  In this paper, we investigate a coverage extension scheme based on orthogonal
random precoding (ORP) for the downlink of massive multiple-input
multiple-output (MIMO) systems. In this scheme, a precoding matrix consisting
of orthogonal vectors is employed at the transmitter to enhance the maximum
signal-to-interference-plus-noise ratio (SINR) of the user. To analyze and
optimize the ORP scheme in terms of cell coverage, we derive the analytical
expressions of the downlink coverage probability for two receiver structures,
namely, the single-antenna (SA) receiver and multiple-antenna receiver with
antenna selection (AS). The simulation results show that the analytical
expressions accurately capture the coverage behaviors of the systems employing
the ORP scheme. It is also shown that the optimal coverage performance is
achieved when a single precoding vector is used under the condition that the
threshold of the signal-to-noise ratio of the coverage is greater than one. The
performance of the ORP scheme is further analyzed when different random
precoder groups are utilized over multiple time slots to exploit precoding
diversity. The numerical results show that the proposed ORP scheme over
multiple time slots provides a substantial coverage gain over the space-time
coding scheme despite its low feedback overhead.
