tailieunhanh - Optical Networks: A Practical Perspective - Part 30

Optical Networks: A Practical Perspective - Part 30. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 260 Modulation and Demodulation Using it can then be shown that the BER see Problem is given by BER q - . 7Q 71 The Q function can be numerically evaluated. Let y -1 BER . For a BER rate of 10-12 we need y 7. For a BER rate of KU9 y 6. Note that it is particularly important to have a variable threshold setting in receivers if they must operate in systems with signal-dependent noise such as optical amplifier noise. Many high-speed receivers do incorporate such a feature. However many of the simpler receivers do not have a variable threshold adjustment and set their threshold corresponding to the average received current level namely 7 Io 2. This threshold setting yields a higher bit error rate given by BER We can use to evaluate the BER when the received signal powers for a 0 bit and a 1 bit and the noise statistics are known. Often we are interested in the inverse problem namely determining what it takes to achieve a specified BER. This leads us to the notion of receiver sensitivity. The receiver sensitivity Psens is defined as the minimum average optical power necessary to achieve a specified BER usually 10 12 or better. Sometimes the receiver sensitivity is also expressed as the number of photons required per 1 bit M which is given by M 2 Psens hfcB where B is the bit rate. In the notation introduced earlier the receiver sensitivity is obtained by solving for the average power per bit Pq Pi 2 for the specified BER say 10-12. Assuming Pq 0 this can be obtained as Psens 2Gm7 Here Gm is the multiplicative gain for APD receivers and is unity for pin photodiodes. First consider an APD or a pin receiver with no optical amplifier in the system. The thermal noise current is independent of the received optical power. However the shot noise variance is a function of PSens- Assume that no power is transmitted for a 0 bit. Then u2 r2ermal and u2 u2hermal u2hot where the shot noise variance Demodulation 261 Figure Sensitivity plotted as a