  In this paper, we design space-time block codes (STBCs) to broadcast the
common information omnidirectionally in a massive MIMO downlink. To reduce the
burden of the downlink channel estimation and achieve partial spatial diversity
from base station (BS) transmit antennas, we propose channel-independently
precoded low-dimensional STBC. The precoding matrix and the signal
constellation in the low-dimensional STBC are jointly designed to guarantee
omnidirectional coverage at any instant time and sufficiently utilize the power
amplifier capacities of BS transmit antennas, and at the same time, achieve the
full diversity of the low-dimensional STBC. Under this framework, several
designs are presented. To provide transmit diversity order of two, a precoded
Alamouti code is proposed, which has a fast symbol-wise maximum-likelihood (ML)
decoding. To provide transmit diversity order of four, three types of STBCs are
proposed, being referred to as precoded orthogonal STBC (OSTBC), precoded
quasi-orthogonal STBC (QOSTBC), and precoded coordinate interleaved orthogonal
design (CIOD), respectively. The last two codes have the same complexity for
pair-wise ML decoding, while precoded QOSTBC has a higher coding gain when the
bit rate is lower than or equal to 4 bps/Hz, and precoded CIOD has a higher
coding gain when the bit rate is higher than 4 bps/Hz. Precoded OSTBC has a
higher decoding complexity and a lower coding gain than the other two codes,
since in the precoded OSTBC the information symbols need to be jointly designed
and decoded. Moreover, a precoded no-zero-entry Toeplitz code and a precoded
no-zero-entry overlapped Alamouti code are also proposed. These two codes can
achieve a higher diversity order with linear receivers.
