MicroStrain by HBK Guide to Optimal RF Performance

This guide is broken up into four sections. Part I reviews some of the basics of 2.4 GHz signals. Concepts introduced in this section will be referred to in the next two sections. Part II provides specifics about the radiation patterns of MicroStrain wireless products. Part III introduces the software tools MicroStrain provides to evaluate the quality of the RF network. Part IV provides some practical tips for deploying MicroStrain wireless networks.

MicroStrain wireless products transmit in the 2.4 GHz ISM band. Operation in this band is available worldwide; however many countries require unique certifications. MicroStrain wireless are products are certified for use in the United States, Canada, the EU, China and many other countries, but not all.
 

The operation of our radios in the 2.4 GHz ISM band is specified by the IEEE 802.15.4 standard. Other standards also operate in this band including WiFi and Bluetooth.

As the graph above shows, the IEEE 802.15.4 standard breaks the 2.4 GHz band into 16 channels, channel 11 through channel 26. The center frequencies of each channel are spaced 5 MHz apart. Each channel is 2 MHz at the -3 dB point, but as the plot shows the full bandwidth of a channel can bleed into adjacent channels, but at very low power.

Other standards such as WiFi and Bluetooth divide up the 2.4 GHz band differently. Operation of neighboring Bluetooth and WiFi signals can cause interference with MicroStrain wireless products; however with some planning and testing these devices can operate alongside MicroStrain devices.

dBm is the power of an RF signal relative to 1 milliwatt. It uses the logarithmic decibel scale (dB).

Here is the formula to convert mW to dBm:

dBm = 10 * log₁₀mW

When selecting transmit power in SensorConnect or monitoring RSSI, the signal strength is often expressed in dBm. The following table provides some useful conversions of dBm to milliwatts.

As the above table shows, a 2.4 GHz receiver can pickup signals as low as -70 dBm or 0.0000001 mW or even lower. These receivers are sensitive, and this is a very good thing because even in ideal conditions – through open air with no obstructions – an RF signal gets greatly attenuated as it travels from the transmitter to the receiver. Obstructions between the transmitter and receiver signals cause additional attenuation.

Since dBm employs the logarithmic scale the following relationships apply:

Decreasing a signal by 3 dB, halves a signal’s power: a 10 mW (10 dBm) signal decreases to 5 mW (7 dBm).

Inversely, increasing a signal by 3 dB doubles a signal’s power: a 10 mW (10 dBm) signal increases to 20 mW (13 dBm).

These relationships will come in handy in the next section where we consider the impact of various obstructions on signal strength.

By walls we mean any material that greatly attenuates a 2.4 GHz signal. For example, water, which is transparent to visible light, is a “wall” to a 2.4 GHz signal.

Though many materials attenuate a 2.4 GHz signal, they do often let some signal through, so many of these materials can be thought of as semi-transparent.

The following table provides an estimate of signal attenuation due to common materials.

For optimal performance, it is important to reduce the number of obstructions between a transmitter and a receiver. In the most favorable conditions, no obstructions will lie between a transmitter and a receiver.

Plastic enclosures are popular for protecting sensor nodes from adverse elements like rain and dust. Many plastics produce minimal attenuation for a 2.4 GHz signal. Plastics are more like windows than walls to a 2.4 GHz signal. But be sure to use plastics with no carbon in them. Carbon strongly attenuates 2.4 GHz signal.

LOS, or line of sight, simply means that there is an unobstructed path between a transmitter and receiver. If possible, all wireless DAQs (G-Link-200, V-Link-200, etc) should have a LOS (unobstructed) view of the gateway (WSDA-2000, WSDA-200).

RSSI stands for Received Signal Strength Indicator. RSSI is the strength of an RF signal as it is received, not as it is transmitted. Therefore, RSSI measurements capture the signal attenuation due to range, obstructions and multipath fading.

RSSI is technically a unitless, relative indicator of signal strength; however, like WiFi, MicroStrain wireless solutions express RSSI in dBm. The larger the RSSI value, the stronger the received signal. Here are some guidelines for what constitutes good RSSI.

RSSI can be read from the node and gateway easily through the diagnostic packet in SensorConnect.

RSSI indicates the strength of a received signal. Signal strength indicates whether or not a transmitter and a receiver are in range of each other. If the RSSI is too low, near say -90 dBm the transmitter and receiver are out of range of each other. Basically the signals cannot be differentiated from intrinsic noise sources in the electronics and the environment. At -80 dBm they are in range, but barely, and one can expect a lot of packet loss due to intrinsic noise. At -60 dBm the transmitter and receiver are well within range of each other.

However, when evaluating the quality of wireless network, good RSSI (-67 dBm and better) does not mean that all the transmitted packets will make it to a receiver. A lot of packet loss can occur even when RSSI is very strong, that is because there are other factors that determine wireless network quality: interferers and multipath. These two sources of error will be discussed in the following sections.

Interferers are any source of unwanted signal or noise in the 2.4 GHz ISM band. Producers of interference include microwaves, bluetooth devices, WiFi, and EMI (electromagnetic interference) from machines.

Interferers cause noise on the RF channel and produce packet loss even when the RSSI is strong. If a user is getting good RSSI readings during a range test in SensorConnect, but still seeing a lot of packet loss, there may be an interferer present. The RF Traffic Analyzer available in SensorConnect can be used to help select quiet RF channels.

Multipath describes the phenomena whereby an RF signal is reflected off of surfaces and therefore takes multiple paths to the receiver. The reflections of the signal can arrive at the receiver out of phase (at slightly different times) with each other and this can cause destructive interference. The signals basically diminish each other or even cancel each other out. This is called multipath fading.

Sometimes moving an antenna a few inches, or even angling it differently in an environment where there is a lot of multipath fading, can cause a huge improvement in signal strength.

Multipath can cause signal loss even in conditions where there is good LOS. If the transmitter and receiver have an unobstructed view of each other but there are a lot of metal surfaces surrounding the path, as you might find on a factory floor, multipath fading can occur.

In situations where there is no LOS, multipath can be helpful. Reflections ensure that the signal bounces around the obstructions and can be “seen” by the receiver.

Antennas provided with MicroStrain wireless solutions do not radiate uniformally (the same in all directions). The graphs in this section detail the directions in which the antennas radiate most strongly and most weakly. The diagrams also take into account any attenuation of the radiation strength due to the wireless node itself. For instance, metal or batteries on the node will attenuate signal strength along that path.

Interpreting the diagrams is simple: the longer the arrow, the stronger the wireless node will radiate in that direction. The colors are there to help users quickly identify short and long arrows.

Includes: G-Link-200-OEM, SG-Link-200-OEM, TC-Link-200-OEM, WSDA-200-USB.

Includes: V-Link-200, RTD-Link-200, TC-Link-200, WSDA-2000, WSDA-200-USB-EXT, all -OEMs with external antennas.

Includes: All non-OEM G-Link-200 products, SG-Link-200.

Coax cable length can make a big difference in the range of a wireless transmission. Coax cables that are too long result in unnecessary signal loss. Signal loss is measured in dB per unit length at a specified frequency. The amount of loss is proportional to cable length.

Check the specs of any coax cable to determine its loss per unit length (dB/100 ft). If a cable is rated to have 32.2 dB/100 ft, ten feet of that cable would produce a 3.22 dB loss. This means that signal received at the antenna at -60 dBm would be reduced to -63.22 dBm at the receiver, approximately a halving of the signal's power.

Per FCC rules, antenna changes not expressly approved by MicroStrain by HBK could void the user’s authority to operate the equipment. The antennas must be the same type, must be of equal or less gain than an antenna previously authorized under the same grant of certification (FCC ID), and must have similar in-band and out-of-band characteristics.

The RF transmit power of MicroStrain wireless nodes can be configured by a user. The method using SensorConnect is shown in the image below.

Transmit power is expressed in dBm. The higher the number, the higher the transmit power. Increasing transmit power can extend the range over which a node can communicate with a base station. The base station must also send commands and timestamps to the node, so be sure to adjust its transmit power as well.

RF transmissions account for the largest portion of energy consumption on a wireless node. Finding the right transmit power can be a balance between battery life and communication range.

LXRS and LXRS+ are wireless communication protocols developed by MicroStrain to ensure reliable, synchronized, and lossless data transmission from multiple wireless sensors.

LXRS is more noise immune, so it typically provides better range than LXRS+. LXRS+ offers more bandwidth, allowing for more data channels and higher sample rates.

In applications where range is an issue, LXRS should be used. LXRS is enabled by default for all wireless nodes shipped from the factory. However, changing between LXRS and LXRS+ is easy. It can be done when starting a node or network sampling, as shown below.

The Received Signal Strength Indicator (RSSI) tells users how strong a signal is as received by the either the node or the gateway’s receiver. RSSI is provided as part of the diagnostic packet which is sent periodically, both when the node is actively sampling and when it is in idle. This indicator is a great way to determine whether or not communication issues are due to a lack of signal strength. SensorConnect can plot RSSI over time.

In SensorConnect a user must select RSSI (Node) and RSSI (Base) in the diagnostic packet, as shown in the image below. The diagnostic packet is enabled by default on all units shipped from the factory.

RSSI (Node) is the signal strength of packets when received by the node (sent from the WSDA). RSSI (Base) is the signal strength of packets when received by the WSDA (sent from the node).

The range test feature in SensorConnect allows users to determine the RSSI and packet success rate between a node and a gateway (WSDA). This feature can be used to determine optimal antenna and or node position and to detect the presence of interferers and multipath fading.

During a range test, the gateway pings the node and the node sends a ping response and RSSI information. The range test provides two important pieces of data:

• RSSI: signal strength of the received ping

• Ping success rate: number of pings successfully received

The range test feature can be accessed through the highlighted tile below

Once a user starts the range test, SensorConnects updates the RSSI (pointed to with arrows) and the ping success rate. The current, min, max, and average detail the node RSSI (top arrow) and the gateway RSSI (bottom arrow).

A usable RSSI is typically -70 dBm and higher. RSSIs measuring -80 dBm and lower are considered poor.

Situations where there is too great a distance between the node and the gateway typically show as a constant, weak RSSI. Interferers and multipath fading effects can change over time leading to large swings in RSSI. This table captures some of the possible scenarios a user may encounter while conducting the range test.

The RF traffic analyzer is a tool within SensorConnect that allows user to select the best channel on which to operate their wireless network. The traffic analyzer uses a connected base station (WSDA) to scan the available RF channels for traffic. The light green bar represents the max RF energy transmitted on that particular channel. The purple bar indicates the average RF energy on that channel. And the dark blue is the current activity on each channel.

This tool can help identify busy channels. It is valuable as part of a site assessment. The rule of thumb is simple: choose the channel with the least activity (lowest bars).

To access the RF traffic analyzer, a user must simply click on their base station and select the RF Traffic Analyzer tile.

Then click the start button in the lower left. The x-axis is divided into 802.15.4 channels. A higher resolution graph is available if you unselect the check box in the lower right.

Here are some simple tips for the successful deployment of a MicroStrain wireless network. These recommendations may not always be possible. Many customers have successfully deployed wireless networks while violating many of these best practices. However, we provide them to help customers understand an ideal deployment.

A note about expectations: MicroStrain wireless products can achieve 1 km+ of range, but this is only in ideal conditions: open air with LOS and no surrounding reflective surfaces while using the maximum transmit power of 20 dBm. Most networks are deployed in regions and environments where these conditions are not present. On a factory floor, for instance, range may be limited to 50 meters or less.

• Use the maximum transmit power permitted in your region. (Transmit power impacts battery life so sometimes it customers set it lower than the maximum permitted.) The table below provides maximums by region based on the current certifications for our products. Please contact MicroStrain support if your region is not listed.

• Raise the antennas off of the ground. We recommend to elevate them to head height or higher.

• Keep the antennas away from nearby obstructions, particulary metal.

• Orient the node antennas so that their most active regions are “pointing” toward the gateways. See section II for details on the active regions of the antennas for each node-type.

• Maintain LOS. Place the node antennas so that there are no obstructions between them and the gateway.

• Use the RF Traffic Analyzer to select a quiet channel.

• Perform a range test on each node in the network. In situations where there is a lot of multipath fading, small adjustments in antenna position may produce big improvements in RSSI and packet success rate.