Space-time-frequency: Diversity for wireless communication.

摘要

Space-time codes (STCs) have demonstrated considerable potential for improving the performance of wireless communication systems. However, previous efforts mainly focus on the flat fading channel. In this dissertation, some important properties are derived for the diversity gain and the coding gain of STCs for frequency selective channels. It is proven that the diversity gain of a STC which provides maximum diversity gain for channels with a given number of taps is robust to a decrease in the number of taps. For coding gain analysis, we categorize the possible types of correlation matrices into several classes based on the type of power delay profile, spatial correlation, and tap correlation. It is demonstrated that spatial correlation, tap correlation and non-uniform power delay profile will cause performance penalty. It is also proven that optimum STCs found for spatially independent and frequency selective channels with uncorrelated taps and uniform power delay profile have coding gains which are robust against a mismatch in correlation structure if the number of taps is fixed.;STC design for frequency selective fading channels using the standard rank and determinant criteria are complicated by the need to know the exact number of channel taps. An alternative design approach is justified here that removes the need for such detailed channel knowledge.;Another way of exploiting space diversity is using a collection of distributed antennas belonging to multiple users under the name of user cooperative diversity. The system model for a wireless ad hoc network employing cooperative diversity is developed in this dissertation. An equivalent multiple-input multiple-output (MIMO) channel with multiple users is developed. Based on this model, basic rate matrix theory is applied to study the average rate regions under various transmission strategies. In particular whether relaying is beneficial is studied for cases with single hop and multi-hop routing where spatial reuse may be employed. The results indicate that cooperative diversity can provide substantial gains for cases with single hop routing. However, if optimum multi-hop routing is employed, cooperative diversity gains are small.