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An OFDM baseband signal is the sum of a number of orthogonal sub-carriers, each sub-carrier being independently modulated (for instance using some type of QAM or PSK) by its own data. This composite baseband signal is typically used to modulate a main RF carrier.
The benefits of using OFDM are that it is easy to filter out noise (if a particular range of frequencies suffers from interference, the carriers within that range can be disabled or made to run slower), and that the upstream and downstream speeds can be varied by allocating either more or fewer carriers for each purpose. Some forms of Rate Adaptive DSL use this feature in real time, so that bandwidth is allocated to whichever stream needs it most.
OFDM modulation and demodulation are typically (as of 2001) implemented using digital filter banks generally using the Fast Fourier Transform (FFT).
When OFDM is used in conjunction with channel coding techniques, it is described as coded orthogonal frequency division modulation (COFDM). As the overhead of doing this in an already digital system is low, and the gains substantial, practical OFDM/DMT systems are all actually COFDM.
Although highly complex, COFDM has high performance under even very challenging channel conditions.
By combining the OFDM technique with error-correcting codes, adaptive equalization and reconfigurable modulation, COFDM has the following properties:
COFDM also generally has a nearly 'white' spectrum, giving it benign electromagnetic interference properties with respect to other signals.
Some COFDM systems use some of the sub carriers to carry pilot signals, which are used for frequency stabilisation, as frequency shifts during the transmission using the main modulation/demodulation process transform into bit errors in the decoded data.
In wide area broadcasting, receivers can benefit from receiving signals from several spatially dispersed transmitters simultaneously, since transmitters will only destructively interfere with each other on a limited number of subcarriers, whereas in general they will actually reinforce coverage over a wide area. This is very beneficial in many countries, as it permits the operation of national single frequency networks, and avoids the replication of program content on different carrier frequencies which is necessary with FM or other forms of radio broadcasting. Also, because effectively the bit rate is slowed down on each sub-carrier, the effects of "ghosting" are much reduced. Such single frequency networks utilise the available spectrum more effectively than existing analogue radio networks.
However, OFDM suffers from time-variations in the channel, or presence of a carrier frequency offset. Moreover, due to the mathematical FFT operation applied at the transmitter, the signal also tends to have a high peak-to-average ratio. All mentioned effects are emphasised when several users are sending data to the same base stationBase stations are low-power multi-channel two-way radios which are in a fixed location. They are typically used by low-power single-channel, two-way radios such as Mobile phone and portable phones. When you talk on such a mobile phone, you (and perhaps do.