On the Transmission Method for Short-Range MIMO Communication

This paper investigates a transmission scheme that is suitable for short-range multiple-input-multiple-output (MIMO) transmission. Since the distance between two array antennas that face each other is comparable with the size of the array antenna aperture in short-range MIMO, the propagation characteristics are greatly different from those in conventional MIMO. Unlike conventional MIMO, the optimal element spacing, which maximizes channel capacity, exists in short-range MIMO. Moreover, the channel capacity with optimal antenna spacing exceeds the ergodic capacity of independent identically distributed (i.i.d.) channels since optimal eigenvalue distribution, which can maximize channel capacity, is obtained in the short-range MIMO. In this paper, we focus on the actual transmission methods, because complex transmission schemes such as eigenmode transmission or maximum-likelihood detection are required to obtain ideal channel capacity. We clarify that the channel capacity obtained by zero forcing (ZF) at the receiver without beamforming at the transmitter is almost the same as that using eigenmode transmission when considering the optimal element spacing. The effectiveness of short-range MIMO communication is also clarified using a 4 × 4 MIMO testbed with actual signals based on the IEEE 802.11n standard. Simulated and measured results show that optimal element spacing is a key parameter in the short-range MIMO communication. We found that designing antenna arrays with optimal element spacing is a very effective approach to achieving a simple hardware configuration.