For frequency division duplex channels, a simple pilot loop-back procedure has been proposed that allows the estimation of the UL & DL channels at an antenna array without relying on any digital signal processing at the terminal side. For this scheme, we derive the maximum likelihood (ML) estimators for the UL & DL channel subspaces, formulate the corresponding Cram\'er-Rao bounds and show the asymptotic efficiency of both (SVD-based) estimators by means of Monte Carlo simulations. In addition, we illustrate how to compute the underlying (rank-1) SVD with quadratic time complexity by employing the power iteration method. To enable power control for the data transmission, knowledge of the channel gains is needed. Assuming that the UL & DL channels have on average the same gain, we formulate the ML estimator for the channel norm, and illustrate its robustness against strong noise by means of simulations.
@article{arxiv.1610.03260,
title = {Channel Training for Analog FDD Repeaters: Optimal Estimators and Cram\'er-Rao Bounds},
author = {Stefan Wesemann and Thomas L. Marzetta},
journal= {arXiv preprint arXiv:1610.03260},
year = {2017}
}
Comments
Submitted to IEEE Transactions on Signal Processing, 12 pages, 7 figures