hi there are few error in my code. kindly fix them i am a newbie.

clc;
close all;
clear all;
%% 
segma_N = 10.^-7;
p_db = 0:1:30; %power
p = 10.^(p_db./10);
R = 0.5;    %Optoelectronic conversion factor 

%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%I = 1; %linspace(0,100,length(p));

AP = @(I)((0.5)*erfc((p.*R.*I)./segma_N)*FI_I(I));
P_e = integral(AP,0,100);
semilogy(p,P_e);
%po = 0.5*erfc((p.*R.*I)/segma_N)*(1.3604e+162);
% P_e = integral(@(I) po(I),0,500);
% semilogy(p,P_e);
hold on
axis([0 30 10^-5 0.5])
grid on 

[Walter Roberson]

FI_I is not defined in your code.