The invention is a satellite communication system used as a ground station to receive and transmit a signal.
Satellite communication (SATCOM) has made wireless local area networks (LAN) ubiquitous in a true sense. SATCOM requires the use of high-gain antennas due to its long communication distance. The conventional SATCOM system uses a large dish antenna that can be mechanically controlled to change a beam direction. Planar antennas that can replace dish antennas have been developed. However, mechanical beam control is still required to establish a link between satellite and ground stations.
Phased arrays are very attractive for satellite communications due to their planar structure and agile electronic beam control. There are several approaches to electronically control a beam. The majority of today's phased arrays rely on phase shifters to steer a beam. Although there have been many efforts for cost and size reduction, the phase shifter is still one of the most expensive parts in the SATCOM ground station system. The use of heterodyne scanning eliminates the need for phase shifters. However, the heterodyne scanning technique requires complex local oscillator (LO) networks, making the technique far from practical.
Moreover, beam steering for these approaches requires a priori knowledge of the satellite location or a feedback system to track the satellite using its beacon signal. This is a problem especially when the ground station is on the move. The ground station needs to continuously adjust the elevation and azimuth angles of radiation using peak-search of the beacon signal.
Smart antennas use digital beam-forming (DBF) techniques to overcome this problem, but this requires power-hungry analog-to-digital converters and digital signal processor circuits. Therefore smart antennas are not suited for SATCOM where the array requires a large number of elements. Needs exist for improved satellite communication systems.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.
The invention solves all the problems mentioned. The system was developed for SATCOM applications. However, it can be applied to other applications where a point-to-point link is required. For example, an unmanned aerial vehicle (UAV) requires a high gain antenna due to the limited fuel or battery power on the vehicle. The invented phased array provides real-time beam control that enhances the communication link between the UAV and ground stations. The phase lock loop circuitry in the new system uses arbitrary frequencies within a designated band for receiving and transmitting. That allows for full-duplex point-to-point communication links to be established using two sets of the invented array, enabling low-power operation of the communication system.
Applications for the new system include:
The new planar phased array antenna system is capable of autonomous beam-steering for both uplink and downlink without relying on digital signal processing. The system takes advantage of a beacon signal sent by a satellite in order to autonomously point a beam directly back at the satellite. Because the beacon and data signals are very close in frequency and illuminate the phased array from the same angle, the geometry phases of the beacon and data signals are the same at each element and can be eliminated by down-converting the data signal using the beacon signal as a local oscillator (LO) signal. The down-converted signals are thus combined in phase without using phase shifters. If the frequency difference between the beacon and communication signals is not negligible, the phase error can be easily compensated using frequency dividers and multipliers. The beacon signal is also used to steer the transmitted signal beam. The transmitting array establishes a beam toward the satellite by up-converting a communication signal using the phase-conjugated beacon signal. As an alternative to using the beacon signal for LO generation the received data signal can be used instead by implementing carrier recovery circuitry. In both cases the beam steering is done autonomously with no delay, because the system does not use any signal processing or feedback control.
The technology has the following characteristics:
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.
In the transmitter array 21, the incoming beacon signal 27 is split off by SAW filters 15 and phase-conjugated, and the output 16 is used as a LO signal provided to mixer 23 to up-convert an outbound IF information signal 22 to an outbound RF information signal 24. Signal 24 is amplified and then transmitted by antennas 13-13n as an uplink RF information signal 29 returned in the direction of beacon 27. Note that the phase of the signal 29 is the conjugation of the beacon signal 27 phase.
As a result, the transmitted signal 29 establishes a beam in the incoming direction of the beacon. Because the system does not rely on signal processing or feed back systems, beam steering is done in real time without any lag. Plural transmitter array elements have identical COTS components and antennas.
When the frequencies of the down-converted RF and beacon signals are given by ωbeacon and ωRF, the mixing process described in the previous paragraph can be expressed with the following equations. Equations 1 and 2 show the down-conversion process.
Case (1) ωbeacon<ωRF
cos(ωbeacont+θn)·cos(ωRFt+φdata(t)+θn)cos((ωRF−ωbeacon)t+φdata(t)) (1)
Case (2) ωbeacon>ωRF
cos(ωbeacont+θn)·cos(ωRFt+φdata(t)+θn)cos((ωbeacon−ωRF)t+φdata(t)) (2)
where θn is the geometry phase at the nth element of the array. The geometry phase is successfully eliminated.
Even if a beacon is not available, the autonomous beam steering is still possible by generating an LO from the received communication signal using carrier recovery circuitry.
The geometry phase is frequency dependant. Thus, if the beacon and communication frequencies are far apart or the directivity of the array is high, the beam pointing error due to the frequency difference is no longer negligible (Equations (3) and (4)).
Case (1) ωbeacon<ωRF
cos(ωbeacont+θn)·cos(ωRFt+φdata(t)+θ′n)cos((ωRF−ωbeacon)t+φdata(t)+θ′n−θn) (3)
Case (2) ωbeacon>ωRF
cos(ωbeacont+θn)·cos(ωRFt+φdata(t)+θ′n)cos((ωbeacon−ωRF)t+φdata(t)+θn−θ′n) (4)
where θn and θ′n are the geometry phases in the beacon and communication signals at the nth element of the array. The relationship between the geometry phases of the beacon and communication signals can be given by:
where ƒComm is the communication frequency and ƒBeacon is that of the beacon (before down-conversion). Therefore the phase of the beacon can be easily adjusted using the N and R counters in the PLL circuit. The N and R counters should be set so that
where ƒComm is the communication frequency and ƒBeacon is that of the beacon.
The transmitter array is based on phase conjugating array technology. The phase conjugating array has the interesting characteristic that it retransmits a signal back to the direction of the beacon. The output signal from the modem is split and applied to each element of the array. In order to transmit a signal in the direction of a satellite, the transmitted signal at each element must have the conjugated phase of the received beacon signal. The phase-conjugating operation can be achieved simply using the heterodyne mixing technique. The modem output signal is up-converted using the phase-conjugated LO generated by the received beacon (Equation (7)).
cos(ωmodemt+φdata(t))·cos(ωLOt−θn)cos((ωmodem+ωLO)t+φdata(t)−θn) (7)
Notice that the lower side band (LSB) of the mixing product is the phase conjugation of the LO signal and the phase will be different at each element as it depends on the phase of the received beacon signal.
The invented array is capable of communicating with different satellites A, B, C simultaneously with little modification. The system diagram of the multibeam steering array is shown in
The invented system can also be used in other types of mobile communications. An unmanned aerial vehicle (UAV) requires a high gain antenna due to the limited fuel or battery power on the vehicle. The new phased array can provide real-time beam control that enhances the communication link between the UAV and ground stations. Because the new system uses arbitrary frequencies within a designated band for receiving and transmitting thanks to the phase lock loop circuitry, full-duplex point-to-point communication link are established using two sets of the new arrays, enabling low-power operation of the communication system.
The receiver schematic diagram shows reception by an antenna element 15, filtering through a band pass filter 242, amplifying 244, down-converting 246 with a local oscillator 248 and amplifying 249.
The new SATCOM system can self-steer a beam toward a satellite that sends a beacon signal. This beam steering technique does not rely on any digital signal processing or algorithm to find the satellite direction. Due to that, the new technique enables agile autonomous beam steering for mobile satellite communication.
The invention has advantages over known systems in that the new satellite communications system is small and low cost and operates with no delay. The new system has multiple beam capability. It operates autonomously and in real time. Geometry phase of the signals are cancelled by mixing the beacon and radio frequency information signals. The new system provides retrodirection to satellites by mixing the satellite beacon with the radio frequency transmitter signal. The phase conjugation enables retrodirection from the transmitter.
The new satellite communications system uses open ended circular wave guide antennas having microstrip-to-slot line transition feeds combined with the antenna structure.
The receiver and transmitter antennas have low return losses in the satellite communication GHz bands.
The transmitter meets Transmit Sidelobe Mandatory Requirements for Intelsat Standard E Antennas. The system meets planned requirements of transmitting 1544 kbps at 3/4 rate from the hub and 512 kbps at 3/4 rate inbound from remotes. QPSK full duplex is provided and required uplink EIRP of 79.6 dBm is met. One watt of power in each element with 8 rows of 32 elements per row is sufficient to achieve the required EIRP.
Advantages of the new satellite communications system include: a flat architecture, the fact that the use of a beacon to generate LO eliminates the need for direction finding, allowing real time agile autonomous beam standing, no need for complicated high-frequency LO networks, a modular design that makes it easy to increase or decrease the array size, beam steering at low frequency, which makes possible the use of economical COTS ICs, no phase shifters, which leads to a reduced overall cost, and possible multi-beam control, allowing communication with several satellites simultaneously.
While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention, which is defined in the following claims.
This invention was made with government support under Contract No. N00014-04-C-0473 awarded by Office of Naval Research, Department of the Navy. The government has certain rights in this invention.
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