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In this paper, Space-Time Block Codes (STBCs) with reduced Sphere DecodingComplexity (SDC) are constructed for two-user Multiple-Input Multiple-Output(MIMO) fading multiple access channels. In this set-up, both the users employidentical STBCs and the destination performs sphere decoding for the symbols ofthe two users. First, we identify the positions of the zeros in the$\textbf{R}$ matrix arising out of the Q-R decomposition of the latticegenerator such that (i) the worst case SDC (WSDC) and (ii) the average SDC(ASDC) are reduced. Then, a set of necessary and sufficient conditions on thelattice generator is provided such that the $\textbf{R}$ matrix has zeros atthe identified positions. Subsequently, explicit constructions of STBCs whichresults in the reduced ASDC are presented. The rate (in complex symbols perchannel use) of the proposed designs is at most $2/N_{t}$ where $N_{t}$ denotesthe number of transmit antennas for each user. We also show that the class ofSTBCs from complex orthogonal designs (other than the Alamouti design) reducethe WSDC but not the ASDC.

STBCs with Reduced Sphere Decoding Complexity for Two-User MIMO-MAC