Handoff procedure in a mobile communication system

1. A mobile terminal, comprising:

a receiver which, in operation, receives a handoff command message for a handoff including a handoff execution condition and a handoff validity timer, wherein the receiver, in operation, receives the handoff command message from a source base station via a downlink carrier of a source radio cell; and

circuitry which, in operation, determines, based on the received handoff execution condition included in the handoff command message, whether or not the mobile terminal is to trigger execution of the handoff;

wherein, when the circuitry determines that the mobile terminal is to trigger the execution of the handoff, the mobile terminal executes the handoff by: (a) performing a random access channel (RACH) procedure to thereby time align an uplink carrier of a target radio cell, and (b) transmitting a handoff complete message thereby indicating completion of the handoff including completion of configuration of the mobile terminal for communication via the target radio cell; and

wherein when the circuitry does not, within a first pre-configured period of time, determine that the mobile terminal is to trigger the execution of the handoff, the circuitry:

discards the received handoff command message; and

re-configures the first pre-configured period of time based on the handoff validity timer.

2. The mobile terminal according to claim 1 , wherein the mobile terminal, prior to receiving the handoff command message, communicates with the source base station via the source radio cell.

3. The mobile terminal according to claim 1 , wherein the handoff execution condition, included in the received handoff command message, corresponds to a timer value; and

the circuitry, in operation, determines whether or not a timer with the timer value has expired, wherein, upon expiration of the timer with the timer value, the mobile terminal is to trigger the execution of the handoff.

4. The mobile terminal according to claim 1 , wherein the handoff execution condition, included in the received handoff command message, corresponds to an indication to trigger the execution of the handoff based on threshold values for signal strength or signal quality of at least one of: the source radio cell or the target radio cell; and

the circuitry, in operation, determines whether or not at least one of: the signal strength or signal quality of the source radio cell falls below one of the threshold values or the signal strength or signal quality of the target radio cell rises above another of the threshold values,

wherein, upon the signal strength or signal quality having fallen below the one of the threshold values or having risen above the another of the threshold values, the mobile terminal is to trigger the execution of the handoff.

5. The mobile terminal according to claim 1 , wherein the handoff execution condition, included in the received handoff command message, corresponds to at least one threshold value for a signal strength or signal quality of at least one of: the source radio cell or the target radio cell; and

the circuitry, in operation, determines whether or not at least one of: the signal strength or signal quality of the source radio cell falls below the at least one threshold value or the signal strength or signal quality of the target radio cell rises above the at least one threshold value,

wherein, upon the signal strength or signal quality having fallen below the at least one threshold value or having risen above the at least one threshold value, the mobile terminal is to trigger the execution of the handoff.

6. The mobile terminal according to claim 5 , wherein the at least one threshold value for the signal strength corresponds to a threshold value for a reference signal received power (RSRP) to be measured by the mobile terminal based on reference signals (RSs) transmitted in at least one of: the source radio cell or the target radio cell; and

the at least one threshold value for the signal quality corresponds to a threshold value for a reference signal received quality (RSRQ) to be measured by the mobile terminal based on the reference signals (RSs) transmitted in at least one of: the source radio cell or the target radio cell.

7. The mobile terminal according to claim 1 , wherein the handoff execution condition, included in the received handoff command message, corresponds to an indication to trigger the execution of the handoff based on out-of-sync events where a channel quality of the source radio cell falls below an out-of-sync threshold (Qout); and

the circuitry, in operation, determines whether or not a number of out-of-sync events exceeds a pre-configured number N,

wherein, in case the number of out-of-sync events exceeds the pre-configured number N, the mobile terminal is to trigger the execution of the handoff.

8. The mobile terminal according to claim 1 , wherein the handoff execution condition, included in the received handoff command message, corresponds to a counter value for out-of-sync events where a channel quality of the source radio cell falls below an out-of-sync threshold (Qout); and

the circuitry, in operation, determines whether or not a number of out-of-sync events exceeds a received counter value,

wherein, in case the number of out-of-sync events exceeds the received counter value, the mobile terminal is to trigger the execution of the handoff.

9. The mobile terminal according to claim 1 , wherein the handoff command message corresponds to an RRCConnectionReconfiguration message.

10. The mobile terminal according to claim 1 , wherein the handoff complete message corresponds to an RRCConnectionReconfigurationComplete message.

11. The mobile terminal according to claim 1 , wherein:

in case the circuitry determines, within a second pre-configured period of time, that the mobile terminal is to trigger the execution of the handoff, the mobile terminal executes the handoff including (a) performing a contention-free RACH procedure based on a random access preamble included in the handoff command message; and

in case the circuitry determines, after expiry of the second pre-configured period of time, that the mobile terminal is to trigger the execution of the handoff, the mobile terminal executes the handoff including (a) performing a contention-based RACH procedure based on a random access preamble randomly selected by the mobile terminal.

12. The mobile terminal according to claim 11 , wherein the received handoff command message includes a RACH validity timer, and

the circuitry, in operation, re-configures the second pre-configured period of time based on the RACH validity timer.

13. The mobile terminal according to claim 1 , wherein after receiving the handoff command message, the mobile terminal detaches from the source radio cell whereby communication with the source base station is stopped.

14. A non-transitory computer readable medium storing instructions that, when executed by a processor of a mobile terminal, cause the mobile terminal to:

receiving a handoff command message for a handoff including a handoff execution condition and a handoff validity timer, wherein the handoff command message is received from a source base station via a downlink carrier of a source radio cell; and

determining, based on the received handoff execution condition included in the handoff command message, whether or not the mobile terminal is to trigger execution of the handoff;

wherein, when the mobile terminal determines that the mobile terminal is to trigger the execution of the handoff, the mobile terminal executes the handoff by (a) performing a random access channel (RACH) procedure to thereby time align an uplink carrier of a target radio cell, and (b) transmitting a handoff complete message thereby indicating completion of the handoff including completion of configuration of the mobile terminal for communication via the target radio cell; and

wherein when the mobile terminal does not, within a first pre-configured period of time, determine that the mobile terminal is to trigger the execution of the handoff, the mobile terminal:

discards the received handoff command message; and

re-configures the first pre-configured period of time based on the handoff validity timer.

15. A method, comprising:

receiving, by a mobile terminal, a handoff command message for a handoff including a handoff execution condition and a handoff validity timer, the mobile terminal, in operation, receives the handoff command message from a source base station via a downlink carrier of a source radio cell; and

determining, by the mobile terminal, based on the received handoff execution condition included in the handoff command message, whether or not the mobile terminal is to trigger execution of the handoff;

wherein, when the mobile terminal determines that the mobile terminal is to trigger the execution of the handoff, the mobile terminal executes the handoff by: (a) performing a random access channel (RACH) procedure to thereby time align qn uplink carrier of a target radio cell, and (b) transmitting a handoff complete message thereby indicating completion of the handoff including completion of configuration of the mobile terminal for communication via the target radio cell; and

wherein when the mobile terminal does not, within a first pre-configured period of time, determine that the mobile terminal is to trigger the execution of the handoff:

discards the received handoff command message; and

re-configures the first pre-configured period of time based on the handoff validity timer.

16. The method according to claim 15 , wherein the mobile terminal, prior to receiving the handoff command message, communicates with the source base station via the source radio cell.

17. The method for performing the handoff according to claim 15 , wherein the handoff execution condition, included in the received handoff command message, corresponds to a timer value, and

determining whether or not the mobile terminal is to trigger the execution of the handoff includes determining whether or not a timer with the timer value has expired, wherein, upon expiration of the timer with the timer value, the mobile terminal is to trigger the execution of the handoff.

18. The method according to claim 15 , wherein after receiving the handoff command message the mobile terminal detaches from the source radio cell thereby stopping communication with the source base station.