Tilting in UCA-Based LoS MIMO Systems

We consider a $4 \times 4$ line-of-sight multiple-input multiple-output (LoS MIMO) system with uniform circular arrays (UCAs) at the transmitter (Tx) and receiver (Rx). First, we show that the optimum design of non-tilted UCAs of four antennas can be generalized to the design of optimum elliptical arrays (EAs). Afterwards, we investigate the effect of tilting the Tx or Rx UCA over different transmit distances. Although it is commonly considered that tilting a UCA at a fixed transmit distance is detrimental, we show that a tilted UCA can be optimum, in terms of capacity, at other transmit distances. We showcase that this can be explained based on the optimum design of EAs of four antennas. Specifically, we provide a valid approximation for the optimum transmit distance when a UCA is tilted by a given angle or, equivalently, for the optimum tilting angle of a UCA at a certain transmit distance. The use of the approximations is validated with simulations, also for the design of UCAs with more than 4 antennas.