**5. Principles and ways to photonics BFN design**

*Array Pattern Optimization*

**No Time-delay unit**

13 Dispersion

14 Linearly

15 Integrated ring resonators

integrated ring resonators

compensation fiber

chirped fiber Bragg grating

*Examples of TTD-based photonics BFNs.*

Phase modulation

Wavelengthdependent recirculating loop

Binary tree, MZM

12 TLS,

developer for PAA.

**Table 4.**

application in RSs of incoming RoF-based 5G networks. **Table 3** lists the primary requirements to a TTD-based MWP BFN from the point of view of a hardware

**Scheme Bandwidth Steering** 

**method, settling time**

TLS 5.9–17 GHz Continuously ±200 ps [35]

2.05 GHz Switchable, 1 ms

3–7 GHz Continuously 300 ps [34]

11.2 GHz Continuously 2500 ps [36]

**Delay range**

630 ps [37]

**Source**

journal and conference contributions have been published.

To meet the requirements of **Table 3**, a TTD-based photonics BFN should have enough bandwidth and delay range and support small level of settling time and crosstalk. Besides, it must be either continuously tunable or switchable with a sufficiently small sampling period. At present, many implementations of TTD-based photonics BFNs exist for PAA application. Such devices are often based on a set of fiber or integrated delay lines, ring resonators, spatial light modulators, semiconductor optical amplifiers, dispersive fibers, and so on. Optical channel is usually formed based on single-carrier technique using untunable laser or on multicarrier one with wavelength division multiplexing (WDM) using tunable laser source (TLS) and spectral multiplexer (MUX). RF-to-optical conversion is advantageously realized with the help of a Mach-Zehnder intensity modulator (MZM), but other types of optical modulators are also used. For reverse optical-to-RF conversion, pinphotodiodes are exclusively utilized. **Table 4** lists the key results of our search using

As one can see from table, the developed MWP-based BFNs provide time delays from tens of picoseconds to units of nanoseconds in the bandwidth up to tens of GHz. The results being presented allow us to conclude that it is possible to meet requirements 1 and 2 of **Table 3** by using a known approach based on the concept of weighted amplitudes and phases [38]. In particular, to precisely control loss and delay time, the optical fibers of a slightly different length (example no 5) and the dispersion effect in standard single-mode (example no 3), dispersion-compensated (examples nos 6 and 13) or photonics crystal fibers (example no 4) were in use at the early stage. Later, with the development of photonics integrated technology, which ensured a significant reduction in a device footprint and simplifying the complexity of feed network (see point 3 of **Table 3**), the switchable integrated silicon waveguides (example nos. 1 and 10) or the ring microresonators (example nos 8, 11, and 15) began to be exploited. In addition, if it is necessary to ensure a continuous adjustment of the delay time, a tunable TLS (example nos 2–4, 6, 8, and 12–14) is in common use. The requirement to reduce the mutual coupling (see point 4 of **Table 3**), usually quantified as crosstalk level, occurs in common elements of

**58**

In the process of design, a developer of new MWP-based RF apparatuses is facing a problem of choosing an appropriate software. As of today, the existing optical and optoelectronic CAD tools (OE-CAD) are not developed like being perfected for three decades CAD tools intended for modeling of RF circuits (E-CAD). On the contrary, operating at symbolic level modern high-power microwave E-CAD tool solves this problem enough simply and with high precision, but there are no models of specific active and passive photonics components in its library. To overcome this problem, we have proposed and validated experimentally a new approach to model a broad class of promising analog microwave radio-electronics systems based on microwave photonics technology. Guided by them, the electrical equivalent circuit models for the different types of semiconductor laser, photodetector, optical modulator, and so on were proposed and verified [39 and refs. cited there]. Using these components, a simple PAA's BFN was proposed and initially studied using NI AWRDE software [9]. Below, continuing work of the direction, we model a typical photonics BFN scheme including a set of switchable optical delay lines (see examples of **Table 4**), and a novel structural and cost-efficient configuration that, following the results of the previous sections, consists of microwave photonics BFN combining wavelength division multiplexing and TTD techniques.

### **5.1 The schematics for simulation**

**Figure 9** shows first photonics BFN schematic for comparison that is a part of 16-element PAA's feed network.

In this case, 16 unmodulated untunable lasers of different wavelengths λ1–λ<sup>16</sup> are used. Using the same RF signal, each transmission channel is converted by the

**Figure 9.**

*16-element RF photonics BFN based on switchable optical delay lines.*

corresponding Mach-Zehnder modulator (MZM) to optical range and shared into 16 branches by optical splitter (OS). Each branch consists of a switchable optical delay lines (ODL). Then, the delayed optical signals are summarized, converted into RF band by a photodiode (PD), and emitted by an ideal isotropic antenna element.

Important drawback of this scheme is the need to use a large number of lasers and MZMs (16 lasers and the same MZMs for a 16-element array), which makes it impractical due to the cumbersomeness and large energy consumption, even for such a relatively small PAA. In addition, according to the results of Section 3, each ODL must provide total delay of at least 71 ps and of digit capacity of at least 5 bits. That is, even when this BFN is implemented in the integrated version (see **Table 4**) using the waveguide material with the lowest losses [40], the difference in losses at the minimum and maximum step will be more than 30 times, which, according to Section 3, will lead to unacceptable distortions of the radiation pattern.

To overcome the above issues, **Figure 10** demonstrates an advanced photonics BFN scheme that is a part of the same 16-element PAA's feed network. In this case, only four unmodulated untunable lasers of different wavelengths λ1–λ4 in accordance to 200 GHz ITU WDM grid are used. Laser emissions are summarized in a spectral multiplexer (MUX), modulated in the common MZM by RF signal, and, through optical circulator, are input to four-channel reflected Bragg grating (RBG). The levels of corresponding delayed signals are recovered by an optical amplifier (OA) with an optical bandpass filter (OBF) after it, and shared into 16 branches by OS. Each branch consists of a 3-bit switchable ODL unit delayed once more optical signals for 2.3, 4.6, and 9.2 ps (see results of Section 3), a spectral demultiplexer (DMUX), 1 × 4 optical switch (OSW), a PD, and a PAA's antenna element. In addition, the schematic of 3-bit binary delay line is shown in **Figure 11**.

## **5.2 Models**

**Figures 12**, **13** demonstrate the equivalent models of the BFN schemes discussed above that are developed using the NI AWRDE microwave electronic CAD tool. The proposed scheme of **Figure 13** contains two units that can be implemented based on PICs: 3-bit optical delay line and a four-channel reflected Bragg grating module. The equivalent model of the first unit is shown in **Figure 14**. The NI AWRDE equivalent model of the second one was proposed and studied in detail elsewhere [41].

### **5.3 Simulation experiments**

With the help of the developed models, a number of simulation experiments were carried out, the main task of which was to check the stability of the proposed scheme for the nonideality of the transmission characteristics of the modules and units that make up its composition in according with the primary requirements to

#### **Figure 10.**

*16-element RF photonics BFN based on a combination of multichannel fiber Bragg grating and switchable optical delay lines.*

**61**

**Figure 12.**

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

a TTD-based MWP BFN (see **Table 3**). The key parameter providing the requirements of point 4 of this table is a crosstalk interference, the permissible level of which in the TTD-based photonics BFN is still poorly studied. For example, in the scheme of **Figure 10**, there are a number of sources of crosstalk interference, including insufficient isolation of the arms of an optical circulator, an OS, and a DMUX. Experiments were carried out on the basis of specific input data received and substantiated in Sections 2 and 3. First, the known and the proposed schemes containing ideally isolated arms were modeled. A comparison of their NRPs showed

*NI AWRDE model of 16-element RF photonics BFN based on switchable optical delay lines.*

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

*The schematic of 3-bit binary delay line.*

**Figure 11.**

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

**Figure 11.** *The schematic of 3-bit binary delay line.*

*Array Pattern Optimization*

corresponding Mach-Zehnder modulator (MZM) to optical range and shared into 16 branches by optical splitter (OS). Each branch consists of a switchable optical delay lines (ODL). Then, the delayed optical signals are summarized, converted into RF band by a photodiode (PD), and emitted by an ideal isotropic antenna element. Important drawback of this scheme is the need to use a large number of lasers and MZMs (16 lasers and the same MZMs for a 16-element array), which makes it impractical due to the cumbersomeness and large energy consumption, even for such a relatively small PAA. In addition, according to the results of Section 3, each ODL must provide total delay of at least 71 ps and of digit capacity of at least 5 bits. That is, even when this BFN is implemented in the integrated version (see **Table 4**) using the waveguide material with the lowest losses [40], the difference in losses at the minimum and maximum step will be more than 30 times, which, according to

Section 3, will lead to unacceptable distortions of the radiation pattern.

tion, the schematic of 3-bit binary delay line is shown in **Figure 11**.

To overcome the above issues, **Figure 10** demonstrates an advanced photonics BFN scheme that is a part of the same 16-element PAA's feed network. In this case, only four unmodulated untunable lasers of different wavelengths λ1–λ4 in accordance to 200 GHz ITU WDM grid are used. Laser emissions are summarized in a spectral multiplexer (MUX), modulated in the common MZM by RF signal, and, through optical circulator, are input to four-channel reflected Bragg grating (RBG). The levels of corresponding delayed signals are recovered by an optical amplifier (OA) with an optical bandpass filter (OBF) after it, and shared into 16 branches by OS. Each branch consists of a 3-bit switchable ODL unit delayed once more optical signals for 2.3, 4.6, and 9.2 ps (see results of Section 3), a spectral demultiplexer (DMUX), 1 × 4 optical switch (OSW), a PD, and a PAA's antenna element. In addi-

**Figures 12**, **13** demonstrate the equivalent models of the BFN schemes discussed above that are developed using the NI AWRDE microwave electronic CAD tool. The proposed scheme of **Figure 13** contains two units that can be implemented based on PICs: 3-bit optical delay line and a four-channel reflected Bragg grating module. The equivalent model of the first unit is shown in **Figure 14**. The NI AWRDE equivalent

model of the second one was proposed and studied in detail elsewhere [41].

With the help of the developed models, a number of simulation experiments were carried out, the main task of which was to check the stability of the proposed scheme for the nonideality of the transmission characteristics of the modules and units that make up its composition in according with the primary requirements to

*16-element RF photonics BFN based on a combination of multichannel fiber Bragg grating and switchable* 

**60**

**Figure 10.**

*optical delay lines.*

**5.2 Models**

**5.3 Simulation experiments**

a TTD-based MWP BFN (see **Table 3**). The key parameter providing the requirements of point 4 of this table is a crosstalk interference, the permissible level of which in the TTD-based photonics BFN is still poorly studied. For example, in the scheme of **Figure 10**, there are a number of sources of crosstalk interference, including insufficient isolation of the arms of an optical circulator, an OS, and a DMUX. Experiments were carried out on the basis of specific input data received and substantiated in Sections 2 and 3. First, the known and the proposed schemes containing ideally isolated arms were modeled. A comparison of their NRPs showed

#### **Figure 13.**

*NI AWRDE model of 16-element RF photonics BFN combining four-channel optical source, four-channel reflected Bragg grating, and 3-bit switchable optical delay lines.*

**Figure 14.**

*NI AWRDE model of 3-bit optical delay line.*

their complete identity. **Figure 15** exemplifies the calculation results of NRP characteristics for the both schemes under testing at the lower, middle, and upper frequencies of the PAA's operating range for beam deflection angles of 45° (a) and 30° (b). Comparison with the results of formal calculations given in Section 3 allows us to draw a conclusion about the correctness of the developed models.

Investigation of the effect of crosstalk interference showed the overall stability of the proposed scheme. **Figure 16** exemplifies the NRPs for the case of a joint effect of crosstalk (CS) in an optical circulator and amplitude asymmetry of the levels at the output of the optical splitter (AiOS). As on can see, their effect causes a phase shift of sidelobes, which leads to an increase in their level. However, their suppression meets the standard requirements for phased arrays.

**63**

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

In the chapter, we explored and demonstrated the availability of using the phased

This work was supported by the Russian Foundation for Basic Research, Grant

The authors declare the lack of the conflict of interest.

array antennas, which were known for a long time in the radar technique, in the incoming fifth-generation wireless communication systems. The study was carried out using a specific example of designing a photonics-steered beamforming network (BFN) of a transmitting-phased array antenna for a remote station operating in the V-band with a 30% fractional bandwidth allocated in the USA as a promising one for future 5G systems. For this goal, we first reviewed the specialties of microwave and millimeter-wave photonics technique in 5G wireless networks of radio-over-fiber architecture. Then, to determine the input data for subsequent design, a theoretical background of array antenna beam steering using ideal models of phase shifters and true-time-delay lines was presented. A brief analysis of updated optical beamforming networks produced on optical fibers, Bragg gratings or photonics integrated circuits, showed the possibility and efficiency of constructing the delay elements required for the device being developed, on the basis of photonics integrated circuits. The developed models and executed simulation of two versions of photonics BFN based on known scheme including set of optical delay lines and a novel structurally and cost-efficient configuration using wavelength division multiplexing and TTD techniques demonstrated the advantages of the proposed scheme from the point of view of the simplicity, key figures of merit, size, weight, and power features.

*Normalized radiation patterns of 16-element BFN combining 4-channel optical source, four-channel Bragg* 

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

**6. Conclusions**

*grating, and 3-bit switchable optical delay lines.*

**Figure 16.**

**Acknowledgements**

**Conflict of interest**

No. 17-57-10002.

**Figure 15.** *Normalized radiation patterns of the both TTD-steered 16-element photonics BFNs.*

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

#### **Figure 16.**

*Array Pattern Optimization*

**Figure 13.**

**Figure 14.**

*NI AWRDE model of 16-element RF photonics BFN combining four-channel optical source, four-channel* 

their complete identity. **Figure 15** exemplifies the calculation results of NRP characteristics for the both schemes under testing at the lower, middle, and upper frequencies of the PAA's operating range for beam deflection angles of 45° (a) and 30° (b). Comparison with the results of formal calculations given in Section 3 allows

Investigation of the effect of crosstalk interference showed the overall stability of the proposed scheme. **Figure 16** exemplifies the NRPs for the case of a joint effect of crosstalk (CS) in an optical circulator and amplitude asymmetry of the levels at the output of the optical splitter (AiOS). As on can see, their effect causes a phase shift of sidelobes, which leads to an increase in their level. However, their suppres-

us to draw a conclusion about the correctness of the developed models.

sion meets the standard requirements for phased arrays.

*Normalized radiation patterns of the both TTD-steered 16-element photonics BFNs.*

*reflected Bragg grating, and 3-bit switchable optical delay lines.*

*NI AWRDE model of 3-bit optical delay line.*

**62**

**Figure 15.**

*Normalized radiation patterns of 16-element BFN combining 4-channel optical source, four-channel Bragg grating, and 3-bit switchable optical delay lines.*
