I have a background in traditional digital and RF wireless communications. Normally, when working with link budgets, I can determine the SNR at the receiver, and then calculate the Eb/No given the bit rate. Finally, based on the modulation, I can calculate the BER for the link. I have moved into optical communications, and although the basic theory is the same, the terminology is very different. In particular, I have read of using photons/bit to determine the BER for an optical system in place of Eb/No as used in RF communications. What I am having trouble doing is equating the two figures of merit (photons/bit and Eb/No). I have seen the same formula used to calculate BER using both figures of merit. For example, the BER for DPSK using non- coherent demodulation is 0.5*exp(-FOM) where FOM is photons/bit for optical communications and is Eb/No for RF communications. Can somebody help me with equating these two values and explaining how the same formula can be used to compute the BER? Thanks, Susheem
Equating Photons/Bit to Eb/No
Started by ●June 21, 2007
Reply by ●June 21, 20072007-06-21
susheemg@yahoo.com wrote:> I have moved into optical communications, and although the basic > theory is the same, the terminology is very different. In particular, > I have read of using photons/bit to determine the BER for an optical > system in place of Eb/No as used in RF communications.One of the main features of the optical communications is that the antipodal signaling can't be used. You can transmit light, but you can't transmit darkness. Thus the communication is basically a sort of on/off keying with the very different noise PDFs in '0' and '1' state. The trivial formulae from the RF textbook based on the AWGN and Euclidean distances are not suitable for this case. The unavoidable source of noise in any optical system is the shot noise, which is the variance of the number of photons per time. This noise is basically the square root of the number of photons per bit.> What I am > having trouble doing is equating the two figures of merit (photons/bit > and Eb/No).It is the incorrect comparison. What is bigger: 1m or 1kg?> Can somebody help me with equating these two values and explaining how > the same formula can be used to compute the BER?Get a book on the optical communication and/or data storage. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●June 22, 20072007-06-22
On Thu, 21 Jun 2007 16:58:57 -0500, Vladimir Vassilevsky wrote:> susheemg@yahoo.com wrote: > > >> I have moved into optical communications, and although the basic >> theory is the same, the terminology is very different. In particular, >> I have read of using photons/bit to determine the BER for an optical >> system in place of Eb/No as used in RF communications. > > One of the main features of the optical communications is that the > antipodal signaling can't be used. You can transmit light, but you can't > transmit darkness. Thus the communication is basically a sort of on/off > keying with the very different noise PDFs in '0' and '1' state. The > trivial formulae from the RF textbook based on the AWGN and Euclidean > distances are not suitable for this case. > > The unavoidable source of noise in any optical system is the shot noise, > which is the variance of the number of photons per time. This noise is > basically the square root of the number of photons per bit.You'll find it in the comms books under "Poisson process".> >> What I am >> having trouble doing is equating the two figures of merit (photons/bit >> and Eb/No). > > It is the incorrect comparison. What is bigger: 1m or 1kg? > >> Can somebody help me with equating these two values and explaining how >> the same formula can be used to compute the BER? > > Get a book on the optical communication and/or data storage. >Or just look up the Poisson process and do the math. It'll be more tedious, but you'll remember it longer. -- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
