Patent Grant
7027837
This application relates to point-to-point radio systems, and more particularly to antenna arrays for point-to-point radio systems.
With the increasing use of point-to-point radio systems the efficient use of the allocated transmission spectrum is a growing concern. To improve spectral efficiency, conventional millimeter wave point-to-point radio systems often utilize sophisticated Quadrature Amplitude Modulation (QAM) and error correcting codes to achieve data rates of up to 7 bits per second per hertz of channel bandwidth. For example, one such system that operates at 28 GHz, uses 256 QAM modulation, a symbol rate of 125 M symbols/second, 20% excess bandwidth, and a rate 7/8 convolutional code concatenated with a (188, 204) byte Reed Solomon block code to achieve a spectral efficiency of about 5.3758 bits/Hz. Recent improvements in modulation techniques and error correction techniques have led to only marginal improvements in spectral efficiency.
A system and method for communicating information between two locations via a wireless microwave link is provided. The system may include at least two antennas, each to transmit information as a narrow beam signal to be directed toward a focal point at a remote location. The antennas may include at least one antenna to transmit a narrow beam signal toward a redirection point different from the focal point. A redirection device located at the redirection point to reflect the narrow beam signal from the at least one antenna element and to redirect the received narrow beam signal toward the receiver. The redirection point is located such that the narrow beam signals from the at least two antenna elements converge and overlap to form an interference pattern proximate to the receiver. The interference pattern includes peaks and nulls that have a peak-to-peak spacing narrower than a width of each of the received narrow beam signals.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.
Each of the antenna arrays 12a and 12b is comprised of two or more antenna elements 16a–16d with at least one redirection device 18a and 18b configured in relation to a corresponding one of the antenna elements 16a and 16c to redirect signals that are communicated between the corresponding antenna element 16a and the other antenna array 12b or 12a. For example, a signal generated from antenna element 16a of antenna array 12a is directed towards a redirection point at which the redirection device 18a is located. The signal is redirected from the redirection device 18a towards the receiving antenna array 12b. The redirection point is selected so that the redirection device 18a is spaced a separation distance from other redirection devices if any, and/or the antenna elements 16b that point directly at the receiving antenna array 12b. The separation distance is chosen to control the spacing of peaks and nulls in the interference pattern that is created at the receiving location by the superposition of the narrow beam signals emitted from antenna array 12a.
If the transmitting antenna elements 24 are placed closer together, then the transmitting array aperture becomes smaller, which in turn widens the main lobe, and the spacing between the nulls and peaks of the interference. The increased separation requirement between the nulls and the peaks of the interference pattern forces the receiving antenna elements 26 need to be widely spaced in order to achieve orthogonality. To attain a symmetric link and maintain orthogonality simultaneously in both directions, the transmitting antenna elements 24 are placed further apart, causing the aperture to increase, the main lobe 20 to narrow, and the distance between the peaks 20 and the nulls 22 to decrease. The spatial repetition frequency of the interference pattern that is created at the receiving antenna elements 26 defines a peak-to-peak spacing that is much narrower than the width of the received narrow beam signals that are generated by the transmitting antenna elements 24. For example, for a 5 kilometer radio link and 38 dBi parabolic dish antennas, the 3 dB beamwidth at the target might be 250 meters across. However, with 28 GHz radio carrier and a transmit array aperture of 10 meters, the peak-to-null spacing would be less than 10 meters.
Referring to
If “x” is the separation distance, “d” is the distance between antenna arrays, and lambda is the radio carrier wavelength, then the following relationship holds for the preferable separation distance:
For example, at 28 GHz, the United States Local Multipoint Distribution Service (US LMDS) band, and various antenna array separations, the following table enumerates the optimum separation distances;
At 5.5 GHz, the United States Unlicensed National Information Infrastructure (US UNII) band, and 10 km, the optimum spacing is 16.51 meters.
For two transmitting antenna elements and two receiving antenna elements, this corresponds to a coupling matrix row of about [1 0]. For a different phasing of the transmitting antennas, the coupling matrix row is about [0 1]. Likewise for a given distance d, an appropriate choice of antenna element/redirection element spacing x, and appropriate choices for transmitter signals phases of N transmitting antennas and N receiving antennas, the coupling matrix can approximate a diagonal matrix. Thus, the cross-coupling matrix between the array of transmitters and the array of receivers is approximately a diagonal matrix with small condition number and is readily invertible. For the ideal special case, the cross-coupling matrix is an identity matrix in which the condition number is 1. Therefore, multiple transmitters and receivers may be advantageously operated in parallel resulting in an increase in the number of operational communication channels between the antenna arrays 12a and 12b. Since the communication channels are approximately orthogonal, the data rates of the independent channels may be added to determine the aggregate rate of information flowing between the antenna arrays 12a and 12b. Spacing the transmitting antenna elements apart by the optimal separation distance improves the benefits of spatial processing. Nearly independent parallel radio channels may be produced that can be readily utilized by adaptive spatial processing to dramatically increase data rate without utilizing more radio spectrum. By setting the interference distance approximately equal to the separation distance, x, so that the receiving antenna array is symmetrical to the transmitting antenna array, nearly independent full-duplex parallel radio channels may be established. When the array element spacing is optimized for the wavelength and distance, required transmit power for a given data rate is minimized.
Although the interference distance is preferably set equal to the separation distance so that a symmetrical link is set up between the antenna arrays 12a and 12b, nearly equivalent interference and separation distances are not required. For example, referring to
The antenna elements 16a–16d are preferably directional antenna elements such as parabolic antenna elements. One such example includes a 30 cm parabolic dish with a gain of 38 dBi and beam width of about 1.8 degrees. All other types of directional antenna elements also may be used such as curve-shaped antenna elements. A curve-shaped antenna element may be used in combination with a curve-shaped redirection device 18a–18b to, in combination, provide the effect of a parabolic or near-parabolic shape.
The redirection device 18 may include devices and objects that may be used to reflect a narrow beam signal. Such devices and objects may include passive reflectors that have flat surfaces, curve-shaped surfaces, and parabolic-shaped surfaces. The redirection device 18 may be a dedicated reflector or an object such as a building that has a reflective surface. In addition, the redirection device 18 may be located in close proximity, for example several meters, to the corresponding antenna element 16b or at a distance such as atop another building. The redirection device 18 may be constructed from flat plate reflectors set at 45 degrees and used in combination with a standard parabolic antenna element 16 that is pointed perpendicular to a point-to-point radio link path extending between the antenna arrays 12a and 12b. Another approach combines curve-shaped elements for both the redirection device 18 and the antenna element 16 that points at the redirection device 18. While curved reflectors are generally more difficult to manufacture than flat plates, the curved elements may provide higher gain, better stiffness, or less weight than flat plates.
The redirection device 18 preferably includes a reflecting surface composed of a reflecting material for reflecting the narrow beam signals. Suitable reflecting surfaces include metallic surfaces, metallized surfaces, screens, grating patterns, and the like.
In conventional systems, two (or more) transmitting radios are typically spaced apart and interconnected by rigid wave-guides to share local oscillator signals so that the transmitting radios can generate the frequency-coherent, phase offset signals that are required for precise beam and null steering. At very high frequencies such as 28 GHz, running rigid wave-guides between multiple radios that are spaced 7 or 8 meters apart may become difficult and expensive. Similar to the transmitting radios, conventional receiving radios also typically share local oscillator signals with each other to facilitate the modem's proper separation and demodulation of the multi-channel received signals from the transmitting radios.
Antenna elements 34a and 34b, associated with respective radios 32a and 32b, generate narrow beam signals that are directed towards another antenna array (not shown). At least one antenna element 34a is pointed at a redirection device 36 instead of directly at the other antenna array. The redirection device 36 is spaced apart from the other antenna 34b by the separation distance described above. The narrow beam signal from the antenna element 34a is redirected from the redirection device 36 directly toward the other antenna array. Using one or more redirection devices 18 enables coherent radios to be physically co-located while obtaining a large aperture antenna array. Physical co-location of the radios 32a and 32b facilitates the sharing of high frequency local oscillator signals and simplifies packaging of multi-channel radios. A receiving antenna array (not shown) may be configured similarly, so that the receiving radios may be co-located, thereby minimizing the difficulty of sharing local oscillator signals.
Shown in
Other embodiments are within the scope of the following claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5991345 | Ramasastry | Nov 1999 | A |
| 20010031647 | Scherzer et al. | Oct 2001 | A1 |
| 20020119790 | Judson et al. | Aug 2002 | A1 |
