**1. Introduction**

Generally, antenna unit is a requisite of any on-air radio frequency (RF) system forming its service area and bandwidth capability. At present, implementing an active phased array antenna (PAA) [1] results in remarkably increased footprint and operation flexibility thanks to electronic beam steering function, which is realized by a beamforming network (BFN). The concept of electronically beam-steered PAA was proposed for radar application more than 50 years ago [2]; and due to the limited requirements for the bandwidth, up to the first decade of this century, microwave diode- or ferrite-based phase shifters were widely used in BFN as control elements. In the course of further development of radar technology, new requirements that

arose referred to the increase in the area of PAA and the sector of beam scanning, also to the expansion of operating frequency range and instantaneous bandwidth [3]. To meet all of them in BFN based on standard phase-shifter approach, a serious barrier has arisen associated with the so-called beam squint effect, which leads to beam expansion and deflection from its intended target [1]. The search for solution of the limited instantaneous bandwidth issue led to the conclusion that the most effective way for radars, both pulsed and continuous probing, is to replace phase shifters in the PAA feed network with time-delay units, which will operate as true time delay (TTD) negating the effect of the finite fill time of PAA aperture [2].

Conceptually, the operation principles of microwave phase-shifter and TTD units are similar, since the both has to adjust a large number of antenna elements to force the electromagnetic wave to add up at a particular angle to the PAA regulating such uniquely related parameters, as phase and time delay. However, in the first case, steering is provided by changing transmission phase angle (phase of S21) of a two-port network, but in the second one, by changing the length of the set of the passive microwave delay lines controlled by pin-diode or transistor switching circuits. So when implemented in the form of microstrip or coplanar microwave lines, it is possible to provide a bandwidth of up to tens of GHz. The main disadvantages of microwave TTD-based BFN are cumbersomeness and large insertion loss, the value of which can vary significantly at each step of the delay. Other shortcoming of microwave implementation of TTD BFN includes crosstalk due to leakage in the microwave switches that results in reflections and irregularities of transmission characteristics.

The progress of radar technique at the beginning of the current century has led to the emergence and development of ultra-wideband radar systems. The typical examples are radars with low probability of interception of signals in which the carrier frequency of signals is rapidly reconstructed during operation in wide ranges, or radars using ultra-wideband probing signals allowing to receive an image of the object in the microwave range and distinguish close targets [4]. In addition, a number of radio electronic systems operating in different frequency bands are installed and simultaneously function on mobile carriers, in particular, the marine ones. In such systems, the application of TTD-based BFN was the only solution, which induced the numerous researches aimed to eliminating the drawbacks noted above. One example of advanced microwave beamforming schemes became so-called Rotman lens that is compact in size and provides true time delay [2]. However, this concept suffers from various additional losses, the main mechanism among which is beam-angledependent scanning loss that could reduce significantly the level of the main lobe of the PAA radiation pattern. Another intriguing concept, which is widely utilizing in the modern receiving PAA, is a processing at an intermediate frequency using a digital BFN [4]. Nevertheless, in the transmission PAA, where the delay is usually introduced into the microwave path, the issue of using the digital BFN is still open.

When creating such systems, combining the demands for various components of complex radar systems and ensuring the effective implementation of the required characteristics allow the use of approaches based on microwave photonics (MWP) technologies [5, 6]. At present, for incoming communication networks of fifth generation (5G), an extremely broad instantaneous bandwidth is required too, that is why ultra-wideband phase shifting or true-time-delay techniques must be used. In addition, enlarging the operating frequency of wireless fronthaul in the millimeter range is the mainstream research topic for 5G [7], which will be addressed in detail separately. On this way, MWP approach is extremely attractive for realizing multifunction PAA's optical BFN due to its superior instantaneous operating bandwidth, immunity to electromagnetic interference, lightweight, and reconfigurability [8].

Recently, we compared by NI AWRDE-based simulation, the three versions of photonics BFN arrangements using optical phase shifters, switchable optical delay lines, and the proposed arrangement based on a combination of multichannel

**49**

**Table 1.**

*The key engineering challenges facing 5G.*

*Design and Optimization of Photonics-Based Beamforming Networks for Ultra-Wide…*

**2. Microwave and millimeter-wave photonics technique in 5G** 

**No Feature Result**

1 Ultra densification Femtocell RoF architecture

2 Utilization of mmWave spectrum Microwave photonics-based circuit design

3 Mobile data traffic explosion 1000-fold factor over present-day systems

comes to the forefront

The next-generation wireless communications network (usually named as 5G) promises to deliver unprecedented data volumes and services for the mobile and fixed users representing both an evolution and a revolution of mobile technologies [10–13]. Some of these technologies are mainly architectural in nature—for example, moving some of the decision-making to the devices themselves (device-centric architectures and smart devices)—while others are more hardware oriented. The increasing demands for broadband services and the transmission of higher data rates have led to consideration of wireless links operating at higher carrier frequencies and extending well into the mmWave-band where total capacity of the single cell can approach some gigabits per second. **Table 1** lists three interconnected engineering challenges facing 5G [10, 14]. The first one is ultradensification of service areas and users. In the result, femtocell radio-over-fiber (RoF) architecture is proposed [15]. The second one includes utilization of mmWave spectrum [7]. Following it, microwave photonicsbased circuit design comes to the forefront. At last, the third one is mobile data traffic explosion. In the result, 1000-fold factor over present-day systems must be reached. As follows from the table, the ambitious goal to increase explosively mobile traffic is able to achieve by solving two global tasks: architectural referred to RoF and technological referred to MWP. Combining millimeter-wave band and RoF network architecture is one of the promising candidates to deliver intensive bitrate traffic with seamless convergence between optical backhaul and wireless fronthaul. In addition, RoF technique allows converting directly a lightwave spectrum to mmWave radio spectrum using a simple MWP-based up-conversion scheme [16], which is important to keep the remote cells flexible, cost effective, and power

fiber Bragg grating and switchable optical delay lines [9]. Continuing work of the direction, in this chapter, we review the worldwide progress referred to designing photonics-based BFN and highlight our last simulation results on design search of optimized photonics BFNs for next-generation ultra-wide millimeter-wave (mmWave) antenna arrays. In particular, Section 2 reviews the specialties of microwave and mmWave photonics technique in 5G wireless networks of radio-over-fiber (RoF) architecture. In addition, Section 3 presents theoretical background of array antenna beam steering using ideal models of phase shifters and TTD delay lines. There is a short analysis of updated photonics beamforming networks produced on optical fibers, Bragg gratings, or photonics integrated circuits (PIC) in Section 4. The principles and ways to optimize photonics BFN design is discussed in Section 5 based on the known photonics BFN scheme including set of optical delay lines and a novel structural and cost-efficient configuration that, following the results of the previous sections, consists of microwave photonics BFN using wavelength division multiplexing and TTD techniques. All schemes are modeled by NI AWRDE CAD

*DOI: http://dx.doi.org/10.5772/intechopen.80899*

tool. Finally, Section 6 concludes the chapter.

**wireless networks of RoF architecture**

*Design and Optimization of Photonics-Based Beamforming Networks for Ultra-Wide… DOI: http://dx.doi.org/10.5772/intechopen.80899*

fiber Bragg grating and switchable optical delay lines [9]. Continuing work of the direction, in this chapter, we review the worldwide progress referred to designing photonics-based BFN and highlight our last simulation results on design search of optimized photonics BFNs for next-generation ultra-wide millimeter-wave (mmWave) antenna arrays. In particular, Section 2 reviews the specialties of microwave and mmWave photonics technique in 5G wireless networks of radio-over-fiber (RoF) architecture. In addition, Section 3 presents theoretical background of array antenna beam steering using ideal models of phase shifters and TTD delay lines. There is a short analysis of updated photonics beamforming networks produced on optical fibers, Bragg gratings, or photonics integrated circuits (PIC) in Section 4. The principles and ways to optimize photonics BFN design is discussed in Section 5 based on the known photonics BFN scheme including set of optical delay lines and a novel structural and cost-efficient configuration that, following the results of the previous sections, consists of microwave photonics BFN using wavelength division multiplexing and TTD techniques. All schemes are modeled by NI AWRDE CAD tool. Finally, Section 6 concludes the chapter.
