Spatial Nulling and MIMO Pre-Processing Design for Dual mmWave Active Array SDR Platform With OTA Effects

This research studies transmit pre-processing designs using two millimeter wave (mmWave) active arrays with a multiple input multiple output (MIMO) software-defined radio (SDR) platform under an over-the-air (OTA) communication emulator. It covers the fast Doppler, line-of-sight (LOS) with small delay effect, and MIMO array interference effects. In this emulator, we propose spatial nulling and MIMO pre-processing cancellation techniques to eliminate the MIMO streams interference. First, differential training sequences are utilized to acquire and compensate for the LOS fast Doppler offset. Next, a maximum power beam scanning scheme via the phase shifters of mmWave arrays is employed to estimate the arrival angles between MIMO array transmitter and receiver stations. Subsequently, the steering vectors of the desired and interference signals via these estimated angles are adopted by the phase-only optimization algorithms to calculate the spatial beamforming weights of the two mmWave arrays, which can null the interference stream and retain the desired stream. Because we observed that the residual MIMO interference lingers in the MIMO receiver even after the spatial nulling processing with the interference mitigation, we further propose a MIMO pre-processing filters using the least square method to cancel the residual MIMO interference. Finally, the measurement results of the two mmWave arrays SDR platform with the fast Doppler and the OTA scenarios confirm that the proposed joint spatial nulling and MIMO pre-cancellation techniques can eliminate the MIMO interference and provide the high-quality error vector magnitude (EVM) performance.