Method and apparatus for handover in heterogeneous cellular networks
A method at a user equipment for handover from a serving cell to a target cell, the method sending a measurement report to the serving cell; and transmitting a reconfiguration complete message to the target cell; wherein the measurement report includes downlink timing measurements for the target cell. Further, a method at a source network element for handover of a user equipment from the source network element to a target network element, the method receiving a measurement report from the user equipment; sending a handover request to the target network element; receiving a handover request acknowledgement from the target network element, the handover request acknowledgement including a reconfiguration message and at least one downlink subframe in which an uplink grant is expected at the target network element for the user equipment; and forwarding the reconfiguration message and at least one downlink subframe to the user equipment.
1. A method at a user equipment for handover from a serving cell to a target cell, the method comprising:
measuring, at the user equipment, a difference between downlink times of arrival for the target cell and the serving cell;
sending to the serving cell the difference between downlink times of arrival for the target cell and the serving cell;
sending to the target cell adjustments to uplink transmit timing at the user equipment based on the difference between downlink times of arrival for the target cell and the serving cell;
receiving a response from the target cell; and
if no response is received, resending the adjustments to uplink transmit timing at the user equipment to the target cell.
2. The method of claim 1 , further comprising, receiving a reconfiguration message from the serving cell, the reconfiguration message including an uplink grant or at least one downlink subframe to receive the uplink grant from the target cell.
3. The method of claim 2 , wherein the reconfiguration message includes a timing advance value for the target cell.
4. The method of claim 1 further comprising adjusting uplink transmit timing at the user equipment based on the difference between downlink times of arrival for the target cell and the serving cell.
5. The method of claim 2 , wherein at least one downlink subframe is a downlink control channel subframe indicating the uplink grant at the target cell.
6. The method of claim 1 , wherein if a maximum number of retransmissions is reached, performing random access procedure for uplink synchronization with the target cell.
7. The method of claim 2 , wherein the uplink grant contains a subframe number.
8. A method at a source network element for handover of a user equipment from the source network element to a target network element, the method comprising:
receiving, from the user equipment, differences between downlink times of arrival for the source network element and the target network element measured by the user equipment;
sending a handover request to the target network element;
receiving a handover request acknowledgement including at least one downlink subframe in which an uplink grant is expected at the target network element for the user equipment; and
forwarding the at least one downlink subframe to the user equipment.
9. The method of claim 8 , wherein the handover request acknowledgement includes a timing advance value for the target network element.
10. A source network element for handover of a user equipment from the source network element to a target network element, the source network element comprising a processor configured to:
receive, from the user equipment, differences between downlink times of arrival for the source network element and the target network element measured by the user equipment;
send a handover request to the target network element;
receive a handover request acknowledgement including at least one downlink subframe in which an uplink grant is expected at the target network element for the user equipment; and
forward the at least one downlink subframe to the user equipment.
11. The source network element of claim 10 , wherein the handover request acknowledgment includes a timing advance value for the target network element.
12. The source network element of claim 10 , wherein the source network element is configured to receive a range of downlink subframes, and forward by providing the user equipment with a subset of downlink subframes within the range of downlink subframes.
13. The source network element of claim 12 , wherein the source network element chooses the subset of downlink subframes within the range of downlink subframes.
14. The source network element of claim 13 , wherein the source network element is further configured to send a confirmation message to the target network element with the subset of downlink subframes.
15. The source network element of claim 10 , wherein the at least one downlink subframe for the uplink grant is a downlink control channel subframe indicating the uplink grant at the target network element.