**2. Microwave and millimeter-wave photonics technique in 5G wireless networks of RoF architecture**

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


#### **Table 1.** *The key engineering challenges facing 5G.*

*Array Pattern Optimization*

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

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

(TTD) negating the effect of the finite fill time of PAA aperture [2].

**48**

efficient. **Figure 1** demonstrates an example of mmWave RoF architecture, which consists of central office (CO), a remote or base station (RS) and wireless subscriber terminals (ST). CO is interactively connected with RS through fiber-optic cable, and RS is interactively connected with ST through wireless link. A typical position of RS is in the center of the service area; that is, for omnidirectional covering, four PAAs with an azimuth of 90° would be an optimal decision.

As shown in a large number of studies [7, 17, 18], mmWave 5G wireless network infrastructure must be erected with a lot of small cell sites controlled by the corresponding RS. In order to avoid inter-interference in these cells, one of the promising approaches is to equip the RS with a beam-steerable PAA (as in **Figure 1**), as has been practiced in radars for many years (see section "Introduction"). According to the estimates, mmWave RS would use PAA with hundreds of antenna elements to form directional beams for transmission and to receive similar beams from adjacent STs and RSs.

To implement effective radio communication within these cells, a number of leading countries have already developed a promising spectrum including mmWave-bands up to 100 GHz. **Figure 2** exemplifies USA assignation ranged from 27.5 to 95 GHz [19]. As follows from the figure, there is a continuous operating band between 57 and 76 GHz (fractional bandwidth of 30%), which will be used by us in the following treatment.

The final topic to be highlighted in this section is to design 5G RS's equipment using microwave and mmWave photonics techniques. Microwave photonics is an interdisciplinary scientific and technological field that combines the domains of microwave engineering and photonics. This field in the last 30 years has attracted immense interest and generated many new R&Ds from both the scientific community and the commercial sector. Emerging applications for 5G networks of RoF architecture indicate that MWP is set to be a subject of increasing importance ([8, 20, 21], and refs. cited there). By common opinion, MWP technology opens the way to super-wide bandwidth transmitting characteristics at lower size, weight, and power as compared with traditional electronic approach [22]. As an example, in a typical arrangement of MWP-based microwave transmitting unit (**Figure 3**), a photonics circuit is inserted between two microwave electronic chains typically including digital-analog converter (DAC), intermediate frequency and power amplifiers (IFA and PA, respectively), and antenna. For forward and reverse transformations of microwave and optical signals, there are two interfacing units at their bounds: electrical-to-optical (E/O) and opticalto-electrical (O/E) converters. Between the interfaces, there are various photonics processing units for frequency up-conversion, filtering, time delaying, beamforming, and so on, of microwave signals in optical domain.

**51**

environment NI AWRDE.

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

*The outcome*. The source data for posterior calculations of PAA are:

**3. Theoretical background of array antenna beam steering**

• the maximum azimuth steering of the main lobe deviation is ±45° (see **Figure 1**)

As noted in the Introduction, phased array antennas are now widely used in radar equipment due to the possibility of fast electron beam scanning and increased failure-resistant feature compared to continuous aperture antennas and mechanical scanning. The application of PAAs in radar allows achieving high speed of viewing the service area and tracking high-speed maneuvering objects [1]. Besides, PAAs ensure the operability of the radar system in a complicated interference situation due to the adaptive formation of a complex-shape radiation pattern [4]. In many cases, the use of array antenna let reduce the weight of the radar system and lower its total cost. In addition to radar, mmWave array antennas capable of operating in ultra-wide frequency range are considered as one of a key enabling technology for designing RS of 5G network, as noted in the previous section. There, a formation of a narrow steered beam by means of the antenna array makes it possible to increase the directive gain to compensate for the attenuation in the mmWave-band. Besides, the use of a narrow beam would reduce the interference effects from other closely spaced transmitters, and also provide the possibility of spatial multiplexing to increase throughput while simultaneously exchanging information with several STs. As described above, electronic scanning in the PAA is provided by a beamforming network, which includes phase shifters, or delay lines. The BFN supports a continuous or discrete beam movement in space due to phase control or signal time delay between the array elements. Below, a short theoretical study using ideal models will be presented pursuing the goal to define the complementary input data for the posterior design of the specific photonics-based BFNs for the ultra-wide mmWave-band PAA exploiting widespread microwave-electronic computer-aided design (CAD)

• the operating band is 57–76 GHz (see **Figure 2**)

*A typical arrangement of MWP-based microwave transmitting unit.*

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

*5G mmWave spectrum allocation of USA assignation.*

**Figure 2.**

**Figure 3.**

**Figure 1.** *An example of mmWave RoF architecture.*

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

#### **Figure 2.**

*Array Pattern Optimization*

STs and RSs.

the following treatment.

and so on, of microwave signals in optical domain.

efficient. **Figure 1** demonstrates an example of mmWave RoF architecture, which consists of central office (CO), a remote or base station (RS) and wireless subscriber terminals (ST). CO is interactively connected with RS through fiber-optic cable, and RS is interactively connected with ST through wireless link. A typical position of RS is in the center of the service area; that is, for omnidirectional cover-

As shown in a large number of studies [7, 17, 18], mmWave 5G wireless network infrastructure must be erected with a lot of small cell sites controlled by the corresponding RS. In order to avoid inter-interference in these cells, one of the promising approaches is to equip the RS with a beam-steerable PAA (as in **Figure 1**), as has been practiced in radars for many years (see section "Introduction"). According to the estimates, mmWave RS would use PAA with hundreds of antenna elements to form directional beams for transmission and to receive similar beams from adjacent

To implement effective radio communication within these cells, a number of leading countries have already developed a promising spectrum including mmWave-bands up to 100 GHz. **Figure 2** exemplifies USA assignation ranged from 27.5 to 95 GHz [19]. As follows from the figure, there is a continuous operating band between 57 and 76 GHz (fractional bandwidth of 30%), which will be used by us in

The final topic to be highlighted in this section is to design 5G RS's equipment using microwave and mmWave photonics techniques. Microwave photonics is an interdisciplinary scientific and technological field that combines the domains of microwave engineering and photonics. This field in the last 30 years has attracted immense interest and generated many new R&Ds from both the scientific community and the commercial sector. Emerging applications for 5G networks of RoF architecture indicate that MWP is set to be a subject of increasing importance ([8, 20, 21], and refs. cited there). By common opinion, MWP technology opens the way to super-wide bandwidth transmitting characteristics at lower size, weight, and power as compared with traditional electronic approach [22]. As an example, in a typical arrangement of MWP-based microwave transmitting unit (**Figure 3**), a photonics circuit is inserted between two microwave electronic chains typically including digital-analog converter (DAC), intermediate frequency and power amplifiers (IFA and PA, respectively), and antenna. For forward and reverse transformations of microwave and optical signals, there are two interfacing units at their bounds: electrical-to-optical (E/O) and opticalto-electrical (O/E) converters. Between the interfaces, there are various photonics processing units for frequency up-conversion, filtering, time delaying, beamforming,

ing, four PAAs with an azimuth of 90° would be an optimal decision.

**50**

**Figure 1.**

*An example of mmWave RoF architecture.*

*5G mmWave spectrum allocation of USA assignation.*

#### **Figure 3.**

*A typical arrangement of MWP-based microwave transmitting unit.*

*The outcome*. The source data for posterior calculations of PAA are:

