A Novel Digitally Polarization Tracking Antenna for Ku-band Mobile Satellite Communication Systems

Transcription

1 8th AIAA Intenational Communications Satellite Systems Confeence (ICSSC-) 3 August - Septembe, Anaheim, Califonia AIAA -87 A Novel Digitally Polaization Tacking Antenna fo Ku-band Mobile Satellite Communication Systems Yoshinoi Suzuki, Fumihio Yamashita and Kiyoshi Kobayashi 3 NTT Access Netwok Sevice Systems Laboatoies, NTT Copoation, Yokosuka, Kanagawa, , Japan and Yozo Takeda 4 SKY Pefect JSAT Copoation, Minato-ku, Tokyo, 7-5, Japan This pape pesents a novel electical polaization tacking method fo mobile satellite communication systems that use linea polaization. It consists of two main technologies; the fist estimates the mobile station antenna s elative polaization angle against that of the satellite s onboad antenna, the second foms counte polaization against the estimated value. We use a Ku-band communication satellite to evaluate the polaization tacking chaacteistics and BER pefomances. These esults show that the poposed antenna offes the desied pefomances. Nomenclatue G i = tansmission coefficient of a DPT eceiving antenna polaization plane i (i = o ) G ti = tansmission coefficient of a DPT tansmitting antenna polaization plane i (i = o ) P = popagation matix R = polaization angle otation matix R = amplitude of polaization efeence signal of hoizontal polaization R i = eceived amplitude of R in a polaization plane i (i = o ) R = amplitude of polaization efeence signal of vetical polaization R i = eceived amplitude of R in a polaization plane i (i = o ) ij = input signal amplitude of channel estimato (i = o, j = o ) S = modulated signal S i = output signal fom a polaization plane i (i = o ) S = eceived signal of efeence antenna at hoizontal plane S = eceived signal of efeence antenna at vetical plane w i = calibation coefficient fo the RF font-end path i (i = o ) = caie angula fequency = otated polaization angle value = elative polaization angle against satellite onboad antenna I. Intoduction ROADBAND mobile satellite communications sevices ae aleady being offeed to passenges on aiplanes, B tains, and vessels via the Ku-band -4. oweve, the eath stations ae quite expensive because highly accuate auto-tacking antennas ae equied. They must tack not only satellite diection but also polaization pecisely so as not to intefee with othe satellite uses and/o othe polaization uses. Polaization tacking makes it difficult to ealize the low-pofile antennas desied fo aibone and/o tain communication. Insufficient polaization tacking yields hamful coss-polaization intefeence and degades fequency utilization efficiency. Reseach Enginee, Wieless Access Netwok Systems Poject, - hikai-no-oka, Yokosuka, , Japan, non-membe. Senio Reseach Enginee, Wieless Access Netwok Systems Poject, - hikai-no-oka, Yokosuka, , Japan, non-membe. 3 Senio Reseach Enginee, Supeviso, Wieless Access Netwok Systems Poject, - hikai-no-oka, Yokosuka, , Japan, non-membe. 4 Manage, Reseach and Development Depatment, -4-4 Akasaka, Minato-ku Tokyo, 7-5 Japan, non-membe. Copyight by the, Inc. All ights eseved.

2 To tackle these poblems, we focused on the electical polaization tacking antenna appoach. It can eliminate the otation mechanism of the antenna feed and attendant otay joints and so can be expected to impove tacking speed while loweing costs. To ealize electical antenna polaization tacking, two main technologies ae needed. The fist estimates the shift in the mobile station antenna s elative polaization angle against that of the satellite s onboad antenna. The second foms counte polaization against the estimated value. In geneal, elative polaization angles against the satellite s onboad antenna ae calculated by antenna position infomation and taget satellite infomation 4-5. oweve, due to low-fequency distubances, the polaization angle estimates become less accuate 6. Aibone antennas, a typical application of electical polaization tacking, employ an analog-contolled polaization fome in the RF band 5. It consists of some RF components such as phase-shifte and vaiable gain amplifie. As highly accuate polaization foming must be ealized by exciting othogonal two polaization planes with appopiate weights, they suffe fom the contol eos tiggeed by the fequency and tempeatue dependence of the RF components. This pape pesents a novel digital polaization tacking (DPT) antenna that achieves high-speed and highaccuacy polaization tacking. Ou poposal digitally implements a elative polaization angle estimation function and a counte polaization foming function. Ou polaization angle estimato, unlike the conventional polaization angle estimato, estimates the elative polaization angle fom the estimated popagation factos of vetical and hoizontal polaization signals fom a base station whose antenna is pecisely aligned in tems of diection and polaization. Ou polaization angle estimation method dispenses with high-pecision sensos needed by the conventional appoach to calculating polaization angle. Futhemoe, to achieve high accuacy polaization foming, ou antenna is equipped with a digital polaization contolle and calibato fo RF components chaacteistics. As ou calibatos can offset the RF components chaacteistics such as tansmission gain and phase, manual adjustment of the RF cicuit is unnecessay. To evaluate the poposed polaization tacking function, we built seveal eath stations and caied out seveal expeiments using a Ku-band communication satellite. As a esult of satellite expeiments, it was confimed that the poposed polaization tacking technique satisfied the egulatoy value on XPD (Coss Polaization Discimination) and dual polaization shaing tansmission among independent eath stations was available in Ku-band mobile satellite communications. II. Requied Pefomance As the taget value with egad to polaization foming ability, we detemined that the XPD had to be at least 7 db unde the ship motion envionment. SAT (ey Small Apetue Teminal) systems, i.e. the well known Kuband satellite communication system with dual polaization use, must satisfy this value in Japan 7. III. Poposed Polaization Tacking Antenna To ealize highly accuate polaization tacking without highly accuate sensos, we popose a novel polaization tacking method. This method consists of two main technologies; the fist is estimation of the s elative polaization angle (θ ) against that of the satellite s onboad antenna using a polaization efeence signal fom the satellite antenna; the second is counte polaization foming against the estimated value. Figue shows a block diagam of the. It consists of a dual polaization antenna, a elative polaization angle estimato, an electical polaization foming tansmitte, and a maximum atio combine. The dual polaization antenna dispenses with a polaization adjustment mechanism and hamonization of the tansmission chaacteistics on the two-path of the RF cicuit. The polaization foming tansmitte and the maximum atio combine counte the polaization mismatch based on θ detected by the elative polaization angle estimato. Pol. ef. signal Modulated signal OMT Pol. Pol. Relative pol. angle estimato Pol. setting Maximum atio combine Tansmitted signal Dual pol. antenna (Relative pol. angle : ) OMT : Otho-Mode Tansduce Pol. Pol. Polaization foming tansmitte Signal in Signal out Figue. Block diagam of poposed digitally polaization tacking (DPT) antenna.

3 A. Relative Polaization Angle Estimation Method To estimate θ, we popose to use a polaization efeence signal fom the satellite antenna. Figue shows the block diagam of the elative polaization angle estimato. It consists of a dual polaization antenna, a two-path RF font-end, a channel estimato and a polaization angle calculato. Figue 3 shows the pinciple of polaization angle estimation. When the DPF antenna eceives the polaization efeence signal, the example is hoizontal polaization (R ), the eceived signal s amplitude on the s polaization planes and (pol. and pol. ) is given by the Eq. (a) and (b). θ value can be calculated by Eq. (). R R cosθ (a) R R sin θ (b) R θ tan () R oweve, the actual input signals amplitudes at the channel estimato (, ) ae affected by the RF font-end and antenna gain imbalances. These factos ae descibed as follows, GR (3a) G R (3b) Digital signal pocesso RF font-end Pol. angle calculato Channel estimato LNA Antenna OMT LNA : Low noise amplifie Gi : Tansmission coefficient of the pol. plane i Figue. Block diagam of a elative polaization angle estimato. pol. plane A/D A/D G LNA G R : Ref. signal (. pol.) R, R : Recieved signals of pol. plane and. plane pol. plane R R : Relative pol. angle. plane Figue 3. Relative polaization angle estimation pinciple of a against the satellite s onboad antenna. whee G and G ae tansmission coefficient of a DPT eceiving antenna polaization plane i (i= o ). θ value can be calculated by Eq. (4). θ G G tan (4) To calculate θ value using Eq. (4), we must specify G and G. oweve, it is difficult to specify these values accuately due to fluctuation in the chaacteistics of the RF devices. We popose a θ calculation method that dispensed with G and G. Ou appoach is to use two othogonal polaization signals as a polaization efeence. When the DPF antenna eceives a vetical polaization efeence signal (R ), input signals amplitude at the channel estimato is descibed as follows. Ou poposal is to calculate θ value by Eq. (6). GR (5a) G R (5b) G G θ tan tan (6) G G 3

4 In Eq. (6), θ is independent of G and G values, but the sign of θ is unknown. Since the initial sign state of θ can be specified when detemining the calibato s coefficients, descibed below, we only have to detect changes in the sign of θ. We focus on the elative phase between eceived signals on the s pol. and pol.. When θ is positive, see Fig. 3, the eceived elative polaization efeence signals, and, ae in evesed phase. This phenomenon is also confimed by Eq. (a) and (b). Meanwhile, when θ is negative, the eceived signals ae in-phase. Theefoe, the change in the sign of θ can be detected by the phase elationship of the eceived signals. B. Electical Polaization Foming Method () Pinciple To electically fom the desied polaization, it is necessay to excite the two othogonal polaization antenna planes with appopiate weights. Figue 4 shows the polaization foming pinciple. The s polaization axis is mismatched by θ fom that of the satellite antenna s polaization axis. In ode to output hoizontal polaization signal fom the, the antenna must be electically otated by θ = θ. Fo this pupose, the s pol. and pol. must output evesed phase signals whose amplitude atio is cosθ to sinθ. These two signals ae spatially in-phase combined in the hoizontal polaization plane (. plane), and canceled out in the vetical polaization plane (. plane). () Implementation and Calibation oweve, the output signal eos in each antenna plane due to tansmitte gain imbalance cause polaization foming eo. Amplitude eo causes polaization angle shift eo and phase eo yields elliptical polaization. Both eos cause signal leakage on the othogonal polaization plane and XPD degadation. Theefoe, hamonization of the tansmission chaacteistics on the two-path of the DPT antenna is vey impotant. Fig. 5 illustates a pomising appoach to ealize digital polaization foming. It consists of a polaization angle shifte, an RF font-end calibato, a two-path RF font-end, and a dual polaization antenna. In ode to detemine the calibato coefficients as shown in Fig. 5, we monitoed signal level eceived by a efeence antenna which is located at the fixed base station. The efeence antenna also tansmits polaization efeence signals. Figue 6 shows the mathematical model used fo detemining the calibato s coefficients. In this figue, R() and P ae the tansmission chaacteistics of the polaization angle otation matix and the popagation matix between the efeence antenna and the, espectively. R() and P ae given by Eq. (7) and (8). Mod. Signal : S pol. plane Polaization angle shifte S : Combined signal S, S : Output signals fom DPT antenna pol. plane and Signal elements of plane. plane pol. plane Digital signal pocesso RF font-end RF font-end Calibato S S D/A D/A : Relative pol. angle. plane Figue 4. Electically polaization foming pinciple using dual polaization antenna. Gt PA PA Antenna OMT PA : igh Powe Amplifie Gt Gti : Tansmission coefficient of the pol. plane i Figue 5. Block diagam of a polaization foming tansmitte. Signal input : S Pol. angle shifte : R() w w Gt Gt Polaization otation : P Figue 6. mathematical model fo calibation. S S S S cos sin R (7) cosθ sin θ sin θ P (8) cosθ 4

5 The outputs signals fom pol. and pol., S and S, ae descibed by Eq. (9), S wg S t w G t w G cos w Gt sin jt t jt R Se Se (9) whee S and indicate the modulated signal and caie angula fequency, espectively; w i and G ti ae the calibation coefficient of the RF font-end calibato and the tansmission coefficient of the polaization plane i (i= o ), espectively. The signals eceived at the efeence antenna ae given by Eq. (). S S ( ) ( ) S w G cosθ cos w G sin θ sin t t jt P Se S w Gt sin θ cos w Gt cosθ sin () To fom. Polaization, we set θ = θ. The intefeence signal, S (θ ), is then given by Eq. (). S jt θ sin θ θ w G w G Se () cos t t To minimize the intefeence signal, S (θ ), the calibation coefficients should be adjusted to meet Eq. (). w G w G () t t We popose a sequential calibation pocedue that nulls S (θ ). At fist, the phase of w is detemined so as to minimize S (θ ). Next, the amplitude of w is detemined so as to null S (θ ). I. Expeiments and Results To confim the poposed techniques, we fabicated a that consisted of a dual-polaization autotacking, RF font-end, and seveal kinds of functions fo polaization tacking. All polaization tacking functions wee implemented on FPGAs (Field Pogammable Gate Aays). Fo channel estimation, we adopted the MMSE (Minimum Mean Squae Eo) algoithm and used a pai of othogonal Gold codes as the / unique wod 8. A dual-polaization auto-tacking antenna with RF font-end was also developed. Table shows the specifications of the antenna. This antenna has no polaization tacking mechanism and each path s tansmission gain diffeence due to no adjustment of the RF devices tansmission chaacteistics. Communication satellite. pol. Figue 7 shows the satellite expeimental setup. It. pol. consisted of a simulated mobile station (MS) Table. Tacking antenna specification. Apetue diamete. m Numbe of polaization (linea) Pointing eo <. degees Antenna gain TX : 4.3 dbi RX : 39.3 dbi EIRP 45 dbw G/T 5 db/k RF output powe 8 W (each pol.) DPT antenna Table. Simulated ship motion. Motion Small Big Cicling Pol. tacking functions Ref. signals geneato Type wave wave and modulato and MODEM Roll º / 6s 5º / 6s 5º / 7s Pitch º / 4s 4º / 5s º / s Mobile station (MS) Base station (BS) Yaw 35º 5º 7º Figue 7. Satellite expeimental setup. 5 Ref. Signals Modulated signal Ship motion simulato Modulated signal Ref. Signals Refeence antenna

6 employing and a base station (BS). The was mounted on a ship motion simulato, which mechanically ceates thee dynamic ship motions (Small wave, Big wave, and Cicling). Table lists the motion specifications. Fist of all, we evaluated the calibation function and static polaization tacking function. In this expeiment, we monitoed the BS s eceived signal level. Figue 8 and Figue 9 show eceived signal specta of. and. polaization befoe and afte calibation, espectively. As shown in Fig. 9, we confimed that polaization intefeence less than the noise level by using the poposed calibation. The calibation esults indicated that the amplitude and phase diffeences befoe calibation wee estimated to be.8 db and 7 degees, espectively pol.. pol Relative Fequency (Mz) Figue 8. Measued spectums befoe calibation. Next, we evaluated the polaization tacking pefomance in atificial ship motion envionments. Fo compaison, we also measued a commecial tacking antenna unde the same motions. Figue and Figue show the measued XPD pefomances. Figue confims both antennas achieved the equied polaization tacking pefomance. (XPD > 7 db) With cicling motion, see Fig., the commecial antenna allowed tacking eo to ceep in. This occued because the maximum motion aound the YAW axis was 5 º within 8 sec., moe than twice the commecial antenna spec. (6 º pe 8 sec). The poposed could maintain antenna polaization tacking even in the face of this exteme yaw ate. These esults confim that ou method offes high speed and high accuacy polaization tacking pol.. pol Relative Fequency (Mz) Figue 9. Measued spectums afte calibation. 45 Commecial 45 Commecial YAW Figue. : : : 3: 4: 5: Elapsed time (minutes) Measued XPD at big wave. Next, we measued the BER pefomance of the link fom the MS to the BS. Fo this measuement, we used a QPSK modulated signal (symbol ate:.8 Mz) with tubo poduct code (R =.66). Figue shows the measuement esults in thee cases; manual polaization adjustment in static condition, polaization tacking mode in static condition and in big wave condition. The degadation in equied Eb/N was about.5 db in each case. Since no significant diffeence was obseved among these thee conditions, it was concluded that the poposed polaization tacking was sufficiently available even in pactical moving envionments. Finally, we pefomed a dual polaization fequency shaing tansmission expeiment. In this expeiment, we measued the BER pefomance of the BS (. pol.) loop-back link. We also tansmitted a polaization intefeence signal fom the BS (. pol.) o the MS (. pol.). Figue 3 shows the esults. The degadations imposed by dual 6 Figue. cicling. : 3: 6: 9: - Elapsed time (minutes) Measued XPD and YAW angle at

7 polaization fequency shaing tansmission wee about.3 db in both cases compaed to single polaization tansmission. We consideed that the degadation was due to the BS s eceived antenna s XPD chaacteistics. These esults show that ou polaization tacking technique is feasible fo dual polaization shaing tansmission using a Ku-band mobile satellite. E- E-3 E-4 E-5 E-6 E Eb/No (db) Figue. BER pefomances (MS to BS). Simulation Manual adjust Static Big wave E- E-3 E-4 E-5 E-6 E-7 Single pol. use Dual pol. use Eb/No (db) Eb/No (db) (a) BS. pol. (b) MS. pol. Figue 3. BER pefomances (BS loop-back).. Conclusion We descibed a digitally contolled polaization tacking antenna fo Ku-band mobile communication satellite sevice. To achieve high polaization tacking ability, we poposed two techniques. One estimates the mobile station antenna s elative polaization angle against that of the satellite s onboad antenna. The othe is counte polaization foming against the estimated value. We evaluated the polaization tacking chaacteistics and conducted tansmission expeiments using Ku-band communication satellite. The esults showed that the developed antenna offes the desied pefomance in tems of XPD and BER. Acknowledgments This wok is elated to eseach sponsoed by the Ministy of Intenal Affai and Communications though gants fo Reseach and Development of the Adaptive Polaization Division Multiplexing (APDM) fo the Satellite Communications. Refeences E. Laase, and W. R. Richads, Deploying Connexion By Boeing SM Satellite Sevices, st AIAA-ICSSC, AIAA 3-35, Oct., 4, Yokohama, Japan. Y. Imaizumi, S. aada, F. Nagase, M. Nakayama, K. Ohata, and M. Ueba, Netwok Pefomances Evaluation fo Ku Band Mobile Satellite Communications System Based on Shinkansen Raiload s Popagation Condition, 5 TC-Fall, Session L., Sep. 5. Dallas, TX. 3. J. Lee, J. M. Kim, B. S. Lee,. Lee, and J. S. Ryoo, Recent Koean R&D in Satellite Communications, IEICE Tans. COMMUN., ol.e9-b, No., pp , Nov J. igaki, and M. Tsuchiya, Design of Isolation Mechanism and Contol Tacking fo Satellite in Maitime Boadband Antenna System, IEICE Tans. B, ol. J9-B, No., pp , Dec S. Nuimua, T. oie,. Sato, I. Naito, K. Kumoi,. Yoshizawa, Y. Konishi, N. Takeuchi, and Y. Shimawaki, Aibone Ku-band Antenna Subsystem fo Satellite Communications, Technical Repot of IEICE SAT4-3, pp.63-68, Oct T. Yoshida, K. Ohata, and M. Ueba, ighly Accuate Inclinomete Robust to Ultalow-Fequency Acceleation Distubances and Applications to Autotacking Antenna Systems fo essels, IEEE Tans. Instumentation and measuement, ol. 58, No. 8, Aug Odinance Regulating Radio Equipment, section 7.., Aticle F. Yamashita, J. Abe, K. Kobayashi, K. Ohata, and K. Ando, aiable Polaization Fequency Division Multiplexing (PFDM) fo Satellite Communications, 5th AIAA-ICSSC, AIAA 7-344, Ap., 7, Seoul, South Koea. 7