Optimal Per-Antenna ADC Bit Allocation in Correlated and Cell-Free Massive MIMO

TL;DR

This paper investigates optimal ADC bit allocation per antenna in Massive MIMO systems to maximize spectral efficiency, revealing different strategies for co-located and cell-free architectures and highlighting energy efficiency gains.

Contribution

It introduces a method for optimizing ADC bit allocation in Massive MIMO, demonstrating significant spectral efficiency and energy efficiency improvements over equal allocation.

Findings

Equal ADC bit allocation is optimal for co-located arrays with low channel variation.

Optimized ADC bit allocation improves spectral efficiency by about 2 bits per user in cell-free systems.

Low-resolution mixed ADCs can increase energy efficiency by up to 30% in cell-free Massive MIMO.

Abstract

In Massive MIMO base stations (BSs), the hardware design needs to balance high spectral efficiency (SE) with low complexity. The level of hardware impairments (HWIs) indicates how strong the signal distortion introduced by hardware imperfections is. In particular, the analog-to-digital converters (ADCs) have an important impact on signal distortion and power consumption. This article addresses the fundamental problem of selecting the optimal hardware quality in the Massive MIMO space. In particular, we examine the optimal HWI and ADC bit allocation per BS antenna to maximize the SE. The results show that in co-located arrays with low channel gain variations across antennas, equal ADC bit allocation is optimal. In contrast, cell-free Massive MIMO systems benefit the most from optimizing the ADC bit allocation achieving improvements in the order of 2 [bit-per-channel-use] per user equipment when using regularized zero-forcing (RZF). In addition, when including the impact of power consumption in cell-free Massive MIMO with RZF, allocating low values of mixed ADC bit resolutions across the BS antennas can increase the energy efficiency up to 30% compared to equal ADC bit allocation.