Space-time signaling in multi-antenna systems

A key leverage for providing capacity and quality in future wireless links is multiple-input multiple-output (MIMO) technology which exploits availability of multiple transmit and multiple receive antennas. Multiple antenna links increase capacity by taking advantage of scattering to introduce new spatial modes into the propagation channel. They also increase reliability by providing diversity through the availability of multiple paths between transmitter and receiver. Signaling, modulation, or error correction coding, for MIMO systems, however, has been primarily limited to multiplexed signaling—for high data rate—and diversity signaling—for high link reliability. In practice, the choice between diversity and multiplexing is unclear and is rate and channel dependent. Further, it is desirable to have the flexibility to distribute the available degrees of freedom between rate and reliability. Therefore, code designs that fall between multiplexing and diversity modes of operation are needed.; This dissertation deals with the problem of coding and modulation for MIMO wireless communication links. Using the probability of symbol error, it is shown that for a fixed rate and power, the choice of multiplexed signaling versus diversity signaling depends to a large degree on the condition number of the matrix channel. The distribution of the matrix condition number in independent identically distributed Gaussian matrix channels is shown to determine the probability that a channel better for multiplexed signaling is obtained. Application of the analysis to a fixed-rate system that switches between both multiplexing and diversity is described. Since practical systems may need the benefits of both multiplexing and diversity signaling, a new space-time code design for MIMO systems is presented. Numerical optimization is used to search a family of code designs to find the code with the best diversity advantage. Parameterizations of the family of codes and procedures for finding those codes are presented in detail. Example code designs for various modulation schemes and numbers of transmit and receive antennas are described and compared in terms of ergodic capacity and bit error rate.