Orthogonal frequency-division multiplexing

In telecommunications, orthogonal frequency-division multiplexing (OFDM) is a type of digital transmission used in digital modulation for encoding digital (binary) data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, used in applications such as digital television and audio broadcasting, DSL internet access, wireless networks, power line networks, and 4G/5G mobile communications.^[1]

OFDM is a frequency-division multiplexing (FDM) scheme that was introduced by Robert W. Chang of Bell Labs in 1966.^[2]^[3]^[4] In OFDM, the incoming bitstream representing the data to be sent is divided into multiple streams. Multiple closely spaced orthogonal subcarrier signals with overlapping spectra are transmitted, with each carrier modulated with bits from the incoming stream so multiple bits are being transmitted in parallel.^[5] Demodulation is based on fast Fourier transform algorithms. OFDM was improved by Weinstein and Ebert in 1971 with the introduction of a guard interval, providing better orthogonality in transmission channels affected by multipath propagation.^[6] Each subcarrier (signal) is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying) at a low symbol rate. This maintains total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.^[7]

Consecutive raised-cosine impulses, demonstrating zero-ISI property; these closely resemble OFDM power spectrum (frequency domain).

The main advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath) without the need for complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to eliminate intersymbol interference (ISI) and use echoes and time-spreading (in analog television visible as ghosting and blurring, respectively) to achieve a diversity gain, i.e. a signal-to-noise ratio improvement. This mechanism also facilitates the design of single frequency networks (SFNs) where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be re-combined constructively, sparing interference of a traditional single-carrier system.

In coded orthogonal frequency-division multiplexing (COFDM), forward error correction (convolutional coding) and time/frequency interleaving are applied to the signal being transmitted. This is done to overcome errors in mobile communication channels affected by multipath propagation and Doppler effects. COFDM was introduced by Alard in 1986^[8]^[9]^[10] for Digital Audio Broadcasting for Eureka Project 147. In practice, OFDM has become used in combination with such coding and interleaving, so that the terms COFDM and OFDM co-apply to common applications.^[11]^[12]

^ Mustafa Ergen (2009). Mobile Broadband: including WiMAX and LTE. Springer Science+Business Media. doi:10.1007/978-0-387-68192-4. ISBN 978-0-387-68189-4.
^ Weinstein, S. B. (November 2009). "The history of orthogonal frequency-division multiplexing". IEEE Communications Magazine. 47 (11). IEEE Communications Magazine ( Volume: 47, Issue: 11, November 2009 ): 26–35. doi:10.1109/MCOM.2009.5307460. S2CID 29001312.
^ Cite error: The named reference chang was invoked but never defined (see the help page).
^ Cite error: The named reference patent was invoked but never defined (see the help page).
^ webe.org - 2 GHz BAS Relocation Tech-Fair, COFDM Technology Basics. 2007-03-02
^ Cite error: The named reference weinstein was invoked but never defined (see the help page).
^ Ahmad R. S. Bahai, Burton R. Saltzberg, Mustafa Ergen, Multi-carrier digital communications - Theory and applications of OFDM. Springer (November 2004 )
^ WO 8800417, Pommier, Daniel & Alard, Michel, "Method and installation for digital communication, particularly between and toward moving vehicle", published 1988-01-14, assigned to Centre national d'études des télécommunications and Telediffusion de France
^ "Principles of modulation and channel coding for digital broadcasting for mobile receivers" (PDF). EBU Technical Review n°224, p.187. August 1987.
^ Le Floch, B.; Alard, M.; Berrou, C. (1995). "Coded orthogonal frequency division multiplex [TV broadcasting]". Proceedings of the IEEE. 83 (6): 982–996. doi:10.1109/5.387096. Archived from the original on 2014-07-03.
^ Akansu, Ali; et al. (1998). "Orthogonal transmultiplexers in communication: a review" (PDF). IEEE Transactions on Signal Processing. 46 (4). IEEE Trans. On Signal Processing, Vol. 46, No. 4, April 1998: 979–995. Bibcode:1998ITSP...46..979D. CiteSeerX 10.1.1.46.3342. doi:10.1109/78.668551.
^ Yang, James Ching-Nung (October 10, 2001). "What is OFDM and COFDM?". Shoufeng, Hualien, Taiwan: Department of Computer Science and Information Engineering National Dong Hwa University. Retrieved 2017-04-16.

[ergen-1] Mustafa Ergen (2009). Mobile Broadband: including WiMAX and LTE. Springer Science+Business Media. doi:10.1007/978-0-387-68192-4. ISBN 978-0-387-68189-4.

[2] Weinstein, S. B. (November 2009). "The history of orthogonal frequency-division multiplexing". IEEE Communications Magazine. 47 (11). IEEE Communications Magazine ( Volume: 47, Issue: 11, November 2009 ): 26–35. doi:10.1109/MCOM.2009.5307460. S2CID 29001312.

[chang-3] Cite error: The named reference chang was invoked but never defined (see the help page).

[patent-4] Cite error: The named reference patent was invoked but never defined (see the help page).

[cobas-5] webe.org - 2 GHz BAS Relocation Tech-Fair, COFDM Technology Basics. 2007-03-02

[weinstein-6] Cite error: The named reference weinstein was invoked but never defined (see the help page).

[7] Ahmad R. S. Bahai, Burton R. Saltzberg, Mustafa Ergen, Multi-carrier digital communications - Theory and applications of OFDM. Springer (November 2004 )

[brevet-8] WO 8800417, Pommier, Daniel & Alard, Michel, "Method and installation for digital communication, particularly between and toward moving vehicle", published 1988-01-14, assigned to Centre national d'études des télécommunications and Telediffusion de France

[EBU-9] "Principles of modulation and channel coding for digital broadcasting for mobile receivers" (PDF). EBU Technical Review n°224, p.187. August 1987.

[10] Le Floch, B.; Alard, M.; Berrou, C. (1995). "Coded orthogonal frequency division multiplex [TV broadcasting]". Proceedings of the IEEE. 83 (6): 982–996. doi:10.1109/5.387096. Archived from the original on 2014-07-03.

[11] Akansu, Ali; et al. (1998). "Orthogonal transmultiplexers in communication: a review" (PDF). IEEE Transactions on Signal Processing. 46 (4). IEEE Trans. On Signal Processing, Vol. 46, No. 4, April 1998: 979–995. Bibcode:1998ITSP...46..979D. CiteSeerX 10.1.1.46.3342. doi:10.1109/78.668551.

[12] Yang, James Ching-Nung (October 10, 2001). "What is OFDM and COFDM?". Shoufeng, Hualien, Taiwan: Department of Computer Science and Information Engineering National Dong Hwa University. Retrieved 2017-04-16.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]