent M-ary frequency shift keying receiver is derived for independent but not necessarily identical diversity branches. The comparison reveals that postdetection schemes perform better than predetection combiners when the average branch signal-to-noise ratio (SNR) exceeds some crossover SNR.

Simple closed-form expressions of outage probability and moment generating function (MGF) are found for a switch and examine combiner (SEC) operating over independent and identically distributed (IID) Hoyt fading channel. Next, using the derived MGF, symbol error probability (SEP) of different M-ary modulation schemes are calculated. In addition, an analytic framework is presented for calculation of optimum switching threshold that ensures minimum outage probability or minimum SEP. The analysis is quite general in the sense that it covers switch and stay combiner (SSC) and Rayleigh fading as special cases.

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Closed-form error probability expressions for a L(≥ 2) branch post-detection switch-and-examine combiner (SEC), using noncoherent M -ary frequency shift keying (ncMFSK) modulation and operating over independent and identically distributed (IID) Hoyt fading channel, are derived. Under similar channel and receiver constraints, derived error rates are compared with results available for pre-detection combining. It was observed that an additional diversity gain is achievable with post-detection scheme when the average channel signal-tonoise ratio (SNR) exceeds some crossover SNR value that depends on modulation order (M ), fading parameter (q), and number of diversity branches (L).