Handover in unlicensed band

1. A method comprising:

receiving, by a wireless device from a first base station, one or more messages indicating:

a handover from a first cell of the first base station to a second cell of a second base station;

a time duration of a monitoring window; and

a time offset of the monitoring window;

determining, based on the time offset and for starting the monitoring window, a monitoring occasion of a downlink control channel of the second cell;

starting the monitoring window, with the time duration, at the determined monitoring occasion;

monitoring, based on the one or more messages indicating the handover and during the monitoring window started at the monitoring occasion with the time duration, the downlink control channel for a downlink control information (DCI) from the second base station;

receiving, based on the monitoring, the DCI from the second base station;

determining a random-access-channel occasion (RO) of the second cell based on the received DCI;

transmitting a preamble to the second base station via the RO; and

transmitting, based on receiving a response to the preamble, a message to the second base station indicating a completion of the handover.

2. The method of claim 1 , wherein the one or more messages comprise a parameter indicating a presence of the DCI in the second cell.

3. The method of claim 1 , further comprising determining periodic ROs indicated by the one or more messages, wherein the periodic ROs do not comprise the RO indicated by the DCI.

4. The method of claim 1 , wherein:

the one or more messages further indicate a downlink reference signal of the second cell;

a demodulation reference signal antenna port, associated with the reception of the DCI via one or more downlink control channels of the second cell, is quasi co-located with the downlink reference signal; and

the RO is associated with the downlink reference signal.

5. The method of claim 1 ,

wherein the DCI is identified by a radio network temporary identifier.

6. The method of claim 5 , wherein the one or more messages further indicate the radio network temporary identifier.

7. The method of claim 5 , wherein the first monitoring occasion is an earliest monitoring occasion of the downlink control channel after the time offset from an end of a reception of a downlink reference signal of the second cell.

8. The method of claim 1 , wherein:

the one or more messages comprise one or more parameters of random access resources of the second cell, the one or more parameters comprising:

one or more preambles comprising the preamble;

one or more time domain resources of the RO; and

one or more frequency domain resources of the RO; and

wherein the RO is determined further based on the one or more parameters of the random access resources indicated by the one or more messages.

9. The method of claim 8 , wherein the determining the RO comprises determining the RO based on:

the one or more time domain resources;

the one or more frequency domain resources;

at least one offset indicated by the DCI; and

a reception timing of the DCI.

10. The method of claim 1 , further comprising:

performing, based on a type of channel access procedure, at least one channel access procedure to access a channel on which the preamble is transmitted, wherein the type is indicated by the DCI; and

determining that the channel is idle before transmitting the preamble.

11. A wireless device comprising:

one or more processors; and

memory storing instructions that, when executed by the one or more processors, cause the wireless device to:

receive, from a first base station, one or more messages indicating:

a handover from a first cell of the first base station to a second cell of a second base station;

a time duration of a monitoring window; and

a time offset of the monitoring window;

determine, based on the time offset and for starting the monitoring window, a monitoring occasion of a downlink control channel of the second cell;

start the monitoring window, with the time duration, at the determined monitoring occasion;

monitor, based on the one or more messages indicating the handover and during the monitoring window started at the monitoring occasion with the time duration, the downlink control channel for a downlink control information (DCI) from the second base station;

receive, based on the monitoring, the DCI from the second base station;

determine a random-access-channel occasion (RO) of the second cell based on the received DCI;

transmit a preamble to the second base station via the RO; and

transmit, based on receiving a response to the preamble, a message to the second base station indicating a completion of the handover.

12. The wireless device of claim 11 , wherein the one or more messages comprise a parameter indicating a presence of the DCI in the second cell.

13. The wireless device of claim 11 , wherein the instructions further cause the wireless device to determine periodic ROs indicated by the one or more messages, and wherein the periodic ROs do not comprise the RO indicated by the DCI.

14. The wireless device of claim 11 , wherein:

the one or more messages further indicate a downlink reference signal of the second cell;

a demodulation reference signal antenna port, associated with the reception of the DCI via one or more downlink control channels of the second cell, is quasi co-located with the downlink reference signal; and

the RO is associated with the downlink reference signal.

15. The wireless device of claim 11 ,

wherein the DCI is identified by a radio network temporary identifier.

16. The wireless device of claim 15 , wherein the one or more messages further indicate the radio network temporary identifier.

17. The wireless device of claim 15 , wherein the monitoring occasion is an earliest monitoring occasion of the downlink control channel after the time offset from an end of a reception of a downlink reference signal of the second cell.

18. The wireless device of claim 11 , wherein:

the one or more messages comprise one or more parameters of random access resources of the second cell, the one or more parameters comprising:

one or more preambles comprising the preamble;

one or more time domain resources of the RO; and

one or more frequency domain resources of the RO; and

wherein the RO is determined further based on the one or more parameters of the random access resources indicated by the one or more messages.

19. The wireless device of claim 18 , wherein the instructions further cause the wireless device to determine the RO based on:

the one or more time domain resources;

the one or more frequency domain resources;

at least one offset indicated by the DCI; and

a reception timing of the DCI.

20. A system comprising:

a first base station;

a second base station; and

a wireless device comprising:

one or more processors; and

a memory storing instructions that, when executed by the one or more processors, cause the wireless device to:

receive from the first base station, one or more messages indicating:

a handover from a first cell of the first base station to a second cell of the second base station;

a time duration of a monitoring window; and

a time offset of the monitoring window;

determine, based on the time offset and for starting the monitoring window, a monitoring occasion of a downlink control channel of the second cell;

start the monitoring window, with the time duration, at the determined monitoring occasion;

monitor, based on the one or more messages indicating the handover and during the monitoring window started at the monitoring occasion with the time duration, the downlink control channel for a downlink control information (DCI) from the second base station;

receive, based on the monitoring, the DCI from the second base station;

determine a random-access-channel occasion (RO) of the second cell based on the received DCI;

transmit a preamble to the second base station via the RO; and

transmit, based on receiving a response to the preamble, a message to the second base station indicating a completion of the handover.