Interference Mitigation via Relaying

This paper studies the effectiveness of relaying for interference mitigation in an interference-limited communication scenario. We are motivated by the observation that in a cellular network, a relay node placed at the cell-edge observes intercell interference that is correlated with intended receiver’s interference. A relaying link can effectively allow their antennas to be pooled together for both signal enhancement and interference mitigation. We model this scenario by a multiple-input multiple-output (MIMO) Gaussian relay channel with a digital relay-destination link of finite capacity, with correlated noises across relay and destination antennas. Assuming compress-and-forward strategy with Gaussian input distribution and quantization noise, we characterize the achievable rate using a coordinate ascent algorithm for jointly optimizing transmit and quantization covariance matrices. For fixed input distribution, the globally optimum quantization noise covariance matrix can be found in closed-form using a transformation for the relay’s observation that simultaneously diagonalizes two conditional covariance matrices by *congruence. For fixed quantization, the globally optimum transmit covariance matrix can be found via convex optimization. This paper further shows that such an optimized achievable rate is to within a constant additive gap of capacity of the channel, and that the optimal structure of the quantization noise covariance enables a characterization of the slope of the achievable rate as a function of the relay-destination link capacity. Finally, this paper studies the improvement in spatial degrees of freedom (DoF) by MIMO relaying in presence of noise correlation and reveals a relation between the DoF gain and the aforementioned slope via a connection to the deterministic relay channel capacity. Index Terms Gaussian MIMO relay channel, noise correlation, compress-and-forward, reverse water-filling, approximate capacity.