The Design of a 360°-Switched-Beam-Base Station Antenna

chapter Abstract The concept of switched-beam antenna (SBA) systems covering an area of 360° for wire- less base station applications is presented. First, a reconfigurable pattern antenna (RPA), which is composed of an omnidirectional slot-antenna array surrounded by an active cylindrical frequency selective surface (FSS), is studied. The behavior of FSS is controlled by PIN diodes which are able to divide the Azimuth plane into six sectors from one com- mon source. Unfortunately for a sector antenna, a huge number of diodes are required which complicates the structure in terms of efficiency and complexity. However, a simple and an efficient SBA configuration based on a hexagonal Fabry-Pérot cavity leaky wave antenna (FPC LWA) arrays is proposed as a solution for RPA problems. A sector-directive beam is generated from a simple patch antenna embedded inside a resonant Fabry-Pérot cavity with specific dimensions which have an influence on beamwidth and radiation efficiency. To increase more sectorization level and channel’s capacity, the proposed sector in FPC LWA arrays can be divided into three subsectors by using an active high- impedance surface (HIS). As a conclusion, SBA based on FPC LWA is the most suitable solution for future wireless communications. covering all azimuth planes. The proposed SBA structure, on FPC LWA, most for future wireless base station applications its


Introduction
Wireless communications are evolving to offer new generations of services such as video, satellite broadcasting, Internet, and mobile networks, which require high throughput to transmit multimedia signals with a good reliability [1]. In fact, these requirements force the improvement of the immunity of base station in front of many constraints such as multipath fading and interferences created between wireless networks due to an omnidirectional radiation pattern [2].
Beamforming is seen as a solution for the problems cited previously to ensure a high quality of service (QoS) and increased channel capacity using smart antenna systems [3].
Basically, a smart antenna system is based on the theory of phased antenna arrays wherein the differential phase between adjacent antennas permits the scan of radiation pattern. Depending on their architectures, these systems are divided into two categories based on transmit strategy: adaptive antennas and switched-beam antenna (SBA) [4].
Adaptive antennas are able to adapt their radiation patterns by exploiting the wireless radio channel at both transmitter and receiver sides with high precision using digital signal processing. Moreover, these adaptive systems have the ability to distinguish between user signal and interferences and also to steer the main beam in the desired direction with insertion zeros, thereby increasing the signal-to-interferences ratio [2].
However, the physical implementation of this system is very complex and expensive and requires high-power consumption and a longer time period to compute the current weight's values (amplitude and phase) for each antenna which is not suitable for high data rate communications [4].
On the other side, the SBA generates multiple fixed beams in a specific area. The behavior of these systems consists in the detection of the user's position by measuring signal strength, selects one of the predefined fixed beams and switches from one beam to another when the user moves [4].
In general, switching operation is assured by a fixed beamforming network, such as Blass matrix [5], Rotman lens [6,7], and Butler matrix [4], connected to a linear antenna array. When interference and useful signal are both located in the same beam, the SBA system is disabled to resolve this scenario compared to the adaptive antennas [8]. Thus, the SBA is less costly, but its complexity and losses increase when the number of beams is increased.
In addition, these two classical smart antenna systems do not provide 360° of coverage due to the printed circuit broad (PCB) technique used on the design of antenna which radiates above the ground plane of the structure allowing only half-plane coverage. Moreover, if the antenna element has an omnidirectional radiation pattern, such as monopole, the directivity will be divided into two opposed directions that are decreasing, which causes the radiated energy.
Two SBA configurations, allowing 360° coverage, are presented in this chapter, which is organized as follows. After the introduction, an RPA is presented using as a technique to create SBA, which is obtained from an omnidirectional source surrounded by an active FSS, wherein active elements are mounted on its surface. However, this type of configuration requires a huge number of switchers such as PIN diodes that complicate the structure. To overcome the use of the PIN diodes and achieve directive beam from one low-gain source, Section 3 presents a simple and an efficient hexagonal SBA configuration based on Fabry-Pérot cavity leaky wave antenna (FPC LWA) arrays. Finally, the last section concludes the chapter.

FSS unit cell
FSS is an artificial electromagnetic band gap (EBG) metamaterial periodic structures which has the ability to control the propagation of electromagnetic waves, as spatial filter, in a specific frequency band [4,9].
This behavior can be used as a beamforming technique for covering an area of 360° and creating a smart antenna. The proposed periodic unit-cell structure presented in Figure 1 is composed of a discontinuous metal strip printed on a flexible substrate where a PIN diode is inserted in the discontinuity [9].
According to the PIN diode state, the strip has a reconfigurable transmission/reflection coefficient [10]. When the diode is turned ON, it is equivalent to a resistor and acts as a reflector. While for OFF state, the diode is equivalent to parallel RC circuit allowing the strip to be transparent to the electric field of a transverse electric (TE) plane wave which is parallel to it [9,10]. A high reflection coefficient R is obtained for the ON state, whereas for the OFF state, a high transmission coefficient T is achieved.   [10]. The surface of the gap should be equal to the diode's dimensions.

Reconfigurable pattern antenna
In fact, RPA is achieved by placing an omnidirectional antenna in the center of the active cylindrical FSS. A sector notion is defined in this design by dividing the cylinder into two semi-cylinders. The first semi-cylinder is activated by turning its PIN diodes "ON" and behaves as a parabolic reflector which defines the directivity of radiation pattern. However, the other diodes are turned OFF and the second semicylinder will be transparent to the electromagnetic waves allowing radiation [10].
So, by controlling PIN diode states, a dynamic FPC antenna is obtained which is able to scan all azimuth planes from an omnidirectional source.

Omnidirectional slot-array antenna
If the source used is a simple dipole, the system will suffer from a low directivity due to the small aperture surface of FSS [11]. To enhance the directivity, authors in [10] have used a dipole array wherein pipes are used between two successive dipoles to ensure impedance matching and in-phase excitation of the array. Again, the use of pipes complicates the system in terms of adaptation.
To overcome problems cited above, a high-gain omnidirectional radiation pattern can be obtained from a printed slot array which is fed by a strip line embedded in sandwich between two substrates with the same thickness as shown in Figure 3. The radiation phenomenon is done from the two ground planes situated at the top and bottom of the structure [9].
Return loss and radiation pattern in H-and E-planes are shown in Figures 4 and 5, respectively. The structure operates at 5.8-GHz central frequency of Industrial Scientific Medical (ISM) band with 1.72% of the bandwidth.   Due to its constant values in all directions and according to Figure 5, the reconfigurable radiation pattern is located in H-plane and will be used to divide the azimuth plane in many fixed sectors.

Switched-beam antenna system
The whole reconfigurable antenna system, which is composed of the slot-antenna arrays surrounded by the active conformal FSS, is given in Figure 6.  A recent study reported in [12] demonstrates that six-sector deployments increase the channel capacity by 70-80% compared to traditional three sector sites. Therefore, FSS parameters, which are the number of strips N, the width of unit cell D x, and the radius R, are studied to satisfy this requirement using the following equations [10]: where P θ is the period angular and, in this case, is equal to 30°.
The activated strip number, whose PIN diodes are turned ON, has an influence on the H-plane beamwidth and equals 4, in the proposed design, to achieve a symmetrical half beamwidth of 70° as shown in Figure 7.
The effect of FSS radius on radiation pattern is presented in Figure 8 where the optimum value found is close to the half wavelength of the operating frequency in free space.    With these optimum parameters of FSS, a directional sector antenna is achieved offering 12 dBi of directivity with a wide beam in H-plane and a narrow beam in E-plane. The proposed antenna is surrounded by 12 strips along the antenna length and each strip requires 36 PIN diodes. As a result, a huge number of active elements are required, which decreases the radiation efficiency and increases the cost and complexity of the system.

An efficient hexagonal switched-beam antenna structure based on Fabry-Pérot cavity leaky wave antenna
SBA system based on a planar FPC LWA is proposed, in this section, as a solution to avoid the use of PIN diode on an RPA system.
In general, a planar resonant FPC is composed from an x-directed horizontal electric dipole (HED) backed with a ground plane and PRS superstrate placed on top of the structure [13,14] as shown in Figure 10.
This type of structure is seen as a leaky parallel waveguide or two-dimensional (2D) leaky wave antenna which is able to produce a pencil beam at broadside under a specific condition [14,15] h where h is the distance between the ground plane and PRS screen, λ 0 is the free space wavelength, ε r permittivity of the medium of propagation, and θ i is the incident wave angle.

Antenna Arrays and Beam-formation
For air-filled cavity (ε r = 1) and at broadside (θ i = 0°), the incident wave propagation has a normal angle to structure providing a distance h equal to half wavelength of the operating frequency in free space.
In this proposed design, a simple patch antenna, printed on the Rogers RT5880 substrate (permittivity ε r = 2.2 and thickness h = 1.575 mm), is used as HED and resonates at the frequency 5.8 GHz. However, PRS is made from an inductive metal strip grating (MSG) [16] screen printed on Rogers RT6010 superstrate (permittivity ε r =10.2 and thickness t = 2.5 mm) with the following parameters: width of strips w = 5 mm and period of strips P = 7 mm) [4,17].
The proposed FPC LWA is presented in Figure 11.

Azimuth division approach
Our sectorization approach consists of dividing the azimuth plane in N sectors where N is defined according to an angle φ formed between two successive cavities FPC LWA forming a specific shape [4,17].
By fixing the size on y-direction, a superstrate portion ΔL is added in two edges to interconnect the two adjacent PRS screens and, in this case, we talk about one-dimension (1D) FPC LWA which enhances the gain of the patch antenna by a factor about 3 dB [4].
Furthermore, PRS length controls the attenuation constant α of the propagation constant k inside cavity which has an influence on the radiation efficiency [18,19].
The leakage rate or attenuation coefficient α and radiation efficiency (η RA ) are obtained using the following formulas, respectively [20]: where S SLL is the side-lobe level (in dB) and L is the total length of PRS along x-direction.
The effect of sector number on antenna radiation efficiency is summarized in Table 1 and Figure 12.
The optimal radiation efficiency is achieved for six and three sectors and its values are close to the optimum radiation efficiency (87%) predicted in [20].
However, a directivity of 11.39 dBi is achieved for six sectors compared with 10.3 dBi for three sectors despite the large length of PRS [4]. This observation can be explained by the existence of an optimal size for the cavity which limits radiation efficiency.
The whole SBA FPC-LWA system is illustrated in Figure 13. It is composed of the hexagonal FPC LWA arrays connected to a Single Pole 6 Throws (SP6T) RF switcher [21] which switches the common input power between six outputs.

Number of sectors
Side-lobe levels (in dB)

Total length of PRS (in mm)
Radiation efficiency (%) The photograph of fabricated prototype is shown in Figure 14.
The simulation and experimental results of return loss are shown in Figure 15. From these curves, it can be concluded that a bandwidth of 100 MHz is obtained for a return loss below 10 dB. Due to the repetitive recurrence of the structure, all coverage directions have the same bandwidth which represents 1.46% of the total bandwidth at the central frequency 5.8 GHz of ISM band.
It can be seen also that measured result is shifted on the left from the simulation caused by the height of cavity which exceeds slightly a half wavelength found by Eq. (3).  Simulated and experimental radiation patterns in the azimuth plane or E-plane are shown in Figure 16 wherein a good agreement is carried out. A sector is defined by 3-dB beam width around 30°,which is the half value found in RPA [4], and according to [22], this beamwidth value allows to decrease the overlap between beams to 9 dB compared to 3 dB (the case of beamwidth of 60°) increasing thus signal-to-noise ratio (SNR).
The activate radiation pattern has a back-lobe level and a side lobe level of 15 dB. Some beams, in measurement, have high SLL which can be explained by the imperfect interconnection between two successive FPC LWA introducing parasite radiations.  It is important to see how the six beams cover all azimuth planes without any crossover between adjacent beams. Figure 17 presents the results of gain in the azimuth plane between 5.6 and 6 GHz found by measurement and simulation using two full-wave analyses Ansoft HFSS [23] and CST Microwave studio [24].
We noted an intersection between the measured value and the simulated one found by HFSS. This observation confirms our previous study which proposed that FPC LWA has a high radiation efficiency as presented in Section 3.1.
As result, a gain around 10 dBi is obtained for both simulation and measurement. However, a simulated value of CST Microwave studio is greater than HFSS by 1 dB, which can be explained by the different methods used in the simulation process.

Sector division
Dividing the sector into subsectors or small cells further enhances the channel's capacity by increasing the sectorization level. As an example, an active high-impedance surface (HIS) based on electromagnetic band gap mushroom-like structure, located on the ground plane of 50 ohms fed coaxial patch antenna [3], can be used as beamforming technique to scan the beam inside sector as shown in Figure 18.
Six PIN diodes are used as a switcher to control the surface-wave propagation according to the equation [3] β where β is phase coefficient of complex constant propagation k, k 0 is the free space wavenumber, and θ is the steered angle of the radiation pattern.
As seen previously in Section 2, the HIS ground plane is divided in two parts: one part acts as a reflector for TE surface wave by activating the corresponding PIN diodes. While other PIN diodes are turned OFF, allowing the propagation of TM surface waves which radiate with patches printed on top of the substrate.
By replacing the PRS of the hexagonal FPC LWA by the proposed active HIS, the whole system divides the Azimuth plane in 18 sectors, according to Figure 20, in simple ways using few PIN diodes [3].

Conclusion
SBA systems based on periodic metal structures have been presented in this chapter showing their ability in 360° of azimuthal coverage for base station applications. First, RPA has been studied which is composed of a linear slot-antenna array surrounded by an active FSS. In this design, PIN diodes are used as a switcher to control FSS response, giving an agility to omnidirectional radiation pattern and create SBA which divides the azimuth plane in six sectors. However, the major drawback of this system consists of a large number of active elements used which introduces losses and complicates the system's architecture.
For that, a hexagonal FPC LWA array has been proposed as solution to overcome problems related to PIN diodes. A high gain and good radiation efficiency are obtained from patch antenna, located inside a planar FPC, to avoid the use of antenna arrays with their complex feeding networks. An external SP6T RF switcher is required to be connected to FPC LWA arrays to switch the beam from a sector to another.
To increase more sectorization level, a reconfigurable HIS ground plane is proposed as a beamforming technique to divide the sector in three small cells. As a result, 18 beams are  achieved covering all azimuth planes. The proposed SBA structure, based on FPC LWA, is the most suitable for future wireless base station applications due to its simplicity.

Author details
Mohamed