# Optical Beamforming Networks for Millimeter-Wave Wireless Communications
## 5 Scalability and Application Potentials
### Scalable Techniques for OBFNs
The propagation losses of Si and InP fabricated by generic foundries is about 1 dB/cm and 2 dB/cm, respectively, while Si\({}_{3}\)N\({}_{4}\) deposited via plasma-enhanced chemical vapor deposition (PECVD), inductively coupled plasma chemical vapor deposition (ICP-CVD) and reactive sputtering (RS) can also achieve a small loss of \(\sim\)1 dB/cm. As a result, Si\({}_{3}\)N\({}_{4}\) material, especially the Si\({}_{3}\)N\({}_{4}\) deposited via LPCVD, is promising for the integration of large-scale OBFNs. More recently, a wideband erbium waveguide amplifier was realized based on Si\({}_{3}\)N\({}_{4}\) deposited via LPCVD [126], which may have greatly promoted the photonic integration on the Si\({}_{3}\)N\({}_{4}\) platform. However, the lack of a modulator and PD hinders the full integration of photonic circuits on a Si  
Figure 21: (**a**) A microcomb source and its spectrum [115] (figure licensed under a Creative Commons Attribution 4.0 License), (**b**) microdisk-based photonic field-programmable gate arrays [118] (figure licensed under a Creative Commons Attribution 4.0 License), and (**c**) MRR-based photonic field-programmable gate arrays [119] (Copyright 2021, AIP Publishing, #5443540906967). platform. One probable approach is to develop a Si-Si\({}_{3}\)N\({}_{4}\) monolithic integration platform as presented in Figure 22, which combines the Si active devices (such as modulator and PD) and Si\({}_{3}\)N\({}_{4}\) passive devices [120,127]. Thus, this Si-Si\({}_{3}\)N\({}_{4}\) photonic platform can capitalize on the advantages of Si\({}_{3}\)N\({}_{4}\) passive devices, Si\({}_{3}\)N\({}_{4}\) waveguide amplifiers and Si active devices. The heterogeneous integration and monolithic integration of photonic circuits and electrical circuits (electro-photonic systems) are prominent for the compactness, energy efficiency and stability of beamforming systems [11]. The heterogeneous integration of an electro-photonic system can present the advantages of Si\({}_{3}\)N\({}_{4}\) deposition via LPCVD, thanks to the separate fabrication of electric circuits and photonic circuits on different wafers. Meanwhile, the monolithic integration of an electro-photonic system can accelerate the innovation of an electro-photonic system on one substrate [128]. This electro-photonic monolithic integration can be implemented on a silicon-on-insulator (SOI) platform and bulk silicon platform, the latter being more CMOS-compatible [128,129]. To integrate Si\({}_{3}\)N\({}_{4}\) devices on these platforms, Si\({}_{3}\)N\({}_{4}\) material should be fabricated via low-temperature processes such as PECVD, ICP-CVD and RS, instead of LPCVD. Therefore, more efforts should be devoted to manufacturing low-loss photonic circuits and waveguide amplifiers via a low-temperature fabrication process for exploiting the advantages of a Si\({}_{3}\)N\({}_{4}\) platform and the monolithic integration of electro-photonic systems. In addition, the linearity of OBFNs is also a key factor for scalability, which is mainly determined by the linearity of modulators and PDs [130, 131, 132]. For microwave photonic systems, such as OBFNs, the linearity of the modulator is critical for the performance of whole system [132]. The linearity of a modulator can be characterized by a spurious free dynamic range (SFDR) defined as the ratio of the maximum RF power which produces third-order intermodulation distortions to noise power. Typically, the SFDR of a silicon-based MZM is smaller than 100 dB-Hz\({}^{2/3}\) at a 1 GHz modulation frequency, exhibiting worse linearity than do LiNbO\({}_{3}\) modulators [133]. The linearity of a modulator can be improved using response compensation techniques of MRR and the Kerr effect in a MZM [132, 133, 134]. Especially, the SFDR of a heterogeneously integrated III-V/Si MZM has been increased to \(\sim\)117 dB-Hz\({}^{2/3}\) at 10 GHz with the assistance of MRR, as shown in Figure 23a. For mmWave wireless communication networks, high-speed and high-power