Transmit diversity analyses for M-QAM in rician fading channels

Abstract

It is well known that the fading phenomenon, which inherently exists in most radio links, constitutes one of the boundary conditions of radio communications design. A widely recognized practice for combating fading in digital communications over such a timevarying channel is to use diversity techniques. More recently, the transmit diversity technique, which may provide the same order of diversity gain as the receive diversity does, has received great attention for some applications where the receive diversity is impractical. It has also been known that M-ary Quadrature Amplitude Modulation (MQAM) is an attractive modulation technique for wireless communications due to its high spectral efficiency. This thesis presents simple bit error rate results for M-QAM transmitted over slow, flat, identically independently distributed (i.i.d) Rician fading channels and using space diversity. We use an approximate formula for bit error rates (BER) for M-QAM modulation over a Gaussian channel. This approximation is bounded within 1 dB for M ≥ 4 and 0 ≤ SNR ≤ 30 dB. Two cases are considered: the first is optimum receive diversity, with channel side information known at the receiver (CSIR), and the second is the simple space-time transmit diversity (STTD) with no channel side information available at the transmitter (CSIT). Recently exact analysis of symbol error rate (SER) has been presented in the literature for M-ary differentially encoded/ differentially decoded phase shift keying (MDPSK) and coherent M-ary phase shift keying (MPSK), transmitted over Rician fading channel using N branch receive diversity with maximal-ratio-combining (MRC). The results are presented in an integral form. Our analysis has followed the same track and the simplicity of the SER expressions used has resulted in simple closed form expression of the SER of N order diversity at Rician fading channel. Our expressions contain only exponential functions. vii Alamouti has presented the well known simple space time transmit diversity (STTD) analysis which has been well received by the scientific community around the world. His work has shown that optimum transmit diversity can be accomplished without the channel state information at the transmitter (CSIT). The results were presented for two transmit antennas over Rayleigh fading channel. The results have been extended to the cases of Nakagami fading channels, and bit error rate analysis was presented for MQAM. However, the results are in the form of Hyper-geometric infinite series. Unfortunately its use leads to numerical un-stability. In this thesis, we have used an approximate expression for the SER of M-QAM over Gaussian channels and obtained a simple closed form expression for the SER for dual branch STTD over Rician channel. On comparing the SER of the exact and the approximate M-QAM scheme under i.i.d Rician fading with STTD, we found that the maximum difference between the two is 0.8 dB at SER of 10-5.