noise capabilities of the channel. This section provides more details on NRZ and RZ transmission and an overview of the most commonly used alternative techniques. NRZ and RZ Transmission: Power Spectral Densities We start by assuming for simplicity that the laser at the transmitter has an infinitesimal linewidth and that the pulses are generated by an ideal external modulator that can provide perfect rectangles in the time domain. In RZ operation we define the duty cycle, E, as being the ratio of the pulse width to the bitslot and in the special case of NRZ operation, is = 1. Both NRZ and RZ signaling then provide power spectra that can be represented in frequency space as having continuous and discrete components (Conradi, 2002; Gallion, 2002). In each case, the continuous components are "sinc-squared" functions. In NRZ the discrete compo- nent is a solitary frequency tone at the carrier frequency, for while in RZ there is a tone at fc plus an infinite number both above and below for which is weighted according to a sinc- squared distribution. The normalized spectra are illustrated in Figure 6.29, which follows Gallion (2002) in showing the continuous and discrete components separately. In both NRZ and RZ signaling the first Os of the continuous components of the spectra are at fc + (B/), where B is the bitrate of the channel. The bandwidth is commonly defined as being the distance between the first two Os and SO it is (2B/) in general and 2B in the case of NRZ modulation. Given that is is less than 1 in RZ operation, the channels have broader bandwidths. As stated in the section on single-channel transmission at the beginning of the chapter, the pulses in both NRZ and RZ modulation tend to differ from perfect rectangles. There are two main reasons for this. First, it is normal to apply filtering to remove the spectral com- ponents outside the range fc + (B/), and when this is done the resulting pulses have nonzero rise and fall times. Second, the practical modulators used in optical communications systems can h