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 time-varying 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 Quad rature Amplitude Modulation (M-QAM) is an attractive modulation technique for wireless communications due to its high spectral efficiency.