Orthogonal frequency-division multiplexing (OFDM) is a digital multi-carrier transmission technique that uses a large number of closely spaced orthogonal sub-carriers. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth. The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example, narrowband interference and frequency-selective fading) without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slow narrowband signals rather than one fast broadband signal.
OFDM has been adopted for many digital telecommunication standards, such as digital audio broadcasting (DAB), digital video broadcasting (DVB) for terrestrial television, and wireless networks like WiFi (IEEE 802.11a/g/n/ac), WiMAX (Wireless Interoperability for Microwave Access) and Long Term Evolution (LTE) networks. OFDM has also been proposed for digital radio Mondiale (DRM) and digital audio radio services (DARS). Currently it is being evaluated by the IEEE for use in standards such as wireless regional area networks (Wireless RAN) and wireless personal area networks.
For communication over broadcast channels that use a single transmitter and multiple receivers, OFDM provides an advantage related to multipath propagation. The relative amount of delay spread severe enough to cause intersymbol interference (ISI) is typically small in relation to the OFDM symbol period, providing resilience to multipath for the receivers. The orthogonality between the closely spaced OFDM subcarriers further reduces the sensitivity to multipath delay spread. Common approaches for broadcast channelcoding are to use a single error correction code across all data sub-streams or to use independent coding for each subcarrier data stream. Time-interleaving is also used to protect against short fade bursts.
History:
The basic concepts of OFDM were developed by Chang in 1966 and published in the Bell System Technical Journal in 1968. It was notuntil the early 1970s that OFDM was implemented in digital communications, when Weinstein and Ebert realized that OFDM could be implemented efficiently using an IDFT/DFT (FFT) algorithm.
Weinstein and Ebert patented the basic ideas of OFDM in 1971 for military communications. In the late 1970s and early 1980s, OFDM was further investigated for applications such as digital audio broadcasting (DAB) and digital television (DTV).
In 1988, a standard for digital audio broadcasting to mobile devices using COFDM (coded OFDM) was published by the European Telecommunications Standards Institute (ETSI). This standard was important in bringing OFDM from theory into practical applications.
In the 1990s, OFDM became an active area of research and development driven by requirements for higher data rates for broadband wireless. Major advances occurred in adapting OFDM to different channel conditions, developing efficient implementation methods, and improving channel coding and modulation schemes.
OFDM was adopted for digital audio/video broadcasting systems and also became the basis for wireless LAN standards including IEEE 802.11a/g. It was selected for the European wireless IP networking standard HIPERLAN/2 and also the wireless MAN standard IEEE 802.16-2004 (WiMAX).
More recently, OFDM has been adopted for cellular networks including 3G standards like EV-DO and 3GPP Long Term Evolution (LTE) for 4G networks. Research continues on OFDM-based 5G systems for improved performance in terms of data rates, coverage and service capabilities.
Key Concepts:
Multi-carrier modulation: OFDM functions by dividing the available spectrum into many narrowband sub-channels or subcarriers. Each subcarrier is modulated with a conventional modulation scheme at a low symbol rate.
Orthogonality: The key to maximum bandwidth efficiency with OFDM is maintaining orthogonality between the subcarriers so that overlapping spectra can be packed closely without inter-channel interference. This is achieved by using sinusoids with frequencies that are integer multiples of a fundamental frequency.
IFFT/FFT implementation: The inverse fast Fourier transform (IFFT) is used at the transmitter to generate the OFDM signal from the sub-carrier signals. At the receiver, the fast Fourier transform (FFT) extracts the sub-carrier signals. This makes the implementation computationally efficient using fast Fourier transform algorithms.
Cyclic prefix: A guard interval or cyclic prefix is added as a copy of the end of the OFDM symbol to prevent inter-symbol interference (ISI). The cyclic prefix must be longer than the delay spread of the channel.
sub-carrier modulation: Each sub-carrier can employ various digital modulation schemes such as BPSK, QPSK, 16-QAM, 64-QAM etc. depending on the required data rate and robustness. Higher order schemes provide higher data rates.
Channel coding: Forward error correction coding is used across OFDM sub-blocks or entire blocks to provide protection against transmission errors. Techniques like convolutional coding, turbo coding and low density parity check (LDPC) coding are used.
Adaptive loading: Sub-carriers can be selectively activated or deactivated based on channel conditions. Data rates can be adapted by changing the modulation levels or error correction for each subcarrier for optimum performance.
OFDM is an efficient multi-carrier technique utilized in many modern high-speed data communication systems due to its ability to cope with severe channel conditions and offer flexibility. It remains an active area of research for next generation networks as higher data rates and more users are supported.
Andrew Stroud