Patent EA-000414-B1 (EA)
Title: Low cost wide area network for data communication with remote of mobile field units
Inventor: FITZGERALD BRENDAN T
IPC: H04W84/02
Filed: 1996-05-22
Abstract: 1. A two-way wireless data communication system comprising: an outbound messaging sub-system for sending outbound messages; an inbound messaging sub-system consisting of network of at least two radio frequency base stations for receiving inbound messages, the radio frequency base stations also continuously sampling, at their respective locations, a set of frequency channels in an inbound radio frequency band, to determine an observed frequency availability level for the set of frequencies for each base station; at least one remote field unit having an outbound message receiver and a tunable inbound message transmitter; and a central control unit, wherein the central control unit uses the outbound messaging sub-system as an outbound link to send an outbound message to the remote field unit, and uses the inbound messaging subsystem as an inbound link to receive an inbound message from the field unit, and wherein the outbound message includes data fields indicating an inbound time and an inbound carrier frequency at which the field unit is to send each inbound message, and wherein the central control unit receives reports of the observed frequency availability levels from the network of radio base stations, and the central control unit selects the inbound carrier frequency such that the inbound message will be received by one or more of the base stations in the inbound messaging sub-system without interference from another communication system. 2. A system as in Claim 1 wherein the inbound messaging sub-system uses the high frequency (HF) radio frequency band as the inbound radio frequency band. 3. A system as in Claim 2 wherein the outbound messaging sub-system is a paging sub-system. 4. A system as in Claim 1 wherein the central control unit additionally selects the inbound frequency by estimating a probability that radio energy at the inbound frequency will propagate. 5. A system as in Claim 2 comprising multiple inbound links implemented by multiple radio base station receivers, and wherein the control unit assigns the carrier frequency depending upon the observed availability of that carrier frequency in each of the multiple radio base station receivers. 6. A system as in Claim 3 comprising multiple outbound messaging sub-systems, and wherein the central control unit first selects an initial outbound messaging sub-system for the outbound link, and if the central control unit does not receive a message on the inbound link at the selected time and carrier frequency, then the central control unit reinitiates the outbound message on a different outbound messaging sub-system. 7. A system as in Claim 1 wherein the inbound message contains data representing a geolocational position of the field unit. 8. A two-way wireless data communication system comprising: a paging sub-system for sending outbound request messages to a remote field unit; an inbound messaging sub-system consisting of at least one radio base station for receiving inbound messages from the remote field unit, the radio base station also continuously sampling, at its respective location, a' set of frequency channels in an inbound radio frequency band, to determine an observed frequency availability level for the set of frequencies for the base station; at least one remote field unit having an outbound message receiver and a tunable inbound message transmitter; and a central control unit, wherein the central control unit uses the paging sub-system as an outbound link to send an outbound message to the remote field unit, and uses the inbound messaging subsystem as an inbound link to receive an inbound message from the field unit, and wherein the outbound message includes data fields indicating an inbound time and an inbound carrier frequency at which the field unit is to send each inbound message, and wherein the central control unit receives reports of the observed frequency availability levels from the network of radio base stations, and the central control unit selects the inbound carrier frequency such that the inbound message will be received by one or more of the base stations in the inbound messaging sub-system without interference from another communication system. 9. A system as in Claim 8 wherein the inbound messaging sub-system uses the high frequency (HP) radio frequency band as the inbound radio frequency band. 10. A system as in Claim 8 comprising multiple inbound links implemented by multiple radio base station receivers, and wherein the control unit assigns the carrier frequency depending upon the observed availability of that carrier frequency in a plurality of the radio base station receivers. 11. A system as in Claim 8 comprising multiple paging sub-systems, and wherein the central control unit first selects an initial paging sub-system for the outbound link, and if the central control unit does not receive a message on the inbound link at the selected time and carrier frequency, then the central control unit reinitiates the outbound message on a different paging sub-system. 12. A system as in Claim 8 comprising multiple paging sub-systems, and wherein the central control unit maintains a data base indicating a last known position for the field unit, and the control unit selects an initial paging sub-system for the outbound link based upon the last known position for the field unit. 13. A system as in Claim 8 wherein the control unit abandons further use of the selected inbound carrier frequency after transmission of the inbound message, to minimize interference with other communication systems which make use of that carrier frequency. 14. A method of operating a communications system to provide ubiquitous wireless data communication services between a message operations center (MOC) and a plurality of remote field units, by using a network of widely distributed radio base station (RBS) sites and multiple paging systems, the method comprising the steps of: A. at a customer calling station, initiating a request to the MOC to communicate with a particular remote field unit, such as a request for the field unit to determine and report its location; B. at the MOC site, i. polling the RBS sites to determine a radio frequency which is presently suitable for the field unit to send an inbound message back to one or more of the RBS sites; ii. determining an available frequency and a time for the field unit response; iii. formatting the time and frequency for the field unit response into an outbound page message; iv. forwarding the page massage to at least one of the paging systems; C. in at least one of the paging system sites, communicating the page message to the field units over the paging system; D. at the selected field unit, i. receiving the page message; ii. formulating a response to the page message by reading data available locally to the field unit; iii. formulating a response as an inbound message back to the radio base stations; iv. encoding the response in the form of a short duration burst message, to minimize the probability of interfering with existing broadcast or other communications at frequencies near the selected frequency; v. at the indicated carrier frequency and time, transmitting the inbound message over the air; E. at the RBS site. i. receiving the inbound message from the field unit over the air; and ii. forwarding the inbound message back to the MOC; F. at the MOC site, forwarding the inbound message back to the customer calling station; and G. at the calling station, receiving inbound message. 15. A method as in Claim 14 wherein the calling stations are connected to the MOC through the land based public switched telephone network (PSTN). 16. A method as in Claim 14 wherein the MOC is connected to the paging systems through the land based public switched telephone network (PSTN). 17. A method as in Claim 14 wherein the MOC communicates with the RBSs over a land-based connection such as the public switched telephone network. 18. A method as in Claim 14 wherein the MOC communicates with the RBSs over a land-based connection such as a leased private telephone circuits. 19. A method as in Claim 14 additionally comprising the step of, at the MOC, alerting one or more of the associated RBSs to expect a response from the indicated field unit at the specified frequency and time. 20. A method as in Claim 19 additionally comprising the step of, at the RBS, if no such message is received as alerted by the MOC, reporting an error to the MOC. 21. A method as in Claim 20 additionally comprising the step of, at the MOC, upon receiving an error report from the RBS, sending the outbound page message using another one of the paging systems. 22. A method as in Claim 14 wherein the step B.iv. additionally comprises the steps of, at the MOC, i. receiving reports of locations of the field units, ii. maintaining a database of last known locations for the field units in the system, and iii. determining the identity of a paging system which is closest in physical location to the last known location of the field unit, and attempting to send the outbound message using that nearest paging system first. 23. A method as in Claim 14 wherein each RBS additionally performs the step of periodically measuring a receive power level in each possible inbound frequency channel. 24. A method as in Claim 14, additionally comprising the step of, at the field unit, collect data to be reported including a latitude and longitude from a geolocation system. 25. A method as in Claim 14 wherein the short duration inbound message is encoded using spread spectrum modulation. 26. A method as in Claim 14 wherein the MOC communicates with the RBSs over a Very Small Aperture Terminal (VSAT) network.