Method and apparatus for device to device communication

1. A Device to Device (D2D) communication method in which a first terminal directly communicates with a second terminal, the D2D communication method comprising:

receiving a first Timing Advance (TA) value from a base station;

including a second TA value that is obtained based on the first TA value in a Scheduling Assignment (SA) signal;

transmitting the SA signal to the second terminal at downlink (DL) timing for receiving a downlink signal of the base station in cellular communication; and

transmitting, when a length of a Cyclic Prefix (CP) that is set for the D2D communication and a length of a CP that is set for the cellular communication are different, data of the D2D communication to the second terminal at first timing that is obtained by applying the second TA value to the DL timing.

2. The D2D communication method of claim 1 , further comprising transmitting, when a CP length for the D2D communication and a CP length for the cellular communication are the same, data of the D2D communication to the second terminal at second timing that is obtained by applying the first TA value to the DL timing.

3. The D2D communication method of claim 1 , wherein a timing adjustment step size for a first cell is the same as a timing adjustment step size for a second cell, even if the first cell to which the first terminal belongs and the second cell to which the second terminal belongs have different cell coverage.

4. The D2D communication method of claim 1 , wherein the transmitting of the SA signal comprises including a first timing adjustment step size that is used by the first terminal in the SA signal.

5. The D2D communication method of claim 4 , wherein the including of a first timing adjustment step size comprises setting a value of 1 bit that is included in the SA signal to a value representing the first timing adjustment step size.

6. The D2D communication method of claim 5 , wherein the first timing adjustment step size is one of 16 multiplied by a sampling time and ½ of a normal CP length, when the CP for the D2D communication is the normal CP, and

the first timing adjustment step size is one of ½ of an extended CP length and ¾ of an extended CP length, when the CP for the D2D communication is the extended CP having a longer length than that of the normal CP.

7. The D2D communication method of claim 1 , further comprising transmitting a cell-specific reference signal for demodulating the SA signal to the second terminal,

wherein a timing adjustment step size of a cell to which the first terminal belongs is transmitted to the second terminal by the base station.

8. The D2D communication method of claim 1 , wherein the transmitting of the SA signal comprises including a terminal identifier of the first terminal that belongs to the first cell in the SA signal,

wherein terminal identifiers of terminals that belong to the first cell and a timing adjustment step size of a first cell are transmitted to the second terminal by the base station.

9. The D2D communication method of claim 1 , wherein the bit number representing the second TA value is smaller than the bit number representing the first TA value.

10. The D2D communication method of claim 1 , further comprising receiving allocation of a resource for transmitting the SA signal and a resource for transmitting data of the D2D communication from the base station.

11. The D2D communication method of claim 1 , wherein the including of a second TA value comprises:

calculating a first timing adjustment value that is used for obtaining timing for transmitting a Physical Uplink Shared Channel (PUSCH) based on the first TA value; and

obtaining the second TA value using Equation 1:

TA2=floor( N _TA/TAS)  [Equation 1]

(floor( ): a rounding down function, N_TA: the first timing adjustment value, TAS: a timing adjustment step size used by the first terminal and having a unit of a sampling time, TA 2 : the second TA value).

12. A Device to Device (D2D) communication method in which a first terminal directly communicates with a second terminal, the D2D communication method comprising:

selecting a first resource for transmitting a Scheduling Assignment (SA) signal and a second resource for transmitting data of the D2D communication among resources;

setting, when first timing for transmitting data of the D2D communication is set to downlink (DL) timing for receiving a downlink signal of a base station in cellular communication, a first Timing Advance (TA) value that is included in the SA signal to 0; and

transmitting the SA signal to the second terminal using the first resource.

13. The D2D communication method of claim 12 , further comprising setting the first timing to the DL timing, when a Radio Resource Control (RRC) state of the first terminal is an RRC idle state as well as when a RRC state of the first terminal is an RRC connected state.

14. The D2D communication method of claim 12 , further comprising:

setting, when an RRC state of the first terminal is an RRC connected state, the first timing to timing that is obtained by applying a first timing adjustment value to the DL timing; and

setting, when an RRC state of the first terminal is an RRC idle state, the first timing to the DL timing.

15. The D2D communication method of claim 14 , wherein the setting of the first timing to timing that is obtained by applying the first timing adjustment value to the DL timing comprises:

calculating, when a Cyclic Prefix (CP) length for the D2D communication and a CP length for the cellular communication are the same, the first timing adjustment value for obtaining Physical Uplink Shared Channel (PUSCH) transmitting timing based on a second TA value that is received from the base station; and

calculating the first TA value based on the first timing adjustment value.

16. The D2D communication method of claim 14 , wherein the setting of the first timing to timing that is obtained by applying the first timing adjustment value to the DL timing comprises:

calculating, when a CP length for the D2D communication and a CP length for the cellular communication are different, a second timing adjustment value for obtaining PUSCH transmitting timing based on a second TA value that is received from the base station;

calculating the first TA value based on the second timing adjustment value; and

calculating the first timing adjustment value based on the first TA value.

17. The D2D communication method of claim 12 , wherein the transmitting of the SA signal comprises including a first timing adjustment step size that is used by the first terminal in the SA signal.

18. The D2D communication method of claim 12 , further comprising including a first timing adjustment step size that is used by the first terminal in a synchronization channel of the D2D communication.

19. A Device to Device (D2D) communication method in which a first terminal directly communicates with a second terminal, the D2D communication method comprising:

receiving a Scheduling Assignment (SA) signal comprising a first Timing Advance (TA) value from the second terminal at first timing;

acquiring a first timing adjustment step size from at least one of a first signal that is received through System Information Block (SIB) or Radio Resource Control (RRC) signaling of a base station, the SA signal, and a D2D communication synchronization channel that is received from the second terminal;

calculating second timing using the first timing, the first TA value, and the first timing adjustment step size; and

receiving data of the D2D communication at the second timing.

20. The D2D communication method of claim 19 , wherein the acquiring of a first timing adjustment step size comprises:

determining, when a Cyclic Prefix (CP) for the D2D communication is a normal CP, a value corresponding to a value of 1 bit that is included in the SA signal among 16 multiplied by a sampling time and ½ of the normal CP length to the first timing adjustment step size having a unit of seconds; and

determining, when a CP for the D2D communication is an extended CP having a longer length than that of the normal CP, a value corresponding to a value of 1 bit that is included in the SA signal among ½ of the extended CP length and ¾ of the extended CP length to the first timing adjustment step size having a unit of seconds.