Photonics-based Fully-Connected Hybrid Beamforming: Towards Scaling Up Massive MIMO at Higher Frequency Bands

Wireless communication at higher frequency bands has attracted significant research interest for fifth generation and beoynd (5GB) wireless networks due to the large amount of unused bandwidth at these frequencies. However, there are substantial challenges associated with higher frequency bands due to the high path loss of the propagation environment and the high power consumption of the transceivers. Hybrid beamforming with massive multiple-input multiple-output (MIMO) has emerged as a solution to these problems by combining the performance and flexibility of digital beamforming with the energy efficiency of analog beamforming. Optical beamforming has recently been considered as an alternative to implement the analog component of a hybrid beamformer, which may offer improvements in size, weight and power consumption in comparison to conventional electronics. This paper proposes a new approach to implement an optical beamforming system based on photonic vector modulators using tunable photonic filters. Our experimental demonstration of the proposed optical beamformer shows that microring resonator (MRR)-based photonic vector modulators can be calibrated to achieve a root-mean-square (RMS) phase error of better than 2◦ and an amplitude error of 0.3 dB. Our findings identify a pathway to realize large scale, fully-connected hybrid beamformers by leveraging the compact and low loss photonics-based weight banks. The photonics-based beamformer can play a pivotal role in scaling up the number of antennas utilized to realize massive and extreme MIMO in 5GB. This is deemed essential specifically at higher frequency bands where conventional electronics fails.