MIMO Characterization on System Level of 5G Microbase Stations Subject to Randomness in LOS

Abstract

Wireless systems have become more and more advanced in terms of handling the statistical properties of wireless channels. For example, the 4G long term evolution (LTE) system takes advantage of multiport antennas [multiple-input multiple-output (MIMO) technology] and orthogonal frequency division multiplexing (OFDM) to improve the detection probability of single bitstream by diversity in the spatial and frequency domains, respectively. The 4G system also supports transmission of two bitstreams by appropriate signal processing of the MIMO subchannels. The reverberation chamber emulates according to previous works rich isotropic multipath (RIMP) and has proven to be very useful for characterizing smart phones for LTE systems. The measured throughput can be accurately modeled by the simple digital threshold receiver, accounting accurately for both the MIMO and OFDM functions. The throughput is equivalent to the probability of detection (PoD) of the transmitted bitstream. The purpose of this paper is to introduce a systematic approach to include the statistical properties of the user and his or her terminal, when characterizing the performance. The user statistics will have a larger effect in environments with stronger line-of-sight (LOS), because the angle of arrival and the polarization of the LOS contribution vary due to the user's orientation and practices. These variations are stochastic, and therefore, we introduce the term random-LOS to describe this. This paper elaborates on the characterization of an example antenna in both RIMP and random-LOS. The chosen antenna is a wideband microbase transceiver station (BTS) antenna. We show how to characterize the micro-BTS by the PoD of one and two bitstreams in both RIMP and random-LOS, by considering the user randomly located and oriented within the angular coverage sector. We limit the treatment to a wall-mounted BTS antenna, and assume a desired hemispherical coverage. The angular coverages of both one and two bitstreams for the random-LOS case are plotted as MIMO-coverage radiation patterns of the whole four-port digital antenna system. Such characterizations in terms of PoD have never been done before on any practical antenna system. The final results are easy to interpret, and they open up a new world of opportunities for designing and optimizing 5G antennas on system level.