Date Added: Jan 2012
It has been recently shown that some Multiple-Input Multiple-Output (MIMO) channels under Orthogonal Space-Time Block Coding (OSTBC) transmissions can not be unambiguously identified by only exploiting the Second Order Statistics (SOS) of the received signal. This ambiguity, which is due to properties of the OSTBC, is traduced in the fact that the largest eigen-value of the associated eigen-value problem has multiplicity larger than one. Fortunately, for most OSTBCs that produce ambiguity, the multiplicity is two. This means that the channel estimate lies in a rank-2 subspace, which can be easily determined applying a first Principal Component Analysis (PCA) step. To eliminate the remaining ambiguity, the authors propose to apply a Constant Modulus Algorithm (CMA).