Medical Coexistence Testing Methods for Bluetooth and WLAN

Publish Date: May 08, 2013 | 0 Ratings | 0.00 out of 5 | Print | Submit your review

Overview

In wireless coexistence scenarios, two or more collocated devices transmit data sharing available time and frequency resources.  The FDA has issued a draft guidance to assist designers with the development of radio-frequency (RF) wireless technology in medical devices. This example program describes 802.11g channel allocation and interference, then includes two example programs: one example to generate 802.11g traffic and another to characterize results. Also included is an existing record/playback LabVIEW VI, as well as a sample .DAT file or 802.11g traffic.

Table of Contents

  1. Wireless Medical Product RF Characterization and Test
  2. Understanding 802.11g Channel Allocation and Interference
  3. Using Software Defined Radios for Coexistence Testing
  4. Additional Information

1. Wireless Medical Product RF Characterization and Test

In wireless coexistence scenarios, two or more collocated devices transmit data sharing the available time and frequency resources.  At the receiver side, due to the broadcast nature, the intended signal and the interfering signals may be superimposed, thus degrading system performance.  The FDA has issued a draft guidance to assist designers with the development of radio-frequency (RF) wireless technology in medical devices.  It emphasizes the need to address wireless coexistence, performance, data integrity, security, and electromagnetic compatibility (EMC).  Because these issues affect all stage of a product's design cycle, each of these issues should be considered during the identification, documentation, and implementation of product design requirements, as well as during design V&V and risk management processes.

To protect against electromagnetic interference, wireless medical devices should limit their RF output to the lowest power necessary to reliably accomplish their intended function.  Typically,  device manufacturers describe in pre-market submissions and in their labeling, the wireless technology and RF specifications, the testing performed and the testing results demonstrated.  This is to show that the wireless functions will operate safely and effectively in the intended environment.

To learn more about the standards one should be aware of in regard to coexistence testing methods that address the grey area between radio coexistence and RF immunity see the document Wireless Medical Device Testing with LabVIEW.  This document describes 802.11g channel allocation and interference, then includes two example programs: one example to generate 802.11g traffic and another to characterize results.  Also included is an existing record/playback LabVIEW VI, as well as a sample .DAT file or 802.11g traffic.

 

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2. Understanding 802.11g Channel Allocation and Interference

A common wireless implementation many designers are considering is the 802.11 standard.  Many modalities to the 802.11 standard exist, one being 802.11g, having channels ranging from 2412 MHz (channel 1) to 2462 MHz (channel 11) in the United States and Canada. Comparing coexistence to the ZigBee 802.15.4 standard, for example, this means that at most 3 non-overlapping 802.11g networks may coexist in a worst case interference scenario for ZigBee, using carrier frequencies at 2412 MHz, 2437 MHZ and 2462 MHz.  If 802.15.4 links are established in the frequencies between them (2425 and 2450 MHz), the resulting frequency offset becomes either 12 or 13 MHz.  Studies have shown that when a ZigBee network in the presence of an 802.11g interferer, high performance can be achieved in most topologies when the frequency offset between 802.15.4 and 802.11g is at least 18 MHz.

This following examples demonstrates why the medical community must test, identify, and label worst case scenarios where a device may not be appropriate for a specific application.  National Instruments RF hardware and software enable a user to test not only the common modalities, but also the various points between.

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3. Using Software Defined Radios for Coexistence Testing

Software defined radios (SDRs) have emerged as a viable option for next generation wireless research and test by enabling users to quickly define a system.  NI RF hardware and software is an excellent choice to emulate real-world conditions.   The NI Vector Signal Generator (VSG) can mimic the transmissions of multiple network nodes, allowing full control over interference parameters along power, frequency and network duty cycle.  With NI LabVIEW, tests are fully repeatable, while testing different protocols requires only a change in VSG software.

Emulating 802.11g

The NI Bluetooth Measurement Suite is a software-defined approach to Bluetooth testing. With this solution, one can generate and acquire Bluetooth signals using vector signal generators (VSGs) and vector signal analyzers (VSAs).  To assist in medical device coexistence testing the following example was provided by the Electromagnetic Compatibility and Design Center (WECAD) at the University of Oklahoma (OU) to simulate 802.11g traffic.

The following file called emulate_80211g.zip contains the following:

As shown in Figure 1, the file 80211g_18.vi will generate traffic with parameters defined by the user.  The NI 5673E Vector Signal Generator provides full control over generation parameters (frequency, power, modulation, etc).
Script Mode allows for generation of modulated waveform for precise time duration, and allows for any desired inter-packet delay time. A script can be hundreds of thousands of lines long which allows for generating hundreds of thousands of packets., while waveform generation can be controlled with hardware or software triggers

Figure 1. LabVIEW VI to generate 80211g traffic using script mode.

 

Another LabVIEW file called inter-arrival times v4.vi is used to characterize the traffic.  It determines the packet length and inter-arrival time distribution for network activity that has been streamed to disk previously using the shipping VI called Stream Binary IQ Data to Disk.vi.

A third core VI in the file called multirun v4.vi.  It is used to analyze multiple files consecutively by calling the VI inter-arrival times v4.vi.  See Figure 2.

An example data file is 80211sampledat.dat.

Figure 2. File Structure of the Folder

 

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4. Additional Information

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.

RF Wireless Coexistence Testing for Medical Devices- This document discusses wireless coexistence test for wireless-based medical devices with example application code written in NI LabVIEW to demonstrate automated test generation. 

Testing BAN (WBAN) Networks and IEEE 802.15.6- This tutorial describes test methodologies and techniques for testing 802.15.6 Body Area Networks (BAN)

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

Originally Authored By: Greg Crouch, National Instruments

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