The Art and Science of Interference Hunting. Paul Denisowski, Applications Engineer

Transcription

1 The Art and Science of Interference Hunting Paul Denisowski, Applications Engineer

2 Topics Overview All about interference Interference hunting tools Identifying and analyzing signals Directivity and propagation Importance of antennas Fundamentals of direction finding Q&A / discussion

3 The changing wireless world More transmitters spectrum is becoming more and more crowded. More mobile devices stationary transmitters are no longer the norm. New modulation types analog signals becoming less common than digital signals. More complex modulation higher order modulation requires a better RF environment. Spectrum refarming moving services to different frequencies requires spectrum clearing and involves different propagation and interference types. Wireless connectivity from nice to have to must have.

4 Effects of interference Interference affects different signals in different ways. Analog audio suffers from noise, static, superimposed audio, etc. In cellular systems interference causes poor voice quality, dropped calls, low data throughput. For data services there is often the inability to initiate, sustain, or use connections at optimal rates. Severity of the effects may not increase linearly with the level of interference, especially for digital modulation (the cliff effect ). Pixilation in a digital video signal

5 Modulated sources Modulated sources are devices which are intended to generate RF signals. Problems occur when these devices are faulty or are operated incorrectly. Even signals from well-behaving transmitters may produce interference due to harmonics, intermodulation, overload, etc. Because modulated sources are meant to carry information, extracting this information can help us identify the signal source.

6 Interference from modulated noise sources Since modulated noise sources usually have a bandwidth of < several MHz or khz, they appear as relatively narrow signals. These signals create interference either by being superimposed on another signal or by generating harmonics, intermodulation, etc. Sometimes these signals will exceed their normal spectral limits due to malfunctions (e.g. a broken Tx filter) or improper operation (overamplification, frequency instability, etc.). A narrowband intermittent source interfering with TV channel 48

7 Harmonics A harmonic of a signal is a copy of that signal appearing at a whole number multiple of the original (fundamental) frequency. For example, a transmitter at 155 MHz can produce harmonics at 310 MHz, 465 MHz, etc. Always check to see if a copy of the interfering signal is a harmonic of another signal. The 4 th harmonic of a signal at MHz appears at 781 MHz (LTE Band 13)

8 Intermodulation Intermodulation results from two or more signals appearing in a nonlinear circuit. Sum and difference frequencies are created from the mixing of fundamentals and harmonics. Because intermod involves the mixing of multiple signals, it will only occur when all component signals are present. Signals at 440 MHz (f1) and 445 MHz (f2) produce intermod products at 435 MHz (2f1 f2) and 450 MHz (2f2 f1) MHz

9 External rectification ( rusty bolt effect ) The junction between two pieces of metal can create a rectifier (diode), especially when corrosion is present. This effect can generate spurious signals that are then radiated by metallic elements in the joint. Towers and guy lines are a good starting point, since they can rust, have long metal elements, and are close to powerful transmitters. Utility poles/wires, metal fences, and gutters are also prime suspects.

10 Repeaters / BDAs Cellular repeaters or bidirectional amplifiers (BDAs) can be used to extend cellular coverage in buildings or in fringe areas. May also be installed on boats. The main interference issues are the retransmission of unwanted signals at the input of the BDA as well as malfunctioning BDAs. Difficult to troubleshoot but a very common source of interference in the cellular bands.

11 Umodulated sources Unmodulated sources are devices which unintentionally generate RF signals. Common sources are electric motors, faulty transformers, vehicle ignition systems, electrical fences, fluorescent lighting, etc. Easy to recognize, often shows up as jumps in the noise floor or a wide, random spectral pattern. Noise in the aircraft band (generated by a nearby electrical motor)

12 Deliberate interference Deliberate interference may be narrowband (e.g. talking on a public safety frequency) or broad-band (jamming). Pirate or unlicensed ( freeband ) operations can also cause issues to licensed users. Sources may be mobile, possibly to avoid detection / radiolocation. Although most businesses and individuals are very cooperative in resolving interference, deliberate interferers will usually deny or conceal their activities. Mobile GPS jamming device

13 Recognizing jammers Jammers are typically easy to identify and locate : strong, broad, always-on signal. Tend to increase the noise floor even outside of their nominal operating range.

14 Interference Hunting Tools Two primary tools : spectrum analyzer and the monitoring receiver. Two major differences between spectrum analyzers and monitoring receivers : Internal architecture Heterodyne principle FFT (Fast Fourier Analysis) Operational features Spectrum analysis functions Task-oriented features Direction finding Offline, remote, and coordinated operation

15 Heterodyne principle The basis for most spectrum analyzers. Input signal converted to an intermediate frequency (IF) using a mixer and a local oscillator (LO). Signal is swept past a fixed-tuned filter to determine resolution bandwidth. Signal is then logarithmically amplified and passed to the display.

16 FFT-based analysis The basis for most monitoring receivers. Converts time domain signals into frequency domain signals using a Fourier transform. Digitizes a sampled signal and applies the Fast Fourier Transform (FFT). Very fast high probability of intercept (POI). Most spectrum analyzers do NOT use FFT analysis : poor dynamic range over large frequency ranges.

17 What does a spectrum analyzer do? Make high accuracy measurements of known signals. User must specify a wide variety of parameters and settings. Measure and display modulation characteristics (e.g. EVM, ACLR). Verify conformance to a standard (e.g. SEM). Heterodyne (swept) principle : very precise measurements, but information may be missed/lost during a sweep. Usually connected to a cable, not to an antenna. Primary use is in a lab / production environment.

18 What does a monitoring receiver do? Makes rapid measurements of unknown signals. Designed for speed and high POI. Signals can be demodulated and monitored. Can make measurements at discrete frequencies (scanning). Received signals almost always impaired/distorted, so features like attenuation, AGC, preselection, etc. needed. Can often be integrated into a direction finding system.

19 Understanding propagation A good knowledge of radio propagation is vital in localizing interference. The distance at which a signal can be received is a function of both power and frequency. Signals also have different characteristics based on frequency. These include multipath, directivity and penetration. Spectrum refarming means that services traditionally found in one portion of the spectrum may now be found in a different portion with different propagation characteristics (700 MHz LTE, digital TV).

20 Directivity and penetration Generally speaking, higher frequencies (GHz range) tend to be line-of-sight and more easily reflected. Penetration of signals into structures can be poor depending on building composition. Lower frequencies (VHF/UHF) may refract or bend around structures and these frequencies penetrate well into buildings. HF signals can propagate for great distances depending on ionspheric conditions. Ground-wave propagation is also possible. Naturally, penetration also is very dependant on transmitter power.

21 Multipath Multipath means receiving a signal from different directions simultaneously. The severity of multipath is also a function of the frequencies involved. Caused by reflections, most commonly in an urban environment. Multipath can make direction finding difficult. Careful selection of monitoring / DF sites can reduce the impact of multipath.

22 Antennas Radio transmitters require antennas. Antenna design/size is related to frequency and function. Antenna direction also useful in identification. Even if an antenna is not in active use, it can act as a reradiator. Check the antenna site for signage and other clues as to owner/purpose. Antennas may be hidden or disguised. Jammer disguised as a pack of cigarettes Cell phone repeater antenna

23 Identifying signals Some of the more common ways of identifying signals are : Pattern of interference Audio demodulation Spectrum characteristics Signal analysis and digital demodulation Online resources Direction finding Signal splatter (overmodulation) Carrier drift

24 Looking for patterns Outdoor sources may change based on weather conditions Important questions to ask in analyzing interference : When does the interference occur? Is the interference constant or intermittent? Does the interference coincide with any other events? If possible, attempt to see if interference can be eliminated by disconnecting or powering down transmitters or other electrical devices. For some types of signals, propagation will change depending on time of day or season.

25 Audio demodulation Audio demodulation means listening to the signal Modulation generally is AM or FM, but there are variations of these (narrow band FM, single sideband, etc.) Recording signals for later analysis / documentation is useful What are we listening for? Station ID / call signs Language and content (what are they saying?) Morse code IDs Even digital signals can sometimes identified this way

26 Spectral analysis The most fundamental display in interference hunting is a spectral display (amplitude vs. frequency). A max hold function is also important in detecting short duration signals or looking for an elevated noise floor. Max hold (red) shows maximum signal amplitude for frequency and also indicates signals that are not currently present

27 Waterfall analysis A waterfall display shows frequency, time, and level information and is extremely useful in analyzing signals. Signal width of 870 MHz signal is 1.4 MHz Duration of jamming signal exactly 9 seconds (T1 T2)

28 Signal analysis / digital decoding The ratio of digital to analog transmitters is increasing at an exponential rate Signal analysis involves modulation type, bandwidth, baud rate, etc. Much easier to locate a digital signal if we know what kind it is.

29 Online Resources The Universal Licensing System of the Federal Communications Commission (FCC) provides access to a database of all licensed transmitters. Searchable by location, frequency, call sign, etc. URL :

30 Example of ULS Results Search results contain license information, contact address and phone, transmitter location, power output, etc.

31 Direction Finding (radiolocation) If a signal cannot be identified via demodulation or signal analysis, locating its source is often the only way to resolve interference DF requires a receiver and a directional antenna. Mapping and triangulation software are also extremely helpful Understand the strengths and weaknesses of various DF techniques and equipment Triangulation of a transmitter based on multiple DF bearings

32 DF equipment DF can be performed using fixed, mobile (vehicle) or portable (handheld) units, or a combination of these. Fixed / mobile units are most useful during initial hunting and when covering large geographical areas Portable units are used best used for the last hundred meters, as well as in cases where vehicle access is not practical

33 DF antennas Directional antennas are a critical component in DF. Handheld antennas should have good directionality and wide bandwidth. Integrated compass/gps allows for greater accuracy when determining location and bearings. More advanced DF methodologies such as Watson-Watt or Correlative Interferometry usually require more elaborate antennas, but provide much higher accuracy. Typical gain pattern of a handheld DF antenna note the directivity Vehicle mounted interferometer

34 Recording bearings For results to be useful, we need to record both position and bearing (azimuth) information. Traditionally, this information was calculated / recorded manually (e.g. using a magnetic compass and a map/ruler) but this is prone to human error. Modern equipment can use GPS and electronic compass data for an automated determination. An example of a handheld DF antenna with an integrated electronic compass and GPS receiver

35 Triangulation The main reason for recording bearings is to combine them in order to locate a transmitter. This is known as triangulation. Bearings are plotted (manually or automatically) on a map, and the transmitter is near the intersection of the lines. Automatic triangulation calculation, including calculated uncertainty radius

36 DF best practices Rotate handheld antennas slowly. Tripod mounts can also help obtain stable / reproducible readings. Binoculars or a telephoto lens are extremely useful for finding and identifying antennas. A good DF location is one away from obstructions. Higher is usually better. Rooftops and top floors of parking garages are good urban DF sites. Be aware of the influence of reflection / multipath.

37 Q&A / Discussion