How to set noise threshold for spectrum sensing?

Hi, I have been doing my research work on spectrum sensing using matlab but the issue, I'm facing in setting noise threshold is how to set variance for detection. I'm using Gaussian distribution and considering awgn case and for that matter,I'm setting variance power 1. Kindly guide me in this.

    % code
function [noise_threshold]=settingnoisethreshold(samples);
    var=1; %variance
    P_fa=.01; %probability of false alarm
    Q=norminv(P_fa);
    gemma=var.^2*(Q*sqrt(2*samples)+samples);
    noise_threshold=gemma;
*Thanks*
*WN*