Bit-Metric Decoding Rate in Multi-User MIMO Systems: Theory

Link-adaptation (LA) is one of the most important aspects of wireless communications where the modulation and coding scheme (MCS) used by the transmitter is adapted to the channel conditions in order to meet a certain target error-rate. In a single-user SISO (SU-SISO) system, LA is performed by computing the post-equalization signal-to-interference-noise ratio (SINR) at the receiver. The same technique can be employed in multi-user MIMO (MU-MIMO) receivers that use linear detectors. Another important use of post-equalization SINR is for physical layer (PHY) abstraction, where several PHY blocks like the channel encoder, the detector, and the channel decoder are replaced by an abstraction model in order to speed up system-level simulations. This is achieved by mapping the post-equalization SINR to a codeword error rate (CER) or a block error rate (BLER). However, for MU-MIMO systems with non-linear receivers, like those that use variants of the sphere-decoder algorithm, there is no known equivalent of post-equalization SINR which makes both LA and PHY abstraction extremely challenging. This important issue is addressed in this two-part paper. A metric called the bit-metric decoding rate (BMDR) of a detector for a set of channel realizations is presented in this part. BMDR is the proposed equivalent of post-equalization SINR for arbitrary detectors. Since BMDR does not have a closed form expression that would enable its instantaneous calculation, a machine-learning approach to predict it is presented. The second part describes the algorithms to perform LA, detector selection, and PHY abstraction using BMDR for MU-MIMO systems with arbitrary detectors. Extensive simulation results corroborating the claims are presented.