Timing a random access UE to relay AP

1. A method in a relay access point (AP) for timing a random access of a user equipment (UE) to the relay AP in a time division duplex (TDD) radio communication network, the relay AP performing a self-backhaul over a backhaul link to a donor AP, the backhaul link and an random access link between the relay AP and the UE sharing a same frequency resource, comprising:

obtaining a first propagation delay information from the donor AP to the relay AP over the backhaul link;

determining a timing advance offset for the UE to perform the random access at least based on the first propagation delay information;

transmitting the timing advance offset to the UE;

when the determined timing advance offset is larger than a length of a guard period for the UE at a downlink-to-uplink switching point, assigning a time period scheduled for transmitting at least one downlink subframe and/or uplink subframe next to the downlink-to-uplink switching point as additional guard period for the UE; and

informing the UE of the assignment.

2. The method of claim 1 , wherein the backhaul link between the relay AP and the donor AP goes through one or more relay APs, the determining step comprises:

calculating an accumulated propagation delay from the first propagation delay information; and

determining the timing advance offset as twice of the accumulated propagation delay.

3. The method of claim 1 , further comprising obtaining a second propagation delay information from the relay AP to the UE, wherein the determining step comprises determining the timing advance offset for the UE to perform the random access based on the first propagation delay information and the second propagation delay information.

4. The method of claim 1 , wherein the timing advance offset is transmitted in an access information table, a radio resource control signaling or a medium access control signaling.

5. The method of claim 1 , wherein the relay AP is a mobile AP, the method further comprises updating the timing advance offset to UE by periodically repeating the obtaining, determining, and transmitting steps therein.

6. A relay access point (AP) configured to time a random access of a user equipment (UE) to the relay AP in a time division duplex (TDD) radio communication network, the relay AP performing a self-backhaul over a backhaul link to a donor AP, the backhaul link and an random access link between the relay AP and the UE sharing a same frequency resource, comprising a processor and a memory, said memory containing instructions executable by said processor whereby said relay AP is operative to:

obtain a first propagation delay information from the donor AP to the relay AP over the backhaul link;

determine a timing advance offset for the UE to perform the random access at least based on the first propagation delay information;

transmit the timing advance offset to the UE;

when the determined timing advance offset is larger than a length of a guard period for the UE at a downlink-to-uplink switching point, assigning a time period scheduled for transmitting at least one downlink subframe and/or uplink subframe next to the downlink-to-uplink switching point as additional guard period for the UE; and

inform the UE of the assignment.

7. The relay AP according to claim 6 , wherein the backhaul link between the relay AP and the donor AP goes through one or more relay APs, the relay AP is operative to:

calculating an accumulated propagation delay from the first propagation delay information; and

determining the timing advance offset as twice of the accumulated propagation delay.

8. The relay AP according to claim 6 , further operative to:

obtain a second propagation delay information from the relay AP to the UE, wherein the second propagation delay information is also a base to determine the timing advance offset for the UE to perform the random access.

9. The relay AP according to claim 6 , further operative to: obtain a second propagation delay information from the relay AP to the UE, wherein the second propagation delay information is also a base to determine the timing advance offset for the UE to perform the random access.

10. The relay AP according to claim 6 , wherein the timing advance offset is transmitted in an access information table, a radio resource control signaling or a medium access control signalling.

11. The relay AP according to claim 6 , wherein the relay AP is a mobile AP, the relay AP is further operative to date the timing advance offset to the UE by periodically repeating the obtain, determine, and transmit steps configured therein.

12. A user equipment (UE) configure to time a random access to a relay access point (AP) in a time division duplex (TDD) radio communication network, the relay AP performing a self-backhaul over a backhaul link to a donor AP, the backhaul link and an random access link between the relay AP and the UE sharing a same frequency resource, comprising a processor and a memory, said memory containing instructions executable by said processor whereby said UE is operative to:

receive a timing advance offset from the relay AP, the timing advance offset being associated with a propagation delay information from the donor AP to the relay AP over the backhaul link;

determine a starting time point, within a guard period at a downlink-to-uplink switching point, to perform the random access based on the timing advance offset;

receive an indication on occupying a time period scheduled for transmitting at least one downlink subframe and/or uplink subframe next to the downlink-to-uplink switching point as additional guard period; and

in response to receiving the indication, reset the guard period at the downlink-to-uplink switching point based on the additional guard period and stopping receiving the at least one downlink subframe and/or performing uplink transmission in the at least one uplink subframe.