SPREAD SPECTRUM RADIO COMMUNICATIONS

Spread Spectrum radio communications offers organizations seeking to collect environmental monitoring information a robust short-range, low cost solution for transmission of 24-hour data. Government agencies and utilities, among others, are relying on spread spectrum to rapidly transmit information on hydrological and meteorological events.

Radio Frequency (RF) organizations, aware of the demand for local radio communications for individual users, have allocated certain frequency bands to be used in a more flexible way. The oldest and most commonly used frequencies are 900 MHz and 2.4 GHz, often called the ISM bands ( Industrial, Scientific and Medical). The main characteristic of these bands is that they are unlicensed, which means that the frequencies are open for public use and require no registration or payment (apart from the radio hardware).

To avoid abuses, these organizations have imposed a set of rules for these frequency bands and only products certified to conform to those rules are allowed to emit in the bands. Among other requirements, the ISM band rules specify that Spread Spectrum technology must be used (either Direct Sequence or Frequency Hopping, see below). Additionally, these rules specify maximum power to be transmitted in- band and out-of-band (to avoid pollution of adjacent bands), and how channels are defined in order to allow the peaceful cohabitation of different systems.

These rules may vary depending on the country: the U.S. FCC allocates both the 900 MHz and 2.4 GHz band with 1 W maximum power, whereas the ETSI ( Europe) allocates only the 2.4 GHz band with 100 mW maximum power (the 900 MHz frequency is used for GSM cell phones). The 2.4 GHz band is available worldwide and the regulations are mostly compatible between the different authorities (usually 80 MHz of bandwidth between 2.4 GHz and 2.48 GHz). The main exception is Japan, which has some additional constraints.

Spread Spectrum is a coding technique for digital radio transmission on multiple radio channels. The technique was originally developed for the US Navy in WWII – it was deploying radio-guided torpedoes but the enemy had learned how to jam the radio channel. The purpose of coding is to turn the information signal into a frequency that more closely resembles radio noise. Noise has a flat spectrum without consistent peaks and can be reduced or eliminated by filtering methods. The spread spectrum coding technique modifies the signal spectrum to spread it out over several frequencies, thereby increasing its bandwidth. Spread Spectrum technology actually "spreads" a transmitted radio signal out over a wide frequency band. The Spread Spectrum receiver does not detect narrow band signals because it is designed to listen to a wider bandwidth at a code sequence generated by transmitting algorithms.

Spread spectrum devices must use one of two different transmission procedures, Direct Sequence or Frequency Hopping.

Direct Sequence Spread Spectrum (DSSS) uses a coding technique for digital radio transmission on multiple radio channels. DSSS technology actually “spreads” a transmitted radio signal out over a wider frequency band. This coding technique modifies the frequency spectrum to spread the signal across may frequencies from its carrier, thereby increasing it bandwidth. The DSSS receiver does not detect the narrow band signals because it is designed to “listen” to a pseudo-random code sequence generated by the transmitter in this wide channel bandwidth. The disadvantage is that the effective RF power is reduced, reducing the radio range and penetrating capability.

Frequency Hopping Spread Spectrum (FHSS) concentrates the radio power on one narrow channel at any one time for a very short duration. It uses a hopping sequence to synchronize with the receiver. The random nature of the hopping sequence across the allocated frequency band of operation, spreads the signal across a wider bandwidth, and reduces the average output power because the RF power is concentrated on one radio channel at any one time. The radio devices "hop" frequencies, and as each device hops to its own pre-set frequency pattern, the patterns of any two devices are different. This is how a large number of devices are able to share the same radio band. If two devices do hop onto the same channel and their transmissions "collide", they will then hop to different channels and re-transmit the message.

Because the frequency hopping technique concentrates the radio power on one channel at any one time, these devices perform better in industrial plants and factories where a strong penetrating radio signal is more important than high data rates.

Unlicensed ISM Frequencies
A license from the Federal Communications Commission (FCC) is not required to operate in the unlicensed ISM frequency bands. However, the user must follow the rules specified in the FCC Code of Federal Regulations (CFR) 47 Part 15. Under those rules, a radio must accept any existing or new interference since others are free to operate radios on the same frequencies.

Although spread-spectrum systems offer the flexibility of license-free operation in four distinct frequency bands (902-928 MHZ, 2400-2483.5 MHZ, 5150-5350 MHZ, and 5725-5825 GHz), and there are many other unlicensed ISM frequency bands, the two frequencies most commonly used for environmental monitoring and SCADA applications are 902 MHz-928 MHz and 2400 MHz-2483.5 MHz.

The 900MHz range allows for better radio propagation and penetration than 2.4GHz, however, the 2.4GHz band is designed for much higher data rates. Wireless LANs are common in the 2.4GHz band where a high data rate over short distance is required. Bluetooth and Wi-Fi communications use the 2.4GHz band. For instrumentation and automation applications, where the amount of data transmitted is small and the radio paths are not congested, the 900MHz band provides better performance.

Advantages of Spread Spectrum Telemetry  
Disadvantages of Spread Spectrum Telemetry