Orthogonal frequency-division multiplexing facts for kids

Kids Encyclopedia Facts

Imagine sending a secret message across a busy room. If you shout it all at once, it might get lost in the noise. But if you break it into many small parts and whisper each part to different friends, who then pass it along, your message is more likely to arrive clearly.

That's a bit like how Orthogonal Frequency-Division Multiplexing (OFDM) works in the world of telecommunications. It's a clever way to send digital information, like the data from your phone or TV, using many different radio waves at the same time.

OFDM is super popular for many modern digital communication systems. You'll find it in:

Digital TV and radio broadcasts
DSL internet connections (the kind that uses phone lines)
Wireless networks like Wi-Fi
Power line networks (sending data over electrical wires)
4G and 5G mobile phone networks

OFDM was first thought up by Robert W. Chang in 1966. Later, in 1971, Weinstein and Ebert added an important improvement called a "guard interval." This made OFDM even better at handling signals that bounce around, like echoes in a big room.

Instead of sending all the data on one main signal, OFDM splits the data into many smaller streams. Each small stream travels on its own "subcarrier" signal. These subcarriers are very close together but are designed so they don't interfere with each other. Think of them as many tiny, organized radio channels all working together. This way, lots of information can be sent at the same time, in parallel.

Imagine these waves as different signals carrying parts of your data. In OFDM, they are carefully spaced so they don't mess each other up.

The biggest benefit of OFDM is its ability to handle tough signal conditions. For example, if your Wi-Fi signal has to go through walls or if a long cable weakens certain frequencies, OFDM can still work well. It's like having many small, slow signals instead of one big, fast one. This makes it easier to fix problems with the signal.

Also, because the individual signals are "slow," there's time to add a small pause, called a "guard interval," between them. This pause helps prevent echoes from one signal from crashing into the next, which is called intersymbol interference (ISI). This also helps create single frequency networks (SFNs), where several transmitters can send the same signal on the same frequency without causing problems. This is great for broadcasting, as it means better coverage.

When OFDM is combined with special error-correction codes and a technique called "interleaving," it's often called Coded Orthogonal Frequency-Division Multiplexing (COFDM). This makes the signal even stronger against errors, especially in mobile situations where signals might fade or change quickly. COFDM was first used for Digital Audio Broadcasting (DAB) in 1986. Today, most OFDM systems use these coding and interleaving tricks, so the terms OFDM and COFDM are often used to mean the same thing.

Where is OFDM Used?

OFDM is used in many different technologies that help us communicate every day.

Wired Connections (like Internet at Home)

ADSL and VDSL: These are common ways to get broadband internet through regular phone lines. They use a version of OFDM called Discrete Multi-tone Transmission (DMT).
DVB-C2: A newer standard for digital cable TV.
Power line communication (PLC): Lets you send internet signals through your home's electrical wiring.
ITU-T G.hn: A standard for high-speed home networks using power lines, phone lines, and TV cables.
DOCSIS 3.1: Used for broadband internet delivery over cable TV lines.

Wireless Connections (like Wi-Fi and Mobile Phones)

Wireless LAN (WLAN): This includes popular Wi-Fi standards like IEEE 802.11a, g, n, ac, and IEEE 802.11ax.
Digital Radio: Systems like DAB/EUREKA 147, DAB+, Digital Radio Mondiale, and HD Radio use OFDM.
Terrestrial Digital TV: Systems like DVB-T and ISDB-T use OFDM to broadcast TV signals over the air.
Mobile TV: Systems like DVB-H and MediaFLO use OFDM for TV on the go.
Ultra-wideband (UWB): Used for very fast, short-range wireless connections.
Wi-SUN: Used for smart networks.

An advanced version of OFDM, called OFDMA, allows many users to share the same wireless connection at the same time. OFDMA is used in:

WiMAX (a type of wireless internet)
3GPP Long Term Evolution (LTE): This is the main technology behind 4G mobile networks.
3GPP 5G NR: The newest 5G mobile network standard.

Why OFDM is Great (and Its Challenges)

OFDM has some cool features that make it very useful, but it also has a few challenges.

Good Things About OFDM

Efficient Use of Airwaves: It packs a lot of data into the available frequency space.
Handles Tough Conditions: It works well even when signals are weak or have echoes, without needing complex fixes.
Resistant to Interference: It's good at ignoring narrow-band interference (like static).
No Intersymbol Interference (ISI): The guard interval helps prevent signals from overlapping and causing errors.
Easy to Build: It uses a common math trick called the fast Fourier transform (FFT), which makes it efficient to create and decode.
Good for Single Frequency Networks (SFNs): It allows many transmitters to send the same signal on the same frequency, improving coverage.

Challenges with OFDM

Sensitive to Movement: If the transmitter or receiver is moving very fast (like in a high-speed train), the signal can get distorted by something called the Doppler effect.
Needs Precise Timing: The receiver and transmitter need to be very well synchronized in terms of frequency.
High Peak Power: Sometimes, all the different subcarrier signals can line up and create a very strong "peak" in power. This means the equipment needs to be able to handle these high peaks without distorting the signal, which can make it less power-efficient.
Guard Interval Reduces Efficiency: While helpful, the guard interval adds extra, empty time to the signal, which slightly reduces how much data can be sent.

How OFDM Works (Simplified)

Orthogonality: The Key Idea

The main idea behind OFDM is "orthogonality." This means that all the different subcarrier signals are designed to be perfectly separate from each other, even though their frequencies are very close. Think of it like different musical notes that can be played at the same time without sounding messy. This special design means they don't interfere with each other, and the receiver doesn't need a separate filter for each one.

This "orthogonality" is achieved by carefully choosing the spacing between the subcarrier frequencies. Each subcarrier completes a whole number of cycles during the time a symbol is sent. This makes it easy for the receiver to pick out each individual subcarrier.

Using the FFT for Speed

OFDM uses a super-fast math trick called the fast Fourier transform (FFT) to make it work efficiently. On the sending side, an "inverse FFT" (IFFT) combines all the separate data streams into one complex signal. On the receiving side, the FFT quickly separates that complex signal back into its individual data streams. This is why OFDM became so popular for wideband communication, as modern computers can do these FFT calculations very quickly.

The Guard Interval: Preventing Echoes

One of the smartest parts of OFDM is the "guard interval." Imagine you're talking in a canyon, and your voice echoes. If you start your next sentence before the echo of the last one dies out, it gets confusing. The guard interval is like a short pause between each "OFDM symbol" (a block of data sent on all subcarriers at once). This pause is longer than most echoes, so the echoes from one symbol don't interfere with the next.

Often, a small copy of the end of the OFDM symbol, called a "cyclic prefix," is put into this guard interval. This helps the receiver correctly process the signal, even with echoes.

Making Equalization Simple

When a signal travels through a channel (like air or a cable), it can get distorted. This is called "channel conditions." Fixing these distortions is called "equalization." In OFDM, because each subcarrier is very narrow, the channel conditions for that subcarrier are usually very simple – they just make the signal a bit stronger or weaker, or shift its timing slightly. This means the receiver only needs to do a simple math step (multiply by a constant number) to fix each subcarrier. This is much easier than fixing a single, wide signal, which would require much more complex calculations.

Some OFDM systems even send special "pilot signals" or "training symbols" that help the receiver figure out the channel conditions and stay perfectly synchronized.

Channel Coding and Interleaving: Fixing Errors

OFDM almost always uses special techniques to correct errors that happen during transmission.

Channel Coding: This adds extra information to the data, like a secret code, that helps the receiver detect and fix errors.
Interleaving: This shuffles the data around before sending it. If a burst of errors happens (like a sudden interference), interleaving spreads those errors out in the data stream. This makes it easier for the error correction codes to fix them, because they work better when errors are spread out rather than all clumped together.

Newer systems use very powerful error correction codes, like turbo codes and LDPC codes, which can get very close to the theoretical limit of how much data can be sent reliably.

Adaptive Transmission: Smart Sending

Some advanced OFDM systems can "adapt" to the channel conditions. If the receiver can send information back to the transmitter about how good the signal is, the transmitter can adjust how it sends data. For example, if a certain subcarrier frequency is having trouble, the system can send less data on that subcarrier or use a stronger error correction code for it. This is like a smart delivery system that knows which roads are bumpy and sends fragile packages on smoother routes.

This "bit-loading" feature is used in ADSL and VDSL internet connections, allowing them to change their speed based on the quality of your phone line.

Sharing the Connection: Multiple Access

While OFDM itself is about sending one stream of data, it can be combined with ways to let many users share the same connection.

OFDMA: This is a popular method where different OFDM subcarriers are given to different users. This means multiple people can use the network at the same time, each getting their own "slice" of the frequency. This is used in WiMAX and 4G/5G mobile networks.
MC-CDMA: Another method that combines OFDM with a technique called CDMA to separate users.

Space Diversity: Better Coverage

OFDM is great for broadcasting because it allows for "single-frequency networks" (SFNs). In an SFN, many transmitters send the exact same signal on the same frequency. This might sound like it would cause interference, but with OFDM's guard interval, the signals from different transmitters can actually combine and make the overall signal stronger. This means better coverage and more efficient use of radio frequencies.

For example, a guard interval of 200 microseconds allows transmitters to be spaced up to 60 kilometers apart in an SFN.

Linear Power Amplifiers: A Challenge

One challenge with OFDM is that sometimes all the different subcarrier signals can line up and create a very high "peak" in power. This is called a high peak-to-average power ratio (PAPR). To handle these peaks without distorting the signal, the equipment (especially the power amplifiers in the transmitter) needs to be very "linear." Linear amplifiers use more power, which can be a challenge for battery-powered devices. Engineers are always working on ways to reduce this PAPR.

OFDM in Action: Real-World Uses

OFDM is a fundamental technology in many of the digital communication systems we use daily.

ADSL Internet

As mentioned, ADSL (Asymmetric Digital Subscriber Line) internet connections use OFDM, calling it discrete multitone modulation (DMT). This is why DSL works well even on long copper phone lines, which can weaken high frequencies. OFDM's ability to adapt to these frequency changes is key.

Powerline Communication

Many devices that send data over your home's electrical wiring use OFDM. The electrical wiring can be very "noisy," so OFDM's ability to adapt to changing conditions is very important here.

Wi-Fi and Wireless Networks

OFDM is widely used in Wi-Fi standards like 802.11a/g/n/ac/ax. These standards allow for different speeds (from 6 to 54 Mbit/s and even faster with newer versions) by using different modulation schemes (like BPSK, QPSK, 16-QAM, and 64-QAM) and error correction rates. This lets your Wi-Fi adapt to how good your signal is.

Digital Radio and TV Broadcasting

Most of Europe and Asia use OFDM for digital TV (DVB-T) and radio (EUREKA 147 DAB, Digital Radio Mondiale).

In the United States, the FCC approved a new digital TV standard, ATSC 3.0, in 2017, which is based on COFDM. This allows for better reception and new features. For digital radio, the US uses HD Radio, which also uses COFDM to add digital audio to AM and FM broadcasts.

COFDM vs. VSB

For a long time, there was a debate between Europe and North America about which digital TV system was better: COFDM (used in Europe) or 8VSB (used in the US). COFDM is generally better at handling echoes and signals bouncing around, and it allows for those useful single-frequency networks. However, newer 8VSB receivers have improved a lot. With the adoption of ATSC 3.0, the US is now also moving towards COFDM.

Ultra-wideband (UWB)

UWB technology, used for very fast, short-range wireless connections, also uses OFDM in some versions, like Multiband OFDM (MB-OFDM).

Flash-OFDM

Flash-OFDM was a mobile broadband technology developed by Flarion (later bought by Qualcomm). It was used in some countries for mobile internet services, but many networks eventually switched to other technologies like CDMA2000 or LTE.

History of OFDM

1957: Kineplex, an early multi-carrier modem, was developed.
1966: Robert W. Chang from Bell Labs published a paper and patented the idea of OFDM.
1971: Weinstein and Ebert suggested using the FFT and adding a guard interval, making OFDM much more practical.
1987: Alard & Lasalle introduced COFDM for digital broadcasting.
1995: The first OFDM-based standard, ETSI Digital Audio Broadcasting (DAB), was created.
1997: The ETSI DVB-T standard for digital TV was released.
1999: The IEEE 802.11a wireless LAN standard (Wi-Fi) adopted OFDM.
2002: IEEE 802.11g for wireless LAN also started using OFDM.
2004: IEEE 802.16 (WiMAX) and ETSI DVB-H standards were released, both using OFDM.
2005: OFDMA became a candidate for the 3GPP Long Term Evolution (LTE) mobile standard.
2007: The first full LTE system, including OFDM, was shown.

Images for kids

This diagram shows how the "cyclic prefix" (a copy of the end of the symbol) is added to the beginning of each OFDM symbol. This helps the receiver deal with echoes.
A simplified diagram of how an OFDM signal is created at the transmitter.
A simplified diagram of how an OFDM signal is received and decoded.
This image shows how different subcarriers (like different channels) are used in an OFDM signal.