Remote Spectrum Monitoring

Introduction to Remote Spectrum Monitoring

With the rapid expansion of wireless communications, the need for robust networks relatively free of interference continues to grow. Capacity will be degraded by the presence of illegal or unlicensed signals that interfere with needed transmissions. These signals can be periodic or present at different frequencies over time, making the discovery and removal of these sources of interference a significant challenge.

A spectrum monitoring system will facilitate the identification and removal of illegal or unlicensed interference signals. 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.

In addition to interference detection, spectrum monitoring is also used to characterize spectrum occupancy. Government regulators and operators are often interested in determining the usage rate for various frequency bands. Monitoring these frequencies provides the information needed to optimize spectrum for maximum utilization. Spectrum can be re-purposed for other applications or multiplexed with other signals using cognitive radio techniques.

Spectrum monitoring can also serve to enforce compliance with government regulations. Police, fire fighters, air traffic control, military and emergency services must all have access to communications free of impediments and distortion. Compliance with spectrum regulations is often enforced by spectrum monitoring.

Key Applications

A key impediment to good network performance is the presence of interference. Sources of interference include illegal or unlicensed broadcasters, repeaters, DECT phones, jammers, wireless microphones and cable TV leakage.

Interference can also come from other cellular networks, particularly along national borders where competing services are subject to different regulatory entities. A spectrum monitoring system will facilitate the identification and removal of interference signals that reduce network capacity. 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.

Satellite earth station monitoring

Clear communication channels is critical to ensure optimal performance. A typical earth station will consist of many satellite dishes, all of which need to be monitored in real time. A multiport remote spectrum monitor enables the spectrum of each satellite uplink or downlink to be monitored for spectral purity and the presence of interferers. Alarms can be automatically triggered on detection of unwanted interferers that would degrade the signal integrity.

Government regulators enforcing spectrum policy

Government regulatory agencies benefit from remote spectrum monitoring because it allows manpower to be effectively multiplied over a wider area and at times when engineers are not normally on duty. Direct savings are realized through reduced time when engineers are on travel, avoiding unnecessary travel, and focusing travel on times and locations where interference is more likely to occur. Prior to rule-making regulatory agencies can use remote spectrum monitoring to analyze and understand real-world scenarios in bands where new services are to be targeted.

Security at military facilities, national borders, utilities, airports and other sensitive sites where monitors are positioned indoors

Reliable communications are critical for both military operations and for testing high-tech systems which rely on wireless command and control. Security at military facilities, national boarders, utilities, airports and other sensitive sites must all have access to communications free of impediments and distortion. Spectrum monitoring is crucial to insure that such facilities remain free of interference.

The role of enforcing in-building public safety needs (e.g.,# occupants in a room) typically falls to the local fire marshal’s staff. They rely on building codes and standards from the National Fire Protection Association (NFPA). Wireless communications standards are covered under NFPA 72 and NFPA 1221. Both standards prescribe periodic measurements to assure communications. For more information go to http://www.nfpa.org .

Historically, public safety communications were mostly based on powerful, external high site transmitters to supply sufficient signal levels to reach into buildings. Modern energy efficient building construction and increasing reliance on anytime, anywhere communications are causing installation of public safety radio enhancement systems. These systems, known in the technical community as Distributed Antenna Systems (DAS) take the external high site signals, amplify them and evenly distribute the signals throughout the building.

Monitor jails/prisons for illegal broadcasts

Contraband cell phones are a growing problem in correctional facilities. Inmates use them to coordinate crimes outside prisons, to threaten and intimidate witnesses and their families, and to coordinate assaults on other inmates. Spectrum monitoring with real-time alerts and location estimation lets correctional officers and prison officials know that illegal phone use is occurring, and helps them focus efforts efficiently during search and seizure actions.

Spectrum Monitoring usage surveys (white space)

The use of vacant spectrum allocated to TV broadcast known as white space, can alleviate the spectrum need while opening the path for dynamic spectrum access. Several measurement campaigns have shown that the TV broadcasting spectrum is mostly used in populated areas. In developing countries, there’s not enough return on investment for broadcasters to provide many simultaneous TV channels. For this reason, these areas are lacking in Internet access.

TV white space can take advantage of the improved propagation capabilities of these frequencies to provide affordable Internet access in rural areas. White spaces are also present in densely populated areas as a consequence of the transition from analog to digital TV. The lower frequencies as compared with the ones used for WiFi which in some places is becoming over crowded. For machine to machine applications and the "Internet of Things" wave, white space has significant advantages both for developed and developing economies.

Airport monitoring for interference

Aviation communications in the area of busy metropolitan airports is especially critical, because the tight timing and coordination of air traffic patterns and runway activity leaves little margin for error. Monitoring for interference in adjacent channels/bands is critical to determine if immediate action is needed.

Spectrum occupancy and frequency band clearing

In an effort to maximize spectrum use efficiency, regulators in many countries have implemented re-banding rules which require users to vacate spectrum and transition to other frequencies. For example: In the US, the 600 MHz frequencies formerly occupied by analog television broadcast will be auctioned to mobile telephony carriers, but the rules for this auction are complex and not all broadcasters will be vacating their channels. This means that carriers who want to bid on spectrum will need to monitor and analyze the performance of their bidding targets to determine if interference from co-channel broadcasters might occur during periods of unusual propagation. Also, there are a large number of wireless microphones in use which operate on the 600 MHz band, and not everyone will know they have to stop using them after the auctions are complete.

Positive Train Control (PTC)

Radio manufacturers have already started to ship their new ITC-R Positive Train Control (PTC) radios, and the first PTC digital radio communications systems are now being deployed and tested in the field. Information is beginning to be gathered on how these new PTC signals propagate, and an understanding of PTC system performance is beginning to emerge. Advanced propagation prediction software has been used to estimate coverage, and verification work is under way using the first deployed base stations to measure and confirm effective RF propagation.

PTC radio systems project commissioning teams from several different railroads are using their own individual approaches, with no final testing procedures defined yet. The focus on PTC RF testing has been mostly on verifying through RSSI (Received Signal Strength Indication) values that the coverage for each base station is similar to what the propagation software predicted. Typically, previously installed base station radios are used to transmit and receive PTC signals, and sometimes locomotive and/or wayside radios are also used in the field to verify coverage to and from the base stations.

Sports venue monitoring

Communications are a critical component of American Football. The NFL employs large numbers of engineers and professional volunteers to ensure that coaches, referees, and medical teams can communicate during games. Stadium security, concessions, parking, and cleaning crews also rely on communications to ensure fans can enjoy a clean and safe event. With the massive adoption of wireless devices by consumers, NFL communication engineers and volunteers have an increasingly difficult task of ensuring that interference does not occur. Remote spectrum monitors help multiply resources and provide a dashboard of all radio activity within a sports venue so the efforts of engineers and volunteers can be focused efficiently.

University and lab research

In areas where certain frequencies are under-utilized, the spectrum can be re-purposed for other applications. Alternatively, little used spectrum can be shared with other applications using cognitive radio techniques. Research is ongoing in many universities and labs throughout the world for new cognitive radio algorithms.

Features and Capabilities of Remote Spectrum Analyzers

Remote Spectrum recording

Typically this is done when a system user complains of interference from an unknown source. In public safety and critical communications users might complain of broken reception (especially in digital voice systems), warbling or squealing (analog heterodyne from co-channel signals), or sporadic interference at certain times of the day or night. Recordings can be correlated with time and location estimation reports from users to determine likely sources and facilitate “spectrum fingerprint” analysis.

Fast and efficient detection and elimination of interference sources

Interference to mobile telephony can cause subscriber frustration leading to “churn” – where a subscriber switches to another carrier. Mobile telephony is replacing traditional wireline telephony for some subscribers – hence interference to mobile telephony can be potentially life-threatening if calls for police, fire, or EMS do not go through. Intentional and unintentional interference to aviation, maritime operations, critical infrastructure, and public safety creates a potentially life-threatening scenario that must be quickly addressed. Resources for this kind of work are expensive, and the work is time-consuming. Understanding the sources of interference, determining if the interference occurs at specific times of the day, and obtaining location estimates for interference sources is critical to maximizing limited engineering resources. With Remote Spectrum Monitoring, it is now possible for the engineer in the field to both verify the interference is occurring while hunting the source and to immediately confirm the true “source” has been disabled.

Geo-location of interference signals
Power of Arrival (POA) and Time Difference of Arrival (TDOA)

Once an interferer or suspected illegal signal is identified, a geo-location algorithm is employed to fix the approximate position of the signal. This enables the user to narrow down the signal location, minimizing the time and expense for pin-pointing its position. A search can be done for alarm violations that occurred at any of the spectrum monitor probes in the network. Using three probes in the vicinity, the interference position can be geo-located. Power of Arrival (POA) algorithms are used to position the interference signal. Three or more probes must be in the vicinity to detect the signal of interest in order to correctly triangulate the position. Time Difference of Arrival (TDOA) is a technique whereby the time of arrival of a signal is precisely measured at multiple (physically separate) receivers. To use TDOA, some distinct signal characteristic must be present on the signal in order to synchronize the time of arrival. Using GPS-based timing, signals can be synchronized to about 10 ƞSec (10 ƞSec corresponds to about 3 meters of inaccuracy). However, other factors such as receiver variations and the presence of multi-path will present additional errors to the measurement.

Maintain history of spectrum activity

In some cases interference signals may exist only at certain times of the day, or certain days of the week. Sometimes an interference signal will cause a severe problem in the network for several hours, only to appear again months later. These issues can be impossible to eliminate unless historical information about the signal of interest is stored.

For difficult to find signals that are mobile or intermittent, logging spectrum history can help identify patterns of the interference signal. Spectrum history can show not only the signals of interest, but also when they occurred. Identifying when signals of interest are present is an important tool to hunting and finding interference.

Generate records of interference events for potential legal action

The detection of illegal or unlicensed broadcast signals is an important application. Illegal broadcasters may set up AM/FM, cellular or other types of transmissions which must be identified and ultimately located. By using spectrum monitors, unlicensed broadcasts can be tracked, processed and stored in a database for further examination and potential use in legal proceedings.

Selecting a Spectrum Monitoring Tool

Many spectrum monitoring tools are available today. With all the instruments available, making an informed choice can be challenging. Here are a few guidelines for selecting a spectrum monitoring instrument, whether it be a receiver or a spectrum analyzer. It is important to pay attention to specifications and features. Missing or sparse specifications are a warning sign of a low quality instrument. Specifications that are present, but poor, can be harder to interpret. At the same time, some specific features may distinguish one tool from another. Here are a few pointers:

High Dynamic Range

A large Dynamic Range allows you to see a small signal when a large signal is also present. This is essential if the signal you are interested in is small and close in frequency to a large signal. This condition is often the case in a communications system. Unfortunately, dynamic range for a spectrum analyzer can be specified in many different ways with wildly different numbers. One example of a very useful Dynamic Range specification is “greater than 106 dB in a 1 Hz RBW”, using the method “2/3(TOI-DANL)”. Overload Indicator It is important that your chosen instrument indicate when its front-end is experiencing overload conditions. A front-end overload is quite possible when receiving signals through an antenna and may be generated by a signal that is not even visible in the current span. Overloads may generate spurious signals (spurs) that appear to be legitimate signals, and these signals are made worse by poor TOI. If you know that an overload is occurring, you can add attenuation, external band-pass filters, or even move to a different location. If you do not know that an overload is happening, you may waste a lot of time investigating signals that are generated inside the instrument itself!

Reliability (hardware watchdog timer)

A remote spectrum analyzer should include a Watchdog timer to insure long-term stability for remotely deployed monitors. Watchdog timers are used to detect and recover from computer malfunctions, where accessibility is limited.

Fast Sweep Speed

Ideally at least 20 MHz instantaneous FFT bandwidth to allow nearly real-time sweeps across a wide spectrum. Capability of sweeping at rates up to 24 GHz/s, will allow the capture of many types of signals. This includes periodic or transient transmissions as well as short “bursty” signals.

Third Order Intercept (TOI)

Instruments with poor TOI will be more susceptible to generating spurious responses (spurs) internally that may be confused with real, external, signals. In other words, the effect of an overload, or near-overload, shows up sooner and is more pronounced. A good TOI figure is in the 15 to 20 dBm range, with two –20 dBm tones and no attenuation. Be wary of instruments that specify TOI with >0 dB attenuation, because this makes the TOI number artificially high.

DANL

Instruments with a poor Displayed Average Noise Level (DANL) may not be able to see a low level signal due to a high noise floor. In any given RBW, their noise floor will be higher than an instrument with a lower DANL. This tends to mask low level signals. A counter-argument is “Well, if the signal is that small, we don’t need to worry about it.” It is good to remember that a signal that is weak in one location may be strong in another location, and you may be in the location where the signal is weak. The DANL is often specified in a 1 Hz bandwidth, which allows you to easily estimate the noise floor of different instruments at any RBW setting. A good DANL is quite helpful when setting up a spectrum monitoring station or network, ensuring that you can effectively monitor signals over a wider area. An example of a good DANL specification is a number less than –160 dBm in a 1 Hz RBW. Remember also that when setting up a monitoring network, having a noise floor or DANL that is 3 dB better may mean that you need only have half as many monitoring stations.

Low Spurious

RF and microwave transmitters often require extensive spur searches to ensure they are not causing potential interfering signals or radiating unintended signals outside of their designated signal parameters. A spectrum monitor with low self-generated spurious signals allows reliable detection of low-level signal for these transmitters under test.

Remote firmware update capability

In the event of an application error or power fluctuation which causes an ongoing interruption in monitor communication, a re-boot policy will bring the remote probe back to its previous state. Under these conditions, the current firmware should automatically reload and on-line operation restored. Instrument settings will then be restored to the previous state.

If for any reason the firmware in the unit becomes corrupted, a warning such as a “Golden” firmware image is used to bring back full operation of the probe. This feature is particularly useful for remote firmware updates. The Golden Image feature provides the user with assurance that the monitor stays in operation when changes are made to the code. Any bug fixes, updates or user requested features can then be remotely sent to the spectrum monitor and safely incorporated.

Low power consumption

In certain instances, users want to deploy a spectrum monitor in remote areas where there is no power available. The use of a battery or solar cell may be required, however, having low power consumption enables remote placements of the spectrum monitor for longer periods of time.

Video Bandwidth Control

Some monitoring tools do not have a Video Bandwidth (VBW) setting. This makes it more difficult to deal with modern digital signals. A VBW filter is a fast way to average the displayed data and lower the noise floor. A user-adjustable VBW filter is an important feature when selecting a tool to monitor in the current RF environment.

User Interface

While no one likes to think that a user interface would make the difference between a usable and an unusable instrument, the user interface truly has a great deal to do with how comfortable you feel with the instrument. Most spectrum analyzers have a user interface that is similar to other spectrum analyzers. While there is not a standard spectrum analyzer user interface, if you learn to use one spectrum analyzer, you know most of what you need to use a different spectrum analyzer. This has been worked out over time and many trials to be an efficient user interface. If you are considering an instrument with an unusual user interface, be sure to carefully consider both the learning curve and how easy it may be to do common tasks—both for yourself and others in your organization.

Spectrum Recording

Not all spectrum measurement devices allow saving traces, saving spectrograms, or save-on-event. If you are in a position where it is necessary to compare measurements or report on measurements, the ability to save traces, either normally, though save-on-event, or within a spectrogram, will be important.

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