Identifying and rectifying interference issues in a mobile environment is a challenging and critical task. Minimizing sources of RF interference is essential for maintaining high-user quality of experience throughout the life cycle. VIAVI offers an exceptional selection of intuitive, industry-leading interference hunting solutions to resolve these issues quickly and reliably.
What is Interference Hunting?
Interference hunting is the process of locating, identifying, and eliminating unwanted sources of interference that degrade the quality of wireless reception on the receiving end. As more devices and users are introduced within the limited RF spectrum, minimizing the impact of interference becomes more challenging. Interference hunting tools and practices put advanced technology to work to reduce the impact on network capacity and subscriber QoE.
Interference issues in wireless networks can result in a higher noise floor on the received channel and a lower signal-to-noise ratio (SNR) that degrades signal quality. Mobile users near a source of interference will experience lower data throughput, limited range, more dropped calls, or poor voice quality.
Types of Interference
Causes of interference can range from damaged RF cables and antenna hardware to channel overlap and timing issues, among a growing list of potential culprits. The variety of interference sources and the need for specialized interference test and measurement methods make it important to distinguish between interference types.
Radio Frequency Interference
Radio frequency interference (RFI) is defined as the effect of unwanted energy from emissions, radiation, conduction, or induction upon the reception of a radio communication system. Radio frequency interference dates to the earliest telegraph and radio communication in the United States when unwanted humming, static, or service interruptions caused by external radio waves or electrical activity were first observed. The causes of RF noise include electrical energy, environmental conditions like thunderstorms, internally generated instrumentation or “thermal” noise, and man-made noise caused by out-of-band emissions, harmonics, and spurious signals.
RF signal interference sources fall within the broad sub-categories of intentional or unintentional radiators of RF energy. Both interference types can contribute in aggregate to the noise floor, making interference analysis more challenging.
- Intentional interference can be produced by everyday objects like portable radios, baby monitors, and cell phones as well as inadequate RF designs or cell tower co-siting practices that adversely affects a receiver’s sensitivity. This category also includes malicious interference signal types and practices such as jamming. Powering down cell phones during takeoff and landing is an example of protecting aircraft communication systems from intentional interference.
- Unintentional interference is generated by devices that produce RF energy as a byproduct of their operation, providing an opportunity for this radiated energy to be “leaked” out to the nearby area. Potential sources of unintentional interference include computer monitors, ovens, and electrical motors. Products which contain a digital timing component (oscillator), including laptop computers and tablets, can also create unintentional interference.
Passive Intermodulation (PIM)
Increased traffic within the finite RF spectrum has inevitably led to higher signal density. Multiband operation has helped to alleviate this bottleneck. However, a condition known as signal interference can occur when multi-frequency RF signals contact corroded hardware or joints between dissimilar metals on or around the cell tower.
The resulting passive intermodulation (PIM) can cause signals from separate wireless communications to be mixed. This can impact mobile networks by lowering the SNR, increasing the bit error rate (BER), and decreasing throughput.
Co-Channel Interference
Another interference category that is a symptom of congestion as operators share and compete for bandwidth is known as co-channel interference (CCI). As the name implies, this problem occurs when multiple signals are present over the same channel. Overlap in cellular coverage patterns can lead to co-channel or on-channel interference, although most cell phones are designed to process at least five simultaneous signal paths without any issues.
- Time division duplex (TDD), with both the uplink and downlink transmission operating over the same frequency, can create interference if stringent timing and synchronization requirements are not met. With more channels being shared, more diligence and frequency planning in network operations is necessary to prevent co-channel interference from disrupting the user experience.
- Adjacent channel interference (ACI) occurs when neighboring frequencies “bleed” into one another. ACI can be caused by improper filtering or tuning. Appropriate guard banding between adjacent channels can mitigate this issue and prevent the loss of bandwidth or noise caused by adjacent interfering signals.
How to Detect RF Interference
Detecting RF interference begins with recognizing the symptoms and analyzing the data. A review of alarms, KPIs, and logs can often reveal whether a weak signal, elevated receive noise floor, or high bit error rate could be due to a hardware failure or configuration issue, rather than interference.
Interference hunting requires a versatile test and measurement tool kit to facilitate the quick location and diagnosis of a wide range of RF interference sources. Among the most useful RF interference hunting practices is the use of a spectrum analyzer at the cell site to determine whether the interference is being generated internally or externally. The spectrum analyzer can be used to analyze any interference signals within the receiver’s bandwidth which are not filtered out. Even if signal strength remains strong, a high noise floor or any intermittent noise could be compromising signal quality.
A real time spectrum analyzer provides advantages over traditional swept tune spectrum analyzers because the continuous overlapping data capture process prevents any intermittent or transient signals from being missed. These elusive signals can be important clues in the radio interference hunting process. Persistent spectrum display allows the rapid data collection of real time spectrum analysis to be presented graphically, using color or brightness to signify the probability of signals appearing at a given frequency.
Once the nature and intensity of the interference has been characterized through a network test and on-site analysis, the position of the source must be identified. The VIAVI InterferenceAdvisor is an intuitive mobile solution that allows an RF engineer or technician to locate an interference source with minimal effort.
A CellAdvisor base station analyzer, or CellAdvisor 5G, connected to a broadband Omni-antenna collects and analyzes signal interference data in transit. Fully automated EagleEye™ software running on a cable-free Android tablet then provides visual and voice prompts to guide investigators to the most probable area of interference origin, saving valuable troubleshooting time. The InterferenceAdvisor system also features gated sweep control and display for TDD interference frequency hunting.
After the geographic area associated with the interference source has been narrowed down, the VIAVI AntennaAdvisor can be used to perform radial signal analysis and generate intersection vectors to triangulate the precise location of unwanted interference. Working in conjunction with the powerful real time spectrum analysis capabilities of the CellAdvisor 5G base station analyzer, rogue RF emission sources can be diagnosed, located, and eliminated quickly.
How to Measure Interference
Measuring interference accurately is the key to correctly diagnosing the type, source, and probable cause. Each wireless communication technology operates within an expected bandwidth and noise power level, measured in dBm. Any type of interference can affect the noise level, making it harder for the RF network to transmit efficiently or maintain adequate coverage at the edge of the network.
- Low-level signal measurement is essential for interference hunting and is directly impacted by the spectrum analyzer settings. Utilizing a low noise/high gain preamplifier, narrowing the resolution bandwidth (RBW) of the RBW filter, and minimizing the input attenuation can improve sensitivity considerably. The dynamic range of the RF signal analyzer, specified in dB, is also important for interference hunting applications because the expected and unwanted RF are best evaluated side by side.
- Wireless networks that continually exhibit spikes greater than -60 dBm above the noise floor or constant amplitudes greater than -80 dBm within the operating band of the spectrum are probably experiencing some form of RF interference. Each source of interference produces a unique signature that experienced RF engineers and technicians use to determine the most probable root cause(s). Spectrum analysis overlayed with network test data such as throughput or BER can be used to quantify the impact of interference on performance.
5G Interference
5G introduces new interference hunting challenges through cell densification and the range limitations of the millimeter wave (mmWave). 5G network operations also leverage frequency division duplex (FDD), dynamic TDD, and other frequency sharing options to optimize throughput and scheduling.
- Interference hunting for 5G requires real time spectrum analysis to capture complex interference signals overlapping the 5G NR signal. Exceptional bandwidth and dynamic range are needed for 5G beam and interference analysis in the FR1 (sub-6GHz) and FR2 (mmWave) frequency ranges. Persistent display is a useful feature for distinguishing 5G uplink interference signals that traditional swept tuned methods can miss.
- RF over CPRI (RFoCPRI) technology uses the CPRI interface to verify control signals and extract user plane traffic or RF (IQ) data. This allows interference signals on mobile devices (uplink) and radio performance data (downlink) to be monitored and analyzed thus simplifying 5G interference troubleshooting and minimizing cell tower climbs.
- The T-BERD/MTS-5800 handheld network tester utilizes advanced RFoCPRI capabilities to identify PIM and other interference issues from an optical access test point. The industry’s smallest 10G handheld network test instrument also supports emerging 5G technologies through eCPRI test workflows, automated Ethernet service activation, and remote radio head testing from the wireless base station.
