Apparatus for mobile unit acquisition in a satellite communication system and method therefor

• US 5,659,545 A
• Filed: 11/15/1994
• Issued: 08/19/1997
• Est. Priority Date: 11/15/1994
• Status: Expired due to Term

• Current Assignee: Google Technology Holdings LLC (Alphabet Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Sowles, Kenneth L., Gregg, Ralph C., Olds, Keith A.
• Primary Examiner(s): Chin, Wellington
• Assistant Examiner(s): Corrielus, Jean B.
• Application Number:
  US08/339,998
• Time in Patent Office: 1,008 Days
• Field of Search: 375/358, 375/354, 375/362, 375/355, 375/268, 375/298, 375/356, 375/357, 370/324, 370/350, 370/501, 370/508, 370/509, 370/519, 370/516
• US Class Current: 370/324
• CPC Class Codes: H04B 7/18558 Arrangements for managing communications, i.e. for setting up, maintaining or releasing a call between stations

Abstract

To obtain service from a satellite communication system, a mobile subscriber unit must acquire a TDMA communication channel. Because the satellites move with respect to the surface of the earth, propagation delays and Doppler shift affect the timing and frequency of received signals. A mobile subscriber unit synchronizes to the frequency and timing windows of a satellite communication station by receiving frequency and timing corrections during channel acquisition. The satellite initially estimates correction factors during initial acquisition requests and further estimates correction factors from synchronization bursts from the subscriber unit.

Claims

1. A method of synchronizing a subscriber unit with a node of a communication system, said method comprising the steps of:

receiving an assignment of a traffic channel, said traffic channel being a time division multiple access (TDMA) frequency channel;

sending a synchronization burst on said traffic channel;

receiving timing and frequency correction data based on said synchronization burst, said timing and frequency correction data calculated by said node by comparing an arrival time of said synchronization burst with a time-slot of said traffic channel, said frequency correction data calculated by comparing a frequency of said traffic channel with a frequency of said synchronization burst; and

adjusting timing and frequency characteristics of said subscriber unit for communication over said traffic channel based on said timing and frequency correction data.

2. A method as claimed in claim 1 further comprising the step of communicating data with said node over said traffic channel within said time-slot and at said frequency of said traffic channel.

3. A method as claimed in claim 1 further comprising the step of sending a second synchronization burst having an adjusted transmit time and adjusted frequency in response to the adjusting step.

4. A method as claimed in claim 1 further comprising the steps of:

selecting a broadcast channel, said broadcast channel transmitting classes of subscriber units for which acquisition is prohibited;

determining if acquisition is permitted based on a class of said subscriber unit; and

sending an acquisition request to said node.

5. A method as claimed in claim 1 further comprising the steps of:

selecting a broadcast channel, said broadcast channel providing frequency and time-slot information for at least two acquisition channels;

randomly selecting one of said acquisition channels; and

sending an acquisition request to said node on said one of said acquisition channels.

6. A method as claimed in claim 1 further comprising the steps of:

receiving a downlink synchronization message from said node indicating whether synchronization has been achieved; and

switching to a traffic mode when said downlink synchronization message indicates that synchronization has been achieved.

7. A method as claimed in claim 6 wherein said traffic channel is configured for bi-phase-shift-keyed (BPSK) modulation, and wherein the switching step includes the step of switching said traffic channel to quadrature phase-shift-keyed (QPSK) modulation.

8. A method as claimed in claim 6 wherein further comprising the steps of:

sending a sync acknowledge message to said node to acknowledge receipt of said downlink synchronization message; and

receiving a traffic mode message from said node that instructs said subscriber unit to switch to a quadrature phase-shift-keyed (QPSK) modulation on said traffic channel.

9. A method as claimed in claim 1 wherein said node is a low-earth orbiting satellite that is moving rapidly with respect to said subscriber unit, and

wherein the receiving an assignment step comprises receiving an assignment of said traffic channel, said frequency of said traffic channel is affected by the satellite'"'"'s movement, and said time-slot of said traffic channel is affected by a changing propagation distance to said satellite.

10. A method of synchronizing a node of a communication system with a subscriber unit, said method comprising the steps of:

receiving a synchronization burst from said subscriber unit on a TDMA traffic channel;

comparing an arrival time of said synchronization burst with a time-slot of said TDMA traffic channel to determine timing correction data;

comparing a frequency of said TDMA traffic channel with a frequency of said synchronization burst to determine frequency correction data; and

sending said timing and frequency correction data to said subscriber unit, said subscriber unit adjusting a transmit time and transmit frequency based on said correction data.

11. A method as claimed in claim 10 further comprising the steps of:

determining if a timing and frequency threshold are met; and

sending a repeat burst message to said subscriber unit when said timing and frequency threshold are not met, said repeat burst message directing said subscriber unit to resend said synchronization burst using said correction data.

12. A method as claimed in claim 10 further comprising the steps of:

determining if a timing and frequency threshold are met;

sending a sync OK message when said timing and frequency threshold have been met; and

switching from a synchronization mode to a traffic mode on said TDMA traffic channel.

13. A method as claimed in claim 12 further comprising the steps of:

waiting for an acknowledgment of said sync OK message from said subscriber unit; and

when said acknowledgment is not received within a predetermined time, sending a switch to traffic mode message to said subscriber unit.

14. A method as claimed in claim 10 further comprising the steps of:

receiving an acquisition request from said subscriber unit;

determining if said acquisition request is within a frequency and timing tolerance;

when said acquisition request is within a frequency and timing tolerance, reserving said TDMA traffic channel for said subscriber unit; and

transmitting to said subscriber unit a traffic channel assignment message which includes a frequency and time slot of said TDMA traffic channel and initial frequency and timing correction data.

15. A subscriber unit that synchronizes to a timing window of a communication station, said communication station moving with respect to earth'"'"'s surface, said subscriber unit comprising:

a transmitter for sending a synchronization burst on a traffic channel to said communication station;

a receiver for receiving an assignment of said traffic channel from said communication station, said traffic channel being a time division multiple access (TDMA) frequency channel, and receiving timing and frequency correction data based on said synchronization burst; and

a processor coupled to said receiver and said transmitter for adjusting timing and frequency characteristics of said transmitter for communicating over said traffic channel based on said timing and frequency correction data.

16. A subscriber unit as claimed in claim 15 wherein said transmitter and receiver includes means for communicating data with said communication station over said traffic channel wherein said data is received by said communication station within a time-slot and at a frequency of said traffic channel.

17. A subscriber unit as claimed in claim 15 wherein said processor includes means for sending a second synchronization burst having an adjusted transmit time and adjusted frequency.

18. A subscriber unit as claimed in claim 15 wherein:

said processor includes means for selecting a broadcast channel, said broadcast channel transmitting classes of subscriber units for which acquisition is prohibited;

said processor includes means for determining if acquisition is permitted based on a class of said subscriber unit and said classes transmitted over said broadcast channel; and

said transmitter includes means for sending an acquisition request to said communication station based on said class.

19. A subscriber unit as claimed in claim 15 wherein:

said processor includes means for selecting a broadcast channel, said broadcast channel providing frequency and time-slot information for at least two acquisition channels, and

includes means for randomly selecting one of said acquisition channels, and wherein

said transmitter includes means for sending an acquisition request to said communication station on said one of said acquisition channels.

20. A subscriber unit as claimed in claim 15 wherein:

said processor includes means for receiving a downlink synchronization message from said communication station indicating whether synchronization has been achieved; and

wherein

said transmitter and receiver include means for switching to a traffic mode when said downlink synchronization message indicates that synchronization has been achieved.

21. A subscriber unit as claimed in claim 20 wherein said traffic channel is configured for bi-phase-shift-keyed (BPSK) modulation, and wherein said transmitter and receiver include means for switching said traffic channel to quadrature phase-shift-keyed (QPSK) modulation.

22. A subscriber unit as claimed in claim 20 wherein:

said transmitter includes means for sending a sync acknowledge message to said communication station to acknowledge receipt of said downlink synchronization message; and

said receiver includes means for receiving a switch to traffic mode message from said communication station.

23. A subscriber unit as claimed in claim 15 wherein said communication station is a low-earth orbiting satellite that is moving rapidly with respect to said subscriber unit, and wherein a frequency of said traffic channel is affected by the satellite'"'"'s movement, and a time-slot is affected by a changing propagation distance to said satellite.