Backoff method and device used for wireless communication system

1. A backoff method used for a lower priority access category (AC), comprising:

detecting a transmission opportunity (TXOP) sharing state of the lower priority AC when the lower priority AC and another AC in a same station (STA) obtain a TXOP at a same time and an internal collision occurs, wherein detecting the TXOP sharing state of the lower priority AC comprises determining whether the lower priority AC is suitable for TXOP sharing according to a TXOP sharing standard, and wherein the TXOP sharing standard comprises at least one of an available amount of transmission capacity in the TXOP and an improvement to a total throughput of the TXOP; and

executing a backoff of the lower priority AC according to the detected TXOP sharing state, wherein executing the backoff of the lower priority AC according to the detected TXOP sharing state comprises:

bypassing a backoff procedure of the lower priority AC when detecting that the TXOP sharing state of the lower priority AC is TXOP sharing; and

keeping a contention window (CW[AC]) of the lower priority AC unchanged and performing data transmissions when detecting that the TXOP sharing state of the lower priority AC is TXOP sharing.

2. The method according to claim 1 , wherein executing the backoff of the lower priority AC according to the detected TXOP sharing state comprises executing the backoff procedure of the lower priority AC when detecting that the TXOP sharing state of the lower priority AC is not TXOP sharing.

3. The method according to claim 1 , wherein when the lower priority AC performs data transmission and a first frame fails to be sent, the method further comprises:

resetting the CW[AC] of the lower priority AC to its minimum contention window (CW_min[AC]) when a number of retransmissions of the lower priority AC reaches a maximum value;

updating the CW[AC] of the lower priority AC to (CW[AC]+1)×2−1 when the number of retransmissions of the lower priority AC does not reach the maximum value and a current CW[AC] is smaller than a maximum contention window (CW_max[AC]); and

keeping the CW[AC] unchanged in the rest of the retransmissions when the number of retransmissions of the lower priority AC does not reach the maximum value and the current CW[AC] is equal to the CW_max[AC].

4. The method according to claim 1 , wherein executing the backoff of the lower priority AC according to the detected TXOP sharing state comprises:

determining whether a backoff time of the lower priority AC is 0 when the lower priority AC obtains the TXOP sharing state;

either bypassing the lower priority backoff procedure or keeping a current backoff time unchanged when the backoff time of the lower priority AC is not 0; and

doubling the CW[AC] of the lower priority AC when the backoff time of the lower priority AC is 0.

5. The method according to claim 4 , wherein the method further comprises:

determining whether data of the lower priority AC is finally sent successfully when the backoff time of the lower priority AC is 0 and its CW[AC] has been doubled; and

either resetting the CW[AC] to a minimum contention window (CW_min[AC]) or keeping a current CW[AC] unchanged when the data of the lower priority AC is finally sent successfully.

6. The method according to claim 1 , wherein executing the backoff of the lower priority AC according to the detected TXOP sharing state comprises doubling the CW[AC] of the lower priority AC when the lower priority AC has the internal collision.

7. The method according to claim 6 , wherein the method further comprises:

determining whether data of the lower priority AC is finally sent successfully; and

either setting the CW[AC] to a minimum contention window (CW_min[AC]) or keeping a current CW[AC] unchanged when the data of the lower priority AC is finally sent successfully.

8. The method according to claim 1 , wherein executing the backoff of the lower priority AC according to the detected TXOP sharing state comprises determining the CW[AC] of the lower priority AC according to a final sending result of data of the lower priority AC when the lower priority AC obtains the TXOP sharing state.

9. The method according to claim 8 , wherein the determining the CW[AC] of the lower priority AC according to the final sending result of the data of the lower priority AC comprises:

either setting the CW[AC] to a minimum contention window (CW_min[AC]) or keeping the current CW[AC] unchanged when the data of the lower priority AC is finally sent successfully; and

keeping the CW[AC] of the lower priority AC unchanged when the data of the lower priority AC is finally sent unsuccessfully.

10. The method according to claim 1 , wherein the TXOP sharing state of the lower priority AC is detected according to a scheduling result of the STA.

11. The method according to claim 1 , wherein the improvement to the total throughput of the TXOP is determined according to at least one of a TXOP channel condition and a package error rate (PER) curve.

12. The method according to claim 1 , wherein executing the backoff of the lower priority AC according to the detected TXOP sharing state ensures fair treatment of the lower priority AC by not repeatedly extending the CW[AC] of the lower priority AC when the internal collision occurs between the lower priority AC and a higher priority AC.

13. A wireless terminal station (STA), comprising:

a computer processor configured to:

detect a transmission opportunity (TXOP) sharing state of a lower priority access category (AC) when two ACs in the STA obtain a TXOP at a same time and an internal collision occurs, wherein detecting the TXOP sharing state of the lower priority AC comprises determining whether the lower priority AC is suitable for TXOP sharing according to a TXOP sharing standard, and wherein the TXOP sharing standard comprises at least one of an available amount of transmission capacity in the TXOP and an improvement to a total throughput of the TXOP; and

execute a backoff of the lower priority AC according to the detected TXOP sharing state, wherein executing the backoff of the lower priority AC according to the detected TXOP sharing state comprises:

bypassing a backoff procedure of the lower priority AC when detecting that the TXOP sharing state of the lower priority AC is TXOP sharing; and

keeping a contention window (CW[AC]) of the lower priority AC unchanged and performing data transmissions when detecting that the TXOP sharing state of the lower priority AC is TXOP sharing.

14. The STA according to claim 13 , wherein the computer processor executes the backoff procedure of the lower priority AC when the TXOP sharing state of the lower priority AC is no TXOP sharing.

15. The STA according to claim 14 , wherein when the lower priority AC performs data transmission and a first frame fails to be sent, the computer processor is further configured to:

reset the CW[AC] of the lower priority AC to its minimum contention window (CW_min[AC]) when a number of retransmissions of the lower priority AC reaches a maximum value;

update the CW[AC] of the lower priority AC to (CW[AC]+1)×2−1 when the number of retransmissions of the lower priority AC does not reach the maximum value and a current CW[AC] is smaller than a maximum contention window (CW_max[AC]); and

keep the CW[AC] unchanged in the rest of the retransmissions when the number of retransmissions of the lower priority AC does not reach the maximum value and the current CW[AC] is equal to the CW_max[AC].

16. The STA according to claim 13 , wherein the computer processor is further configured to:

determine whether backoff time of the lower priority AC is 0;

either bypass the lower priority backoff procedure or keep a current backoff time unchanged when the backoff time of the lower priority AC is not 0; and

double the CW[AC] of the lower priority AC when the backoff time of the lower priority AC is 0.

17. The STA according to claim 16 , wherein the computer processor is further configured to:

determine whether data of the lower priority AC is finally sent successfully; and

either reset the CW[AC] to a minimum contention window (CW_min [AC]) or keep a current CW[AC] unchanged when the data of the lower priority AC is finally sent successfully.

18. The STA according to claim 16 , wherein the computer processor is further configured to:

determine whether data of the lower priority AC is finally sent successfully;

reset the CW[AC] of the lower priority AC when the data of the lower priority AC is finally sent unsuccessfully, and wherein a resetting method comprises one of:

reducing a current CW[AC] by half when the current CW[AC] does not reach a maximum contention window (CW_max[AC]); and

keeping the current CW[AC] unchanged when the current CW[AC] has reached the CW_max [AC].

19. A system of a wireless local area network, comprising:

at least two stations (STAs), wherein in the STAs, at least one STA is capable of performing backoff of a lower priority access category (AC) and is configured to:

detect a transmission opportunity (TXOP) sharing state of the lower priority AC when two ACs of the STA obtain a TXOP at a same time and an internal collision occurs, wherein detecting the TXOP sharing state of the lower priority AC comprises determining whether the lower priority AC is suitable for TXOP sharing according to a TXOP sharing standard, and wherein the TXOP sharing standard comprises at least one of an available amount of transmission capacity in the TXOP and an improvement to a total throughput of the TXOP; and

execute the backoff of the lower priority AC according to the detected TXOP sharing state, wherein executing the backoff of the lower priority AC according to the detected TXOP sharing state comprises:

bypassing a backoff procedure of the lower priority AC when detecting that the TXOP sharing state of the lower priority AC is TXOP sharing; and

keeping a contention window (CW[AC]) of the lower priority AC unchanged and performing data transmissions when detecting that the TXOP sharing state of the lower priority AC is TXOP sharing.