Relay X Channels without Channel State Information at the Transmit Sides: Degrees of Freedom

This paper focuses on the two-user relay-assisted X channel with no channel state information (CSI) available at the transmitter side. Two relaying modes, namely half-duplex decode-and-forward (DF) and cognitive relays, are considered and the degrees of freedom (DoF) are characterized. It is shown that assisted by a half-duplex DF relay that is equipped with 2M antennas, the X channel with two M-antenna users has 4M/3 DoF, which is achievable through interference alignment (IA). Furthermore, it is shown that in this channel, an M-antenna cognitive relay (with non-causal access to information streams) provides 2M DoF using interference cancellation (IC) technique. In this setting, IC outperforms interference alignment in the cognitive relay mode, since the latter achieves 4M/3 DoF

MIMO Y channel with imperfect CSI: Impact of training and feedback overhead

Signal space alignment with network coding (SSA-NC) is a cooperative transmission strategy that, under some conditions such as full channel state information (CSI), provides maximum degrees of freedom (DoF) in two-way relay networks. However, obtaining full CSI for SSA-NC requires a significant amount of training and feedback overhead. This paper focuses on the multiple-input multiple-output (MIMO) Y channel and explicitly considers the required training and feedback at the users and relay. We derive the optimum signaling overhead of this channel to maximize the effective sum-rate of the SSA-NC scheme. We further investigate an important question: under what conditions does the SSA-NC scheme have a better sum-rate performance compared to other popular schemes with lower complexity such as multi-user MIMO (MU-MIMO) and time division multiple access (TDMA)? Since different schemes require different amounts of training/feedback overhead, our result demonstrates how the channel coherence interval plays an important role in their relative performance. Specifically, the SSA-NC scheme outperforms the other schemes only when the coherence interval is sufficiently large. This result highlights the importance of properly taking the training and feedback overhead and channel coherence interval into account when designing the communication scheme for the MIMO Y channe

Relay X channels without channel state information at the transmit sides: Degrees of freedom

This paper focuses on the two-user relay-assisted X channel with no channel state information (CSI) available at the transmitter side. Two relaying modes, namely half-duplex decode-and-forward (DF) and cognitive relays, are considered and the degrees of freedom (DoF) are characterized. It is shown that assisted by a half-duplex DF relay that is equipped with 2M antennas, the X channel with two M-antenna users has 4M/3 DoF, which is achievable through interference alignment (IA). Furthermore, it is shown that in this channel, an M-antenna cognitive relay (with non-causal access to information streams) provides 2M DoF using interference cancellation (IC) technique. In this setting, IC outperforms interference alignment in the cognitive relay mode, since the latter achieves 4M/3 DoF