# Combating Massive MIMO Channel Aging by Orthogonal Precoding

## TL;DR

This paper proposes an orthogonal precoding method to mitigate channel aging effects in massive MIMO vehicular communications, significantly improving reliability despite outdated channel information.

## Contribution

It introduces a novel orthogonal precoding approach combined with massive MIMO to counteract channel aging in high-mobility scenarios.

## Key findings

- Performance improves by two orders of magnitude in bit-error rate
- Orthogonal precoding enhances channel hardening effects
- Method is effective with outdated channel state information

## Abstract

In this work we investigate ultra-reliable low-latency massive multiple-input multiple-output (MIMO) communication links in vehicular scenarios, where coherence between uplink and downlink cannot be assumed. In such scenarios the channel state information obtained in the uplink will be outdated for the following downlink phase. To compensate for this channel aging we will utilize orthogonal precoding with two-dimensional precoding sequences in the time-frequency domain within an orthogonal frequency division multiplexing system. The channel hardening effect of massive MIMO transmission decreases, due to channel aging, with increasing frame duration and increasing velocity, while the channel hardening effect of orthogonal precoding (OP) increases with increasing time- and frequency-selectivity of the wireless communication channel. By combining massive MIMO and OP we can show by numeric link level simulation that the performance with outdated channel state information in terms of bit-error rate versus signal-to-noise ratio can be improved by two orders of magnitude.