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Tackling Interference in High Reliability Wireless Industrial Control Systems

By European Editors

Contributed By Digi-Key's European Editors

Wireless controls are a huge advantage for industrial systems, but there are some key challenges to overcome. This article looks at the challenge interference presents to high reliability industrial control systems and the different frequencies and wireless protocols that are implemented in a variety of transceiver devices and modules from Decawave, Linx Technologies, Digi and Atmel.

There are many different ways to minimize interference in wireless control systems for industrial automation. Designers can trade link budget, distance, frequency and protocols to get the most reliable wireless links possible.  Interference can come from a wide range of sources, from wide band electrical noise to other wireless systems operating nearby.

Protocols are one way to optimize the link, using code division multiplexing (CDMA) to minimize the effect of lost symbols. Forward error correction and cyclic redundancy checking (CRC) are now routinely added to maintain the integrity of the data, but they can take up valuable bits in the payload.

Spread spectrum and frequency hopping techniques are also being used to minimize interference. Spreading the signal across a range of frequencies again reduces the impact of interference at any one frequency. Or the link can detect a problem and automatically shift to another band to avoid the interference in a frequency-hopping scheme.

At the same time designers can trade off the range delivered by these techniques, which can be up to 12 km for some systems, to provide a higher link budget within a factory site that can be less vulnerable to other signals. 

All these techniques have knock-on effects for the frequency spectrum used. The sub-GHz 868 MHz and 902 MHz bands are crowded with many different types of links that mean spread spectrum or frequency hopping is not viable, while the 2.4 GHz band is home to the low-power ZigBee protocol but also has to accommodate Wi-Fi and Bluetooth and also tackle common interference from microwave ovens and other industrial systems.

As an example of the challenges, there are only a few ZigBee channels that do not overlap with Wi-Fi (channels 15, 20, 25, and 26) and so have negligible interference, while for each Wi-Fi channel there are four overlapping ZigBee channels. The reduction in the PER (Packet Error Rate) has a close relationship with the distance between interference source and receiver and differences of center frequencies (between interference source and receiver), presenting some significant challenges for system designers using the 2.4 GHz band.

Instead of tackling this head on, Decawave is using a combination of the 3.5 GHz to 6.5 GHz band and ultra-wideband protocols to deliver higher data rates that are more immune to interference. DecaWave’s DW1000 chip is a complete single-chip CMOS Ultra-Wideband IC based on the IEEE802.15.4-2011 standard. This is the first in the ScenSor (Seek Control Execute Network Sense Obey Respond) family of parts, operates at data rates of 110 kbps, 850 kbps and 6.8 Mbps, and, as a result of the higher frequencies, can also locate tagged objects both indoors and outdoors to within 10 cm.

Diagram of DecaWave DW1000 transceiver

Figure 1: The block diagram of the DW1000 transceiver.

The technology addresses both high reliability links for precise indoor location and communication for factory automation, especially in remote or difficult-to-access locations. Because the DW1000 allows both accurate measurement of time and data communications to occur simultaneously, it can be used for a wide variety of applications by developers of Real Time Location Systems (RTLS) and Indoor Positioning Systems, as well as the Internet of Things and Wireless Sensor Networks.

Suppliers of factory automation equipment can incorporate the technology into automation and monitoring tools with location precision of 10 cm versus 3 – 5 m for Wi-Fi RTLS. Using the higher frequencies also provides data rates up to 6.8 Mbit/s compared to 250 kbit/s for ZigBee and 1 Mbit/s for Wi-Fi.

The protocol used is the 802.15.4a standard, which is a combination of burst position modulation (BPM) and binary phase shift keying (BPSK). The combined BPM-BPSK is used to modulate symbols, with each symbol being composed of a burst of ultra-wideband pulses that reduce the vulnerability to interference at any one particular frequency. The chip also combines six channels of frequency division (FDMA) with code division CDMA techniques that use two different codes per channel to further optimize the channel link and reduce interference. This is then combined with integrated FEC and CRC error correction to ensure that interference does not impact on the signal.

The technology also has built-in immunity to multipath interference, as the band of frequencies in the pulse do not reflect well and dissipate more easily.

The DW1000 uses a single supply voltage of 2.8 V to 3.6 V and has a transmit mode current from 31 mA and receive mode current from 64 mA for low-power operation.

Down in the sub-GHz band, Linx Technologies has developed a transceiver for reliable long-range remote control and sensor applications. The TRM-900-TT consists of a highly optimized Frequency Hopping Spread Spectrum (FHSS) RF transceiver and integrated remote control transcoder. The FHSS system allows higher power with less interference and so gives a longer range than narrowband radios.

Operating in the 902 to 928 MHz frequency band, the module achieves a typical sensitivity of ‑112 dBm. The base version is capable of generating +12.5 dBm transmitter output power and achieves a range of over 2 miles (3.2 km) for a line of site link in typical environments with 0 dB gain antennas. A high-power version outputs +23.5 dBm, achieving up to 8 miles (12.8 km). 

The RF synthesizer contains a VCO and a low-noise fractional-N PLL. The VCO operates at twice the fundamental frequency to reduce the spurious emissions that cause interference and so allow for the longer range. The receive and transmit synthesizers are integrated, enabling them to be automatically configured to achieve optimum phase noise, modulation quality and settling time.

The receiver incorporates highly efficient low-noise amplifiers that provide up to –112 dBm sensitivity, and Linx has developed advanced interference blocking techniques that make the transceiver extremely robust when in the presence of interference in the sub-GHz band.

Modules such as the XBee from Digi allow designers to move across both the 2.4 GHz and 900 MHz bands using the 802.15.4 protocol. These embedded RF modules have a common footprint shared by multiple platforms, including multipoint and ZigBee/Mesh topologies with both 2.4 GHz and 900 MHz solutions. Developers deploying the XBee can substitute one XBee for another, depending upon dynamic application needs with minimal development, with the 2.4 GHz versions for global deployment and the 900 MHz versions for longer range or environments that need more immunity to interference.

Image of Digi XBee module

Figure 2: The Digi XBee module has the same footprint for both 2.4 GHz and 900 MHz implementations.

Interference is one key reason for developers moving to modules. The modules provide both protection against EMI interference through shielding, but also have optimized antenna path designs to reduce the interference from the rest of the electronics and from external sources. 

Atmel’s ATZB-S1-256-3-0-C ZigBit low-power 2.4 GHz module is a traditional ZigBee module that combines a low-power AVR 8-bit microcontroller and a high data rate transceiver that provides high data rates from 250 kb/s up to 2 Mb/s, frame handling, high receiver sensitivity and high transmit output power to give a robust wireless communication. The module is designed for wireless sensing, monitoring, control, and data acquisition applications.

Image of Atmel’s ATZB-S1-256-3-0-C ZigBit module

Figure 3: Atmel’s ATZB-S1-256-3-0-C ZigBit module.

To tackle interference, the IEEE802.15.4 standard supports two PHY options based on DSSS (Direct sequence spread spectrum). The 2.4 GHz PHY uses Q-QPSK modulation, whereas 780/868/915 MHz uses BPSK (binary phase shift keying) modulation, and both these can offer good BER (bit error rate) performance. To highlight the challenges of using frequency hopping at these lower frequency bands, the 802.15.4 physical layer offers thirty-one channels, four in the 780 MHz band for China (802.15.4c), one in the 868 MHz band for Europe, ten in the 915 MHz for North America, and sixteen in the 2.4 GHz throughout of the world.

Sometimes the interference has to be addressed within the device itself. The WL1835MOD from Texas Instruments combines both Wi-Fi MIMO and Bluetooth 4.0 links within a single device that presents key challenges in managing cross channel interference.

Diagram of Texas Instruments' WL1835MOD

Figure 4:  TI’s WL1835MOD tackles the interference between Wi-Fi and Bluetooth operation on the same chip.

The chip includes the integrated 2.4 GHz power amplifiers (PAs) for the Wi-Fi as well as a baseband processor that handles 802.11b/g and 802.11n data rates with 20 MHz or 40 MHz SISO (single antenna) and 20 MHz MIMO (multiple antenna) designs, as well as the Bluetooth radio front-end.

To do this requires a new advanced coexistence scheme. This works at the MAC-level to coordinate the use of all of the bandwidth in the 2.4 GHz band. At any time, all of the available bandwidth can be dedicated to either 802.11 or Bluetooth, as long as one or the other is idle. For example, when no Bluetooth communication is taking place, all of the bandwidth can support 802.11n communications at speeds up to 54 Mbit/s. Or, when the 802.11 radio is idle, all of the bandwidth in the 2.4 GHz range can be devoted to Bluetooth communications. To ensure the quality of certain types of critical communications, mostly audio channels, the coexistence solution can intelligently set different priorities depending on the time-sensitive nature of the communication.

Conclusion

There are many ways to minimize the impact of interference: moving out of the crowded bands, using spread spectrum and frequency hopping techniques, and boosting the link with more sensitive receivers and higher power transmitters and layouts optimized to reduce the impact of external signals. All of this allows the industrial automation equipment designer to trade-off link budget and link distance to implement the highly reliable links they need. 

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European Editors

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Digi-Key's European Editors