Generalized signal alignment for high mobility OFDM channels

Generalized signal alignment (GSA) is recently proposed as a new transmission framework to achieve the maximum degrees of freedom in multi-user MIMO two-way relay channels. In this paper, we apply GSA in high mobility OFDM systems. To handle the inter-carrier interference (ICI) resulted from large Doppler frequency shift effectively, we propose subcarrier extension (SE)-GSA. Its idea is to treat each subcarrier as a virtual antenna and then apply GSA in the extended virtual MIMO channel. We take the MIMO two-way X relay channel as the system model. It well models to the scenario where a high speed train enters the edge of two adjacent cells and two mobile users inside the train are communicating with the base stations (BSs) through a relay station deployed on the train carriage. By jointly designing the compression matrix and precoding matrices in the virtual antenna domain, both inter-user interference and inter-carrier interference can be managed together. To reduce the computation complexity at the relay, a new method is proposed to design the precoding matrix at each source node. Simulation results demonstrate that the proposed SE-GSA achieves 25% and 66% higher throughput than the conventional signal alignment (SA) and time division duplex (TDD) transmission schemes, respectively, at high SNR with the train moving speed of 500 km/h.