Hybrid Analog-Digital Transmit Beamforming for Spectrum Sharing Backhaul Networks

Next generation wireless backhauling networks are meant to share the same spectrum resources in order to deal with the exponential base station data rate demands. One alternative is to consider a very aggressive frequency reuse among backhaul links and implement interference mitigation techniques. This paper deals with the problem of analog-digital transmit beamforming under spectrum sharing constraints for backhaul systems. In contrast to all-digital designs where the spatial processing is done at baseband unit with all the flexible computational resources of digital processors, analog-digital beamforming schemes require that certain processing is done through analog components, such as phase-shifters or switches. These analog components do not have the same processing flexibility as the digital processor but; on the other hand, they can substantially reduce the cost and complexity of the beamforming solution. Precisely, with an hybrid analog-digital scheme the number of radiofrequency chains can be reduced by extending the processing through the analog part and; therefore, reducing the overall cost and digital bandwidth requirements. This work presents the joint optimization of the analog and digital parts that results in a non-convex, NP-hard and coupled problem. In order to solve it, an alternating optimization with a penalized convex-concave method is proposed. According to the simulation results, this novel iterative procedure is able to find a solution that behaves close to the fully-digital beamforming upper bound scheme. All in all, despite the computational complexity of the proposed scheme is relatively high, it is adequate for backhauling networks where nodes are static and the beamforming weights do not need to be updated on a frame basis.Grant numbers : This work has received funding from the Spanish Ministry of Economy and Competitiveness (Ministerio de Economia y Competitividad) under project TEC2014-59255-C3-1-R (ELISA); and from the Catalan Government (2014SGR1551).© 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works