Limitations Of Modulation In Isi

980 WordsAug 14, 20174 Pages
Due to the spectacular growth of the wireless services and demands during the last years, the need of a modulation technique that could transmit high data rates at high bandwidth efficiency strongly imposed. The problem of the inter–symbol interference (ISI) introduced by the frequency selectivity of the channel became even more imperative once the desired transmission rates dramatically grew up. Using adaptive equalization techniques at the receiver in order to combat the ISI effects could be the solution, but there are practical difficulties in operating this equalization in real-time conditions at several Mb/s with compact, low-cost hardware. ORTHOGONAL FREQUANCY DIVISION MULTIPLEXING is a promising candidate that eliminates the need…show more content…
Slow data rate (and long symbol duration) on each subchannel merely means that the effects of ISI are severely reduced. This is in fact the basic idea that lies behind ORTHOGONAL FREQUANCY DIVISION MULTIPLEXING . Transmitting the data among a large number of closely spaced subcarriers accounts for the “frequency division multiplexing” part of the name. Unlike the classical frequency division multiplexing technique, ORTHOGONAL FREQUANCY DIVISION MULTIPLEXING will provide much higher bandwidth efficiency. This is due to the fact that in ORTHOGONAL FREQUANCY DIVISION MULTIPLEXING the spectra of individual subcarriers are allowed to overlap. In fact, the carriers are carefully chosen to be orthogonal one another. As it is well known, the orthogonal signals do not interfere, and they can be separated at the receiver by correlation techniques. The orthogonality of the subcarriers accounts for the first part of the ORTHOGONAL FREQUANCY DIVISION MULTIPLEXING name. In a mobile radio environment, the signal is carried by a large number of paths with different strength and delays. Such multipath dispersion of the signal is commonly referred as “channel-induced ISI” and yields the same kind of ISI distortion caused by an electronic filter [2]. In fact, the multipath dispersion leads to an upper limitation of the transmission rate in order to avoid the frequency selectivity of the channel or the need of a complex adaptive equalization in the receiver. In
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