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Unformatted text preview: 248 Modulation and Demodulation In practice, the NRZ format is used in most high-speed communication systems, ranging from speeds of 155 Mb/s to 10 Gb/s. Scrambling is widespread and used in most communication equipment ranging from PC modems to high-speed telecom- munications links. High-speed computer data links (for example, Fibre Channel, which operates at 800 Mb/s, and Gigabit Ethernet, which operates at 1 Gb/s) use line codes. See Chapter 6 for a discussion of these protocols. The RZ format is used in certain high-bit-rate communication systems, such as chirped RZ or DM soliton systems (see Section 2.6.1). In these systems, the pulse occupies about half the bit interval, though this is usually not precise as in digital/electronic communication. The use of RZ pulses also minimizes the effects of chromatic dispersion (see Section 5.7.2). RZ modulation with pulses substan- tially shorter than the bit interval is used in soliton communication systems (see Sec- tion 2.6). The pulses need to be very short in such systems because they must be widely separated (by about five times their width) in order to realize the dispersion-free propagation properties of solitons. 4.2 Subcarrier Modulation and Multiplexing The optical signal emitted by a laser operating in the 1310 or 1550 nm wavelength band has a center frequency around 10 14 Hz. This frequency is the optical carrier frequency. In what we have studied so far, the data modulates this optical carrier. In other words, with an OOK signal, the optical carrier is simply turned on or off, depending on the bit to be transmitted....
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This note was uploaded on 01/15/2011 for the course ECE 6543 taught by Professor Boussert during the Spring '09 term at Georgia Tech.
- Spring '09