Massive MIMO Downlink Transmission for Multiple LEO Satellite Communication

We investigate massive multiple-input multiple-output (MIMO) downlink (DL) transmission for multiple low-earth-orbit satellite communication systems. We establish the signal and channel models, and reveal that the signals received by each user terminal (UT) are typically asynchronous in time and frequency. We propose a spatial linear receive processing for signal extraction and perform time and frequency compensations at each UT to achieve synchronized signal. We prove that the single data stream transmission from each satellite to each UT is optimal to maximize the ergodic sum rate. Therefore, without loss of optimality, we can reduce the joint design of the transmit covariance matrices and receive vectors for spatial linear processing to that of the precoding vectors and receive vectors, which we refer to as joint precoder and receiver design (JPRD). We devise a weighted minimum mean-square error (WMMSE) based JPRD algorithm by using the statistical channel state information. Further, we approximate the optimal design with an ergodic sum rate upper bound, for which the optimality of single data stream transmission still holds. We derive a condition under which the inter-satellite interference can be eliminated, and develop a low-complexity WMMSE based JPRD algorithm with the upper bound. Simulation results validate the proposed approaches.