Interference Hunting

Remote Spectrum Monitoring Benefits in Maintaining Communication at Maritime Ports

June 13, 2018

Maritime ports are a major component of the nation’s freight transportation system, as they handle 75% of America’s international trade by volume, according to an annual report to the United States Congress. Moving the estimated $900 billion worth of goods transported to and from the U.S. each year by container ships requires tremendous synchronicity between arriving and departing ships, as well as the trained personnel at ports responsible for smooth entry and exit.

This doesn’t even take into account the thousands of cruise ships whose ports of entry and departure stretch the east and west coasts, as well as the Gulf of Mexico. Maintaining the safety of passengers, as well as goods being imported and exported requires reliable communications between those aboard the vessels and the ones on land.

Mobile networks are key to the success of the maritime industry because they provide the means by which maritime safety data, ship position reporting and weather forecasts, as well as other information, can be passed to ships at sea. Remote spectrum monitoring tools are being implemented that provide cost-efficient methods of ensure the operation of these critical networks.

Reliable communications are no easy task due to the complex RF environments associated with our nation’s ports. From operational considerations such as in- and out-bound vessels, dockside logistics, and freight movement to safety issues, including perimeter management and dockside protection, numerous signals are being transmitted. There is also the issue of illicit signals being transmitted that jam transmission – whether intentional or not – that can wreak havoc on port operations.

Unintentional Interference

Radio frequencies are typically allocated by country, so devices built for one country or region can cause interference in other regions of the world. Many channels are utilized in open water which can cause interference for port operations. Often, devices onboard a ship arriving from across the ocean are mistakenly left on when the vessel enters a port of call. The potential for adverse effects increases as the boat approaches the shoreline. During these times, laws and regulations of the host port are applicable. If an electronic device that is not authorized for use is energized while the vessel is docked and waiting for unloading and loading, the resulting interference could cause multiple issues and service outages.

Of course, in today’s world there is also the threat of intentional interference by nefarious groups who wish to cause disruption or worse to our maritime ports. Early detection and geolocation of these sources can greatly reduce or eliminate interference that can hinder network performance and upset port operations.  

Remote Monitoring Solution

Government regulators often use outdoor monitors, such as the Anritsu Remote Spectrum Monitor MS27102A (figure 1), along with its Vision™ software, to enforce spectrum policies and protect our ports. These remote tools provide early detection of suspicious signals and conditions, among many other capabilities, including:

• Geolocation of interference signals
• Maintain a history of spectrum activity
• Set power threshold levels to automatically generate alarms
• Automated reports on network health

Figure 1

Many ports will install three or more remote spectrum monitors at strategic terrestrial locations to achieve precise geolocation. Such a configuration makes it easier and more efficient to identify a vessel or group of ships that are causing interference. Deployment of the proper spectrum monitoring solution will allow maritime professionals to take swift and appropriate actions to restore optimal network operation.

Performance Parameters

There are a few essential criteria that should be considered when choosing a remote spectrum monitor for maritime ports. Considering the outdoor environments where probes are positioned in a permanent or semi-permanent location for radio surveillance and monitoring, durability is critical. Effective tools should also be capable of sweep rates up to 24 GHz/s, which allows for the capture of many signal types, including periodic or transient transmissions, as well as short “bursty” signals. Another critical specification is 20 MHz instantaneous FFT bandwidth to capture wideband, real-time signal activity for subsequent post-processing

To learn more about remote spectrum monitoring for maritime applications, you can download this free application note. 

Tips to Manage Interference in Densified Networks

May 23, 2017

Mobile operators continue to densify their networks to meet the ever-growing consumer demand for high-bandwidth services. They have deployed DAS, small cells and similar solutions to maximize the RF spectrum and maintain customer satisfaction as mobile usage continues to rise.

The integration of these solutions helps to alleviate bandwidth pressure points on mobile networks, helping them achieve stated KPIs set forth by operators. For field technicians and engineers responsible for the performance of these networks, test equipment remains a priority. To meet the verification processes associated with DAS, macro cells, and small cells, test instruments need to evolve and offer new capabilities.

In today’s post, we will answer questions about network densification and how test solutions are meeting the challenges they have created.

Q: As networks densify with DAS, small cells and other solutions, how is testing evolving?

A: Densification makes capacity increasingly important, especially with LTE. Because of this, the focus is on received signal quality and spectrum assurance. There are a variety of ways to ensure a clean uplink to significantly improve capacity. This focus on capacity also means, in some cases, new measurement techniques are required. DAS systems, such as the one shown in figure 1, are a prime example of shifting measurement techniques.

Figure 1

Q: What makes DAS testing different from other network elements?

A: The number of tests drastically multiplies in DAS. Typical tower installation tests consist of antennas, splitters and other passive RF components. These tests can have up to 150 sweeps to determine return loss, distance-to-fault, cable loss, and PIM. Some additional fiber tests may also be necessary. These tests can be done with traditional processes.

There are considerably more tests associated with DAS, as procedures may require testing three or four frequency bands on every cable. Even a medium-sized DAS install could require several thousand tests. For example, a football stadium can have as many as 15,000 tests, which means that the testing, troubleshooting, and reporting process can take months.

Q: How will testing transform to address the increased complexity of DAS?

A: Because of the large number of tests needed, existing processes based on tower techniques don’t scale, so new procedures need to be developed. Automation of field tests, as well as reporting, is currently possible, and is one big step towards making DAS testing more effective.

Q: How do you deal with the increased amount of interference opportunities brought by densification?

A: The first step is to understand that interference is a receive issue. Signals that reach a radio’s receiver will affect the radio’s front end, even if these signals aren’t wanted. These unwanted signals create a reduction in the radio’s sensitivity, causing increases in bit error rate (BER), frame rate, dropped calls and other negative effects. This occurrence is called receiver desense or desensitization. If severe enough, it’s known as receiver blocking.

Interference does not have to be on a receive channel. It just must arrive at the input of a receiver, and it can be on a different frequency than the receive channel. In other words, it can be in band, but it doesn’t have to be on channel. This makes the receive pre-filter very important, as it can be selected to block out-of-channel, in-band, signals. If an unwanted signal does reach the receiver’s input, however it can cause desense.

Q: How do interference sources change from macro cells to small cells?

A: The biggest difference between macro cells and small cells is that small cells are likely closer to the source of interference. Since RF signals exhibit path loss, the closer a receiver is to the interference, the stronger the interfering signal will be. Small cells, because of their multiple locations, will be closer to some RF sources. They will be affected by interference that would not affect macro cells.

Anritsu offers various solutions for conducting more efficient interference hunts. Starting with the simple methods, there are traditional direction-finding tools. Using a spectrum analyzer and a Yagi antenna (figure 2), the user can look for the strongest signal and triangulate. There are also tools that can actually put a map on a spectrum analyzer. A higher level is car-based signal location, where a user can drive around and seek the strongest signal. This method tends to lessen the effect of multi-path, an issue in dense urban areas where small cells are likely to be deployed.

Figure 2

Another solution is remote monitoring, because interfering signals aren’t always present. A signal’s occurrence times can be characterized through short-term monitoring. All Anritsu spectrum analyzers are web-enabled, meaning they can be controlled from a distance via web browser. So, a user can configure one at a site and leave it for weeks. They can check the site remotely to save time and money, and maintain network performance.

There is also long-term monitoring. A set of headless spectrum analyzers can be either temporarily or permanently put in place. Software for these devices allows users to monitor and maintain signal quality through their networks as aspects change. The devices can be comfortably monitored remotely.

Q: What impact does densification have on spectrum assurance?

A: Spectrum assurance is an umbrella term for a family of spectrum monitoring, interference hunting and signal mapping tools that can be used in-building and outdoors. Anritsu is developing these in response to our customers’ requirements for the capacity associated with LTE and densification.

Q: Where do you see testing in the future?

A: We’re going to be moving closer to real time. Right now, cloud-based solutions are tremendously efficient. The test automation for DAS is cloud-based and that will continue. There will be more control, remote expertise and remote dispatch in the near future, as well.

Q: Monitoring allows you to identify and fix a problem. Can it also allow operators to optimize the use of the network resources?

A: Yes. One example is a service-assurance solution such as Master Claw, a big-iron software application. Another is a software application that works with long-term RF monitoring probes to help characterize when interfering signals occur and how they appear. Through triangulation, this application helps determine the interfering signal’s location and provides all the necessary information to find it.

There is much to learn about DAS, interference and the test tools necessary to address network densification. Anritsu has published an application note that explains DAS testing procedures, as well as an interference hunting application note. Anritsu has also designed a poster that provides examples of interference sources. All are helpful resources for field engineers and technicians.

Tools for Network Operators to Protect Spectrum Investment from Costly Interference

December 13, 2016

Cellular operators have spent billions of dollars purchasing rights to use allocated frequency spectrum. Carriers and other spectrum users – such as public safety organizations, military branches, and broadcasters – all need to protect their investments and quality of service from interference.

The importance of keeping networks free of interference will only become more imperative. Bandwidth usage is exploding, evident by reports that global demand for wireless capacity will increase six-fold by 2019 (figure 1). This will raise the stakes on carriers and the like to identify and repurpose under-utilized spectrum. It will also mean that mitigating interference is not only currently a top priority for optimizing network performance and maximizing revenue, it will be for years to come. To maintain spectrum integrity and optimize network performance, field engineers and technicians will need to utilize various tools.

  Figure 1

Spectrum Monitoring

Keeping a virtual eye on the spectrum 24/7/365 is a necessity in today’s wireless world. Spectrum monitoring tools discover and facilitate the removal of interference sources by alerting operators of patterns of unwanted signal activity, characterizing unwanted signal behavior, and locating the interference source. Systems such as the one shown in figure 2 generate a greater return on the spectrum investment; maximize network capacity; automate methods of surveillance, interference detection, and policy enforcement; and deliver greater flexibilities and cost efficiencies to network management.

  Figure 2

There are other benefits realized with spectrum monitoring. It can be used to characterize spectrum occupancy so operators can determine usage rate for targeted frequency bands, as well as to identify under-utilized spectrum for potential sharing or repurposing.

Spectrum monitoring is an invaluable tool for satellite operators, government regulators and mobile carriers. For example, satellite operators may have to pay a penalty if customer service is jeopardized. This is particularly true for satellite farms home to TV/radio stations, corporate networks, maritime and in-flight systems, and mobile communications providers. Spectrum monitoring tools can monitor modulation error ratio (MER) and trigger alarms when there is a threshold violation to help maintain specified performance.

Government regulators can use spectrum monitors to identify unlicensed use of spectrum, interference from adjacent frequency bands, and spectrum occupancy. Other applications also include illegal transmissions (i.e. prisons, military installments, borders) as well as to detect drones and their location.

Spectrum monitoring may have the biggest payback for cellular operators for spectrum assurance. In the recent AWS-3 auctions for 65 MHz of spectrum in the U.S., $44.9 billion was raised. As gaudy a number as this is, it falls short of similar auctions in the U.K. and Germany, which generated $50 billion and $98 billion, respectively. Spectrum monitoring tools help prevent interference from lowering the return on this sizeable investment.

Software

Spectrum monitoring can deliver the biggest benefits if dedicated software is part of the overall solution. A standard software tool will allow for remote control of multiple spectrum monitors and automated measurements, as well as provide valuable insights such as spectrum history and geo-location. Advanced tools build off the standard model by adding demodulation capabilities for signal analysis and identification. Metrics such as C/N, EVM, MER and identification codes can also be established by these higher-end software tools.

Mobile Interference Hunting

In cases when field engineers and technicians need to be deployed, they have the benefit of advanced interference hunting tools. Traditionally, a spectrum analyzer and a Yagi antenna are the equipment taken into the field. Directional antennas have trouble differentiating between the direct signal and reflections, and can lead even the most experienced user astray. They also have issues with RF shadows caused by buildings or terrain. Multi-path affects them as well, leading to somewhat erratic power measurements as the antenna is moved around.

Mobile interference hunting tools such as the one in figure 3 change the entire process. It takes many measurements per minute, averages them, and plots the result – all while the user is driving. There is no need to stop the car, get out and take a bearing, drive to a new location and repeat the process. Because so many power measurements are taken and averaged, multi-path does not affect the results. Reflections tend to be eliminated, because the reflected signal travels a longer path and so has more path loss, as well as loss when reflected. RF shadowing becomes apparent, since areas of low signal power can quickly be spotted, and either allowed for or ignored. Also, since the better mobile interference hunter tools use channel power to conduct measurements, they can locate signals that wander in frequency, such as oscillating cell phone boosters.

 Figure 3

Tools such as the ones outlined in this post can help safeguard against interference that can lower the return on the massive spectrum investment. To learn more about these tools, visit our spectrum monitoring and mobile interference hunting pages.

Test and Measurement Strategies for LTE-Unlicensed

April 12, 2016

To even a casual observer, it’s obvious that the world’s appetite for wireless data is voracious and seemingly never satisfied. Globally we consumed 8 Exabytes per month in 2014. By 2020 the monthly average will spike to 71 Exabytes (figure 1), and there simply isn’t enough licensed spectrum to support this amount of growth.

Global mobile data consumption will reach 71 Exabytes per month by 2020 [Source: “Carrier & Public Wi-Fi, July 2015, Mobile Experts LLC] Figure 1

Licensed spectrum is also very expensive – commercial wireless carriers in the U.S. spent a record-setting $49 billion on spectrum in the AWS-3 auctions, and it’s expected that competition (and pricing) for the upcoming 600 MHz auction will easily break that mark. Given these economics, it should come as no surprise that commercial carriers have been aggressively moving forward with plans to deploy a modified form of LTE into the unlicensed bands.

A Potential Solution: LTE-Unlicensed

Known generally as LTE-Unlicensed, the technology is an adaptation of standard LTE that’s been modified to help it co-exist with the standards we normally associate with unlicensed spectrum – notably Wi-Fi™. LTE-Unlicensed uses techniques such as transmitter duty-cycling (ceasing transmission for a period of time) and listen-before-talk (avoiding transmission when other RF signals are detected in the channel) to co-exist with IEEE 802.11ac systems and other incumbent technologies.

LTE-Unlicensed will be part of the upcoming 3GPP Release 13 standard, and will be deployed in the U-NII (Unlicensed National Information Infrastructure) bands (figure 2) that span most of the 5 GHz spectrum. Trials are already underway in the U.S., and once the standard is released wide deployment is expected over the next two years – primarily in dense urban centers and public venues, such as shopping centers and entertainment arenas and stadiums. For IT managers tasked with tracking down interference to existing wireless networks, identifying LTE-Unlicensed signals will require some creative testing strategies.

The U-NII Bands at 5 GHz offer different channel bandwidths to both Wi-Fi and LTE-U Figure 2

Spectrum Analysis Techniques

Because of its transmitter duty-cycling and listen-before-talk behavior, LTE-Unlicensed cannot be analyzed by existing LTE analyzers. Eventually, solutions for direct testing of LTE-Unlicensed will be available, but for now testing will need to be done using general purpose test equipment such as spectrum analyzers like the one shown in figure 3. Unfortunately, LTE-Unlicensed and 802.11ac Wi-Fi are both OFDM (Orthogonal Frequency Division Multiplexing) signals, which support high-order modulation such as QAM. Channel bandwidths for both LTE-Unlicensed and 802.11ac can vary from 20 MHz to 80 MHz.

Figure 3: Anritsu Spectrum Master handheld analyzer

When viewed on a spectrum analyzer, OFDM signals will produce a characteristic “Bart’s Head” trace, but without demodulation it’s impossible to determine if an observed OFDM signal is LTE-Unlicensed or 802.11ac. Combining a spectrum analyzer with a smartphone or tablet Wi-Fi sniffer app (figure 4) is a workaround for this – if you see a Bart’s Head on the spectrum analyzer and your Wi-Fi sniffer doesn’t show a Wi-Fi SSID for that channel, it’s probably an LTE-Unlicensed transmitter. Be aware that not all Wi-Fi sniffer apps will identify SSIDs set to non-broadcast, so be sure to get an app that supports non-broadcast detection.

Smartphone Wi-Fi analyzer apps can help rule out LTE-Unlicensed signals Figure 4

Also, since both LTE-Unlicensed and 802.11ac transmissions are not 100% duty-cycle, you should choose a high-quality spectrum analyzer capable of tracking transient signals, such as the Spectrum Master™ MS2720T.

Anritsu offers a number of solutions and educational tools to help field engineers and technicians hunt and locate interference sources. To learn more, visit a specialized interference hunting web page.

Remote Spectrum Monitoring Systems Help Optimize Network Performance

November 18, 2015

Operators have spent billions of dollars acquiring RF spectrum to increase capacity of their networks to best deliver high-bandwidth 4G services. Unfortunately, the return on that sizeable investment can be put in jeopardy by the presence of interference that can degrade network operation and frustrate consumers.

To optimize performance, reduce customer churn, and maximize their network investment, operators need to eliminate (at best) or minimize (at least) interference. As field engineers and technicians know all too well, this is easier said than done, due to the presence of illegal or unlicensed signals that can be periodic or present at different frequencies over time. For these reasons, a spectrum monitoring system is a valuable investment.

There are a number of benefits associated with a spectrum monitoring system, which is specifically designed to facilitate the identification and removal of illegal or unlicensed interference signals. Most importantly, by monitoring spectrum on a continual basis, problem signals can be identified as they occur in real time. Patterns of unwanted signal activity can also be examined, providing an efficient way to characterize and locate the source of the interference problem.

Monitoring these frequencies with the best possible tools will give operators the information they need to optimize spectrum for maximum utilization. Spectrum monitoring systems can detect a wide variety of signals. In this post, we will discuss three main categories – intentional interference (including illegal or unlicensed broadcasts), accidental interference, and occupancy.

Intentional Interference

Illegal AM/FM and video broadcasts are found in many parts of the world. These signals can be generated by pirated broadcast equipment or by using over-powered CB radios. Figure 1 shows a table listing interference complaints per year registered by the UK government communications regulator. “Critical service” refers to interference reports affecting life services communications.

Figure 1 (click for full size)

Additionally, jammers are sometimes used for applications such as preventing students from cheating on tests, stopping employees from taking phone calls on company time or to prevent inmates from making illicit calls from prisons. Jammer signals can often leak into the wider environment, interfering with other legitimate services. Mitigating these types of interference has become a high priority with government regulators.

Accidental Interference

A wide variety of accidental interference can be seen in the RF spectrum. A common problem is cable TV leakage, which exists both from cable signals seeping into the outside environment, as well as from external signals bleeding into the cable system. This problem has been enhanced with the transmission of cable signals into frequency bands used by broadcasters and cellular operations such as the 700 MHz LTE band.

DECT phones also cause interference problems, particularly when people bring their wireless devices along when moving across borders. DECT frequencies vary in different countries, providing the potential for interference when transported. Figure 2 shows spectrum used in the U.S. for certain cellular frequencies. DECT phones brought by travelers from other countries can often cause interference. Other sources of interference include cellular signals (due to antenna tilt or azimuth errors), repeaters oscillating, wireless microphone problems, and power equipment.

Figure 2 (click for full size)

Occupancy

With the rapid demand for available spectrum from both public and private sectors, new ways are being investigated to allow more efficient use of various frequency bands. Much of the spectrum is potentially underutilized, providing the opportunity to re-purpose existing spectrum with additional applications. Spectrum occupancy measurements quantify the amount of usage of frequency bands over a given period of time (figure 3). Remote spectrum probes are used to monitor a band of frequencies to record spectral histories as a function of time.

Figure 3 (click for full size)

As you can see, interference can come from many different sources. Spectrum monitoring systems can be used to efficiently monitor all these signals and maintain optimal network performance. To learn more about remote spectrum monitoring tools, download this application brief.

Seven Questions to Answer Before Your Next Interference Hunt

 September 4, 2015

One of the biggest challenges associated with finding interferences is that it can come from so many sources. The cause can be from the inside the base station, due to poor connections, or external sources such as a rusty fence or jammer.

Engineers and field technicians equip themselves with a variety of test tools to help in this daily quest to find the cause of interference that can lower network performance. Today’s handheld spectrum analyzers have features such as Spectrograms to accurately analyze signals. A variety of antennas are available that can be easily attached to the analyzer to help track and locate interference sources. Recently, handheld interference hunters (figure 1) equipped with a GPS receiver, antenna, electronic compass, and a user-selectable preamplifier have been developed to further improve the process.  

 
figure 1

The most invaluable tool, however, is knowledge. Before starting any hunt, you should ask some important questions. The answers will probably help you locate the interference source and correct it before it causes dropped calls, slow loading and buffering issues that will frustrate operators and consumers alike.

 1.     Is It On-Channel Interference?

Operators often assign frequency or codes in a tiling pattern to help control cell overlaps. Factors that can cause excessive overlap and subsequent interference include:

• Antenna tilt
• Valleys
• Antennas mounted on high buildings
• Better than expected signal propagation over water
• Errors in frequency settings
• Excessive multi-path

The first three factors all have a common cause – the antenna is transmitting further than intended because it is aimed too high. Water allows radio waves to propagate better than over land.

Frequency setting errors are more prevalent than you might imagine. If a cellular operator has 500 cell sites and each cell has nine radios – not uncommon – there are 4,500 radios to maintain. If all of them are not set to the right frequency at all times, you can have interference.

Typically, phones can handle four or five signal paths and still get signal gain. If there are more paths, however, they are seen as interference.

 2.     Can In-band Interference Be My Problem?

If the interfering signal is within the pass band of the receiver’s receive filter, receiver desense can occur. These signals can be generated from other carriers, IMD products, or harmonics. The key takeaway is that a signal only needs to make it through any receive filter to the front end of the radio receiver to cause interference.

 3.     Are External Sources Causing Impulse Noise?

Impulse noise is created whenever a flow of electricity is abruptly started or stopped. A surprising variety of items can create impulse noise:

• Lighting suppression devices
• Electrical motors from elevators or the like  
• Electric fences
• Power lines, which may arc and spark
• Light dimmers

Most of these impulse noise sources affect the lower frequencies, generally below 500 MHz. Micro-arcing or fritting is the exception, since it is generated by the RF signal and can affect reception at any frequency. It is typically very broadband, measuring more than 1 GHz wide.

 4.     Can Harmonics Throw My Signal Off?

Governing bodies normally regulate the power level of harmonics, which are multiples of an RF carrier. Equipment, however, does fail or fall out of specification. Many of those failure mechanisms create high harmonic levels. Also, if the original broadcast is at a high power level, even legal harmonics can be powerful enough to create problems.

Another cause can be a damaged transmitter. If only half of the specified sine wave is transmitted, high power harmonics will be created across much of the RF spectrum. This type of harmonic display is called a comb or “picket fence” signal.

 5.     Is Interference Due To Passive Intermodulation (PIM)?

Also called the Rusty Bolt Effect, PIM is caused when two or more strong RF signals combine in a non-linear device, such as a transistor, diode, or even the crystals found in corrosion or rust. It can be caused by a rusty fence, rusty bolts, corroded rooftop air conditioners, or even a rusty barn roof. Of course, it’s also possible that loose connectors in an antenna feed line or poorly configured transmitters can be the cause.

PIM has become so prominent that we have established a separate blog, The PIM Source. You can learn much more about the topic by reading those posts.

 6.     How Close Am I To Finding A Near-Far Problem?

In the case where a wide area RF coverage is overlaid with a smaller area coverage, and the two operating frequencies are close enough to give receivers a problem, the nearby, in-band-but-off-frequency signal can overload a receiver trying to listen to the weaker signal. The near-far problem can also happen between cell towers, as long as the mobile device can’t make a handover. This may occur when a device from one operator is broadcasting a strong signal to reach a distant cell tower. If a cell tower operated by another operator is near the mobile device, that second carrier’s receiver may be temporarily desensed by the mobile device.

 7.     Is Someone Causing Intentional Interference?

Intentional interference sources can be caused by jammers. This is a common issue in shopping malls, where employers want to ensure employee productivity, as well as in cars or at military bases. Recognizing this fact early on can help when conducting a hunt in these environments.

Once you have answers to these questions, you can select and use the test solution(s) most effective to locate interference sources in your particular scenario. This can be a handheld spectrum analyzer or a mobile interference hunting system such as the one shown in figure 2.

 

figure 2

To learn more about these test solutions, as well as read more in-depth answers to the questions posed in this post, download a free application note.

 

 

Selecting the Proper Antenna(s) for Your Interference Hunt

July 23, 2015

The test tool box used by field technicians and engineers to establish and maintain wireless network quality is becoming as robust as the systems themselves. Because of network complexity, selecting the proper tools for your job can save time, money, and some headaches.

One example is choosing the best possible antenna when embarking on an interference hunt. The correct one will have you on the path to quickly solving the problem. Selecting the wrong one will lead you down a series of dead ends. Today’s post will focus on the antenna types and when you should use them.

Yagi

A Yagi antenna (figure 1) is typically a very solid choice, especially in direction finding scenarios. One of the biggest pros of a Yagi antenna is that it has a very good radiation pattern, so you clearly know when it is pointing at the interfering signal. It also has a solid front-to-back ratio to reduce the likelihood that you will be misled by signals that are transmitted from behind the antenna. Low sidelobes, which help prevent you from chasing reflections that will waste time and frustrate you, are another trait of Yagi antennas. The fairly narrow frequency band of a Yagi antenna can limit its effectiveness in certain applications. For example, if you are working with a broad set of receive frequencies, you may need too many Yagi antennas for it to be practical.

Figure 1: A Yagi antenna is particularly effective in direction-finding scenarios.

Log Periodic

The biggest advantage of a log periodic (LP) antenna is that it has a very broad frequency range, with some covering frequencies from below 500 MHz to above 6 GHz. There are even LP antennas that cover up to 18 GHz and higher. LP antennas aren’t as directional as their Yagi cousin, but they do have good front-to-back ratio. If you need to work over a very wide range of frequencies, an LP antenna is a good choice, although it will take more time to find emitters due to it having a wider beamwidth than a Yagi. The best way to deal with this limitation is to move the antenna side to side and discover where the signal dips a given amount on each side of the swing. The halfway point between the two swings is the best direction estimate to the signal source.

Bi-blade

Good directivity is a hallmark of these directional antennas. The drawback is that they only have a moderate front-to-back ratio. This latter concern can be marginalized by using your body as a shield. Anritsu offers a bi-blade antenna that integrates with its MA2700A Handheld InterferenceHunter™ for highly effective interference hunting.

Omni-directional

If you are doing a mobile hunt, an omni-directional antenna may be your best bet, as it is often mounted on a car roof using a magnetic disk. It is typically used when seeking the strongest signal, such as when using the Anritsu MX280007A Mobile InterferenceHunter™.

Rubber Duck

A small, compact and mobile antenna, the rubber duck is convenient when carrying equipment in the field. Unfortunately, these antennas are not that effective. They have a short range, as well as a high Q factor, which narrows their bandwidth. Also, be careful that the antenna doesn’t pick up signals radiating from your instrument, rather than those being hunted.

Maximizing the Antenna

Another tool in your test box should be an antenna handle (figure 2), especially if you are conducting a hunt on foot. It preserves the antenna pattern and its ergonomic design puts less stress on your hand and arm, a nice advantage for long days in the field. A strap is usually available with the antenna handle and can actually free your hands to do other tasks. The trigger on the handle makes it extremely easy to take a bearing, or switch the antenna preamp on or off. Other advantages include that any small and light RF filter can easily be added before the pre-amp in the handle, and it can be easily integrated with all types of antennas.

Figure 2: An antenna handle such as the Anritsu MA2700A Handheld InterferenceHunter has a GPS receiver and antenna, electronic compass, and user-selectable preamplifier, all in an ergonomic design.

The handle also integrates a magnetic compass and GPS, so that position and direction can be easily relayed to the spectrum analyzer to simplify acquiring directional bearings. The built-in pre-amp makes it easier to send a sufficiently strong signal to the spectrum analyzer.

Conclusion

As this post shows, understanding the conditions in which you will be conducting an interference hunt will help determine the antenna to use. Selecting the proper accessory to your spectrum analyzer will help you locate the interference source faster and make your job a little easier.

To learn more about interference hunting tools, please watch this free webinar.

Take the Right Steps in a Successful Interference Hunt

June 26, 2015

There are many interference sources that field technicians must deal with on a daily basis. Simply, interference is anything that makes a radio receiver’s job more difficult – signal noise, EMI, radios, even multipath propagation in some cases. The list of potential interferers is even longer and can include other radios, Wi-Fi routers, CATV, transformers and anything connected to electricity. Intentional sources such as jammers are also a common cause.

Interference often causes big problems in network performance. It adversely affects receiver performance, leading to customer complaints due to coverage drops and slower throughput. All of this can weaken a service provider’s bottom line. To make sure operators receive the best possible return on their network investment, interference sources must be found and eliminated quickly. In this post, we will outline three steps to help you efficiently locate and remove interference sources.

Step 1: Get a Good Look at the Signals
Once you suspect an interference problem, the first step is to take measurements with your handheld spectrum analyzer to get a good look at the signals. Today’s handheld spectrum analyzers offer a variety of measurement and analysis tools that a generation ago seemed unfathomable in a field instrument. These features can be used to find even the most difficult sources, including broadband noise, transient signals, and moving emitters.

For broadband noise caused by things such as fluorescent lighting ballasts, jammers and CATV signals, you can use a handheld spectrum analyzer to look in the guard band between allocated channels. Trace averaging and narrow VBW are useful to see the average power of the noise. Be sure that the noise isn’t coming from your TMA/LNA. If you can bypass that amplifier and the noise level remains high, you can be reasonably confident that the source is external.

Using the correct tools can help you find interference sources faster and more easily.

For CW and CW-like signals such as oscillating repeaters/BDAs, a spectrum analyzer with a very narrow RBW of <100 Hz can make your job easier. If the signal is at the same frequency as your traffic, the narrow RBW will drop the modulated signal amplitude for a clearer view of the interfering signals. You can also use a Min-Hold feature, which is particularly effective for CW-like interfering signals because it helps to ignore the bursty uplink. Anritsu’s mobile interference hunting system makes this even easier by automatically resetting the Min-Hold every few seconds. This is particularly beneficial when hunting in a vehicle, as it eliminates the need to repeatedly press the Reset Trace button every time you change directions.
 
Transient signals can be caused by repeaters that turn themselves off when oscillation is detected, as well as other bursty types caused by signals on the move such as jammers in cars, repeaters in boats or trucks, or arcing. Key spectrum analyzer capabilities that can help track these sources down include:

• Spectrogram – This can be used to gather data over multiple days, which can then be analyzed on a computer to give you a better understanding of what you might see during your hunt.
• Max Hold – A nice feature that makes it is easier to see and locate intermittent signals.
• Save-on-Event – For very long signal recording, Save-on-Event is preferred over a Spectrogram, as you can record the signal just when it’s present and exceeds a mask level.
• Burst Detect – This feature can be used to capture signals as short as 200 µs. It can be particularly effective when combined with a Spectrogram and Save-on-Event.

Getting a good early look at the signal(s) is essential, as it can shave considerable time off of your hunt. Why? Here are four reasons:

1. Helps eliminate local problems such as PIM, loss or reflections caused by an antenna or feedline, as well as the radio – both transmitter and receiver.
2. Determines when interference happens. Locating an interfering signal that is constant is simple. If it’s only on Sunday morning, you know when you have to look for it (and you might start by checking emissions from churches.)
3. Acquire a reference trace, so you know what to look for in the field.
4. Gain an initial estimate of the emitter position by comparing the power on adjacent sectors. This is explained further in step two.  

Step 2: Estimate Emitter Position
Step two is to get an estimate of the emitter position. In cellular networks you can make a measurement at the base station to determine the rough direction of the emitter by comparing the amplitude in different sectors. The emitter will be in the general direction of the sector with the highest power, and closer to the direction of the next-highest-power sector than others. For networks without sectorized antennas, you may need to use a directional antenna, such as a high-gain Yagi, to determine the initial direction.

While this estimate will not pinpoint the source of interference perfectly, it will be good enough for you to drive to the area the interference source calls home. If you can compare the power from several different base stations, you can also make a simple estimate of the emitter’s distance between them. Again, the emitter will generally be closer to the base station with the higher received power.

Step 3: Locate the Source
To refine your estimate and locate the source, you can use several techniques. One is signal survey, in which you record amplitudes once you arrive at the target area to determine the strongest signal locale. Find the location with the largest amplitude and then repeat the procedure with a finer grid. Signal surveying can be very effective, especially when a mobile interference hunting solution, such as the one in Figure 1, is employed. Using such a solution will make this technique faster because it helps direct you where to go rather than surveying an entire area.

 

Figure 1 (MIH) – A mobile interference hunting solution can save time when on the hunt for interfering signals. 

Triangulation is another technique that is usually used for long distances of at least a half mile. In this method, you acquire multiple bearings from several widely spaced positions. Using multiple bearings is especially beneficial to avoid being fooled by large reflections that are common in high multipath environments. The triangulation technique is much easier to perform with a spectrum analyzer that has mapping capability. A good tip is to verify the bearing from several nearby locations to reduce the effects of multipath and shielding caused by buildings and vegetation.
 
Successive approximation, also known as homing, can be the fastest technique, especially indoors or when the emitter is nearby. You find the strongest signal using a spectrum analyzer and Yagi antenna, and travel along the bearing line. When a peak in signal strength is detected, you acquire a new bearing and go in that direction. This approach is not recommended when conducting the hunt from a vehicle for two reasons. One, it’s difficult and unsafe to hold the antenna out the window of a moving truck or car. The second is because multipath and path loss are more common due to buildings and vegetation.

Spectrum analyzers also have a number of tools to help in successive approximation. A dual trace mode allows for normal and max-hold traces; you can use this to help get you pointed in the right direction. Simply rotate the antenna 360° horizontally, then continue until you see the maximum signal strength again. A signal strength meter and interference mapping tools – both of which are common in handheld spectrum analyzers – are also helpful in narrowing the area in which the emitter is located.
 
In the real-world, you will probably employ all three of these methods. For example, you can start by performing triangulation from several cellular base stations, or just get an initial bearing from one. Then, determine the general area of the emitter location using the signal survey process. Using mobile interference hunting software and mapping tools are particularly helpful, as they allow you to take a more “intelligent” survey. Bearings aren’t collected, but rather the software uses power measurements from many locations to help lead you to the emitter. For the final step, you may use a handheld analyzer and directional antenna to find the actual device that’s emitting the unwanted signal.

Use the Right Tools
No matter the technique employed, you can follow certain interference hunting tips to make it easier and faster to locate the source of the network issues. Always remember to look in all directions. Consider multipath, as it can provide faulty analysis and lead you to incorrect conclusions. Most importantly, using the proper test tools can make your job easier. Select a handheld spectrum analyzer that is loaded with interference analysis options such as those discussed in this post. The analyzer should also have a broad enough frequency range to see the signal, and high sensitivity and dynamic range. Antennas should also be part of your took kit. Training courses can help you be best prepared when you are out in the field, as well.

Finding interference sources continues to be one of the biggest issues for field technicians. By taking the steps outlined and selecting the proper test tools, you can locate the problem and fix it faster. To learn more, view an online webinar on interference hunting.

 

Tips on Identifying Interference to Optimize Cellular Network Performance

June 12, 2015

If you’re like most field engineers and technicians responsible for cellular network quality, you view interference as the bane of your existence. An early indication that interference may be adversely affecting your wireless network is poor switch statistics. Once you’ve received this bad news, determining the cause becomes priority number one. Often, it requires you driving out to the base station to see if it is one of myriad of potential causes – bad feedlines, a broken TMA, poor transmitter power, passive intermodulation (PIM), or something else.  In this blog we’ll focus on external interference—the only possibility left when you’ve ruled out those other sources.

Handheld spectrum analyzers have become a frequent passenger on the journey to optimum network performance. For example, when searching for external causes of interference, they are used to acquire a spectrum reference for future analysis, as well as to view the spectrum over time to monitor fluctuations. Our inaugural post is written to help you use a handheld spectrum analyzer to spot interference, characterize interference types, and identify signals to help you ensure cellular network operation meets design specifications.

Spotting Interference
The first step to determine if your cellular network is suffering from interference is to compare real-world performance to KPI targets. If the network is not up to snuff, you can turn on your handheld spectrum analyzer (figure1) and conduct an interference hunt to determine if the problem is internal and/or nearby interference.

Figure 1: The Spectrum Master MS2720T is an example of handheld spectrum analyzers well suited for interference hunting.

The first and best place to start hunting is at the receiver input. This might be from a test port or via a CPRI link, for example. Another approach is to turn off the base station and disconnect the antenna. A third alternative is to have a separate receive antenna that is as high as the base station antenna. It’s best to measure the signal after the pre-filter (if the receiver has one) to have a “visual ID” early on so you are sure you are on the same signal later. Once you have eliminated internal sources, such as bad transmit power or PIM, as potential causes you can concentrate on external factors.

Uplink, Downlink Concerns
Interference can occur at both uplink and downlink frequencies, however it is a bigger concern for the uplink because it can adversely affect the base station receiver and ruin an entire network sector. That is not to say you shouldn’t be concerned with interference in the downlink. In fact, our experiences have shown that while most carriers focused solely on uplink interference in the past, there are now initial indications they are addressing downlink interference, as well.

It is a bit more difficult to see an interfering signal in the downlink, because the large base station transmit signal tends to mask it. To help make the interfering signal visible in the presence of the BTS signal, you have some options:

1.     Directional antenna – This can help by up to 10 dB or so for a small Yagi. A panel antenna, as used by the base station, can give you 20 dB or more of gain, at the expense of size.

2.     Min-Hold – A feature in some spectrum analyzers that can reduce the modulation on the base station signal by as much as 30 dB. If the interferer is CW, it will stay at the same amplitude.

3.    Filters – If you are somewhat fortunate, the interfering signal will be at a frequency near the BTS signal, rather than on top of it. While this will still affect UE receivers, it will let you use the spectrum analyzer (RBW) or other (narrow-band cavity) filter to see the interferer separate from the BTS transmitter.

4.    Drive Test – Data acquired via this process will show the location of dropped calls. This is a good place to start looking for the interferer, as the BTS signal will be relatively low, and the interfering signal relatively high.

Characterizing Interference
An important step to remember is to characterize the type of interference before disconnecting from the receiver’s signal. Simply adjust the spectrum analyzer to best view the signal by using the pre-amp, reference level, span, and resolution bandwidth controls. This will allow you to observe the signals’ shape, bandwidth, and behavior. Frequency drift, amplitude changes, and frequency hopping are key attributes to analyze because they:
   
1.    Provide a clue as to the emitter causing the interference
2.    Indicate when the signal is on and off
3.    Guide you on how to set up the spectrum analyzer when signal hunting
4.    Let you capture a reference trace to compare what you see later in the field
   
The spectral shape of the signal is another clue; one that is perhaps more important than those listed above. If you’ve seen the shape before, you probably recognize it. For example, cable TV has a very distinct pattern, with repeating channels every 6-8 MHz, depending on the country.
 
Instrument Features You Need
We mentioned the advantages of Min-Hold when locating interference in the downlink; it’s also useful for the same reason when looking at interference in the uplink. Another helpful tool in field spectrum analyzers is Max-Hold, which allows you to create an envelope so it is easier to locate intermittent or “bursty” signals. In these scenarios, a spectrum analyzer with a Burst Detect feature is also advantageous, as it can capture emitters as narrow as 200 µs.

When searching for interfering signals with long intervals between appearances, a “Save on Event” capability comes in handy. This feature uses a mask automatically generated from the “normal” signal and only saves a trace when something unusual appears. Once saved, the traces can be examined for time-of-appearance, and signal characteristics, so you can predict when the interfering signal will appear, rather than waste time taking measurements when the interference is not present.

Figure 2: Spectrogram displays can show field technicians how signals change over time.

Another useful capability is a Spectrogram, which shows how signals change over time. The signal in Figure 2 is clearly shown to be unstable in frequency through Spectrogram. This sort of oscillation in frequency is characteristic of a repeater with insufficient isolation between its input and output antennas. This is one example how Spectrogram helps identify a variety of signals.

To locate signals that don’t belong on the input to a receiver, it’s important to know which signals are supposed to be there. If you know who might be putting out interfering signals, you can save a lot of time. Once again, the handheld spectrum analyzer comes in handy, as it can be used to help identify the signal source.

It may be possible to demodulate the signal and listen for a station ID call sign from a TV or radio station, or a Morse code station ID at the start of a page. For those of you who are experienced, audio demodulation can give significant clues on the type of signal being generated. A good example is power line arcs, which generate a very distinctive audio signal. Sometimes, a signal can be identified by its frequency and location using government databases. For instance, the United States Federal Communications Commission (FCC) maintains a data base of signals and locations, linking them to owners, with contact information.

You can learn much more about locating interference sources by downloading a free application note on interference hunting techniques.