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(11) | EP 0 000 038 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Method and apparatus for cancelling interference between area coverage and spot coverage antenna beams |
| (57) The present invention relates to method and apparatus for substantially cancelling
interference between signals using the same frequency spectrum which are received
via overlapping area coverage (12) and spot coverage (e.g. 14a) antenna beams. At
the transmitter where the overlapping area and spot coverage beams are concurrently
transmitted, a predetermined portion of the area coverage signal is coupled into each
of the spot beam signals which will be overlapped at the associated spot beam receiver
area by the area coverage beam. The predetermined portion coupled into each spot beam signal will have a magnitude and phase to substantially cancel the area coverage signal at the associated spot beam receiver area. |
[0001] The present invention relates to method and apparatus for effecting substantial cancellation of interference between a first and a second signal transmitted concurrently in a first and a second antenna radiated beam, respectively, where the first and second signals include different informational content and use the same frequency spectrum and the first and second beams overlap each other in the area of a receiver which is to receive only the first signals.
[0002] In a domestic satellite communication system the coexistence of spot and area coverage beams can be desirable. For example, a separate spot coverage beam can be used for communication between the satellite and each high traffic ground station while an area coverage beam can be used for communication between the satellite and a plurality of low traffic ground stations under conditions where it might not be desirable to interconnect the individual low traffic ground stations to a nearest high traffic-ground station for access to the satellite system. To avoid signal degradation and permit separation of the overlapping spot coverage and area coverage beams, especially at each spot coverage receiving station, a typical prior art technique would be to use separate bandwidths or polarizations, if possible, for the spot coverage beams and the area coverage beam. Using separate bandwidths, however, results in inefficient use of the frequency spectrum and different polarizations may not be available where dual polarized beams are already used by each of the beams of the satellite system.
[0003] Various techniques have been devised to suppress interference between two beams arriving at a receiver from separate directions. In this regard see, for instance, U.S. Patents 2,520,184; 3,094,695; 3,369,235 and 3,987,444. Since the area and spot coverage beams transmitted from a satellite arrive at each spot beam ground station from the same direction, techniques for separating signals arriving from different directions are not usable.
[0004] An alternative technique to enable rece ption of only one signal of a plurality of signals concurrently received from a plurality of transmitters at an FM receiver would be to modulate the carrier of each transmitter with a separate frequency to provide a unique address that is assigned to an associated receiver as disclosed, for example, in U.S. Reissue Patent Re. 27,478. Such arrangement may be applicable to FM communication systems but does not appear applicable to a digital communication system.
[0005] The problem remaining in the prior art is to provide a technique which permits overlapping spot and area coverage beams which use the same frequency band to be separated at an overlapped receiving station.
[0006] The foregoing problem is solved according to the invention by the method characterized by the step of, at the transmitter, coupling a predetermined portion of the second signal to be transmitted in the second beam into the signal to be transmitted by the first beam, said predetermined portion of the coupled-in second signal having a magnitude and phase to cancel substantially, after propagation in the first beam to the receiver, the second signal which arrives in the second beam at the receiver. For practizing the above recited method, the invention provides for a transmitter characterised by a first antenna capable of transmitting the first beam with a predetermined field pattern Es(θ) in the direction of the receiver which is to receive only the first signals; a second antenna capable of transmitting the second beam with a predetermined filed pattern EA(D) which overlaps said first beam field pattern in the area of the receiver which is to receive only the first signals; a first transmission line capable of delivering the signal to be transmitted in the first beam to the first antenna; a second transmission line capable of delivering the signal to be transmitted in the second beam to the second antenna, and a coupler disposed between the first and second transmission lines arranged to couple a predetermined portion of the second signal propagating in the second transmission line into the first transmission line for transmission in the first beam, the predetermined portion of the second signal coupled into the first transmission line having a magnitude and phase to substantially cancel the signal in the second beam arriving at the first beam receiver.
[0007] The present invention has been described primarily in relationship to a satellite communication system to enable the concurrent use of an area coverage satellite radiated beam and a plurality of spot coverage satellite radiated beams where all of the .beams use the same frequency spectrum and the spot coverage beams are received within the area encompassed by the area coverage beam. However, it will be understood that such description is exemplary only and is for the purpose of exposition and not for purposes of limitation. It will be readily appreciated that the inventive concept described is equally applicable to other radiated wave transmission systems - which comprise two or more beams which have different destinations but interfere with each other at one or more of the destinations.
[0008] In the drawings:
FIG. 1 diagrammatically illustrates a satellite communication system for providing both an area coverage beam and a plurality of spot coverage beams between the satellite and the associated ground receiver stations;
FIG. 2 illustrates an arrangement according to the present invention to effect interference cancellation of the area coverage beam at each of the spot coverage receiver stations;
FIG. 3 is a curve illustrating the antenna pattern of a spot coverage beam and a modified area coverage beam in the area of a spot coverage ground station according to the present invention;
FIG. 4 is a curve illustrating the Signal-to-Interference ratio at the ground stations between a spot coverage beam and the modified area coverage beam in accordance with the arrangement of FIG. 2;
FIG. 5 is a curve illustrating the power spectrum of a 4φ- PSK signals for a 300 Mbauds spot beam and two 75 Mbauds area beams in accordance to the present invention.
[0009] In FIG. 1, a satellite communication system is illustrated wherein the present invention is especially useful to permit the concurrent transmission from a satellite 10 of both an area coverage beam 12 and a plurality of spot coverage beams of which, for example, three beams 14a, 14b and 14c are shown with all beams being able to use the same frequency spectrum. Spot coverage beams 14a, 14b and 14c are shown radiating from antennas 15a, 15b and 15c, respectively, and directed at respective ground areas 16a, 16b and 16c which include for example, high traffic ground stations 17a, 17b and 17c, respectively. Area coverage beam 12 is shown radiating from an antenna 13 and directed at a ground area 18 which includes both the ground areas 16a, 16b and 16c and a plurality of low traffic ground stations of which, for example, four stations 19a-19d are shown. In the satellite communication system of FIG. 1, each of the high traffic ground stations 17a-17c communicates with satellite 10 via a separate spot beam 14a-14c, respectively, while the low traffic ground stations 19a-19d communicate . with satellite 10 via common area coverage beam 12 using any suitable technique to assure that a particular message will be processed by only the appropriate one of stations 19a-19d. Such arrangement permits low traffic ground stations 19a-19d to communicate with satellite 10 under conditions where it is not advantageous to connect a low traffic ground station 19 to a nearby one of high traffic ground stations 17a-17c.
[0010] It can be seen from FIG. 1 that when area coverage beam 12 and spot coverage beams 14a-14c are transmitted concurrently and use the same frequency spectrum that each of ground stations 17a-17c will receive both the associated one of spot coverage beams 14a-14c and area coverage beam 12 since these beams emmanate from approximately the same point and most probably the same antenna rather than separate antennas as shown in FIG. 1. Under such conditions the use of prior art arrangements such as, for example, side lobe suppression arrangements to select a wave received from a particular direction over waves received from other directions is not feasible.
[0011] The concurrent transmission of area coverage beam 12 and a plurality of spot coverage beams 14a-14c using the same frequency spectrum can be effected in accordance with the present invention by the arrangement shown in FIG. 2. For purposes of explanation, S represents the signal intended for a particular spot beam antenna 15 with a field pattern Es(θ). More particularly, signals Ssa, Ssb and Ssc propagate in waveguide 21a, 21b and 21c, respectively, to respective antennas 15a, 15b and 15c for radiation to respective ground stations 17a-17c via spot coverage beams 14a, 14b and 14c, respectively. The field pattern Es(o) for each of the spot coverage beams 14 is assumed to be of Gaussian shape as, for example, in the main lobe of a paraboloid fed by a corrugated feedhorn, and is given by:
where E (0) is in the magnitude of the field along the axis of each spot coverage beam 14. Additionally, SA represents the signal intended for area coverage beams 12 and is shown propagating in waveguide 21d to antenna 13 for radiation to ground stations 19 via area coverage beam 12 which has a field pattern EA(θ) which is given by
where EA(0) is the magnitude of the field along the axis of area coverage beam 12.
[0012] Since EA(θ) represents the field pattern over area 18 of FIG. 1, it is desirable to produce a "hole" in EA(θ) in the areas 16a-16c where the spot coverage beams 14a-14c exist such that EA does not interfere with each of the Es patterns. In accordance with the present invention, interference between the signal SA transmitted via area coverage beam 12 and each of signals Ssa, Ssb and Ssc transmitted via spot coverage beams 14a, 14b and 14c, respectively, is substantially reduced at each of the spot beam ground stations 17 by coupling a portion of the area coverage signal, SA, propagating in waveguide 21d, into each of the spot coverage signals S Ssb and Ssc propagating in waveguides 21a-21c, respectively, using respective directional couplers 22a, 22b and 22c. To accomplish such interference cancellation at each of ground stations 17, each of couplers 22a-22c should preferably have a negative coupling coefficient of approximately between one and two times the value of
. For example, for a negative coupling coefficient of 1.21, the radiated signal for area beam 12 and one of spot beams 14a-14c in the vicinity of the associated spot beam ground station 17 then becomes
Since ES(0) » EA(0), Equation (3) can be simplified to
The normalized power patterns for both a spot and the area coverage beams are
and are shown in FIG. 3. From FIG. 3 it can be seen that the spot coverage beam 14 remains unchanged when received at associated area 16 whereas the area coverage beam 12 is significantly reduced in the spot coverage beam region 16.
[0013] If it is assumed that 4φ-PSK modulation of the same baud rate is used in both beams and that the Effective Instantaneous Radiated Power (EIRP) at beam peaks are the same, i.e., < EA(O)SA 2> = < Es(O)s 2>, the signal to interference ratio (S/I) at the ground defined by PA/ps or Ps/PA is shown in FIG. 4 by a solid line, where PA = received power of SA ( EA(θ)[1-1.21
]SA2) and PS = received power of SS (ESs(θ)SS ). From FIG. 4, it can be seen that if S/I > 14 dB is acceptable, the far field region breaks down to
The blackout region is that area which is serviceable by neither the area beam nor the spot beam because of mutual interference between the two beams. The.traffic terminating in the blackout region at the edge of each of spot beam regions 16 may have to be trunked on the ground via other stations in the neighboring region.
[0014] If advantage is taken of the spectrum shape of the 4φ-PSK signal, the blackout region can be reduced or the S/I may be increased. For example, the capacity of the area coverage beam can be reduced by a factor of two and the modulations can be placed at the edges of the allocated 500 MHz bandwidth of the satellite downlink. The power spectrums of a 300 Mbauds spot coverage peam and two 75 Mbauds area beams are shown in FIG. 5. It should be noted that a ground station 19, intended to receive the area coverage beam 12, will have a receiving filter having characteristics which follow either spectrum A1 or A2. Therefore, the received interference power of SS is reduced by about 6 dB due to this offsetting of modulation spectrum. Similarly, a ground station 17 intended to receive Ss will have a receiving filter having characteristics which follow spectrum S in FIG. 5. The received power of SA is reduced by about 9 dB compared to that of S .
[0015] Taking into account both the S/I improvement obtained by spectrum offsetting (FIG. 5) and the antenna pattern discrim- ination, the resultant (Ps/PA)' and (PA/Ps)' are shown by a dashed line in FIG. 4.
[0016] In FIG. 4 it can be seen that the blackout region is reduced using spectrum offsetting and antenna pattern discrimination. Again for S/I > 14 dB, the regions for (Ps/PA)' and (PA/Ps)' becomes:
Compared to the previous case using only the arrangement of FIG. 2, the blackout region has been reduced to (1.85 - 1.2)2/ (2.25 - 1)2 = 27 percent. Or, if maintaining the same blackout region, the minimum S/I in the serviceable region would be higher than 20 dB.
at the transmitter
(a) coupling a predetermined portion of the second signal to be transmitted in the second beam into the signal to be transmitted by the first beam, said predetermined portion of the coupled-in second signal having a magnitude and phase to cancel substantially, after propagation in the first beam to the receiver, the second signal which arrives in the second beam at the receiver.
(b) providing a signal capacity for the second beam which is less than the signal capacity of the first beam; and
(c) modulating the second beam signal in a manner to divide the power spectrum for the second beam signal into two portions with each portion disposed both within the frequency spectrum of the first beam and near separate edges of said frequency spectrum.
a first antenna (15) capable of transmitting the first beam (14) with a predetermined field pattern (Es(θ) in the direction of the receiver which is to receive only the first signals;
a second antenna (13) capable of transmitting the second beam with a predetermined field pattern EA (θ) which overlaps said first beam field pattern in the area of the receiver which is to receive only the first signals;
a first transmission line (21a) capable of delivering the signal to be transmitted in the first beam to said first antenna
a second transmission line (21d) capable of delivering the signal to be transmitted in the second beam to said second antenna
a coupler (22a) disposed between said first and second transmission lines arranged to couple a predetermined portion of the second signal propagating in said second transmission line into said first transmission line for transmission in the first beam, said predetermined portion of the second signal coupled into said first transmission line having a magnitude and phase to substantially cancel the signal in the second beam arriving at the first beam receivero
, where E (0) and EA(0) are the magnitude of the fields along the axes of the first and second antenna radiated beams, (14,12) respectively.
the second beam (12) is provided with a capacity which is less than the signal capacity of the first beam (14); and the transmitter
a modulator capable of modulating the second beam signal in a manner to divide the power spectrum for the second beam signal into two portions with each portion disposed both within the frequency spectrum of the first beam and near separate edges of said frequency spectrum.