SATELLITE SIGNALLING SYSTEM

Abstract: This invention relates to a satellite signalling system. A ground station (70) receives paging signals from an earth-bound terminal, and an up-link radio transmitter/antenna (76) transmits paging signals and control signals to a satellite in earth orbit. An up-link receiver (78) coupled to the satellite receives the paging and control signals. A down-link transmitter (86) is coupled to an antenna whose beam may be directed in a predetermined sequence to a plurality of locations on earth as paging signals are transmitted to earth.

Description: SATELLITE SIGNALLING SYSTEM F eld of the Invention This invention relates to satellite signalling systems having signal coverage over a very wide area. The present invention is particularly, though not exclusively, concerned with a satellite signalling system such as may be used in a satellite paging system. Background of the Invention The present state-of-the-art satellite paging systems comprise a fixed position antenna mounted on a satellite in synchronous orbit. Typically in such systems, the antenna and the beam pattern of the antenna are both fixed so that the spot or the "footprint" of the signal beam covers that portion of the globe which is "visible" to the satellite; that is, approximately one-third of the earth's surface. The ground level strength of the signal transmitted by the antenna is fairly uniform across the covered area; however, the signal strength is very weak. This creates a problem, even for the highest powered synchronous satellites, since the ground level signal strength is too weak to allow for in-building paging coverage. For example, for a synchronous satellite with a radiated power of 400 watts, a state-of-the-art selective call paging receiver using the POCSAG paging protocol at a bit rate of 512 bits per second would only have 2 dB of signalling margin; that is, the average signal strength on the ground would be only 2 dB above the minimum level to activate a paging device. One solution would be to use low bit rate signals of 60 baud or less to reduce the bandwidth of the paging receiver. However, this makes the paging system extremely expensive and impractical in view of the small number of users that can be serviced by the paging system. That is, the low bit rate could only support a small volume of paging traffic. Thus, it is an object of the present invention to provide an improved satellite signalling system wherein the above disadvantages are overcome or at least alleviated. SUMMARY OF THE TWVEN TON In accordance with the present invention, there is provided a satellite signalling system comprising a ground station for receiving paging signals from an earth-bound paging terminal, an up-link radio transmitter and antenna for sending paging signals and control signals to a satellite, a satellite in orbit around the earth, an up¬ link receiver on the satellite for receiving the paging signals and control signals, and a down-link radio transmitter coupled to an antenna with a steerable beam, the satellite being arranged to control the position of the beam of the down-link antenna, the beam of the antenna being directed in a predetermined sequence to a plurality of locations on the earth as the paging signals are transmitted to the earth. It will also be appreciated that since the antenna is arranged to be movable in a predetermined sequence, the signal beam can be sequentially directed at specific geographical areas which, as a result, enables the antenna to scan a very wide area and thus provide continental or global coverage. In another aspect of the invention, the paging receivers in the system contain battery saver circuits that synchronize their ON-OFF switching operation with the motion of the down-link beam such that the receivers are "ON" or fully operational only when the beam is transmitting to the local area of the pager. A satellite signalling system in accordance with the present invention will now be described by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of a prior art satellite paging system. FIG. 2 is a diagram of a satellite paging system in accordance with the present invention. FIG. 3 is a diagram illustrating the operation of the inventive satellite paging system. FIG. 4 is a flow chart which illustrates satellite beam operation. FIGS. 5A-5C are diagrams illustrating batches of signals transmitted by inventive satellite paging system. FIG. 6 is a block diagram of a typical paging receiver suitable for use in conjunction with the inventive satellite paging system. FIGS. 7A-7B are timing diagrams illustrating the operation of the battery saver circuit in the paging receiver shown in FIG. 6. FIGS. 8A-8B are flow charts which describe the operation of the pager in FIG. 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring firstly to FIG. 1, a prior art satellite paging system 2 comprises a fixed antenna 4 mounted on a satellite 6 in synchronous orbit around the earth 8. The operation of the antenna 4 is controlled by the satellite 6. The antenna 4 is arranged so that the signal beam 10 (whose periphery is represented by the dotted lines 9 and 11) transmitted from the antenna 4 has a beam area which covers the portion of the earth's surface which is "visible" to the satellite 4. The beam is generally a spot beam. As has been stated above, the satellite paging system 2 cannot provide in-building paging coverage unless the data rate is reduced to approximately 60 baud or less since the signal strength at the earth's surface is very weak. Paging signals from a ground based terminal 31 are transmitted via antenna 33 to a receiving antenna 30 on satellite 6. This is represented by radio frequency (RF) up-link 32. Referring now also to FIG. 2, in order to circumvent the above described problems, a satellite paging system 20 according to a preferred embodiment of the present invention comprises a movable or steerable antenna 22 mounted on a satellite 24 in synchronous orbit around the earth 26. The movement and operation of the movable antenna 22 is controlled by the satellite 24. The movable antenna 22 is arranged so that the transmitted signal beam 27 (represented by dotted lines 21 and 23 in FIG. 2) has a small enough beam area, which is in the preferred embodiment the spot area 28, to provide sufficient signal strength at the earth's surface to allow for in-building paging coverage. Since the ground level signal strength varies as approximately the second power of the radius of the beam on the earth, reducing the radius by a factor of ten from 12,874.52 kilometers to 1,287.452 would provide 100 times, or 20 dB, more ground level signal. For example, a synchronous satellite with 400 watts of effective radiation power (ERP) and a spot beam diameter of 1,287.452 kilometers would produce a ground level signal strength that is 22 dB above the threshold for 512 baud POCSAG signalling. This is sufficient to provide good in-building paging coverage. The steerable antenna 22 can be moved by the satellite 24 so that the spot area 28 can be directed towards specific geographical locations on the earth's surface, and thus the satellite 24 can scan the whole portion of the earth's surface which is "visible" to the satellite 24. The movable or steerable antenna 22 scans each of the geographical locations in a predetermined sequence and for a predetermined time. The techniques for implementing an electronically steerable antenna are well known in the antenna art and are not duplicated here. For example see U.S. Patents Nos. 4,489,325 and 4,595,926. The satellite paging system, according to the present invention, operates in the following manner. Referring to FIG. 3, paging signals from ground base terminal 70 and control signals from a ground station satellite controller 74 are applied to up-link transmitter 76. Up-link signals, consisting of paging data signals and signals required to control the operation of a satellite which in turn control the operation of movable antenna 72, are transmitted from up-link transmitter 76 to up-link receiver 78 via transmitting antenna 80 and receiving antenna 72. It should be appreciated that up-link data can be transmitted at a much higher data rate than that used for down-link transmissions. The output of up-link receiver 78 is coupled to satellite control system 81 and satellite paging controller 82. A paging signal generator 84 receives information and control signals from satellite paging controller 82 and transmits paging information of a type previously described via down-link transmitter 86 and movable antenna 88 to a pager user 90 carrying paging receiver 92. Movable antenna 88 is directed by an antenna control system 94 coupled to satellite paging controller 82 to scan specific geographic locations on the earth's surface in a predetermined sequence and for a predetermined time. When the beam of movable antenna 22 is directed at a particular location, down-link transmitter 86 will transmit paging signals in the batch form previously described and corresponding to the specific location during the time slot assigned to that location via paging signal generator 84. The pagers in the paging system operate to synchronize their battery savers to the appropriate batch in the signal transmissions from the satellite using the synchronization code of the appropriate signal as will be described hereinbelow. Once the pager has been synchronized and addressed, it will decode the paging signals. FIG. 4 is a flow chart which describes the satellite beam steering operation. After the system has been initialized (102) , the next beam location is selected (104) . The beam is moved to the selected location (106) and the satellite beam activated (112) . The paging signals which are to be directed to that specific location are then transmitted (114) for a predetermined period of time (116) . When time has elapsed, the satellite beam is turned off (118) and if the system is not disabled (120) , the next beam location is selected (104) . FIG. 5A shows an exemplary representation 40 of one of the signals transmitted from the satellite antenna 22. The signal 40 is a time-division multiplexed binary paging signal and comprises a sequence of batches 42-50. Each of the batches 42-50 corresponds to a specific geographical location within the scanning area covered by the satellite 24. For example, batch 42 may correspond to New York, batch 44 Boston, batch 46 Chicago, etc. Each batch is transmitted to the appropriate location on a sequential basis. For purposes of example, the POCSAG signalling protocol is used for the signalling format in each batch. FIG. 5B illustrates that each batch (e.g., 42) comprises a preamble 62 followed by N frames 64. Finally, FIG. 5C shows an exemplary representation 61 of one of the frames in FIG. 5A. The frame 61 comprises a synchronization code 63 followed by a unique area identification code 65 that corresponds to the geographical location covered by the batch transmission. The identification code 65 is followed by address signals 66 for addressing a predetermined pager. The format of the frame 61 enables the operation of the battery savers in the pagers to be synchronized to the scanning sequence of the movable antenna 24 by using the synchronization code 63. A pager operating within a particular geographical location, once it has been synchronized, will only be powered up in order to decode paging signals if the satellite 24 is transmitting to its geographical location. This provides for good pager battery life. In addition, the identification code 65 of the frame 61 identifies the current operating location to the pager. Thus, if the pager is outside its normal operating area, the identification code 65 provides the pager with location information which can be displayed on a display or can be used to enable different operating modes of the pager, for example, the "roaming" mode. FIG. 6 is a block diagram of a paging receiver suitable for use within the inventive satellite paging system. It is a conventional selective call paging receiver which includes a radio receiver 122 which receives signals via antenna 120. The output of radio receiver 122 is applied to a microcomputer decoder 124 which processes the information contained in the received signals. As can be seen, microcomputer decoder 124 communicates with output annunciator 126, code plug address and option memory 128, and the user via controls on the pager. The operation of a paging receiver of the type shown in FIG. 6 is well known and is described in U.S. Patent No. 4,518,916 issued May 21, 1985, entitled "Universal Paging Device with Power Conservation." To function within the inventive satellite paging system, microprocessor decoder 124 is programmed to decode the signalling protocol shown in FIGS. 5A, 5B, and 5C. Specifically, the decoder is programmed to search for a signal from the satellite and then to synchronize its decoding operations to the timing of the motion of the beam. To synchronize to the signalling waveform, the decoder first remains on until it decodes the down link signal. It then synchronizes its decoding operation with the received signal using well-known synchroniza ion techniques. Having done this, the receiver then synchronizes the operation of its own battery saving circuits with the predetermined timing of the down -link transmission of the satellite using information that can be contained in either the program ROM of the microcomputer decoder or in the code plug and option memory. As a result, the receiver battery savers only activate the pager during the intervals when the satellite is transmitting to the geographic area in which the pager resides. For example, FIG. 7A again shows preamble 62 followed by a synchronization code 63 which is in turn followed by an area identification portion 65 in batch 42 (FIG. 5A) . The area identification portion 65 and an address number 2 130 form what is referred to as GROUP 1. Each subsequent group contains first and second addresses as is shown in FIG. 7A. Synchronizaton code 63 is repeated after the eighth group. Thus, if the receiver is programmed to respond to the GROUP 4 addresses, the battery saver circuits in the receiver will only activate the pager during its search operation 132 during which time the receiver will search for preamble 62 and synchronization code 63, during time interval 134 when the GROUP 4 addresses are being transmitted, and then again during each synchronization code interval 136. This is shown in FIG. 7B. Of course, the page is not activated during batches 44, 46, 48, and 50 corresponding to other geograhical locations . FIGS. 8A and 8B are flow charts which describe the operation of the receiver. After initialization, the receiver is turned on (140) and searches for the bit synchronization code (142) for a predetermined search period (144) after which the receiver is turned off (145) . If bit synchronization is achieved prior to termination of this predetermined search period (146) , the receiver will begin its search for the synchronization code (148) . Again, assuming the synchronization code is detected (150) prior to the expiration of a predetermined search period (152), the location code will be decoded (154) and the battery saver timing circuitry synchronized (156) . The receiver will then begin searching for an address signal (158) . Should an address signal be detected (160) , and alert signal will be generated (162) . If an address signal is not detected, the receiver will continue to search for an address signal until the battery saver timing circuits indicate that it is time to battery save (164) . If an end of batch has not been detected (166) , the timer will be set to wait for the next frame (168) . If an end of batch has been detected, a timer will reset to wait for the next batch (170) . In either case, the receiver will be turned on (172) after an appropriate time (174) . It will be appreciated that by making use of a steerable antenna, the claimed invention can provide practical in-building paging coverage over a very wide area. In addition, the scanning operation of the beam can be used to provide wide area coverage, as well as to prolong pager battery life when the operation of the pager battery saver is synchronized to the movement of the beam. It will be further appreciated that the claimed invention could be used to provide true nationwide or continent-wide paging coverage. In addition, it could also be used in conjunction with existing land-based paging systems to provide for true nationwide coverage when a paging subscriber leaves his usual geographical location. What is claimed is:

Claims: Claims 1. A satellite signalling system comprising: a satellite; and an antenna coupled to said satellite for providing a signal beam having a small beam area, said satellite being arranged to control the position of said antenna, and means for directing said signal beam in a predetermined sequence to a plurality of locations on earth. 2. A satellite signalling system according to claim 1 wherein said signal beam comprises a data signal having a plurality of signal batches arranged in a predetermined sequence, each one of said batches corresponding to one of said locations and having an identification code identifying the one location. 3. A satellite signalling system according to claim 1 wherein said small beam area is no greater than 800 square miles. 4. A satellite signalling system according to claim 1 further comprising a portable selective signal receiver. 5. A satellite signalling system according to claim 2 further comprising a portable selective signal receiver located within one of said locations, said portable selective signal receiver being arranged to selectively respond only to the batch corresponding to said location in said signal beam. 6. A satellite signalling system according to claim 2 further comprising a portable selective signal receiver, wherein on decoding said location identification code, said portable selective signal receiver can determine its location. 7. A satellite signalling system comprising: an antenna having a steerable beam; a first receiver for receiving communication and control information; and control means coupled to said antenna and said first receiver for controlling said antenna to sequentially direct said steerable beam to a plurality of locations. 8. A system according to claim 7 further comprising a first transmitter for transmitting said communication and control information to said first receiver. 9. A system according to claim 7 further comprising a second transmitter coupled to said antenna for transmitting paging signals . 10. A system according to claim 9 further comprising at least one selective call receiver for receiving said paging signals. 11. A system according to claim 10 wherein said paging signal comprises a data signal having a plurality of signal batches arranged in a predetermined sequence, each of said batches corresponding to one of said locations. 12. A system according to claim 11 wherein each of said batches has an identification code for identifying one of said locations . 13. A system according to claim 12 wherein said at least one selective call receiver in one of said locations responds only to the one of said plurality of batches corresponding to said location. 14. A method for paging comprising: transmitting paging and control information to a satellite receiver; steering a beam of an antenna coupled to the satellite sequentially to a plurality of locations; and transmitting paging information from said satellite to paging receivers in at least one of said plurality of locations . 15. A method according to claim 14 wherein said step of transmitting paging and control information comprises transmitting a plurality of signal batches arranged in a predetermined sequence, each of the batches corresponding to one of said locations. 16. A satellite paging system comprising: a satellite; first means for transmitting paging information to said satellite; at least one pager receiver; and second means for transmitting paging signals from said satellite to a plurality of locations in a predetermined sequence, said at least one pager receiver residing in one of said plurality of locations. 17. A satellite paging system according to claim 16 wherein said paging signals comprise a plurality of signal batches arranged in a predetermined sequence, each one of said batches corresponding to one of said plurality of locations . 18. A satellite paging system according to claim 17 wherein each of said batches corresponding to one of said plurality of locations includes an identification code for identifying said one of said plurality of locations . 19. A satellite paging system according to claim 17 wherein said at least one pager receiver selectively responds to only the batch corresponding to the location in which the pager receiver resides . 20. A satellite paging system according to claim 17 wherein said second means includes a moveable antenna for providing a signal beam having a small beam area, said antenna being positioned for directing a signal beam to said plurality of locations in said predetermined sequence. 21. A selective call radio receiver for receiving information from a satellite comprising: a receiver section; and battery saver circuitry for activating said receiver section when said satellite is transmitting information to a predetermined region in which said radio receiver resides and for deactivating said receiver section when said satellite is transmitting to regions other than said predetermined region.

Inventors: DAVIS WALTER LEE, DAVIS, WALTER, LEE
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1990
Main classification: H04B7
IPC codes: H04B7, H04W84