on in the bit pattern. The overall consequence is a greater probability of the 1s and Os crossing to the "wrong" side of the threshold current at the instant of decision, as illustrated in Figure 6.13. In common with the noise phenomena described in an earlier section on optical system noise, we can compensate for the lower Q values due to chromatic dispersion by increasing the power of the pulses incident on the detector; when expressed in dB, the increase is the dispersion power penalty. The maximum chromatic dispersion power penalty for accept- able system performance depends on the datarate, the nonlinear optical effects, the modula- tion format, whether forward error correction is used, and various other factors. However, common values are 1 to 2 dB. If we can assume the use of NRZ transmission, high extinc- tion at the transmitter, and negligible transmitted pulse chirp, the power penalty can be Chapter 6 Optical Fiber Transmission expressed as a function of E, the fraction of a bit period in which pulse spreading occurs. E is 0.306 and 0.491 for power penalties of 1 dB and 2 dB, respectively (Ramaswami & Sivarajan, 2002). A simple relationship between transmission span, L, bitrate, B, and signal bandwidth at the transmitter, AA, follows from equation 6.26 and allows us to estimate the operating limits. If we interpret AT as the inverse of the bit period, 1/B, we have |D| L B A2 E. The frequency bandwidth, Av (in Hz), is normally higher than the bitrate (Gallion, 2002), and we can include a factor, b, which depends on the modulation format being used SO that Av = bB. Substitution for AA from equation 6.1 yields B² 22 Ec/b (6.30) where a is the central wavelength of the pulses. The modulus of D results from the fact that pulse broadening occurs for both positive and negative dispersion coefficients. Equation 6.30 shows that the maximum propagation length varies inversely with the square of the bitrate, and there are two contributions to this dependence. First, by Fourier transform considerations, the pulse b