Method and mobile node for determining a point in time for transmissions

1. A method in a mobile node, of determining when to transmit a signal, wherein the mobile node operates in a wireless communication system comprising a first cell and a second cell, where the mobile node is synchronized to the first cell, the method comprising:

determining a timing reference based on a reception of a downlink transmission in the first cell;

transmitting the signal, to be received in the second cell, at a point in time based on the timing reference and a timing advance value of the first cell;

receiving a timing advance value for the second cell in response to the transmitted signal;

using the received timing advance value for the second cell for further transmissions of signals to be received in the second cell; and

determining a time of start of transmission of a downlink sub-frame in the second cell, wherein the determining the time of start of transmission of the downlink sub-frame in the second cell is based on the timing reference and the timing advance value for the second cell,

wherein the timing advance value for the second cell is half the timing advance value of the first cell plus an offset value, and

wherein the offset value corrects asymmetry in propagation times between uplink and downlink transmissions.

2. The method of claim 1 , wherein the timing reference is the time of reception of a sub-frame in the first cell and where the point in time is based on the timing reference minus half the timing advance value of the first cell.

3. The method of claim 1 , wherein the timing reference is the time of reception of a sub-frame in the first cell minus the timing advance value for the first cell and where the point in time is based on the timing reference plus half the timing advance value of the first cell.

4. The method of claim 1 , wherein the timing reference is the time of reception of a sub-frame in the first cell minus half the timing advance value for the first cell and where the point in time is based on the timing reference.

5. The method of claim 1 , wherein the offset value is received by the mobile node from the wireless communication system.

6. The method of claim 1 , wherein the determination of the timing reference is based on the start of the reception of a sub-frame in the first cell.

7. The method of claim 1 , wherein the determination of the timing reference is based on the end of the reception of a sub-frame in the first cell.

8. The method of claim 1 , wherein the determination of the timing reference is based on a specified location within a sub-frame received in the first cell.

9. The method of claim 1 , wherein the signal comprises a random access preamble.

10. The method of claim 9 , wherein the random access preamble is of preamble format 4.

11. A mobile node, for determining when to transmit a signal, wherein the mobile node is configured to operate in a wireless communication system comprising a first cell and a second cell, where the mobile node is synchronized to the first cell, the mobile node comprising:

a processing circuitry configured to determine a timing reference based on a reception of a downlink transmission in the first cell;

a transmitter configured to transmit the signal, to be received in the second cell, at a point in time based on the timing reference and a timing advance value of the first cell; and

a receiver configured to receive a timing advance value for the second cell in response to the transmitted signal;

wherein the processing circuitry is further configured to:

use the received timing advance value for the second cell for further transmissions of signals to be received in the second cell, and

determine a time of start of transmission of a downlink sub-frame in the second cell, wherein the determining the time of start of transmission of the downlink sub-frame in the second cell is based on the timing reference and the timing advance value for the second cell,

wherein the timing advance value for the second cell is half the timing advance value of the first cell plus an offset value, and

wherein the offset value corrects asymmetry in propagation times between uplink and downlink transmissions.

12. The mobile node of claim 11 , wherein the timing reference is the time of reception of a sub-frame in the first cell and where the point in time is based on the timing reference minus half the timing advance value of the first cell.

13. The mobile node of claim 11 , wherein the timing reference is the time of reception of a sub-frame in the first cell minus the timing advance value for the first cell and where the point in time is based on the timing reference plus half the timing advance value of the first cell.

14. The mobile node of claim 11 , wherein the timing reference is the time of reception of a sub-frame in the first cell minus half the timing advance value for the first cell and where the point in time is based on the timing reference.

15. The mobile node of claim 11 , wherein the offset value is received by the mobile node from the wireless communication system.

16. The mobile node of claim 11 , wherein the determination of the timing reference is based on the start of the reception of a sub-frame in the first cell.

17. The mobile node of claim 11 , wherein the determination of the timing reference is based on the end of the reception of a sub-frame in the first cell.

18. The mobile node of claim 11 , wherein the determination of the timing reference is based on a specified location within a sub-frame received in the first cell.

19. The mobile node of claim 11 , wherein the signal comprises a random access preamble.

20. The mobile node of claim 19 , wherein the random access preamble is of preamble format 4.