Multibeam antennas can provide increased wireless communications capacity with enhanced spectral efficiency and higher quality of service. One approach to designing such an antenna involves the use of space division multiple access (SDMA) techniques. SDMA methods provide high user capacity in a limited frequency spectrum without any major technological changes.1

SDMA techniques, which are implemented by most wireless service providers for optimum utilization of available spectrum, is typically restricted to three sectors in a 360-deg. coverage area. But using a multibeam antenna system, the number of sectors covered can be increased to as many as 48. Because the system's beamforming network can reuse available frequencies and mitigate interference, it can result in a greater number of subscribers with improved quality of service for a wireless network's service area.

The system can transmit data, voice, and video signals in multiple directions and at long distances without the need for repeater stations. The result is that network operating costs are minimized and reliability, quality, and subscriber numbers are all increased substantially. Instead of using a short-distance (low-gain) omnidirectional antenna, a long-distance (high-gain) narrow-beam directional antenna is used. Normally, a longdistance antenna would increase the number of subscribers in a single direction, but not allow subscribers in other directions from using the system. The system proposed in this article solves this problem through the use of multibeam techniques that can either simultaneously or sequentially re-employ a high-gain narrowbeam antenna to effectively achieve the spherical coverage of an omnidirectional antenna with a substantial increase of subscribers in all directions. A further increase in capacity can be achieved by frequency-reuse techniques.

The multibeam system is a hardware solution based on a phased-array antenna and proprietary Optibeam beamforming networks developed by Electromagnetic Technologies Industries, Inc. (ETI, www.etiworld.com). Because the hardware solution eliminates the need for software programming and external power sources, it is ideal for use in harsh environments.

The major components of the proposed multibeam antenna system are the antenna and the beamforming network. The antenna consists of small antenna elements, such as dipole or patch antennas, arranged in an array. The beamformer provides the required signal phase to all antenna elements in order to generate beams in various directions. The design parameters for both components are equally critical in achieving required performance for the multibeam antenna system.

The antenna used in the proposed system is based on patch antennas arranged in an array. The patch antennas are based upon proven microstrip high-frequency printed-circuit technology. The advantages of using a patch element in such an array arrangement include compact size, low manufacturing cost, low weight, ease of installation, and high reliability. Each element is fed with different signal amplitude and phase according to the desired direction of electromagnetic (EM) radiation. The different phases of the radiating elements then combine in the antenna's far field to form a narrow beam. The proposed antenna is designed as a linear phasedarray antenna system with equal interelement spacing and progressive phase shift across the array.2,3

The spacing between each element is maintained at a half-wavelength (λ/2} at the center frequency. The feed point is selected to be at the center line of the patch, although the exact position is determined by the experimental results of input reflection measurements performed with a high-frequency vector network analyzer (VNA). In addition to the feed point, the shape of each patch is carefully selected to achieve a voltage standing wave ratio of less than 1.50:1 at the frequency range of interest. For improved performance in the frequency range of interest, the feedpoint was selected to be slightly higher than the center point. Additional design parameters for the patch antenna elements include:

Resonant frequency = 3.7
GHz Substrate height = 0.030 in.
Substrate dielectric constant = 2.2
Patch antenna length = 1.575 in.
Patch antenna width = 0.710 in.
Feed point located slighter higher than the patch center point
Polarization = vertical

Many patch antennas are arranged linearly on a single dielectric substrate to achieve an azimuthal beamwidth of 15 deg. and a vertical beamwidth of 7 deg. The design of a four-beam antenna requires a minimum of four arrays of patch antenna elements.4 A four-beam system based on the proposed techniques was designed for antenna gain of 26 dB, front-to-back ratio of greater than 30 dB, and sidelobe levels of 20 dB lower than than main lobe level. The performance of a four-beam antenna design was verified by means of measurements using a commercial microwave VNA, with the results shown in Fig. 1 for a full sweep from 2.0 to 4.5 GHz. The operating range of the antenna system was found to be 3.2 to 4.2 GHz with VSWR of less than 1.50:1.

DESIGNING THE BEAMFORMER
A beamformer is a complex network comprised of passive microwave components. It is used to provide the required phase and amplitude of signals between the antenna and systems transceivers. A beamforming network shapes the beams from antenna arrays and steers their directions electronically without need for mechanical motion. Such an electronically steered beamforming network can be designed by considering time- or frequency-domain analysis of the antenna elements and associated electronic components. For the proposed multibeam antennas system, frequency- domain analysis was applied to the design of beamforming networks for broadband applications.5-8

In order to minimize the RF signal loss and maintain signal properties, such as phase and amplitude, a beamforming network is typically placed closed to the antenna assembly or integrated with the antenna assembly. In the present example, the beamformer was placed near the antenna and the phase across the array maintained using phase-matched cables (Fig. 2). These phase-matched cables provide phase-matching accuracy of 1 deg. across the desired frequency band. Each 36-in. length of cable contributes less than 0.5 dB insertion loss.

In this example, the beamformers were designed using a combination of quadrature couplers, microwave hybrids, and phase shifters in order to accomplish the phase requirements of generating four beams in a 60-deg. sector. Fully symmetrical 90-deg. hybrid junctions can be used for vector additions to create desired phase weights. The hybrids are integrated into the assembly, taking advantage of their inherent impedance transformations and reducing the overall insertion loss by minimizing the use of matching transformers.

To demonstrate the design approach, a four-beam antenna beamformer was designed for use in the 3.4-to-3.6-GHz band. Its performance was measured with a model N5230A vector network analyzer from Agilent Technologies (www.agilent.com) connected to the U3042A multiport test setup from Agilent Technologies, which also operates in the 3.4-to-3.6- GHz range. Figures 3, Figure 4, and Figure 5 show the results of a typical eight-beam beamforming network based on this design approach.

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The multibeam antenna system's radiation pattern was measured in an open environment in the frequency range of 3.4 to 3.6 GHz. The beamformer was connected to the antenna using phase-matched RF cables. The beamformer's input ports were supplied with four different center frequencies, each with a 7-MHz-wide channel, centered at 3.440, 3.480, 3.520 and 3.580 GHz. The input RF power used in the testing was +5 dBm and the received power from the antenna and beamformer combination was measured at a 200-m distance using a spectrum analyzer as a receiver. The received signal power was measured at every 1.0 deg. on a circle drawn with 200-m radius considering a four-beam antenna as the center of the circle. This practical radiation pattern is presented in Fig. 7. A theoretical radiation pattern is also shown in Fig. 6, generated by means of a MATLAB software simulation.

Based on the analysis of the fabricated four-beam antenna system, it should be possible to provide full 360-deg. coverage for wireless communications by using six such antenna systems. Potential application of the multibeam antenna approach is in Worldwide Interoperability for Microwave Access (WiMAX) and cellular networks. The approach can drastically increase the user capacity and spectral efficiency of such communications networks.

A SDMA-based multibeam antenna system such as the design presented here can dramatically increase communications network capacity and throughput by means of frequency reuse. The design approach is straightforward, with performance verified by means of commercial test equipment in an outdoor environment. The measured results compare favorably with theoretical results produced by MATLAB software simulations.

REFERENCES
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2. C. A. Balanis, Antenna Theory: Analysis and Design, 2nd ed., pp. 249-337, 722-779.
3. J. D. Kraus, Antennas, 2nd ed., McGraw-Hill, New York, 1988.
4. J. R. James and P. S. Hall, Handbook of Microstrip Antennas, INSPEC, Inc., June 1988, two-volume edition, ISBN-10: 0863411509.
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