Method of expanding transmission range and base station apparatus using the same

1. A method of expanding, by a small cell Base Station (BS), a transmission range in a heterogeneous network, the method comprising:

estimating a number of success devices which successfully access the small cell BS through a contention based on an idle Random Access Opportunity (RAO);

determining a transmission range expansion distance by comparing the estimated number of success devices with an actual number of measured success devices; and

calculating a power increment necessary to expand a transmission range by incorporating the transmission range expansion distance based on a signal intensity value to which path loss values of the small cell BS and a macro BS have been applied.

2. The method of claim 1 , wherein estimating of the number of success devices which successfully access the small cell BS comprises:

calculating a ratio of idle RAOs in a contention frame of a previous random access process,

estimating a number of devices that attempt access using the calculated idle RAO ratio inversely, and

estimating the number of success devices based on the estimated number of devices which attempt access.

3. The method of claim 2 , wherein the number of success devices in a specific i-th contention frame is calculated by a multiplication of a success probability value according to

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where L pico,i is a contention frame size value, {circumflex over (N)} PUE,i is the estimated number of devices which attempt access to the small cell BS in the i-th contention frame, and {circumflex over (P)} succ,i is a probability that devices of {circumflex over (N)} PUE,i are able to be successfully accessed when the devices perform a contention in a contention frame having a size of L pico,i .

4. The method of claim 3 , further comprising calculating a number of RAOs which maximizes the number of success devices when a random contention is performed in order to allocate resources corresponding to the number of devices which attempt access, wherein the number of RAOs is a size of a contention frame.

5. The method of claim 4 , wherein a number of RAOs which maximizes the number of success devices when the random contention is performed is calculated based on an optimum contention frame size calculated by differentiating a number of RAOs which maximizes a success probability of

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6. The method of claim 5 , wherein the number of RAOs which maximizes the number of success devices when the random contention is performed is calculated by taking into consideration the calculated optimum contention frame size and a maximum contention frame size which is able to be allocated.

7. The method of claim 2 , wherein the determining of the transmission range expansion distance comprises:

determining whether a difference between the estimated number of success devices and the number of measured success devices exceeds a threshold,

calculating a current transmission range, in response to a determination that the difference between the estimated number of success devices and the number of measured success devices exceeds the threshold, and

calculating the transmission range expansion distance based on the current transmission range.

8. The method of claim 7 , wherein the transmission range expansion distance is calculated using a ratio of a maximum number of devices which are able to be accommodated by the macro BS and a transmission radius of the macro BS.

9. The method of claim 1 , further comprising:

determining a transmission power increment in which a signal intensity value of the small cell BS and a signal intensity value of the macro BS become equal, and

calculating transmission power in a next contention frame by taking into consideration a maximum value and minimum value of the transmission power based on the determined transmission power increment.

10. The method of claim 1 , wherein a RAO value is calculated by multiplying a number of Physical Random Access CHannels (PRACH) by a number of preambles.

11. A small cell Base Station (BS) apparatus in a heterogeneous network, comprising:

a processor configured to:

estimate a number of success devices which successfully access the small cell BS through a contention based on an idle Random Access Opportunity (RAO); and

determine a transmission range expansion distance by comparing the estimated number of success devices with an actual number of measured success devices,

wherein a transmission range of the small cell BS is expanded based on the determined transmission range expansion distance.

12. The small cell BS apparatus of claim 11 , wherein the processor is further configured to:

calculate a ratio of idle RAOs in a contention frame of a previous random access process,

estimate a number of devices that attempt access using the calculated idle RAO ratio inversely, and

estimate the number of success devices based on the estimated number of devices which attempt access.

13. The small cell BS apparatus of claim 12 , wherein the estimated number of success devices in a specific i-th contention frame is calculated by a multiplication of a success probability value calculated according to

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where L pico,i is a contention frame size value, {circumflex over (N)} PUE,i is the number of devices which attempt access to the small cell BS in the i-th contention frame, and {circumflex over (P)} succ,i is a probability that devices of {circumflex over (N)} PUE,i are able to be successfully accessed if the devices perform a contention in a contention frame having a size of L pico,i .

14. The small cell BS apparatus of claim 13 , wherein the processor is further configured to calculate a number of RAOs which maximizes the number of success devices when a random contention is performed in order to allocate resources corresponding to the number of devices which attempt access.

15. The small cell BS apparatus of claim 12 , wherein the processor is further configured to:

determine whether a difference between the estimated number of success devices and the number of measured success devices exceeds a threshold,

calculate a current transmission range, in response to a determination that the difference between the estimated number of success devices and the number of measured success devices exceeds the threshold, and

calculate the transmission range expansion distance based on the current transmission range.

16. The small cell BS apparatus of claim 15 , wherein the transmission range expansion distance is calculated using a ratio of a maximum number of devices which are able to be accommodated by a macro BS and a transmission radius of the macro BS.

17. The small cell BS apparatus of claim 11 , wherein the processor is further configured to calculate a power increment necessary to expand the transmission range by incorporating the transmission range expansion distance based on a signal intensity value to which path loss values of the small cell BS and a macro BS have been applied.

18. The small cell BS apparatus of claim 17 , wherein the processor is further configured to determine a transmission power increment in which a signal intensity value of the small cell BS and a signal intensity value of the macro BS become equal, and calculate transmission power in a next contention frame by taking into consideration a maximum value and minimum value of the transmission power based on the determined transmission power increment.

19. A heterogeneous network apparatus, comprising:

a macro Base Station (BS) configured to receive random access requests from devices among a plurality of devices; and

a small cell BS configured to estimate a number of success devices which access the small cell BS based on an idle Random Access Opportunity (RAO), determine a transmission range expansion distance by comparing the estimated number of success devices with an actual number of measured success devices in order to process random access requests from the devices while coexisting with the macro BS, and expand a transmission range of the small cell BS based on the determined transmission range expansion distance.