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Home > Wireless Medical Device Coexistence
Wireless Medical Device Coexistence
Nickolas J. LaSorte, Hazem H. Refai, Donald M. Witters Jr., Seth J. Seidman, Jeffrey L. Silberberg
Created 2011-08-10 18:10
[1]
Wireless Medical Device Coexistence [2]
August 10, 2011
By: Nickolas J. LaSorte, Hazem H. Refai, Donald M. Witters Jr., Seth J. Seidman, Jeffrey
L. Silberberg
[3]
Find more content on: Wireless Technology [4]
Tests to assess the risks associated with coexistence of wireless technologies are
necessary for safe and effective RF wireless medical devices.
At any given time, a typical home or hospital uses a number of wireless systems (e.g.,
IEEE 802.11a/b/g/n, or WiFi; Bluetooth; ZigBee; cordless phones) operating on the same
industrial, scientific, and medical (ISM) band.1,2 Given the increasing use of wireless, RF
wireless medical devices and other wireless systems operating nearby can interfere with
each other. If a collision between their respective transmissions occurs, data packets
transmitted by medical devices could be delayed or blocked, potentially interfering with
timely transmissions of critical data. Techniques such as retransmission and forward error
correction might no longer be sufficient to overcome interference and spectrum
congestion. Hence, methods to design and test wirelessly enabled medical devices for
risks associated with coexistence of wireless technologies are essential for innovative,
safe, and effective RF wireless medical devices.
Although there is some overlap between electromagnetic compatibility (EMC) and wireless
coexistence, differences exist. Wireless coexistence is the ability of one wireless system
to perform a task in an environment where other systems that may or may not be using
the same set of rules can also perform their tasks.3 EMC is the ability of a device to
function properly in its intended electromagnetic environment without introducing
excessive electromagnetic energy that could interfere with other devices. Manufacturers of
electrically powered medical devices routinely test their equipment to applicable national
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and international consensus safety standards. EMC test results are often used to support
safety claims to regulatory agencies such as FDA. Less well-known are the issues and
concerns associated with wirelessly enabled medical devices, although this is changing
thanks to FDA’s guidance document on wireless medical devices.4
To date, no consensus standards adequately address the risks associated with wireless
coexistence for medical devices and systems. Current methods of evaluating wireless
coexistence use ad hoc test methods that vary widely among device manufacturers and
test facilities. Moreover, current medical device EMC standards have no requirements or
test procedures to assess the performance of systems containing RF receivers in the
presence of in-band transmitters. This article examines the limitations of present medical
device EMC standards for coexistence evaluation, identifies factors to be examined when
testing for coexistence, and discusses the status of plans to develop a wireless
coexistence test method.
Limitations of Medical Device EMC Standards
International Electrotechnical Commission (IEC) 60601-1-2— a collateral to IEC 60601-1,
the general safety standard for medical electrical equipment—is the primary standard
used for EMC testing of nonimplanted and nonin-vitro diagnostic electrical medical
devices.5,6 Even if the wireless technology is considered part of the essential performance
and function of the device, meaning its absence would result in unacceptable risk, the
testing and requirements in these standards and the related documents do not fully
address the characteristics and performance associated with the wireless technology.
One major limitation of the IEC 60601-1-2 standard is that RF receivers are exempt from
immunity testing in the exclusion band (passband). This exemption is granted because a
test signal in the passband of a traditional RF receiver would be expected to cause
interference. The exclusion band is defined in 3.10 of IEC 60601-1-2 as the:
…frequency band for intentional receivers of RF electromagnetic energy that
extends from -5 % to +5 % of the frequency, or frequency band, of reception
for frequencies of reception greater than or equal to 80 MHz and from -10 % to
+10 % of the frequency, or frequency band, of reception for frequencies of
reception less than 80 MHz”
For a wireless medical device system operating at 2.45 GHz, the exclusion band covers
the entire 2.4 GHz ISM band. In other words, a medical device with RF wireless
technology is not required by IEC 60601-1-2 to be able to maintain wireless
communication when subjected to RF signals in its passband. Therefore, as published,
this standard does not provide means for assessing the performance of the medical
device wireless communication system. A new draft of Edition 4 of IEC 60601-1-2 would
require that medical equipment remain safe (i.e., provide basic safety and essential
performance) when exposed to an RF immunity test signal in the passband but still would
not assess nonsafety performance under these conditions.7
No Standard RF Wireless Coexistence Test
Wireless coexistence can also be defined as the ability of multiple wireless systems to
share the same or adjacent frequency spectrum without undue interaction or interference
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affecting performance and transmission or reception of signals and data. IEEE 802.15.2
discusses computer modeling and design issues for improving wireless coexistence
between IEEE 802.15 and IEEE 802.11 communication systems.8 The document does
not, however, specify a method or pass-fail criteria for testing coexistence, leaving a gap
in the ability to assess the characteristics and performance of wireless systems.
Due to concern for patient safety and medical device effectiveness, FDA drafted a
guidance document to assist in the design and testing of RF wireless technology in
medical devices.4 The document calls attention to general risks and concerns for devices
operating in the crowded RF spectrum—particularly in the ISM frequency bands. Other
risks, including those that might be affected by quality of service, data integrity, wireless
security, and EMC of wireless technologies and medical device functions, are also
outlined. In addition, the document provides information to assist in preparing regulatory
submissions. FDA remains active and has an interest in the development of standards
and evaluation methods for determining and validating the performance of RF wireless
technology in medical devices in general and wireless coexistence in particular.9
Considerations for RF Wireless Medical Devices and Wireless
Coexistence
The following are major factors influencing the coexistence of wireless medical devices
operating in the presence of other heterogeneous wireless networks, including radio
channel characteristics, antenna and signal polarization, frequency bands, separation
distances, and cochannel and adjacent-channel interference. These factors should be
considered when developing a standardized test protocol for RF wireless coexistence.
Radio Channel Characteristics. The environment in which an RF wireless medical
device is evaluated for coexistence is critical and must be well-characterized to mimic the
expected environment and typical deployment. Medical device deployments include those
that can be connected to or implanted in stationary or mobile patients. Device deployment,
configuration, and environment affect the wave propagation through the wireless channel.
Two such wave propagation scenarios, line-of-sight (LOS) and nonline-of-sight (NLOS),
must be examined. LOS testing can be performed in an anechoic chamber, ensuring
reproducibility by isolating the medical device and its wireless technology from spurious
interference. Conversely, NLOS testing is performed outside an anechoic chamber to
account for the effects of reflection-caused multipath on the received RF signal.
Performance in an NLOS deployment depends upon radio channel path loss among the
transmitter and receiver, frequency band, and transmission power. In an NLOS test setup,
as the signal-to-interference ratio of the wireless medical device decreases, the likelihood
of interference from nearby wireless systems increases.10,11 The delay in the traffic flows
is also exponentially related to the signal-to-noise ratio of the wireless medical device
network under test, which would cause an increase in latency for the wireless medical
device data.12,13
This type of testing for the NLOS configuration outside of an anechoic chamber raises the
possibility of collateral phenomena, including reflections from nearby structures causing
multipath. To account for this, multiple test arrangements of the transmitter and receiver
terminals should be considered. For each test setup, the wireless channel can be
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Figure 1. LOS Test Setup for Wireless Medical Device Telemetry
System
characterized by finding its path loss attenuation and power delay spread, improving test
result reproducibility from one testing environment to another.
The wireless medical device transceivers should be evaluated separately (see Figure 1)—
each separately exposed to one or multiple interfering wireless networks. Various
interference phenomena arise depending on whether the interfering wireless network is in
the proximity of the wireless medical device transmitter or receiver. When a medical
device receiver is surrounded by an interfering network or networks, packet collisions
increase at the receiver (i.e., the hidden terminal effect). In contrast, when a medical
device transmitter is surrounded by one or more interfering networks, channel utilization
decreases (i.e., the exposed terminal effect). Decreased channel utilization is the result of
busy channel sensing followed by increased backoff (contention) windows.
Polarization. Polarization and cross-polarization of both the wireless medical device and
interfering terminals should be considered when testing for coexistence. For example, if
the RF fields from the wireless medical device are intended to be horizontally polarized
when operated, the interfering network should be deployed in both horizontal and vertical
polarizations because an antenna is never 100% polarized in a single mode. Additionally,
radio channel multipath could cause cross polarization in an NLOS test environment.
Cochannel and
Adjacent-
Channel
Interference.
Cochannel and
adjacent-
channel
interference
must be
considered to
effectively
characterize the
probability of
packet
collision.14,15 The
effect of
adjacent-
channel
interference on
the bit-error rate
has been shown to strongly depend on the frequency offset between the channel under
test and the interfering carriers.16,17 The correlation between spatial distance and channel
spacing to control interference between concurrent transmissions in a wireless sensor
multichannel network has also been investigated.18 The impact of cochannel interference
was tested experimentally, and performance was analyzed for IEEE 802.11g networks.19
The impact of adjacent channel interference in 802.15.4 (ZigBee) networks has also been
studied.14 Coexistence testing was performed with a single interfering terminal and
multiple interfering terminals in different combinations, including two interferers in the
same adjacent channel, two interferers in two different adjacent channels, and three
interferers in the same adjacent channel. It has been suggested that the key to
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Transmitter
Power Initial
Distance
m
Minimum
Distance
m
<600 mW 1 0.25
600 mW–2
W20.5
2 W–8 W 3 1
Table 1. Initial and minimum
test distances specified by
ANSI C63.18.
determining coexistence in heterogeneous networks is the study of the effects of multiple
interferers.20
While a number of studies were limited to a specific wireless technology, the
aforementioned practices are recommended across various wireless technology
platforms. Hence, to evaluate a medical device for RF wireless coexistence, testing should
consider a pair (server,client) of interfering terminals configured to operate on the
cochannel and subsequently on the two immediate adjacent channels of the wireless
medical device under test. Does one pair of interfering terminals operating on the
cochannel frequency of the medical device present the worst-case scenario? The answer
depends on the duty cycle (i.e., packet generation and transmission rate) of the interfering
terminals. If the duty cycle is high, the cochannel will be proportionally highly utilized. If
multiple pairs of interfering terminals are configured to operate on the cochannel, the
exposed terminal problem arises, resulting in cochannel underutilization.
Most wireless technologies (e.g, WiFi and ZigBee) employ carrier sensing multiple access
(i.e., CSMA/CA protocol) to avoid packet collision. This regulates channel accessibility by
permitting one terminal to transmit at any given time. Testing with multiple interfering
terminals—where one pair of terminals is set on the cochannel frequency and two others
are set on two different adjacent channels—is highly advantageous, as it evaluates
performance of medical devices designed to automatically select an alternate channel
when interference exists on its current channel. Testing will evaluate the accuracy of the
new selection and the time required for the medical device to switch to the new channel.
Distance. The distance between the interfering wireless
terminal or network and the RF wireless medical device
transmitter or receiver requires consideration because
the likelihood of interference with the medical device
increases as the signal-to-interference ratio
decreases.10,11 Adjusting the distance between the
medical device transmitter or receiver and the
interfering wireless terminal or network is one way to
control the signal-to-interference ratio. The initial and
minimum test separation distances recommended by
ANSI C63.18 should be used, as shown in Table 1.21
Initial and minimum distances are determined based on
the transmitting power of the interfering terminals. The
initial test distance is calculated to expose the device
under test to approximately 3 V/m. The minimum test distances were calculated so that
the device under test would be exposed to approximately 20 V/m. In the C63.18 test
method, if there is no interference at the initial distance, the distance between the wireless
medical device receiver and interfering networks is systematically reduced until
interference occurs or the minimum test distance is reached. Auto-power-leveling
algorithms should be disabled in interfering terminals. Allowed maximum power should be
configured for each interfering wireless device used. The C63.18 test procedure
recommends configuring transmitters to maximum output power. If interference is present
at the initial distance, then the distance between the medical device terminal and the
interfering network terminals is increased until interference ceases. Results are most
reproducible when the output power of the interfering terminals does not change during
the test. The results obtained at various separation distances are used to characterize the
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wireless medical device and inform policies and procedures for EMC management in
healthcare facilities.
Medical Wireless Transmission Parameters
The transmission parameters of the wireless medical device (e.g., packet size, polling
window, clear channel assessment threshold, and duty cycle) can alter the outcome of
coexistence testing. Studies have shown that as the packet size increases, the probability
of packet loss increases.10,11,22 Studies have also shown that as the polling window
increases, the probability of packet loss decreases.11,22 Additionally, results indicate that
when the interference level is below a certain level specified by the device sensitivity, the
channel is sensed as idle, or clear, and the interference does not affect
communication.10,11,14,22 Duty cycle, or channel utilization, is mainly dependent on the
amount of traffic generated and transmitted by the interfering wireless networks. Studies
show that as the interfering device or network increases its duty cycle, the victim network
packet loss ratio increases, causing either temporary or permanent interference.10,11,22
Therefore, two transmission parameter settings should be used during coexistence
testing: typical or manufacturer-suggested default settings and worst-case-scenario
settings (e.g., as suggested by previous work published in the literature).
Case Study
Assimilating the aforementioned coexistence factors into a test protocol is dependent
upon the medical device under test. A case study is offered to serve as an example. Of
note is that a test setup is unique to a specific wireless medical device or system.
The wireless medical system under test was a telemetry system implementing a ZigBee
chipset to establish bi-directional communication between two parts of the system,
hereafter designated as device #1 and device #2. ZigBee is a low-power communication
protocol with a bit rate of 250 kilobits per second in the 2.4 GHz band. When using
ZigBee, selecting channel 13 or 21 for coexistence testing is suggested, the justification
for which is explained below. Channel 21, i.e., 2.455 GHz, was used by the medical
wireless system for this case study.
The RF wireless medical device system was first baselined under static conditions without
interference. The test layout is shown in Figure 1. Medical devices were separated by a
distance of 5 m and elevated on a wood table to a height of 1 m. The separation distance
and height were chosen based on the typical deployment of the wireless medical
telemetry system.
The wireless medical system transmission parameters were evaluated using two
configurations: typical settings (i.e., default manufacturer settings) and worst-case-
scenario settings, as explained previously. The medical device system communicated a
predetermined number of packets to establish a statistical baseline of the wireless link for
each transmission parameter setting. To characterize the wireless medical telemetry
system for this case study, the measured parameters consisted of packet loss ratio, delay,
and throughput. These parameters are used to assess the quality of service provided by
the wireless medical device. The desired quality of service is dependent upon the critical
functions specified by the medical device manufacturer.
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After establishing a baseline, the wireless medical device system was evaluated for
coexistence with other wireless technologies. An IEEE 802.11g network, e.g. Wi-Fi, was
deployed around medical device #2. The 802.11g network consisted of up to three pairs,
each operating on a different channel. It should be noted that auto-power-leveling
algorithms are disabled in the 802.11g network, and the maximum power allowed by the
wireless network standard is used. The initial separation distance between the medical
device and the interfering network was determined based upon the transmission power of
the interfering network, which is less than 600 mW. The ANSI C63.18 table (Table 1)
suggested that the initial distance should be 1 m.
For the first set of coexistence tests, only a single 802.11g network was introduced to the
wireless medical device network. Communication was first established between the
wireless medical devices and then in the 802.11g network. In an attempt to cause
interference with the wireless medical device, the testing parameters were: channels 1-11,
data rate of 1-26 Mbit/s, horizontal and vertical polarization, variable separation distance
between the medical device and interfering network and a single 802.11g interfering
network. After an adequate number of packet transmission attempts between the medical
devices, the interfering 802.11g network was turned off. If interference occurred during
testing, the amount of time was determined that the wireless medical device telemetry
system needed to restore communication after the cause of interference was removed.
After all variables were tested, the positions of medical device #1 and medical device #2
were exchanged, and testing was repeated so that each medical device was tested
separately with the interfering network.
The RF wireless medical device system was then exposed to three 802.11g wireless
networks simultaneously. Channels 9, 10, and 11 were chosen. The channel selection
allows co-channel interference (10) and two different adjacent channel interferences (9,
11) to be tested for the wireless medical device telemetry system. The data rate,
separation distance, and polarization of the three 802.11g networks were variables that
were evaluated. Again, medical device #1 and medical device #2 were separately
exposed to the interfering network.
The NLOS test setup was then performed. The experimental methodology for LOS and
NLOS testing was the same, i.e., the RF wireless medical telemetry system was first
baselined without an interfering network and then interfering networks were introduced
into the environment.
A LOS and an NLOS test setup were then repeated with different interfering networks in
the same frequency band as the wireless medical telemetry system. Examples include
ZigBee, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n, Bluetooth, and cordless phones,
which transmit at 2.4 GHz.
The wireless medical network did not experience a loss of network connectivity during the
LOS testing with one or three 802.11g wireless networks operating simultaneously in the
same unlicensed spectrum band at a separation distance of 0.25 m between the
interfering networks and the wireless medical device under test. All of the wireless
functions operated as intended.
The wireless medical network experienced a loss of network connectivity during the NLOS
testing when one or three 802.11g wireless networks were operating simultaneously in the
same unlicensed spectrum band with a separation of 3 m or less between the interfering
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networks and the wireless medical device under test. When the separation distance was
greater than 3 m, network connectivity was restored to the wireless medical network.
When the throughput (duty cycle) of the 802.11g network was decreased, network
connectivity was restored to the wireless medical network at a separation distance of 1 m.
It was also observed that shorter wireless medical device packets had a higher probability
of successful transmission while longer data packets had a lower probability of successful
transmission.
General Considerations for Coexistence Testing
The RF wireless parameters discussed and the recommendations made in this article are
intended to serve as a starting point for medical equipment manufacturers and healthcare
organizations in assessing the coexistence of RF wireless communication in modern
dynamic electromagnetic environments. Coexistence testing is different from EMC RF
immunity testing, under which the RF immunity of a medical device is characterized over a
range of frequencies.
Pass-fail criteria for coexistence testing must be identified and need to be quantified in
terms of the critical functions of the device under test as determined by risk assessment.
The following parameters can be considered:
Packet error rate.•Latency.•Jitter.•Network throughput.•
In other words, coexistence testing should demonstrate that the effects of the wireless
medical device on nearby RF wireless equipment and networks is minimal and effects of
nearby RF wireless equipment and networks on the functions of the wireless medical
device would not result in unacceptable risk to the patient or user.
Status
Several groups interested in RF wireless coexistence testing of wireless medical devices
have begun work in this area. A working group of ANSI-accredited committee C63 held a
preliminary teleconference in January 2011, and FDA sponsored a teleconference in
February 2011. The University of Oklahoma Wireless and Electromagnetic Compliance
and Design Center, in Tulsa, has an evolving test protocol based on the parameters listed.
The University of Oklahoma is currently experimentally testing medical devices for
coexistence. Plans for future work include proposing that ANSI-accredited committee C63
establish an official working group to develop consensus coexistence standards or
recommended practices.
References
1. N. LaSorte, Y. Burette, H. Refai, “Experimental Characterization of Electromagnetic
Propagation of a Hospital from 55-1950MHz”, in Proceedings of IEEE Asia-Pacific
Symposium on Electromagnetic Compatibility, April, 2010, pp. 826 – 829.
2. N. LaSorte, W. Barnes, H. Refai, “Characterization of the Electromagnetic Environment
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in a Hospital and Propagation Study”, in Proceedings of IEEE International Symposium on
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Nickolas LaSorte is pursuing a PhD in electrical engineering at the University of
Oklahoma. Hazem H. Refai is an associate professor in the university’s school of electrical
and computer engineering and founding director of the WECAD Center. Seth Seidman is
a research electrical engineer at FDA. Donald Witters is chairman of the CDRH EMC and
Wireless Group. Jeffrey L. Silberberg is senior electronics engineer for CDRH, Office of
Science and Engineering Laboratories, Division of Electrical and Software Engineering.
Author:
Nickolas J. LaSorte, Hazem H. Refai, Donald M. Witters Jr., Seth J. Seidman, Jeffrey L.
Silberberg
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