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Agilent
Techniques and Trends in Signal
Monitoring, Frequency Management
and Geolocation of Wireless Emitters
Application Note
Monitoring RF signals in a wireless evolved into cooperative networks
environment is often required by of low-cost sensors that collectively
a variety of wireless equipment monitor the wireless spectrum over a
operators, facility and test managers large geographic area. This applica-
and government agencies. Signal tion note reviews various issues,
monitoring applications can range techniques and associated equipment
from compliance of carrier-specific required for signal monitoring and fre-
transmissions to the discovery quency management of RF spectrum
and location of unknown or illegal in the VHF/UHF frequency range. The
transmitters. Traditional methods for goals and automation requirements
signal monitoring rely on high perfor- for various monitoring applications
mance spectrum analyzers and digi- will be discussed and the concepts
tizers often operating as a standalone of implementing a distributed sensor
system. With the current widespread network for determining the geoloca-
availability of broadband connectiv- tion of a wireless "emitter" will be
ity, signal monitoring systems have introduced.
Introduction Monitoring the frequency spectrum in a wireless environment for known and
unknown RF signals is required by a variety of equipment operators and govern-
ment agencies. Applications can range from carrier-specific measurements
to wide bandwidth spectrum searching and data logging. In all cases, the
spectrum or signal monitoring equipment requires several basic characteristics
such as a broad range of frequency coverage, high-speed channel scanning,
high frequency resolution and dynamic range, data storage and some level of
system automation for determining a course of action when a signal of interest
is detected. In some applications, spectrum monitoring is required to ensure
compliance with local regulatory requirements while other applications require
discovery of unknown transmitters or "emitters". The discovery process may
involve uncovering the type of signal including duration of transmission, number
of occurrences, carrier frequency, bandwidth, and modulation type and emitter
geolocation. Figure 1 shows a typical monitoring system that may contain fixed,
stationary and mobile receivers placed throughout a geographic area. Several
receivers may be networked together to improve the performance and localiza-
tion accuracy of the overall system.
Figure 1. Signal monitoring and surveillance system
2
Frequency Management Signal monitoring systems configured for the frequency management of licensed
or unlicensed spectrum typically operate over a known set of RF carrier frequen-
cies and modulation characteristics. These systems are used to verify compli-
ance and coexistence with other wireless systems. Typical users interested in
frequency management include government agencies, wireless service providers
including cellular operators, broadcasters, first responders, transportation agen-
cies for navigation and communication, and military installations. In addition,
regulatory agencies that manage spectrum utilization, licensing and coordination
of spectrum allocation across national and international regions often establish
a network of monitoring stations that cover highly-populated areas. Agencies
such as the International Telecommunication Union (ITU) have designated the
ITU Radiotelecommunications (ITU-R) organization to manage the RF spectrum
and satellite transmission at a global level. National/provincial regulatory
agencies, such as the FCC, NTIA, OFCOM, SRRC, and ANFR, manage spectrum
utilization at the national level. These agencies need a good understanding of
spectrum utilization as license revenue may be lost and they need to uncover
and mitigate potential system-to-system interference.
Surveillance Signal monitoring systems configured for the surveillance of unknown or
unfriendly transmissions require measurements of signals that occur sporadi-
cally over short periods of time and often require extraction of the intelligence
contained within the transmission. Surveillance of wireless signals is rapidly
expanding in the areas of law enforcement and correctional facility administra-
tion, boarder and coastal security and military intelligence. In many applications,
eavesdropping in the form of signal demodulation is required to extract vital
intelligence information for use by the military, national security agencies and
law enforcement. These types of systems monitor signals originating from both
indoor and outdoor locations. Direction Finding (DF) and geolocation are usually
associated with these types of systems as signal recovery and knowledge of
the emitter location is desirable. In the government and military areas, these
transmissions are often characterized in a category called Signals Intelligence
(SIGINT).
Interference Management Signal monitoring systems configured for the interference management of
known and potentially harmful signals require specific measurements for a
variety of different applications. Some applications require signal monitoring
over a large geographic area while some may be limited to the confines of a
building or individual room. For example, in test ranges where complex systems,
such as aircraft, can be studied for EMI and EMC, signal monitoring equipment
may be used to understand potential interference emanating from the aircraft
as different subsystems are activated. In some applications, where wireless
signals are generally known to cause interference to sensitive equipment, such
as specialized instruments installed within a hospital or testing facility, signal
monitoring becomes very important to the proper operation of the equipment.
For example, healthcare administrators often impose restrictions on the use of
cellular handsets within their emergency and intensive care facilities. Studies
have shown that transmission from cellular devices in close proximity to sensi-
tive equipment can obstruct the proper operation of equipment such as an
electroencephalogram (EEG) monitor [1, 2]. As it is difficult to prevent mobile
handsets from being carried into these specialized facilities, it may be necessary
to monitor the RF cellular spectrum and trigger an alarm when an undesired
signal transmission is discovered.
3
Emitter Geolocation Identifying the location of a target emitter is highly desirable especially in
surveillance and interference management applications. Direction Finding (DF)
and geolocation methods are traditionally based on receivers attached to highly
directional antennas. Received signal strength, triangulation and/or angle of
arrival (AOA) techniques can be used to accurately locate a transmitter in two
and possibly three dimensional space. Increased accuracy can be achieved by
increasing the number of monitoring receivers and adding GPS-assisted sample
timing and positioning of the receivers. Other geolocation techniques include
time difference of arrival (TDOA) and correlation based methods that use digital
processing of signals that are simultaneously captured by multiple receivers.
The timing among the multiple receivers in these systems can be coordinated
using GPS assistance or the IEEE 1588-based network timing protocol pioneered
by Agilent and approved by the IEEE in 1992.
The challenge in any signal monitoring system is to quickly detect, identify and
possibly locate a distant non-cooperative signal which may be intermittent, be
of short duration, and/or have low received power. The trend in wireless com-
munications is toward digital modulation schemes, higher carrier frequencies
and wider signal bandwidths. The higher carrier frequency results in larger path
loss between the target emitter and the monitoring system making it more diffi-
cult to recover the desired signal due to lower signal to noise ratio (SNR) at the
receiver. In addition, wider signal bandwidths will result in lower power spectral
density at the receiver again making it difficult to detect the desired signal. In
many cases, signal monitoring systems based on a single measurement point
within a wireless environment may be inadequate for these emerging technolo-
gies and proper signal recovery may require the coordinated effort of multiple
receivers or sensors being repositioned closer to the target emitter.
Equipment and The most basic configuration for a signal monitoring system includes a
receiver, antenna, low noise amplifier, output display, and possibly, some level
methods for signal of software automation for signal searching and data storage. A traditional
monitoring swept-tuned spectrum analyzer can provide a minimum set of requirements for
the monitoring receiver. The spectrum analyzer is a very flexible platform with
a broad frequency range, high dynamic range and graphical display with limit
line capability for setting amplitude level detection thresholds. Typically a low
noise amplifier (LNA) is placed between the antenna and analyzer to improve
the sensitivity of the spectrum analyzer which increases the signal amplitude
and lowers the noise figure of the measurement system. Most high performance
spectrum analyzers, such as the Agilent PSA and MXA series analyzers, have
options for an internal LNA. Many spectrum analyzers have built-in analog
demodulation capability but often their use requires the re-tuning of the analyz-
er's center frequency and span to match the signal of interest. When changing
the analyzer's frequency settings it is important that the instrument can rapidly
tune the instrument's internal local oscillators (LO) otherwise the probability of
intercepting an intermittent signal of short duration may be reduced. Other types
of receivers designed specifically for signal monitoring applications may use fast
tuning LOs and high-speed digitizers to rapidly measure the frequency content
using FFT signal processing. For example, the Agilent E3238S is configured with
up to six dedicated FFT processors operating in parallel to achieve exceptionally
fast spectral survey rates. When selecting the receiver architecture for surveil-
lance and signal monitoring it is often necessary to examine the features and
the performance of the measurement system for the proposed application. Table
1 shows many of the important characteristics required for a basic monitoring
receiver.
4
Table 1. Desired characteristic for a basic signal monitoring receiver
Characteristic Function
Broad frequency range Start and stop RF frequency range
Fast survey rates Fast tuning local oscillators and FFT processing for narrow RBWs
High sensitivity LNA and narrow RBW settings
Good selectivity RBW shape
Downconverted and/or detected analog output with wide instantaneous bandwidth.
IF output and/or video output
Useful for handoff receiver applications
Graphical display Visual aid in signal identification. Limit line capability.
Local or remote computer control Programmable control. Connectivity through LAN, USB, IEEE-488
There are numerous receiver architectures that can be used to achieve the
characteristics described in table 1. For example, figure 2 shows the block
diagram of a super-heterodyne architecture found in many traditional spectrum
analyzers. The input RF signal is filtered and downconverted to an intermedi-
ate frequency (IF) using a mixer and local oscillator (LO). A broad range of RF
frequencies can be measured by sweeping the LO and measuring the signal
amplitude after the IF filter (also known as the RBW filter) in a spectrum
analyzer. In signal monitoring applications, it is desired to quickly sweep the
receiver's LO in order to capture intermittent signals and increase the probability
of intercept (POI). When resolution and sensitivity requires the use of a narrow
RBW, the sweep time proportionally increases resulting in a potentially reduced
POI. To overcome this sweep time limitation, many receiver architectures use a
digital IF and perform IF filtering in the digital domain. Digital filtering can offer a
large improvement in sweep time when compared to their analog counterparts.
Digital signal processing (DSP) at the IF also provides a convenient path to flex-
ible demodulation capabilities should the measured signals require additional
analysis and identification. Figure 2 also shows a separate IF path through an
analog-to-digital (ADC) converter where the signal amplitude is detected and
processed using DSP techniques.
Figure 2. Block diagram of a super-heterodyne receiver
5
Equipment and methods for signal In systems requiring demodulation of the measured signals, the instantaneous
monitoring (continued) bandwidth of the IF must be wider than the modulation bandwidth of the signal
otherwise a portion of the occupied spectrum will be attenuated. In traditional
AM and FM analog communication systems, the signal's instantaneous band-
width were typically much less than 200 kHz. In this case, an IF filter approxi-
mately matched to the channel spacing of the analog modulated system, such
as 30 kHz, would properly pass the desired signal and provide good receiver
sensitivity due to the relatively narrow IF bandwidth. With the desire for higher
data rates and the introduction of digital modulation schemes, the signal's
instantaneous bandwidth increases to 5-20MHz for many emerging wireless
systems such as WiMAXTM1 and 3GPP LTE. As the instantaneous bandwidth
increases, the receiver's IF filter bandwidth also needs to increase if the signal
is to be properly demodulated and identified. Unfortunately, the wider IF band-
width results in a proportionally reduced SNR into the demodulator. To over-
come the SNR limitations, the monitoring system can be modified to increase
the signal level into the receiver by increasing the preamplifier gain, increasing
the antenna gain or positioning the monitoring system in closer proximity to
the emitter. In practice, these techniques have limitations of their own. For
example, increasing the preamplifier gain may introduce undesired intermodula-
tion distortion (IMD) products when the receiver is operated in the presence of
other signals with higher amplitudes. Antenna gain can be increased resulting
in a highly directional antenna with an increase in the antenna's physical size
and a potential reduction in operating bandwidth. Physically positioning the
monitoring system closer to the emitter may not be practical for a number of
reasons including conditions when the emitter's location is unknown over a
large geographic area. Consequently increasing the number of receivers in the
surrounding environment will tend to increase the total system cost unless a set
of low-cost sensors can be placed at a higher density to alleviate many of the
SNR issues when monitoring wideband, high-carrier frequency signals.
A traditional rack-mounted surveillance system, configured around a con-
ventional spectrum analyzer or VXI-based receiver, is typically installed in
a weatherproof shelter or building and interconnected to rooftop antennas
through low-loss coaxial cables. These typically standalone systems may also
contain several handoff receivers for demodulation and data storage of specific
signals of interest. The handoff receiver takes the downconverted analog IF, and
working in parallel with the primary receiver, demodulates the AM or FM signal
of interest while not interrupting the search function of the primary receiver. For
signal monitoring over a large frequency range, various antenna types may be
required to cover the complete range of interest. In this case, an RF multiplexer
is connected to the receiver and switched between one of several antennas
externally mounted to the facility or vehicle.
In contrast to the traditional approach, a lower-cost network-ready receiver,
also referred to as an "RF sensor", can be used as a downconverter and
signal acquisition system capable of transferring sampled IQ data over a wired
network to a remote system controller for signal processing, data archiving and
demodulation. A typical low-cost RF sensor, such as the Agilent N6841A, is a
small self-contained weatherproof receiver that can be easily pole-mounted,
1. ("WiMAX," "Fixed WiMAX," "Mobile
rack-mounted, vehicle-mounted or configured into a man-portable system. To
WiMAX," "WiMAX Forum," the WiMAX
Forum logo, "WiMAX Forum Certified,"
increase receiver flexibility, the RF sensor is typically configured with "software
and the WiMAX Forum Certified logo are defined" functionality and a wideband digital IF architecture. Figure 3 shows a
trademarks of the WiMAX Forum. All other simplified block diagram of the Agilent N6841A RF sensor. The sensor has two
trademarks are the properties of their antenna inputs for local connection to broadband and/or diversity antennas.
respective owners) The system also includes a set of banded pre-selection filters.
6
Equipment and methods for signal These selectable filters are useful when searching for small signals in the
monitoring (continued) presence of high power transmissions and designed to reduce sensor cost and
improve reliability. Downconversion to IF is performed using tuner architecture
similar to a traditional spectrum analyzer. The digitized IF implements a digital
downconverter (DDC) for processing the sampled IF down to baseband. The
completely digital IF of the N6841A has a variable bandwidth up to 20MHz
to accommodate a variety of wireless technologies and modulation types.
Embedded software controls the receiver's triggering, FFT operations and
memory capture. Sampled time-stamped data is transferred over the network
to a remote server where signal identification and data logging is performed.
The receiver's internal clocks can be controlled by the IEEE 1588 network tim-
ing or optional GPS. The general concept for implementing a distributed signal
monitoring system is to deploy a higher density of low-cost RF sensors placed
physically closer to the intended emitters and to have all the advanced signal
processing functions operate on the sampled data at a common, centrally
located server.
Figure 3. The N6841A RF Sensor block diagram combines a VHF/UHF Receiver with
software-defined signal processing
7
Receiver Location and The location of the signal monitoring equipment and the associated antenna(s)
will have a great effect on the overall system performance. Attenuation of the
Proximity Gain propagating signal, also referred as path loss, and nearby interference can
impact the receiver's ability to detect the energy from a target emitter. Path loss
is a function of the RF carrier frequency and the relative distance between the
emitter and the receiver. At higher carrier frequencies, the path loss increases
and it may be necessary to locate the receiver in close proximity to the emitter.
Interference from the surrounding environment may also influence the receiver's
performance. For example, when a receiver is placed in close proximity to
a television broadcast station, cellular base station and/or radar system,
significant interference can be induced from spurious emissions, harmonics and
intermodulation distortion [3]. These effects may also include receiver front-end
overload produced from these nearby high power transmitters. It is important
to initially monitor the spectrum around the proposed vicinity of the receiver to
quantify the influence that these interferers and high power systems may have
on receiver performance.
The receive antennas in a signal monitoring system are typically placed high
on towers, buildings or hills to reduce the multipath effects introduced by the
surrounding environment [4]. Ideally, antennas should be separated from sur-
rounding metallic objects by a distance of several wavelengths otherwise the
expected antenna pattern may become distorted [3]. Even the metallic mast that
the antenna is attached can greatly influence the gain pattern [5]. Also other
antennas in the nearby vicinity can alter the antenna pattern and reduce system
performance in unexpected ways. Proper placement of the antenna is crucial
to the overall performance of the monitoring system especially in applications
where a limited number of high performance receivers are located over a wide
geographic area. On the other hand, systems based on low-cost RF sensors
allow relaxed antenna requirements resulting from the proximity gains achieved
using a higher density of receivers. Figure 4 shows a roof-mounted RF sensor
connected to a broadband antenna with a second antenna placed on a separate
mast. The sensor is placed relatively close to the antennas to reduce cable loss
that could degrade the noise figure of the system.
Antenna 1
GPS Antenna
Antenna 2
RF Sensor
Figure 4. RF sensor and antennas configured on a rooftop installation
8
Receiver Location and Proximity Gain When the emitter location is unknown, it is desirable to use antennas with
(continued) omni-directional patterns for terrestrial applications. Unfortunately omni-direc-
tional antennas have low gain, approximately 0dBi, and do not help to improve
the receiver's SNR. Increasing antenna gain may improve the SNR but the
resulting antenna pattern will favor signal reception into a particular direction.
Unless a highly directional (high gain) antenna is physically or electronically
scanned into the direction of the emitter, it is possible that an unknown emitter
may be missed due to low receive SNR.
Higher RF carrier frequencies often used in modern wireless communications
such as cellular and WLAN, result in an increase in the free space loss when
compared to similar systems operating at lower VHF/UHF frequencies. At
these higher carrier frequencies, it may be necessary to locate the monitoring
antenna/receiver closer to the emitter in order to maintain a reasonable level of
SNR. The signal improvement achieved when reducing the separation between
the emitter and the receiver is referred to as "proximity gain". For example,
assume that two communication systems are operating over the same distance
between the emitter antenna and the signal monitoring antenna. One system
is operating with a RF carrier frequency of 100MHz with a 20 kHz modulation
bandwidth. The second system is operating at 2.4 GHz with a 20 MHz modula-
tion bandwidth. What is the measured SNR for each system assuming identical
transmit power, antenna gains, cable loss and receiver noise figure? What are
the main contributors to the SNR difference? In order to answer these questions
and to estimate the performance of each system, the SNR is calculated using
the following equation (1).
SNR = [PT + GT