Communications: directive radio wave systems and devices (e.g. – Directive – Including a steerable array
Reexamination Certificate
2001-03-22
2003-04-15
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a steerable array
C342S372000, C342S375000
Reexamination Certificate
active
06549164
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an antenna system adapted to provide alignment of several antenna apertures in time to enhance the signal-to-noise ratio in antenna systems.
BACKGROUND OF THE INVENTION
Steerable beam antenna systems typically consist of two basic types-reflector antennas and phased arrays. Although other antenna types exist, such as lens antennas, reflector and phased array antennas are by far the two most common.
Reflector antennas are simple and well understood and make up a significant portion of high gain antenna systems. In order to steer a reflector antenna, a mechanical movement of the entire antenna is usually necessary, although other means such as mechanical or electrical displacement of the feed have also been used. The structure which supports the reflector surface must provide certain precision to maximize the gain of the reflector. Surface deformation considerations can also cause the structural requirements to increase significantly as the size of the antenna increases.
In phased array antennas, the beam is steered electronically and the speed of beam motion is considerably faster than for a reflector antenna, especially for large regions of coverage. However, phased array antennas have several drawbacks. For example, they are typically much more expensive than reflector antennas, the signals sent and received at each element of the array must be phase and time aligned, and the gain of a phased array antenna decreases as the beam is steered off of the antenna boresight.
Current methods for automatic phase aligning a signal compare the phase of two signals using phase detectors, then adjust a phase adjuster associated with one of the signals until a phase difference is no longer detected. This phase alignment of signals from individual elements enhances the signal to noise ratio of the combined received signal from the array antenna. However, if the antenna is receiving a broadband signal, either a high data rate or composite multiple channel signal phase alignment may not result in an optimized signal to noise ratio, since phase alignment can occur in integer wavelength offsets. For relatively small phased array panels, phase alignment alone may be sufficient, as the distance between the center elements and edge elements may not be large enough to result in an integer wavelength offset between signals received at the elements. However, as the panel size in a phased array increases, the distance between the center and edge elements may be large enough to result in an integer wavelength offset between signals from the center elements and edge elements. Thus, for relatively large phased array panels, or widely spaced panels the signals also need to be aligned in time as well as phase to achieve an optimized signal-to-noise ratio for such a system. Signals which require both time and phase alignment to achieve an optimized signal to noise ratio are referred to as broadband signals. To compensate for this possibility, current methods for time aligning signals include using one or more reference signals at different frequencies to determine the required time offset. An external reference signal is sometimes used, which is a signal from a source external to the antenna apparatus which has a set, known frequency which is used to determine time offset between elements in the array. Alternatively, an internal reference signal may be used, which is a signal generated from within the antenna apparatus which is used to determine time offset between elements in the array.
Such a method is described in U.S. Pat. No. 5,041,836. In such a system an external beacon signal separate from the received signal is used to determine the amount of time adjustment required for each antenna element. The separate signals from the elements are first phase aligned, then the beacon signal is checked for phase alignment. The phase detector output will be proportional to the frequency ratio of the received signal and the beacon signal times the number of wavelengths of time delay difference in the received signal at the elements. While this system is successful in time aligning a broadband signal, it has drawbacks. For example, the maximum time delay error which can be detected is a function of the ratio of the frequency of the received signal and the frequency of the beacon signal. Thus, in an example shown in the above-mentioned patent, if the frequency of the received channel is chosen from 7.25-7.31 Ghz, and the beacon frequency is either 7.590 or 7.615 Ghz, the maximum time difference detectable is +/−11 wavelengths, and the maximum uncertainty in the absolute position of the elements must be within 18 inches. If larger time differences or uncertainties in position are required in an application, additional beacon frequencies may be used, or a larger difference in the received signal and beacon signal frequency can be used. If additional beacon signals are used, additional hardware is required, and if a larger difference in frequencies is used ambiguity may result in the smallest time delay bits. Thus, while allowing time alignment of broadband signals, this method requires additional hardware associated with the use of the one or more beacon frequencies, and is limited by ambiguity issues.
Digital hardware may also be used to determine required time offsets needed for each element of an antenna system. In such a case, a digital signal processor analyzes the signals from each element and determines the amount of phase and time shift for each element required to phase and time align all of the elements. In such a case, the digital processing hardware must be used, which can increase the cost of the system, and may also be limited by the signal processing capacity of the digital signal processor.
As mentioned above, the gain of a phased array decreases as the beam is steered off boresight. Due to this decrease in gain, phased array antennas typically are limited to scanning up to 60 conical degrees off the antenna boresight. Additionally, arrays are typically scaled to compensate for this scan loss by adding additional elements or amplifiers, which increases the cost of such an antenna. In order to increase the region of coverage beyond 60 degrees, often several apertures are used with each separate aperture including a separate phased array antenna. In such a case, the separate apertures are placed at angles to one another, with the signal being handed off from one aperture to an adjacent aperture when the scan angle to the first aperture becomes too large. The addition of other apertures allows scan angles beyond 60 degrees, with the signal typically being handed off between adjacent apertures at a scan angle to where the power level is equal between the adjacent faces. While this technique allows larger regions of coverage, several problems can be encountered when a beam is handed off between apertures. For example, phase coherency can be lost, bit synchronization can be lost, and there can be carrier and data drops during a signal handoff between apertures.
SUMMARY OF THE INVENTION
In accordance with the present invention, an antenna apparatus is disclosed that can determine phase and time delay between elements of a single phased array, or between apertures of a multiple aperture phased array antenna without the need for an independent external or internal reference signal. The phase and time delay can be determined using only a single received signal. Thus, there is no need for a separate beacon signal to be received at the antenna apparatus, nor is there a need to generate a separate reference signal within the antenna apparatus. The antenna apparatus includes an array of antenna elements for a single panel antenna, or multiple apertures in a multi-panel antenna. The elements or apertures are connected to at least a receive system which adjusts the received signal from each element or aperture to bring the signal into time and phase alignment. These same adjustments may then be used in a transmit mode to enhance a signal transmitted from the
Moosbrugger Peter
Paschen Dean Alan
Ball Aerospace & Technologies Corp.
Phan Dao
Sheridan & Ross P.C.
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