Tuesday, April 2, 2013



OFDM spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own. OFDM is sometimes called multi-carrier or discrete multi-tone modulation.

The OFDM transmission scheme has the following key advantages:
·         Makes efficient use of the spectrum by allowing overlap
·         By dividing the channel into narrowband flat fading subchannels, OFDM is more
·         resistant to frequency selective fading than single carrier systems are.
·         Eliminates ISI and IFI through use of a cyclic prefix.
·         Using adequate channel coding and interleaving one can recover symbols lost due to
·         the frequency selectivity of the channel.
·         Channel equalization becomes simpler than by using adaptive equalization
·         techniques with single carrier systems.
·         It is possible to use maximum likelihood decoding with reasonable complexity
·         As discussed in OFDM is computationally efficient by using FFT techniques to
·         implement the modulation and demodulation functions.
·         In conjunction with differential modulation there is no need to implement a channel estimator.
·         Is less sensitive to sample timing offsets than single carrier systems are.
·         Provides good protection against co channel interference and impulsive parasitic noise.

In terms of drawbacks OFDM has the following characteristics:
·         The OFDM signal has a noise like amplitude with a very large dynamic range; therefore it requires RF power amplifiers with a high peak to average power ratio.
·         It is more sensitive to carrier frequency offset and drift than single carrier systems are due to leakage of the DFT.

It is used for wireless as well as wireless access.
Some of the Major Tecnologies using OFDM : 802.11a WLAN, WiMax, LTE

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