Sector Free and Sub-Connected Precoding for Millimeter Wave Massive FD-MIMO Systems

Hybrid precoding architectures can solve the achievable sum-rate and energy consumption problems in massive MIMO systems. This makes millimeter wave (mm-Wave) as a strong contestant to enable magnitude greater spectrum than current cellular frequency bands. Therefore, desirable hybrid precoding with enough antenna gain should be designed for the mmWave system to provide promising high data rates. However, most previous works focus on fully-connected structure, where sub-connected structure can be the breakpoint between sum-rate and energy consumption. Compared to ULA (Uniform Linear Array), the URA (Uniform Rectangular array) and the UCA (Uniform Cylindrical Array) can provide beams in the vertical domain in addition to the horizontal domain with a uniform beam coverage in the horizontal 360-degree domain, thus making it full dimensional MIMO (FD-MIMO). In this paper, a sub-connected structure is exploited, first the whole antenna array problem is broken into sub-array problems then each sub-array is optimized using an iterative procedure and later interference cancellation between sub-arrays over sector free base station (BS) is performed to find the analog precoder and then a digital precoder. Simulation results show that the proposed hybrid precoding solution achieves better sum-rate and energy efficiency when compared with other existing hybrid precoding designs. And also showing the advantage of having a sector free UCA as compared to the UPA.