Designing robust microwave connections can rarely be accomplished with off-the-shelf solutions. The best and most reliable links are optimized to overcome the challenges imposed by the environmental elements, such as weather, terrain, atmosphere, and other factors.M.J. Martin
During the heat of the summer months, there is a phenomenon that impacts microwave radio signals and actually distorts these radio waves so much that connections are lost. The phenomenon is caused by heat of the day. It comes in two forms – Thermal Ducting and Stratification.
Microwave radio signals need line of sight connectivity in order to work properly. If the line of sight path is obstructed or lost, then the links fail. This situation occurs in both point-to-point links and point-to-multipoint links. The longer the path, the greater the potential for signal loss due to atmospheric events like thermal ducting and stratification. Below are some simple definitions of these heat phenomenon for your consideration.
Thermal Ducting is a bending of microwave signals over a distance due to the rising columns of hot air created by irregular surfaces on the earth. It is common to see thermal ducting caused by roadways and highway surfaces.
Thermal ducting causes the link to fade and then be lost completely. The source signal is bent away by the heated columns of air and therefore it does not reach the receive antenna.
It is observed that rising temperature plays a key role where ducts occurred above 30% in the summer months and less than 7% in the spring, autumn, and winter months. It is due to an increase in temperature, especially in summer and autumn months, where humidity gradients play an essential role in creating a higher frequency of duct.
Humidity is a major concern is the atmospheric layers when large scale dusts occur over great areas. However, local ducting can happen when the pathway crosses over a road or surface that reflects the heat differently compared to the typical surfaces along the signal path.
For example, let us consider a path of 30 kilometres in a rural area. Most of the earth surface along this path is vegetation such as grass, trees, bushes, and other common land cover. Then the path crosses a 4-lane highway. The highway reflects and heats the air very differently compared to the grass surfaces. This results in rising columns of hot, heated air off of the road surface. AS the microwave signal moves over the road, the heated column causes it to rise or bend. This bending breaks the link to the receive antenna resulting in loss of signal.
It is rare to happen at night and peaks from 11:00 am to 5:00 pm during the hottest times of the day.
Stratification is when the atmosphere heats at different temperatures at different elevations. It creates layers of heat. The different temperatures allow the microwave signal to pass but the speed of the signal is altered in each layer. The receiving radio has a difficult time to reconstruct the source signal due to the disparity in arrival times on the source signal. It cannot reconstruct the source signal since the different times make it impossible to reconcile.
Again, this sort of problem is worse over longer microwave radio links of 30, 40, or 50 kilometres.
The graphic is a dramatization of the effect to help visualize what is observed. Even sophisticated radio equipment with advanced adaptive equalization technology may have difficulty rebuilding the source signal at the receiver due to jitter from phase and amplitude errors.
In radio systems with higher order modulation techniques, such as 32 QAM, 64 QAM, or 128 QAM and higher, are especially susceptible to this problem. Lower order modulation equipped microwave radios with simple modulation, such as QPSK or 16 QAM when equipped with Forward Error Correction (FEC) will be more robust.
The above radios problems can be further exasperated when multipath problems exist. A reflected wave causes a phenomenon known as multipath, meaning that the radio signal can travel multiple paths to reach the receiver. Typically, multipath occurs when a reflected wave reaches the receiver at the same time in opposite phase as the direct wave that travels in a straight line from the transmitter.
In wireless telecommunications, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings. The effects of multipath include constructive and destructive interference, and phase shifting of the signal.
Other Forms of Interference
What is radio frequency interference?
Radio frequency (RF) is an electromagnetic wave that oscillates between the audio and infrared in the electromagnetic spectrum. Many natural RF signals exist in nature and indeed travel throughout the universe. But typical radio frequency interference (RFI) is defined as any “unwanted” signal received by a device that prevents clear or best “wanted” signal reception. This means that RFI can affect any signal receiving system such as IoT, SCADA, televisions, computers, audio, and security systems, and even automatic garage door openers. Example of interference can include:
- In-Band Interference
- Self Interference
- Adjacent Band Interference
- Out of Band (Harmonic) interference
What is Transmitter Harmonic Interference?
Transmitter harmonic interference is due to non-linear characteristics in a transmitter. The harmonics are always fundamental frequency multiples and the non-linear design of the final output stage of the transmitter. If the harmonic signal falls within the passband of a nearby receiver and the signal level is of sufficient amplitude, it can degrade the performance of the receiver
What is Transmitter Spurious Output Interference?
Transmitter spurious output interference can be attributed to many different factors in a transmitter. The generation of spurious frequencies could be due to non-linear characteristics in a transmitter or possibly the physical placement of components and unwanted coupling. If a spurious signal falls within the passband of a nearby receiver and the signal level is of sufficient amplitude, it can degrade the performance of the receiver
What is Receiver Desensitization Interference?
Receiver desensitization interference occurs when an undesired signal from a nearby “off-frequency” transmitter is sufficiently close to a receiver’s operating frequency. The signal may get through the RF selectivity of the receiver. If this undesired signal is of sufficient amplitude, the receiver’s critical voltage and current levels are altered and the performance of the receiver is degraded at its operating frequency. The gain of the receiver is reduced, thereby reducing the performance of the receiver. A high-power transmitter can be operating several megahertz away from the receiver frequency and/or its antenna can be located several thousand feet from the receiver’s antenna and still cause “desense” interference.
What is Transmitter Noise Interference?
Transmitter noise interference occurs because a transmitter radiates energy on its operating frequency as well as frequencies above and below the assigned frequency. The energy that is radiated above and below the assigned frequency is known as sideband noise energy and extends for several megahertz on either side of the operating frequency. This undesired noise energy can fall within the passband of a nearby receiver even if the receiver’s operating frequency is several megahertz away. The transmitter noise appears as “on-channel” noise interference and cannot be filtered out at the receiver. It is on the receiver’s operating frequency and competes with the desired signal, which in effect, degrades the operational performance.
There are all sorts of forms of interference that prevent radio signals from doing what they are designed to do. Once you understand the impairment issues, you can construct solutions that directly address them or reduce the potential for these issues to occur. There is no substitute for a comprehensive design that is intensely engineered to greatly add to the robustness of the radio connections.
About the Author:
Michael Martin is the Vice President of Technology with Metercor Inc., a Smart Meter, IoT, and Smart City systems integrator based in Canada. He has more than 40 years of experience in systems design for applications that use broadband networks, optical fibre, wireless, and digital communications technologies. He is a business and technology consultant. He was a senior executive consultant for 15 years with IBM, where he worked in the GBS Global Center of Competency for Energy and Utilities and the GTS Global Center of Excellence for Energy and Utilities. He is a founding partner and President of MICAN Communications and before that was President of Comlink Systems Limited and Ensat Broadcast Services, Inc., both divisions of Cygnal Technologies Corporation (CYN: TSX). Martin served on the Board of Directors for TeraGo Inc (TGO: TSX) and on the Board of Directors for Avante Logixx Inc. (XX: TSX.V). He has served as a Member, SCC ISO-IEC JTC 1/SC-41 – Internet of Things and related technologies, ISO – International Organization for Standardization, and as a member of the NIST SP 500-325 Fog Computing Conceptual Model, National Institute of Standards and Technology. He served on the Board of Governors of the University of Ontario Institute of Technology (UOIT) [now OntarioTech University] and on the Board of Advisers of five different Colleges in Ontario. For 16 years he served on the Board of the Society of Motion Picture and Television Engineers (SMPTE), Toronto Section. He holds three master’s degrees, in business (MBA), communication (MA), and education (MEd). As well, he has three undergraduate diplomas and five certifications in business, computer programming, internetworking, project management, media, photography, and communication technology. He has completed over 30 next generation MOOC continuous education in IoT, Cloud, AI and Cognitive systems, Blockchain, Agile, Big Data, Design Thinking, Security, Indigenous Canada awareness, and more.