Spread-Spectrum Radio Transmission

Spread-Spectrum Radio Transmission

Spread-spectrum radio transmission is a technique originally developed by the military to solve several communication problems. Spread-spectrum improves reliability, reduces sensitivity to interference and jamming, and is less vulnerable to eavesdropping than single-frequency radio.

As its name suggests, spread-spectrum transmission uses multiple frequencies to transmit messages. Two techniques employed are frequency hopping and direct sequence modulation.

Frequency hopping switches (hops) among several available frequencies (see Figure 3.18), staying on each frequency for a specified interval of time. The transmitter and receiver must remain synchronized during a process called a hopping sequence in order for this technique to work. Range for this type of transmission is up to two miles outdoors and 400 feet indoors. Frequency hopping typically transmits at up to 250 Kbps, although some versions can reach as high as 2 Mbps.

Direct sequence modulation breaks original messages into parts called chips (see Figure 3.19), which are transmitted on separate frequencies. To confuse eavesdroppers, decoy data also can be transmitted on other frequencies. The intended receiver knows which frequencies are valid and can isolate the chips and reassemble the message. Eavesdropping is difficult because the correct frequencies are not known, and the eavesdropper cannot isolate the frequencies carrying true data. Because different sets of frequencies can be selected, this technique can operate in environments that support other transmission activity. Direct sequence modulation systems operating at 900 MHz support bandwidths of 2-6 Mbps.