5. Design principles and ways of integrated photonics-based millimeter-wave array beamformers

In general, photonics-based BFNs for PAAs have many potential advantages over their electrical counterparts [18, 19, 22], such as small size, low weight, no

Figure 9.

Advances in Array Optimization

Figure 10.

120

NRP at the center RF of 40.25 GHz using phase matrix (top) or time delay matrix (bottom).

NRP at the lower RF of 37.0 GHz using phase matrix (top) or time delay matrix (bottom).

susceptibility to electromagnetic interference, and, especially, wide instantaneous bandwidth, and squint-free array steering while using TTD concept. This section first reviews the state of the art in mmWave photonic beamforming concepts and technologies and their potential application in multiple-beam antennas. Following it an updated schematic of multiple-beam mmWave array feed networks using photonics integrated circuit (PIC) of optical Butler matrix is proposed and modeling by well-known software tool VPIphotonics Design Suite [23].

To date several optical beamforming architectures have been proposed using different technological implementations [10] such as free-space optics, fiber optics, or integrated optics. Among them, integrated photonic beamformers (IPBF) are of particular interest from the point of view of compactness and moderate implementation costs [21, 24–28]. In addition, their attractiveness is expected to increase as the RF signal frequency increases up to mmWave. Today, a number of reviews and research papers are devoted to the study of building principles for 5G NR small cells in the mmWave band [13, 21, 29, 30]. Table 3 highlights the main design principles and ways for mmWave IPBF.

• One of the most promising techniques for designing an RS's PAA is to use

Design of Reconfigurable Multiple-Beam Array Feed Network Based on Millimeter-Wave…

Generalized block diagram of photonics-based mmWave multiple-beam array feed network.

Analysis of the publications referenced in Table 3 allows us to draw a generalized block diagram of photonics-based mmWave multiple-beam array feed network

As follows from the Figure, the principal units are the laser sources (LS), optical modulators (OMs) performing the operation of electro-optical conversion, and the intensity of the output signal for which is controlled by the mmWave transmitter (TX). The output optical signals of the OMs are fed to a spatial distribution unit based on 88 optical Butler matrix. A photoreceiver unit (PRU) is connected to its outputs performing the operations of reverse optical-to-electrical conversion and amplification of the mmWave electrical signal to a level sufficient for reliable radio communication within the pico-cell of Figure 2, which is performed using the array antenna (AA). Note that the uplink channel between UT and RS is designed in a similar way and can be simplified using the reciprocity property of the Butler

In this work, the subject of the study is a mmWave multiple-beam array feed network, and the device of the study is an integrated optical Butler matrix. A tool for the computer simulation is the well-known commercial software VPIphotonics Design Suite™. In the course of the research, first of all, the accuracy of creating a mmWave 88 integrated OBM is checked. Then, the transmission quality of a mmWave multiple-beam array feed network using this OBM through the downlink channel for one of four sectors of the pico-cell RS (see Figures 2 and 7) is analyzed by the simulation in VPI and MATLAB software. Table 4 lists the reference data for the integrated OBM under study and the setup for its characterization. In addition,

According to the outcomes in the previous section, when analyzing with the help of MATLAB software, before modeling the integrated OBM using VPIphotonics Design Suite environment, it is worth checking the phase shifts provided by the equivalent delay elements based on integrated waveguides. Figure 13 depicts the model that consists of one delay-less arm and the four arms with library models of TriPleX-based integrated waveguides (ng = 2.016) providing phase shift of 22.5°, 45°, 67.5°, and 90°, correspondingly (see Figure 3a for the reference), and setup for the simulation experiments. In addition, there are two instrumental library models in the setup. The first one imitates optical transmitting module including library

Table 5 lists the reference data for the array feed network under analysis.

IPBFs based on a multiple-beam Butler matrix.

DOI: http://dx.doi.org/10.5772/intechopen.89076

for downlink channel of RS, which is shown in Figure 12.

5.1 Reference data for the simulation

5.2 CAD models and setups

matrix.

123

Figure 12.

The review of the referred sources allows us to conclude the following:



#### Table 3. Examples of mmWave IPBF.

Design of Reconfigurable Multiple-Beam Array Feed Network Based on Millimeter-Wave… DOI: http://dx.doi.org/10.5772/intechopen.89076

#### Figure 12.

susceptibility to electromagnetic interference, and, especially, wide instantaneous bandwidth, and squint-free array steering while using TTD concept. This section first reviews the state of the art in mmWave photonic beamforming concepts and technologies and their potential application in multiple-beam antennas. Following it an updated schematic of multiple-beam mmWave array feed networks using photonics integrated circuit (PIC) of optical Butler matrix is proposed and modeling by

To date several optical beamforming architectures have been proposed using different technological implementations [10] such as free-space optics, fiber optics, or integrated optics. Among them, integrated photonic beamformers (IPBF) are of particular interest from the point of view of compactness and moderate implementation costs [21, 24–28]. In addition, their attractiveness is expected to increase as the RF signal frequency increases up to mmWave. Today, a number of reviews and research papers are devoted to the study of building principles for 5G NR small cells in the mmWave band [13, 21, 29, 30]. Table 3 highlights the main design principles

The review of the referred sources allows us to conclude the following:

• The direction of mmWave IPBF is at the initial stage of its development. There are a small number of publications related to the research and development of

• There are two approaches to ensuring delays in an IPBF. The first is based on the transit time through the planar waveguide. The disadvantage of this method is the relatively large length of the waveguide, which leads to an attenuation of the signal and an increase in the dimensions of the beamformer.

However, this method is often used due to the ease of implementation.

• The second approach involves the use of optical ring resonators. Its main disadvantage is narrowing the bandwidth with increasing group delay time, which leads to the necessity of cascading elements to obtain feasible delays. Nevertheless, with the help of ring resonators, it is possible to obtain an order

Binary with 22 switches Narrowband

14 TTD binary tree 8.7 GHz at

Independent phase and amplitude control, four channels

22 Butler matrix Approximately

Scheme Bandwidth Steering

42.7 GHz

90 GHz

200 MHz

Narrowband, 60.8 GHz

161 TTD binary tree 2.5 GHz Thermal tuning 1200 ns [33]

88 Blass matrix — —— [33]

method, settling time

Switchable, 4 bit, 20 ns

Thermo-optic effect

Delay range

Thermal tuning 172.4 ps [32]

Fixed 100 ps [21]

15.7 ps [31]

45° [34]

Source

well-known software tool VPIphotonics Design Suite [23].

IPBF in the field of telecommunications.

of magnitude larger delay values.

No. Time delay unit

1 Integrated waveguide

2 Optical ring resonator

3 Integrated waveguide

4 Optical ring resonator

5 Integrated waveguide

6 Integrated PLC waveguide

Examples of mmWave IPBF.

Table 3.

122

and ways for mmWave IPBF.

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Generalized block diagram of photonics-based mmWave multiple-beam array feed network.

• One of the most promising techniques for designing an RS's PAA is to use IPBFs based on a multiple-beam Butler matrix.

Analysis of the publications referenced in Table 3 allows us to draw a generalized block diagram of photonics-based mmWave multiple-beam array feed network for downlink channel of RS, which is shown in Figure 12.

As follows from the Figure, the principal units are the laser sources (LS), optical modulators (OMs) performing the operation of electro-optical conversion, and the intensity of the output signal for which is controlled by the mmWave transmitter (TX). The output optical signals of the OMs are fed to a spatial distribution unit based on 88 optical Butler matrix. A photoreceiver unit (PRU) is connected to its outputs performing the operations of reverse optical-to-electrical conversion and amplification of the mmWave electrical signal to a level sufficient for reliable radio communication within the pico-cell of Figure 2, which is performed using the array antenna (AA). Note that the uplink channel between UT and RS is designed in a similar way and can be simplified using the reciprocity property of the Butler matrix.

### 5.1 Reference data for the simulation

In this work, the subject of the study is a mmWave multiple-beam array feed network, and the device of the study is an integrated optical Butler matrix. A tool for the computer simulation is the well-known commercial software VPIphotonics Design Suite™. In the course of the research, first of all, the accuracy of creating a mmWave 88 integrated OBM is checked. Then, the transmission quality of a mmWave multiple-beam array feed network using this OBM through the downlink channel for one of four sectors of the pico-cell RS (see Figures 2 and 7) is analyzed by the simulation in VPI and MATLAB software. Table 4 lists the reference data for the integrated OBM under study and the setup for its characterization. In addition, Table 5 lists the reference data for the array feed network under analysis.

#### 5.2 CAD models and setups

According to the outcomes in the previous section, when analyzing with the help of MATLAB software, before modeling the integrated OBM using VPIphotonics Design Suite environment, it is worth checking the phase shifts provided by the equivalent delay elements based on integrated waveguides. Figure 13 depicts the model that consists of one delay-less arm and the four arms with library models of TriPleX-based integrated waveguides (ng = 2.016) providing phase shift of 22.5°, 45°, 67.5°, and 90°, correspondingly (see Figure 3a for the reference), and setup for the simulation experiments. In addition, there are two instrumental library models in the setup. The first one imitates optical transmitting module including library


TriPleX technology. The internal scheme of the galactic module is presented in Figure 15. In addition, the setup of Figure 14 includes two instrumental library

Design of Reconfigurable Multiple-Beam Array Feed Network Based on Millimeter-Wave…

models, which are the same as in Figure 13.

DOI: http://dx.doi.org/10.5772/intechopen.89076

Figure 13.

Figure 14.

Figure 15.

125

Equivalent delay elements of integrated OBM.

The model and setup for simulation of 88 PIC-based OBM.

The internal scheme for the galactic module G of a quadrature optical hybrid.

## Table 4.

The reference data for the OBM under study and the setup for its characterization.


## Table 5.

The reference data for the array feed network under analysis.

models of laser source and optical modulator EA controlled by RF generator tuning in the band of 37.5–41.0 GHz. The second one imitates optical receiving module including library models of PIN photodiode and RF network analyzer recording amplitude and phase RF signal distribution at the photodiode output. One can see their relevant parameters in Table 4.

Then, Figure 14 depicts the model and setup of 88 OBM that in according to Figure 3a contains the models of quadrature optical hybrids (QOH) and library models of the straight waveguide as a phase shifter.

Due to the lack of a suitable library model in this software tool, QOH is designed as a so-called "galactic" module G, containing, in accordance with a typical circuitry of an electrical analog, library models of two optical X-couplers and two optical straight waveguides with 90° phase shift. Both elements are carried out based on

Design of Reconfigurable Multiple-Beam Array Feed Network Based on Millimeter-Wave… DOI: http://dx.doi.org/10.5772/intechopen.89076

TriPleX technology. The internal scheme of the galactic module is presented in Figure 15. In addition, the setup of Figure 14 includes two instrumental library models, which are the same as in Figure 13.

Figure 13. Equivalent delay elements of integrated OBM.

Figure 14. The model and setup for simulation of 88 PIC-based OBM.

Figure 15. The internal scheme for the galactic module G of a quadrature optical hybrid.

models of laser source and optical modulator EA controlled by RF generator tuning in the band of 37.5–41.0 GHz. The second one imitates optical receiving module including library models of PIN photodiode and RF network analyzer recording amplitude and phase RF signal distribution at the photodiode output. One can see

Parameter Value Number of optical inputs 8 Number of optical outputs 8 Band of RF carrier frequencies 37.5–41.0 GHz Input RF power 11 to 26 dBm Material platform for IPBF TriPleX (Si3N4/SiO2) [35] PIN photodiode Responsivity 0.92 A/W

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Laser source Optical carrier 193.1 THz

Optical modulator Principle Electro-absorption

Parameter Value Overall number of mobile UTs in the pico-cell 72 Number of mobile UTs in one sector 18 Number of PAA sectors 4 (see Figure 7) Number of PAA beams in one sector 6 Number of RF carrier frequencies 6 Band of RF carrier frequencies 38.0–40.5 Spacing of RF carrier frequencies 0.5 GHz

The reference data for the OBM under study and the setup for its characterization.

Dark current 100 nA 3 dB bandwidth 50 GHz Optical input power <3 mW

Average power 50 mW Linewidth 10 kHz

Spectral range C band Modulation index 0.5 Chirp factor 0

Modulation type Intensity, double sideband

Then, Figure 14 depicts the model and setup of 88 OBM that in according to Figure 3a contains the models of quadrature optical hybrids (QOH) and library

Due to the lack of a suitable library model in this software tool, QOH is designed as a so-called "galactic" module G, containing, in accordance with a typical circuitry of an electrical analog, library models of two optical X-couplers and two optical straight waveguides with 90° phase shift. Both elements are carried out based on

their relevant parameters in Table 4.

Table 4.

Table 5.

124

models of the straight waveguide as a phase shifter.

The reference data for the array feed network under analysis.

downlink channel. Table 7 exemplifies the simulation results of phase error values

Design of Reconfigurable Multiple-Beam Array Feed Network Based on Millimeter-Wave…

Finally, a simulation experiment for the mmWave multiple-beam array feed network of Figure 16 was carried out. Figure 17 exemplifies the calculation results of the back-baffled normalized radiation patterns generated at the central and two extreme frequencies of the input RF band based on the data for the amplitude and

Normalized radiation patterns for the mmWave multiple-beam array feed network under test (a) at 39.5 GHz

for channel A6 (see Figure 16) at the corresponding outputs.

DOI: http://dx.doi.org/10.5772/intechopen.89076

Figure 17.

127

(b) at 38 GHz (c) at 40.5 GHz.

Figure 16. The model and setup for simulation of mmWave multiple-beam array feed network.

The module of Figure 15 contains a set of PIC library models, such as two Y-branches (YB), four straight waveguides (SW) including two SW for 90° phase shift, and two compensating SW with equivalent phase shift of 360°, six 90° waveguide bends (WB), one waveguide crossing element (WC), and two X-couplers (XC).

Finally, Figure 16 depicts the model and setup for the mmWave multiple-beam array feed network under study that contains the model of 88 OBM (see Figure 14) with six inputs because as shown in subsection 3.1, the extreme beams generated by the Butler matrix (A2 and A7 in Figure 3) have a significantly greater width and less directivity than the others do (see Figure 4). In addition, there are two instrumental library models in the setup. The first one imitates optical transmitting module including library models of laser source and six optical modulators controlled by six RF generators, the RF carriers of which are allocated in the band of 38.0–40.5 GHz. The second one imitates optical receiving module including library models of eight PIN photodiodes and eight RF network analyzers recording amplitude and phase RF signal distributions at the photodiode outputs. One can see their relevant parameters in Tables 4 and 5.
