3. Theoretical background of multiple-beam array antenna beam steering

As noted in chapter 2, mmWave array antennas capable of operating in ultrawide frequency range are considered as one of the key enabling technologies for designing RS of 5G NR network. There, a formation of a narrow steered beam by means of a PAA makes it possible to increase the directive gain to compensate for the excessive loss in the mmWave-band. Besides, the use of narrow beams would reduce the interference effects from other closely spaced mobile terminals and provides the possibility of spatial multiplexing to increase throughput while simultaneously exchanging information with several RSs.

Generally, electronic scanning in the PAA is provided by a beamforming network, which includes phase shifters or delay lines [1]. The BFN supports a continuous or discrete beam movement in space due to phase control or signal delay between the array elements. In our previous work devoted to the study of the PAA BFN [10], a single-beam PAA with electron scanning of the radiation pattern was considered. Nevertheless, for 5G pico-cells in conditions of simultaneous communication with a large number of terminal units, using a set of multiple-beam antenna (MBA) is considered to be a more practical way. PAAs based on MBA have greater functionality, but they are very complex, bulky, energy-consuming, and expensive devices. These factors limit their use to date mainly in special-purpose radars and unique satellite communication stations, for example, in satellite arrays of the iridium global mobile communication system [15]. There, PAA of the transponder has 106 channels and forms 16 fixed beams covering the contour-shaped

• Dynamic beamforming is employed, and hence, it mitigates higher path loss at

Due to these benefits, 5G mmWave is suitable for mobile communication over sub-6 GHz wireless technologies. The main disadvantages of 5G mmWave commu-

• Millimeter-wave goes through different severe losses such as penetration, rain attenuation, and even foliage. This limits distance coverage requirement in 5Gbased cellular mobile deployment. Moreover, path loss is proportional to the frequency squared. It supports about 200–300 m in outdoors based on channel

• Power consumption is higher due to the greater number of RF modules and antennas. To avoid this drawback, hybrid architecture, which has fewer RF chains than the number of antennas, needs to be used at the RS receiver chain.

These disadvantages must be considered during 5G mmWave link budget cal-

The source data for posterior calculations of multi-beam PAA in a pico-cell are:

• The base station is located on a separate mast of 3 m high in the geometric

• The elevation angle for the PAA under study must be such that the dead area

• The overall azimuth angle for the PAA under study is 360°.

center of the service area (see Figure 2).

around the mast does not exceed 1 m.

The drawbacks mentioned above led to the need for a radical change in the architecture of access networks compared to 4G LTE. In particular, instead of macro-cells, a multistage configuration was introduced, additionally containing micro-cells and pico-cells [3, 14]. In this direction, a newer RoF-based access networks of FiWi architecture is considered as the most promising approach (see item 3 of Table 1) ([9, 11]). The reason is that the important drawback for the implementation of the wired links, for example, of Fiber-to-the-Home (FTTH) architecture is feasible for fixed UTs only. In contrast, current wireless access networks of 4G LTE that provide a flexible communication with a relatively simple infrastructure cannot meet growing in geometric progression demands to increase the capacity of mobile systems. The most promising technique to meet it, which is actively discussed in the referred publications, is to expand the operating frequency band and to apply multi-position digital modulation of a radio frequency (RF) carrier through fiber fronthaul to simplify pico-cell RS layout. Figure 2 illustrates a typical pico-cell in a large city. The mmWave wireless network is managed from a remote station including one unidirectional PAA for downlink and uplink

• 5G mmWave networks support multi-gigabit backhaul up to 400 m and

mmWave frequencies.

Advances in Array Optimization

nication are the next:

culation.

channels.

112

2.1 The outcome

cellular access up to 200–300 m [13].

conditions and RS antenna height above the ground.

• It supports only LoS that limits the cell coverage.

the Earth's area. Each satellite has three such PAA, each of which forms its own sector. Thus, a set of 48 fixed satellite beams covers the Earth's area of about 4000 km in diameter.

As noted in [10], an appropriate beamforming scheme focusing the transmitted and/or received signal in a desired direction in order to overcome the unfavorable path loss is one of the key enablers for cellular communications in mmWave frequency bands. Depending on its layout, the beamforming weights required to form the directive beam could be applied in the digital or analog domain. Generally, digital beamforming provides a higher degree of freedom and offers better performance at the expense of increased complexity and cost because separate digital-toanalog converters, and analog-to-digital converters are required per each RF chain. Analog beamforming, on the other hand, is a simple and effective method of generating high beamforming gains from a large number of antennas but less flexible than digital counterpart.

For analog MBAs, BFN on the basis of multipole microwave circuits are usually applied. In particular, multipoles based on the Butler and Blass schemes are in common use since they are more compact than quasi-optical BFNs. In addition, they can be performed on printed circuit boards decreasing BFN's cost, size, weight, and power (C-SWaP) characteristics that are critical challenges in communication system design. For example, Butler matrix-based BFNs are exploited in the abovementioned iridium system. Currently, fixed-beam PAAs that use matrix BFNs based on a parallel circuitry (Butler matrix) and a serial circuitry (Blass matrix) [1] are being developed for photonics compatible mmWave small cell RSs of incoming 5G NR mobile communication networks.

demonstrates the block diagram (a) and BFN beam rosette (b) of the 8-element

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

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

The number of inputs of the matrix is equal to the number of outputs. The amplitude-phase distribution at the outputs of the Butler matrix is described by the

An <sup>¼</sup> <sup>1</sup>

Examples of the normalized radiation patterns for 4�4 (a) and 8�8 (b) Butler matrix.

ffiffiffiffi N <sup>p</sup> <sup>X</sup> N

cos <sup>φ</sup><sup>i</sup> <sup>¼</sup> <sup>i</sup> � <sup>N</sup> <sup>þ</sup> <sup>1</sup>

4�4 (a) and 8�8 (b) Butler matrix calculated by MATLAB software.

m¼1 e �j 2π

where N is the number of channels; m and n are the number of the inputs and outputs, respectively. It should be noted that Eq. (1) is essentially a fast Fourier

When connected to a linear equidistant PAA of N omnidirectional element, the Butler matrix forms N orthogonal beams, symmetrically located relative to the normal, with maxima in the azimuth directions φ<sup>i</sup> measured from the PAA broad-

> 2 � � λ

where λ is the operating wavelength and L is the PAA aperture. Moreover, the

Thus, due to the simplicity of the design and a relatively small number of elements, the Butler matrix is used in tasks that do not require the possibility of arbitrarily setting beam directions, for example, in covering the wide service sector of a wireless system.

The Blass matrix consists of directional couplers connected to the inputs and outputs using transmission lines with different fixed delays. The matrix can be used to supply signals to the PAA with an arbitrary number of elements; the number of

beams intersect each other at a level of �4 dB. As it follows from Eq. (2), the direction of the beams deviates when λ varies, that is, a so-called squint effect is observed. Besides, the fan of orthogonal beams shrinks with decreasing λ/L that is clearly seen in Figure 4 illustrating the normalized radiation patterns (NRP) for

<sup>N</sup>ð Þ <sup>m</sup>�<sup>1</sup> ð Þ <sup>n</sup>�<sup>1</sup> (1)

<sup>L</sup> , i <sup>¼</sup> <sup>1</sup>::N, (2)

Butler matrix.

Figure 4.

transform.

3.2 Blass matrix

115

side and determined by the formula:

following formula:

Following this, below, a short theoretical study using ideal models is presented pursuing the goal to define the optimum RS's omnidirectional antenna construction, type, and configuration of multi-beam matrix for its BFN and the input data for the posterior design and optimization of the specific photonics-based BFN for the mmWave-band PAA exploiting widespread computer-aided design (CAD) tools.

First, following [1], the schematics and characteristics of Butler and Blass matrixes are discussed below.

### 3.1 Butler matrix

The traditional RF-band layout of Butler matrix consists of quadrature hybrids, fixed phase shifters, and transmission lines between them. A matrix can be used to feed a PAA; the number of elements of which is a multiple of degree 2. Figure 3

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

Figure 4. Examples of the normalized radiation patterns for 4�4 (a) and 8�8 (b) Butler matrix.

demonstrates the block diagram (a) and BFN beam rosette (b) of the 8-element Butler matrix.

The number of inputs of the matrix is equal to the number of outputs. The amplitude-phase distribution at the outputs of the Butler matrix is described by the following formula:

$$A\_n = \frac{1}{\sqrt{N}} \sum\_{m=1}^{N} e^{-j\frac{2}{N}(m-1)(n-1)}\tag{1}$$

where N is the number of channels; m and n are the number of the inputs and outputs, respectively. It should be noted that Eq. (1) is essentially a fast Fourier transform.

When connected to a linear equidistant PAA of N omnidirectional element, the Butler matrix forms N orthogonal beams, symmetrically located relative to the normal, with maxima in the azimuth directions φ<sup>i</sup> measured from the PAA broadside and determined by the formula:

$$\cos \varphi\_i = \left( i - \frac{N+1}{2} \right) \frac{\lambda}{L}, i = \overline{1.N}, \tag{2}$$

where λ is the operating wavelength and L is the PAA aperture. Moreover, the beams intersect each other at a level of �4 dB. As it follows from Eq. (2), the direction of the beams deviates when λ varies, that is, a so-called squint effect is observed. Besides, the fan of orthogonal beams shrinks with decreasing λ/L that is clearly seen in Figure 4 illustrating the normalized radiation patterns (NRP) for 4�4 (a) and 8�8 (b) Butler matrix calculated by MATLAB software.

Thus, due to the simplicity of the design and a relatively small number of elements, the Butler matrix is used in tasks that do not require the possibility of arbitrarily setting beam directions, for example, in covering the wide service sector of a wireless system.

#### 3.2 Blass matrix

The Blass matrix consists of directional couplers connected to the inputs and outputs using transmission lines with different fixed delays. The matrix can be used to supply signals to the PAA with an arbitrary number of elements; the number of

the Earth's area. Each satellite has three such PAA, each of which forms its own sector. Thus, a set of 48 fixed satellite beams covers the Earth's area of about

As noted in [10], an appropriate beamforming scheme focusing the transmitted and/or received signal in a desired direction in order to overcome the unfavorable path loss is one of the key enablers for cellular communications in mmWave frequency bands. Depending on its layout, the beamforming weights required to form the directive beam could be applied in the digital or analog domain. Generally, digital beamforming provides a higher degree of freedom and offers better performance at the expense of increased complexity and cost because separate digital-toanalog converters, and analog-to-digital converters are required per each RF chain. Analog beamforming, on the other hand, is a simple and effective method of generating high beamforming gains from a large number of antennas but less

For analog MBAs, BFN on the basis of multipole microwave circuits are usually

abovementioned iridium system. Currently, fixed-beam PAAs that use matrix BFNs based on a parallel circuitry (Butler matrix) and a serial circuitry (Blass matrix) [1] are being developed for photonics compatible mmWave small cell RSs of incoming

Following this, below, a short theoretical study using ideal models is presented pursuing the goal to define the optimum RS's omnidirectional antenna construction, type, and configuration of multi-beam matrix for its BFN and the input data for the posterior design and optimization of the specific photonics-based BFN for the mmWave-band PAA exploiting widespread computer-aided design (CAD) tools. First, following [1], the schematics and characteristics of Butler and Blass

The traditional RF-band layout of Butler matrix consists of quadrature hybrids, fixed phase shifters, and transmission lines between them. A matrix can be used to feed a PAA; the number of elements of which is a multiple of degree 2. Figure 3

(a) Block diagram of 88 traditional Butler matrix and (b) corresponding BFN beam rosette.

applied. In particular, multipoles based on the Butler and Blass schemes are in common use since they are more compact than quasi-optical BFNs. In addition, they can be performed on printed circuit boards decreasing BFN's cost, size, weight, and power (C-SWaP) characteristics that are critical challenges in communication system design. For example, Butler matrix-based BFNs are exploited in the

4000 km in diameter.

Advances in Array Optimization

flexible than digital counterpart.

5G NR mobile communication networks.

matrixes are discussed below.

3.1 Butler matrix

Figure 3.

114

inputs can also be arbitrary and is determined by the required number of beams to be formed. The block diagram of the Blass matrix for three inputs and eight outputs, as well as the BFN beam rosette is shown in Figure 5.

The amplitude-phase distribution at the outputs of the Blass matrix with N inputs is determined by the delays of the transmission lines τmn and the levels of the signals branched off each of the directional couplers amn according to the formula:

$$A\_n = \sum\_{m=1}^{N} a\_{mn} e^{-j\alpha \tau\_{mn}},\tag{3}$$

should be abandoned in order to avoid creating significant interference outside the service sector. Thus, the 4�4 matrix makes it possible to effectively exploit only two beams, which is not enough for spatial multiplexing of communication channels under the conditions illustrated in Figure 2; it is necessary to use an 8�8 matrix with six active channels. A fan using six beams allows covering a sector of the order of 50° for the �4 dB level (see Figure 4), which provides a full 360° coverage with

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

According to [1], the radiation pattern of a PAA Dð Þ θ, φ is determined by the radiation pattern of a single antenna element f (θ, φ) and the array factor F(θ, φ) by

> fð Þ¼ θ, φ fð Þθ ∗ fð Þ φ fð Þ¼ φ const, <sup>f</sup>ð Þ¼ <sup>θ</sup> <sup>1</sup> <sup>þ</sup> cosð Þ <sup>π</sup> cos <sup>θ</sup> sin θ

For a one-dimensional linear equidistant 8�1 PAA with a distance between

Fð Þ¼ θ, φ Fð Þθ ∗ Fð Þ φ Fð Þ¼ θ const,

> Ane j 2πf c n λ0 <sup>2</sup> cos <sup>φ</sup>,

<sup>F</sup>ð Þ¼ <sup>φ</sup> <sup>X</sup> 8

Configuration of the antenna system for the mmWave pico-cell remote station under study.

n¼1

Dð Þ¼ θ, φ fð Þ θ, φ ∗ Fð Þ θ, φ , (4)

(5)

(6)

four PAAs mounted at 90° relative to each other, as shown in Figure 7.

Calculation of the radiation pattern in the elevation plane for one-dimensional PAA.

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

where θ is an elevation angle and φ is an azimuth.

the formula

Figure 6.

elements d = λ0/2

Figure 7.

117

For a half-wave dipole,

where m is the input number n is the output number.

Due to the fact that the RF signal from the input port sequentially passes through several directional couplers for feeding all the PAA elements, each coupler in the matrix must has the strictly defined value of the branch ratio, which greatly complicates the design. The configuration of the Blass matrix requires a larger number of directional couplers than Butler matrix, which increases its cost and often degrades the C-SWAP characteristics. However, due to the use of delay lines, the beams do not deviate from their position when the wavelength λ varies as it happens using the Butler matrix (see Eq. (2)). For this reason, the Blass matrix is better feasible for ultrawide band systems with a fractional bandwidth of more than 20%, as well as in systems requiring specific beam placement, for example, in satellite broadcasting equipment. Based on this outcome, in the course of further consideration of 5G mmWave MBA beam steering, only the BFN based on the Butler matrix will be studied.
