  The optimal decoder achieving the outage capacity under imperfect channel
estimation is investigated. First, by searching into the family of nearest
neighbor decoders, which can be easily implemented on most practical coded
modulation systems, we derive a decoding metric that minimizes the average of
the transmission error probability over all channel estimation errors. This
metric, for arbitrary memoryless channels, achieves the capacity of a composite
(more noisy) channel. Next, according to the notion of estimation-induced
outage capacity (EIO capacity) introduced in our previous work, we characterize
maximal achievable information rates associated to the proposed decoder. The
performance of the proposed decoding metric over uncorrelated Rayleigh fading
MIMO channels is compared to both the classical mismatched maximum-likelihood
(ML) decoder and the theoretical limits given by the EIO capacity (i.e. the
best decoder in presence of channel estimation errors). Numerical results show
that the derived metric provides significant gains, in terms of achievable
information rates and bit error rate (BER), in a bit interleaved coded
modulation (BICM) framework, without introducing any additional decoding
complexity.
