Spectrally efficient multiuser space-time modulation

To achieve full transmit diversity for a single user with N_T antennas the coherence time of the channel has to be at least N_T time-slots long. The same minimum number of slots (or dimensions) guarantees full transmit diversity for every user in a "narrowband" multiuser system in which all users communicate simultaneously and occupy the same bandwidth. However, with an increase in the number of users, an increasing signal-to-noise ratio (SNR) is required of each user to achieve the same error probability as in the single-user case. We showed earlier that this SNR penalty can be alleviated, and even eliminated for sufficiently high SNR, with only a marginal increase of the bandwidth and a judicious design of "spreading matrices" that spread a single-user space-time constellation or (block-) code. In other words, such signaling exploits multiuser diversity and ensures that each user enjoys single-user like performance for sufficiently high SNR. For instance, for N_T-transmit antennas per user and a single-user space-time constellation that requires only the minimum time-slots N_T, no more than N_T+1 slots are required for the common signal space of any number of users. We present here simplified design criteria to obtain the spreading sequences/matrices. We also show that the optimum multiuser receiver can be implemented by the (generalized) sphere decoder of Damen et al. (see IEE Electronics Letters, vol.36, no.2, p.166-167, 2000).