Hybrid analog-digital transmit beamforming for spectrum sharing satellite-terrestrial systems

This paper deals with the problem of hybrid analog-digital transmit beamforming under spectrum sharing constraints. Hybrid multiantenna architectures are ideal for high throughput wireless links, where full-digital solutions are extremely expensive due to their the high number of radiofrequency chains and full-analog architectures require complex and lossy beamforming networks. In contrast to the recent works regarding mm-wave precoding, we consider the case where the transmitter is required to limit its array gain to certain angles-of-departure where non-intended receivers are located. This is of great importance in the 18 GHz band where wireless backhaul systems can eventually share the spectrum with satellite systems, leading to a substantial reduction of the spectrum license cost. We propose a general optimization framework based on an alternate analog-digital optimization that can consider any arbitrary sub-array scheme (i.e. interleaved, localized, etc.). At each stage of the analog-digital optimization a quadratically-constraint-quadratic-program needs to be solved and it requires the use of novel non-smooth methods. Numerical results show the performance of our method for different scenarios and sub-array schemes.