Switching communication modes (direct and indirect UE access)

1. An apparatus comprising:

a radio transceiver that communicates with a relay unit;

a processor that:

decides to handover the relay unit;

determines a group of remote units associated with the relay unit, wherein each remote unit in the group has a Layer 2 device-to-device (“D2D”) connection with the relay unit and exchanges Radio Resource Control (“RRC”) and Non-Access Stratum (“NAS”) signaling with a mobile communication network via the Layer 2 D2D connection with the relay unit;

selects a target base unit of the mobile communication network based on the relay unit and the group of remote units, wherein the target base unit supports handover of the relay unit;

determines a first subgroup of remote units from among the group of remote units that cannot be supported by the target base unit, wherein the target base unit supports handover of a second subgroup of the remote units, the second subgroup comprising a remainder of the group of remote units associated with the relay unit; and

sends, to a remote unit in the first subgroup of remote units, a command to resume communication with the mobile communication network by switching its communication mode from a mode using the Layer 2 D2D connection with a relay unit to a communication mode using an Uu interface with the mobile communication network, wherein the remote unit exchanges RRC and NAS signaling directly with the mobile communication network without using the relay unit.

2. The apparatus of claim 1 , wherein the processor further:

requests a measurement report from one or more remote units in the first subgroup of remote units; and

executes handover of the relay unit and of the remote units not included in the first subgroup to the target base unit in parallel with receiving one or more measurement reports from the first subgroup of remote units.

3. The apparatus of claim 1 , wherein determining the group of one or more remote units associated with the relay unit comprises the processor identifying one or more remote units that are in a radio resource control (“RRC”) connected state and that have an RRC connection whose path includes the relay unit.

4. The apparatus of claim 1 , wherein selecting the target base unit based on the relay unit and the group of one or more remote units comprises the processor identifying a number of remote units a base unit can support and selecting a base unit supporting the largest number of remote units as the target base unit.

5. The apparatus of claim 1 , wherein the relay unit is a layer 2 (“L2”) relay, wherein the relay unit and each remote unit in the group of one or more remote units has its own radio resource control (“RRC”) connection with the apparatus and its own non-access stratum (“NAS”) connection with the mobile communication network.

6. The apparatus of claim 1 , wherein determining the first subgroup of remote units that cannot be supported by the target base unit for communicating with the mobile network via the relay unit comprises the processor identifying one or more of remote units of the group which the target base unit cannot support due to mobility limitations of the identified one or more of remote units.

7. The apparatus of claim 1 , wherein determining the first subgroup of remote units that cannot be supported by the target base unit comprises the processor determining from a handover response from the target base unit that resource limitations at the target base unit prevent supporting the group of one or more remote units associated with the relay unit.

8. The apparatus of claim 1 , wherein sending the command to resume communication with the mobile communication network without using the relay unit comprises the processor sending a radio resource control (“RRC”) message to each remote unit in the first subgroup of remote units via the relay unit, the RRC message instructing a remote unit in the first subgroup to resume communication with a target base unit in the mobile communication network.

9. The apparatus of claim 1 , wherein the processor further requests measurement reports from each remote unit in the first subgroup of remote units in response to determining the first subgroup of remote units, wherein the processor sends the command to resume communication with the mobile communication network without using the relay unit in response to receiving measurement reports from each remote unit in the first subgroup of remote units and in response to determining a target base unit for each remote unit based on the received measurement reports.

10. The apparatus of claim 1 , wherein the processor further:

receives a radio resource control (“RRC”) initial message from a second remote unit via the relay unit;

determines that the second remote unit is restricted from using the apparatus; and

sends to the second remote unit a command to resume communication with the mobile communication network by using a second base unit and without using the relay unit.

11. A method comprising:

serving a relay unit in a mobile communication network;

deciding to handover the relay unit;

determining a group of remote units associated with the relay unit, wherein each remote unit in the group has a Layer 2 device-to-device (“D2D”) connection with the relay unit and exchanges Radio Resource Control (“RRC”) and Non-Access Stratum (“NAS”) signaling with a mobile communication network via the Layer 2 D2D connection with the relay unit;

selecting a target base unit of the mobile communication network based on the relay unit and the group of remote units, wherein the target base unit supports handover of the relay unit;

determining a first subgroup of remote units from among the group of remote units that cannot be supported by the target base unit, wherein the target base unit supports handover of a second subgroup of the remote units, the second subgroup comprising a remainder of the group of remote units associated with the relay unit; and

sending, to a remote unit in the first subgroup of remote units, a command to resume communication with the mobile communication network by switching its communication mode from a mode using the Layer 2 D2D connection with a relay unit to a communication mode using an Uu interface with the mobile communication network, wherein the remote unit exchanges RRC and NAS signaling directly with the mobile communication network without using the relay unit.

12. The method of claim 11 , further comprising:

requesting a measurement report from a remote unit in the first subgroup of remote units; and

completing handover of the relay unit and of the remote units not included in the first subgroup to the target base unit in parallel with receiving one or more measurement reports from the first subgroup of remote units.

13. The method of claim 11 , wherein determining the group of one or more remote units associated with the relay unit comprises identifying one or more remote units that are in a radio resource control (“RRC”) connected state and that have an RRC connection whose path includes the relay unit.

14. The method of claim 11 , wherein selecting the target base unit based on the relay unit and the group of one or more remote units comprises identifying a number of remote units a base unit can support and selecting a base unit supporting the largest number of remote units as the target base unit.

15. The method of claim 11 , wherein the relay unit is a layer 2 (“L2”) relay, wherein the relay unit and each remote unit in the group of one or more remote units has its own radio resource control (“RRC”) connection and its own non-access stratum (“NAS”) connection with the mobile communication network.

16. The method of claim 11 , wherein determining the first subgroup of remote units that cannot be supported by the target base unit for communicating with the mobile communication network via the relay unit comprises identifying one or more of remote units of the group which the target base unit cannot support due to mobility limitations of the identified one or more of remote units.

17. The method of claim 11 , wherein determining the first subgroup of remote units that cannot be supported by the target base unit comprises determining from a handover response from the target base unit that resource limitations at the target base unit prevent supporting the group of one or more remote units associated with the relay unit.

18. The method of claim 11 , wherein sending the command to resume communication with the mobile communication network without using the relay unit comprises sending a radio resource control (“RRC”) message to each remote unit in the first subgroup of remote units via the relay unit, the RRC message instructing a remote unit in the first subgroup of remote units to transition to resume communication with a target base unit in the mobile communication network.

19. The method of claim 11 , further comprising requesting measurement reports from each remote unit in the first subgroup of remote units in response to determining the first subgroup of remote units, wherein sending the command to resume communication with the mobile communication network without using the relay unit occurs in response to receiving measurement reports from each remote unit in the first subgroup of remote units and in response to determining a target base unit for each remote unit based on the received measurement reports.

20. The method of claim 11 , further comprising:

receiving, at a base unit, a radio resource control (“RRC”) initial message from a second remote unit via the relay unit;

determining that the second remote unit is restricted from using the base unit; and

sending to the second remote unit a command to resume communication with the mobile communication network by using a second base unit and without using the relay unit.