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

TL;DR

This paper designs space-time block codes for two-user MIMO-MAC that reduce sphere decoding complexity by identifying zero positions in the R matrix and providing conditions for their existence, improving decoding efficiency.

Contribution

It introduces new conditions and explicit constructions for STBCs that lower average sphere decoding complexity in two-user MIMO-MAC systems.

Findings

Proposed STBCs achieve reduced average SDC.

Identified zero positions in R matrix for complexity reduction.

Complex orthogonal designs reduce worst-case SDC but not average SDC.

Abstract

In this paper, Space-Time Block Codes (STBCs) with reduced Sphere Decoding Complexity (SDC) are constructed for two-user Multiple-Input Multiple-Output (MIMO) fading multiple access channels. In this set-up, both the users employ identical STBCs and the destination performs sphere decoding for the symbols of the two users. First, we identify the positions of the zeros in the $\textbf{R}$ matrix arising out of the Q-R decomposition of the lattice generator 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 the lattice generator is provided such that the $\textbf{R}$ matrix has zeros at the identified positions. Subsequently, explicit constructions of STBCs which results in the reduced ASDC are presented. The rate (in complex symbols per channel use) of the proposed designs is at most $2/N_{t}$ where $N_{t}$ denotes the number of transmit antennas for each user. We also show that the class of STBCs from complex orthogonal designs (other than the Alamouti design) reduce the WSDC but not the ASDC.