# Optical Beamforming Networks for Millimeter-Wave Wireless Communications
## 5 Scalability and Application Potentials
### Scalable Techniques for OBFNs  
Among the above architectures, MRR group delay architectures, MZI delay architectures, PC delay architectures, integrated time-time delay selection architectures and integrated phase shifter array architectures and matrix architectures are promising for small-scale integration. Nevertheless, these architectures are still of a relatively large size and demand a large number of delay units. For example, a 16 \(\times\) 1 MRR group delay archi \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Schemes Performance** & **Size** & **Weight** & **Power Consumption** & **Loss** & **Bandwidth** & **Multi-Beamforming** \\ \hline I. Fiber dispersion delay & + & + & + & + & +++ & + \\ architectures & ++ & ++ & ++ & ++ & ++ & ++ \\ II. MRR group delay architectures & ++ & ++ & ++ & ++ & ++ & ++ \\ III. MMI delay architectures & ++ & ++ & ++ & ++ & ++ & ++ \\ IV. PC delay architectures with & ++ & +++ & + & + & ++ & ++ \\ integrated PC & + & + & + & ++ & ++ & ++ \\ V. Time delay selection & + & + & + & +++ & ++ & ++ \\ architectures with integrated & ++ & ++ & ++ & ++ & ++ & ++ \\ optical switches and delay lines & & & & & & \\ VII. Time delay selection & ++ & ++ & ++ & ++ & ++ & + \\ architectures with integrated & ++ & ++ & ++ & ++ & ++ & + \\ wavelength de-multiplexer & & & & & & \\ VIII. Polarization-modulated & + & + & + & ++ & + & ++ \\ phase shifter architectures & & ++ & ++ & ++ & + & ++ \\ IX. Integrated phase shifter array & ++ & ++ & ++ & ++ & + & ++ \\ architectures & ++ & ++ & ++ & ++ & + & ++ \\ X. Matrix architectures & ++ & ++ & ++ & ++ & + & +++ \\ \hline \hline \end{tabular}  
* The more “+” appears, the better the corresponding performance is.
* power consumption is evaluated by the number of lasers and the integration level in various architectures.  
\end{table} 
Table 1: Comparison of scalability for various OBFN architectures. tecture is adopted to form and steer the beam received by the 4 \(\times\) 4 2D PAA, which needs 20 MRRs to provide the accurate delays for 16 antennas (Figure 9). It is difficult to scale up this architecture for massive PAAs, since a thermal compensation circuit and a delay control circuit are required for each MRR except for the complexity of delays. One approach is to use the wavelength multiplexing technique as addressed earlier, dramatically decreasing the total number of MRRs from 20 to 8 for the 4 \(\times\) 4 2D PAA. However, a large amount of lasers, including an integrated laser array, are required for the wavelength multiplexing technique which greatly increases the footprint and cost. One solution is to use microcomb source (Figure 21a [115]) which can largely reduce the size and has the flexibility to meet the demands of different 2D PAAs by changing number of comb lines [56; 116]. Furthermore, combining wavelength multiplexing with mode multiplexing, the channel number of integrating OBFNs can be further increased by several times [117], relaxing the requirement of a microcomb source. In addition, photonic field-programmable gate arrays based on microdisk (Figure 21b) and MRR (Figure 21c) can achieve a small footprint, time delay selection, wavelength filtering and reconfiguration simultaneously [118; 119]. These structures have potential to be applied in large-scale OBFNs. To sum this up, microcomb sources and programmable structures may provide high scalability for OBFNs used in PAAs with massive antenna elements. For small-scale integration, a material platform with very low loss is vital for large-scale OBFNs. Currently, photonic integration circuits are predominantly implemented on platforms of silicon (Si), silicon nitride (Si\({}_{3}\)N\({}_{4}\)) and indium phosphide (InP). Table 2 lists the propagation losses of these material platforms [120; 121; 122; 123; 124; 125], in which it is exhibited that the Si\({}_{3}\)N\({}_{4}\) deposited via low-pressure chemical vapor deposition (LPCVD) has the smallest propagation loss, of less than 0.1 dB/cm.