Design Criteria of Uniform Circular Array for Multi-User MIMO in Rural Areas

When multi-user multiple-input multiple-output (MU-MIMO) is applied to predominantly line-of-sight (LoS) environments, such as in the case of fixed wireless access in rural areas where a central access point (AP) equipped with an antenna array with $N_{\mbox{\tiny AP}}$ antenna elements serves $N_{\mbox{\tiny UT}}$ user terminals (UTs) each equipped with a single antenna, the problem of ill-conditioned channels arises. This paper investigates the performance of zero-forcing precoding based MU-MIMO downlink when the AP is equipped with a uniform circular array (UCA) in an LoS environment. The performance is analyzed as a function of the spacing and the number of AP UCA antenna elements for $N_{\mbox{\tiny AP}} \geq N_{\mbox{\tiny UT}}$. The analysis reveals a complex yet orderly pattern of the performance nulls indicating different optimal antenna spacing for different number of antenna elements. The performance nulls can be largely eliminated by employing $N_{\mbox{\tiny AP}} \geq 2 N_{\mbox{\tiny UT}}$.