  We consider the problem of channel estimation for amplify-and-forward two-way
relays assuming channel reciprocity and M-PSK modulation. In an earlier work, a
partially-blind maximum-likelihood estimator was derived by treating the data
as deterministic unknowns. We prove that this estimator approaches the true
channel with high probability at high signal-to-noise ratio (SNR) but is not
consistent. We then propose an alternative estimator which is consistent and
has similarly favorable high SNR performance. We also derive the Cramer-Rao
bound on the variance of unbiased estimators.
