How to choose wireless technology for industrial applications
Wireless technologies are rapidly being adopted for simple monitoring and control to supervisory control and data acquisition (SCADA). Wireless provides highly reliable data communications in harsh and interference-heavy environments. Compared with traditional cable-based circuits, wireless technology offers advantages, including increased flexibility, easy installation, and cost savings.
Wireless has quickly grown from a convenience used in our everyday consumer lives to a means of collecting information and controlling systems in the most demanding industrial applications. With rapid developments in wireless, the technology can go farther, communicate faster, and provide increased reliability and security. Thanks to these improvements, many industrial users are adopting wireless technology. While wireless might not be right for every application, it certainly has its place in the industrial world.
Knowing a little about general RF (radio frequency) characteristics, such as distance and speed, transmission methods, and the various wireless technologies available, will help in making an informed decision about which technology or product a facility should deploy.
General RF characteristics
It is important to have a fundamental understanding of RF prior to evaluating different transmission methods and wireless technologies. All wireless systems operate on some frequency, communicate some distance, and offer some over-the-air (OTA) data rate. These properties impact each other, so it is important to find the balance between them to satisfy the needs of a particular application.
Frequency is the key identifier for a radio. The frequency on which a radio operates defines if the system is license-free or if a license from the local regulatory body such as the FCC (Federal Communications Commission) is required. In the United States, there are three generally used license-free frequency bands: 900 MHz, 2.4 GHz, and 5 GHz. These are commonly called the ISM (industrial, scientific, and medical) bands, allotted by the FCC for license-free radio operation. To use one of these license-free bands, the FCC requires that a radio operate within specific guidelines including some spread spectrum technology. See “Transmission methods,” below.
If a radio does not operate in one of the specified license-free bands, then an application must be submitted to the FCC to use a radio. This type of radio is considered a licensed radio and typically uses fixed frequency technology (also see the “Transmission methods” section).
Frequency defines if a radio is licensed or license-free and can play a large part in transmission distance. In radio networks, once a signal has left an antenna and is communicated over the air, energy continues to be lost over the communications path. This general energy loss is considered free space loss, which is a function defining lost energy at a frequency over a distance. Chart 1 shows free space loss at popular radio frequencies over distance. This chart shows that as frequency and distance increase, so does free space loss.
Fundamental principle: Using a technology with a lower frequency will result in greater communication distances.
Over-the-air (OTA) data rate
A radio does not operate at one frequency; instead, it operates in a frequency band. This band can be used in its entirety or in smaller sections, typically called channels. Radios that operate in the license-free bands are not allowed to use the whole band and must use channels. The spectrum that the radio operates in (whether it is an entire band or specific channels) is considered the radio’s channel bandwidth.
A high-speed radio uses a wider channel bandwidth regardless of the transmission method. Channel bandwidth refers to the amount of data that can be transmitted by radio signal. It is measured in bytes transferred over a specific prescribed period of time (kbps or Mbps). Higher speed communications require a wider bandwidth, making high-speed radios more susceptible to interference. This is because there is higher probability of existing interference over the used band and because there is less energy per data bit.
Energy per bit is the amount of energy available to send each bit of data over the air. The slower the transmission rate, the higher the energy level per bit. Higher energy per bit results in greater achievable transmission distance. Therefore, longer range and higher interference immunity result from reducing the transmission speed.
Fundamental principle: A general trade-off must be made between distance and over-the-air (OTA) data speed. The slower the OTA speed, the farther the radio system can communicate.
Different wireless transmission methods have various unique characteristics. These variations result in the ability to communicate different amounts of data at varying distances. One or more of the following transmission modes are used in each of the wireless technologies defined below.
Fixed frequency transmits a signal on one frequency with a specific channel width (usually very narrow). Fixed frequency radios typically have high power transmitters and require a license to operate.
A strong interference can affect a fixed frequency transmitter if it is in or near the channel. The licensing requirement prevents a nearby system from operating in the same channel, reducing the likelihood for interference.
Frequency-hopping spread spectrum (FHSS) transmits radio signals by rapidly switching a carrier among many frequency channels by using a pseudorandom sequence known by both transmitter and receiver. FHSS tolerates interference because a transmission will immediately re-send on the next hop if it is blocked on a channel.
Direct-sequence spread spectrum (DSSS) broadcasts transmission signals that spread over the channel bandwidth of a device’s transmitting frequency. User data is combined with a “spreading code” before it is sent over the air, creating a wide band transmission. Interference is primarily rejected in the demodulation process in the receiver. When the spreading code is removed to extract the user data, the noise signal is simultaneously suppressed.
Orthogonal frequency-division multiplexing (OFDM) broadcasts on multiple subcarrier frequencies simultaneously. Each subcarrier is essentially a narrow band transmission, summarily allowing high data rates to be achieved. OFDM is flexible in coping with severe channel conditions. Because of the higher complexity of OFDM transmission, a variety of methods handle interference. Narrow band interference is tolerated because of the high number of interleaved subcarriers and channel coding mechanisms similar to DSSS.
A variety of wireless technologies suit different applications. Each technology below uses a frequency and one or more of the transmission methods previously discussed. Chart 2 below shows the ideal industrial application(s) for each technology. Understanding different wireless technologies will make it easier to select the proper technology for a specific application.
IEEE 802.15.4 (WirelessHART/ISA 100.11a/ZigBee)
Frequency: Generally 2.4 GHz
Distance: Generally <300 feet
- Intended for inexpensive, self-organizing mesh networks that require low data rates
- Resulting network uses small amounts of power, allowing individual devices to run for up to 5 years on the original battery
Proprietary spread spectrum
Frequency: Depends on manufacturer
Distance: Depends on manufacturer
- Radio technology is unique to the device manufacturer and will not operate with another manufacturer’s devices
- Customizable to fit a specific application
- Available in a wide price range based on features and performance
Frequency: 400 MHz
Distance: Typically 2-50 miles
- Requires a frequency license from the local regulatory body (such as the FCC) to operate on one, fixed frequency
- Used for long-distance links because it uses high-power transmitters (up to 5 W)
Frequency: 850/1900 MHz (USA)
Distance: Around the world
- Cellular technology that is globally available, providing access to anywhere in the world that cellular servers exist
- Requires a service plan
Frequency: 2.4 GHz
Distance: Generally <500 ft
- Used in phones, printers, headsets, and other products
- Industrial products use Bluetooth technology to send I/O, serial, or Ethernet data
WLAN (IEEE 802.11 a/b/g)
Frequency: 2.4 GHz/5 GHz
Distance: Generally 1,000-3,000 ft
- Provides high-speed wireless data networking up to 54 Mbps
- Enables networking for static and mobile applications
Know the differences
Different wireless technologies are designed for various applications, so it is important to understand the differences. Knowing these differences allows the user to make an informed decision about the correct technology for a particular application. In addition, it is useful to understand general principles of wireless communication. That is, generally, the lower the frequency and OTA data rate used, the greater the distance that can be achieved.
Wireless communication is no longer just for the consumer industry. The technology has its place in industrial applications. Wireless can replace cables, reduce installation times, and increase productivity by providing the ability to access data that might previously have been unobtainable.
– Also read from Control Engineering:
Control Engineering industrial wireless coverage – www.controleng.com/wireless
– Ira Sharp is product marketing lead specialist-wireless for Phoenix Contact, www.phoenixcontact.com/wireless.