Subarray-Cooperation-Based Multi-Resolution Codebook and Beam Alignment Design for mmWave Backhaul Links

The conflict between heavy training overhead and large beamforming gain is a bottleneck to deploy millimeter-wave (mmWave) communications for the next-generation wireless networks. In response, this paper proposes a subarray-cooperation-based multi-resolution codebook design and the corresponding beam alignment scheme, which enables the mmWave backhaul links to quickly align the desired beam pairs under single-dominant-path channels. The proposed codebook is of a hybrid digital/analog structure, in which the digital component selects subarrays and handles the coupling impacts among the selected subarrays, and the analog component shapes essential sub-beams using phase-shifter groups. In particular, the analog beamformers are configured as equal-size discrete Fourier transform vectors, which are easy to implement. The resulting beam alignment method adaptively selects initial layers according to different transmission signal-to-noise ratios, guaranteeing sufficient success ratios on angle detection. The numerical results evaluate the effectiveness of the proposed scheme.