Spread Spectrum and Frequency Hopping Technology



What is Difference Between Spread Spectrum and Frequency Hopping?


Most communication technologies we are familiar with—radio,television, two-way radios—use what is called narrowband communications. Each station or channel operates over a very thin slice of the radio spectrum. Because the station is assigned that particular band, and the FCC ensures that no other broadcasters in the local area use that same band through licensing, there is no interference. The range of each station is limited, so the same frequency can be re-used a great distance away without interference. Because many devices might use the ISM bands in a local area, additional technology is required to keep the various signals from interfering with each other.

Fortunately, a technology has been developed over the past fifty years which permits such bandwidth "sharing." This technology provides a way to spread the radio signal over a wide "spectrum" of radio frequencies, minimizing the impact of narrowband interference. In most cases, only small parts of the transmission are corrupted by any interference, and coding techniques allow that data to be recaptured. This technology is now generally known as spread spectrum.

There are currently two different spreading techniques used. Both use a coded pattern of communication. A receiving unit is synchronized to use the same pattern and successfully receive the transmission. Any other radio unit hears the signal as noise because it is not programmed with the appropriate coding. The two techniques are called frequency hopping spread spectrum and direct sequence spread spectrum.



Frequency Hopping Technology

Frequency Hopping Spread Spectrum (FHSS). The United States military developed a radio technology called spread spectrum during the 1950s and 1960s. Obviously, the first concern was ensuring that radio transmissions were not intercepted. The second concern was to ensure that guided missile communications were not jammed by enemy radio transmissions.

Though developed and implemented by the U.S. military, the problem was first addressed by Hedy Lamarr, a famous actress of Austrian descent in the 1930s and 1940s. She and a music composer, George Antheil, patented the idea in 1940. She was so far ahead of her time in conceptualizing the idea that she never received any monetary rewards for her patent. The patent license expired before government and commercial implementation of the concepts occurred.

A communications signal (voice or data) is split into separate parts. Instead of transmitting a signal continuously over one narrow frequency band, the several parts are transmitted separately over a wide spectrum of radio frequencies. A defined, but random-appearing pattern of non- sequential bands is used, with successive parts being transmitted over the next frequency band in the pattern. On the other end, a receiver is configured to receive the signals in the same pattern. The radio receiver then reassembles the pieces into the original signal. Since many distinct patterns can be developed, it is possible to have multiple radios transmitting at the same time, but never at the same frequency at the same time.

The process of jumping quickly from one frequency to another is called frequency hopping.

And, therefore, the technique is called frequency hopping spread spectrum. Frequency hopping has two benefits. Electricalnoise —random electromagnetic signals which are not part of anycommunications signal—will only affect a small part of the signal. Also, the effects of any other forms of radio communications operating in narrow bands of the spectrum will be minimized. Any such interference that occurs will result in only a slightly reduced quality of voice transmission, or a small loss of data. Since data networks acknowledge successful receipt of data, any missing pieces will trigger a request to transmit the lost data.

Spread Spectrum Technology

Direct Sequence Spread Spectrum (DSSS). Direct sequence spreading is very different from frequency hopping. Instead of splitting a data signal into pieces, direct sequencing encodes each data bit into a longer bit string, called a chip . Usually, 11 to 20 bits are used for the chip, depending on the application. Because the military requires a much higher degree of security, it generally uses much longer chips—even a long as 1,000 to 10,000 bits! An eleven-bit chip is illustrated below.


 
                                                   0=10010010110
                                                   1=01101101001



Notice that the binary string encoding a 0 has the opposite form as the string encoding a 1— where a "1" is used in one chip, a zero is used in the other. The chip is then used to modulate (change) the signal generated by the radio transmitter, spreading the signal out over a wide band of frequencies. The receiveruses the same code and so listens for the unique signature across the frequency spectrum. It then decodes the signal back to the original data.

This is a simplified explanation of a very technical subject, but hopefully it gives you an idea of how spread spectrum works. The gist is that spread spectrum technology allows multiple radio signals to operate in an open, unlicensed band with a minimum of interference. It also provides security for the transmission.

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