Space-time coding for asynchronous cooperative relay networks

Space-time coding (STC) is a promising technique to achieve transmit diversity in Multiple-Input-Multiple-Output (MIMO) systems. Recently, space-time coding in a distributed fashion (DSTC) for cooperative relay networks attracts a lot of research interests. The design of space-time coding for cooperative relay networks has some new challenges which are different from the design of space-time coding for MIMO systems. The synchronization issue is one of the most important problems in the design of distributed space-time coding for cooperative communication systems since the cooperative systems are asynchronous in nature, e.g., there may exist timing errors and multiple frequency offsets in the cooperative systems. In this dissertation, we will discuss and study several designs of space-time coding for cooperative systems with different system models and in different asynchronous scenarios.;In the first topic, we attempt to design very simple space-time coding schemes for cooperative systems with timing errors in Amplify-and-Forward (AF) mode. One of the major purposes is to make the relay nodes as simple as possible so that the relay nodes only need to implement very simple operations with a very low complexity. Another major purpose is to achieve full diversity and fast Maximum-Likelihood (ML) decoding at the receiver despite the existence of the timing errors.;Except for the timing errors, the problem of multiple frequency offsets is another kind of asynchronous error which should be solved as well. In the second topic, we investigate the scenario that the timing errors and the multiple frequency offsets are existed simultaneously in the Decode-and-Forward (DF) cooperative systems. We adopt OFDM to combat the timing errors and propose an intercarrier interference (ICI) Self-Cancellation scheme to suppress the ICI caused by multiple frequency offsets.;In the third topic, we consider the physical layer security issue in cooperative systems with timing errors. In order to avoid illegal interception of the eavesdropper in a cooperative system, we propose a distributed differential encoding/decoding scheme combined with deliberate signal randomization to achieve Low Probability of Interception (LPI) as well as to obtain full diversity regardless of the timing errors.