Wireless communication for remote monitoring is becoming standard practice in the industrial space. With the price of copper cable and labor continuously increasing and the reliability of wireless communication becoming stronger, the use of wireless in applications that were previously manual is becoming more common. Technology choice is important for wireless applications because each technology has its appropriate environment for use. Since most monitoring applications are not time critical and require the communication to travel long distances through challenging terrain, a more robust wireless technology is the best fit.
Designing a reliable radio system involves many aspects. Much of this work is completed well before the system is installed or even purchased. An engineer should consider factors such as:
- Network topology
- Number of nodes
- The amount of data that will be passed
- The connection medium of the radio network
Each factor is a vital piece of the radio network and choosing the radio technology. If anything is overlooked, the network reliability could be compromised.
Proprietary radio technology has become popular for monitoring remote input/output (I/O) because of its flexibility and robustness. This technology allows analog and discrete I/O signals, serial data communication and Ethernet communication to travel reliably across large distances. Because proprietary technology is limited to a particular manufacturer, it does require that the entire radio network be from a single manufactur
Many proprietary systems use frequency-hopping spread-spectrum (FHSS) technology. On a basic level, all FHSS radios function similarly, but each manufacturer adds its own features and functions to stand out from the rest. These features can range from multiple interface options on board the radio, to hot-swappability, to added encryption on the wireless link.
FHSS radios typically operate on the 900 megahertz (MHz) or 2.4 gigahertz (GHz) frequency band. The 900 MHz band is popular in North America because its free space loss is less than that of other unlicensed radio frequencies. It also has less radio frequency (RF) congestion than others—such as 2.4 GHz, which is used for Wi-Fi systems and other commercial products used in homes (see Figure 1).
Figure 1. The free space RF attenuation increases in proportion to the frequency.Couple the low free space loss with the ability to transmit up to 1 watt of power (the maximum allowed by the FCC) and the radio system can transmit multiple miles. The 2.4 GHz versions of FHSS radios have become more popular in areas such as Europe, in which the 900 MHz band is used by the government and, therefore, not open for public use.
Technology
FHSS uses many different individual frequencies or channels in a pseudorandom pattern. This way, an interference signal only blocks one or a few neighbored individual frequencies—no matter how high the level—so at least some portion of the communication continues (see Figure 2).
FHSS transmits radio signals by rapidly switching a carrier between frequency channels using a pseudorandom sequence known to both the transmitter and receiver. FHSS tolerates interference because a transmission will immediately resend on the next hop if it is blocked by a channel.If disturbances worsen, only the data throughput is reduced in the FHSS system. In other technologies, such as direct-sequence spread spectrum (DSSS), however, communication may be completely blocked. The number of frequencies used within the pseudorandom hopping pattern depends on further settings and mechanisms, such as the exclusion of certain frequency ranges (blacklisting) for coexistence management or the use of several frequency groups (RF bands) to optimize parallel operation.
Hardware
Industrial radios use two basic types of receiver designs: direct conversion and superheterodyne. A direct conversion receiver accepts the radio signal and then directly processes it to extract the original data. The simpler architecture of a direct conversion receiver results in a lower-cost radio but sacrifices some performance, especially in the critical aspect of noise rejection when operated in harsh industrial environments. A radio receiver’s ability to reject interference has a direct correlation to its range, coexistence with other radio systems and throughput.
A superheterodyne radio receiver uses frequency mixing to convert a received RF signal to a lower frequency. The intermediate frequency (IF) is easier to process than the original signal. This provides opportunities for additional stages of filtering and greatly improves selectivity, which is the receiver’s ability to select the desired signal from a noisy environment. This also increases the receiver’s sensitivity. Therefore, a superheterodyne receiver significantly improves performance in industrial environments, although the increased complexity of the design impacts the cost.
Features
Since all FHSS radios, in their basic form, function in the same way, manufacturers build in different functions and features to distinguish their products from others. The added features can be as simple or as complex as the manufacturer likes, but manufacturers need to keep in mind the benefit that the feature will have versus the impact that it will have on the overall performance of the communication. Below are some features that can help maximize performance and security in an industrial wireless radio system:
Encryption and authentication—The encryption method varies from manufacturer to manufacturer. One encryption option is to follow the Advanced Encryption Standard (AES) and an authentication of the data in accordance with request for comments (RFC) 3610. AES encryption ensures that hackers cannot extract the content of theoretically captured data packets. A designated password (Pre-Shared-Key) generates the 128-bit key, and all network devices must recognize this password. The authentication of transmitted data packets rates as highly as encryption (see Figure 3). The simplest way to attack a wireless system is to listen to a message, change it and feed it back into the network. Therefore, the message must come from a guaranteed source, such as an authenticated transmitter. For this, messages have a continuous code, which must not repeat. The code for this method can be programmed so that an attacker would have to wait more than 1,000 years before the code repeats.
Figure 3. Encryption and authentication are two steps that can add layers of security to a
wireless network.Channel blacklisting—This is the ability to blacklist frequency ranges, allowing users to plan coexistence with other systems. To do this, the system recalculates the frequency-hopping patterns according to the blacklisted areas.
Adjustable data rates for higher receiver sensitivity—Reducing the data rate can increase receiver sensitivity. If a transmission uses a low data rate, every bit transmits with the transmission power (P) for a longer time (t) than for a transmission using a high data rate. Therefore, the energy per bit (Ebit = P • tbit) is four times lower when the data rate measures four times as high. A higher energy per bit results in a higher system gain. This shows in the increased receiver sensitivity. A four-times-lower data rate results in a system gain of about 6 decibels per milliwatt (dBm), which effectively doubles the range of a radio link (see Figure 4).
Figure 4. The lower the energy rate, the higher the data per bit.Diagnostics—Users want greater access to network information, and diagnostics provide the vital information that users want on the state of their wireless networks. Receiving information—such as network structure, channel statistics and signal strength—are just a couple examples of information that could be available.
An industrial wireless system is characterized by its exceptional adaptability to the desired industrial application. It requires a high degree of reliability, ruggedness, security and flexibility. Wireless technology developed specifically for industrial use is based on the requirements of industrial infrastructure applications and closes the gap between specific sensor networks, such as WirelessHART, and the high-speed technology WLAN.
