**2. Design factors of beamforming phased array antenna**

Phased array antenna is a multiple antenna system in which antenna elements are fed coherently with variable phase or amplitude control to provide for pattern shaping [14]. The concept of phased antenna array was introduced in military applications in the 1940s and first used for military applications for several decades. It improved the reception and transmission patterns of antennas and enabled the antenna system to be electronically steered to receive or transmit information from a particular direction without mechanically moving the structure, which cannot be obtained by any single antenna type. Furthermore, in applications for long distance communication, an antenna design with very high directive characteristics is required. As a single antenna usually provide low directivity and gain with wide radiation pattern [15], an array of antennas with high directivity gain is usually used to meet that demand. Recent growth in civilian radar-based sensors and communication systems has drawn increasing interest in utilizing phased array technology for commercial applications. In most cases, the antenna elements in an array are identical to conveniently adjust the directivity and other parameters of the array. Besides, the antenna elements can be arranged in different geometries to create different beams. The complexity of system increases according to the array geometry. The total beam of the array is a combination of the fields radiated by each antenna elements. The fields of elements will interfere constructively to reinforce in desired directions and interfere destructively to cancel in undesired directions in order to provide directive patterns. The influence of element fields on total beam is considered as array factor and the beam steering principle is based on the change of array factor. In addition, feeding techniques also contribute to the array performance and feed network is the most important component since it supplies the signals to the whole antenna structure and determines the amplitude and phase of electromagnetic waves in order to create the desired beam.

#### **2.1 Array geometry**

identification (RFID) [4], wireless local area networks (WLAN) [5], ultrawideband [6], ultra-sound [7], magnetic positioning [8], and audible sounds [9]. Among them, WLAN-based approach appears to be among the most suitable solutions owing to its wide range and the popularity of equipment. Nowadays, WLAN devices can be found in almost all civil and commercial properties due to their ease of installation and usage. The number of WLAN devices has reached billions and is continuously increasing day by day, as a result, the study in WLAN-based indoor positioning promise to be applied and spread in the near future. Besides, with the current 5G supports, this indoor positioning technology can even attain to longer

*Advanced Radio Frequency Antennas for Modern Communication and Medical Systems*

For WLAN-based approach, indoor localization techniques are classified into RSS scene analysis, ToA, TDoA, RToF, and AoA [1]. While the highly unstable feature of RSS in indoor environments is the major challenge of RSS scene analysis technique and ToA, TDoA, and RToF are based on a precise clock synchronization of devices, the AoA technique requires a directional antenna design to estimate the

There have been several antenna designs for AoA-based indoor localization presented in past few years. Giorgetti and Cidronali [10] introduced a switchedbeam directional antenna, including six circular antennas to cover six areas in a room. Rzymowski et al. [11] also introduced an antenna design using 12 passive elements electrically steerable parasitic array radiator antenna with one active monopole in the center of the ground plane. By controlling single-pole, single-throw switches, parasitic elements connect to the ground and act as reflectors, which change the main lobe's directions. With 12 passive elements, this antenna has 12 different directions. Kamarudin et al. [12] proposed reconfigurable antennas using PIN diodes to switch lumped components such as inductors and capacitors in order to change the structure of antennas and switch between four beams. Bui et al. [13] built a switch-beam array antenna based on 4 4 Butler matrix to create four beams toward four angles. It is found that these designs are based on switching between the limited number of predefined beams, which limits the resolution in determining the location of object and leads to significant errors in positioning. For indoor localization, the system needs to determine the location of object with high accuracy. Therefore, an antenna design with high-resolution steering beam ability is very essential in the AoA technique. As discussed above, the current antenna designs for indoor localization system mainly focus on switched-beam antenna structures. With these structures, the resolution of angle of arrival primarily depends on the number of antenna elements. Therefore, to create a system with high resolution of beam scanning, the number of antennas as well as switching elements such as PIN diode, FET also increases, which makes the system more cumbersome and complex. In order to improve the accuracy for AoA-based indoor localization, the aim of this chapter is to study and develop a phased array antenna at Wi-Fi band (2.4 to 2.484 GHz), capable of finely steering beam without

This chapter addresses the subject of a beamforming phased array antenna, which plays the vital role for AoA-based indoor positioning system. The array antenna is designed to steer the main beam in high resolution without the increase of antenna elements as well as switching elements. The design of this antenna array has focused on developing the controllable 360° continuous reflection type phase shifter. This chapter is organized as follows. In Section 2, we explain the principles of forming beam and several methods to feed antennas and to control the phase differences for phased array antennas. In Section 3, we discuss the structure of phased array antennas used for practical indoor localization, advancing from

reach, greater capacity, high accuracy, and reliability.

relative angle of object to reference points.

increasing the number of antennas.

**96**

As mentioned in [15], the geometrical configuration of the overall array may be linear, planar, circular, spherical, etc. For each case, the effective field distribution and mutual coupling will be different from one another. A linear phased array is where the antenna elements are aligned along a straight line, called the array axis. Normally, identical antennas with equal distances are selected in order to simplify calculation as well as beam control. An array of identical elements all of identical magnitude and each with a progressive phased is referred to as a uniform array [15]. The distance between two antenna elements is called element spacing d in **Figure 1a**. For linear phased array, the main beam can only scan along the x-axis, and the angle θ represents the "angle of arrival" of the radio waves. A planar phased array antenna is a set of antennas located in a plane with equal spacing between N elements (dy) in each column and M elements (dx) in each row, as shown in **Figure 1b**. By adjusting the magnitude and phase of incoming wave for each antenna, the main beam of this structure can be steered along two x- and y-axis. Therefore, it can provide a 2D angular scan, both horizontal ϕ and vertical θ scans. With this advantage, planar phased array antennas are being used in smart antenna

If a linear array has uniform amplitude, distances between two adjacent elements are identical with identical powers traveling to antennas. The *AF* is given as

*Total field patterns of two dipole antenna array with λ/4 element spacing and different phase excitation*

where *N* is the number of antenna elements, *d* is the element spacing, *β* is the phase difference of incident waves to antennas, and *θ* changes from 0 to 2*π*. The

control the maximum value of *AF* through controlling the phase difference *β*. That is the method to steer the main beam in the phased array antenna system. For AoAbased indoor localization, the use of uniform amplitude and spacing linear array is enough. For non-uniform amplitude linear array, planar array, circular array, and spherical array, *AF* is quite complicated and not really necessary but already intro-

When steering main beam toward the desired direction, some side lobes happen to be substantially larger in amplitude and reach the level of the main lobe. The lobe of maximum radiation toward unintended direction is known as a grating lobe which is undesirable in phased array antenna applications. During the transmission, if the energy does not focus in the desired direction, that means it is allocated in the direction of the grating lobes, the transmission distance is significantly reduced. In smart antenna systems where the user's direction is determined, the presence of grating lobes can cause the system to misidentify the user that means while transmitting, unwanted users are also treated as expected user, thus it affects information security or interferes other users. In a whole, to avoid the grating lobes,

1

where *λ* is the wavelength of operating frequency and *θ* is the angle between main beam direction and z-axis **Figure 1a**. However, in this chapter, the angle *θ* = 90° is used as original angel of main beam direction, so angle of main beam will be (*θ* – 90°). The scanning main beam angle range is designed from �60 to 60°.

*e j n*ð Þ �<sup>1</sup> ð Þ *kd cosθ*þ*<sup>β</sup>* (2)

2*πd*

<sup>1</sup> <sup>þ</sup> j j *cos<sup>θ</sup>* (3)

� �, which enables to

*AF* <sup>¼</sup> <sup>X</sup> *N*

*Beamforming Phased Array Antenna toward Indoor Positioning Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.93133*

maximum value of array factor is at an angle of *<sup>θ</sup><sup>m</sup>* <sup>¼</sup> *cos* �<sup>1</sup> *λβ*

element spacing *d* is required to meet the following condition:

*d λ* ≤

duced in [10, 11].

**Figure 2.**

*β =* �*90° [15].*

**2.3 Grating lobe**

**99**

*n*¼1

**Figure 1.** *Phased array geometry: (a) linear, (b) planar, (c) circular, and (d) spherical [15].*

and beamforming antenna system. In circular phased array, antennas also lie on the same plane but are arranged on a circular ring of radius *r* with N equally spaced elements (**Figure 1c**). Circular arrays are basically 1D linear arrays but in circular form and can steer beam in 2D. A spherical phased array consists of antenna elements arranged on the surface of sphere with uniform or non-uniformly spacing (**Figure 1d**). The element spacing is defined as the distance between two adjacent elements along the curved surface. With such geometry, spherical phased array may radiate in any direction and achieve omnidirectional or isotropic coverage. However, due to difficulties in model, design as well as fabrication, this geometry seems to not receive much less attention than above geometries.

#### **2.2 Array factor**

Another important factor – Array Factor (*AF*) depends on some parameters such as the number of antennas in array, geometrical arrangements, relative magnitude, phase difference, and element spacing. It is a mathematical factor representing the relationship between total field of array and the field of single element. If *Es* is the field of a single antenna and *AF* is the array factor, the total field *Etotal* at the farfield of the array can be calculated as [15]:

$$E\_{\text{total}} = E\_s \times AF \tag{1}$$

In case the array of two dipole antennas with element spacing *<sup>d</sup>* <sup>¼</sup> *<sup>λ</sup>* 4, the total fields are different corresponding to different phase excitation β shown in **Figure 2**. *Beamforming Phased Array Antenna toward Indoor Positioning Applications DOI: http://dx.doi.org/10.5772/intechopen.93133*

**Figure 2.** *Total field patterns of two dipole antenna array with λ/4 element spacing and different phase excitation β =* �*90° [15].*

If a linear array has uniform amplitude, distances between two adjacent elements are identical with identical powers traveling to antennas. The *AF* is given as

$$AF = \sum\_{n=1}^{N} e^{j(n-1)(kd\cos\theta + \beta)}\tag{2}$$

where *N* is the number of antenna elements, *d* is the element spacing, *β* is the phase difference of incident waves to antennas, and *θ* changes from 0 to 2*π*. The maximum value of array factor is at an angle of *<sup>θ</sup><sup>m</sup>* <sup>¼</sup> *cos* �<sup>1</sup> *λβ* 2*πd* � �, which enables to control the maximum value of *AF* through controlling the phase difference *β*. That is the method to steer the main beam in the phased array antenna system. For AoAbased indoor localization, the use of uniform amplitude and spacing linear array is enough. For non-uniform amplitude linear array, planar array, circular array, and spherical array, *AF* is quite complicated and not really necessary but already introduced in [10, 11].

#### **2.3 Grating lobe**

and beamforming antenna system. In circular phased array, antennas also lie on the same plane but are arranged on a circular ring of radius *r* with N equally spaced elements (**Figure 1c**). Circular arrays are basically 1D linear arrays but in circular form and can steer beam in 2D. A spherical phased array consists of antenna elements arranged on the surface of sphere with uniform or non-uniformly spacing (**Figure 1d**). The element spacing is defined as the distance between two adjacent elements along the curved surface. With such geometry, spherical phased array may radiate in any direction and achieve omnidirectional or isotropic coverage. However, due to difficulties in model, design as well as fabrication, this geometry seems

*Advanced Radio Frequency Antennas for Modern Communication and Medical Systems*

Another important factor – Array Factor (*AF*) depends on some parameters such as the number of antennas in array, geometrical arrangements, relative magnitude, phase difference, and element spacing. It is a mathematical factor representing the relationship between total field of array and the field of single element. If *Es* is the field of a single antenna and *AF* is the array factor, the total field *Etotal* at the far-

In case the array of two dipole antennas with element spacing *<sup>d</sup>* <sup>¼</sup> *<sup>λ</sup>*

fields are different corresponding to different phase excitation β shown in **Figure 2**.

*Etotal* ¼ *Es* � *AF* (1)

4, the total

to not receive much less attention than above geometries.

*Phased array geometry: (a) linear, (b) planar, (c) circular, and (d) spherical [15].*

field of the array can be calculated as [15]:

**2.2 Array factor**

**98**

**Figure 1.**

When steering main beam toward the desired direction, some side lobes happen to be substantially larger in amplitude and reach the level of the main lobe. The lobe of maximum radiation toward unintended direction is known as a grating lobe which is undesirable in phased array antenna applications. During the transmission, if the energy does not focus in the desired direction, that means it is allocated in the direction of the grating lobes, the transmission distance is significantly reduced. In smart antenna systems where the user's direction is determined, the presence of grating lobes can cause the system to misidentify the user that means while transmitting, unwanted users are also treated as expected user, thus it affects information security or interferes other users. In a whole, to avoid the grating lobes, element spacing *d* is required to meet the following condition:

$$\frac{d}{d\lambda} \le \frac{1}{1 + |\cos \theta|}\tag{3}$$

where *λ* is the wavelength of operating frequency and *θ* is the angle between main beam direction and z-axis **Figure 1a**. However, in this chapter, the angle *θ* = 90° is used as original angel of main beam direction, so angle of main beam will be (*θ* – 90°). The scanning main beam angle range is designed from �60 to 60°.

Expression (3) indicates that element spacing *d* in this case is smaller than 0.53λ. Therefore, to avoid grating lobes in scanning angle sector from 60 to 60°, an element spacing not more than 0.53λ is chosen.

#### **2.4 Mutual coupling**

When signals are transmitted to antenna arrays, the antenna elements will interfere each other, which is the so-called mutual coupling effect. The amount of coupling depends on the radiation characteristic, relative orientation of each antenna and spacing between elements. As discussed in [15], even if both antennas are transmitting, partial energy radiated will be received by other antennas because of the directional characteristics of practical antennas. Part of the incident energy on antenna elements may be backscattered in different directions, thereby allowing them to behave as secondary transmitters. In many cases, it is very complex to analyze and difficult to predict this effect but the coupling must be taken into account because of its significant contribution.

phase shifters is also cumulative, which can be an issue in the design of array with a large number of elements. Calculating and controlling the phase shift value of the phase shifters become elaborate as they depend on the length of the feed lines. In parallel feed, which are often called corporate feeds, the input signal is divided in a parallel tree network to all the antenna elements as shown in **Figure 3b**. The parallel feed is composed of power dividers and phase shifters followed by antenna elements. Power dividers split input signal to *N* signals with same amplitudes and same phases at output ports, then phase shifters control phase shift to each antenna. Thanks to the parallel structure, lengths of transmission lines do not affect phase shift. Therefore, it eliminates the major disadvantage of series feed, operating at only one frequency. Furthermore, because of parallel structure, the phase shifts and losses of phase shifters do not affect each other, which lead to simpler control circuit. However, the disadvantage of parallel feed is cumbersome as discussed above. Moreover, in applications requiring long transmission, transceiver modules including power amplifier, low noise power amplifier, and switch, are used. Sometimes, in array antenna system forming the beam based on amplitude control, power amplifiers with variable gain ability are needed. In practice, variable

*(a) Series feed network for phased array antenna; (b) parallel feed network for phased array antenna [14].*

*Beamforming Phased Array Antenna toward Indoor Positioning Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.93133*

**Figure 3.**

**Figure 4.**

**101**

*4 4 Butler matrix network [20].*

When two radiating elements are positioned along the E-plane, very small spacing exhibits the smallest coupling isolation, while the H-plane exhibits the small coupling for large spacing [15]. By selecting the correct distance, these fields can be decomposed to surface waves, and the spacing at which on plane coupling overtakes the other one also depends on the electrical properties and the geometrical dimensions of the microstrip antenna. In general, the element spacing should be designed to reduce the adverse effects of mutual coupling. According to [16], the spacing is recommended to be between 0.33λ and 0.5λ.

#### **2.5 Feed network**

The angle of radiation beams mainly depends on the range of phase difference at the feed lines to the antenna array. Meanwhile, the phase difference is directly generated from the feed network. In electronically scanned arrays or phased arrays, the feed networks for phase difference generation are typically realized using microwave circuit types such as Hybrid Coupler, Delay Line, Crossover, Power Divider, Phase Shifter, … Generally, all types of feed networks can be classified into the categories: constrained feeds, space feeds, and hybrid feeds. In an example for the space feed network, a lens array is fed by a single horn antenna located at an expected distance from the array [14] with the phase control at every element in the lens. The main advantage of this configuration is to reduce the cost and weight of the system compared with using hybrid feed, therefore it is applicable to lower cost ground-based arrays as well as very large space-based radar and communication system. However, this configuration is quite complex and requires the precision mechanical system to use the phase control at the objective aperture. Therefore, we select constrained feed which is commonly used for antenna array. The constrained feed can then be categorized into two basic types: series feed and parallel feed.

In series feed, antenna elements are placed in series along the feed line, and phase shifters can be inserted series either antennas or feed line, as shown in **Figure 3a**. The input signal is fed from one end of the feed network and then coupled serially to each antenna element. The compactness and low loss are two main advantages that make series feed more attractive than parallel feed. Additionally, the number of required phase shifters is also less than ones in parallel feed. However, bandwidth limitation is the main disadvantage of series feed. As the feed line is also treated as a delay line, the phase shifts on feed line are different at different frequencies. Therefore, the series feed only operates at designed frequency. Moreover, when phase shifter is placed on the feed line, the loss through

*Beamforming Phased Array Antenna toward Indoor Positioning Applications DOI: http://dx.doi.org/10.5772/intechopen.93133*

**Figure 3.** *(a) Series feed network for phased array antenna; (b) parallel feed network for phased array antenna [14].*

phase shifters is also cumulative, which can be an issue in the design of array with a large number of elements. Calculating and controlling the phase shift value of the phase shifters become elaborate as they depend on the length of the feed lines.

In parallel feed, which are often called corporate feeds, the input signal is divided in a parallel tree network to all the antenna elements as shown in **Figure 3b**. The parallel feed is composed of power dividers and phase shifters followed by antenna elements. Power dividers split input signal to *N* signals with same amplitudes and same phases at output ports, then phase shifters control phase shift to each antenna. Thanks to the parallel structure, lengths of transmission lines do not affect phase shift. Therefore, it eliminates the major disadvantage of series feed, operating at only one frequency. Furthermore, because of parallel structure, the phase shifts and losses of phase shifters do not affect each other, which lead to simpler control circuit. However, the disadvantage of parallel feed is cumbersome as discussed above. Moreover, in applications requiring long transmission, transceiver modules including power amplifier, low noise power amplifier, and switch, are used. Sometimes, in array antenna system forming the beam based on amplitude control, power amplifiers with variable gain ability are needed. In practice, variable

**Figure 4.** *4 4 Butler matrix network [20].*

Expression (3) indicates that element spacing *d* in this case is smaller than 0.53λ. Therefore, to avoid grating lobes in scanning angle sector from 60 to 60°, an

*Advanced Radio Frequency Antennas for Modern Communication and Medical Systems*

When signals are transmitted to antenna arrays, the antenna elements will interfere each other, which is the so-called mutual coupling effect. The amount of coupling depends on the radiation characteristic, relative orientation of each antenna and spacing between elements. As discussed in [15], even if both antennas are transmitting, partial energy radiated will be received by other antennas because of the directional characteristics of practical antennas. Part of the incident energy on antenna elements may be backscattered in different directions, thereby allowing them to behave as secondary transmitters. In many cases, it is very complex to analyze and difficult to predict this effect but the coupling must be taken into

When two radiating elements are positioned along the E-plane, very small spacing exhibits the smallest coupling isolation, while the H-plane exhibits the small coupling for large spacing [15]. By selecting the correct distance, these fields can be decomposed to surface waves, and the spacing at which on plane coupling overtakes the other one also depends on the electrical properties and the geometrical dimensions of the microstrip antenna. In general, the element spacing should be designed to reduce the adverse effects of mutual coupling. According to [16], the spacing is

The angle of radiation beams mainly depends on the range of phase difference at

the feed lines to the antenna array. Meanwhile, the phase difference is directly generated from the feed network. In electronically scanned arrays or phased arrays, the feed networks for phase difference generation are typically realized using microwave circuit types such as Hybrid Coupler, Delay Line, Crossover, Power Divider, Phase Shifter, … Generally, all types of feed networks can be classified into the categories: constrained feeds, space feeds, and hybrid feeds. In an example for the space feed network, a lens array is fed by a single horn antenna located at an expected distance from the array [14] with the phase control at every element in the lens. The main advantage of this configuration is to reduce the cost and weight of the system compared with using hybrid feed, therefore it is applicable to lower cost ground-based arrays as well as very large space-based radar and communication system. However, this configuration is quite complex and requires the precision mechanical system to use the phase control at the objective aperture. Therefore, we select constrained feed which is commonly used for antenna array. The constrained feed can then be categorized into two basic types: series feed and parallel feed. In series feed, antenna elements are placed in series along the feed line, and phase shifters can be inserted series either antennas or feed line, as shown in **Figure 3a**. The input signal is fed from one end of the feed network and then coupled serially to each antenna element. The compactness and low loss are two main advantages that make series feed more attractive than parallel feed. Additionally, the number of required phase shifters is also less than ones in parallel feed. However, bandwidth limitation is the main disadvantage of series feed. As the feed line is also treated as a delay line, the phase shifts on feed line are different at different frequencies. Therefore, the series feed only operates at designed frequency. Moreover, when phase shifter is placed on the feed line, the loss through

element spacing not more than 0.53λ is chosen.

account because of its significant contribution.

recommended to be between 0.33λ and 0.5λ.

**2.4 Mutual coupling**

**2.5 Feed network**

**100**

gain also enables to compensate the loss on phase shifter, of which loss value is not small.

Butler matrix is also an approach usually studied in phased array antennas [17–19]. The Butler matrix is a type of beamforming network and first described by Jesse Butler and Ralph Lowe in [20]. It has *N* inputs and *N* outputs; with *N* is usually 4, 8, and 16. One input signal fed from one of input ports goes through components of Butler matrix like couplers, phase shifters, crossovers in order to create phase difference of wave at N output ports, which combines with antenna elements to create beams with different predefined directions, as shown in **Figure 4**. Because beam direction is predefined, this system is also considered as a switched beam system. Nevertheless, there are two main drawbacks when using Butler matrix. First of all, the complexity and dimension significantly increase when *N* increases. That is why Butler matrix network is hardly designed with the number of input ports of 16 or more. Secondly, the number of beams is limited. With N inputs, this system can only create *N* beams with different directions, hence it is not suitable to provide beams with high resolution scan angle.
