Low Cost Phased Array Antennas for BWA Applications
Today, most broadband wireless access (BWA) customer premise equipment (CPE) use fixed-beam antennas. These are generally either reflector (dish), or flat-panel array antennas. These antennas have generally been selected based on value - meeting a minimum set of technical requirements at a minimum cost. To some degree, aesthetics is factored into the value equation, sometimes resulting in the selection of flat-panel antennas, even though they may be somewhat more expensive than reflectors for the same technical performance.
Now, a new factor has been added to the antenna value equation - the ability to minimize service calls through self-installation and self-healing, and to support remote system reconfiguration. That is, the value of a steerable beam antenna.
With current technology, the beam-steering feature can be realized by using mechanically-steered antennas, such as gimbaled dish or flat-plate antennas. However, these antennas cost significantly more than fixed versions, have reduced reliability due to mechanical complexity and operation in extreme outdoor environments, have relatively slow scan speeds, and often have very poor aesthetics. In fact, the environment often requires that a large radome be used to enclose the gimbaled antenna to shield it from ice, snow and wind. The resulting structure may be so large that many customers are unwilling to accept the antenna at all, they may be restricted by zoning regulations, or commercial property owners may charge a premium to permit their installation.
A more desirable form of the steerable antenna is a typical, conventional phased array. It consists of an array of small antennas in which phase shifters or delayers, arranged in a beamformer, are used to control the relative phases or time delays of the signals feeding the antennas. These phases or delays are varied so that the radiation pattern of the antenna array is reinforced in a desired direction and suppressed in undesired directions. The relative amplitudes of, and constructive and destructive interference effects among the signals radiated by the individual antennas determine the radiation pattern of the array. A phased array may be used to point a fixed radiation pattern beam, or to scan a beam rapidly in azimuth or elevation.
Phased arrays offer low physical profile for an attractive, flush or near-flush installation, and they permit extremely rapid beam-steering with few or no moving parts, as the phase-control devices are usually based on electronic components. Unfortunately, for a conventional phased array to achieve two-dimensional scanning as would be required for BWA CPEs, the electronic components in the transmit/receive (T/R) module must be replicated at each antenna element. These typically include a power amplifier, a low noise amplifier, a phase shifter and two circulators or T/R switches. In addition, a separate control line must address each T/R module, and the antenna control unit (ACU) must have the processing power to rapidly calculate phase values for all elements, and the I/O capacity to address them. Because such arrays can have tens to thousands of elements, the result is a very expensive antenna.
Because of the technical and cost limitations outlined here for existing steerable antennas, there is currently no acceptable steerable antenna solution for commercial applications where high reliability and relatively fast scan are required, and where low cost is paramount. These applications include BWA, and others such as broadband SATCOM user equipment. The latter includes antennas to track non-geostationary earth orbit (GEO) satellites, as well as potentially switching between GEO satellites.
FlexScan Low Cost Phased array
FlexScan™ technology fills the commercial market need for steerable, high gain, low cost phased array antennas. FlexScan can provide array gain between 14 and 36 dBi, depending on frequency and the size of the array. The beam pointing angle of the FlexScan array may be steered up to 50° off broadside in either one or two dimensions, and the beam pointing angle may be changed in as little as 10 ms.
FlexScan phased arrays are based on patent-pending e-tenna technology for variable delay lines, true-time-delay beamformers and integrated arrays. A block diagram for a typical FlexScan phased array is shown in Figure 1 .
Fig. 1 FlexScan phased array block diagram.
The FlexScan variable delay line (VDL) device is a transmission line whose propagation velocity can be adjusted via electrical control. It can be designed to achieve constant time delay over a very broad frequency range, as evidenced by the linearity of measured insertion phase for one such device, as shown in Figure 2 . This particular VDL achieves constant delay from 90 MHz to 18 GHz, implying a very broad operating bandwidth. FlexScan VDLs can be controlled by either an analog voltage or current, permitting infinitely fine (analog) delay adjustment. They can be fabricated using primarily printed-circuit techniques, resulting in a thin, low cost device that is readily integrated into a larger circuit such as an array beamformer or corporate power divider. The FlexScan VDL does not use any active RF devices, and thus achieves low RF loss, low passive intermodulation distortion and low cost. Overall losses are similar to fixed transmission lines, and are dominated by losses in the dielectric and conductor materials used in fabrication.
A key feature and design premise in achieving low cost for the FlexScan array is that the RF losses in the VDL are low enough that the array beamformer, built up out of VDL devices and fixed transmission lines, can be RF-passive and low loss, and thus require no amplification within the beamformer or between the beamformer and the array elements. As seen in the FlexScan block diagram, RF transmit or receive amplification occurs only within the radio (if co-located with the array), or in an optional IF converter. In either case, a single RF module replaces tens to hundreds of distributed T/R modules.
Fig. 2 Linear phase vs. frequency for FlexScan VDL.
Another cost-saving simplification in the FlexScan phased array is achieved by a unique arrangement of VDL devices within the FlexScan true-time-delay beamformer. This configuration permits two-dimensional scanning with any number of array elements by using only four unique control signals, which correspond to +x, -x, +y and -y control directions. This greatly simplifies the ACU and reduces its cost. Because of the infinitely-variable delay control of FlexScan VDLs, the resulting array has no inherent quantization error for beam pointing, as a conventional phased array typically does.
Actual beam pointing accuracy depends on physical details of the ACU, including the digital-to-analog converters within it, and the software and algorithms that select the beam position. Preferably, some quality-of-service (QoS) measure will be available from the radio, so that closed-loop feedback may be used to locate, fine-tune and maintain the beam pointing angle. In an environment with low co-channel interference, the QoS measure could be as simple as an automatic gain control voltage. A built-in test equipment (BITE) circuit may also be used to optimize the control signals, especially in cases where absolute-angle or open-loop beam pointing is required.
FlexScan phased array antennas are being developed, initially for applications in the MMDS band (2.6 GHz), ISM band (2.4 GHz) and other point-to-multipoint (PMP) bands (including 3.5 GHz). Demonstration arrays have been built for the MMDS and ISM bands. Measured antenna pattern data from this array, showing three scan positions covering ±30° in azimuth, are shown in Figure 3 .
Fig. 3 Scanned-beam patterns for FlexScan demonstration array at 2.4 GHz.
These demonstration arrays were built to validate engineering concepts and for field trial use. Engineering development is now underway to develop prototypes, starting with the MMDS band. Specifications for an initial FlexScan array are shown in Table 1 .
A fully-integrated, one-dimensionally scanned prototype to meet these specifications was completed in the first quarter of 2002, with a two-dimensionally scanned prototype to follow in the second quarter. Production availability is projected for the third quarter of 2002.
2.596 to 2.690
15° to 25°
Field of view
±45° Az, ±30° El
13" x 13" x 1.2"
BWA systems require high gain CPE antennas to ensure reliable, high data-rate communication. Parabolic reflector, or dish antennas, are typically used today. Because of their narrow beamwidths, they are difficult to align accurately. In addition, they are often considered unattractive to the point that significant effort is expended to hide them. There is a market need for an alternative antenna that can be pointed remotely and automatically, and which is low profile, unobtrusive and aesthetically pleasing. Such electronically steerable antennas offer numerous system performance advantages associated with dynamic system reconfiguration and load balancing, along with cost savings from eliminating service calls at initial installation and throughout the life of the system.
To date, no such antenna has existed, other than extremely expensive phased arrays used in aerospace and military applications. That paradigm has now been broken with the development of the FlexScan line of products. These low cost electronically steered arrays offer true-time-delay beamforming for ultra-broad bandwidth. Based on patent-pending e-tenna intellectual property, these antennas offer high gain, steerable beam performance for self-installing, self-aligning, self-healing and tracking applications in BWA and other markets.
e-tenna Corp., Laurel, MD (240) 456-4104.
Circle No. 301