Adaptive modulation for multiple antenna channels

We consider the use of adaptive modulation scheme for multiple transmit and multiple receive antenna system with instantaneous channel information known to both the receiver and the transmitter. We derive an efficient bit allocation algorithm which maximizes the transmission rate for a given transmit power. The algorithm is generally a greedy algorithm; however, we derive a sufficient condition for the bit allocation algorithm to be globally optimal, which is found to be satisfied in all digital modulation schemes. It is found that when uncoded M-ary QAM is used with a target symbol error probability of 10(-5) there is about 9 dB gap between the channel capacity and the throughput of adaptive modulation.