Optimal Channel Training in Uplink Network MIMO Systems

TL;DR

This paper analyzes optimal channel training duration in uplink network MIMO systems with multiple base stations and limited backhaul, optimizing spectral efficiency without prior CSI at BSs or CS.

Contribution

It derives the optimal training time considering backhaul constraints and path losses, providing analytical results validated by simulations for small systems.

Findings

Optimal training fraction depends on backhaul capacity and path losses.

Analytical results are accurate even for small system sizes.

Trade-off between training duration and data transmission efficiency.

Abstract

We consider a multi-cell frequency-selective fading uplink channel (network MIMO) from K single-antenna user terminals (UTs) to B cooperative base stations (BSs) with M antennas each. The BSs, assumed to be oblivious of the applied codebooks, forward compressed versions of their observations to a central station (CS) via capacity limited backhaul links. The CS jointly decodes the messages from all UTs. Since the BSs and the CS are assumed to have no prior channel state information (CSI), the channel needs to be estimated during its coherence time. Based on a lower bound of the ergodic mutual information, we determine the optimal fraction of the coherence time used for channel training, taking different path losses between the UTs and the BSs into account. We then study how the optimal training length is impacted by the backhaul capacity. Although our analytical results are based on a large system limit, we show by simulations that they provide very accurate approximations for even small system dimensions.