Editing Orthogonal frequency-division multiplexing (section)

=== Linear transmitter power amplifier ===
An OFDM signal exhibits a high [[Crest factor|peak-to-average power ratio (PAPR)]] because the independent phases of the subcarriers mean that they will often combine constructively. Handling this high PAPR requires:
* A high-resolution [[digital-to-analog converter]] (DAC) in the transmitter
* A high-resolution [[analog-to-digital converter]] (ADC) in the receiver
* A linear [[signal chain]]

Any non-linearity in the signal chain will cause [[intermodulation distortion]] that
* Raises the noise floor
* May cause inter-carrier interference
* Generates out-of-band spurious radiation

The linearity requirement is demanding, especially for transmitter RF output circuitry where amplifiers are often designed to be non-linear in order to minimise power consumption. In practical OFDM systems a small amount of peak clipping is allowed to limit the PAPR in a judicious trade-off against the above consequences. However, the transmitter output filter which is required to reduce out-of-band spurs to legal levels has the effect of restoring peak levels that were clipped, so clipping is not an effective way to reduce PAPR.

Although the spectral efficiency of OFDM is attractive for both terrestrial and space communications, the high PAPR requirements have so far limited OFDM applications to terrestrial systems.

The crest factor CF (in dB) for an OFDM system with ''n'' uncorrelated subcarriers is<ref name="Kaehs">{{cite web|url=https://karriere.rohde-schwarz.de/career-cdn-pull/rs-common/fileadmin/customer/downloads/PDF/Huellkurve_eng.pdf|title=The Crest Factor in DVB-T (OFDM) Transmitter Systems and its Influence on the Dimensioning of Power Components|author=Bernhard Kaehs|date=January 2007|publisher=Rohde & Schwarz|archive-url=https://web.archive.org/web/20140705103705/http://cdn.rohde-schwarz.com/dl_downloads/dl_application/application_notes/7ts02/7TS02_2E.pdf|archive-date=2014-07-05|url-status=dead}}</ref>

:<math> CF = 10 \log_{10} ( n ) + CF_c </math>

where CF<sub>c</sub> is the crest factor (in dB) for each subcarrier.
(CF<sub>c</sub> is 3.01&nbsp;dB for the sine waves used for BPSK and QPSK modulation).

For example, the DVB-T signal in 2K mode is composed of 1705 subcarriers that are each QPSK-modulated, giving a crest factor of 35.32&nbsp;dB.<ref name="Kaehs" />

Many PAPR (or [[crest factor]]) reduction techniques have been developed, for instance, based on iterative clipping.<ref>{{cite journal |last1=Wang |first1=Y.-C. |last2=Luo |first2=Z.-Q. |title=Optimized Iterative Clipping and Filtering for PAPR Reduction of OFDM Signals |journal=IEEE Transactions on Communications |date=January 2011 |volume=59 |issue=1 |pages=33–37 |doi=10.1109/TCOMM.2010.102910.090040|s2cid=2487860 }}</ref>
Over the years, numerous model-driven approaches have been proposed to reduce the PAPR in communication systems. In recent years, there has been a growing interest in exploring data-driven models for PAPR reduction as part of ongoing research in end-to-end communication networks. These data-driven models offer innovative solutions and new avenues of exploration to address the challenges posed by high PAPR effectively. By leveraging data-driven techniques, researchers aim to enhance the performance and efficiency of communication networks by optimizing power utilization.
<ref>{{cite conference |last1=Huleihel |first1=Yara |last2=Ben-Dror|first2=Eilam |last3=Permuter |first3=Haim H. |title=Low PAPR Waveform Design for OFDM Systems Based on Convolutional Autoencoder |conference=2020 IEEE International Conference on Advanced Networks and Telecommunications Systems (ANTS) |date=2020 |pages=1–6}}</ref>


The [[dynamic range]] required for an FM receiver is {{nowrap|120 dB}} while DAB only require about {{nowrap|90 dB.}}<ref name=dabpr>{{cite book|title=Digital Audio Broadcasting: Principles and Applications of DAB, DAB + and DMB |url=https://books.google.com/books?id=T0KSa6w3qH4C&pg=PA333 |page=333 |year=2009 |last1=Hoeg |first1=Wolfgang |last2=Lauterbach |first2=Thomas |publisher=John Wiley & Sons |edition=3rd |access-date=2013-07-04|isbn=9780470746196 }}</ref> As a comparison, each extra bit per sample increases the dynamic range by {{nowrap|6 dB.}}