RECEIVERNOISEANDBITERRORRATEdoc.docVIP

RECEIVER NOISE AND BIT ERROR RATIO The optical receiver converts incident optical power into electrical current through the photo-diode. The relation between the optical power and the photo-current is linear, i.e. Where is the responsivity of the photo-detector, is the photo-current and is the incident optical power. The interaction of photon with the matter in a photo-detector however is a statistical process. That means the linear relationship is valid in an average sense. If we go to micro level there are fluctuations in the photo-current around the mean. These fluctuations are called the noise in the receiver. There are fundamental mechanisms of noise: Shot noise: This is due to statistical fluctuations in the optical signal itself and the statistical interaction process. Thermal noise: This is due to the thermal motion of the electrons inside the electronic circuits and the amplifiers following the photo-detector. The presence of noise degrades the signal quality. For the analog system the signal quality is measured by the parameter, signal-to-noise ratio (SNR), and for a digital system the signal quality is measured by the parameter, bit error ratio (BER). The analysis of noise therefore very important in a communication link. Here first we investigate the properties of the two noises mentioned above and then calculate the BER for the digital signals. SHOT NOISE The shot noise is because of the random fluctuation in the photo-carrier generation. The shot noise was first investigated by Schottky in 1918. In the presence of the noise the photo current can be written as Where is the instantaneous current of the photo detector, and is current fluctuation due to the shot noise. The noise current is stationary random process with Poisson statistics. The spectral density of the shot noise is practically constant over a very wide band and hence the noise is almost white. The variance of the shot noise is Where, q is the electro