Orthogonal frequency-division multiplexing (OFDM) is a very flexible and efficient modulation technique that is at the heart of all major wireless and wired standards used or in development today. It was first described by R.W. Chang in 1966 at Bell Labs but has been widely used for only the last 15 years. Examples of standards that use OFDM are LTE, LTE-Advanced, WiMAX, Digital Audio and Video Broadcast, WLAN, and ADSL.
OFDM is a special form of the frequency-division multiplexing (FDM) multicarrier modulation technique. In multicarrier systems, the information to be transmitted is split into multiple smaller chunks and transmitted independently. A common analogy is the shipment of goods using multiple small boxes compared to using only one huge box. The smaller boxes are easier to manipulate, and if some boxes are lost or damaged in transit, the receiver still gets the remaining boxes.
Why OFDM Is Efficient?
OFDM, like FDM, separates the channel bandwidth into multiple narrow-band subcarriers to carry the information. To prevent adjacent carrier interference (ACI), traditional FDM systems require small gaps or guard bands between the carriers where no information can be transmitted. This results in a waste of spectrum. To solve this problem, OFDM uses special subcarriers that are all orthogonal to each other. This not only permits the removal of the guard bands, but since the subcarriers are completely unrelated, they can even overlap each other. This is why OFDM is so bandwidth efficient.
The use of narrow-band sub-channels compared to a single wideband channel makes the system very resistant to channel fading, which greatly reduces the complexity of the receiver equalizer that is required. A typical OFDM receiver uses only a single-tap equalizer per subcarrier.
Cyclic Prefix (CP) To Prevent Inter-Symbol Interference (ISI)
To prevent inter-symbol interference (ISI) caused by the propagation channel, OFDM systems insert a cyclic prefix (CP) before each symbol to be transmitted. To preserve orthogonality, the end of the current symbol is transmitted before each symbol. As long as the length of the CP is at least equal to the length of the multipath channel, all copies of the current symbol are received before the start of the useful part of the next symbol, thus preventing ISI.
To generate an OFDM signal, the data to be transmitted is first channel coded and modulated. Typical modulations include QPSK, 16-QAM and 64-QAM. This serial stream of symbols is then split into N parallel streams, where N corresponds to the number of sub-channels used in the system. An N-point inverse fast Fourier transform (IFFT) is then used to create the OFDM symbols (the use of the FFT/IFFT makes the implementation very computationally efficient). The CP is then inserted at the beginning of the symbol, and the resulting signal is transmitted.
At the receiver, all the operations are performed in the reverse order, except that a FFT is used instead of an IFFT. Some additional processing blocks are required to recover the carrier frequency and the symbol timing synchronization.
The Benefits Of OFDM
The OFDM is a very efficient modulation technique that can achieve very high throughput by transmitting on a great number of carriers simultaneously. It is also very spectrally efficient because of the proximity of the subcarriers. While being relatively simple and efficient to implement, to design a robust system that prevents ISI caused by multipath propagation and ICI caused by Doppler shift and timing errors, a good knowledge of the propagation conditions is required to make the right selection of the OFDM parameters. OFDM can also be used with MIMO to further improve the throughput for a given bandwidth.