Omnidirectional precoding based transmission in massive MIMO systems

Common signals in public channels of cellular systems should be transmitted omnidirectionally from the base station (BS) to ensure cell-wide coverage. In this paper, we propose an omnidirectional precoding (OP) based transmission for public channels in massive MIMO systems, which leads to a significant reduction in the downlink pilot overhead while providing omnidirectional signaling. For this OP based transmission, we present three necessary conditions that the OP matrix should satisfy, to meet the requirements for omnidirectional transmission for reliable cell-wide coverage, equal average power on each antenna to sufficiently utilize all the available power amplifier capacities of BS antennas, and achievable ergodic rate maximization for the i.i.d. channel, respectively. Then, several examples of the OP matrix satisfying the three necessary conditions simultaneously are designed by utilizing the Zadoff-Chu sequence and its properties. We also analyze the system performance in terms of achievable ergodic rate, outage probability, and peak-to-average power ratio (PAPR) for these designs. It is shown that a dedicated design of the OP matrix has the following advantages: 1) it asymptotically maximizes the achievable ergodic rate, maximizes the achievable diversity order, and minimizes the outage probability in the large-scale array regime, not only for the i.i.d. channel, but also for spatially correlated channels; 2) it preserves the PAPR of the transmitted signal after precoding.