Method and device for transmitting a synchronization signal in a wireless communication system

1. A method of transmitting a signal, which is transmitted by an access point in a wireless communication system, comprising:

generating a frame for synchronization of a timer of a station associated with the access point; and

transmitting the frame for the synchronization,

wherein the frame for the synchronization is transmitted before or after every target beacon transmission time,

wherein the frame for the synchronization comprises a timestamp field indicating a value of a timer of the access point and an offset field indicating an offset amounting to a difference between the target beacon transmission time and a transmission time of the frame for the synchronization, wherein the offset is determined as a maximum clock error of the timer of the station, wherein the maximum clock error is calculated from a time information of a listen interval field transmitted when the station is associated with the access point, and wherein the time information comprises an interval for the station to awake to receive a beacon frame in power save mode.

2. The method of claim 1 , wherein the frame for the synchronization is transmitted to all stations associated with the access point.

3. The method of claim 1 , further comprising:

receiving a request for the frame for the synchronization from the station; and

transmitting the frame for the synchronization in response to the request independently from the target beacon transmission time.

4. The method of claim 3 , further comprising:

comparing the maximum clock error, which is calculated from the time information included in the request, of the timer of the station to the offset; and

if the offset is smaller than the maximum clock error, resetting the offset to the maximum clock error.

5. The method of claim 4 , wherein the frame for the synchronization in response to the request further comprises a field indicating a transmission rate.

6. An apparatus for transmitting a signal in a wireless communication system, comprising:

an RF (radio frequency) unit; and

a processor generating a frame for synchronization of a timer of a station associated with the apparatus, the processor transmitting the frame for the synchronization,

wherein the frame for the synchronization is transmitted before or after every target beacon transmission time,

wherein the frame for the synchronization comprises a timestamp field indicating a value of a timer of the apparatus and an offset field indicating an offset amounting to a difference between the target beacon transmission time and a transmission time of the frame for the synchronization, wherein the offset is determined as a maximum clock error of the timer of the station, wherein the maximum clock error is calculated from a time information of a listen interval field transmitted when the station is associated with the apparatus, and wherein the time information comprises an interval for the station to awake to receive a beacon frame in power save mode.

7. A method of receiving a signal, which is received by a station in a wireless communication system, comprising receiving a frame for synchronization of a timer from an access point,

wherein the frame for the synchronization is transmitted before or after every target beacon transmission time,

wherein the frame for the synchronization comprises a timestamp field indicating a value of a timer of the access point and an offset field indicating an offset amounting to a difference between the target beacon transmission time and a transmission time of the frame for the synchronization, wherein the offset is determined as a maximum clock error of the timer of the station, wherein the maximum clock error is calculated from a time information of a listen interval field transmitted when the station is associated with the access point, and wherein the time information comprises an interval for the station to awake to receive a beacon frame in power save mode.

8. The method of claim 7 , wherein the frame for the synchronization is broadcasted to all stations associated with the access point.

9. The method of claim 7 , further comprising:

transmitting a request for the frame for the synchronization to the access point; and

receiving the frame for the synchronization in response to the request independently from the target beacon transmission time.

10. The method of claim 9 , wherein the time information to calculate the maximum clock error of the timer of the station is included in the request, wherein if the offset is smaller than the maximum clock error, the frame for the synchronization including the offset reset to the maximum clock error is received.

11. The method of claim 9 , wherein the frame for the synchronization in response to the request further comprises a field indicating a transmission rate.

12. An apparatus for receiving a signal in a wireless communication system, comprising:

an RF (radio frequency) unit; and

a processor synchronizing a timer by receiving a frame for synchronization of the timer of the apparatus,

wherein the frame for the synchronization is transmitted before or after every target beacon transmission time,

wherein the frame for the synchronization comprises a timestamp field indicating a value of a timer of the apparatus and an offset field indicating an offset amounting to a difference between the target beacon transmission time and a transmission time of the frame for the synchronization, wherein the offset is determined as a maximum clock error of the timer of the station, wherein the maximum clock error is calculated from a time information of a listen interval field transmitted when the station is associated with the apparatus, and wherein the time information comprises an interval for the station to awake to receive a beacon frame in power save mode.