Exploitation of the redundancy induced by Alamouti coding for MIMO signals' blind demodulation

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 is not 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 cannot 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 reestablish 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.

► Blind demodulation of MIMO communications signals using Alamouti coding in an interception context.
► Proposition of a new criterion in order to exploit the Alamouti code redundancy and used with a Blind sources separation (BSS) criterion.
► Mathematical study of the stationary points of the new global criterion.
► This new global criterion allows to recover the Alamouti code structure in output after separation.
► This new global criterion allows also to raise one phase ambiguity contrary to a BSS algorithm used alone.