RF Wireless Coexistence Testing for Medical Devices

1. Radio-Frequency (RF) Wireless Technology in Medical Devices

Medical device manufacturers are increasingly incorporating radio-frequency (RF) wireless technology into their medical devices. Communication is accomplished using either published IEEE standards or proprietary protocols, many of which operate at unlicensed frequencies in the Industrial, Scientific and Medical (ISM) bands or in the Medical Implant Communication Service (MICS) band. Testing to assess the risks associated with wireless medical devices is a growing concerning for the Food and Drug Administration (See References (FDA) [1, 2]).

To date, limited consensus standards address the risks associated with wireless coexistence for medical devices and systems.  Most current methods of evaluating wireless coexistence use test methods that vary widely among device manufacturers. Moreover, current medical device electromagnetic compatibility (EMC) standards often do not define requirements or test procedures to assess the performance of systems containing RF receivers in the presence of in-band interference.

Coexistence Concern Background

Wireless coexistence can be defined as the ability of multiple heterogeneous wireless systems to share the same or adjacent frequency spectrum without undue interaction or interference affecting performance and transmission or reception of signals and data.  Coexistence is a growing concern in wireless communication standards. One of the first standards that dealt with coexistence was 802.16.2-2001 [3], which recommended guidelines and deployment practices for minimizing interference among fixed broadband wireless access systems. It covered frequencies from 10-66 GHz. IEEE 802.16.2-2004 [4] superseded it. IEEE 802.15.2-2003 [5] then recommended practices for coexistence among personal area networks and other selected wireless devices operating in unlicensed frequency bands. 

That same year, IEEE 802.15.4-2003[6] recommended factors that should be taken into account to allow for coexistence.  Found in Annex E. IEEE 1900.2-2008 [7], it recommended interference analysis criteria for measuring interference between wireless systems. It lists an exhaustive list of coexistence factors in the physical layer and medium access control layer that should be taken into account. The standard also suggests a structure for a coexistence report.

The IEEE 802.19 Wireless Coexistence Working Group [8] is a technical advisory group that was created based on the success of 802.15.2, to act as a coexistence advisory committee, and to act as a coexistence advisory committee across all of IEEE 802. Its primary focus is IEEE 802 standards operating in the unlicensed bands. Currently, the 802.19 working group is working on a recommended practice for methods for assessing coexistence of wireless networks. Each of these standards [3-8] does adequate work analytically laying out guidelines for determining coexistence; however, lack experimental coexistence setups to evaluate interference among the standards. To address the above concern, the American National Standards Institute (ANSI) C63, subcommittee 8 formed a working group to seek solutions, known as C63.27.

FDA Guidance Documents Radio Frequency Wireless Technology in Medical Devices

The FDA's draft document, Radio‐Frequency Wireless Technology in Medical Devices; Jan. 3, 2007, asks manufacturers many questions concerning wireless coexistence in their electronic medical device.  Two important topics include:

1. Explain all devices to be included in the coexistence test plan and justify their choice and how they represent a reasonable worst case scenario.  Include all pertinent information such as transfer power, frequency, modulation, data rate, data flow, protocol, security, etc for your device and all possible interfering devices. 

2. Explain how the wireless coexistence testing was performed and how the testing addresses the possible risks from other wireless devices.  Important parameters are separation distances, number of interferers, location, orientation, etc.

Using this documents as a guideline, as well as Annex E. IEEE 1900.2-2008 [7] as a template list of coexistence factors in the physical layer and medium access control layer, will assist in RF coexistence test plan management.

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2. Medical RF Coexistence Factors

Coexistence among wireless devices is dependent on three main factors: 1.) frequency, 2.) space, and 3.) time. In terms of frequency, the probability of coexistence increases as the frequency separation of channels increases between wireless networks. In terms of space, the probability of coexistence increases as the signal-to-interference-ratio (SIR) of the intended received signal increases. In terms of time, the probability of coexistence increases as the overall channel occupancy of the wireless channel decreases. 

An important factor to achieving coexistence lies in the ability to at least control one of the three factors. Coexistence is possible given one of the three following conditions: 1.) adequate frequency separation between wireless networks; 2.) sufficient distance between wireless networks, effectively decreasing the SIR in each; or 3.) relatively low overall occupancy of the wireless channel. 

The remainder of this document discusses an example non-line-of-site (NLOS) and a line-of-site (LOS) test setup, and references a NI LabVIEW software examples for coexistence testing.  

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3. Using NI PXI Hardware for Medical RF Coexistence Test

The test setup below is for a medical device that utilizes ZigBee in the 2.4 GHz ISM band for its wireless telemetry.  The explanation is described for a system test setup at the Wireless and Electromagnetic Compatibility and Design Center (WECAD) at the University of Oklahoma (OU) 

Line-of-sight (LOS) and Non-line-of-sight (NLOS) Testing

During the compliance testing phase for medical devices, the transmit power of the wireless medical device may not easily be adjustable. In that case the path loss of the wireless signal is adjusted for the test setup. The wireless medical devices should be separated in which the received signal strength (RSS) at the receiver is at its minimum while packet error rate (PER) remains at 0%. This emulates that the receiver is being placed on the outer edge of the transmitter’s cell area, creating a worst-case scenario. The IEEE 802.15.4 standard [9] specifies that a compliant device shall be capable of achieving a sensitivity of -85 dBm or better. To achieve a symbol error rate of 10e^-5 for quadrature phase shift keying (QPSK) modulation, a 13.5 dB SNR ratio must be maintained [10]. Taking these requirements into account and compensating for the transmitter’s power amplifier variation, which will vary among devices, the RSS measured at the medical device receiver is suggested to be -70 dBm. A baseline test should be conducted to validate a 0% PER among wireless medical devices.

The test setup, shown in Figure 1, is utilized to control the received signal strength for the medical device under test and can be performed outside of an anechoic chamber. It is suggested to first perform an ambient scan at each point where the wireless medical devices are tested to identify background noise in the 2.4 GHz ISM band. The background noise should be kept below -85 dBm. It should be noted that the dimensions of the test setup are arbitrary. Emphasis is placed on the RSS of the RF signal at the receiver. 

Two alternatives are available for test setup dimensions: line-of-sight (LOS) and non-line-of-sight (NLOS). The transmission range of ZigBee can be from 20 to 100 meters in a LOS deployment. An anechoic chamber this size is not always readily available or financial viable. An alternative, as utilized at the University of Oklahoma WECED, is to perform coexistence testing in a NLOS test setup while still maintaining the repeatability of the tests. Toscano [11] showed that the RSS for IEEE 802.15.4 is directly related to the distance and are also quite stable, as the coefficient of variation was below 2% in almost all the performed measurements.

Figure 1. Wireless Electromagnetic Compliance and Design (WECAD) Center at the University of Oklahoma (OU)

Testing in a Multipath Environment Non Line of Site (NLOS)

In a multipath environment non line-of-site (NLOS), time delay spread is known to cause bit error in the form of inter-symbol interference. A large time delay spread cannot be neglected, thus the transmitted signals will suffer from frequency selective fading which will cause irreducible bit error [12]. However, if the symbol rate of the transmitter is lower than the coherent bandwidth, the adverse effect of the channel time delay spread on the received signal can be neglected and the multipath propagation causes only transmitted signal fading, and Gaussian noise becomes the dominant factor causing bit error [13]. For ZigBee transmissions, the wireless network is capable of working in a reverberation chamber and is only seriously limited for a value of Q-factor above 5000, which is greater than anything typically found outside a reverberation chamber [14]. The ZigBee PER is below 1% with a Q factor of 1000.  Johnson et al [15] measured Q in harsh military related environments and found no Q factor greater than 1000. Typically, a 1000 Q-factor should not be present in any practical environment in which a wireless medical device operates. Thus, a NLOS ZigBee network test setup the delay spread of the ZigBee wireless network can be neglected as a contributor to bit error.

Test Setup for a ZigBee based Medical Device

All wireless nodes should be placed on wooden tables at a height of 1 meter from the ground. The separation distance between the interfering wireless network using the NI PXIe-5652 and the wireless medical device under test are determined by ANSI C63-18 [16]. Initial and minimum distances, shown in Table 1, are based on the transmitting power of the interfering wireless network emulated by the NI PXIe-5652.  The NI PXIe-5663E is placed near the wireless medical device under test to monitor the duty cycle and average power of the interfering network.

Table 1. Initial and Minimum Distances Required by ANSI C63-18

It is suggested to evaluate the wireless medical receiver and transmitter to the interfering network. Various interference phenomena occurred depending on whether or not the interfering wireless network is in proximity to the transmitter or receiver of the wireless network under test. When a receiver is surrounded by an interfering network, packet collisions can increase at the receiver, i.e., the hidden terminal effect. In contrast, when a transmitter is surrounded by an interfering network, channel utilization can decrease, i.e., the exposed terminal effect.

A vector signal transceiver (VST) is a new class of instrumentation that combines a vector signal generator (VSG) and vector signal analyzer (VSA) with FPGA-based real-time signal processing and control. The NI VST from National Instruments also features a user-programmable FPGA, which allows custom algorithms to be implemented directly into the hardware design of the instrument. This instruments can be used in place of both the PXI5662/3 instruments.

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4. Using NI LabVIEW Software for Medical RF Coexistence Test

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. Studies have also shown that as the polling window increases, the probability of packet loss decreases. 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.

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. 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 this document).

The following link is an example NI LabVIEW application to characterize wireless channel activity for duty cycle, inter-packet interval distribution and network activity to assist RF coexistence test. NI LabVIEW with its graphical programming paradigm allows a developer complete control of RF waveforms for coexistence testing.   

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5. Additional Information

Learn more by visiting http://www.ni.com/medical/

WECAD

The Wireless Electromagnetic Compliance and Design (WECAD) Center at the University of Oklahoma (OU) has developed a coexistence environment and testing protocol for various wireless technologies, including IEEE 802.11a/b/g/n (WiFi), Bluetooth, ZigBee, ultra wideband (UWB), and cordless phones, under controlled parameters of modulation, transmission power, and frequency channels. The Center’s objective is to develop a wireless coexistence framework/methodology for medical devices employing RF communication and operating on the unlicensed 2.4 GHz ISM band.  

RF Record and Playback

RF Record and Playback of medical RF signals- National Instruments RF record and playback systems combine PXI RF signal analyzers and RF signal generators with RAID arrays for high-speed, long-duration recording and playback.  This platform can be used to measure common wireless technologies in such environments as Hospitals:

• Commercial/ Public radio services (FCC)
• Wireless Medical Telemetry Service (WMTS)
• Cell phones, Wireless handheld computers
• Wireless local area networks (802.11.a/b/g) (future 802.11.ac)
• Personal area networks including 802.15.1 (Bluetooth), 802.15.4 (Zigbee)
• RF Identification (RFID) Bar code readers

Testing Emerging Medical RF Standards

Testing BAN (WBAN) Networks and IEEE 802.15.6 - A Body Area Network (BAN) is a communications method optimized for low power consumption.  Wireless body networks will emerge as a key technology in providing real-time health monitoring and diagnoses of many life threatening diseases. This tutorial describes test methodologies and techniques for testing. The IEEE 802.15.6 defines both a PHY and MAC layer that can satisfy BAN needs. It can use existing Industrial Scientific Medical (ISM) bands as well as frequency bands approved by national medical and regulatory authorities.

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6. References:

1.  D. Witters, S. Seidman, H. Bassen, “EMC and Wireless Healthcare,” IEEE 2010 APEMC, April 2010, pp. 5-8.
2.  N. LaSorte, H. Refai, D. Witters, S. Seidman, J. Silberberg, “Wireless Medical Device Coexistence,” Journal of Medical Electronics Design, 2011.
3.  IEEE Recommended Practice for Local and Metropolitan Area Networks Coexistence of Fixed Broadband Wireless Access Systems, IEEE Standard 802.16.2-2001.
4.  IEEE Recommended Practice for Local and Metropolitan Area Networks Coexistence of Fixed Broadband Wireless Access Systems, IEEE Standard 802.16.2-2004.
5.  IEEE Recommended Practice for Information Technology—Telecommunications and Information Exchange Between Systems—Local and Metropolitan Area Networks—Specific Requirements Part 15.2: Coexistence of Wireless Personal Area Networks With Other Wireless Devices Operating in Unlicensed Frequency Bands, IEEE Standard 802.15.2-2003.
6.  IEEE Standard for Telecommunications and Information Exchange Between Systems—LAN/MAN Specific Requirements—Part 15: Wireless Medium Access Control and Physical Layer Specifications for Low rate Wireless Personal Area Networks, IEEE Standard 802.15.4-2003.
7.  IEEE Recommended Practice for the Analysis of In-Band and Adjacent band Interference and Coexistence Between Radio Systems, IEEE Standard 1900.2-2008.
8.  http://ieee802.org/19/
9.  IEEE Std 802.15.4-2006, “Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs),” 2006.
10. J. R. Barry and E. A Lee, Digital Communication: Third Edition. Springer, 2003.
11. E. Toscano and L. Lo Bello, “Cross-channel interference in IEEE 802.15.4 networks,” Proc. IEEE WFCS 2008, May 2008, pp. 139–148.
12. C. I. Chuang, “The Effects of Time Delay Spread on Portable Radio Communications Channels with Digital Modulation,” IEEE JSAC, vol. SAC-5, no. 5, June 1987, pp. 879–89.
13. Y. Chen and J. C. Chuang, “The Effects of Time Delay Spread on TCM in Portable Radio Environments,” Proc.  IEEE Conference on Universal Personal Communications, Nov. 1995, pp. 133–37.
14. D. Hope, J. Dawson, A. Marvin, M. Panitz, C. Christopoulos, P. Sewell, “Assessing the Performance of ZigBee in a Reverberant Environment Using a Mode Stirred Chamber,” Proc. IEEE EMC, Aug 2008, pp. 1-6.
15. D. Johnson, M. Hatfield, M. Slocum, T. Loughry, A. Ondrejka, R. Johnk, and G. Freyer, “Phase II demonstration test of the electromagnetic reverberation characteristics of a large transport aircraft,” Naval Surface Warefare Center, Dahlgren, Virginia, USA, Tech. Rep. NWSCDD/TR-97/84, September 1997.
16. Institute of Electrical and Electronics Engineers, Inc. American national standard recommended practice for on-site ad hoc test method for estimating radiated electromagnetic immunity of medical devices to specific radio-frequency transmitters. (standard C63.18). Piscataway, NJ: IEEE, 1997.

Originally Authored By: Greg Crouch, National Instruments