Orthogonal frequency-division multiplexing

Orthogonal frequency-division multiplexing
Name	Orthogonal frequency-division multiplexing
Caption	An illustration of the overlapping, orthogonal subcarriers in the frequency domain.
Invented	1960s
Inventors	Robert W. Chang, Bell Labs
Related	Frequency-division multiplexing, Discrete Fourier transform

Orthogonal frequency-division multiplexing. It is a method of digital signal modulation that divides a high-data-rate stream into numerous slower, parallel sub-channels. This technique is fundamental to modern broadband communication systems, providing robustness against frequency-selective fading and intersymbol interference. Its development was pioneered by researchers like Robert W. Chang at Bell Labs, and it now forms the core of major wireless and wired standards worldwide.

Principles of operation

The core principle involves splitting a single data stream across a large set of closely spaced, orthogonal subcarriers. Unlike conventional frequency-division multiplexing, these subcarriers overlap spectrally without interfering, maximizing spectral efficiency. A key component is the guard interval, typically implemented as a cyclic prefix, which is inserted between symbols to mitigate multipath propagation effects. This structure allows the receiver, often using a simple equalizer, to recover the transmitted data reliably even in challenging radio frequency environments.

Mathematical foundations

The mathematical basis for the generation and reception of signals is the inverse discrete Fourier transform and the discrete Fourier transform, respectively. This relationship allows for highly efficient implementation using the fast Fourier transform algorithm, developed by James W. Cooley and John Tukey. The orthogonality condition requires that the subcarrier spacing is precisely the reciprocal of the symbol period, a concept formalized in work by Leonard J. Cimini. This precise spacing ensures the integral of the product of any two different subcarriers over a symbol period is zero.

Advantages and disadvantages

A primary advantage is exceptional resilience to multipath interference and frequency-selective fading, common in urban environments and digital television broadcasting. It also offers high spectral efficiency due to overlapping subcarriers and simplifies channel equalization. However, a significant disadvantage is a high peak-to-average power ratio, which demands linear power amplifiers in transmitters, reducing efficiency. The system is also sensitive to carrier frequency offset and Doppler shift, which can disrupt orthogonality, a challenge for high-mobility applications.

Applications

This modulation scheme is the foundation for a vast array of contemporary wireless and wired standards. In wireless, it is used in IEEE 802.11 networks, 4G systems like Long-Term Evolution, and 5G NR. For broadcasting, it is employed in Digital Audio Broadcasting, Digital Video Broadcasting standards such as DVB-T, and the ATSC 3.0 standard. Wired applications include asymmetric digital subscriber line and power-line communication systems. It is also integral to the physical layer of WiMAX, defined by the IEEE 802.16 family.

Implementation and standards

Practical implementation is dominated by efficient digital signal processor and field-programmable gate array designs that execute the necessary FFT operations. Major standardization bodies have incorporated it into their specifications, including the 3rd Generation Partnership Project for LTE Advanced, the International Telecommunication Union for IMT-Advanced, and the European Telecommunications Standards Institute for DVB-T2. Key implementation challenges, such as synchronization and phase noise mitigation, are addressed in the design of integrated circuits for consumer devices like smartphones and routers.

Historical development

Early concepts were outlined in the 1960s, with a key patent filed by Robert W. Chang of Bell Labs in 1966. Practical application was limited until the 1980s, when advances in very-large-scale integration made the required FFT processors feasible. Its adoption in civilian systems began with Digital Audio Broadcasting in Europe and the IEEE 802.11a standard in the late 1990s. The work of individuals like John A. C. Bingham at Stanford University and organizations like the COST 207 project helped refine the technique for commercial use in mobile telephony and broadcasting.

Category:Telecommunication theory Category:Modulation techniques Category:Signal processing