Transceiver Pair Designs for Multiple Access Channels under Fixed Sum   Mutual Information using MMSE Decision Feedback Detection
  In this paper, we consider the joint design of the transceivers for a multiple access Multiple Input and Multiple Output (MIMO) system having Inter-Symbol Interference (ISI) channels. The system we consider is equipped with the Minimum Mean Square Error (MMSE) Decision-Feedback (DF) detector. Traditionally, transmitter designs for this system have been based on constraints of either the transmission power or the signal-to-interference-and-noise ratio (SINR) for each user. Here, we explore a novel perspective and examine a transceiver design which is based on a fixed sum mutual information constraint and minimizes the arithmetic average of mean square error of MMSE-decision feedback detection. For this optimization problem, a closed-form solution is obtained and is achieved if and only if the averaged sum mutual information is uniformly distributed over each active subchannel. Meanwhile, the mutual information of the currently detected user is uniformly distributed over each individual symbol within the block signal of the user, assuming all the previous user signals have been perfectly detected. 
Wenwen Jiang, Jian-Kang Zhang and Kon Max Wong