# Optical Beamforming Networks for Millimeter-Wave Wireless Communications
###### Abstract  
With the rapid data growth driven by smart phone, high-definition television and virtual reality/augmented reality devices and so on, the launched 5G and upcoming 6G wireless communications tend to utilize millimeter wave (mmWave) to achieve broad bandwidth. In order to compensate for the high propagation loss in mmWave wireless communications and track the moving users, beamforming and beamsteering are indispensable enabling technologies. These have promising potential to be realized through the use of optical beamforming networks (OBFNs) that have a wider bandwidth and smaller size, lower power consumption, and lower loss compared to those of their electric counterparts. In this paper, we systematically review various OBFN architectures using true time delays and optical phase shifters, as well as discuss performances of different architectures, scalable technologies that promote the advancement of OBFNs, and the application potentials of OBFNs. Two-dimensional OBFNs with discrete components or integrated optical devices have been elaborated, in addition to one-dimensional architectures. Moreover, the state-of-the-art technologies relative to reducing the size, loss and nonlinearity of OBFNs have also been discussed here. optical beamforming; beamsteering; wireless communication; millimeter wave; phase array antenna; true time delay; optical phase shifter; 5G; 6G +
Footnote †: journal: Applied  
those of antenna elements, which are too expensive to implement in PAAs with massive antenna elements [9; 14]. These issues hinder the utilization of electric beamforming networks in mmWave communications with broad bandwidth and compact antenna arrays. One promising solution is to use OBFNs that have inherent advantages in enabling broadband wireless communications, thanks to the wide bandwidth of optical devices and multi-dimensional multiplexing capability of optical signals. With photonic integration technology, OBFNs also possess the potential advantages of small size, low weight, low power consumption and low loss [1; 14; 15]. OBFNs are used to phase tune or induce a time delay in the radio frequency (RF) signal at each antenna element with optical phase shifters or true time delay (TTD) components, and further control the beam pattern of PAAs. The implementations of OBFNs in PAAs can be dated back to the 1970s [16; 17]. Early demonstrations of OBFNs are mainly based on TTD architectures which adopt discrete devices to build fiber-optic or free-space beamforming systems [18; 19; 20; 21]. These architectures are easy to be realized by using commercially mature components; however, their bulkiness inhibits their integration with antennas especially in mmWave wireless communications. OBFNs with optical phase shifters predominantly produce RF signals via coherent beating at photodetectors (PDs), which have a relatively small tuning range and bandwidth [16; 22; 23; 24; 25]. Moreover, OBFN architectures combining TTDs and phase shifters have also been demonstrated to have superiorities in reducing the cost and complexity of TTD architectures designed for PAAs with limited bandwidth [20; 21]. In recent years, OBFNs with integrated TTDs or integrated phase shifters have received much concern and exhibited advantages in compactness, high scalability and low power consumption [26; 27; 28; 29; 30; 31]. Several application scenarios of OBFNs such as indoor coverage and mobile fronthaul have been proposed [29; 32; 33; 34; 35; 36; 37; 38]. Up to now, reviews about OBFNs have paid more attention on TTD architectures [8; 12; 15; 20; 21; 39], but the roles and advancements of phase shifter architectures have not been discussed. Furthermore, relative advancements in materials, optical devices and electro-photonic integrations, which may improve the scalability of OBFNs, have not been discussed either.