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Finding the perfect wireless frequency

02 April 2018

Scott Keller, founder and CEO at SignalFire Wireless Telemetry, offers some advice on determining the best wireless frequency for remote monitoring and control systems.

Remote monitoring and control systems rely on radio communications to integrate a variety of sensors, nodes and gateways to a wireless network for the transmission of data.

A large number of radio-based telemetry systems use communication networks based on a 2.4GHz frequency band. Zigbee, WirelessHART, ISA 100 and other 802.15.4 systems, for example, operate in the 2.4GHz wireless frequency spectrum. While this is suitable for short range building and plant environments, 2.4GHz systems have limitations when performing in outdoor applications and a lower frequency is a better option. A 915MHz system, for example, will provide a more reliable data transmission over longer ranges in challenging environments.

The operating range or distance over which the wireless system must perform will determine the best frequency band. When assessing the use of a 2.4GHz or 915MHz system, consider transmitter power; radio antenna design; interference; and data rates.

In the US and Canada, 915MHz systems can transmit powers up to 1 Watt and have antennas that can double that range (+6dB). While 2.4GHz systems can operate at the same power levels in the US and Canada, most do not exceed the global requirement of 0.1 Watts. Global restrictions on 2.4GHz frequency further limit performance by measuring radiated power – no antenna gain is permitted for a full power (0.1 Watt) system in most countries.

With operating parameters being equal, a 915MHz system offers about 2.6 times the range of a 2.4GHz system. In most cases, the 915MHz supports longer range between nodes, beneficial for wireless sensor control networks that cover large geographic areas of hundreds of square miles.

Interference
Most wireless sensor control systems are subject to two types of interference – physical and electromagnetic. Physical interference can be broken down:

Building attenuation: Either a 2.4GHz or 915MHz solution works efficiently in buildings.
Vegetative attenuation: The impact of trees and other vegetation can significantly impact link quality. Evergreens are worse than deciduous trees and wet foliage will degrade link quality even further. A 2.4GHz system will experience significant foliage attenuation problems due to the moisture in vegetation as it operates close to the frequency that vibrates water molecules. Rain attenuation: Rain attenuation is modest for both a 2.4GHz and 915MHz frequency. Fog and snow are slightly lower. Rain, however, will significantly exacerbate the foliage attenuation at 2.4GHz due to the high absorption of the 2.4GHz energy by water.

Electromagnetic attenuation
Both frequencies use standard techniques to manage radio interference. The choice of methodology will depend on system design.

• Zigbee, WirelessHART, and ISA 100 at 2.4 GHz use Direct Sequence Spread Spectrum (DSSS) as a method of using multiple frequencies to avoid narrowband interference. This technique preserves high data rates until the interference band spreads to a point where data rates degrade quickly.
• While DSSS works at 915MHz, SignalFire implements a Frequency Hopping Spread Spectrum (FHSS) that ‘hops’ over narrowband interference. This technique assures communications, albeit at a lower data rate, where DSSS fails.

Data rate
Data rate will significantly impact range. Most 2.4GHz systems transmit at 200 – 500Kbits/sec, while a SignalFire Remote Sensing System operates at 915MHz transmits at 10Kbits/sec. Doubling the data rate causes a 3dB loss in link budget. When compared to a 200Kbits/sec system, this provides an advantage of about 12dB (6dB= a doubling of range).

Conclusion
While no single frequency will provide the perfect solution to supporting the communications of a wireless telemetry or mesh network. While working well in building and plant environments at ranges measuring in tens of meters, a 2.4GHz frequency poses limitations in longer ranges and under certain outdoor conditions. A 915 MHz frequency offers a more reliable data transmission platform for outdoor applications where data must be reliably transmitted hundreds to thousands of meters under different weather conditions. The choice will ultimately depend on system operating requirements.


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