Spatial multiplexing and space-time codes are competing ways ofextracting capacity from MIMO wireless systems. We present examples ofboth approaches and study their performance, in particular in the caseof correlated channels. Next, we show that multiplexing and diversityoriented schemes (like STC) react differently to the correlationstructure of MIMO arrays.
We address the problem of finding an optimal combination ofmultiplexing and diversity in a MIMO system. We develop a combiningapproach in the form of an optimal spatial assignment of antennas, inorder to multiplex space-time coded symbol blocks. We call this schemeSMAL, and develop it in two versions, first for the case wheninstantaneous channel information is available at the transmitter, andsecond when only long-term correlation statistics are known. Weinvestigate the performance of both versions in the practical casewhen correlation is not uniform across all antenna pairs (e.g. inlinear arrays).
The SMAL scheme is tested for two levels of correlated fading betweenneighbouring antenna elements. A comparison between the two versions shows that the instantaneous SMAL is especially useful at low levels of correlated fading. Under conditions of heavily correlated fading the correlation-based approach performs just as well and is preferredbecause of its lower complexity.
We also vary the number of transmit and receive (N,M) antennas, andtest the SMAL scheme for the two MIMO systems N=M=4 and N=M=6.
For the instantaneous version of SMAL and low correlation, we showimprovements of over 2 dB over the case of random pattern selection ,at a certain target bit-error rate, both for N=M=4 and N=M=6.With the statistical SMAL for high correlation, a performance gain of almost 5 dB is shown in the case of N=M=4.
The development and results of the SMAL scheme have also resulted inthe submission of a conference article to NORSIG 2003.