Data multiplexing for diversity operation
In a satellite mobile telecommunications system based on the GSM standard, and using a TDMA frame structure, discontinuous transmission (DTX) mode is used to take advantage of the substantial silences which occur during normal speech. In this mode, traffic channel (TCH) data is not sent, but control of the link between a ground station and a user terminal is maintained by sending control channel data bursts (SACCH) together with silence descriptor (SID) frames. To avoid high peak to mean values of transmission power at the satellite, the emission time of the bursts is controlled so as to be uniform over the available sending opportunities. Conventional techniques for doing this cannot be used in a system which includes diversity operation. Therefore, the burst transmission time at the ground station is set in dependence on a reference provided by the user terminal which is a modified version of the reference generated for contention resolution of random access requests.
1. A method of data multiplexing in a mobile telecommunications system in which a ground station is in communication with each of a plurality of user terminals via a respective logical channel, each logical channel capable of carrying link control data to a respective user terminal over diverse communication paths using a multiple access scheme which include time division, in which a frame comprises a sequence of time slots, each time slot being associated with a logical channel, the link control data being sent in bursts in selected time slots, the method comprising:
for each channel, setting the data burst transmission time from the ground station such that data bursts carrying link control data relating to the same channel and traveling over diverse paths to the same user terminal, arrive at the user terminal within a predetermined number of frames, and
distributing the ground station transmission time of data bursts carrying link control data for different channels over a plurality of frames, such that each frame which carries link control data includes link control data relating to a single logical channel only.
2. A method according to claim 1 , comprising setting the data burst transmission time from the ground station such that data bursts carrying link control data relating to the same channel and traveling over diverse paths to the same user terminal, arrive at the user terminal within the same frame.
3. A method according to claims 1 or 2 , comprising setting the ground station burst transmission time in accordance with transmission control information (CREF) provided by the user terminal.
4. A method according to claim 3 , comprising sending the transmission control information in a channel request message from the user terminal to the ground station.
5. A method according to claims 1 or 2 , including setting the ground station burst transmission time in accordance with transmission control information (CREF) provided at the ground station.
6. A method according to claim 3 , wherein the transmission control information comprises information relating to the frame number at which the control data is to be transmitted.
7. A method according to claim 1 , comprising setting the data burst transmission time irrespective of the time slot number TN.
8. A method according to claim 1 , wherein the transmission control information comprises a random reference (CREF, CREF) constrained to the number of frames over which the control data can be multiplexed.
9. A method according to claim 1 , wherein the link between the user terminal and the ground station operates in a discontinuous transmission (DTX) mode.
10. A method according to claim 1 , comprising initiating the transmission of the control data from the ground station to the user terminal over diverse paths.
11. A method according to claim 1 , wherein the mobile telecommunications system comprises a satellite system.
12. A method according to claim 1 , comprising distributing the ground station data burst transmission time so as to achieve a substantially uniform distribution of data bursts over the available frames.
13. A method according to claim 1 , comprising distributing the ground station data burst transmission time so as to achieve a substantially random distribution of data bursts over the available frames.
14. A method of data multiplexing in a mobile telecommunications system in which a ground station is in communication with each of a plurality of user terminals via a respective logical channel, each logical channel capable of carrying link control data to a respective user terminal over diverse communication paths using a multiple access scheme which includes time division, in which a frame comprises a sequence of time slots, each time slot being associated with a logical channel, the link control data being sent in bursts in selected time slots, the method comprising:
for each channel, setting the data burst transmission time from the ground station such that data bursts carrying link control data relating to the same channel and traveling over diverse paths to the same user terminal, arrive at the user terminal within a predetermined number of frames, and
distributing the ground station transmission time of data bursts carrying link control data for different channels substantially randomly over a plurality of frames.
15. A method according to claim 14 , comprising setting the data burst transmission time from the ground station such that data bursts carrying link control data relating to the same channel and traveling over diverse paths to the same user terminal, arrive at the user terminal within the same frame.
16. A method according to claim 14 , comprising setting the ground station burst transmission time in accordance with transmission control information (CREF) provided by the user terminal.
17. A method according to claim 16 , comprising sending the transmission control information in a channel request message from the user terminal to the ground station.
18. A method according to claim 16 , wherein the transmission control information comprises information relating to the frame number at which the control data is to be transmitted.
19. A method according to claim 16 , wherein the transmission control information comprises a random reference (CREF, CREF′) constrained to the number of frames over which the control data can be multiplexed.
20. A method according to claim 14 , including setting the ground station burst transmission time in accordance with transmission control information (CREF) provided at the ground station.
21. A method according to claim 14 , comprising setting the data burst transmission time irrespective of the time slot number TN.
22. A method according to claim 14 , wherein the link between the user terminal and the ground station operates in a discontinuous transmission (DTX) mode.
23. A method according to claim 14 , wherein the step of distributing substantially random includes using a pseudo-random distribution.
24. A method according to claim 14 , wherein the step of distributing substantially randomly includes using a predetermined sequence of numbers which serves to distribute the link control data over the available frames.
25. A link control signal for maintaining link control between a user terminal and a ground station in a mobile telecommunications system in which a ground station is in communication with each of a plurality of user terminals via a respective logical channel, each logical channel capable of carrying link control data to a respective user terminal over diverse communication paths using a multiple access scheme which includes time division, in which a frame comprises a sequence of time slots, each time slot being associated with a logical channel, the link control data being sent in bursts in selected time slots, the signal being configured such that data bursts carrying link control data relating to the same channel arrive at the user terminal within a predetermined number of frames, and such that data bursts carrying link control data for different channels are distributed over a plurality of frames, such that each frame which carries link control data includes link control data relating to a single logical channel only.
26. A signal according to claim 25 , wherein the signal is configured such that data bursts carrying link control data relating to the same channel arrive at the user terminal within the same frame.
27. A user terminal for use in a mobile telecommunications system in which a ground station is in communication with each of a plurality of user terminals via a respective logical channel, each logical channel capable of carrying link control data to a respective user terminal over diverse communication paths using a multiple access scheme which includes time division, in which a frame comprises a sequence of time slots, each time slot being associated with a logical channel, the link control data being sent in bursts in selected time slots, the user terminal comprising means for providing transmission control information to the ground station to control the setting of data burst transmission times at the ground station, in dependence on which data burst transmission times are set at the ground station such that data bursts carrying link control data relating to the same channel and traveling over diverse paths to the same user terminal, arrive at the user terminal within a predetermined number of frames, and such that the ground station transmission time of data bursts carrying link control data for different channels is distributed over a plurality of frames, such that each frame which carries link control data includes link control data relating to a single logical channel only.
28. A user terminal according to claim 27 , wherein said transmission control information comprises a randomly generated reference.
29. A user terminal according to claim 27 , wherein said transmission control information is a subset of a reference generated for contention resolution of requests on a random access channel.
30. A user terminal according to claim 29 , wherein said subset comprises the randomly generated reference constrained to the number of sending opportunities.
31. A ground station in a mobile telecommunications system in which the ground station is in communication with each of a plurality of user terminals via a respective logical channel, each logical channel capable of carrying link control data to a respective user terminal over diverse communication paths using a multiple access scheme which includes time division, in which a frame comprises a sequence of time slots, each time slot being associated with a logical channel, the link control data being sent in bursts in selected time slots, the ground station comprising means for setting data burst transmission times in dependence on transmission control information (CREF, CREF′), such that data bursts carrying link control data relating to the same channel and traveling over diverse paths to the same user terminal, arrive at the user terminal within a predetermined number of frames, and such that the ground station transmission time of data bursts carrying link control data for different channels is distributed over a plurality of frames, such that each frame which carries link control data includes link control data relating to a single logical channel only.
32. A ground station according to claim 31 , wherein the predetermined number of frames comprises a single frame.
33. A ground station according to claim 31 , wherein the transmission control information is provided by a user terminal (CREF) or generated at the ground station (CREF′).