Abstract:
An apparatus and method, for use with the satellite-based communications network, for improving the reliability and speed at which communication between a user terminal and the network is established. The apparatus and method arranges data of a channel request message transmitted from a user terminal to a satellite in the satellite-based network to insure that the most critical data for establishing communication between the user terminal and the satellite-based network is received at the satellite during the appropriate receiving time frame window. The channel request message includes a first data group necessary for establishing a communication link for which information is transmitted between the apparatus and the network, and a second data group including information for decreasing the amount time necessary to establish the communication link. The first data group is positioned at the center of the channel request message, with portions of the second data group at opposite ends of the channel request message. The time at which the user terminal transmits the channel request message is set based on a location of the apparatus within a spot beam, to take into account the appropriate propagation delay time for the message to travel from the apparatus to the satellite in the network, thus assuring that at least the first data group of the channel request message is received at the satellite during an appropriate receiving time frame window.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of a U.S. provisional application of Anthony Noerpel, Chandra Joshi and Stephanie Demers entitled “Techniques to Deliver the Key Information of the Channel Request Message from the User Terminal to the Network”, Ser. No. 60/110,278, filed on Nov. 30, 1998, the entire contents of which are incorporated by reference herein. 
     Related subject matter is disclosed in a copending U.S. patent application of Chandra Joshi, Anthony Noerpel, Neeraj Tewari, Gerald Stelzer, David Roos and Chi-Jiun Su entitled “System and Method for Implementing Terminal to Terminal Connections via a Geosynchronous Earth Orbit Satellite”, Ser. No. 09/115,098, filed on Jul. 13, 1998, and in a copending U.S. patent application of Stephanie Demers, Michael Parr, Anthony Noerpel and David Roos entitled “Method and System for Position Determination Using a Geostationary Earth Orbit Satellite”, Ser. No. 09/200,133, filed on Nov. 25, 1998, the entire contents of both applications being expressly incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention: 
     The present invention relates to an apparatus and method, for use with the satellite-based communications network, for improving the reliability and speed at which communication between a user terminal and the network is established. More particularly, the present invention relates to an apparatus and method for arranging data of a channel request message transmitted from a user terminal to a gateway station via a satellite in the satellite-based network to insure that critical data for establishing communication between the user terminal and the satellite-based network is received at the satellite during the appropriate receiving time frame window. 
     2. Description of the Related Art: 
     A satellite communications network, such as a geosynchronous earth orbit mobile communications network, comprises at least one geosynchronous earth orbit satellite, a ground-based advanced operations center (AOC) and spacecraft operations center (SOC) associated with the satellite, at least one ground-based gateway station (GS), and at least one user terminal (UT), which is typically a hand-held or vehicle mounted mobile telephone. The satellite enables the user terminal to communicate with other user terminals, or with other telephones in the terrestrial public switched telephone network (PSIN), via the gateway stations under the control of the gateway stations. The AOC provides system-wide resource management and control functions for its respective satellite, and the SOC controls on-orbit satellite operations for its respective satellite. 
     When a user terminal is operated to establish a radio resource connection, it generates and transmits a channel request message to the network on a random access channel (RACH) at a frequency assigned by the gateway station to a spot beam covering an area in which the user terminal is located. Typically, a channel request message includes data used to represent the mobile user terminal, as well as contention resolution and timing synchronization information. 
     To increase the likelihood that the channel request message is received at the satellite during a receiving time frame, the transmitter of the user terminal takes into account an estimated time which will elapse between the point in time when the channel request message is transmitted and the point in time when the channel request message is received at the satellite, which is known as the propagation delay. Typically, a user terminal will calculate, based on information broadcast by the network, the propagation delay for a distance measured from a location on the earth&#39;ss surface at the center of the spot beam in which the user terminal is located to the satellite, which is orbiting at about 22,000 miles above the earth&#39;s surface. Because the distance from the surface of the earth at the equator to the satellite is less than the distance from the surface of the earth in the extreme northern and southern hemispheres to the satellite, the propagation delay for a message sent from a user terminal close to the equator is less than that for a message sent from a user terminal in, for example, northern Europe. 
     Also, if the user terminal is at a distance from the center of the spot beam, the propagation delay is different than at the center of the spot beam, because the distance from the user terminal to the satellite is different. However, some user terminals are equipped with global positioning system (GPS) equipment and could thus better estimate their location with respect to the center of their respective spot beam. Also, some user terminals can estimate their location with respect to the center of their respective spot beam based on the power level at which the user terminal receives the spot beam. In either instance, these type of user terminals can better estimate the propagation delay for the channel request message, and therefore increase the likelihood that the channel request message will be received at the satellite during a receiving time frame. Moreover, because the duration of a conventional channel request message is a fraction of the duration of a receiving time frame at the satellite, the entire channel request message can generally be received within the desired time frame. 
     To decrease call set up time, it is desirable to include in the channel request message information pertaining to the position of the user terminal making the call, the called party number, the cause for establishing the call, the service provider identification, and so on, in addition to the contention resolution and synchronization information. However, this additional information increases the overall length of the channel request message, and thus increases the likelihood that a complete channel request message will not be received by the satellite during a receiving time frame window. That is, if the instant at which the channel request message is transmitted is not accurately synchronized with the receiving time frame window, some of the information in the channel request message will not reach the satellite during the receiving time frame window, and will thus be lost. Consequently, if information necessary for call set up, such as contention resolution and timing synchronization information, is not received by the satellite, the call cannot be set up. Furthermore, even if the user terminal resends the channel request message, unless the timing difference between transmission of the channel request message and the appearance of the receiving time frames at the satellite is resolved, the network still cannot set up a call. 
     Accordingly, a need exists for an apparatus and method which enables a user terminal to transmit additional call set-up information in a channel request message to accelerate call set-up time between the user terminal and satellite-based communications network, while also assuring that the information necessary for call set-up is received intact by the satellite in the network. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an apparatus and method for generating and arranging call set-up information in a channel request message transmitted by a user terminal to a gateway station in a satellite-based communications network via a satellite in the network to assure that the information necessary for establishing communication between the user terminal and the network is received intact during a receiving time frame established at the satellite. 
     Another object of the invention is to provide an apparatus and method for generating and arranging call set-up information in a channel request message transmitted by a user terminal to the gateway station via a satellite to assure that the information necessary for establishing communication between the user terminal and the gateway station, as well as information for accelerating call set-up, are received intact during a receiving time frame established at the satellite. 
     A further object of the invention is to provide an apparatus and method for setting the transmission time of a channel request message from a user terminal to a satellite based on the position of the user terminal within a spot beam propagating from the satellite to assure that the channel request message is received at the satellite during a receiving time frame. 
     These and other objects of the present invention are substantially achieved by providing an apparatus, such as a mobile telephone or other user terminal, for use in a satellite-based communications network, for generating and transmitting a channel request message. The apparatus comprises a channel request message generator which generates a channel request message including a first data group for establishing a communication link over which information is transmitted between the apparatus and the network, and a second data group including information for decreasing the amount time necessary to establish the communication link. The first data group includes contention resolution data and synchronization pattern data. The second data group includes position data representing a position of the apparatus, identifier data identifying the apparatus, service provider data representing a service provider for the apparatus, second identifier data identifying a second apparatus with which the apparatus is attempting to communicate through the network, and call establishment cause data representing a cause for which said apparatus is attempting to establish communication with the second apparatus, such as in response to a page, an alert, an emergency call, a terminal location update, and the like. 
     The apparatus further comprises a transmitter which transmits the channel request message to the network within a time period, such that an initial portion of the second data group is transmitted before any of the first data group, and a remaining portion of the second data group is transmitted after all of the first data group. The transmitter sets the starting time of the time period for transmission so that at least all of the first data group, and preferably also all of the second data group, of the transmitted channel request message are received at a satellite of the communications network during a channel request message receiving time frame at the satellite. In particular, the apparatus controls the transmitter to set the starting time of the time period for transmission of the channel request message based on a location of the apparatus within a spot beam and, in particular, based on the location of the apparatus with respect to the center of the spot beam, to take into account the appropriate propagation delay time for the message to travel from the apparatus to the satellite. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, advantages and novel features of the invention will be more readily appreciated from the following detailed description when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic block diagram of a satellite communications network according to an embodiment of the present invention; 
     FIG. 2 is a schematic diagram illustrating a satellite in the satellite communications network shown in FIG. 1, projecting a plurality of spot beams onto different regions on the surface of the earth; 
     FIG. 3 is a block diagram of an example of a user terminal used in the satellite communications network shown in FIG. 1; 
     FIG. 4 illustrates an example of a channel request message format transmitted by the user terminal shown in FIG. 3; 
     FIG. 5 is a more detailed illustration of the channel request message shown in FIG. 4, which illustrates symbols grouping in relation to the time slots making up the channel request message; 
     FIG. 6 is a diagram illustrating an example of the coverage areas provided by spot beams generated by a satellite of the network shown in FIG. 1 onto different geographic locations of the earth; and 
     FIG. 7 is a timing diagram illustrating an example of the difference in propagation delay for frames of information being transmitted to a satellite in the network shown in FIG. 1 by user terminals located at different geographic regions on the surface of the earth. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an example of a satellite communications network  100  according to an embodiment of the present invention. The satellite communications network includes at least one satellite  102  which is, for example, a geosynchronous earth orbit satellite, and at least one primary gateway station  104 . The primary gateway station  104  comprises an advanced operations center (AOC)  106 , a central operations support system (OSS)  108 , a satellite operation center (SOC)  110  and a gateway station  112  which provides access to a public switched telephone network (PSTN)  114 . The PSTN  114  provides access to the public land mobile network (PLMN) and public switched data network (PSDN)  118 . Optionally, the gateway station  112  can include a local operations support system (LOSS)  120 . 
     The network  100  further includes a national gateway  122  comprising a gateway station  124  which optionally includes a local OSS  126 . The national gateway  122  provides access to a PSTN  128 , which provides access to a PLMN  130  and a PSDN  132 . Further details of the primary gateway station  104  and national gateway station  122  and their respective components and operations are set forth in copending U.S. patent application Ser. No. 09/115,098, referenced above. 
     The satellite communications network  100  further includes at least one user terminal  134 , such as a hand-held telephone or vehicle-mounted telephone, which can operate in the geosynchronous earth orbit satellite communications network  100  discussed above, as well as in a GSM or other cellular telephone networks. As shown in FIG. 2, the satellite  102  generates at least one spot beam  136  onto a pre-determined geographic location of the surface of the earth. In this example, the satellite  102  generates a plurality of spot beams  136  onto different respective geographic locations on the earth&#39;s surface. A user terminal  134  communicates with the satellite over, for example, an L-band frequency designated by a particular spot beam  136  covering an area of the earth at which the user terminal  134  is located. The primary gateway station  102  and national gateway station  122  each communicate with the satellite  102  over, for example, a Ku-band frequency identified as a feeder signal  138  in FIG.  1 . Communications can be originated by a user terminal  134 . Alternatively, a call can originate from a telephone serviced by, for example, a PSTN, or from a telephone serviced by a GSM or other cellular networks. Details of the communication and control operations performed by the primary gateway station  104  and national gateway station  122  in relation to the satellite  102  and user terminal  134  to establish communication between two user terminals  134 , or between a user terminal  134  and a terrestrial terminal such as a telephone serviced by a PSTN  114  or  128 , are disclosed in U.S. patent application Ser. No. 09/115,098. 
     FIG. 3 is a block diagram illustrating a typical user terminal  134  included in the satellite communications network  100 . As disclosed in U.S. patent application Ser. No. 09/115,098, user terminal  134  includes a satellite module  140 , a satellite antenna  142 , a cellular module  144 , and a user interface module  146 . The satellite module  140  is coupled to the user interface module  146 , the cellular module  144 , and the satellite antenna  142 . The satellite antenna  142  can be a physically small antenna, such as a helix type antenna, or any other suitable antenna. The satellite module  140  includes a modem and TDMA unit  148 , an RF coder and decoder (codec)  150 , a burst transmitter  152 , a receiver  154 , and a transmit or receive (T/R) switch  156 . The modem  148  is connected to the RF codec  150 , and the RF codec  150  is connected to the burst transmitter  152  and to the receiver  154 . The T/R switch  156  is connected to the burst transmitter  152 , the receiver  154 , and the satellite antenna  142 , and allows the user terminal  134  to either transmit data or receive data. 
     The modem  148  converts speech or data samples to and from channel symbols using quadrature phase shift key modulation (QPSK). QPSK is preferably performed digitally by an application-specific integrated circuit or alternatively on a commercially available digital signal processor. The RF codec  150  converts channel symbols from the modem  148  into baseband I and Q signals that are transmitted to the burst transmitter  152 . In the receive direction, the RF codes  150  processes an IF signal  153  from the receiver  154  for input to the modem  148 . 
     The burst transmitter  152  converts the I and Q signals from the RF codec  150  up to a desired frequency, preferably an L-band frequency, for transmission by the first antenna  142 . The receiver  154  converts a received L-band signal from the first antenna  142  into the IF signal  153  sent to the RF codec  150 . 
     The user terminal  134  also includes a synthesizer  158  that provides a fixed local oscillator (LO) signal for the RF codec  150 . The synthesizer  158  is connected to the receiver  154  and to the cellular module  144 , and includes a variable local oscillator for channel tuning within the satellite module  140  and generates data clock signals for the modem  148 . Both the fixed local oscillator and the variable local oscillator within the synthesizer  158  may be adjusted based on commands from either gateway  104  or  124 , or from another user terminal. 
     The user interface module  146  includes an audio and codec unit  159 , a voice processing unit  160 , a controller  162 , an input/output (I/O) interface  164 , and a memory  166 . Preferably, each element within the user interface module  146  communicates with the other user interface elements. The voice processing unit  160  includes a voice transcoder that performs source coding to compress the digital 164 Kb/s PCM signal. Specifically, an encoder running on a programmable digital signal processor, such as a low delay CELP encoder, compresses the 64 Kb/s PCM signal into approximately a 3.6 Kb/s encoded signal. Alternatively, the encoder may be a multibased excited (MBE) type 3.6 Kb/s encoder that is well known to those skilled in the art. 
     The controller  162  provides a multitasking firmware environment for monitoring and controlling the mobile terminal hardware. The controller  162  may occupy the same processor as the voice transcoder or may optionally be disposed on a separate processor. Preferably, the controller  162  includes an I/O interface  164  that provides a communication interface with a user. The I/O interface  164  includes a keypad for entering data such as a phone number, a display, a data port for digital communication such as a facsimile transmission, and a smart card interface, such as that specified for GSM. 
     The cellular module  144  allows the user terminal  134  to communicate with a cellular system over a second antenna  161 . The second antenna  161  preferably is a linearly polarized whip meeting cellular system standards, and the cellular module  144  uses standard components, such as a GSM chip set, known to those skilled in the art. The user terminal  134  can operate in a first mode where the user terminal  134  functions as a conventional cellular phone, and in a second mode where the terminal  134  communicates with the satellite  102 . Also, a battery  168  is provided for portable operation of the user terminal  134 . 
     As described above, the user terminal  134  provides dual-mode operation, that is, either cellular or satellite operation. Also, the user terminal  134  is mobile and provides high quality digital voice. Furthermore, the user terminal  134  permits paging and messaging, transmission at a 2400 or 4800 bps data rate, or higher, via the data port, and provides a convenient cellular-like interface. The user terminal  134  may transmit on a single channel using a single time slot within a carrier signal allowing many other user terminals  12  to transmit over the same carrier. Thus, the user terminal  134  efficiently transmits over L-band spot beam  136  frequency resources. 
     When a user terminal  134  initiates communications or, in other words, places a call, the user terminal  134  generates and transmits a channel request message on a RACH within a frequency assigned to the spot beam  136  having a coverage area in which the user terminal  134  is located. Specifically, the controller  162  reads information from the memory and database  166 , and controls the modem and TDMA unit  148 , RF coder and decoder  150 , and burst transmitter  152 , to generate and transmit the channel request message on the RACH. 
     An example of a channel request message  170  that is generated and transmitted by the user terminal  134  is shown in FIGS. 4 and 5. That is, as indicated in FIG. 4, the channel request message  170  includes two general types of data, namely, Class 1 and synchronization data  172  and Class 2 data  174 . More specifically, Class 1 and synchronization data  172  includes data for contention resolution and timing synchronization between the user terminal  134  and satellite  102  to enable the user terminal  134  to gain access to the satellite communications network  100  via satellite  102 . Class 2 data  174 , on the other hand, includes data to accelerate call set up time, which includes information such as the called party number, location of the user terminal  134  placing the call, identification of the service provider for the user terminal placing the call, the cause for establishing the call, such as a response to a page, an alert, an emergency call, a terminal location update, and so on. 
     As indicated in FIGS. 4 and 5, the Class 1 and synchronization data  172  includes data bits which are grouped within the center of the RACH burst or, in other words, between a two groups of Class 2 data bits which are positioned at each end of the channel request message  170  transmitted in the RACH burst. The Class 1 and synchronization data bits are heavily coded, such as by ⅛ coding, while the Class 2 data bits are less heavily coded, such as by ½ coding. In this example, the Class 1 and Class 2 bits prior to coding include 139 bits total, with 16 bits being Class 1 type data bits and 123 bits being Class 2 type data bits. Additional data bits such as guard data, tail data, unique word data, continuous wave data and dummy data, may also be included in the channel request message as demonstrated below. These additional bits, along with the existing 16 Class 1 type data bits and 123 Class 2 type data bits, are encoded by either rate ⅛ coding or rate ½ coding, as appropriate, which results in the channel request message  170  including 702 data bits making up 351 symbols (each symbol being made up of two data bits). The symbols are segregated into nine time slots of 39 symbols each, with the Class 1 and synchronization data  172  being positioned within the central 5.4 timeslots of the message  170 . 
     As shown in FIG. 5, in particular, specific numbers of symbols make up certain data in the channel request message  170 . That is, in viewing the channel request message  170  from left to right, 2.5 symbols (5 bits) make up guard data G, 68 symbols (136 bits) make up class 2 data C 2 , eight symbols (16 bits) make up Class 1 and tail data CIT, and 17 symbols (34 bits) make up unique word data UW. The unique word data is followed by 32 symbols (64 bits) making up additional CIT data, followed by 32 symbols (64 bits) making up continuous wave CW data, and another 16 symbols (32 bits) making up more CIT data. The following 16 symbols (32 bits) make up Class 1 and tail data repeated ClTr, followed by 32 symbols (64 bits) making up more CW data, another 32 symbols (64 bits) making up more ClTr data, 17 symbols (34 bits) making up UW data, and 8 symbols (16 bits) making up more CITr data. 
     The next 67 symbols (134 bits) make up more C 2  data, followed by a group of 3.5 symbols (7 bits) including one dummy D symbol and 2.5 G symbols. In this example, all UW data bits, CW data bits, C 2  data bits and G data bits, and the single D data bit, have been rate ½ coded, while the CIT data bits and ClTr data bits have been rate ⅛ coded. 
     As indicated, the central 5.4 time slots comprising 210 symbols (420 bits) make up the Class 1 and synchronization data  172 , while 70.5 symbols (141 bits) on each end of the Class 1 and synchronization data  172  make up the Class 2 data  174 . The 5.4 time slots of Class 1 and synchronization data are positioned at the center of the channel request message  170  to assure that all the Class 1 and synchronization data  172  is always received during a receiving time frame “window” at the satellite  102  regardless of the location of the user terminal  134  within a spot beam  136 . 
     That is, as exemplified in the diagram of FIG. 6, for spot beams covering the geographic regions of the earth close to the sub-satellite point, the satellite  102  has a receiving time frame windows (RACH window size) of 12 time slots each. However, for elongated spot beams at the edge of coverage, the satellite  102  has a receiving time frame windows (RACH window size) of 18 time slots each. To assure that the channel request message is received during the appropriate time frame window at the satellite  102  for a particular spot beam in which the user terminal  134  is located, the user terminal  134  must take into account the amount of propagation delay before transmitting the channel request message  170 . This propagation delay, from the satellite to the center of the spot beam in which the user terminal  134  believes it is located, is calculated by the user terminal  134  based on information broadcast by the network  100 . 
     As discussed above in the background section, and as shown in the timing diagram of FIG. 7, a channel request message  170 , which travels at the speed of light when transmitted from the user terminal  134 , will take a certain amount of time, known as the propagation delay, to reach the satellite  102  which is orbiting the earth at approximately 22,000 miles above the equator. For example, the propagation delay for a channel request message  170  transmitted from a user terminal  134  located on the equator to satellite  102  is about 116.67 milliseconds. However, the propagation delay between a user terminal  134  located in a northern region of the earth, such as Northern Europe, is about 135 milliseconds. 
     Furthermore, these propagation delay estimates are based on the distance between the satellite  102  and a center of a particular spot beam in which the user terminal  134  is most probably located. Hence, depending on the distance of the user terminal  134  from the center of its respective spot beam, a variation in propagation delay of up to about ±6 milliseconds can exist. This differential delay is unknown to the user terminal  134  without feedback from the satellite  102 , and could result in a portion of the transmitted channel request message  170  failing to reach the satellite  102  during a receiving frame time period window, thus resulting in some of the channel request message  170  being unreceived by the network  100 . 
     The user terminal  134  includes, for example, global positioning system (GPS) equipment which enables the user terminal  134  to determine its location on the surface of the Earth to within a tolerance limited by the accuracy of the GPS equipment. Also, the user terminal  134  can further estimate its location within a particular spot beam based on the relative power of the spot beam  136  that the user terminal  134  receives, as described in U.S. patent application Ser. No. 09/200,133, referenced above. Once the user terminal  134  determines its location within the spot beam  136  which it will employ to establish communication with the network  100 , the user terminal  134  corrects the propagation delay value, which was calculated for the channel request message about to be transmitted, between the center of the spot beam and the satellite  102  based on information broadcast by the network  100 . 
     In particular, the user terminal  134  calculates the propagation delay differential, in symbols, based on the location of the user terminal  134  in relation to the center of the spot beam. The user terminal  134  then compares the result of this calculation to the closest value out of a plurality of possible values, namely, seven possible values representing 0, ±47, ±94, and ±141 symbols, which are stored in memory  166 . Based on the result of this comparison, the user terminal delays its burst transmission by the appropriate number of symbols to assure that the 9-timeslot channel request message  170  is received by the satellite  102  during the appropriate receiving time frame window which, as described above, is either 12 or 18 timeslots long, depending on the particular spot beam. Additionally, this correction factor is stored in the channel request message  170 , so that the gateway station  104  can take this correction information into account when transmitting signals to the user terminal  134 . 
     Accordingly, by correcting the transmission time for the channel request message  170 , the user terminal  134  insures that at least the Class 1 and synchronization bits  172  are received at the satellite  102  during the appropriate receiving time frame window. Hence, even if some of the Class 2 bits  174  at the ends of the channel request message  170  are not received during the receiving time frame window and are lost, the network  100  can still establish initial communication with the user terminal  134 . The network  100  can then send back a message to the user terminal  134  to request the user terminal  134  to retransmit the channel request message after having corrected for the timing discrepancy. Therefore, although call set-up time may be longer due to the loss of certain Class 2 data, set up of the call will still occur. 
     Finally, if the user terminal  134  is unsuccessful in establishing communication with the network  100  on a particular spot beam being received by the user terminal  134  at the highest power level, the user terminal  134  can attempt to establish communication with the network  100  by transmitting the channel request message  170  during a RACH burst within a frequency of a spot beam being received at the second strongest power level. If unsuccessful, the user terminal  134  can transmit the channel request message  170  at a frequency of the spot beam being received at the third highest power level, and so on, until communication is established. However, when changing spot beams, the user terminal  134  will have to recalculate the appropriate propagation delay and propagation delay correction for the spot beam being used. 
     Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.