# Random Pilot and Data Access for Massive MIMO Spatially Correlated Rayleigh Fading Channels

## TL;DR

This paper proposes a novel device grouping and pilot set allocation algorithm for uplink massive MIMO systems operating over spatially correlated Rayleigh fading channels, improving channel estimation and spectral efficiency.

## Contribution

It introduces a device grouping method based on channel covariance orthogonality and a pilot allocation scheme tailored for correlated channels, enhancing performance over traditional methods.

## Key findings

- Improved mean square error of channel estimation (MSE-CE)
- Enhanced spectral efficiency compared to traditional schemes
- MSE-CE close to theoretical lower bound in high SNR regimes

## Abstract

Random access is necessary in crowded scenarios due to the limitation of pilot sequences and the intermittent pattern of device activity. Nowadays, most of the related works are based on independent and identically distributed (i.i.d.) channels. However, massive multiple-input multiple-output (MIMO) channels are not always i.i.d. in realistic outdoor wireless propagation environments. In this paper, a device grouping and pilot set allocation algorithm is proposed for the uplink massive MIMO systems over spatially correlated Rayleigh fading channels. Firstly, devices are divided into multiple groups, and the channel covariance matrixes of devices within the same group are approximately orthogonal. In each group, a dedicated pilot set is assigned. Then active devices perform random pilot and data access process. The mean square error of channel estimation (MSE-CE) and the spectral efficiency of this scheme are derived, and the MSE-CE can be minimized when collision devices have non-overlapping angle of arrival (AoA) intervals. Simulation results indicate that the MSE-CE and spectral efficiency of this protocol are improved compared with the traditional scheme. The MSE-CE of the proposed scheme is close to the theoretical lower bound over a wide signal-to-noise ratio (SNR) region especially for long pilot sequence. Furthermore, the MSE-CE performance gains are significant in high SNR and strongly correlated scenarios.