Design and Implementation of Spatial Nulling and MIMO Pre-Cancellation on a Dual mmWave SDR Testbed for LEO Satellite Communications

This research studies transmit pre-processing designs using dual millimeter wave (mmWave) active arrays with multiple input multiple output (MIMO) software defined radio (SDR) platform for a low Earth orbit (LEO) satellite communication emulator. It involves the fast Doppler, line-ofsight (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. Then, 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. Next, 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 dual mmWave array, which can null the interference stream and retain the desired stream. After the spatial nulling processing with the interference mitigation, the residual MIMO interference still exists in the MIMO receiver. We further propose MIMO pre-processing filters using the least square method to cancel the residual MIMO interference. Finally, the measurement results of the dual mmWave array SDR platform with the fast Doppler and the over-the-air (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.