Performance of MIMO spatial multiplexing in indoor line-of-sight environments

Abstract

Spatial multiplexing in a MIMO system can achieve high speed transmission in i.i.d. Rayleigh channels. However, actual communications can be conducted in LOS environments where we have higher received signal power, whereas channels may be correlated. Although many MIMO measurement cam- paigns have been conducted so far, conventional investigations are not considered to be sufficient from wireless engineering view- point. In this paper, we examine bit error rate (BER) of spatial multiplexing as a function of total transmit power in indoor LOS environments. This consideration is done using 4 × 4 MIMO channel measurements. We compare the BER performance with the one in NLOS environments. Then, we investigate also the BER performance for a case where the channel state information is available also at a transmitter. It has been shown that the BER performance is generally better in the LOS environments than in the NLOS ones despite higher fading correlations, and that the performance in the LOS environments tends to change largely depending on a MIMO system configuration. We have also considered the effect of mutual coupling in antennas to the performance, and relations between eigenvalue distributions and the performance. I. INTRODUCTION Spatial multiplexing using a multiple-input multiple-output (MIMO) system can achieve high speed transmission with- out increasing a frequency bandwidth. When channel state information is available only at a receiver, each transmit antenna sends an independent signal with equal power. This is a space division multiplexing (SDM) technique. Many simulations assume independent and identically distributed (i.i.d.) Rayleigh channels. The i.i.d. Rayleigh channels give excellent performance. This implies that the channels exist in nonline-of-sight (NLOS) environments with many scatterers. However, actual communications can be conducted in line- of-sight (LOS) environments. In LOS environments, we have higher received signal power, whereas channels may be corre- lated. So far, many MIMO measurement campaigns have been conducted, and channel capacities have been examined for a given average SNR value at a receiver end. It has been shown that the channel capacity is higher in NLOS situations than in LOS ones (1), (2). However, there are two problems. First, in NLOS environments, we need higher transmit power to have the same average SNR at a receiver comparing with LOS environments. The performance should be compared under conditions with the same transmit power. Second, the channel capacity is the uppermost limit given by the information theory. Bit error rate (BER) performance is more practical from wireless engineering viewpoint. In this paper, we examine the BER performance of 4 × 4 MIMO SDM as a function of transmit power in indoor LOS environments. The BER is obtained by computer simulations using MIMO channel measurements. We compare the per- formance with the one in NLOS environments. Furthermore, we investigate the BER performance of an eigenbeam-space division multiplexing (E-SDM) system also known as a SVD- based MIMO system (3), (4) where the channel state infor- mation is available also at a transmitter end. We consider also the effect of mutual coupling in antennas, and relations between BER performance and eigenvalue distributions of channel related matrices. II. MEASUREMENT SETUP