MIMO-OFDM multiple access with variable amount of collision

We consider frequency selective Multiple-Input Multiple-Output (MIMO) multiple access fading channels with perfect channel knowledge in the receiver and no channel state information in the transmitters. Assuming that each of the users employs Orthogonal Frequency Division Multiplexing (OFDM), we introduce a multiple access scheme which allows to gradually vary the amount of user collision in signal space by assigning different subsets of the available OFDM tones to different users. The corresponding multiple access schemes range from FDMA (each OFDM tone is assigned to at most one user) to CDMA (each OFDM tone is assigned to all the users). We quantify the effect of signal space collision between users by computing ergodic capacity regions for joint and single user decoding. In the case of joint decoding, we prove that irrespectively of the spatial receive fading correlation, the ergodic capacity region obtained by a fully collision-based scheme is an outer bound to any other OFDM-based multiple-access strategy, where the users collide only on subsets of the available tones. For single user decoding, we find that the amount of collision maximizing the capacity region depends critically on the number of transmit and receive antennas and the users' receive fading correlation.