Figure 9.12 shows the basic concept of an SCM system. The input to the transmitter consists of a mixture of N independent analog and digital baseband signals. These signals can carry either voice, data, video, digital audio, high-definition video, or any other analog or digital information. Each incoming signal S_i (t) is mixed with a local oscillator (LO) having a frequencyf_i. The local oscillator frequencies employed are in the 2-to-8-GHz range and are known as the subcarriers. Combining the modulated subcarriers gives a composite frequency-division-multiplexed signal which is used to drive a laser diode.