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.
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.
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.