Secure signal and space alamouti scheme

Abstract

A method is proposed that secures the conventional Alamouti space-time block code against an eavesdropper by use of both signal and space rotations. Signal rotation is realized by applying a random rotation angle to the conventional signal constellation. This is followed by a second random phase rotation of the Alamouti codeword. The received signal strength levels between legitimate users are used to exchange the secret key that produces the random rotations at the receiver. This is possible due to the availability of short-term reciprocity of the radio channel. Both signal and space rotation angles are chosen such that the peak-to-average ratio (PAR) of the transmit signal is not increased. Firstly, we show that the rotation of the codeword alone compromises system security under receive diversity, especially when the eavesdropper is close to the legitimate user. Then, through the analysis of mutual information rate and bit error rate, we show that by the addition of signal rotation, it is possible to reduce the capacity of the eavesdropper to zero, even when receive diversity is enhanced.