OFDM Basics

Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation technique that has recently found wide adoption in a widespread variety of high-data-rate communication systems, including digital subscriber lines, wireless LANs (802.11a/g/n), digital video broadcasting, and now WiMAX and other emerging wireless broadband systems.

The basic idea of Multicareer modulation is quite simple. The ISI would be zero only if symbol time of the siganl is larger than the channel delay. But the modern wireless networks with broadband links providing several mega-bits per second (e.g. 802.11a promises 54 Mbps), which has very less symbol time than channel dealy causes ISI. But it may not be practical to implement an equalizer at all because of overwhelming complexity caused by the high speed link.

On the other hand, if we could somehow reduce the symbol rate so that ISI becomes negligible, while still maintaining the required information bit rate, equalization becomes unnecesssary.

One way to do this is simply to increase the level of modulation in an M-ary pulse modulation scheme but there is a limit on how large M can be. Because as M increases, the spacing between each sample decreases and so it would be difficult at the doecoder side to decoded the weak signal.

The other way to increase the symbol interval is through parallel transmission over many orthogonal channels. This will widen the symbol time larger than channel delay and ISI can be avoided. These individual substreams can then be sent over parallel subchannels, maintaining the total desired data rate, so the subchannels experience relatively flat fading. Thus, the ISI on each subchannel is small. Moreover, in the digital implementation of OFDM, the ISI can be completely eliminated through the use of a cyclic prefix. Such orthogonal carriers can be easily generated using IFFT operation.

In the above figure Tb is useful OFDM simbol time and Tg is cyclic prefix, which is nothing but replica of last few bits in the symbol