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Downlink Transmit Design in Massive MIMO LEO Satellite Communications

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Document pages: 30 pages

Abstract: Low earth orbit (LEO) satellite communication systems have attractedextensive attention due to their smaller pathloss, shorter round-trip delay andlower launch cost compared with geostationary counterparts. In this paper, thedownlink transmit design for massive multiple-input multiple-output (MIMO) LEOsatellite communications is investigated. First, we establish the massive MIMOLEO satellite channel model where the satellite and user terminals (UTs) areboth equipped with the uniform planar arrays. Then, the rank of transmitcovariance matrix of each UT is shown to be no larger than one to maximizeergodic sum rate, which reveals the optimality of single-stream precoding foreach UT. The minorization-maximization algorithm is used to compute theprecoding vectors. To reduce the computation complexity, an upper bound ofergodic sum rate is resorted to produce a simplified transmit design, where therank of optimal transmit covariance matrix of each UT is also shown to notexceed one. To tackle the simplified precoder design, we derive the structureof precoding vectors, and formulate a Lagrange multiplier optimization (LMO)problem building on the structure. Then, a low-complexity algorithm is devisedto solve the LMO, which takes much less computation effort. Simulation resultsverify the performance of proposed approaches.

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