Overcoming Interference is Critical to Success in a ...

Find us at www.keysight.com

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.


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.


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


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.


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?


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


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.

This information is subject to change without notice. © Keysight Technologies, 2018, Published in USA, July 17, 2018, 5992-3095EN


Learn more at: www.keysight.com

For more information on Keysight Technologies’ products, applications or services,

please contact your local Keysight office. The complete list is available at:

www.keysight.com/find/contactus

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.

Bluetooth and the Bluetooth logos are trademarks owned by Bluetooth SIG, Inc., U.S.A. and licensed to Keysight Technologies, Inc.

http://www.keysight.com

http://www.keysight.com

http://www.keysight.com/find/contactus

Overcoming Interference is Critical to Success in a ...

Documents

Transcript of Overcoming Interference is Critical to Success in a ...