Massive multiple-input multiple-output (MIMO) technology is one of the key enablers in the fifth generation mobile communications (5G), in order to accommodate growing traffic demands and to utilize higher super high frequency (SHF) and extremely high frequency (EHF) bands. In the paper, we propose a novel transmit precoding named “nonlinear block multi-diagonalization (NL-BMD) precoding” for multiuser MIMO (MU-MIMO) downlink toward 5G. Our NL-BMD precoding strategy is composed of two essential techniques: block multi-diagonalization (BMD) and adjacent inter-user interference pre-cancellation (IUI-PC). First, as an extension of the conventional block diagonalization (BD) method, the linear BMD precoder for the desired user is computed to incorporate a predetermined number of interfering users, in order to ensure extra degrees of freedom at the transmit array even after null steering. Additionally, adjacent IUI-PC, as a nonlinear operation, is introduced to manage the residual interference partially allowed in BMD computation, with effectively-reduced numerical complexity. It is revealed through computer simulations that the proposed NL-BMD precoding yields up to 67% performance improvement in average sum-rate spectral efficiency and enables large-capacity transmission regardless of the user distribution, compared with the conventional BD precoding.

In order to meet various requirements for the 5th generation mobile communication, a high SHF wideband massive-MIMO system has been widely studied which offers wide system bandwidth and high spectral efficiency. A hybrid beamforming configuration which combines analog beamforming by APAA (Active Phased Array Antenna) and digital MIMO signal processing is one of the promising approaches for reducing the complexity and power consumption of the high SHF wideband massive-MIMO system. In order to realize the hybrid beamforming configuration in high SHF band, small size, low power consumption and precise beam forming over the wide-band frequency range are strongly required for RF frontend which constitutes analog beam former. In this paper, a compact RF frontend module for high SHF wideband 5G small cell base station is proposed. This RF frontend module is prototyped. Various key components of the RF frontend module are fabricated in 15GHz band, and measured results show that high RF performances are able to meet the requirements of RF frontend.