Method for transmitting downlink in unlicensed band

1. A method for downlink transmission in an unlicensed band, the method comprising:

determining, by a first base station (BS)in a first group, a first delay time to initiate first clear channel assessment (CCA) in the unlicensed band from a reference time;

determining, by a second BS in the first group, a second delay time to initiate second CCA in the unlicensed band from the reference time, wherein the first delay time is longer than the second delay time;

checking, by the first BS, whether a channel is idle by performing the first CCA during a first CCA duration after the first delay time, wherein the first CCA duration is defined in inverse proportion to the first delay time;

checking, by the second BS, whether the channel is idle by performing the second CCA during a second CCA duration after the second delay time, wherein the second CCA duration is defined in inverse proportion to the second delay time; and

simultaneously performing, by the first and second BSs, downlink transmissions during the same time when the channel is idle.

2. The method of claim 1 , further comprising:

determining, by a third BS in a second group, a third delay time to initiate third CCA from the reference time,

wherein the third delay time is longer than the first delay time; and

checking, by the third BS, whether the channel is busy by performing the third CCA during a third CCA duration after the third delay time, wherein the third CCA duration is defined in inverse proportion to the third delay time,

wherein downlink transmission is not performed by the third BS when the channel is busy while the first and second BSs perform the downlink transmissions simultaneously.

3. The method of claim 1 , wherein the first delay time and the second delay time are determined based on a parameter specific to the first group.

4. The method of claim 1 , wherein the first delay time and the second delay time are determined at each reference time.

5. A wireless communication system, comprising:

a first base station (BS) in a first group configured to determine a first delay time to initiate first clear channel assessment (CCA) in the unlicensed band from a reference time; and

a second BS in the first group configured to determine a second delay time to initiate second CCA in the unlicensed band from the reference time,

wherein the first delay time is longer than the second delay time,

wherein the first BS checks whether a channel is idle by performing the first CCA during a first CCA duration after the first delay time,

wherein the first CCA duration is defined in inverse proportion to the first delay time,

wherein the second BS checks whether the channel is idle by performing the second CCA during a second CCA duration after the second delay time,

wherein the second CCA duration is defined in inverse proportion to the second delay time, and

wherein the first and second BSs simultaneously perform downlink transmissions during the same time when the channel is idle.

6. The wireless communication system of claim 5 , further comprising:

a third BS in a second group configured to determine a third delay time to initiate third CCA from the reference time,

wherein the third BS checks whether the channel is busy by performing the third CCA during a third CCA duration after the third delay time,

wherein the third CCA duration is defined in inverse proportion to the third delay time,

wherein the third delay time is longer than the first delay time, and

wherein downlink transmission is not performed by the third BS when the channel is busy while the first and second BSs perform the downlink transmissions simultaneously.

7. The wireless communication system of claim 5 , wherein the first delay time and the second delay time are determined based on a parameter specific to the first group.

8. The wireless communication system of claim 5 , wherein the first delay time and the second delay time are determined at each reference time.