function X = Okumura_Hata(frq_car, hgt_rx, hgt_tx, dit, zone)
%----------------Okumura-Hata 模型-------------

%--------------參數說明-----------
%   frq_car                載波頻率，    單位：MHz，范圍[150, 1500]
%   hgt_rx                 接收天線高度，單位：米，范圍[1, 10]
%   hgt_tx                 發射天線高度，單位：米，范圍[30, 200]
%   dit                    發射端與接收端的距離，單位：千米，范圍[1, 20]
%   zone                   接收端所處的環境，范圍[1, 2, 3, 4, 5]
%        = 1               表示處于中小城市
%        = 2               表示處于大城市，frq_carrier <= 300MHz
%        = 3               表示處于大城市，frq_carrier >= 300MHz
%        = 4               表示處于郊區傳播環境中
%        = 5               表示處于開闊鄉村區域
%  Also see: Okumura-Hata.

%  Designed by 李金倫, SWJTU, 2005.5.21 

%-------------初始參數-------------
% frq_car = 1600;
% hgt_rx  = 1.5;
% hgt_tx  = 30;
% dit     = 1.5;
% zone    = 5;

if  nargout>1,
    error('Too many output arguments.');
end
if nargin <5,
   error('Wrong number of input arguments.');
end

Lf = sqrt(8.7)*randn(1);
%Lf = 0;

L = 69.55 + 2.16*log10(frq_car) - 13.82*log10(hgt_tx) + (44.90 - 6.55*log10(hgt_tx))*log10(dit) + Lf;

switch zone
    case 1
        alfa = (1.10*log10(frq_car) - 0.70)*hgt_rx - (1.56*log10(frq_car) - 0.80);
        Ls   = L - alfa;
    case 2
        alfa = 8.29*(log10(1.54*hgt_rx))^2 - 1.10;
        Ls   = L - alfa;
    case 3
        alfa = 3.20*(log10(11.75*hgt_rx))^2 - 4.97;
        Ls   = L - alfa;
    case 4
        Ls   = L - 2*(log10(frq_car/28))^2 - 5.40;
    case 5
        Ls   = L - 4.78*(log10(frq_car))^2 - 18.33*log10(frq_car) - 40.98;
end

%----輸出----- 
if nargout == 1,
        X = Ls;
else
   disp(Ls);
end