Exploitation of the redundancy induced by Alamouti coding for MIMO signals' blind demodulation
Résumé
Blind algorithms are interesting in communications interception context, particularly Blind Source Separation (BSS) algorithms, for Multiple Input Multiple Output (MIMO) systems, and allow to estimate transmitted symbols by using few information on the transmitted data. But they introduce an arbitrary rotation on each separated output flow and possible permutations between these flows. Here we suppose that an Alamouti coding is used at emission. Without precaution the structure of the code isn't found again in the BSS output because of arbitrary phases and possible rows permutation. If the code structure is lost, the redundancy introduced by the Alamouti code can't be exploited to decrease both the bit error rate (BER) and the mean square errors (MSE). In order to recover the code structure in the BSS output, we propose a new criterion, associated with a BSS criterion, which exploits the Alamouti redundancy. This criterion allows to restablish the Alamouti code structure and to obtain the same phase indeterminacy on each row of the separated data, so this proposed algorithm enables to decrease the ambiguities number inherent in the traditional BSS algorithms and like this, finally, to decrease the BER and the MSE.