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# 4.9 Summary

Orthogonal frequency division multiplexing (OFDM), which divides a communication channel into narrow subchannels (each with its own subcarrier) provides a spectrally efficient and flexible way to use the channel. OFDM has become an almost ubiquitous part of modulation, both in wired and wireless communication.

The use of QAM modulation for the subcarriers of OFDM is also ubiquitous. Using various orders of QAM (depending on noise conditions) further adds to the flexibility of OFDM.

In DSL broadband, a version of OFDM called Digital Multitone (DMT) is used both because of its spectral efficiency and its adaptability to unpredictable and variable noise conditions.

In 4G, Orthogonal Frequency Division Multiple Access (OFDMA) is used as a flexible way to share access to a radio channel among multiple, transient users. This is done by grouping subchannels into units of resource which are allocated to users dynamically as fluctuating demand requires.

In WiFi, OFDM is exploited as a way of making very efficient use of the available channels. The subchannels are not used for access (as in 4G), but selective nulling of subchannels is used to ensure that transmission power is properly restricted. Multiple access in WiFi is based on a ‘listen before transmitting’ protocol known as Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA).

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