Industrial Wireless Technologies and their Applications for Protection Automation & Monitoring

Authors: Palak Parikh, Justin Smith, and Michael Pilon, GE Grid Solutions

Unlicensed Spectrum

No license is required to operate in unlicensed frequency bands, so long as the equipment is approved for use. These unlicensed wireless bands are officially referred to as "industrial, scientific and medical (ISM)" radio bands. Worldwide, there are many different unlicensed bands, with common frequency bands located at 2.4 GHz and 5.8 GHz. In North America, Central America, and South America, another popular band is the 902-928 MHz ISM band. (Figure4).

The benefits of operating an industrial wireless network on an unlicensed channel are:

  • No licenses & No cost. Because no license is required, a user can operate a network as per the application requirements
  • Bandwidth. Unlicensed frequency bands typically offer large amounts of bandwidth (>20 MHz). Licensed wireless channels are typically narrow bandwidth (narrowband) or require a considerable investment to purchase (cellular)

The downside to unlicensed / ISM operation is that the user may have to share this large bandwidth with several other simultaneous users

While indoor Wi-Fi networks can provide multi-megabit per second throughputs, long range 900 MHz ISM systems typically provide less than 1 Mbps. For example, a 900 MHz ISM band system with proper antennas and antenna height can provide 250 kbps (shared) to 100 devices located within 5 miles of the access point. In the unlicensed bands there are many different types of equipment with a wide variety of performance. Since industrial and distribution protection applications most often involve connecting end devices to networks, we will look at two use cases: short range/high throughput over Wi-Fi, and long range/medium throughput over 900 MHz radio.

Wi-Fi:  Unlicensed wireless networks are often privately owned and operated within the 2.4 GHz and 5.8 GHz bands. The large bandwidth and GHz carrier frequencies mean that these networks are well suited for indoor operation requiring high data throughput.

With their lower transmit power and higher frequencies (high frequencies propagate less distance), unlicensed networks tend to be short-range (300 meters or less) with a medium number of nodes per network (10-50 devices).

As Wi-Fi has matured it has become much more secure and trusted for industrial applications. As such, it has become a popular way to achieve short range and high throughput connectivity; Wi-Fi is often updated and upgraded with improvements to speed and QoS. For most current industrial use cases, IEEE 802.11 g/n are the most popular. (Table 2).

Long Range 900 MHz:  With greater transmit power and lower carrier frequencies (lower frequencies propagate a greater distance), the 900 MHz ISM band is often used for outdoor/long-range applications. While still used for indoor applications as well, large networks of 900 MHz ISM equipment for monitoring distant assets is common. These networks tend to be built in a similar fashion to licensed wireless systems: point-to-multipoint topology, access point antennas mounted high on a tower, remote directional antennas, and large numbers of remote devices (100-200).

Wireless equipment used in the unlicensed band is designed to share the air waves with other equipment. It is important to realize that while the equipment will tolerate interference, performance "mileage" varies depending on how many other networks are operating nearby, with range, throughput, and latency all affected by congested ISM bands.

For industrial outdoor use-cases, the 900 MHz unlicensed band provides a good combination of throughput, range, and robustness. The greater transmit power, lower carrier frequency, and lower bandwidth results in equipment in this band providing reliable long-range communications. The best equipment for this band features frequency hopping, robust receiver design, and several modulation choices - all allowing for reliable communications in this shared spectrum. Since the unlicensed band is characterized by proprietary solutions, performance varies depending on the technology used. Below we show typical performance for equipment designed for outdoor / long-range use (Table 3).

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