11.8.11 Why Doesn't Every Communication System Simply Use the Highest Frequency Available?
- Why Do Engineers Want Higher Frequencies?
- Why Doesn't More Bandwidth Solve Every Problem?
- Why Are Low Frequencies Still Used?
- Why Do Mobile Networks Use Several Different Frequency Bands?
- Why Are Millimetre Waves Becoming Popular?
- Why Doesn't Satellite Communication Always Use Lower Frequencies?
- Does Antenna Size Influence Frequency Choice?
- Does Cost Affect Frequency Selection?
- How Do Regulations Influence Frequency Choice?
- What Will Happen in the Future?
- Is There Such a Thing as the "Best" Frequency?
- What Should You Remember?
Short Answer
Higher frequencies can support much wider bandwidths and therefore much higher data rates, but they also introduce significant engineering challenges. As frequency increases, radio waves become more dependent on line-of-sight propagation, atmospheric attenuation increases, rain becomes more significant, diffraction becomes less effective, and communication range generally decreases for a given transmitter power. The most suitable operating frequency therefore depends upon the particular application. Successful communication engineering is not about using the highest possible frequency—it is about selecting the frequency that provides the best balance between coverage, capacity, reliability, and cost.
Why Do Engineers Want Higher Frequencies?
Over the past century there has been a steady migration towards higher parts of the radio spectrum.
Early wireless systems operated at relatively low frequencies because transmitters and receivers were technically limited. As electronic technology improved, communication systems progressively moved into the VHF, UHF, microwave, and now millimetre-wave bands. The principal reason is simple. Higher frequencies generally offer much larger allocations of radio spectrum. More spectrum allows more information to be transmitted.
In an age of video streaming, cloud computing, autonomous vehicles, and billions of connected devices, this additional capacity is enormously valuable.
Why Doesn't More Bandwidth Solve Every Problem?
Although higher frequencies provide greater bandwidth, propagation becomes progressively more difficult.
Several changes occur simultaneously. Radio waves:
- diffract less readily;
- become increasingly dependent upon line-of-sight paths;
- experience greater atmospheric attenuation;
- suffer increased rain fade;
- require more accurate antenna alignment; and
- often cover smaller geographical areas.
The additional bandwidth therefore comes at the cost of reduced propagation performance.
Every communication system must balance these competing effects.
Why Are Low Frequencies Still Used?
Some communication applications place much greater importance on coverage than on data rate.
Examples include:
- maritime communication;
- aviation communication;
- military communication;
- emergency services;
- navigation systems;
- AM broadcasting; and
- submarine communication.
These systems often operate at relatively low frequencies because long wavelengths provide excellent propagation over large distances and around obstacles.
Although the available bandwidth may be modest, reliable communication is often far more important than very high transmission speed.
Why Do Mobile Networks Use Several Different Frequency Bands?
Modern cellular networks provide an excellent illustration of engineering compromise.
Low-frequency bands provide:
- larger cell sizes;
- better building penetration;
- wider rural coverage; and
- fewer base stations.
Higher-frequency bands provide:
- greater capacity;
- wider bandwidth;
- higher user data rates; and
- better frequency reuse.
Rather than choosing one or the other, mobile operators employ multiple frequency bands simultaneously.
Low frequencies provide broad coverage, while higher frequencies supply additional capacity where user density is greatest.
Why Are Millimetre Waves Becoming Popular?
The newest wireless technologies increasingly employ frequencies above approximately 24 GHz.
These millimetre-wave bands provide exceptionally wide bandwidths capable of supporting multi-gigabit data rates. Such frequencies are attractive for:
- dense urban broadband;
- fixed wireless access;
- wireless backhaul;
- industrial automation;
- virtual reality;
- autonomous vehicles; and
- future 6G systems.
Their principal limitation is comparatively short communication range.
Millimetre-wave networks therefore rely upon many closely spaced base stations rather than a small number of widely separated towers.
Why Doesn't Satellite Communication Always Use Lower Frequencies?
Satellite communication also demonstrates the trade-offs involved.
Lower-frequency bands such as L-band provide excellent propagation through rain and clouds. Higher-frequency bands such as Ku- and Ka-band provide much larger bandwidths and therefore much greater communication capacity. Modern satellite systems often employ several frequency bands simultaneously. Navigation satellites, for example, operate mainly in L-band because reliability is essential. Broadband satellites increasingly use the Ka-band because high capacity outweighs the additional rain attenuation.
Again, there is no universally best frequency.
Does Antenna Size Influence Frequency Choice?
Very strongly.
Efficient antenna dimensions are directly related to wavelength. At low frequencies, antennas may become impractically large. At higher frequencies, compact antennas become possible. Portable radios, smartphones, Wi-Fi equipment, automotive radar, and satellite terminals all benefit from the smaller antennas made possible by shorter wavelengths.
The relationship between wavelength and antenna size has therefore been one of the major drivers behind the migration towards higher frequencies.
Does Cost Affect Frequency Selection?
Economic considerations are often just as important as technical ones.
Higher-frequency systems frequently require:
- more base stations;
- tighter antenna alignment;
- more sophisticated electronics;
- additional propagation modelling;
- higher installation costs; and
- greater maintenance.
Lower-frequency systems generally require fewer sites but may need larger antennas and may support fewer simultaneous users.
Engineers therefore evaluate both technical performance and total life-cycle cost before selecting an operating frequency.
How Do Regulations Influence Frequency Choice?
The radio spectrum is a shared international resource.
Governments allocate different frequency bands for different purposes. Some are reserved for:
- broadcasting;
- aviation;
- maritime services;
- defence;
- satellite communication;
- scientific research; or
- amateur radio.
Consequently, engineers cannot simply choose any convenient frequency. Every communication system must operate within nationally and internationally allocated spectrum while avoiding harmful interference to other users.
Regulation therefore plays an important role in frequency selection.
What Will Happen in the Future?
The demand for wireless capacity continues to increase rapidly.
Future communication systems will almost certainly expand into still higher frequency bands, including sub-terahertz frequencies. These systems will offer enormous bandwidth but will also face even greater propagation challenges. Artificial intelligence, adaptive beamforming, massive MIMO, intelligent reflecting surfaces, and advanced coding techniques are expected to play increasingly important roles in overcoming these limitations.
Even so, lower-frequency communication systems will remain essential for wide-area coverage and highly reliable communication.
Is There Such a Thing as the "Best" Frequency?
One of the most important lessons in communications engineering is that there is no universally best operating frequency.
Every part of the radio spectrum offers a different balance between:
- communication range;
- bandwidth;
- antenna size;
- atmospheric attenuation;
- propagation reliability;
- infrastructure cost; and
- spectrum availability.
Successful communication systems are designed by matching these characteristics to the application's requirements rather than pursuing the highest possible frequency.
What Should You Remember?
- Higher frequencies generally provide wider bandwidths and support higher data rates.
- Lower frequencies usually provide greater communication range and more favourable propagation characteristics.
- Increasing frequency reduces diffraction and increases dependence on line-of-sight propagation.
- Atmospheric attenuation and rain fade become increasingly important at microwave and millimetre-wave frequencies.
- Antenna size decreases as frequency increases, making portable communication equipment practical.
- Modern communication networks frequently use multiple frequency bands simultaneously to balance coverage and capacity.
- The best operating frequency is determined by engineering trade-offs rather than by bandwidth alone.