Space-time coding for narrowband multiuser composite fading systems

Space-time code design for single user systems has been extensively researched. However, code design for multiuser systems is more complicated since different users may be operating in different fading environments. In this paper, we derive two code design criteria for multiuser composite fading systems in which some users have quasi-static fading channels, while others have rapid fading channels (i.e., the fading coefficients are independent over symbols). The two criteria involve the rank and non-zero eigenvalues of a distance matrix A. We show that A is the sum of two matrices, one characterizing quasi-static fading and the other characterizing rapid fading. Based on the criteria, we obtain optimal space-time trellis codes for a two-user QPSK system through exhaustive search. We also show by simulation that the new codes have better performance than existing space-time block codes and space-time trellis codes in composite fading channels.