Method of adjusting coverage of small cell for efficient use of cell resources

1. A method of adjusting coverage of a small cell for efficient use of cell resources, the method comprising:

operation (S 100 ) of starting an operation of a small cell access point (AP);

operation (S 110 ) of finding, by the small cell AP, a number N_UE of pieces of user equipment (UE) in the small cell, and determining whether the number N_UE of pieces of UE in the small cell is equal to or smaller than a maximum N_ex of a number of pieces of UE for expanding the coverage;

operation (S 120 ) of expanding, by the small cell AP, the coverage and performing the method again beginning with operation (S 110 ) when it is determined in operation (S 110 ) that the number N_UE of pieces of UE in the small cell is equal to or smaller than N_ex;

operation (S 130 ) of determining, by the small cell AP, the number N_UE of pieces of UE in the small cell is within a range from the maximum N_ex of the number of pieces of UE for expanding the coverage to a minimum N_sh of a number of pieces of UE for shrinking the coverage when it is determined in operation (S 110 ) that the number N_UE of pieces of UE in the small cell is neither equal to nor smaller than N_ex, and performing the method again beginning with operation (S 110 ) when the number N_UE of pieces of UE in the small cell is not within the range from N_ex to N_sh;

operation (S 140 ) of estimating, by the small cell AP, current coverage when it is determined in operation (S 130 ) that the number N_UE of pieces of UE in the small cell is within the range from N_ex to N_sh;

operation (S 150 ) of estimating, by the small cell AP, positions of the pieces of UE in the small cell;

operation (S 160 ) of determining, by the small cell AP, whether a number N_edge of users satisfying edge user conditions is equal to or larger than a threshold Nedge_Thresh of a number of cell edge users for shrinking the coverage, and performing the method again beginning with operation (S 110 ) when N_edge is neither equal to nor larger than Nedge Thresh; and

operation (S 170 ) of reducing, by the small cell AP, the coverage when it is determined in operation (S 160 ) that N_edge is equal to or larger than Nedge_Thresh, and performing the method again beginning with operation (S 110 ).

2. The method of claim 1 , wherein operation S 140 of estimating, by the small cell AP, the current coverage comprises:

finding a relational expression among a received reference signal received power (RSRP), a reference signal (RS) transmission power, and a pathloss estimation function when the pathloss estimation function with respect to a distance d from a macrocell to the small cell is PLm(d)

P _RS. m −RSRP m ( d )=PL m ( d )  [Expression 1]

where P_RS.m is an RS transmission power of the macrocell, and

RSRPm(d) is an RSRP at a position apart from the macrocell by the distance d;

calculating the distance d between the macrocell and the small cell AP using an inverse function of the pathloss estimation function

d =PL m −1 ( P _RS. m −RSRP m ( d ));  [Expression 2]

applying a condition that an RSRP received from the macrocell be identical to an RSRP received from the small cell AP at an edge of the small cell coverage

RSRP m ( d−r )=RSRP f ( r ); and  [Expression 3]

calculating a radius r of the coverage using an expression below because an RSRP equals a value obtained by subtracting a pathloss from the RS transmission power

PRS.m −PL m ( d−r )= PRS.f −PL f ( r ).  [Expression 4]

3. The method of claim 1 , wherein operation S 150 of estimating, by the small cell AP, the positions of the pieces of UE in the small cell comprises calculating, by the small cell AP, an estimated distance di using a timing advance value TA.i for UE i according to an expression below,

di= 1/2(TA. i*c )  [Expression 5]

where c is a speed of light, 3.0×10 8 m/s.

4. The method of claim 1 , wherein operation S 150 of estimating, by the small cell AP, the positions of the pieces of UE in the small cell further comprises calculating, by the small cell AP, the estimated distance di between the UE i and the small cell, and then determining, by the small cell AP, the UE i as an edge user when a distance from an edge of the coverage to the UE i is shorter than Δr.

5. The method of claim 1 , wherein operation S 150 of estimating, by the small cell AP, the positions of the pieces of UE in the small cell comprises determining the UE i as an edge user when the distance from the edge of the coverage to the UE i is shorter than Δr based on conditions below

di >( r−Δr )

1/2(TA. i*c )>( r−Δr )

TA. i >2/ c ( r−Δr ).  [Expression 6]

6. The method of claim 1 , wherein operation S 170 of reducing, by the small cell AP, the coverage comprises:

decreasing, by the small cell AP, a transmission power by ΔP per a unit time interval t_interval to shrink the coverage;

calculating, by the small cell AP, a reference signal (RS) transmission power candidate value PRS.candidate within a maximum/minimum coverage range that is applicable using Expression 8 below

PRS .candidate=max(min( PRS .max_cov, PRS−ΔP ), PRS .min_cov);  [Expression 8]

calculating a final RS transmission power for shrinking the cell coverage using an expression below and applying the calculated final RS transmission power

PRS .set=max(min( PRS .max, PRS .candidate), PRS .min).  [Expression 10]

7. The method of claim 1 , wherein operation S 120 of expanding, by the small cell AP, the coverage comprises:

increasing, by the small cell AP, a transmission power by ΔP per a unit time interval t_interval to expand the coverage;

calculating, by the small cell AP, a reference signal (RS) transmission power candidate value PRS.candidate within a maximum/minimum coverage range that is applicable using Expression 12 below

PRS .candidate=max(min( PRS .max_cov, PRS+ΔP ), PRS .min_cov); and  [Expression 12]

calculating a final RS transmission power for expanding the cell coverage using an expression below and applying the calculated final RS transmission power

PRS .set=max(min( PRS .max, PRS .candidate), PRS .min).  [Expression 14]