EP0786826A2

EP0786826A2 - Intermodulation scattering communications apparatus

Info

Publication number: EP0786826A2
Application number: EP97300507A
Authority: EP
Inventors: Arnold L. Berman; James D. Thompson; Michael I. Mandell
Original assignee: Hughes Aircraft Co; HE Holdings Inc
Current assignee: DirecTV Group Inc
Priority date: 1996-01-29
Filing date: 1997-01-28
Publication date: 1997-07-30
Also published as: EP0786826A3

Abstract

Multiple beam, multi-frequency communication apparatus (10) that provides for significant intermodulation scattering improvements for multiple beam, multi-frequency communications systems implemented using directly radiating active phased array (15), or multi-feed reflector systems fed from composite, multiple shared transmit amplifiers (13). The multiple bearn, multi-frequency communications apparatus (10) comprises a defocused antenna array (15) having a plurality of antenna elements (15a) and a plurality of feeds (15b). A hybrid amplifier structure (18) is coupled to the feeds (15b) of the defocused antenna array (15) and shares the plurality of transmit amplifiers (13) among the feeds (15b) of the antenna array (15). Assignment apparatus (20) is coupled to the hybrid amplifier structure (18) for assigning particular transmit amplifier (13) to be contributors to particular beams (16) radiated by the antenna array (15) in response to a desired beam profile (11). Improvements result from judicially combining the effects of the shared transmit amplifiers (13) (that carry all the signals in the frequency reuse system), with phasing effects inherent in rhe multiple beam radiating structure in order to realize the desired zonal frequency reuse pattem 21. An optimal assignment algorithm implemented in the assignment apparatus (20) results in a uniform distribution of power over the transmit amplifiers (13), resulting in corresponding improvements in system performance in the far field.

Description

BACKGROUND

The present invention relates generally to communications systems, and more particularly, to intermodulation scattering communications satellite systems.
Multiple beam communications systems obtain one of their major advantages by re-using an assigned frequency band many times for signals in different beams within the overall field-of-view of the system's antenna. In these multiple beam, multiple carrier systems, each transmitted signal (or beam) is assigned a frequency, and an associated spatial destination address, (i.e., azimuth and elevation coordinates). In conventional systems, intermodulation products created at a particular frequency are radiated in the same directions as desired signals at the same frequency.
The existence of intermodulation scattering, wherein under certain conditions signal beams are directed to desired targets, and intermodulation beams are directed into space, was first described by Sandrin in "Spatial Distribution of Intermodulation Products in Active Phased Array Antennas", IEEE Transactions on Antennas and Propagation, November, 1973. However this paper provides no description of how to realize the necessary conditions in practice.
These conditions were later realized at the assignee of the present invention for the special case of a multiple fan beam frequency addressable array. In this case intermodulation spreading improvements were achieved within a desired field of view by controlling parameters of multiple coverage regions, and by using a frequency addressing algorithm.
During the development of the Inmarsat III communications satellite system by the assignee of the present invention, it was widely believed that intermodulation spreading improvements would occur as a matter of course if a Butler matrix was used to combine outputs of multiple amplifiers to provide for power sharing. Unfortunately, this was shown not to be the case.
Accordingly, it is an objective of the present invention to provide for communications systems that exhibit improved intermodulation scattering performance. It is a further objective of the present invention to provide for multiple beam, multi-frequency communications systems comprising radiating active arrays, or multi-feed reflector systems fed from multiple shared transmit amplifiers that have improved intermodulation scattering performance.

SUMMARY OF THE INVENTION

To meet the above and other objectives, the present invention provides for significant intermodulation scattering improvements for general cases of multiple beam, multi-frequency communications systems realized with directly radiating active arrays, or multi-feed reflector systems fed from composite, multiple shared transmit amplifiers. These two types of systems, when implemented using the principles of the present invention, both provide significant intermodulation scattering improvements within a desired field of view.
More specifically, the present invention provides for satellite-based multiple beam, multiple carrier communications systems employing transmitters that provide enhanced system performance by scattering intermodulation products with respect to desired signals at the same frequency, thereby improving the overall signal-to-noise ratio of the communications links. For systems required to operate at a given noise power ratio (NPR), some portion of the NPR improvement may be allocated to allow satellite transmitters to operate closer to saturation at higher levels of efficiency, resulting in a further improvement in system performance.
The key to improving system carrier-to-intermodulation or NPR performance that is provided by the present invention is to include design features in the communications system that deliberately disassociate the spatial address from the frequency address as the intermodulation products are created. This in turn requires mixing together of many signal reuses of the frequency band in a set of common (nonlinear) transmit amplifiers, and individual signal phasings within the set of transmit amplifiers determine the final spatial destination of each signal. When these multiple signals are subsequently radiated by the antenna, the specific signal components from different amplifiers and radiating elements reinforce and are focused to form the desired beams, while the majority of intermodulation products are scattered over the spatial field-of-view by virtue of amplifier and beamforming matrix interconnections and the 2π modulus of the phase function.
Key aspects of the present invention thus include the disassociation of the frequencies and spatial addresses for the intermodulation (but not the signals), and the ratio of the number of cells in the system to the frequency reuse. For satellite-based communications applications, efficient systems have a very high number of reuses per frequency, a high number of cells in the frequency reuse pattern, and a relatively small fraction of the field-of-view assigned to each frequency sub-band.
The present invention is generally applicable to multiple beam, multiple signal satellite-based communications systems. The present invention is particularly useful in mobile communications systems where the number of signals and the number of frequency reuses are very high, and where the efficiency of the satellite-based transmitter is a critical component of the overall system.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:

Fig. 1 is a block diagram illustrative of a forward link of a multiple beam reflector system utilizing a shared transmitter in accordance with the principles of the present invention;
Fig. 2 illustrates a computational block diagram for of the system of Fig. 1;
Fig. 3 illustrates a typical antenna coverage plot over Africa for the multiple beam reflector system of Fig. 1;
Fig. 4 illustrates details of cell coverage for the plot of Fig. 3 showing cell number versus a seven cell frequency reuse pattern;
Fig. 5 illustrates a typical antenna coverage plot over Asia for the multiple beam reflector system of Fig. 1;
Fig. 6 is a corresponding block diagram illustrative of a forward link of an active phased array communications system in accordance with the principles of the present invention;
Fig. 7 illustrates a typical nine cell frequency reuse pattern;
Fig. 8 illustrates a typical stacking arrangement for arranging the nine cell frequency reuse pattern of Fig. 7; and
Fig. 9 illustrates a partial listing of successful frequency patterns that have been used in a reduced to practice embodiment of the system of Fig. 6.

DETAILED DESCRIPTION

Referring to the drawing figures, and by way of illustration, two realizations of forward link transmitters 10 in accordance with the principles of the present invention are shown and described with reference to Figs. 1 and 6 that may be used with a multiple beam reflector communications system 10a and an active phased array communications system 10b, respectively. Referring to Fig. 1, the transmitter 10a comprises a defocused antenna array 15 that includes a plurality of antenna elements 15a, and a hybrid amplifier structure 18 having a set of transmit amplifiers 13 and a beamforming matrix 14, such as a Butler matrix 14, for example, that share the plurality of transmit amplifiers 13 among feeds 15b for each element 15a of the antenna array 15. The transmitter 10 further comprises assignment apparatus 20 (assignment algorithm 20) that may be implemented in a signal processor 17 or a channelization, beamforming and signal routing matrix 12 that is derived from and corresponds to the assignment algorithm 20, depending upon the system. The assignment apparatus 20 or corresponding matrix 12 assigns particular transmit amplifiers 13 to be contributors to particular beams 16 radiated by the antenna array 15 in response to a desired beam profile 11 applied to inputs 12a of the signal processor 17 or channelization, beamforming and signal routing matrix 12, respectively.
A computational block diagram for the system 10a of Fig. 1 is shown in Fig. 2. A feed excitation matrix 21 receives a traffic profile corresponding to signals to be radiated to each beam (n_b) and produces the desired feed excitations (n_f) 22. This is accomplished for each beam in the system using the corresponding beam profile H. A permutation matrix and plurality of Butler matrices (having n_f inputs and n_a outputs receives the beams from the plurality of feeds 22 and couples them to a plurality of amplifiers (n_a) 13. Outputs of the amplifiers 13 are coupled to a permutation matrix and plurality of Butler matrices 14 (having n_a inputs and n_f outputs). Outputs of the permutation matrix and plurality of Butler matrices 14 are coupled by way of a plurality of feeds 23 (n_f) to a feed to center of beam matrix 24 (having n_f inputs and n_b outputs for example) that outputs the signal received in the center of each beam. By way of example, the number of beams (n_b) may be 243, the number of antenna feed elements (n_f) may be 153, and the number of amplifiers (n_a) may be 160.
Fig. 3 illustrates an antenna coverage plot over Africa for the multiple beam reflector system 10a while Fig. 4 illustrates details of cell coverage for the plot of Fig. 3 showing cell number versus a seven cell frequency reuse pattern. Fig. 5 illustrates an antenna coverage plot over Asia for the multiple beam reflector system 10a.
Referring to Fig. 6, illustrates a forward link of an active phased array communications system 10b in accordance with the principles of the present invention. The system 10b comprises a multibeam receiver 30 that receives signals on each of its input beams. Outputs of the multibeam receiver 30 are coupled to a signal processor 40, or forward link processor 40. The forward link processor 40 generates a plurality of output signals corresponding to the number of beams (n_b) that are coupled by way of a plurality of fixed upconverters 41 to a beamformer 14, such as may be provided by a Butler matrix beamformer 14, for example. Outputs of the beamformer 14 are coupled by way of a plurality of amplifier modules 42 that each include an adjustable phase shifter 43, an adjustable attenuator 44, and a power amplifier 45, and a plurality of output filters 46 to a plurality of antenna elements 15a of an antenna array 15. In addition, the system 10b includes a frequency reference generator 50 that comprises a plurality of frequency reference sources 51 whose frequency outputs are coupled by way of a plurality of summing devices 52 to a plurality of local oscillator generators 53 and a plurality of local oscillator distribution circuits 54 that distribute the reference frequency signals to local oscillators of the forward link. Second outputs of the summing devices 52 are coupled to return local oscillator generators (not shown).
A typical nine cell frequency reuse pattern 21 used in a prototype active phased array communications system 10b of Fig. 6 that has been reduced to practice is shown in Fig. 7, while a typical stacking arrangement for arranging the nine cell frequency reuse pattern 21 is shown in Fig. 8. The frequency reuse pattern 21 shown in Fig. 7 comprises a typical coverage pattern for each frequency sub-band in the far field (on earth relative to a satellite-based communications system, for example). The frequency reuse pattern 21 is shown as a three-by-three square cell pattern, but is to be understood that a hexagonal or other regularly shaped cell pattern, for example, may also be used.
Multiple beam communications systems reuse an assigned frequency band many times for signals in different beams within the overall field-of-view of the antenna 15. In multiple beam, multiple carrier systems, each signal is assigned a frequency, and an associated spatial destination address (azimuth and elevation coordinates). To improve system carrier-to-intermodulation or NPR performance, the present invention deliberately disassociates the spatial address from the frequency address as the intermodulation products are created. This requires mixing together of many signal reuses of the frequency band in the set of common transmit amplifiers 13. Disassociation is achieved for the multiple beam reflector communications system, for example, by using the channelization, beamforming and signal routing matrix 12 in combination with the beamforming matrix 14. Disassociation is achieved for the active phased array communications system, for example, by using the signal processor 17 which implements an algorithm corresponding to the channelizaion, beamforming and signal routing matrix 12 in combination with the beamforming matrix 14.
Individual signal phasings within the set of transmit amplifiers 13 determine the final spatial destination of each signal. When these multiple signals are radiated by the antenna 15, the specific signal components from different transmit amplifiers 13 and associated radiating elements 15a reinforce and are focused to form the desired beams 16, while the majority of the intermodulation products are scattered over the spatial field-of-view by virtue of interconnections between the transmit amplifiers 13 and beamforming matrix 14 and the 2π modulus of the phase function.
By way of example, in the case of the multiple beam reflector system, the shared transmit amplifiers 13 are broken into subsets (such as may be provided by 8x8 Butler matrix modules, for example) while the frequency reuse parameter for the cell pattern 21 may be set to 7 (relatively prime to 8). The resulting distribution of signals forming each beam 16 among the relatively prime (8x8) transmit amplifiers 13 then scatters the resulting intermodulation products. Thus, the resulting average improvement approached a limiting value of 10 log 7 (dB).
In the case of the active phased array system, each signal that is to be transmitted is present in every transmit amplifier 13, and a linear phase progression is formed across the transmit amplifiers 13 to determine the spaced destination of each signal. A nine cell reuse pattern 21 on earth, such as is shown in Fig. 7, for example, may be permuted to randomize the regularity of the cell structure, such as in the manner shown in Fig. 8, for example. In the case of the nine cell reuse pattern 21, the resulting improvement approaches a limiting value of 10 log 9 (dB). Fig. 9 illustrates a partial listing of successful frequency patterns that have been used in a reduced to practice embodiment of the present invention.
In the above-described two system examples. the overall improvements result from judicially combining the effects of the shared transmit amplifiers 13 (that carry all the signals in the frequency reuse system), with the phasing effects inherent in the multiple beam radiating structure in order to realize the desired zonal frequency reuse pattern 21 on earth. For the multiple beam reflector system, an essential element is the permutation matrix describing which Butler matrix 14 output each feed element 15a connects to. An optimal assignment algorithm 20 results in a uniform distribution of power over the transmit amplifiers 13, resulting in corresponding improvements in system performance (NPR) in the far field. An optimal assignment algorithm computes the optimal permutation matrix.
Thus, communications apparatus and transmitters that exhibit improved intermodulation scattering performance have been disclosed. It is to be understood that the described embodiment is merely illustrative of some of the many specific embodiments which represent applications of the principles of the present invention. Clearly, numerous and varied other arrangements may be readily devised by those skilled in the art without departing from the scope of the invention.

Claims

Multiple beam, multiple frequency communications apparatus (10) characterized by:
a antenna array (15) comprising a plurality of antenna elements (15a);

a plurality of transmit amplifiers (13);

a beamforming matrix (14) coupled between the plurality of transmit amplifiers (13) and the plurality of feeds (15b) of the antenna array (15); and

assignment apparatus (20) coupled to the plurality of transmit amplifiers (13) for assigning particular transmit amplifiers (13) to be contributors to particular beams (16) radiated by the antenna array (15) in response to a desired beam profile (11).
The apparatus (10) of Claim 1 wherein the hybrid amplifier structure (18) is characterized by a plurality of transmit amplifiers (13) and a Butler matrix (14).
The apparatus (10) of Claim 1 wherein the assignment apparatus (20) is characterized by a signal processor (17).
The apparatus (10) of Claim 1 wherein the assignment apparatus (20) is characterized by a channelization, beamforming and signal routing matrix (12).
Multiple beam, multiple frequency communications apparatus (10) having reduced intermodulation products, said apparatus (10) characterized by:
a defocused antenna array (15) comprising a plurality of antenna elements (15a) having a respective plurality of feeds (15b);

a plurality of tansmit amplifier (13);

a beamforming matrix (14) coupled between the plurality of transmit amplifiers (13) and the plurality of feeds (15b) of the antenna array (15) for forming a plurality of beams that each have a spatial address and a frequency address and that have a desired beam profile (11); and

assignment apparatus (20) coupled to the plurality of transmit amplifiers (13) for assigning particular transmit amplifiers (13) to be contributors to selected beams (16) radiated by the antenna array (15) in response to the desired beam profile (11);

and wherein the assignment apparatus (20) in combination with the beamforming matrix (14) disassociates the spatial address from the frequency address of each of the plurality of beams as intermodulation products are created.
The apparutus (10) of Claim 5 wherein the hybrid amplifier structure (18) is characterized by a plurality of transmit amplifiers (13) and a Butler matrix (14).
The apparatus (10) of Claim 5 wherein the assignment apparatus (20) is characterized by a signal processor (17).
The apparatus (10) of Claim 5 wherein the assignment apparatus (20) is characterized by a channelization, beamforming and signal routing matrix (12).