Transmission Scheme for Asymmetric MIMO Two-way Relay with Channel Estimation Error

In this paper, we consider the transmission scheme for asymmetric decode-and-forward (DF) multi-input multi-output (MIMO) two-way relay system with channel estimation error. In order to transmit with asymmetric data rates from the relay to the receivers according to their individual link qualities, we propose a novel transmission scheme for the broadcast phase by employing a priori bit information. The proposed scheme employs multi-antenna and network coding at the relay. The idea is the weaker link receiver exploits a priori bit information in each transmit symbol, so that it only needs to decode on a subset of the transmit symbol constellation. Subject to the same bit error rate constraint, the weaker link receiver can decode at lower signal-to-noise ratio compared to the stronger link. The different signal labeling used for mapping bits to symbols at the relay cause different minimum Euclidean distance, so it is shown to be crucial for the performance at the receivers. Subsequently, we investigate the effect of the channel estimation error on the system performance. Simulations show that the proposed transmission scheme can be applied to practical scenarios with asymmetric channel qualities.