ission Basics where GT and GR are the transmitter and receiver antenna gains, respectively, in the direction from the transmitter to the receiver, and a is the wavelength of the signal. For long-distance optical fiber links, the received signal can become very small because of the attenuation (see Chapter 6). Intermediate repeaters or amplifiers can be used to over- come the attenuation losses in the channel. Boosters can also be used to preamplify the signal before the receiver. However, special care must be taken at the receiver not to overload the decision circuitry. Delay Distortion All signals suffer from delay after being transmitted through a channel. A perfect channel will provide at the output an identical delayed version of the input signal. However, distor- tion occurs because of the different speeds of the signal frequencies, causing signal spread- ing. Specifically for digital data, this spreading will cause adjacent bits to overlap in time. To overcome this distortion effect, an equalization stage can be implemented just after the channel output. Figure 2.10 illustrates these stages in the receiver block Delay distortion can be considered a type of noise because of the deformation effect it produces on the signal transmitted. Different kinds of noise and their effect on the decision process will be explained in the next subsection. Noise and Interference Data transmission from transmitter to receiver is subject to much impairment that causes errors in the received data. These undesirable impairments can occur at the transmitter, in the channel, or at the receiver itself. They can be broadly classified into interference and noise. Two main forms of interference are adjacent-channel interference and cochannel inter- ference. In adjacent-channel interference, signals in adjacent frequency bands have compo- nents outside their allocated ranges, and these components can interfere with a wanted signal. This can be avoided by introducing guard bands between the allocated frequency ranges. A guard band is a