ECC Newsletter May 2017
Spectrum engineering: helping to ensure efficient and interference-free wireless communications
Congestion of the radio spectrum is growing with an ongoing rise in the demand for more wireless services. More people are using mobile communications, Wi-Fi, wireless microphones and short range devices – such as car key fobs – than ever before. National communications regulators are faced with the challenge of identifying new frequencies for new uses while preventing interference to existing users of the spectrum.
In the past it was easier to find suitable frequencies with sufficient separation to avoid interference. Then, there were fewer applications available and data rates were lower, but significant advances in technology and resulting increases in demand for spectrum in recent years have made this more challenging. There is now a need to pack more applications into a limited set of frequencies, and to explore novel ways for different users and applications to share spectrum. It is therefore becoming increasingly important to be able to understand and quantify the causes and likelihood of interference occurring between different systems.
Figure 1: Congested frequency allocations in Europe – several applications need to share the same frequencies (see the graphical search function on EFIS for full details and definitions)
Causes and effects of interference
Interference occurs when a transmission from one system disrupts the reception of signals at the receiver of another nearby system. It can occur between systems operating on the same frequency - this is known as co-channel interference – or between systems in frequencies that are close – this is known as adjacent channel or adjacent band interference. It is worth noting that there are other types of interference such as intermodulation but they are not covered here.
Co-channel interference is a result of the interfering signal "drowning out" the victim signal as it has sufficiently high energy at the receiver to impair reception. Consider as a simple analogy a conversation between two people across a room being disrupted by another louder person.
In adjacent channel interference there are two main causes of interference:
- Energy from the transmitter "overflowing" into adjacent frequencies. This is due to the design of the transmitter, which is not able to sufficiently filter out leakage into adjacent frequencies, and is known as unwanted emissions
- The receiver picking up signals from outside its own frequency. This is due to the design of the receiver, which is not able to distinguish between energy received on its own frequency and other frequencies. This is known as blocking or receiver selectivity
Figure 2: Causes of interference: unwanted emissions and blocking
In practice both of these can occur simultaneously. Sometimes it is necessary to improve the design of both the transmitter and receiver to prevent interference, which is becoming increasingly important as the spectrum becomes more congested.
As a simple example consider a "noisy neighbour" scenario between two adjacent houses, both with open windows. Reduction in out-of-band emissions can be compared to the noisy neighbour (interferer) closing their windows to reduce the noise leaving the building. Improving the receiver selectivity can be compared to the quiet neighbour (victim) closing their windows to stop the noise coming in. There are other possible solutions such as the noisy neighbour reducing the volume, or the neighbours discussing to agree reasonable limits on the noise. Equivalent solutions also exist in the radio spectrum world.
The impact of interference to the victim user depends on the type of system. In broadcasting or voice telephony any amount of interference can result in a noticeable disruption of the signal such as loss of picture or audio. In applications such as mobile data or Wi-Fi networks interference generally results in a reduction in the victim user's data rate, the severity of which depends on how much interference power is received.
How can interference be predicted?
In order to avoid the possibility of interference, national authorities and international bodies, (including the ECC) need to quantify the likelihood of interference when proposing a new use of a frequency band. This is typically done through theoretical calculations known as sharing studies, which usually refer to in-band studies, and compatibility studies, which refer to adjacent band studies. Theoretical studies are necessary because it is not always possible to perform measurements on real systems, particularly in cases where the systems are still under development. Two types of studies are commonly used:
- Deterministic studies based on fixed parameters, using the Minimum Coupling Loss method. This is a worst-case assessment of interference. The results usually determine the minimum required separation distance (in space or in frequency) between two systems to avoid interference.
- Statistical studies based on variable parameters, using the Monte Carlo method. This is a more realistic assessment, which takes into account the real-world variation and randomisation of certain parameters such as the relative positioning of systems. The result is a probability of interference for the scenario under investigation, which can be compared against a relevant threshold to determine if the level of interference is considered to be a problem or not.
These studies can be performed using a range of software tools. An example is SEAMCAT for Monte Carlo analysis, which is an open-source software tool used for ECC studies.
In some cases the assumptions used in the studies can be validated through laboratory measurements of real systems, or through field measurements.
If the results of studies show that interference may occur, it may be necessary to investigate different mitigation techniques to minimise the risk of interference. This could include specification of additional filtering to be applied to the transmitter or receiver, additional frequency separation between both systems, restrictions on the usage of the new system such as limits on maximum transmit power or geographical restrictions on where the new system can be used. In addition, with the emergence of new technologies, new and innovative sharing solutions are being explored. For example, techniques such as geolocation and licensed shared access (LSA) have been recently assessed by the ECC.
This overall process, including the prevention of interference by quantifying the risk through studies, and the cure through identifying suitable mitigation, is known as spectrum engineering.
The ECC's role in ensuring efficient use of the radio spectrum
The ECC's role in spectrum engineering is two-fold:
- It ensures harmonised use of the radio spectrum across Europe. This means producing common regulations for use of spectrum in specific frequency bands in all CEPT countries, to allow wireless communications equipment to be sold across Europe. This results in cheaper devices to produce and lower costs for the operator and ultimately the user.
- It protects against cross-border interference. As propagation of radio waves doesn't stop at national borders, it is necessary to produce recommended co-ordination procedures to ensure interference does not occur between neighbouring countries.
The ECC's Working Group Spectrum Engineering (WG SE) and its project teams are responsible for conducting sharing and compatibility studies to meet these aims.
Challenges in predicting the unknown
One of the major challenges in spectrum engineering is in attempting to predict the future based on uncertain information. Sharing studies need to have accurate input assumptions in order to produce meaningful and reliable results. The phrase "garbage in, garbage out" is often quoted in this context. But in many cases spectrum engineers are working with future technologies where not all the parameters can be defined in advance of the deployment of the new technology. In such cases it is important to conduct a range of studies based on different input assumptions to ensure that all possibilities are covered. Cooperation with relevant standardisation groups, in particular ETSI, the European Telecommunications Standards Institute, is important to ensure suitable harmonised equipment standards are produced. They allow for coexistence between systems.
It is also important to ensure the studies and subsequent reports are clear and transparent, to ensure that the results can be understood in the correct context, and reproduced (by making available the underlying calculations, such as SEAMCAT workspaces) if necessary.
Project Team SE 21 is currently working on ways to better characterise both transmitter unwanted emissions and also receiver parameters to ensure accurate studies can be produced with more favourable conditions to allow future efficient use of spectrum.
Future opportunities and new sharing techniques
The wireless world is evolving rapidly with increasing demand for new uses and higher data speeds, particularly mobile applications. This means spectrum engineers need to consider new and innovative ways to ensure efficient use of spectrum.
The ECC identified the need to enable new spectrum sharing techniques in its strategic plan for 2015-2020, as well as the need to promote the use of higher frequency bands to meet the needs of new demands.
In this context, one of the examples the ECC is currently studying is the potential introduction of the 5th generation of mobile communications (5G) in various frequency bands from 24-86 GHz, referred to as millimetre wave bands. These bands are much higher in frequency than those currently used for mobile, so they bring a new set of sharing challenges with existing users of these bands, which include satellite and scientific services and fixed links, among others.
However, these higher frequencies result in shorter link distances and allow new smart antenna technologies to be used, both of which help to reduce the risk of interference and can facilitate new opportunities for sharing. The possibility of more flexible regulation in these bands is also being investigated to increase sharing possibilities.
By improving the accuracy of sharing studies and investigating novel techniques for sharing spectrum, the ECC can continue to ensure efficient use of the radio spectrum, even in today’s challenging environment of increased demand for wireless applications.
Further reading
Additional information can be found in the following articles:
The role of Spectrum Engineering: an essential element to using spectrum efficiently, June 2011
An introduction to SEAMCAT, July 2016
Peter Faris, Spectrum Expert, European Communications Office
