espect to NRZ, SO reducing the impact of chromatic dispersion and PMD. The duobinary method was first applied to electrical digital signals. It combines two successive binary pulses in the digital stream to form a multilevel electrical signal. To easily express the duobinary encoding, let us indicate the binary bit stream with bh,k where h is the progressive number that indicates the position of the bit in the bit stream, k is zero if the bit is a zero, otherwise it is a progressive number that indicates how many ones are passed in the bit stream. Just to give an example in the bit stream 1001101, the bits are coded in this way: b1,1 = 1, b2,0 = 0, = 1, Transmission Systems Architectures and Performances Duobinary FIGURE 6.42 Example of duobinary coding. Using this notation the duobinary signal d where h is the progressive number of the duobinary symbols, can be expressed as follows: dh=(-1)hb (6.103) that is, if the bit is a zero, the code transmits a zero, if it is a one, the code transmits alter- natively +1 and -1. An example of duobinary coding is reported in Figure 6.42. Naturally in intensity modu- lation, duobinary is realized adding a bias to the modulated signal SO that three different levels of power (one of them generally equal to zero) are used. The spectrum of duobinary signal is slightly shrunk with respect to the spectrum of ordinary NRZ and much more compact than the spectrum of RZ, especially when small duty cycles are used to tolerate more pulse spreading without generating ISI. Moreover, at the transmitter duobinary modulation can be performed by using a dual drive MZ modulator in conjunction with electrical low pass filters posed on the modula- tor's driving ports and the receiver is simply a standard IM-DD receiver, with the thresh- old in between the zero and the first of the two levels corresponding to a one. The advantage in terms of dispersion tolerance is not SO strong, and the application of these modulations' format is, at present, not as widespread as it was forecasted a few years 6.