lows them to be spaced very closely. For a good graphic representation of OFDM, see Telephony Online's article "Building Future Networks with MIMO and OFDM" at http://telephonyonline.com/wireless/technology/mimo_ofdm_091905/ Adaptive modulation-An adaptive modulation system supports a variety of modulation schemes. The quality of the radio path determines which modulation technique is most appropriate. Techniques supported include 16-QAM, 64-QAM, QPSK, and DPSK. OFDM overcomes most of the problems with both FDMA and TDMA, includ- ing nonefficient use of spectrum and the requirement for clocking or synchroniza- tion with TDMA. OFDM splits the available bandwidth into many narrowband channels (typically 100 to 8,000), referred to as subcarriers. OFDM offers improved resistance to noise and interference at particular fre- quencies as well as immunity to frequency-selective fading. Information can be reconstructed without receiving the entire transmission band. OFDM is almost always used in conjunction with channel coding or an error correction technique. One approach is referred to as Coded Orthogonal Frequency Division Multiplexing (COFDM). COFDM is widely used in Europe and other places where the DAB stan- dard has been adopted for digital radio broadcasting and for terrestrial digital TV, as in the DVB-T standard. COFDM is also used in ADSL transmission. The major ben- efit of COFDM is that it makes broadcasts relatively immune to multipath distortion and signal fading due to atmospheric conditions or passing aircraft. It is a complex technique, but by combining OFDM with error-correcting codes, adaptive equaliza- tion, and reconfigurable modulation, COFDM provides many beneficial properties, including resistance to multipath, phase distortion, fading, and burst noise. OFDM also offers improved bandwidth efficiency because it can support more users on the same transmission channel by using different spreading codes. OFDM Chapter 13 Wireless Communications Basics makes it easy to filter out noise: If a particular 