Soft Decision Decoding of the Orthogonal Complex MIMO Codes for Three and Four Transmit Antennas

TL;DR

This paper introduces novel low-complexity soft decision decoders for orthogonal complex MIMO codes with three and four transmit antennas, extending maximal ratio combining techniques and improving receiver performance for FEC decoding.

Contribution

It proposes new soft decision decoding schemes for orthogonal MIMO codes beyond two antennas, enhancing performance with minimal complexity.

Findings

Extended MRC to all orthogonal codes up to four Tx antennas

Significant performance improvement with low complexity

Enhanced soft decision feedback to FEC decoders

Abstract

Orthogonality is a much desired property for MIMO coding. It enables symbol-wise decoding, where the errors in other symbol estimates do not affect the result, thus providing an optimality that is worth pursuing. Another beneficial property is a low complexity soft decision decoder, which for orthogonal complex MIMO codes is known for two transmit (Tx) antennas i.e. for the Alamouti code. We propose novel soft decision decoders for the orthogonal complex MIMO codes on three and four Tx antennas and extend the old result of maximal ratio combining (MRC) to cover all orthogonal codes up to four Tx antennas. As a rule, a sophisticated transmission scheme encompasses forward error correction (FEC) coding, and its performance is measured at the FEC decoder instead of at the MIMO decoder. We introduce the receiver structure that delivers the MIMO decoder's soft decisions to the demodulator, which in turn cranks out the logarithm of likelihood ratio (LLR) of each bit and delivers them to the FEC decoder. This makes a significant improvement on the receiver, where a maximum likelihood (ML) MIMO decoder makes hard decisions at a too early stage. Further, the additional gain is achieved with stunningly low complexity.