Unitary Alamouti code design from APSK signals with fast maximum-likelihood decoding algorithm

Unitary orthogonal space-time codes, constructed from Alamouti's scheme, has the advantage of its fast maximum-likelihood (ML) decoding but it requires signal constellations to be phase shift keying (PSK). In this paper, unitary space-time codes are constructed from Alamouti's scheme with amplitude/phase shift keying (APSK) signal constellations. We show that the unitary space-time codes from Alamouti's scheme with APSK signals have better diversity products than those with PSK signals. Our proposed rate 4 bits/s/Hz code has about 2 dB gain over the one of the same rate but with PSK signals at bit error rate (BER) of 10^-3 with one receive antenna. Additionally, our proposed codes have fast ML decoding algorithm, whose complexity is comparable to that of unitary orthogonal space-time codes.