What is this "frequency-hopping"?

Frequency-hopping, spread spectrum (FHSS) radio is the method by which we move our GENERALCOMMUNICAIONS.COM radio control signals from transmitter to receiver. The other two types of radios available on the market today are fixed frequency and direct sequence. Fixed-frequency radios require a government-issued license in order to operate and have a fixed bandwidth in which to send their signals. Direct sequence spread spectrum (DSSS) radios do not need a license and are allowed to operate in the ISM band (DSSS are typically used for moving data files - please refer to our Spread Spectrum tutorial white paper). Frequency-hopping radios are also license free and GENERALCOMMUNICAIONS.COM has designed ours to handle smaller amounts of data, but reliably in very high interference environments. FHSS radios "hop" around all the different frequencies available to them in the ISM frequency band and transmit small amounts of data repeatedly. By changing frequencies every 20 milliseconds, the GeneralCommunications.com FHSS radios avoid most interference that may be present within its frequency band, and by sending data over and over again, gets the data through, even if some packets may have been lost to interference.

Can I use more than 1 radio control in the same area?

Yes please. Our frequency-hopping technology ensures that many GENERALCOMMUNICAIONS.COM radios can co-exist without interfering with each other. If we have a bunch of FHSS radios operating in the same area, the signals will invariably "bump" into each other, but the random manner in which they "hop" around the frequency band never allows them to interfere with each other long enough to interrupt the signal.

What about the receivers? How many types are there? How do I know which one to get?

Depending on the transmitter you choose, you may have the option of one or more different receivers, all roughly the size of an encyclopedia and weighing no more than 7 pounds. They are all shock-mounted with a NEMA 4X environmental enclosure rating and have external BNC antenna connections. All are designed to operate at 900MHz. Here are a few of GENERALCOMMUNICAIONS.COM's receiver options: 

FHSS has turned out to be more reliable in industrial applications for some practical reasons, and here are two of them.

First, FHSS industrial modems typically enable the user to choose from a large number of data channels (e.g. 150 or more), and within each data channel the modem will hop between a large number of frequency channels (e.g. 75 or more in the US). With this flexibility, FHSS modems can be programmed to hop around virtually any electrical interference or interfering object(s), and antenna diversity (see Dual Antenna) also helps in that process. For example, if transmission is blocked at one frequency, then the modem will automatically hop to the next frequency in the pattern, so reliable communication is maintained. Second, FHSS modems remain synchronized at all times, so their latency can be very low, and short latency data transmission is often required in industrial control applications.

Spread Spectrum is a coding technique for digital radio transmission and was originally developed for the military. The purpose of coding is to turn the information signal into a frequency that would resemble something more like noise. Noise has a flat consistent 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 whereby 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 the narrow band signals because it is designed to listen to a wider bandwidth at a code sequence generated by transmitting algorithms.

Direct sequence spread spectrum systems modulate a narrow band carrier by a code sequence used to change the carrier phase of the transmitted signal. The speed of this code sequence is measured in "chips" per second and referred to as the chipping rate. The spreading of a direct sequence signal is a function of the chips per bit and the information transmitted by a spread spectrum radio using this method is recovered by multiplying the received signal with a copy of the code sequence.

In Frequency Hopping Spread Spectrum systems, the carrier frequency of the transmitted signal changes in accordance with the pseudo-random code sequence. A pre-determined algorithm is utilized to maintain the synchronization between radios as they hop sequentially through a set pattern of frequencies determined by the algorithm.

Spread spectrum technology offers advantages where data communications reliability and integrity are important. The ability to share the same frequency band with other users and dramatically enhanced data security represents additional benefits. Sequence codes generated provide a decrease in crossover connection properties enabling individual groups of transceivers to operate side by side without interfering with other units. Multiple signals can be transmitted at the same time on the same frequency.

Some of the most important attributes of spread spectrum technology are:

• Secure data communications
• Military influenced Anti-jamming capabilities
• Outside interference rejection
• Multiple access capability
• Multi-channel interference protection
• Low probability of intercept

Frequency Hopping Spread Spectrum: In general, modern industrial wireless modems utilize frequency hopping spread spectrum (FHSS) technology rather than direct sequence spread spectrum (DSSS) technology.