Method and apparatus for radio link monitoring on sidelink

1. A method performed by a first user equipment (UE), the method comprising:

setting up unicast transmission on a radio sidelink;

sending a request message to a second UE, wherein the request message comprises a request that the second UE transmit data for monitoring;

receiving a reference signal from the second UE; and

monitoring the radio sidelink based on the data and the reference signal.

2. The method of claim 1 , wherein the reference signal comprises at least one periodical sidelink reference signal, and the radio sidelink is monitored based on the transmission of the at least one periodical sidelink reference signal.

3. The method of claim 2 , wherein the setting up the unicast transmission comprises exchanging a configuration of the at least one periodical sidelink reference signal via vehicle to vehicle radio resource control signaling.

4. The method of claim 2 , wherein the setting up the unicast transmission comprises: transmitting a direct communication request message, receiving a direct communication accept message, transmitting an access stratum level configuration, and receiving an access stratum level configuration complete message, wherein the monitoring the radio sidelink comprises the second UE starting to transmit the at least one periodical sidelink reference signal after receiving the direct communication accept message, or after transmitting the access stratum level configuration, or after receiving the access stratum level configuration complete message.

5. The method of claim 2 , wherein the monitoring the radio sidelink comprises the second UE stopping transmission of the at least one periodical sidelink reference signal in the condition that the unicast transmission is released or a radio link failure happens on the radio sidelink.

6. The method of claim 1 , wherein the reference signal is contained in a semi-persistent scheduling resource, and the radio sidelink is monitored based on the transmission of the reference signal contained in the semi-persistent scheduling resource.

7. The method of claim 6 , wherein the setting up unicast transmission comprises exchanging a configuration of the semi-persistent scheduling resource via vehicle to vehicle radio resource control signaling.

8. The method of claim 6 , wherein the setting up unicast transmission comprises: transmitting a direct communication request message, receiving a direct communication accept message, transmitting an access stratum level configuration, and receiving an access stratum level configuration complete message, wherein the monitoring the radio sidelink comprises the second UE starting to transmit or activate the Semi-Persistent Scheduling resource after receiving the direct communication accept message, or after transmitting the access stratum level configuration, or after receiving the access stratum level configuration complete message.

9. The method of claim 6 , wherein the monitoring the radio sidelink comprises the second UE starting transmission of the semi-persistent scheduling resource in the condition that there is no existing semi-persistent scheduling configuration for data transmission or the existing semi-persistent scheduling configuration for data transmission does not meet monitoring requirement.

10. The method of claim 6 , wherein the monitoring the radio sidelink comprises the second UE stopping transmission of the semi-persistent scheduling resource in the condition that the unicast transmission is released or a radio link failure happens on the radio sidelink.

11. The method of claim 6 , wherein the monitoring the radio sidelink comprises the second UE stopping or deactivating transmission of the semi-persistent scheduling resource in the condition that a new semi-persistent scheduling configuration for data transmission is configured and the new semi-persistent scheduling configuration for data transmission or a combination of the new semi-persistent scheduling configuration for data transmission and existing semi-persistent scheduling configuration for data transmission meets monitoring requirement.

12. The method of claim 1 , wherein the monitoring the radio sidelink comprises, in the condition that there is no data transmission or there is not enough data transmission for monitoring for a time period, the second UE transmitting data for monitoring.

13. The method of claim 1 , wherein the radio sidelink is monitored based on a combination of the transmission of a periodical reference signal and the transmission of data.

14. The method of claim 13 , wherein the monitoring the radio sidelink comprises, in the condition that there is no data transmission or there is not enough data transmission for monitoring for a time period, the second UE transmitting the periodical reference signal.

15. The method of claim 13 , wherein the monitoring the radio sidelink comprises, in the condition that there is enough data transmission for monitoring, the second UE stopping transmission of the periodical reference signal.

16. A first user equipment (UE), comprising:

at least one memory; and

at least one processor coupled with the at least one memory and configured to cause the first UE to:

set up unicast transmission on a radio sidelink;

send a request message to a second UE, wherein the request message comprises a request that the second UE transmit data for monitoring;

receive a reference signal from the second UE; and

monitor the radio sidelink based on the data and the reference signal.

17. The first UE of claim 16 , wherein the reference signal comprises at least one periodical sidelink reference signal, and the processor monitors the radio sidelink based on the transmission of the at least one periodical sidelink reference signal.

18. The first UE of claim 17 , wherein the at least one processor is configured to cause the first UE to set up the unicast transmission by: exchanging a configuration of the at least one periodical sidelink reference signal via vehicle to vehicle radio resource control signaling.