On multiple accessing for frequency-selective MIMO channels

We consider frequency-selective Multiple-Input Multiple-Output (MIMO) multiple access fading channels under the assumption that each of the users employs Orthogonal Frequency Division Multiplexing (OFDM), the multiple antenna transmitters have no channel knowledge and the multiple antenna receiver has perfect knowledge of all channels. The family of (MIMO) multiple access schemes previously introduced in [1] allows to gradually vary the amount of user collision in frequency by assigning different subsets of the available OFDM tones to different users and hence ranges from FDMA (each OFDM tone is assigned to at most one user) to CDMA (each OFDM tone is assigned to all the users). It was demonstrated in [1] that under joint decoding CDMA (full collision in frequency) outperforms any other multi-access strategy implementing a variable amount of collision. In practice, however, minimizing the amount of user collision in frequency is desirable as this minimizes the receiver complexity incurred by having to separate the interfering signals. In this paper, we systematically study the impact of user collision in frequency on the sum capacity achieved by the multiple access schemes described in [1]. Our analysis shows that the impact of collision on spectral efficiency depends critically on the channel's spatial fading statistics and the number of antennas. We then systematically identify scenarios where the performance gap (in terms of spectral efficiency) between CDMA and FDMA becomes negligible and hence little collision is needed to achieve high sum capacity. Finally, an asymptotic (in the number of users) analysis is used to exactly quantify the performance gap between CDMA and FDMA.