Tips for Performing Precision Interference Measurements in the Field

Today we will discuss the various types of interference that operators will encounter in existing and new wireless environments. Here we introduce effective measurement techniques and instrument requirements for interference testing using modern high-performance spectrum analyzers such as Keysight FieldFox analyzers. The article also discusses the classification of various types of interference, including in-band, co-channel, out-of-band, and adjacent channel interference. Make accurate measurements anytime, anywhere.

Microwave system operators often encounter interference from cellular systems and data links. Due to the scarcity of wireless spectrum resources, wireless communication systems often need to operate with a limited amount of wireless interference. Many existing wireless systems control and operate portions of the spectrum through licenses from regulatory agencies. Licensed operations authorize service providers to determine which technology to use for the services they provide and to prevent harmful interference from other wireless services and service providers. Licensed wireless systems will operate over a wide range of RF and microwave carrier frequencies. Licensed systems include LTE cellular systems (usually operating in carrier frequencies below 2 GHz), direct broadcast satellites (downlink: 12 GHz; uplink: 17 GHz), and point-to-point backhaul systems (operating in the 23 GHz band). When trying to squeeze a large number of users into a limited frequency band of licensed spectrum, co-channel and adjacent channel interference is often generated within the system. On the other hand, unlicensed operations are generally considered one of the open access resources, and when spectrum demand increases, system interference will also increase, affecting the quality of service for all users. Examples of unlicensed systems include the popular Wi-Fi, Bluetooth®, and ZigBee systems that use the 2.4 GHz band. Many frequency bands are allocated for both licensed and unlicensed operations. For example, in the United States, many radar platforms use the 3.1 to 3.3 GHz band, including airborne systems such as AWACS, synthetic aperture radar (SAR), and shipborne systems such as Aegis. Within the 200 MHz band, unlicensed operations can use portions of the 3.26 to 3.267 GHz band. In addition, as demand for spectrum continues to grow in the public sector, commercial, and non-commercial applications, wireless interference issues are expected to grow as new wireless systems become available. For example, new generation cellular communication systems will deploy “femtocells” in coverage networks, with the potential to interfere with macrocell downlinks.

Another example is a system using Dynamic Spectrum Access (DSA), where these opportunistic wireless systems act as a second user and temporarily use unused spectrum until the primary operator starts transmitting. The second DSA user acts as interference to the primary user until the second user is reallocated to another available spectrum segment. These DSA technologies are sometimes referred to as cognitive radio (CR) and white space.

Increased spectrum usage requires more advanced measurement tools

In order to improve spectrum efficiency, some countries and regions are attempting to reallocate spectrum based on consumer demand. The U.S. government recently decided to reallocate spectrum to open up up to 500 MHz of new spectrum for mobile and fixed broadband applications. During a transition period of several years, existing systems may need to be relocated (in the 1755 to 1850 MHz band[5]), resulting in interference between existing and new systems that may not disappear until the relocation is complete. While all of these existing and new systems are trying to maximize spectrum efficiency, there is a growing need for more advanced measurement tools to test, monitor, and manage the level of interference between different wireless systems. These measurements typically require field testing near the system receivers using rugged, lightweight, portable test instruments that are comparable in performance to traditional benchtop instruments.

Interference and spectrum access

Interference exists in the wireless channel of any wireless system, which may reduce the reception rate of the specified signal. When the power level of the interference signal received by the wireless system is greater than the specified signal, its service quality will be reduced or even cause service interruption. When multiple wireless systems use the same wireless spectrum, it is possible to generate an "interference event". The IEEE defines an interference event as "a situation where the interference exceeds a quantitative threshold level", which can be set based on amplitude, frequency, time and/or system performance. When investigating the type and origin of electromagnetic interference in a dynamic wireless environment, high-performance spectrum analyzers (such as FieldFox) are essential tools that can measure the power level of interference signals based on time, frequency and location.

Since interference testing often requires taking measurements and collecting data in the environment surrounding a wireless system, a portable, battery-powered spectrum analyzer is a great way to conduct field testing in these harsh environments. In Figure 1a, a technician is using a handheld spectrum analyzer to conduct field testing near a noisy CATV amplifier. In this example, the spectrum analyzer is connected to a directional antenna via a short coaxial cable. The measurements displayed by the analyzer can be adjusted for cable loss and antenna gain. Directional antennas measure changes in signal amplitude by pointing at the surrounding environment. Based on this change, the spectrum analyzer can identify the location of interfering transmitters. In Figure 1b, FieldFox is connected to the feeder of a cellular base station to ensure that its return loss is within specifications, as poor cable and antenna performance can cause network interference.

Interference in wireless systems can come from a variety of intentional, unintentional, and incidental radiators. An intentional radiator is defined as a device that contains an active transmitter that generates an electromagnetic signal at a specific RF/microwave carrier frequency and a specific output power level. Examples of intentional radiators include cell phones, radars, and WLAN devices. Unintentional radiators may use RF/microwave signals, such as wireless receivers that do not intentionally act as transmitters but radiate signals unintentionally. Incidental radiators do not use RF/microwave signals but may radiate or modulate RF/microwave signals during operation, such as electric motors and fluorescent lights. While this technique and measurement application can be used for any type of radiator, this application note will focus on measurements of licensed and unlicensed intentional radiators that coexist in the spectrum and may interfere with the operation of the target wireless system.

Licensed wireless systems minimize interference by isolating multiple users in the time, frequency, and/or space domains. Unlicensed systems know that interference will exist and try to share the spectrum in a friendly manner with all users by utilizing the time, frequency, and/or space domains as much as possible. In unlicensed bands, coordination between multiple wireless devices is generally not allowed, and wireless devices often have to measure channel energy before transmitting signals in a "listen before talk" protocol, such as IEEE 802.11 systems.

Figure 1a. FieldFox connected to a directional antenna used to locate sources of wireless interference.

Figure 1b. FieldFox connected directly to the feeder of a wireless communication system.

Figure 2 shows an example of an “over-the-air” measurement using FieldFox and an externally connected omnidirectional antenna. The figure shows that the measured spectrum in the UHF band supports both licensed and unlicensed signals. The lower band covers the downlink portion of the U.S. cellular system. The upper band shows the unlicensed spectrum, which contains emissions from wireless communication devices regulated by FCC Part 15 and non-telecom devices in industrial, scientific and medical (ISM) applications. For this measurement, FieldFox was equipped with an internal preamplifier to increase measurement sensitivity and a 0 dB internal input attenuator to further improve the analyzer’s noise floor. Cursors and the associated cursor table are used to display the start and end frequencies for each allocated band. The measurements shown in the figure were captured using FieldFox and saved as an image file. Measurement sweeps can also be saved to the analyzer’s internal memory, micro SD, or USB drive. Recording measurements is important for capturing intermittent signals and performing further analysis, including channel power, occupied bandwidth, adjacent channel power, and other interference analysis.

Figure 2 also shows that the clean separation of the two frequency ranges is very effective in preventing different types of systems from interfering with each other. Given that spectrum is such a precious resource, the frequency range between the downlink and ISM bands shown in the figure (labeled “Other” in the figure) has been allocated to other types of wireless systems, including commercial navigation and land mobile wireless systems. As shown in the figure, at this particular measurement location and with this particular instrument setup, it is difficult to measure the signal energy output by these “other” systems.

Figure 2. Results of an over-the-air measurement of the UHF spectrum using FieldFox, showing a portion of the licensed cellular bands and the unlicensed Industrial, Scientific, and Medical (ISM) bands.

Interference testing is extremely important at airports, seaports and other locations where interference can disrupt the reception of wireless and satellite signals. Spectrum users near borders must also pay special attention, as wireless transmissions can cause interference across borders, and regulations vary from country to country. Organizations such as the International Telecommunication Union (ITU) develop wireless standards “to ensure that next-generation networks can achieve perfect global communication and interoperability,” but finding common frequency bands that cross national borders is difficult.