Beamspace MIMO-NOMA for Millimeter-Wave Communications Using Lens Antenna Arrays

The recent concept of beamspace multiple-input multiple-output (MIMO) is capable of significantly reducing the number of radio-frequency (RF) chains required by millimeter-wave (mmWave) massive MIMO systems. However, the fundamental limit of the existing beamspace MIMO is that, the number of supported users cannot be higher than the number of RF chains using the same time-frequency resources. To break this limit, beamspace MIMO is integrated with non-orthogonal multiple access (NOMA) in the proposed MIMO-NOMA system in this paper, where the number of supported users can be higher than the number of RF chains. To reduce the inter-beam interference, a transmit precoding (TPC) scheme based on the principle of zero-forcing (ZF) is designed. Furthermore, a dynamic power allocation scheme is proposed for maximizing the achievable sum rate. Moreover, a low-complexity iterative optimization algorithm is conceived for dynamic power allocation. Simulation results show that the proposed beamspace MIMO-NOMA achieves a higher spectrum and energy efficiency than the existing beamspace MIMO for mmWave communications.