Aligning timing for direct communications

1. A method, comprising:

compensating for timing misalignment of subframes associated with at least one of a first user equipment (UE) or a second UE, wherein the first UE and the second UE are in direct UE-to-UE communication, and a timing for the first UE and a timing for the second UE are derived from an eNode B (eNB); and

transmitting, by the first UE, subframes including guard periods, wherein each guard period is at least one of one or more contiguous symbols at a beginning of a subframe or one or more contiguous symbols at an end of a subframe.

2. The method of claim 1 , wherein compensating for timing misalignment comprises compensating for a timing offset between the timing of the first UE and the timing of the second UE.

3. The method of claim 2 , wherein compensating for a timing offset comprises adjusting a transmit time.

4. The method of claim 3 , further comprising:

receiving, from the first UE, a timing reference signal (RS);

measuring, by the second UE, the timing offset between the first UE and the second UE; and

adjusting, by the second UE, the transmit time of the second UE using the timing offset.

5. The method of claim 4 , wherein resources used for the timing RS are signaled, by the eNB, to the first UE and the second UE.

6. The method of claim 3 , further comprising:

transmitting, by the second UE, a timing RS;

receiving, from the first UE, the timing offset between the first UE and the second UE; and

adjusting, by the second UE, the transmit time of the second UE using the timing offset.

7. The method of claim 6 , wherein resources used for the timing RS are signaled, by the eNB, to the first UE and the second UE.

8. The method of claim 1 , further comprising:

determining one or more contiguous physical resource block (PRB) groups allocated to the IDS; and

for each group of contiguous PRBs, allocating, as guard subcarriers, at least one of subcarriers with lowest indices in said each PRB group or subcarriers with highest indices in said each PRB group.

9. The method of claim 2 , wherein compensating for timing offset comprises adjusting a receive timing.

10. The method of claim 9 , further comprising:

receiving, from the first UE, a timing RS;

and

adjusting, by the second UE, the receive timing for the second UE.

11. The method of claim 10 , wherein resources used for the timing RS are signaled, by the eNB, to the first UE and the second UE.

12. The method of claim 1 wherein a guard period is one or more contiguous symbols at an end of a subframe, and the method further comprises: determining a sounding reference signal (SRS) is being transmitted in the subframe; and transmitting the SRS in the last symbol of the subframe, wherein transmitting the SRS overrides the last symbol of the guard period.

13. The method of claim 12 , further comprising adding a guard period at a beginning of a subframe following the subframe including the SRS.

14. The method of claim 1 , further comprising:

receiving, from the eNB, by the first UE, information identifying a size of a physical downlink control channel (PDCCH) for a subsequent subframe; and

adding, by the first UE, a guard period in a beginning of a subframe used for direct UE-to-UE communication to the second UE, the guard period being equal to or greater than the size of the PDDCH for the subsequent subframe.

15. A second UE, comprising:

one or more processors configured to:

compensate for timing misalignment of subframes associated with at least one of a first user equipment (UE) or the second UE, wherein the first UE and the second UE are in direct UE-to-UE communication, and a timing for the first UE and a timing for the second UE are derived from an eNode B (eNB); and

transmit, by the second UE, subframes including guard periods, wherein each guard period is at least one of one or more contiguous symbols at a beginning of a subframe or one or more contiguous symbols at an end of a subframe.

16. The second UE of claim 15 , wherein compensating for timing misalignment comprises compensating for a timing offset between the timing of the first UE and the timing of the second UE.

17. The second UE of claim 16 , wherein compensating for a timing offset comprises adjusting a transmit time.

18. The second UE of claim 17 , the processors further operable to:

receive, from the first UE, a timing reference signal (RS);

measure, by the second UE, the timing offset between the first UE and the second UE; and

adjust, by the second UE, the transmit time of the second UE using the timing offset.

19. The second UE of claim 18 , wherein resources used for the timing RS are signaled, by the eNB, to the first UE and the second UE.

20. The second UE of claim 17 , the processors further operable to:

transmit, by the second UE, a timing RS;

receive, from the first UE, the timing offset between the first UE and the second UE; and

adjust, by the second UE, the transmit time of the second UE using the timing offset.

21. The second UE of claim 20 , wherein resources used for the timing RS are signaled, by the eNB, to the first UE and the second UE.

22. The second UE of claim 15 , the processors further operable to:

determine one or more contiguous physical resource block (PRB) groups allocated to the IDS; and

for each group of contiguous PRBs, allocate, as guard subcarriers, at least one of subcarriers with lowest indices in said each PRB group or subcarriers with highest indices in said each PRB group.

23. The second UE of claim 16 , wherein compensating for timing offset comprises adjusting a receive timing.

24. The second UE of claim 23 , the processors further operable to:

receive, from the first UE, a timing RS;

and

adjust, by the second UE, the receive timing for the second UE.

25. The second UE of claim 24 , wherein resources used for the timing RS are signaled, by the eNB, to the first UE and the second UE.

26. The second UE of claim 15 , wherein a guard period is one or more contiguous symbols at an end of a subframe, and the processors further operable to:

determine a sounding reference signal (SRS) is being transmitted in the subframe; and transmit the SRS in the last symbol of the subframe, wherein transmitting the SRS overrides the last symbol of the guard period.

27. The second UE of claim 26 , the processors further operable to add a guard period at a beginning of a subframe following the subframe including the SRS.

28. The second UE of claim 15 , the processors further operable to:

receive, from the eNB, by the first UE, information identifying a size of a physical downlink control channel (PDCCH) for a subsequent subframe; and

add, by the first UE, a guard period in a beginning of a subframe used for direct UE-to-UE communication to the second UE, the guard period being equal to or greater than the size of the PDDCH for the subsequent subframe.