Communication system with variable length data frames
A wireless communications system in which an access point (AP) transmits a downlink signal for reception by one or more subscriber unit(s) (SU) and the subscriber unit (SU) transmits an uplink signal for reception by the access point (AP). The downlink signal from the access point (AP) carries syncronisation bursts ( 2 ) at predetermined times and data contained in frames interleaved between the synchronisation bursts. Each synchronisation burst comprises an offset pointer ( 4 ) to the start of the subsequent downlink frame.
1. A wireless communications system in which an access point (AP) transmits a downlink signal for reception by a subscriber unit (SU) and the SU transmits an uplink signal for reception by the AP, and in which the downlink signal carries synchronization bursts at predetermined times and data contained in variable length frames interleaved between the synchronization bursts, each synchronization burst comprising an offset pointer to the start of the subsequent downlink frame.
2. A communications system according to claim 1 , in which each synchronization burst comprises a predetermined correlation sequence detectable by the SU in order to synchronization to the downlink signal.
3. A communications system according to claim 1 , in which each downlink frame comprises a header carrying a pointer to the start of the following downlink frame.
4. A communications system according to claim 3 , in which the header in each downlink frame comprises a downlink description, describing the contents of a data portion of the downlink frame.
5. A communications system according to claim 1 , in which each downlink frame comprises a header carrying a pointer to the start of the following uplink frame.
6. A communications system according to claim 5 , in which a downlink frame further comprises an extra pointer to the start of a second uplink frame.
7. A communications system according to claim 5 , in which the header in each downlink frame comprises an uplink description, describing the contents of a data portion of the following uplink frame.
8. A communications system according to claim 1 , in which the access point (AP) can schedule a wide training slot on the uplink into which the subscriber unit (SU) can transmit a training burst, the AP responding to the training burst by feeding back power control and timing information to the SU.
9. A communications system according to claim 8 , in which the SU assesses the downlink channel and assumes that the uplink channel is the same as the downlink channel in order to power control and pre-distort the training burst for reception by the AP.
10. A communications system according to claim 8 , in which the AP broadcasts information about the system on the downlink to allow the SU to power control and pre-distort the training burst for reception by the AP.
11. A communications system according to claim 1 , in which the AP can schedule a narrow training slot on the uplink into which the SU can transmit a training burst, the AP responding to the training burst by feeding back channel information to the SU.
12. A communications system according to claim 1 , in which the AP can schedule a contention slot on the uplink into which the SU can transmit a contention burst comprising its SU identifier (SUID), to which the AP responds by requesting the SU to transmit a request for an uplink bandwidth requirement.
13. A communications system according to claim 12 , in which the SU pre-distorts the contention word for reception by the AP, by calculating the pre-distorted contention word off-line and storing it in memory for transmission in the contention slot.
14. A communications system according to claim 1 , in which the SU pre-distorts uplink transmissions for reception by the AP.
15. A communications system according to claim 1 , in which the AP transmits the downlink for reception by a plurality of SUs and the plurality of SUs can transmit uplink signals for reception by the AP.
16. A communications system according to claim 15 , in which each SU fully pre-distorts uplink transmissions made in uplink slots which have not been scheduled specifically to it.
17. A communications system according to claim 16 , in which the AP does not require an equalizer to decode the fully pre-distorted uplink transmissions.
18. A communications system according to claim 15 , in which all SUs can decode a lowest order of modulation and one or more SUs can decode a higher order of modulation, in which each frame header is modulated in the lowest order of modulation, and in which different elements of the contents of a downlink frame are modulated using different orders of modulation, the elements being arranged in the frame in increasing order of modulation.
19. A communications system according to claim 1 , in which data are carried on the downlink and the uplink in asynchronous transfer mode (ATM) cells.
20. A communications system according to claim 19 , in which acknowledgements (ACKs) are sent after reception of ATM messages on the uplink or downlink, each message comprising one or more sequentially-numbered ATM cells, and each ACK following a message in which reception of one or more cells failed identifies the number of the first failed cell in the message to enable retransmission of failed cells.
21. A communications system according to claim 19 , in which acknowledgements (ACKs) are sent after reception of ATM messages on the uplink or downlink, each message comprising one or more sequentially-numbered ATM cells, and each ACK following a message in which reception of one or more cells failed comprises a bit-map identifying failed cells in message to enable retransmission of failed cells.
22. An access point for a communications system as defined in claim 1 .
23. A subscriber unit for a communications system as defined in claim 1 .
24. A communications system in which a downlink signal carries synchronization bursts at predetermined times and data contained in variable length frames interleaved between the synchronization bursts, each synchronization burst comprising an offset pointer to the start of the subsequent downlink frame.
25. A method for wireless communication between an access point and a plurality of subscriber units, comprising the steps of:
transmitting from the access point (AP) a downlink signal carrying synchronization bursts at predetermined times and data contained in variable length frames interleaved between the synchronization bursts, each synchronization burst comprising an offset pointer to the start of the subsequent downlink frame; and
at each subscriber unit (SU) receiving the downlink signal and using the synchronization bursts to synchronize thereto and using at least one of the offset pointers to locate the start of the subsequent downlink frame.
26. A method according to claim 25 , comprising the steps of;
transmitting at the start of each downlink frame a header containing a pointer to the start of the next downlink frame and a downlink description describing events carried by the frame; and
at each SU reading the header at the start of each downlink frame to decode the events in the frame, as appropriate, and to locate the start of the next downlink frame.
27. A method according to claim 25 , comprising the steps of;
transmitting from the SUs an uplink carrying data in frames, events in the uplink frames being scheduled by the AP;
transmitting at the start of each downlink frame a header containing an uplink offset pointer and an uplink description respectively describing the offset to the start of the next uplink frame and the events carried by that frame; and
at each SU using the offset pointer to locate the start of the next uplink frame and transmitting events in the uplink scheduled for that SU.