Deformed Antenna Pattern Compensation Technique for Multi-beam Antennas for Broadband and Scalable Mobile Communications Satellite

Transcription

1 Deforme Antenna Pattern Compensation Technique for Multi-beam Antennas for Broaban an Scalable Mobile Communications Satellite Yoshinori SUZUKI 1, Satoshi HARADA 2, Kiyoshi KOBAYASHI 3, Masazumi UEBA 4 1 NTT Access Network Service Systems Laboratories, NTT Corporation 1-1 Hikarinooka, Yokosuka, Kanagawa, Japan, suzuki.yoshinori@lab.ntt.co.jp 2 NTT Access Network Service Systems Laboratories, NTT Corporation 1-1 Hikarinooka, Yokosuka, Kanagawa, Japan, haraa.satoshi@lab.ntt.co.jp 3 NTT Access Network Service Systems Laboratories, NTT Corporation 1-1 Hikarinooka, Yokosuka, Kanagawa, Japan, kobayashi.kiyoshi@lab.ntt.co.jp 4 NTT Access Network Service Systems Laboratories, NTT Corporation 1-1 Hikarinooka, Yokosuka, Kanagawa, Japan, ueba.masazumi@lab.ntt.co.jp Abstract To create a next-generation mobile satellite communication system that offers large communication capacity, the onboar antenna system must be a multi-beam system consisting of a light weight 2-m class reflector an a light weight 1-beam class antenna fee system. We clarify that the antenna gain ecrease create by the reflector surface istortion in orbit is relately large. This paper presents a eforme antenna pattern compensation metho that uses the minimum circuits amount. Valiity of the propose metho is confirme by antenna pattern calculations an experiments on a fabricate array-fe reflector antenna. 1. INTRODUCTION Mobile satellite communications systems have been rapily moving towar the broaban service. We previously reporte the concept of an S-ban next generation mobile satellite communication system for omestic service. We estimate that the total amount of traffic carrie by such systems in Japan will be 1 Gbps; our intention is to realize a cost-effecte system in S-ban as a 1-ton-class geostationary satellite [1], [2]. The require capacity of 1 Gbps can be achieve by employing a multi-beam system that allows high antenna gain beams an extremely high levels of frequency reuse. Moreover, it is require that the RF power resource allocate to each beam vary to match the beam s traffic. Two major technologies are important in achieving the propose system. The first one is an ultra light-weight large antenna reflector, whose aperture iameter is about 2 m. The secon is a light-weight 1-beam class antenna fee system. We have stuie the technologies available to satisfy such requirements. The electric performance an function of the antenna esigne using these technologies are escribe in previous reports [3], [4]. Our prior stuies ientifie the problem that the antenna patterns are eforme by the reflector surface istortion in orbit an antenna gain ecrease of the target area is relately large. The eforme antenna patterns can not be compensate by conventional mechanical methos, because the reflector surface istortion fails to keep the relate positions of the forme beams. In array-fe reflector multi-beam antennas, since each beam is forme by an appropriate exciting weight for each target area, the eforme antenna patterns can be compensate by electrical metho. However, a very large number of variable phase-shifters is neee to compensate the eforme antenna patterns. This paper presents a novel electrical approach to the compensation of eforme antenna patterns. The compensation function is realize with the minimum of aitional evices. To achieve the propose metho, we propose here two important technologies: highly precise etection of beam pointing errors an a metho of etermining phase shift compensation value. The valiity of the propose methos is confirme by antenna pattern calculations an experiments on a fabricate array-fe reflector antenna. 2. REQUIREMENTS FOR ONBOARD ANTENNA SYSTEMS AND DESIGNED ANTENNA As escribe above, our aim is to realize an S-ban 1-tonclass geostationary satellite that offers up to 1 Gbps. This is 3 times the capacity of the current mobile communication satellites. Achieving 1 Gbps capacity in the 3 MHz banwith allocate to MSS (Mobile Satellite Service) in S-ban requires frequency reuse in excess of 2 times. Figure 1 1

2 shows an example of beam allocation for the system. Current S-ban service coverage is covere by 69 beams. Each beam offers at least 43.5Bi at its ege of coverage (EOC). The onboar antenna system must allocate RF power to each beam accoring to the beam s traffic. To meet the system requirements, we stuie an onboar multi-beam reflector antenna [3], [4]. Designe antenna was an off-focal type array-fe reflector antenna whose configuration is shown in Fig. 2(a). Fig. 2(b) shows an image of ultra-light weight large reflector which consists of a large scale cable network system an rib structure with tenons an suffers from large eformation [3]. The antenna parameters are shown in Table 1. Since the antenna fee system must be lightweight while achieving high antenna gain an highly RF power istribution flexibility between beams, it employs two of our techniques [4]. The first one is to apply beam group concept. It set up beam groups; in each group, all beams are excite using the same amplitue weight. This improves antenna gain over the entire coverage area compare with the conventional array-fe reflector antenna an realizes RF power istribution flexibility between beams. The 2n technique is a beam-forming metho that combines beams from ifferent beam-groups so as to improve RF power istribution flexibility between beams. The antenna fee system that uses our techniques is much lighter than conventional multi-beam antenna fee systems. Calculate antenna patterns of the North-West-Ege, the South-West-Ege an the South-East-Ege are shown in Figure 1. These beams EOC gains are 44.12, an 43.91Bi, respectely. We confirme by calculations that the antenna gain requirements were satisfie. D f Parabolic Reflector Array Feeer Xf (a): Antenna Configuration focus (b): An Image of Reflector Fig. 2: Designe Array-fe Multi-beam Reflector Antenna TABLE 1: DESIGNED ANTENNA PARAMETERS Frequency S-ban Aperture iameter : D (m) 19. Focal length : f (m) 13.3 Defocus length : (m) 1.2 Offset angle : (egrees) 43.5 Fee element Microstrip patch Fee element spacing (mm) 12 Number of elements 64 Number of beams DEFORMED ANTENNA PATTERN BY REFLECTOR SURFACE DISTORTION IN ORBIT Fig. 1: Beam Allocation an Calculate North-East ege, South-East ege an South-West ege Antenna Patterns (Contours: -2B, -1B an -2B below peak) The antenna patterns are egrae by satellite attitue fluctuation, reflector misalignment, reflector surface istortion, an others. These factors generally shift entire antenna patterns while maintaining the relate positions of the beams. Such isplacement can be compensate by conventional mechanical methos. Thermal istortion of reflector surface in eclipse, however, yiels efocusing an curvature variation of the reflector, antenna patterns are either expane or shrunk. The relate positions of the forme beams are not kept. The eforme antenna patterns by 2-m class reflector surface istortion are really significant in the service area ege beams. 2 International Symposium on Antennas an Propagation ISAP 26

3 We show by calculations that the impact of surface istortion is significant. To moel the thermal shrinkage surface that shoul be allowe for, we assume that the focus of the reflector shifte by 1mm away from its esign position. Calculate antenna patterns for the before an after eforme by reflector surface istortion are shown in Figure 3. The antenna patterns are for the North-East-Ege, the South-West-Ege an the South-East-Ege; the pointing errors are quite obvious. These beams EOC gains of each target area are Bi, 43.Bi an 42.82Bi, respectely. These changes break the antenna gain requirements relately large an force the evelopment of a new compensation metho. Before an after eform ae evices are variable phase-shifters connecte to each fee element, phase-shift controller, an pointing error etector. variable phase-shifters for compensation output terminals for each beam fixe BFN for beam1 fixe BFN for beam2 fixe BFN for beamn Pointing error etector (more than 3 points) Detector outputs Caluculation of eforme factors an phase shift compensation value θi LNA fee element Control comman Phase-shifters controller parabolic reflector Fig. 4: Block Diagram of Array-fe Reflector Multi-beam Antenna with Deforme Antenna Pattern Compensation Function Before an after eform Before an after eform Fig. 3: Calculate North-East ege, South-West an South-East ege Antenna Patterns (Contours: -2B below peak of before an after eforme by reflector surface istortion) 4. DEFORMED ANTENNA PATTERN COMPENSATION METHOD A. Concept of Deforme Antenna Pattern Compensation Metho In array-fe reflector multi-beam antennas, since each beam is forme by an appropriate exciting weight for each target area, eforme antenna patterns can be compensate by electrical metho. However, it is necessary to ajust the exciting weight of each beam inepenently an a very large number of variable phase-shifters must be neee. We propose here a novel antenna pattern compensation metho. The concept is to achieve the compensation function with the minimum of aitional evices. Figure 4 shows a block iagram of an array-fe reflector multi-beam antenna that uses the eforme antenna pattern compensation metho. The In orer to compensate the gross eformation of all antenna patterns with common phase-shifters, a metho for etermining each phase shift compensation value is a very important. We assume that there are two characteristic factors an calculate phase compensation for each factor. The first one, efine as irectity shift factor, inicates the whole movement of the antenna patterns; the relate positions of the beams are maintaine. The secon factor, efine as scale fluctuation factor, inicates the expansion or shrinkage of the entire antenna patterns. These eforme factors are extracte from beam pointing error. Since the irectity shift factor consists of azimuth an elevation coefficients an the scale fluctuation factor has one coefficient, we have to nee more than three pointing error etection points. Figure 5 shows our eforme antenna patterns compensation flow. The compensation phase shifters are then controlle accoring to the calculate factors. Two technologies form the heart of the propose metho. The first one is a metho of etermining phase shift compensation value. The secon is the highly precise etection of beam pointing errors. The etails of the two technologies are escribe below. International Symposium on Antennas an Propagation ISAP 26 3

4 start Detection of pointing errors (more than 3 observations) Extraction of eforme factors irectity shift Calculation of each phase shift compensation value Control phase-shifters for compensation value scale factor fluctuation Calculation of each phase shift compensation value Fig. 5: Propose Deforme Antenna Patterns Compensation Flow B. Metho of Determining Phase Compensation Values To compensate the eforme antenna patterns with common phase-shifters, we calculate irectity shift factor; D = ( az, el ) an scale fluctuation factor; S using etecte pointing error signals at more than three observations. Figure 6 shows the concept of using the control phase to hanle each eformation factor. In terms of the irectity shift factor, the phase-shift values that yiel the virtual inclination of the aperture of the array feeer are etermine. The phase compensation values for this factor of each fee element _i is gen as follows, θ _ i = ( α δ az + β δ el )( ri r ) where az an el inicate the irectity shift of azimuth an elevation angle, r i an r inicate a i-th fee element position an a center position of the fee array, an are coefficient, respectely. In terms of the scale fluctuation factor, the phase-shift values that yiel a virtual curvature of the aperture are etermine. The phase compensation values for this factor of each fee element s_i is gen as follows, θ s _ i = γ { r i r ( S) r r ( S )} where r inicate an original point of the virtual aperture, is a coefficient, respectely. Then, the total phase shift compensation values of each fee element is gen as follows, θ = θ + θ. i _ i s _ i C. Highly Precise Detection Metho of Beam Pointing Errors It is well known that the monopulse metho offers a simple configuration an achieves very precise beam pointing error etection. It is use in tracking antennas an raar systems [5]. However, the monopulse metho is not irectly applicable to our antenna since it is structurally an electrically asymmetrical. In orer to apply the monopulse metho to our antenna, our proposal sets virtual axes to realize electrical symmetry. Figure 7 shows our esigne array feeer layout an virtual axes for the South-East ege beam. The virtual axes are set so that the recee power level of each fee element group ie by the axes become the same. Figure 8 shows a block iagram of beam pointing errors etector. This obtains the azimuth an elevation pointing error signals as well as the forme beam signal. Signal levels versus azimuth an elevation pointing error are calculate by the propose technique. As an example, calculation result in the South-East-Ege beam is shown in Figure 9. The plots confirm the signal level has monotonous change within a range of over -.3 to +.3 egrees. It is large enough to etect expecte beam pointing error. Yf esign axes virtual axes for the South-East ege beam Xf C A Fig. 7: Array Feeer Layout an Virtual Axes for the South-East Ege Beam B D : fee element w : weight(amp. / phase) compensation of irectity shift array center compensation of scale fluctuation aperture virtual aperture w a1 w a2 w b1 w b2 w c1 w c2 w 1 w 2 comb comb comb comb A B C D 18egHYB 18egHYB r' virtual aperture original point of virtual aperture aperture phase shift compensation value Fig. 6: Concept of the Control Phase-Shifting for Each Deforme Factors 18egHYB 18egHYB Output terminal of foming beam (Σ : A+B+C+D) Output of El : A+B-C-D error signals Az : A-B+C-D Fig. 8: Block Diagram of a Beam Pointing Error Detector 4 International Symposium on Antennas an Propagation ISAP 26

5 Azimuth Error (egrees) Elevation Error (egrees) Fig. 9: Calculate Signal Levels versus Azimuth an Elevation Pointing Error in South-East Ege Beam D. Simulation Result Calculate antenna patterns for the before an compensate eforme reflector surfaces are shown in Figure 1. The antenna patterns of the North-East-Ege, South-West-Ege an the South-East-Ege are shown. Table 2 summarizes the calculate antenna patterns for the as-esigne, eforme, an compensate reflector surfaces. It shows that our propose metho can aequately compensate the eforme antenna patterns to satisfying the antenna gain requirements. As esigne an compensate eform Beam position TABLE 2: SUMMARY OF SIMULATION RESULT North East Ege South East Ege South West Ege Requirement of EOC gain (Bi) 43.5 As esigne EOC gain (Bi) Deforme EOC Gain (Bi) Gain ecrease (B) Pointing shift (egrees) Az. : +.18 El. : +.63 Az. : -.54 El. : +.31 Az. :. El. : -.25 Directity shift (egrees) Az. : +.54, El. : +.3 Scale fluctuation.929 Compensate EOC gain (Bi) Compensation gain (B) EXPERIMENTAL VERIFICATION OF FUNDAMENTAL PERFORMANCE USING AN ARRAY-FED REFLECTOR MULTI-BEAM ANTENNA A. Manufacture Array-fe Reflector Antenna To verify the funamental performance of the propose antenna pattern compensation metho, we fabricate an arrayfe reflector multi-beam antenna; its block iagram is shown in Figure 4. An overview an esign parameters are shown in Figure 11 an Table 3, respectely [6]. Three beams can be inepenently forme with the same elevation angle. Each beam is forme by a beam forming network which controls the amplitue an phase of each fee element. Parabolic reflector As esigne an compensate eform As esigne an compensate eform Array Feeer Fixe BFN Fig. 1: Calculate North-East ege, South-West ege an South-East ege Antenna Patterns (Contours: -2B below peak of as esigne an compensate eforme antenna patterns) Phase-shifters for compensation Amplifiers Output terminals Fig. 11: Fabricate Array-fe Multi-beam Reflector Antenna International Symposium on Antennas an Propagation ISAP 26 5

6 TABLE 3: FABRICATED ANTENNA PARAMETERS Frequency (GHz) Aperture iameter : D (m) 1.4 Focal length : f (m) 1.15 Defocus length : (m).5 Offset angle : (egrees) 45. Fee element Helical with cup Fee element spacing (mm) 9 Number of elements 19 (triangular lattice) Number of beams 3 B. Experimental Verification Results The experimental verification examine the two antenna eformations factors (irectity shift factor an scale fluctuation factor). The fabricate antenna subjecte to farfiel measurements. The fabricate antenna was set on a turntable in a semi-open type raio anechoic chamber about 1.6 Km from the transmission antenna. The results are shown in Figure 12. Fig.12 (a) shows the compensation of the irectity shift factor. All antenna patterns were shifte in the same irection by the phase control. Fig.12 (b) shows the compensation of the scale fluctuation factor. All antenna patterns were basically scale by the phase control. These plots confirm that the propose eforme antenna pattern compensation metho offers the funamental functions neee before compensation after compensation Angle (egrees) (a): Compensation of the Directity Shift Factor before compensation after compensation Angle (egrees) (b): Compensation of the Scale Fluctuation Factor Fig. 12: Measure Antenna Patterns before an after Phase Control Compensation 6. CONCLUSIONS We propose a novel eforme antenna pattern compensation metho that requires the minimum of aitional evices. To realize the propose metho, we propose two techniques: the highly precise etection of beam pointing errors an a metho of etermining the phase shift compensation values. We calculate antenna patterns to confirm the valiity of the propose metho. The results showe that our metho can compensate antenna pattern eformation so that the antenna gain requirements are satisfie. The funamental performance of the propose metho was experimentally verifie using a fabricate antenna with three beams. We inten to conuct more extense experiments on the propose metho an further enhance its performance. ACKNOWLEDGEMENT This work is partially supporte by The SCOPE (Strategic Information an Communications R&D Promotion Programme) by the Ministry of Internal Affairs an Communications. REFERENCES [1] M. Ueba, K.Ohata, J. Mitsugi, an M. Umehira, Broaban an Scalable Mobile Satellite Communication System for Future Access Networks, Proc. 22n AIAA International Communications Satellite Systems Conference, , May 24, Monterey, CA. [2] M. Ueba, K. Ohata, A. Meguro, an K. Kobayashi, A Feasibility Stuy of Broaban an Scalable Mobile Satellite Communication System for Ubiquitous Networks, Proc. 23r AIAA International Communications Satellite Systems Conference, I214, Sep. 25, Roma, Italy. [3] A. Meguro, S. Haraa, an M. Ueba, Structural Characteristics of an Ultra-light Large Antenna Reflector onboar Communication Satellite, 55th International Astronautical Congress, IAF-4-I1., Sep., 24, Vancouver, Canaa. [4] Y. Suzuki, A. Meguro, an M. Ueba, Phase-array Multi-beam Antenna Feeer with Beam-group Configuration for Broaban an Scalable Mobile Satellite Communication Systems, Proc. 23r AIAA International Communications Satellite Systems Conference, I15, Sep. 25, Roma, Italy. [5] G. Maral an M. Bousquet, Satellite Communications System, John Wiley & Sons, Lt., 21. [6] Y. Suzuki, S. Haraa, A. Meguro, an M. Ueba, Experimental verification of eforme antenna patterns compensation metho for next generation mobile communication satellite antennas, IEICE Technical Report, SAT26-6, pp.29-32, May International Symposium on Antennas an Propagation ISAP 26