Post on 15-Apr-2022
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W H I T E P A P E R
Overcoming Interference is Critical to Success in a Wireless IoT World
Ensuring reliable wireless network performance in the presence of many smart devices, and on potentially overcrowded radio bands requires a solid plan for coexistence test.
As radio bands filled with wireless Internet of Things (IoT) devices utilizing different
protocols—802.11, Bluetooth® and others, unexplained communications failures
have become commonplace, even when signal strength seems sufficiently high for
good operation. This issue is especially problematic in healthcare environments
where dense deployments of wireless IoT devices operating on different bands
and using different radio protocols is the norm. These communication dropouts
can often be attributed to several causes, in particular, issues with coexistence.
Coexistence is defined as the ability of wireless equipment to operate in the
presence of other equipment using dissimilar operating protocols. The only way to
ensure reliable wireless network performance and in turn, succeed in the wireless
IoT world—especially in healthcare environments—is to properly test for radio
coexistence. That task is often easier said than done.
Real-world exampleHealthcare facilities are the most challenging RF environments in the civilian world. Here’s why:
• There are a wide range of wireless medical devices in use, as well as smart phones and wearables for personal use.
• Many radio protocols are incompatible and do only a marginal job of detecting other signals.
• Critical medical applications require complete and uninterrupted connectivity to process medical alerts and transfer large amounts of data quickly. Any disruption could negatively impact a patient’s outcome
Ensuring reliable wireless operation given these factors and in severely overcrowded radio conditions, makes coexistence test absolutely essential.
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Why Coexistence Test?Coexistence concerns are driven by three key factors: increased use of wireless
technology for critical equipment connectivity, intensive use of unlicensed or shared
spectrum, and higher deployment rates of sensitive equipment like intravenous infusion
pumps, pacemakers, and defibrillators. These factors directly impact how reliable
medical device communications are.
Coexistence problems are found in many environments. Common EMI and EMC tests
only measure the proper operation of circuits against fixed standards of emissions
across intended and unintended frequencies. Protocol compliance tests, such as those
from the Wi-Fi Alliance, ensure that networks of similar devices using the same standard
can communicate and share the channel, following the rules of that standard.
However, one problem remains—different standards are unable to cooperatively share
channels. A Bluetooth device using frequency hopping spread spectrum (FHSS), for
example, cannot detect and understand an 802.11 transmission using Orthogonal
frequency-division multiplexing (OFDM) or direct sequence spread spectrum (DSSS)
modulation, on the same radio frequencies.
Efforts have been made to improve coexistence in these standards, but to date, no
active cooperation has been defined. Bluetooth, for example, can sense which of its
frequency hopping channels are experiencing a high error rate, and drop those channels
from its hopping sequence. However, this indirect method of channel sharing is only
marginally effective in crowded environments where many different devices operate.
Coexistence ChallengesThree techniques are commonly used to improve coexistence of devices and networks.
Each of them face unique challenges when it pertains to healthcare environments.
Technique #1: Physical separation
Physical separation improves network operations by reducing the signal strength of
two radio networks. By placing the two networks in different locations, each network
experiences a weaker signal from the other. This increases the probability that they will
both be able to operate simultaneously without errors (Figure 1). One technique used
to implement physical separation is known as the “capture effect.” In this technique,
a receiver locks onto the stronger of two signals on the same frequency, while being
immune to interference from weaker signals.
Co-existence testing 101
Co-existence testing measures how well a given device (or network) can operate in the presence of devices using other protocol standards on the same or nearby frequencies. Unlike compliance testing, which ensures a device/network complies with the requirement of a given standard, coexistence testing evaluates the performance of a device/network in a mixed-signal environment. Without appropriate coexistence testing, the reliability of the device and network in dense deployment environments, with many protocols operating simultaneously, would be highly questionable.
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Figure 1: Insufficient physical separation causes interference.
Challenge: This technique works well for some modulation types, but not all. As an
example, an FM broadcast station may be able to use the same channel used in another
town, assuming the stations are separated by 50 miles. But, this technique does not
work well in healthcare environments utilizing the 2.4 GHz Industrial, Scientific and
Medical (ISM) band. Although the ISM signal is much lower power, and the range is
much shorter than FM broadcast, hundreds to thousands of wireless IoT devices on this
band may be operating throughout the facility. For this reason, it is impractical to expect
physical separation as an effective technique to reduce interference.
Factor #2: Frequency separation
Frequency separation is a technique used to improve the performance of mixed wireless
networks. Essentially, when one network operates on a different frequency from another
network, interference between the two networks is reduced, whether or not they are
located close to one another.
Challenge: While a valid technique, it is not always effective when it comes to the
2.4-GHz ISM band, which is occupied by overlapping Bluetooth, ZigBee, and 802.11
channels (Figure 2).
WiFi AP2
ZigBee 1
ZigBee 2
WiFiHotspotPhone
WiFi AP3
WiFi AP1
WiFiClient Device
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Bluetooth does offer one interference coping mechanism; by dropping certain channels
from the hop sequence when a high error rate is experienced on those channels. This
technique is a form of frequency separation. While it is a step in the right direction, it is
based on the level of errors experienced by the Bluetooth device and not on intelligent
agreement between the Bluetooth and Wi-Fi equipment. Consequently, it is not an
optimal solution for Bluetooth networks.
Factor #3: Time separation
Time separation is a technique whereby transmissions are sent and received at
different times to avoid collisions. This is possible because most radio networks are
not transmitting 100% of the time, using only small chunk of time to transmit. As the
volume of data increases; however, the radio channel is occupied more of the time,
and simultaneous transmissions and collisions occur more frequently (Figure 3). Some
protocols, like Wi-Fi, have collision avoidance mechanism that can detect other Wi-Fi
signals and take turns using the radio channel.
Figure 3: Different radio networks utilize different protocols, causing collisions when transmitting simultaneously.
Figure 2: Overlapping channels in 2.4 GHz ISM band.
802.11 a/g/n.. (WiFi)
802.15.4 (ZigBee)
802.15.2 (Bluetooth)
Non-overlapping channels (2.4 GHz) 3 16 79
Bandwidth 22 MHz 5 MHz 1 MHz
WiFi WiFi WiFi WiFi!! ZigBee ZigBeeBluetooth Bluetooth
Time
Challenge: Most radio standards are not designed to detect other network
transmissions and cooperatively share channels. To make matters worse, as
more data is transferred, more time is spent sending data and the corresponding
acknowledgments (ACKs). This increases the chance of a device “deaf” to other
protocols transmitting during a critical data transfer, resulting in colliding transmissions
that can potentially cause errors and the need to retransmit data.
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Designing the Coexistence TestImplementing one or more techniques is essential to delivering reliable device and
network performance. Testing these implementations is critical to producing the highest
quality device. A good coexistence test plan should include the following steps:
Step 1: Characterize expected RF environment
To characterize the expected RF environment, field measurements of the frequency
band of interest must be performed. Because device digital transmissions are very
short, a traditional swept spectrum analyzer is often ineffective for this task. The digital
transmissions can come and go before the sweep even reaches the frequency in
use, and thus will not be detected. Additionally, channel utilization cannot be reliably
measured with a swept spectrum analyzer.
To make accurate field measurements, a Real-Time Spectrum Analyzer (RTSA) is
recommended, as it allows the device designer to continually sample spectrum with
a high-speed ADC. A real-time fast Fourier transform (FFT) can then be performed
to convert the data into a spectral view and identify the types of signals present. To
accurately model the environment for the device under test (DUT), its crucial to know
what signals are present in the target environment. It may also be useful to know the
strength of those signals and rates of transmission (spectrum utilization).
Step 2: Choose your test signals
After identifying the signals that are present, the type and number of signals needed
to generate or model the coexistence test must be selected. This may mean choosing
three different tiers of test signals, for example, one single Wi-Fi network passing data at
the lowest tier, two Wi-Fi and one single Bluetooth signals at higher data rates, and then
at the highest level, three Wi-Fi and five Bluetooth signals.
Step 3: Define the functional wireless performance
The next step is to identify the functional wireless performance (FWP) of the DUT. FWP
is the metric used to determine success or failure of the DUT in a certain environment. It
defines the important behavior required of the DUT in its radio channel.
A list of the device’s required functions will need to be determined, including: device
startup and connection to the wireless network in the healthcare facility, successful
sending of status reports, five data exchanges per minute while roaming between
access points (APs), etc. The functional wireless performance requirements will depend
very much on the type and application of the DUT and its defined normal operating
behavior.
For optimal field measurements:
• Utilize a RTSA to continually sample spectrum
• Perform a real-time FFT to identify the types of signals present
• Identify the strength of the signals present and their rates of transmission
To optimally define FWP:
• Understand the device type, its application and normal operating behavior
• Determine the required functions the device must perform, i.e. what does an intravenous infusion pump need to do to coexist?
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Step 4: Choose the test’s physical format
There are four possible ways to configure test equipment for coexistence testing,
according to IEEE/ANSI C63.27 and AAMI TIR69. Each configuration is made up of
similar components: the DUT, the device that connects or pairs with the DUT, competing
network devices, and a spectrum analyzer. The configuration to use depends on
practical considerations, such as access to an external antenna connection on the
DUT, and whether or not the device will operate in a MIMO network or whether it has
directional antennas.
• Conducted test method
This method is performed entirely with coaxial cables connecting the test equipment
and the DUT. It is the most quantitatively accurate test, but the least realistic at
simulating the operating environment. It is best suited for tuning the DUT hardware
and firmware, and for measuring improvements resulting from the changes.
• Multiple chamber test method
This method uses multiple non-reflective chambers to provide a calibrated field at
the location of the DUT. It allows the actual antennas for the device to be included
in the test and radiated path loss to be controlled. Additionally, the shielding
effectiveness of this method eliminates other potential interference sources.
• Radiated-anechoic chamber (RAC) test method
This test method uses a single larger anechoic test chamber to ensure that the
environment does not decrease the repeatability of results. This method provides
additional information about the way in which the DUT fails. Assuming the pattern
of the radiating antenna is known, it is possible to determine which part of the DUT
is susceptible to interference. With this information, the design can be revised,
mitigating the risk due to interference.
• Radiated Open Environment (ROE) Test Method
In this open-air test method, all equipment is placed in an open area, presumably
with limited active radio networks other than those used in the testing. Of all four
methods, this is the least quantitative test, but it is also the most realistic. However,
it is vulnerable to disruption from unexpected signals, and is therefore the least
repeatable method.
To choose the right test format:
• Make the selection based on practical considerations
• Select the method that matches the accuracy you require or how accurately the method simulates the operating environment
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Running the Coexistence TestUnlike electromagnetic compatibility (EMC) testing, coexistence test is not a pass or fail
test. It focuses on the probability that the DUT can meet its FWP under the conditions of
the test. For medical device tests, it provides a measure of risk that a device will or will
not perform properly under the modeled conditions.
Depending on the format of the test, device designers can vary the conditions of
physical separation, frequency separation, data rates, and formats that are pre-defined
in the FWP. As an example, a device designer might choose to vary the DUT signal
strength at the receiving device from -40 to -90 dBm in steps of 5 dBm. The interfering
signal can be varied over the same range of signal strength, increasing the interfering
signal data rates at each step until the DUT fails to achieve the desired FWP goals. Each
step test may be run for a certain period, looking for failures to meet FWP targets. The
result will be a chart of Likelihood of Coexistence as defined in the test design.
Regardless of the test format, the actual RF test conditions throughout the test are
measured and documented using a spectrum analyzer. A Real Time Spectrum Analyzer
(RTSA) that can continually sample the spectrum with a high-speed analog-to-digital
converter (ADC) and then perform real-time fast Fourier transform (FFT) to convert the
results to a spectral view will be needed to see the radio signals and quantify the test
environment.
Page 8This information is subject to change without notice. © Keysight Technologies, 2018, Published in USA, July 17, 2018, 5992-3095EN
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ConclusionWith more than 20 billion wireless devices predicted to be connected by the year
2020, interference between these devices is inevitable. Coexistence test offers a way
to test the ability of a device to operate in the RF environment, populated by many
different wireless standards. The IEEE/ANSI C63.27 document standardizes how
this type of testing is to be performed, while the AAMI TIR69 document tunes this
standard for medical applications based on risk.
The FDA is expected to issue recommendations for coexistence testing of wireless
equipment used in healthcare soon. Regardless of the application in question—
healthcare, industrial monitoring and control, or environmental measurement—and
what radio spectrum is employed, the methods defined in these documents can be
used to measure the actual performance of a wireless device in a mixed wireless
environment. As the impact of interference on medical devices can be life threatening,
device makers would be well advised to use one of these methods to quantify and
mitigate the risk of interference.
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