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Pumps & Systems, October 2007

Frequency Hopping Spread Spectrum (FHSS) radio was invented during World War II by the military for communicating information and strategic plans to allied forces in a way that would counter enemy efforts to "jam" or intercept traditional radio communication frequencies.

The flexibility of the technology, reliability of communication links and the ability for spread spectrum to overcome interference has made it a preferred technology source for many decades. Now, FHSS is being used in applications across the planet.

Technology Overview

FHSS is accomplished by hopping within a specified bandwidth. These frequency bands are commonly referred to in the U.S. as ISM (industrial, scientific, and medical) bands. The ISM bands are, in turn, often referred to as 900-MHz, 2.4-GHz, and 5.8-GHz.

The most commonly used ISM band for industry is the 900-MHz band, which actually covers the 902-MHz to 928-MHz spectrum. This 902-MHz to 928-MHz spectrum can be divided into sub-frequencies. Some manufacturers can divide the spectrum into as many as 112 sub-frequencies.

A spread spectrum diagram showing a typical frequency hopping pattern. The numbers above the waveform peaks (1 through 16) represent a typical frequency hopping pattern. The pattern is the result of a sequential preset algorithm designed to maintain synchronization between the master and remote radio modems.

Additionally, the hopping patterns used to avoid interference should be robust and flexible enough that, in the event of extreme interference, the radio can and will hop away from interference at a speed that any unwelcome networks or thieves cannot intercept or disrupt communication.

The sophistication behind FHSS technology is that the radio can hop within the specified band as quickly as possible from one frequency to another in order to avoid radio interference and send the required data in packets.¹ The faster a radio hops across the spectrum, the more likely it will find channels with no interference.

When using FHSS the rule of thumb is, "He who hops fastest wins." A simple but accurate rule! If a radio hops 180 times per second, it is 18 times more likely to find an open channel than a radio that hops 10 times per second.

Additionally, it is critical that a radio device be able to accommodate the communication speed of the device it is connected to (PLC or RTU, etc.) and the communication interface (serial, Ethernet, 4-mA to 20-mA, or 1-V to 5-V).

It is also important that the radio be able to verify real-time communication of individual messages and be able to report if the message was received correctly or if the delivery was made with errors. One should never select a radio model or manufacturer that cannot determine and eliminate messages containing errors. The process used to check the integrity of a block of data is CRC (Cyclic Redundancy Check). A minimum CRC of 32 bit is recommended.

Do not accept a technology that employs error correction, as this leaves the decision to correct data errors and pass data up to the manufacturer. Instead, select a technology provided that automatically discards "bad" messages and resends the original message.

A radio can also have the capability to monitor the performance of the communication network (diagnostics). This feature allows the user to continually monitor, in real-time, the performance of the communications network and, if needed, use the diagnostic data to troubleshoot any performance deficiencies.

Performance Overview and Expectations

FHSS radios range from slow hopping, short range devices (such as those commonly found in home security systems) to high speed, long range mission critical devices. The upper end of these devices can communicate reliably at ranges in excess of 40-mi using no more than 1-W (30-dB) of power.