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We consider beamformer optimization for user equipment (UE) relaying cooperation in heterogeneous cellular networks (HCNs), where the interference from femto cells nearby aggravates the signal-to-interference-plus-noise ratio (SINR) of a macro UE (MUE). A femto UE acts as a relay over device-to-device (D2D) uplink and forward to the MUE, the composite of desired signal and interference to improve the post SINR of the MUE. The beamformers of the UE relay and MUE collaborate to make a balance between the two signals so that the output signal after multistage maximal-ratio combining (MS-MRC) yields a sufficient post SINR for the desired signal. We first consider the case with a single interfering femto access point (FAP) and show that the beamforming for dual-stage MRC can effectively handle the interference. Then, the idea is generalized to the case of two interfering FAPs with a three-stage MRC. A geodesic geometry view allows us to parameterize the BF design with the angles that the involved channel vectors make. This approach makes the optimization of the BF simpler, reduces the feedback overhead, and provides better insight into the problem at hand. Simulations are carried out to validate the performance of the UE relaying cooperation in HCN environments.
Hwang et al. (Mon,) studied this question.