21 Facts About OFDM

COFDM was introduced by Alard in 1986 for Digital Audio Broadcasting for Eureka Project 147.

Conceptually, OFDM is a specialized frequency-division multiplexing method, with the additional constraint that all subcarrier signals within a communication channel are orthogonal to one another.

Cyclic prefix, which is transmitted during the guard interval, consists of the end of the OFDM symbol copied into the guard interval, and the guard interval is transmitted followed by the OFDM symbol.

The reason that the guard interval consists of a copy of the end of the OFDM symbol is so that the receiver will integrate over an integer number of sinusoid cycles for each of the multipaths when it performs OFDM demodulation with the FFT.

Reason why interleaving is used on OFDM is to attempt to spread the errors out in the bit-stream that is presented to the error correction decoder, because when such decoders are presented with a high concentration of errors the decoder is unable to correct all the bit errors, and a burst of uncorrected errors occurs.

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Classical type of error correction coding used with OFDM-based systems is convolutional coding, often concatenated with Reed-Solomon coding.

However, OFDM can be combined with multiple access using time, frequency or coding separation of the users.

In OFDM-based 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.

Key features of some common OFDM-based systems are presented in the following table.

The fact that OFDM can cope with this frequency selective attenuation and with narrow-band interference are the main reasons it is frequently used in applications such as ADSL modems.

The United States has rejected several proposals to adopt the COFDM-based DVB-T system for its digital television services, and for many years has opted to use 8VSB exclusively for terrestrial digital television.

One of the major benefits provided by COFDM is in rendering radio broadcasts relatively immune to multipath distortion and signal fading due to atmospheric conditions or passing aircraft.

Also, COFDM allows single-frequency networks, which is not possible with 8VSB.

COFDM is used for other radio standards, for Digital Audio Broadcasting, the standard for digital audio broadcasting at VHF frequencies, for Digital Radio Mondiale, the standard for digital broadcasting at shortwave and medium wave frequencies and for DRM+ a more recently introduced standard for digital audio broadcasting at VHF frequencies.

Some forms of COFDM already offer this kind of hierarchical modulation, though BST-OFDM is intended to make it more flexible.

Fast low-latency access with seamless handoff orthogonal frequency-division multiplexing, referred to as F-OFDM, was based on OFDM and specified higher protocol layers.

Flash-OFDM was marketed as a packet-switched cellular bearer, to compete with GSM and 3G networks.

The Flash-OFDM network was switched off in the majority of Slovakia on 30 September 2015.

In VOFDM, assume is the vector size, and each scalar-valued signal in OFDM is replaced by a vector-valued signal of vector size,.

An example of W-OFDM uses the Perfect Reconstruction Cosine Modulated Filter Bank and Extended Lapped Transform is used for the wavelet TF.

W-OFDM is an evolution of the standard OFDM, with certain advantages.