A Generalized MMSE Detection with Reduced Complexity for Spatially   Multiplexed MIMO Signals
  In multiple-input multiple-output (MIMO) spatially multiplexing (SM) systems, achievable error rate performance is determined by signal detection strategy. The optimal maximum-likelihood detection (MLD) that exhaustively examines all symbol candidates has exponential complexity and may not be applicable in many practical systems. In this paper, we consider a generalized minimum mean square error (MMSE) detection derived from conditional mean estimation, which in principle behaves equivalently to MLD but also includes a linear MMSE detection as a special case. Motivated by this fact, we propose a low-complexity detection which significantly reduces the number of examined symbol candidates without significant error rate performance degradation from MLD. Our approach is to approximate the probability density function (pdf) of modulated symbols that appears in the exact conditional mean expression such that the decision metric can be cast into a partially closed form. It is found that uniform ring approximation in combination with phase shift keying (PSK) and amplitude phase shift keying (APSK) is promising, as it can achieve a performance even comparable to MLD, while its complexity is linear when the number of transmit antennas is two. 
Makoto Tanahashi and Hideki Ochiai
cs.IT math.IT