I want to get BER approximately 10^-4 with the variation of L but how?

clc;
%clear all;
%close all;
L_0=10;  %Average outer scale of turbulance in m.
c_n=5*10^(-10);  %Structure constant of refractive index in m^-2/3.
c=3*10^8; % Speed of light in m/s.
f=20*10^9; % Frequency in HZ.
lamda=c/f; 
L=1000;%Equivalent Path length through the turbulence area
 del_ky=0.01;

[EbN0_dB]=[10 15 20 25 30 35 40 45 50 55];
 sigma=((0.307*(c_n).^2)*((2*3.1416/lamda)^1.17)*(L^(1.833)))-((0.742*(c_n)^2)*((2*23.1416/lamda)^0.17)*((L^2.833)*(L_0^2)));

for i= 1:10
    snr=10.^(EbN0_dB(i)/10);
    ky=0:del_ky:10;  
    pe=1/2.*erfc(ky.*sqrt(snr/2));
%     sigma=1;
%     sigma=sigma*10^(-13);
    ky=ky*10^(-13);
%     pdf_ky=((-ky.^2)/(2*sigma^2));
    pdf_ky=(1/(sqrt(2*3.1416)*sigma))*exp((-ky.^2)/(2*sigma^2));
    prod=pe.*pdf_ky;
    ber(i)=trapz(prod)*del_ky;
end
 CDF_ky=trapz(pdf_ky)*del_ky
 ber=ber./CDF_ky;

semilogy(EbN0_dB,ber)
% plot(pdf_ky)
  ylabel('Bit Error Rate(BER)')
  xlabel('Signal to Noise Ratio(SNR)');
 %variance_ky=sigma^2
% semilogy(variance_ky,L)