er of fact, these complex electronic equalization methods rely on high speed electronics, frequently on specific ASICS, SO that the cost of OFDM is no more SO high with respect to this kind of advanced IM-DD systems. Since orthogonality among nearby channels is assured by the carrier, any modulation format can be used in principle on the subcarriers of an OFDM signal. Since OFDM is also used to increase the efficiency in the use of the available bandwidth, it is often used in conjunction with multilevel modulation on the subcarriers. A frequently used modulation format is M-QAM OFDM. 9.4.2 Polarization Division Multiplexing Considering multilevel modulation, we have seen that several multilevel formats exploit polarization to convey information. This not only allows a high efficiency to be reached by the multilevel constellations, but also requires a certain degree of complexity at the transmitter and at the receiver. Polarization division multiplexing (PDM) is a way to exploit polarization with a smaller complexity: it consists in transmitting two channels at the same frequency but on orthogonal polarizations. The transmitter does not need, at least in principle, a polarization modulator and the receiver is simpler than an M-PolSK or an M-4QAM receiver. Intuitively, the greater weakness of PDM is the need of tracking the incoming polar- ization: nonlinear depolarization, polarization differential losses, and similar phenomena affect the channel recovery at the receiver in the same measure they affect M-PolSK or M-4QAM systems. 9.4.3 Channel and Pulse Polarization Diversity If there is no need of using polarization to convey information, polarization diversity can be used to decrease interference between nearby channels in the WDM comb. If this technique is used, nearby channels are transmitted on orthogonal polarizations SO that, as far as polarization evolution during fiber propagation can be considered linear, they do not interfere at the receiver. As a matter of fact, since linear polarization evolution durin