Orthogonal frequency-division multiplexing (OFDM) is a digital communication technique initially developed for use in cable television systems. OFDM is similar to the broadcasting technique known as frequency division multiplexing (also known as FDM), which uses a multitude of transmitters and receivers to send information on different frequencies over a single wire, such as an electrical power cable. 

The first use of OFDM was by Bell Labs in 1984, and it has since become widely used in wireless applications such as mobile telephony and broadband communications. In wireless communications, OFDM has become an alternative to single-carrier modulation techniques such as frequency division multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA). It is used in applications including digital video broadcasting, digital audio broadcasting, digital cable television, and orthogonal frequency division multiplexing line (OFDML) as well as ADSL.

 

In digital wireless systems, M-ary OFDM, or 2 M-ary OFDM (M=2) is used to carry the data streams of multiple users. In an M-ary system, the spectrum is divided into a set of Sub-bands such that each Sub-bands is orthogonal. For example, in an 8 MHz bandwidth system, the spectrum may be split into eight 1 MHz Sub-bands.

These eight sub-bands are typically assigned to individual channels (digital ‘sub-channels’), but they can also be used to encode data streams from several users (analog ‘sub-carriers’).

In communication systems, OFDM has become an alternative to single-carrier modulation techniques such as frequency division multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA).

Uses of OFDM:

• OFDM is used in Digital radio, Digital Radio Mondiale, digital audio broadcasting, and satellite radio.
• OFDM is used in Wired data transmission, Asymmetric Digital Subscriber Line (ADSL), Institute of Electrical and Electronics Engineers (IEEE) 1901 powerline networking, and cable internet providers.

M-ary Amplitude shifting:

Multilevel (M-ary) amplitude shift coding (ASK) transmits a symbol representing N=log 2 M bits of information. A signal by Mary Ask Keep the carrier’s frequency in one of the discreet levels during the time of the symbol T_s for the representation of n = log 2 m logical track signals for the transmission of information.

Encoding M-ary OFDM is difficult. If the system must transmit a single stream, it can use SSB modulation, but this requires precise synchronization of all Sub-carriers.

It cannot use simple equalizers because each subcarrier has its own frequency offset. As a result, the rejection can occur due to echo off adjacent Sub-carriers that have different frequencies. It is thus important to mitigate the effect of inter-carrier interference by synchronous detection and correction.