8.15.2 What Is Frequency-Division Multiple Access (FDMA)?
- What Is Frequency-Division Multiple Access?
- Why Is It Called Frequency-Division Multiple Access?
- How Does FDMA Work?
- Why Doesn't Every User Share the Same Frequency?
- What Are Guard Bands?
- Why Must Frequencies Remain Accurate?
- What Happens When All Channels Are Busy?
- Does Every User Receive the Same Bandwidth?
- Where Is FDMA Used?
- Was FDMA Used in Early Cellular Networks?
- How Is FDMA Used in Satellite Communications?
- What Is Intermodulation?
- Why Are Satellite Power Amplifiers Operated Below Saturation?
- What Are the Advantages of FDMA?
- What Are the Disadvantages of FDMA?
- Is FDMA Still Important Today?
- Why Is FDMA Important?
Description
Explore how FDMA assigns each user a dedicated frequency channel for continuous transmission. Learn where FDMA is used, why guard bands are required, and how amplifier nonlinearity and intermodulation influence system performance.
Introduction
When radio communication systems first began supporting multiple users, engineers faced a fundamental challenge. Radio spectrum is a limited resource, yet many users often need to communicate simultaneously. If two transmitters were to operate on exactly the same frequency at the same time and within range of one another, their signals would interfere, making reliable communication impossible.
The earliest practical solution was straightforward: assign each user a different frequency. Provided sufficient separation existed between adjacent channels, users could communicate continuously without interfering with one another. This approach became known as Frequency-Division Multiple Access (FDMA) and formed the foundation of many early radio, mobile, satellite, and military communication systems.
FDMA is one of the simplest multiple-access techniques to understand because each user is allocated a dedicated portion of the available spectrum for the duration of the communication. There is no need for precise timing synchronization between users, and each transmission occurs continuously rather than in bursts.
Although more spectrally efficient techniques such as TDMA, CDMA, and OFDMA have largely replaced FDMA in many modern wireless systems, it remains an important technology. It continues to be used in satellite communications, aeronautical radio, maritime services, conventional land-mobile radio, and numerous specialized communication systems. Understanding FDMA also provides an excellent introduction to the principles of multiple access.
What Is Frequency-Division Multiple Access?
Frequency-Division Multiple Access (FDMA) is a multiple-access technique in which each user is assigned an exclusive frequency channel.
All users may transmit simultaneously. However, because each occupies a different portion of the available spectrum, their transmissions do not normally interfere with one another.
Each user retains exclusive use of the assigned frequency until the communication session ends or the channel is released.
Why Is It Called Frequency-Division Multiple Access?
The available radio spectrum is divided into numerous frequency channels.
Each channel is allocated to a different user. Unlike Frequency-Division Multiplexing, which combines several information streams within one transmission system, FDMA assigns different frequencies to independent users competing for access to the communication medium.
The communication resource being divided is therefore access to the available frequency spectrum.
How Does FDMA Work?
Suppose a communication system has access to a frequency band that is 10 MHz wide.
This bandwidth might be divided into one hundred channels, each occupying approximately 100 kHz. When a user wishes to communicate, the network assigns one of these channels. During the communication:
- the user occupies that frequency continuously;
- other users occupy different frequencies;
- all users transmit simultaneously; and
- the receiver simply tunes to the assigned channel.
Once communication ends, the frequency becomes available for another user.
Why Doesn't Every User Share the Same Frequency?
If several independent transmitters operated on exactly the same frequency simultaneously, their signals would combine at the receiver.
Unless special multiple-access techniques such as CDMA or OFDMA were employed, the receiver would generally be unable to separate the different transmissions.
Assigning each user a separate frequency avoids this problem by ensuring adequate spectral separation.
What Are Guard Bands?
No practical transmitter or receiver has perfectly sharp frequency characteristics.
Every transmitted signal occupies a small range of frequencies. To prevent adjacent channels from overlapping, narrow unused frequency ranges known as guard bands are inserted between neighbouring channels. Guard bands:
- reduce adjacent-channel interference;
- simplify receiver filter design;
- accommodate frequency drift; and
- improve communication reliability.
The disadvantage is that guard bands consume valuable spectrum without carrying useful information.
Why Must Frequencies Remain Accurate?
Every transmitter must remain within its allocated channel.
If a transmitter drifts outside its assigned frequency:
- neighbouring channels may experience interference;
- receiver performance deteriorates;
- spectrum regulations may be violated; and
- communication quality decreases.
For this reason, modern transmitters employ highly stable frequency references, often derived from crystal oscillators or other precision timing sources.
What Happens When All Channels Are Busy?
An FDMA system has a fixed number of available frequency channels.
If every channel is already assigned, additional users cannot begin transmitting immediately. Depending upon the system design, they may:
- wait until a channel becomes available;
- receive a busy indication;
- be placed in a queue; or
- have their communication request rejected.
This limitation illustrates one of the principal disadvantages of FDMA.
Does Every User Receive the Same Bandwidth?
Usually yes.
Each frequency channel is normally assigned the same bandwidth. For example, a land-mobile radio system may allocate 12.5 kHz or 25 kHz to every user. Similarly, satellite systems often divide transponder bandwidth into channels of equal size.
Although this simplifies network operation, it may not always utilize the available spectrum efficiently because different users often require different data rates.
Where Is FDMA Used?
FDMA continues to appear in many communication systems.
Examples include:
- conventional two-way radio systems;
- aeronautical communications;
- maritime radio services;
- satellite communication systems;
- analog cellular networks; and
- many military communication systems.
Although newer technologies dominate public mobile networks, FDMA remains important wherever continuous communication and operational simplicity are required.
Was FDMA Used in Early Cellular Networks?
Yes.
The first generation of cellular telephone systems, commonly known as 1G, relied primarily upon FDMA. Each mobile subscriber received a dedicated radio channel for the duration of the telephone call. This approach worked well while subscriber numbers remained relatively modest. As demand increased, however, more spectrally efficient techniques became necessary.
Second-generation systems therefore introduced TDMA and CDMA, while later generations adopted OFDMA and SDMA.
How Is FDMA Used in Satellite Communications?
Satellite systems frequently divide transponder bandwidth into multiple frequency channels.
Different earth stations may transmit simultaneously using separate carrier frequencies. FDMA remains attractive because:
- communication is continuous;
- synchronization requirements are relatively simple;
- independently operated earth stations can coexist easily; and
- the technique is well suited to analog and digital transmissions.
Many satellite systems also combine FDMA with other access techniques to improve overall capacity.
What Is Intermodulation?
One important consideration in FDMA systems is intermodulation.
When several carriers pass simultaneously through a nonlinear amplifier, new unwanted frequencies may be generated. These intermodulation products can fall within neighbouring communication channels, producing interference.
The likelihood of intermodulation increases as:
- the number of carriers increases;
- amplifier nonlinearity increases;
- transmitter power increases; and
- carrier spacing decreases.
Satellite communication systems pay particular attention to intermodulation because many earth stations often share the same satellite transponder.
Why Are Satellite Power Amplifiers Operated Below Saturation?
To reduce intermodulation distortion, satellite transponders carrying multiple FDMA carriers usually operate with output back-off.
Operating below maximum output power improves amplifier linearity. Although this reduces power efficiency, it significantly decreases interference between neighbouring channels and improves overall system performance.
This trade-off is one of the defining characteristics of multicarrier satellite communication systems.
What Are the Advantages of FDMA?
Frequency-Division Multiple Access offers several important advantages.
These include:
- simple implementation;
- continuous transmission;
- low timing requirements;
- predictable performance; and
- compatibility with both analog and digital signals.
These characteristics contributed greatly to its widespread adoption during the early years of wireless communications.
What Are the Disadvantages of FDMA?
FDMA also has several limitations.
These include:
- guard-band overhead;
- fixed bandwidth allocation;
- inefficient use of idle channels;
- limited spectral efficiency; and
- susceptibility to intermodulation distortion.
As communication demand increased, these limitations encouraged the development of more advanced multiple-access techniques.
Is FDMA Still Important Today?
Very much so.
Although many public mobile networks have migrated to OFDMA and related technologies, FDMA continues to serve an important role in numerous communication systems. Its simplicity, reliability, and continuous operation make it particularly attractive for specialized radio systems and many satellite applications.
Furthermore, understanding FDMA provides the conceptual foundation for more advanced multiple-access techniques introduced later in this chapter.
Why Is FDMA Important?
Frequency-Division Multiple Access was one of the first practical techniques that enabled multiple independent users to share limited radio spectrum efficiently. It provided the basis for many early wireless communication systems and demonstrated that careful spectrum management could dramatically increase communication capacity.
Although newer techniques achieve greater spectral efficiency, FDMA remains widely used because of its simplicity, robustness, and proven performance. Many of the engineering principles first encountered in FDMA—including channel allocation, guard bands, frequency stability, and interference management—continue to influence the design of modern communication systems.
Summary
Frequency-Division Multiple Access allows multiple independent users to communicate simultaneously by assigning each user an exclusive frequency channel. Continuous transmission, straightforward implementation, and modest synchronization requirements made FDMA the dominant multiple-access technique during the early development of wireless communications.
While modern systems increasingly employ more spectrally efficient techniques such as TDMA, CDMA, and OFDMA, FDMA continues to play an important role in satellite communications, conventional radio systems, and many specialized communication networks. Understanding its operation provides an essential foundation for studying the evolution of multiple-access technologies.
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