Normally, when we dealing with power signal in frequency domain we usually use power spectral density. My question is why is that? Why we define the power P(f) this way. http://img21.imageshack.us/img21/6786/powerec.jpg [Broken] This makes no sense to me, Why don't we just Fourier transform p(t) therefore we also get P(f) http://img198.imageshack.us/img198/8603/power2a.jpg [Broken] I just don't get the concept of power spectral density. Please help!