Paging system providing continvous synchronization of simulcast delay

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TECHNICAL FIELD OF THE INVENTION

The present invention pertains in general to a paging system, and more particularly, to a satellite link for transmitting directly to paging transmitters at select paging sites to provide a simulcast transmission with virtually zero phase shift between transmitters within the footprint of the satellite.

BACKGROUND OF THE INVENTION

Paging systems have conventionally utilized a number of data links to transmit paging information from a central paging terminal to select pagers. Typically, paging information is coded with a unique identification number and then transmitted to all paging receivers, with the receivers only receiving information having a specific identification number associated therewith. However, paging transmitters can be located in any locale with the primary objective of a paging system being to deliver the signals to all paging receivers at virtually the same time in what is sometimes referred to as a Simultaneous Broadcast mode, or "simulcast".

In order to effect a simultaneous broadcast of the information, a radio link is provided with the information modulated onto a carrier and then the carrier transmitted to all the paging receivers. However, due to practical constraints, multiple paging transmitters are required at any given locale to provide sufficient coverage. Therefore, one problem that has been presented in the design of paging systems is that of independently transmitting from a plurality of transmitting antennas. It can be appreciated that when a receiver is proximate to one transmitter in an array of transmitters, the receiver will lock on to the stronger signal. However, a problem arises when a receiver is disposed equidistant between two transmitters with the power from each of the two transmitters being essentially the same magnitude. In this situation, data is being simultaneously received from two transmitters, and the phase difference between the data received from both transmitters must be relatively close. If not, an error may occur. It is accepted in the industry that any phase difference between two transmitters that would result in greater than one-half bit difference in the data received at a receiver midway between the two transmitters would cause an error in the reception of the data. Therefore, data transmitted from the two transmitters must be phased such that a smaller error is present.

In conventional systems, information is typically received at a paging terminal and then transmitted to the separate paging transmitters through either land lines or a radio control link. With land lines, there are delays inherent in the land line and, in the radio control links, delays occur as a result of path length, atmospheric conditions, etc. In each of these systems, some adjustment in phase must be made at the paging transmitter location to compensate for this phase. This is a conventional method, but does present problems in that the phase "drifts" over time as a function of degradation and parts, change in atmospheric conditions, etc. Therefore, the phase at each of the paging transmitters must be adjusted on a periodic basis.

In some conventional systems, the link between the paging terminal and the radio controlled link to the paging transmitters can be a satellite link. Typically, the paging terminal is connected to a satellite uplink which transmits the information to the satellite when a separate receiver is disposed elsewhere to receive the satellite information, decode it and modulate it onto the carrier of the radio control link for transmission to the paging transmitters. Typically, the radio control link is a "line of sight" link that has a large number of disadvantages with respect to terrain, atmospheric conditions, etc. Another disadvantage is that specific frequencies must be obtained from the appropriate regulatory bodies in order to utilize the radio controlled link. In crowded areas, these are difficult to obtain, due to the high demand for the various channels.

SUMMARY OF THE INVENTION

The present invention disclosed and claimed herein comprises a paging system. The paging system includes a satellite having a predetermined footprint for transmitting paging information on a downlink channel. A plurality of paging sites are provided that are disposed in the footprint of the satellite. At each of the paging sites, a satellite receiver is disposed for receiving the paging information for the satellite. The paging information received at each of the satellite receivers is received at substantially the same time. A paging transmitter is disposed at each of the paging sites for receiving the paging information on the output of the satellite receiver and transmitting the received paging information on a paging frequency within a predetermined locale proximate to each of the paging sites. A paging receiver is then operable to receive the paging information from the paging transmitter when it is within the locale of an associated one of the paging transmitters.

In another aspect of the present invention, the predetermined adjacent ones of the paging sites have a substantial overlap such that if one of the paging transmitters fails in its associated predetermined locale, the paging transmitter at the adjacent one of the paging sites will provide sufficient field strength to be received by a paging receiver in the predetermined locale associated with the failed one of the transmitters. Further, a backup system is provided for being disposed at a fixed location. The backup system receives the paging information and then outputs it to a link transmitter. The link transmitter transmits the paging information over a link frequency to select ones of the paging sites. A link receiver disposed at the select paging sites, is operable to receive the paging information from the link transmitter over the link frequency. An arbiter circuit is provided at each of the select sites for selecting the output of the link receiver for input to the paging transmitter when the output of the satellite receiver falls below a predetermined threshold.

BRIEF DESCRIPTION OF TEE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying Drawings in which:

FIG. 1 illustrates a diagrammatic view of a prior art system;

FIG. 2 illustrates a block diagram of the system of the present invention;

FIG. 3 illustrates a diagrammatic view of the simulcast operation of the present system;

FIG. 4 illustrates a diagrammatic view of one of the regional paging systems of the present invention;

FIG. 5 illustrates a field strength pattern for three adjacent paging transmitters;

FIG. 6 illustrates a diagrammatic view of the field strength for two of the paging transmitters;

FIG. 7 illustrates a block diagram of a paging transmitter;

FIG. 8 illustrates a block diagram of-a-paging transmitter and radio control link antenna;

FIG. 9 illustrates a more detailed block diagram of the paging transmitter and radio control link receiver;

FIG. 10 illustrates a block diagram of the post link receiving signal processor; and

FIG. 11 illustrates a block diagram of the delay circuit for delaying the data received from the radio control link.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is illustrated a diagrammatic view of the prior art paging system. The paging system includes a plurality of antenna towers 10 that are disposed at select locations in a particular locale. The particular location in which any of the towers 10 is located is a function of the desired coverage of a given system. Each of the towers 10 has disposed thereon a paging transmitter antenna 12 and a radio control link (RCL) receiving antenna 14. Each of the RCL receiving antennas 14 is operable to receive from an RCL transmitting antenna 16 paging information that is converted to a paging frequency and transmitted out from the paging antennas 12 to a pager 18. The pager 18 can be anywhere within the transmitting locale of any of the paging antennas 12, which transmitting locale is defined as the maximum distance at which the signal can be received by a receiver.

A regional paging terminal 20 is provided for receiving message information from either a satellite receiver 22 or a DID telephone line 24. The message information received by the paging terminal 20 consists of messages that are directed toward users of the systems through their associated pagers 18. The information is received from telephones in a public telephone network and then coded at a central location and transmitted to the paging terminal 20 through either a satellite link or the telephone trunk 24.

The satellite link consists of a geosynchronous satellite 26 which transmits the information through a downlink 28 to the satellite receiver 22 via a receiving antenna 30. The information transmitted to the satellite 26 is transmitted from the central location (not shown) through a separate uplink. By utilizing the satellite 26, a number of regional paging terminals 20 can have information transmitted thereto.

After receiving the message information, the paging terminal 20 outputs the information to a radio controlled (RCL) transmitter 32. This is then transmitted to all of the paging antennas 12 through the RCL transmitting antenna 16. In addition to the paging antennas 12, a diagnostic receiving antenna 34 is provided that is connected to a diagnostic receiver (DR) 36. The antenna 34 is disposed within the field of a number of the paging antennas 12 to receive transmissions therefrom and process them through conventional diagnostic routines. This information is transmitted back to the paging terminal 20 through a land line 38.

One disadvantage to the prior art system is that the path length between the transmitting antenna 16 and the RCL receiving antenna 14 varies. In addition, the delay associated with transmission through a medium such as air varies as a result of atmospheric conditions, terrain, etc. It is important that the data is transmitted from each of the paging antennas 12 at approximately the same time such that when one of the pagers 18 is disposed equidistance between two of the transmitters 12, such that the signal level is essentially equal, there will be no phase difference between the two received signals. This is an important aspect of any simulcast. In order to adjust for the phase, the conventional receiver/transmitter associated with each of the towers 10 has some delay adjustment associated therewith. This delay adjustment varies over time and with seasons and, as such, periodic maintenance is required on each of the towers 10 to insure that this phasing is correct.

Referring now to FIG. 2, there is illustrated a block diagram of the paging system of the present invention. The geosynchronous satellite 26 is utilized to receive from a satellite uplink 40 the paging information. This is distributed to regional paging systems (RPS) 42 through a downlink 44. Each of the RPSs 42 are connected to an interregional data link network (IDLN) 46, which is operable to connect all of the RPSs 42 and the satellite uplink 40.

Referring now to FIG. 3, there is illustrated a block diagram of one of the regional paging systems 42. Each of the towers 10 has associated therewith a satellite receiving antenna 48 and a satellite receiver 50. In the preferred embodiment, the satellite 26 and the associated downlink 44 operate in the Ku-band. In addition, each of the towers 10 has a paging transmitter 52 associated therewith that transmits a simulcast signal out through the paging antenna 12 associated therewith.

The satellite 26 receives the message information on an uplink 56 from an uplink station 58. The uplink station 58 transmits the information to the uplink 56 through a satellite dish 60 on the dedicated uplink channel. The uplink 58 and its associated satellite link 60 are typically located in a central location with all of the information transmitted thereto on a land based line 62. The land based line 62 is interfaced with a paging terminal 64 that is disposed at the location of the regional paging system 42. The paging terminal 64 is operable to interface with the uplink 58 to transmit local phone calls with message information to the uplink 58 for transmission up to the satellite 26, and also to perform certain diagnostic functions.

The paging terminal 64 is interfaced through a Digital Diagnostic Controller (DDC) 66 to a diagnostic receiver 68 and a link transmitter 70. The diagnostic receiver 68 has a receiving antenna 72 for receiving transmissions from the paging antennas 12 and transmitting the information back to the DDC 66 through a land based line 74. The link transmitter 70 is operable as a backup to transmit the paging information through a transmitting antenna 76 to a link receiver 78 that is operable to receive the transmitted information on a link receiving antenna 80. Therefore, a radio controlled link is provided by the link transmitter 70 and the link receiver 78. The link receiver 78 is only associated when certain ones of the towers 10 that are determined to be critical to operation in the event that satellite transmission fails. This can occur as a result of a satellite going down, the uplink failing, etc.

The system of FIG. 3 has a number of advantages in that the satellite signal is directly transmitted to each of the towers 10. This provides a significant advantage in that the footprint of the satellite and the inherent characteristics thereof result in a zero time difference between receipt of the data at each of the satellite antennas 48. As such, no delay need be accounted for when transmitting the information from the paging transmitters 52 on the paging antennas 12. Therefore, it can be assumed that the signals have been transmitted from the towers 10 and the associated paging transmitters 52 on an equal phase and equal time basis.

Another advantage to utilizing this type of system is that no additional radio-controlled link hardware is required other than for backup. It is only necessary to place one of the satellite receivers 50 onto the tower 10 in association with the transmitter 52 and align the satellite dish 48 to provide optimum reception. No alignment of a radio controlled link is required and no delay adjustment is required, with the exception of the backup system, which is only disposed on a few of the towers 10. Further, no license from the appropriate regulatory bodies is required, nor are any intermodulation or RFI studies required.

For diagnostic purposes, a modem 82 can be associated with any of the satellite receivers 50 and paging transmitters 52 to provide diagnostic capabilities. This modem is connected through a land line 84 to a personal computer 86 having an associated modem. This personal computer 86 runs a diagnostic routine and interfaces with the paging terminal 64. This is a conventional system to allow the terminal 64 to interface with any one of the towers for diagnostic purposes.

Referring now to FIG. 4, there is illustrated a diagrammatic view of the system of FIG. 3 illustrating the satellite receiving dishes 48 and the antenna towers 10. The antenna towers are illustrated in a particular region, the region controlled by the paging terminal 20. In operation, the paging terminal 20 is disposed in a particular region. The paging terminal 20 is interfaced with the local telephone system through the DID trunk 24. Telephone calls coming into the paging terminal 20 will allow an individual to access one of the users of the systems through their pager 18. The phone call is made and a message logged onto the paging terminal 20. These messages are queued up and then transferred to the IDLN 46 through a network trunk 88. This paging information is then queued up at the IDLN 46 and transmitted to the uplink and satellite control 58 for output on the satellite dish 60 to the satellite 26. All of the regions that are covered by the system are routed to the IDLN for transmission up to the satellite 26. The satellite 26 then transmits down to each of the satellite dishes 48 on the antenna towers 10 for each of the regions. Although separate regions are described, it should be understood that a single very large paging terminal could be utilized to receive the telephone calls and their associated messages for the various pagers 18. However, it is more efficient to provide some diversity with respect to the paging terminal and utilize a large number of them. This allows for a more efficient interface with the local telephone networks. If it is desired to communicate between regional paging terminals, TNPP network is utilized.

In certain regional systems, it is desirable to extend the reach of a given system to another area on a limited basis. These areas are referred to as "spurs". This is illustrated by a box with a dotted line 90. Spur 90 contains an antenna tower 10 having an associated satellite dish 48 and paging transmitter antenna 12. The local phone is fed into a remote concentrator (RTC) 92 to receive the phone calls at the local area. These are then transmitted to a network I/O 94 to interface the calls with the paging terminal 20 at a particular region or directly with the IDLN 46. In operation, a given spur 90 would be associated with a particular paging terminal. It would then only be necessary to somehow connect the phone call with the paging terminal 20 at its location. This could be accomplished merely by providing a long distance number such that the caller interfaced directly with paging terminal 20. However, from a marketing standpoint, it is more desirable to provide a local phone number and collect the calls and route them to either the IDLN 46 or the paging terminal 20. In any event, once the calls are collected and the messages organized, they are all routed to the IDLN 46 for transmission to the satellite.

In some situations, it may be desirable to have the regional systems occupy separate channels on the downlink 44. This can be accomplished by providing totally separate channels at the transmitter on satellite 26 or, more preferably, multiplexing a given channel for each regional paging system 42 in the system. This could utilize FDM, TDM and CDM modulation techniques. In this manner, the information received at the IDLN 46 can be discriminated into the various locales.

In one embodiment of the present invention, it is desirable to cover the entire footprint of the satellite 26 with antenna towers 10 having their associated satellite receivers and dishes 48 aligned with the satellite. In this manner, all the antenna towers 10 in the footprint can receive the satellite transmission encoded with the various messages and with an effective time difference of substantially "0". Therefore, all the antenna towers 10 in a given system could effectively receive all the pages to the entire system. The disadvantage to this system is with regard to throughput. Since the messages are output in a serial manner, a large number of pagers on a system would result in a large number of messages being queued up at peak times. When this occurs, it could take a significant amount of time for a message to reach its destination pager. Therefore, it is desirable to either utilize a plurality of channels from the satellite and divide the system up into regions or to use a single channel and time multiplex the channels with the system broken up into various regions. This will allow messages to be output in a parallel manner to more users at any given time.

The radio control link 32 is only directed to a number of key towers to minimize its overall expense. In operation, the RCL 32 provides a backup function that is only utilized whenever it is determined that the satellite dish 48 is not receiving information from the satellite 26. In this mode, the local paging terminal 20 will activate the associated RCL 32 through an interconnecting control line, such as a local land line 93, to transmit the paging information directly to the antenna towers 10. In this mode, the information will not be transmitted down from the satellite 26 and received by the tower 10. However, it may still be transmitted to the uplink station 58.

In addition to the RCL 32, there is also provided a diagnostic link monitoring receiver (DLMR) 96 that receives the transmission from the satellite 26 on a satellite dish 98. Satellite dish 98 is identical to the satellite dish 48, in addition to the associated satellite receiver 50 being similar. The DLMR