How to Calculate Power Spectral density of random signal.

Hi I have a dataset of EEG signal which is Excel format which has the time and voltage values of the signals. I have added white noise function to the dataset and plotted it. I need to calculate the Power spectral density of the noisy signal.

dataset=xlsread('input signal.xlsx','sheet1','A1:B770');
t = dataset(:,1);
V = dataset(:,2);
subplot(3,1,1);
plot(t,V);
title('original signal');
X = [t V+1*randn(length(V),1)];
xlswrite('noisyEEG.xls',X);
subplot(3,1,2);
plot(X(:,1), X(:,2));
xlabel ('time');
ylabel('voltage');
title('noisy Signal');