**4. Microwave photonics beamforming networks**

As follows from Section 3, the optimal way to design ultra-wideband PAAs of mmWave-band is to steer the radar beam with TTD-based photonics BFN. Below, we will briefly review the current level of MWP BFN in order to select, using the data in **Table 2**, the optimal principle and scheme of its construction for efficient

**57**

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

1 Suppressing sidelobe level Preserve the directive gain of the

2 Null control Reduce the effects of interference

3 Simplifying the complexity of feed network Cost and power efficiency

**Scheme Bandwidth Steering** 

main beam

**method, settling time**

<1 μs

optical pump power

hopping 1 ns

switched, 20 ms

binary, 6-bit

thermooptical

Switchable, sampling period 120 ps

Switchable, 4 bit, 20 ns

8–12 GHz Continuously ±31 ps [27]

8–12 GHz Continuously ±43.3 ps [29]

8–12 GHz Discrete

56 GHz Continuously,

4–8 GHz Continuously,

6–18 GHz Polarization

18–26.5 GHz Switchable,

2.5 GHz Continuously,

3–7 GHz Switchable, few nanoseconds

Narrowband 1GHz

Narrowband, 42.7GHz and jamming signals

Enlarge the PAA's figures of merit

**Delay range**

Up to 2.5 ns

200 ps [25]

— [26]

6–178.4 ps [28]

443.3 ps [30]

1200 ps [31]

15.7 ps [33]

34 ps [33]

[32]

±200 ps 360 ps

**Source**

[24]

**No Requirement Result**

4 Reducing the mutual coupling between BFN

*Primary requirements to optical TTD-based microwave photonics BFNs.*

Binary, 2 × 2 switches, MZM

TLS and separate pump laser

TLS with MUX

Spatial light modulators

TLS, 8 channels, MZM

8 × 8 2D delay matrix

TLS, binary tree, 1 × 8, MZM

2D, 2 × 2 switches

Binary with 2 × 2 switches, MZM

Binary with 2 × 2 switches, MZM

TLS, four fiber channels, MZM

elements

**Table 3.**

**No Time-delay unit**

waveguide

chirped fiber Bragg grating

length fibers, 5 bit

compensation fiber

1 Silicon

2 Linearly

3 Dispersive fiber prism

4 Dispersive photonics crystal fibers

5 Match in

6 Dispersion

7 Holographic grating

8 Integrated ring resonators

9 FBG prism and fiberoptic delay lines matrix

10 Integrated waveguide

11 Integrated ring resonatorbased delay lines

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

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


#### **Table 3.**

*Array Pattern Optimization*

*FoMs for BSDL with various number of bits.*

**Table 2.**

**Figure 8.**

One possible way to characterize the amount of distortion is to simulate all possible scan angles and calculate maximum sidelobe level (SLL) using the above calculated SP values [4]. Peak and average values obtained through the entire scan range can be considered as figures of merit (FoMs) that determine the performance of BSDL. **Table 2** lists the results of calculations carried out by the above technique. **Figure 8** exemplifies the distortions for BSDLs with different number of bits,

**Number of bits SP (ps) Relative peak SLL (dB) Relative average SLL** 

4 4.7 −3.3 −5.2 5 2.3 −8.1 −10.9 6 1.1 −10.7 −12.4

**(dB)**

When using a 4-bit BSDL, a sharp increase in the level of the sidelobes, a decay in directivity, and a deviation of the beam position from the desired one are observed. However, it is acceptable to use a 5-bit BSDL with a sampling period of

1.The use of phase shifters in ultra-wideband antenna array leads to beam squint

2.Minimum interelement distance for an array that is operable at the frequency

3.The maximum time delay required to ensure scanning range of ±45° is 70.8 ps for the array under consideration and 5-bit BSDL is feasible to simplify TTDbased BFN construction. They provide a sampling period of 2.3 ps and a relative average sidelobe level of −10.9 dB over the entire scanning range (see **Table 2**).

As follows from Section 3, the optimal way to design ultra-wideband PAAs of mmWave-band is to steer the radar beam with TTD-based photonics BFN. Below, we will briefly review the current level of MWP BFN in order to select, using the data in **Table 2**, the optimal principle and scheme of its construction for efficient

which affect NRPs in different directions of scanning range.

*Examples of radiation pattern distortion due to quantization errors.*

To summarize, the following outcomes could be concluded:

range of 57–76 GHz without grating lobes is 2 mm.

**4. Microwave photonics beamforming networks**

phenomenon; so only TTD-based BFN is suitable in such case.

**56**

2.3 ps, for this case.

*Primary requirements to optical TTD-based microwave photonics BFNs.*



#### **Table 4.**

*Examples of TTD-based photonics BFNs.*

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 developer for PAA.

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 journal and conference contributions have been published.

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

**59**

**Figure 9.**

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

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

the optical channel, for example, in optical splitters or multiplexers. Its effect in photonics BFNs has been still poorly studied and will be considered in Section 5.3.

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

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

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

combining wavelength division multiplexing and TTD techniques.

**Figure 9** shows first photonics BFN schematic for comparison that is a part of

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

**5.1 The schematics for simulation**

16-element PAA's feed network.

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

the optical channel, for example, in optical splitters or multiplexers. Its effect in photonics BFNs has been still poorly studied and will be considered in Section 5.3.
