Channel Capacity Evaluation for Urban MIMO Systems Using Path Visibility

Multiple-Input Multiple-Output (MIMO) and Multi-User MIMO (MU-MIMO) systems are expected to improve the channel capacity over the limited bandwidth of existing wireless communication networks. The effects on the channel capacity characteristics of Single-User MIMO (SU-MIMO) systems in urban scenarios have been previously studied. In this paper, we first clarify that the larger number of antennas cannot contribute the improvement in the channel capacity in urban SU-MIMO scenarios due to the very high spatial correlation. Next, we focus on the MU-MIMO transmission because it can discriminate multiple users by the difference of Angle of Arrival (AOA). We compare the Multi Access Channel (MAC) capacity in the uplink to the capacity in SU-MIMO by considering the same total numbers of transmitting and receiving antennas between SU-MIMO and MU-MIMO. When the same numbers of the antenna elements are used, the better performance is obtained by urban MU-MIMO systems, unlike in the identical independent distributed (iid) scenarios which are generally assumed in MIMO transmission. Finally, the parameter called path visibility introduced in our previous study is playing an important role once again. It is defined as the probability that the direct wave can be received at the receiving antenna and has been used to determine the channel capacity characteristics and provide guidance in determining the appropriate range for the base station (BS) installation in urban SU-MIMO scenarios. In this study, we clarify that only this single parameter, path visibility, can also be used to determine the improvement in the channel capacity by MU-MIMO in urban scenarios, while both of SNR and spatial correlation must be considered as the indicative parameters on the performance of SU/MU-MIMO in conventional studies.