Differential space time block codes using QAM for four transmit antennas

We develop differential space time block codes (STBC) using non-constant modulus constellations for four transmit antennas. The proposed method improves on the conventional differential STBC techniques because of the larger minimum distances of non-constant modulus constellations. The transmitted signals are modulated using pulse amplitude modulation (PAM). Encoding is similar to that of the conventional differential STBC, while the receiver is different. The signal at the receiver is divided by the estimated channel power and then decoded using a conventional QAM decoder which treats pairs of transmitted PAM symbols transmitted from two transmit antennas as one QAM symbol. For transmission rates greater than 2 bits/channel use, the proposed method outperforms the conventional differential STBC.