Information Lossless Space-Time Coding for Multiple Access Systems

In this paper we consider multiple access systems where users and access points are equipped with multiple antennas. In order to exploit some of the MIMO potentials we have to resort to space-time coding. Unfortunately, such a processing may induce severe loss in terms of achievable information rates. In this paper we prove the necessary and sufficient condition ensuring that the space-time coding is information lossless, in the sense that it does not induce any modification in certain regions of achievable rates. In particular information lossless property is guaranteed if each user makes use of a trace-orthogonal design (TOD), that is a linear space-time code whose encoding matrices are orthogonal with respect to the trace inner product. Noteworthy, users can also use the same set of encoding matrices.