Low-complexity space-time codes for high and low-priority streams

We examine the design of space-time codes that allow simple encoding and decoding of high and low-priority streams of data. A desirable multi-stream space-time code has the property that accurate soft bit information can be computed easily for one stream without having to decode the remaining streams. This allows, for example, a high-priority stream to be decoded optimally with complexity that is not adversely affected by the presence of a low-priority stream. Conditions that allow simple decoding are provided, and these conditions are demonstrated with some examples. We look specifically at two-stream codes, and compare performance with standard single-antenna hierarchical two-stream methods.