Downlink Precoding With Magnitude CSI for Noncoherent Reception-Based MIMO Systems

The Internet of Things (IoT) requires wireless communications with low-complexity terminals and high-capacity multiple-access capability. This article focuses on the downlink precoder design for multiuser multiple-input–multiple-output (MIMO) systems with only magnitude channel state information (mCSI). The system applies a complementary block coding <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\{s,1-s\}$ </tex-math></inline-formula> at the transmitter and an energy difference detector at the receivers. With the equivalent channel model of the proposed MIMO system based on noncoherent reception, a zero-forcing precoding scheme is presented depending on the mCSI instead of the full channel information. Further derivation shows that the statistical channel covariance matrix can replace this mCSI to achieve a stable precoding scheme. Through postprocessing signal-to-interference-plus-noise ratio (SINR) analysis, an optimized precoding waveform is derived. Under total power constraints, the obtained SINR processing gain increases nonlinearly with the number of transmit antennas, but this gain is not proportional as in coherent MIMO. In addition, a user selection strategy is proposed to mitigate the effect of channel correlation on precoding, and the overall SINRs are proved to be significantly improved. Numerical results verify the spatial-division multiple access (SDMA) of the noncoherent precoding and the effectiveness of the proposed user selection scheme.