Interference hunting in cities: Choose angle of arrival over power

Anyone who has taken a scanning receiver or spectrum analyzer recently and characterized the RF environment will be aware that unwanted transmitters are pervasive in today’s mobile networks. They degrade network capacity and end-user quality of experience, causing lost revenue for operators and dissatisfaction for subscribers. Therefore, finding and eliminating interference has become a priority for mobile network operators and regulators.

When interference occurs, the source of the RF emission can be located by measurement of radio wave characteristics. There are many methods for radiolocation of transmitters but the two main ones are angle of arrival (AOA), also known as automatic direction finding (or DF), and power of arrival (POA). Before we examine the merits of each method, let’s consider how radio waves propagate.

RF propagation

Most RF interference occurs in urban environments. Nearly always, the direct path between the emitter and receiver is obscured by buildings. This situation is referred to as non-line-of-sight (NLOS) where the signal you are trying to locate is reflected, diffracted and scattered due to multipath propagation from various directions before arriving at the receiving antenna.

As you can see from the diagram above, the main waves, reflected from scattering objects near the emitter, arrive at the receiver within a defined sector. The secondary waves, reflected from scattering objects near the receiver, arrive from all directions randomly. Crucially, the average angle of arrival from which most of the RF energy arrives at the receiver is toward the true direction of the emitter.

Start interference hunting with power

Power of arrival (POA)-based radiolocation systems work by continuously measuring and averaging the signal level received by an omnidirectional antenna, usually installed on a vehicle roof. After gathering a sufficient amount of level information, the position of the emitter can then be estimated.

POA is the simplest method available for radiolocation from a moving vehicle: it only requires an omnidirectional antenna together with a global navigation satellite system (GNSS), such as GPS, connected to a monitoring receiver or spectrum analyzer. It is based on the measurement and comparison of the signal level received at various measurement locations and the position of the emitter is then estimated by means of algorithms. All algorithms used for POA are based on the following preconditions:

• The signal level decreases with distance following the log-distance path loss model selected.
• The propagation environment fits the model selected and does not change during the mission.
• The emitter features an omnidirectional horizontal radiation pattern and radiates with constant power.

This method is accurate and fast in line-of-sight (LOS) scenarios but, in contrast to angle of arrival (AOA), requires taking signal level measurements at several locations in order to give a first estimation of the emitter’s direction. For NLOS scenarios, the received signal level fluctuates independently of the distance to the emitter because of multipath propagation, and POA systems find it difficult to determine the true direction of the emitter. This can be partially overcome by increasing the number of measurement locations and using a handheld direction antenna.

AOA-based interference hunting in cities

Angle of arrival (AOA) automatic direction finders (DF) take spatial and temporal samples from the received wavefront and apply digital signal processing according to the particular DF method selected to generate bearings every few milliseconds. Several DF methods, such as the Watson-Watt, Doppler and correlative interferometer are based on the measurement of phase angle differences between multiple antenna elements, typically arranged in a circular antenna array.

The basic principle of the correlative interferometer involves comparing the measured phase differences with the phase differences obtained for a DF antenna system of known configuration at a known wave angle. As we have seen already, the average angle of arrival from which most of the RF energy arrives at the receiver is toward the true direction of the emitter.

In general, AOA-based systems provide three different methods to locate emitters:

• Triangulation requires a network of two or more DF stations (typically, this is not available).
• Homing is based on manual direction finding, usually by means of a handheld directional antenna (this is the most popular method applied to interference hunting but very vulnerable to multipath propagation).
• Running fix, the method used for DF of interferers from a moving vehicle combines bearings of the same emission from different locations to conduct offline triangulation.

The performance of AOA-based radiolocation systems is essentially independent of irregularities in the radiation pattern of the emitter and has the ability to manage different signal modulation types and bandwidths without degrading its performance. Additionally, AOA-based systems are immune to changes in power level radiated by the emitter. The AOA method does not require path loss models and therefore is not affected by changes in the propagation environment.

Required equipment

Automatic direction finding by means of AOA requires a complex receive antenna system consisting of multiple antenna elements and RF components, a GNSS receiver and a PC capable of running software such as R&S®MobileLocator that can process hundreds of DF results per minute and produce a map display of the results

The receiver itself, such as the R&S®DDF007 portable direction finder or R&S®PR200 portable monitoring receiver is able to quickly detect, analyze, and locate interference signals in real time. It uses FFT receiver technology and intelligent processing to bring a new expert approach to the challenge of locating and identifying sources of RF interference in mobile networks and can be used in combination with the R&S®MobileLocator software to generate heat maps and bearings of an emitter’s location in the most demanding environments.

Interference hunting in a nutshell

Although POA works well when you have a clear line of sight to your emitter, for urban interference hunting, AOA outperforms POA in terms of speed, accuracy and effectiveness. In addition, AOA is more adaptable to external factors such as irregular transmit antenna patterns, changes in the wave propagation environment and different modulation schemes and bandwidths. Therefore, although interferometer DF systems such as AOA require more sophisticated equipment, they provide the most efficient, robust and adaptable mean of geolocation of interferers in the city.

For more in-depth reading, download our white paper “Why AOA outperforms POA in urban interference hunting”.