Outage Performance Analysis of Widely Linear Receivers in Uplink Multi-user MIMO Systems

TL;DR

This paper analyzes the outage performance of widely linear receivers in uplink multi-user MIMO systems with real-valued modulation, highlighting their advantages over conventional receivers in massive machine type communication scenarios.

Contribution

It provides an analytical outage performance characterization of WL receivers, including SIC variants, in uplink multi-user MIMO systems with real-valued modulation schemes.

Findings

WL receivers achieve higher diversity gain than conventional linear receivers.

WL-SIC receivers increase coding gain without additional diversity gain.

Coding gain of WL-SIC improves with more users, surpassing CL-SIC under certain conditions.

Abstract

This paper considers the application of widely linear (WL) receivers in an uplink multi-user system using real-valued modulation schemes, where the cellular base station (BS) with multiple antennas provides connectivity for randomly deployed single-antenna users. The targeted use case is massive machine type communication (mMTC) with grant-free access in the uplink, where the network is required to host a large number of low data rate devices transmitting in an uncoordinated fashion. Four types of WL receivers are investigated, namely the WL zero-forcing (ZF) and the WL minimum meansquared error (MMSE) receivers, along with their enhanced versions employing successive interference cancellation (SIC) with channel-dependent ordering, i.e., the WL-ZF-SIC and WL-MMSE-SIC receivers. The outage performances of these receivers are analytically characterized in the high signal-to-noise ratio (SNR) regime and compared to those of conventional linear (CL) receivers using complex-valued modulation schemes. For the non-SIC receivers, we show that, when compared to the CL counterparts, the WL receivers yield a higher diversity gain when decoding the same number of users and have the same diversity gain but a decreased coding gain when the number of users is nearly doubled. The outage performance analysis of WL-SIC receivers is facilitated by the marginal distribution of ordered eigenvalues of a real-valued Wishart matrix. It is shown that the SIC operation with channel-dependent ordering brings no additional diversity gain to the WL receivers but instead increases the coding gain. Moreover, the coding gain of WL-SIC receivers grows as the number of users increases and even exceeds that of CL-SIC receivers under suitable conditions.