Omnidirectional STBC Design 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 two omnidirectional space-time block codes (STBCs), being referred to as precoded Alamouti code and precoded quasi-orthogonal STBC (QOSTBC), respectively, to provide omnidirectional signaling with spatial diversity for public channels in massive MIMO systems. Both of these two schemes are with low downlink pilot overhead and can guarantee omnidirectional transmission and equal-power transmission per antenna simultaneously, to ensure reliable cell-wide coverage and sufficiently utilize all the available power amplifier capacities at BS antennas, respectively. The precoded Alamouti code provides diversity order 2 and has fast single-symbol maximum-likelihood (ML) decoding. The precoded QOSTBC provides a higher diversity order 4, but at the expense of a higher decoding complexity, since the ML decoding must be done on each pair of the modulation symbols.