Practical multi-user transmission design in millimeter wave cellular networks: Is the joint SDMA-TDMA technique the answer?

Millimeter wave (mmWave) systems will likely employ large antenna arrays at both the transmitters and receivers. A natural application of antenna arrays is simultaneous transmission to multiple user equipments (UEs), which requires multi-user precoding at the transmitter. However, hardware constraints and operation loads make it challenging to apply conventional pure MIMO digital precoding or even the hybrid analogue and digital precoding techniques at mmWave. This paper tries to answer the question - Is high complex multi-user digital precoding needed in mmWave systems? With this goal, we propose a joint SDMA-TDMA technique to make the UEs' transmission orthogonal in a multi-user single-cell mmWave system by classifying the UEs into different orthogonal groups in either space or time domain. By employing this joint SDMA-TDMA technique, both the resulting SINR and achievable rate coverage probability are analyzed based on stochastic geometry. Numerical results imply that the proposed technique enables a Gbits/second high data rate per UE but with very low complexity and overhead.