Fast ML Detection for Quasi-Orthogonal STBCs of Rate 2 in Under-Determined Systems [Transactions Papers]

A fast maximum-likelihood (ML) detection scheme is proposed for the important quasi-orthogonal Space-Time Block Codes (STBCs) of symbol rate 2 for three and four transmission antennas. In particular, the proposed scheme is applicable even to the under-determined system where the number of transmission antennas is larger than that of the receive antennas. The half of the symbols of the transmission matrix are detected by two dependent double-layer sphere decoders, which are referred to as dual double-layer sphere decoding in this paper. The remaining half of the symbols are detected by single-symbol detection, of which complexity is extremely low. Numerical results demonstrate that the proposed new detection method improves the detection speed up to ten times depending on the Signalto- Noise Ratio (SNR) compared to the state-of-the-art scheme.