A Criterion for Switching From SDMA to Hybrid SDMA/TDMA to Resolve Multi User Conflicts

The utilization of pure space-division multiple access (SDMA) in mobile communications systems relies on the assumption that the array propagation vectors of different user equipments (UEs) differ by a minimum angle to obtain reasonable propagation conditions. However, when serving several UEs, this hypothesis does not hold with probability one, especially for channels with a strong line-of-sight (LOS) component, and it is beneficial w.r.t. maximizing the sum rate to switch to a different, truly orthogonal, multiple access scheme like time-division multiple access (TDMA) or frequency-division multiple access (FDMA). In this work, we use the array propagation vector correlation of two UEs to decide whether the utilization of pure SDMA is advantageous. For this reason, we derive an optimal analytical decision threshold based on this correlation for a two UE scenario. In addition, we extend this criterion numerically to find a practical decision criterion for the assignment of transmission groups in multi UE scenarios for a hybrid SDMA/TDMA approach and apply a linear-model based estimation approach.