Timing alignment in an LTE system

1. A method for uplink time alignment in a cellular communications system, comprising:

transmitting a random access preamble to the controlling node as part of a contention based random access procedure;

receiving a random access response from the controlling node in response to the random access preamble, wherein the random access preamble comprises a second timing advance value;

after receiving the random access response, determining whether a first timing advance value received from the controlling node before transmission of the random access preamble is valid;

if the first timing advance value is valid, transmitting to the controlling node in accordance with the first timing advance value after receiving the random access response; and

if the first timing advance is not valid, transmitting to the controlling node in accordance with the second timing advance value.

2. The method of claim 1 , further comprising:

receiving the first timing advance value from the controlling node; and

before transmitting the random access preamble, transmitting to the controlling node in accordance with the first timing advance value.

3. The method of claim 1 , according to which the user terminal uses the first timing advance value if the user terminal wins the contention based random access procedure.

4. The method of claim 1 , wherein:

the first timing advance value is associated with a timer; and

determining whether the first timing advance value is valid comprises determining whether the timer is running.

5. The method of claim 4 , further comprising starting the timer after receiving the first timing advance value from the controlling node.

6. The method of claim 1 , wherein the user terminal comprises a Long-Term Evolution (LTE) UE and the controlling node comprises an LTE eNodeB.

7. The method of claim 6 , wherein the method is used in an LTE Random Access procedure.

8. The method of claim 1 , wherein transmitting the random access preamble to the controlling node as part of the contention based random access procedure comprises:

determining that the user terminal is not allocated resources for contacting the controlling node; and

in response to determining that the user terminal is not allocated resources, transmitting the random access preamble on a physical random access channel to the controlling node.

9. The method of claim 1 , wherein transmitting the random access preamble to the controlling node as part of the contention based random access procedure comprises:

randomly selecting one of a plurality of preambles available for contention based random access; and

transmitting the randomly-selected random access preamble to the controlling node.

10. The method of claim 1 , wherein transmitting data to the controlling node in accordance with the first timing advance value comprises starting transmission of the data at a time that is offset according to the first timing advance value relative to a nominal transmission time associated with the controlling node.

11. A user terminal for transmitting data in a cellular communications system, the user terminal comprising:

a transmitter arranged for transmitting to a controlling node in accordance with a timing advance value; and for transmitting a random access preamble to the controlling node as part of a contention based random access procedure;

a receiver arranged for receiving a random access response from the controlling node in response to the random access preamble, wherein the random access preamble comprises a second timing advance value; and

one or more circuits arranged to:

determine whether a previously received first timing advance value is valid when the second timing advance value is received;

if the first timing advance value is valid, control the transmitter to continue to transmit to the controlling node in accordance with the first timing advance value after receiving the random access response; and

if the first timing advance is not valid, control the transmitter to transmit to the controlling node in accordance with the second timing advance value.

12. A user terminal for use in a cellular communications system, the user terminal comprising:

a transmitter configured to request communication with a controlling node in a contention based random access procedure by transmitting a random access preamble to the controlling node; and

a receiver configured to receive, in response to the random access preamble, a random access response with a second timing advance value;

one or more control circuits configured to:

determine whether a first timing advance value, received before the random access response, is valid at receipt of the second timing advance value; and

ignore the second timing advance value if the first timing advance value is determined to be valid and thereby control the transmitter to apply the first timing advance value to transmissions to the controlling node, and otherwise control the transmitter to apply the second timing advance value to transmissions to the controlling node.

13. The user terminal of claim 12 , wherein the one or more control circuits are configured to control the transmitter to apply the first timing advance value if the contention based procedure is won by the user terminal.

14. The user terminal of claim 12 , further comprising:

a timer that is associated with the first timing advance value;

wherein said one or more circuits is configured to the determine the validity of the first timing advance value in relation to whether the timer is running.

15. The user terminal of claim 14 , wherein said one or more control circuits are configured to start the timer when receiving the first timing advance value from the controlling node.

16. The user terminal of claim 12 , wherein said transmitter is configured to transmit said random access preamble to the controlling node on a physical random access channel.

17. The user terminal of claim 12 , wherein the one or more control circuits are configured to control the transmitter to apply the first timing advance value to transmissions to the controlling node by controlling the transmitter to start transmission of the data at a time that is offset according to the first timing advance value relative to a nominal transmission time associated with the controlling node.

18. The user terminal of claim 12 , wherein the user terminal comprises a Long-Term Evolution (LTE) UE.

19. The user terminal of claim 12 , wherein the one or more control circuits are configured to use the first timing advance value, if the first timing advance value is valid, regardless of receipt of a second timing advance value.

20. A method in a user terminal configured for use in a wireless communication network, said method comprising:

receiving a first timing advance value at the user terminal;

performing a contention-based random access procedure, in which the user terminal attempts to access the network and receives a second timing advance value;

determining whether the first timing advance value is valid at receipt of the second timing advance value; and

ignoring the second timing advance value and continuing to use the first timing advance value at the user terminal for transmitting to the network on the uplink if the first timing advance value is determined to be valid, and otherwise using the second timing advance value at the user terminal for transmitting to the network on the uplink.