Space Time Coding with Unequal Modulations

Use of unequal modulation schemes in a system utilizing space time block coding is investigated. Employing mixed modulations in a space-time system offers finer granularity in the system performance without the receiver complexity increase associated with a larger set of modulation formats. It is shown that for the optimum performance of the system, the power per stream should be adjusted accordingly. Another consideration is that the stream using the lower modulation order has a higher robustness, so it is possible to reduce the complexity of detection process by using maximum-likelihood detection only on the stream with the higher modulation order.