7.11.9 What Is Orthogonal Frequency-Division Multiplexing (OFDM)?
- What Is Orthogonal Frequency-Division Multiplexing?
- Why Is It Called Orthogonal Frequency-Division Multiplexing?
- What Does Orthogonal Mean?
- How Does OFDM Work?
- Why Use Many Slow Subcarriers Instead of One Fast Carrier?
- What Is Multipath Propagation?
- What Is Inter-Symbol Interference?
- What Is the Cyclic Prefix?
- Why Can OFDM Subcarriers Overlap?
- How Is OFDM Generated?
- What Modulation Is Used on Each Subcarrier?
- What Is Adaptive Modulation?
- Where Is OFDM Used?
- What Are the Advantages of OFDM?
- Does OFDM Have Any Disadvantages?
- Is OFDM Used Together with Other Technologies?
- Why Is OFDM Important?
Description
Discover how Orthogonal Frequency-Division Multiplexing divides high-speed data into many low-speed orthogonal subcarriers. Learn why OFDM is highly resistant to multipath propagation and why it forms the basis of Wi-Fi, LTE, 5G, digital television, and many broadband communication systems.
Introduction
As wireless communication systems evolved during the late twentieth century, engineers faced a growing challenge. Users demanded ever-higher data rates, but higher transmission speeds made radio systems increasingly vulnerable to one of the most troublesome propagation effects: multipath propagation.
In cities, radio signals rarely travel directly from transmitter to receiver. Instead, they reflect from buildings, vehicles, hills, and other obstacles. As a result, several delayed copies of the same signal often arrive at the receiver simultaneously. At low data rates these delays may have little effect, but as transmission speeds increase they cause neighbouring symbols to overlap, producing inter-symbol interference (ISI) and reducing communication reliability.
One solution might be to design increasingly sophisticated equalizers capable of correcting these distortions. Engineers instead developed an elegant alternative. Rather than transmitting one very fast stream of data, they divided the information into many much slower streams. Each stream was transmitted on its own closely spaced subcarrier, making each symbol much longer in duration and therefore far less susceptible to multipath delay.
This technique became known as Orthogonal Frequency-Division Multiplexing (OFDM). Today it forms the foundation of many of the world's most important communication systems, including Wi-Fi, LTE, 5G, digital television, digital radio, broadband fixed wireless systems, and numerous satellite communication standards.
What Is Orthogonal Frequency-Division Multiplexing?
Orthogonal Frequency-Division Multiplexing (OFDM) is a multiplexing and modulation technique that divides a high-speed digital data stream into many lower-speed streams.
Each stream is transmitted simultaneously using its own closely spaced subcarrier. Because the subcarriers are orthogonal, they can overlap in frequency without causing mutual interference.
This allows OFDM to achieve excellent spectral efficiency while remaining highly resistant to multipath propagation.
Why Is It Called Orthogonal Frequency-Division Multiplexing?
The name reflects its three defining characteristics.
Frequency-Division indicates that the available channel bandwidth is divided into many individual subcarriers. Multiplexing refers to the simultaneous transmission of many independent data streams. Orthogonal describes the mathematical relationship between the subcarriers.
Although neighbouring subcarriers overlap spectrally, their orthogonality allows the receiver to separate them perfectly under ideal conditions.
What Does Orthogonal Mean?
Two signals are orthogonal if they do not interfere with one another, even though they occupy the same time interval.
Mathematically, the product of two orthogonal waveforms averages to zero over the symbol period. This property allows the receiver to distinguish one subcarrier from another even when their spectra overlap.
Orthogonality is the key innovation that distinguishes OFDM from conventional Frequency-Division Multiplexing.
How Does OFDM Work?
The incoming data stream is divided into many parallel streams.
Each stream modulates a separate subcarrier using a digital modulation scheme such as:
- BPSK;
- QPSK;
- 16-QAM;
- 64-QAM; or
- 256-QAM.
All of the modulated subcarriers are transmitted simultaneously.
At the receiver, the subcarriers are separated and recombined to reconstruct the original high-speed data stream.
Why Use Many Slow Subcarriers Instead of One Fast Carrier?
The duration of each transmitted symbol is inversely proportional to the symbol rate.
By dividing the information among many subcarriers, each subcarrier operates at a much lower symbol rate.
Consequently:
- symbol durations become much longer;
- delayed echoes represent a much smaller fraction of each symbol period;
- inter-symbol interference is greatly reduced; and
- receiver equalization becomes much simpler.
This is one of the principal reasons OFDM performs so well in multipath environments.
What Is Multipath Propagation?
Multipath propagation occurs when several copies of the same transmitted signal reach the receiver by different paths.
These paths may involve reflections from:
- buildings;
- mountains;
- vehicles;
- the ground; or
- other structures.
Because the paths differ in length, the signals arrive at slightly different times.
These delayed copies may interfere with one another, particularly at high data rates.
What Is Inter-Symbol Interference?
Inter-symbol interference (ISI) occurs when delayed energy from one symbol overlaps the following symbol.
The receiver then finds it more difficult to determine where one symbol ends and the next begins. ISI becomes increasingly severe as data rates increase.
Because OFDM lengthens the symbol duration, it substantially reduces the impact of ISI.
What Is the Cyclic Prefix?
One of OFDM's most important features is the cyclic prefix.
Before transmission, a short portion of each OFDM symbol is copied and inserted at its beginning. This guard interval absorbs most delayed reflections caused by multipath propagation.
Provided the reflected signals arrive within the cyclic-prefix interval, they do not cause inter-symbol interference.
The cyclic prefix therefore contributes greatly to OFDM's excellent performance.
Why Can OFDM Subcarriers Overlap?
In conventional Frequency-Division Multiplexing, guard bands are required between adjacent channels.
OFDM eliminates these guard bands by ensuring the subcarriers remain mathematically orthogonal. At the centre frequency of every subcarrier, all neighbouring subcarriers have zero amplitude. Consequently:
- spectral overlap occurs;
- interference does not occur;
- bandwidth is used more efficiently; and
- spectral efficiency increases significantly.
This elegant property is one of OFDM's greatest strengths.
How Is OFDM Generated?
Although OFDM appears mathematically complex, modern digital hardware generates it efficiently using the Inverse Fast Fourier Transform (IFFT). The receiver performs the complementary operation using the Fast Fourier Transform (FFT). These algorithms simultaneously process hundreds or thousands of subcarriers with remarkable efficiency.
Without FFT technology, practical OFDM systems would be far more difficult to implement.
What Modulation Is Used on Each Subcarrier?
Each subcarrier may employ its own digital modulation scheme.
Common choices include:
- BPSK;
- QPSK;
- 16-QAM;
- 64-QAM; and
- 256-QAM.
Modern systems often adjust the modulation on individual subcarriers according to local channel conditions, thereby maximizing overall throughput.
What Is Adaptive Modulation?
Not every subcarrier experiences identical propagation conditions.
Some frequencies may suffer greater fading or interference than others. Many OFDM systems therefore employ adaptive modulation. Subcarriers experiencing favourable conditions use higher-order modulation. Subcarriers experiencing poorer conditions automatically switch to more robust modulation.
This improves both reliability and spectral efficiency.
Where Is OFDM Used?
OFDM has become one of the most widely used multiplexing techniques in modern communications.
Applications include:
- Wi-Fi;
- LTE;
- 5G;
- digital television;
- digital audio broadcasting;
- fixed wireless broadband; and
- many satellite communication systems.
Its ability to provide high data rates in difficult propagation environments explains its widespread adoption.
What Are the Advantages of OFDM?
OFDM offers numerous important advantages.
These include:
- excellent resistance to multipath propagation;
- high spectral efficiency;
- efficient implementation using FFT algorithms;
- compatibility with adaptive modulation; and
- support for very high data rates.
These characteristics have made OFDM particularly well suited to broadband wireless communications.
Does OFDM Have Any Disadvantages?
Like every engineering solution, OFDM also involves trade-offs.
Possible disadvantages include:
- relatively high peak-to-average power ratio (PAPR);
- sensitivity to frequency errors;
- sensitivity to phase noise;
- synchronization requirements; and
- increased digital signal-processing complexity.
Modern communication systems incorporate sophisticated techniques to minimise these limitations.
Is OFDM Used Together with Other Technologies?
Very often.
Modern broadband systems commonly combine OFDM with:
- MIMO antenna systems;
- adaptive modulation;
- forward error correction;
- beamforming; and
- dynamic resource allocation.
Together, these technologies provide the exceptional capacity and reliability expected from modern wireless communication systems.
Why Is OFDM Important?
Orthogonal Frequency-Division Multiplexing represents one of the most significant advances in modern digital communications. By dividing a high-speed transmission into many slower orthogonal subcarriers, OFDM overcomes one of the greatest obstacles to broadband wireless communication: multipath propagation.
Its combination of high spectral efficiency, robustness, and implementation practicality has made OFDM the foundation of many of today's most important communication systems. Whether using Wi-Fi at home, watching digital television, or connecting to a 5G network, users are likely benefiting from OFDM technology.
Summary
Orthogonal Frequency-Division Multiplexing divides a high-speed data stream into many parallel low-speed subcarriers that are mathematically orthogonal. This allows the subcarriers to overlap spectrally without interfering with one another, providing excellent spectral efficiency while greatly reducing the effects of multipath propagation.
Combined with cyclic prefixes, FFT processing, adaptive modulation, and advanced channel coding, OFDM has become one of the enabling technologies of modern broadband communications and now underpins Wi-Fi, LTE, 5G, digital broadcasting, and many other high-capacity communication systems.
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