Further Reading 279 modulation is needed for transmission at high bit rates over long distances. Some form of line coding or scrambling is needed to prevent long runs of 1s or 0s in the transmitted data stream to allow the clock to be recovered easily at the receiver and to maintain DC balance. Subcarrier multiplexing is a technique in which many signals are electronically multiplexed using FDM, and the combined signal is used to modulate an optical car- rier. Multilevel modulation schemes are more spectrally efficient than on-off keying; optical duobinary signaling is an example of such a scheme. A simple direct detection receiver looks at the energy received during a bit inter- val to decide whether it is a 1 or 0 bit. The receiver sensitivity is the average power required at the receiver to achieve a certain bit error rate. The sensitivity of a simple direct detection receiver is determined primarily by the thermal noise in the receiver. The sensitivity can be improved by using APDs instead of pin photodetectors or by using an optical preamplifier. Another technique to improve the sensitivity as well as the channel selectivity of the receiver is coherent detection. However, coherent detec- tion is susceptible to a large number of impairments, and it requires a significantly more complicated receiver structure to overcome these impairments. For this reason, it is not practically implemented today. Clock recovery is an important part of any receiver and is usually based on a