In this paper, we investigate a coverage extension scheme based on orthogonalrandom precoding (ORP) for the downlink of massive multiple-inputmultiple-output (MIMO) systems. In this scheme, a precoding matrix consistingof orthogonal vectors is employed at the transmitter to enhance the maximumsignal-to-interference-plus-noise ratio (SINR) of the user. To analyze andoptimize the ORP scheme in terms of cell coverage, we derive the analyticalexpressions of the downlink coverage probability for two receiver structures,namely, the single-antenna (SA) receiver and multiple-antenna receiver withantenna selection (AS). The simulation results show that the analyticalexpressions accurately capture the coverage behaviors of the systems employingthe ORP scheme. It is also shown that the optimal coverage performance isachieved when a single precoding vector is used under the condition that thethreshold of the signal-to-noise ratio of the coverage is greater than one. Theperformance of the ORP scheme is further analyzed when different randomprecoder groups are utilized over multiple time slots to exploit precodingdiversity. The numerical results show that the proposed ORP scheme overmultiple time slots provides a substantial coverage gain over the space-timecoding scheme despite its low feedback overhead.
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