Sum-Rate of Cell Free Massive MIMO Systems With Power Amplifier Non-Linearity

Zahra Mokhtari,Rui Dinis

doi:10.1109/access.2021.3120925

Abstract

Cell free (CF) massive multiple input multiple output (mMIMO) has been suggested as a key solution to meet the high data rate demands of future wireless communications. Studying the performance of these systems in practical scenarios such as in the presence of hardware impairments is of vital importance. In this paper, we study the effect of power amplifier non-linearity on the uplink and downlink sum-rate of CF mMIMO systems. We derive closed form expressions for the uplink and downlink achievable sum-rates of orthogonal frequency division multiplexing (OFDM) based CF mMIMO systems. Our results show that in the uplink the sum-rate does not increase unlimitedly as the number of access points (AP) increases, being upper bounded, contrarily to the ideal linear case. In fact, the rate of each user is limited by the distortion signal of its power amplifier. However, in the downlink, the sum-rate of the system with non-linear power amplifiers is not bounded and increases unlimitedly with the number of APs. Our results also indicate that for the same signal to distortion ratio (SDR) at the power amplifier output or the same normalized saturation level of the power amplifiers, the relative downlink sum-rate degradation is lower than the relative uplink sum-rate degradation (both with respect to their corresponding values in the ideal linear case). In fact, our results confirm that the user side power amplifier non-linearity has higher impact on the system performance than the power amplifier non-linearity on the AP antennas.

Highlights

Cell free (CF) massive multiple input multiple output has been suggested to overcome the limitations of conventional cellular structures and to reach the data rate demands of sixth generation (6G) wireless communications [1]
The results show that the sum-rate is upper bounded by the carrier frequency offset (CFO) even when the number of access points (AP) tends to infinity
We assume that the power delay profile between each AP antenna and each user is the same, except in a scalar factor that differs due to different distances of the users from an AP

Summary

INTRODUCTION

Cell free (CF) massive multiple input multiple output (mMIMO) has been suggested to overcome the limitations of conventional cellular structures and to reach the data rate demands of sixth generation (6G) wireless communications [1]. Dinis: Sum-Rate of Cell Free Massive MIMO Systems With Power Amplifier Non-Linearity number of antennas in cell free massive MIMO systems, cheaper radio frequency (RF) chains must be used to provide a cost-effective system [2], [9]. This increases the risk of the system’s performance being hit by hardware impairments. To the best of our knowledge, there is no work studying the effect of power amplifier non-linearity on the sum-rate of CF mMIMO systems. We derive the sum-rates considering OFDM waveform at both uplink and downlik to see how the performance degrades in presence of power amplifier non-linearity.

SYSTEM MODEL

DOWNLINK TRANSMISSION

GENERAL BEHAVIOUR OF A PRACTICAL POWER

PERFORMANCE RESULTS

CONCLUSION