Device to-device (D2D) communications

1. A user equipment (UE) operable to perform device-to-device (D2D) communications, the UE having one or more processors configured to:

identify D2D data to be transmitted from the UE using a D2D radio bearer of the UE, the D2D data being identified when the UE is in a radio resource control (RRC) idle mode;

determine that the UE is limited to using a defined resource allocation mode one to communicate the D2D data from the UE;

initiate a service request procedure at a non-access stratum (NAS) of the UE, the service request procedure triggering an RRC layer of the UE to perform an RRC connection establishment procedure with an evolved node B (eNB) to switch the UE from the RRC idle mode to an RRC connected mode;

receive an uplink (UL) grant from the eNB for transmission of the D2D data from the UE; and

send the D2D data using the UL grant provided by the eNB, the D2D data being transmitted from the UE using the D2D radio bearer of the UE.

2. The UE of claim 1 , wherein the one or more processors are further configured to receive the UL grant from the eNB in response to sending a buffer status report (BSR) from a medium access control (MAC) layer of the UE to the eNB, the BSR being sent to the eNB after the UE has switched to the RRC connected mode.

3. The UE of claim 2 , wherein the one or more processors are further configured to use one or more configuration parameters associated with the defined resource allocation mode one for sending the BSR from the MAC layer of the UE to the eNB, the one or more configuration parameters including at least one of a periodic D2D BSR timer or a D2D BSR retransmission timer.

4. The UE of claim 1 , wherein the one or more processors are further configured to perform the RRC connection establishment procedure at the RRC layer of the UE in order to create a signaling radio bearer for the UE.

5. The UE of claim 1 , wherein the one or more processors are further configured to send a service request message to the eNB using a signaling radio bearer of the UE, the service request message for requesting establishment of cellular radio bearers for the UE.

6. The UE of claim 1 , wherein the one or more processors are further configured to determine that the UE is limited to using the defined resource allocation mode one based on a system information block (SIB) broadcast from the eNB.

7. The UE of claim 1 , wherein the D2D radio bearers are present for the UE when the D2D data is identified to be transmitted from the UE, wherein the D2D radio bearers operate using a defined radio access technology (RAT) standard.

8. The UE of claim 1 , wherein the D2D data is generated by a D2D application on the UE.

9. The UE of claim 1 , wherein the D2D radio bearers are present when the UE is in the RRC idle mode.

10. The UE of claim 1 , wherein the UE operates in an Evolved Universal Terrestrial Radio Access Network (E-UTRAN).

11. The UE of claim 1 , wherein the one or more processors are further configured to send an RRC connection request message to the eNB during the RRC connection establishment procedure, the RRC connection request message including an indication that the UE is to switch to the RRC connected mode in order to perform D2D communications.

12. The UE of claim 1 , wherein the UE is barred from performing the D2D communications for a defined period of time when the eNB is enforcing access class barring for the UE.

13. A method for performing device-to-device (D2D) communications, the method comprising:

identifying, at a user equipment (UE), D2D data to be communicated to a destination UE, the D2D data being identified when the UE is in a radio resource control (RRC) idle mode with an evolved node B (eNB);

determining that the UE is limited to using a defined resource allocation mode one to communicate the D2D data to the destination UE;

initiating a service request procedure at the UE, the service request procedure triggering the UE to perform an RRC connection establishment procedure with the eNB to switch the UE from the RRC idle mode to an RRC connected mode;

receiving an uplink (UL) grant from the eNB for communicating the D2D data to the destination UE; and

sending the D2D data to the destination UE using the UL grant provided by the eNB, the D2D data being communicated to the destination UE using D2D radio bearers of the UE.

14. The method of claim 13 , further comprising receiving, at the UE, the UL grant from the eNB in response to sending a buffer status report (BSR) from the UE to the eNB, the BSR being sent to the eNB after the UE has switched to the RRC connected mode.

15. The method of claim 14 , further comprising sending the BSR from the UE to the eNB using one or more configuration parameters associated with the defined resource allocation mode one, the one or more configuration parameters including at least one of a periodic D2D BSR timer or a D2D BSR retransmission timer.

16. The method of claim 13 , further comprising sending a service request message from the UE to the eNB using a signaling radio bearer of the UE, the signaling radio bearer being established for the UE during the RRC connection establishment procedure, the service request message for establishing cellular radio bearers for the UE.

17. The method of claim 13 , further comprising determining that determine that the UE is limited to using the defined resource allocation mode one based on a system information block (SIB) broadcast from the eNB.

18. The method of claim 13 , further comprising performing cell reselection based on whether neighboring cells of the UE support D2D communications.