On the Search for High-Rate Quasi-Orthogonal Space-Time Block Code

TL;DR

This paper introduces a novel algebraic approach and graph-based search to find high-rate quasi-orthogonal space-time block codes, achieving rates above 1 with full diversity, and analyzes their decoding complexity and limitations.

Contribution

It develops an algebraic structure for QO-STBCs and a graph-based search algorithm to discover high-rate codes with rates exceeding 1, including the first full-diversity codes above rate 1.

Findings

Maximum code rate limited to 5/4 with full diversity

Maximum code rate limited to 4 with lower diversity

Decoding can be performed on two separate symbol groups

Abstract

A Quasi-Orthogonal Space-Time Block Code (QO-STBC) is attractive because it achieves higher code rate than Orthogonal STBC and lower decoding complexity than nonorthogonal STBC. In this paper, we first derive the algebraic structure of QO-STBC, then we apply it in a novel graph-based search algorithm to find high-rate QO-STBCs with code rates greater than 1. From the four-antenna codes found using this approach, it is found that the maximum code rate is limited to 5/4 with symbolwise diversity level of four, and 4 with symbolwise diversity level of two. The maximum likelihood decoding of these high-rate QO-STBCs can be performed on two separate sub-groups of symbols. The rate-5/4 codes are the first known QO-STBCs with code rate greater 1 that has full symbolwise diversity level.