Types of Spread Spectrum
Frequency Hopping Spread Spectrum (FHSS) has the frequency of the transmitted message periodically changed (or hopped). The transmitter hops frequencies according to a pre-set sequence (or hop sequence). The receiver either stays synchronized with the transmitter hopping, or is able to detect the frequency of each transmission.
FHSS can hop rapidly, several times per message, but generally it transmits a complete message (or data packet) and then hops. Each transmitter hops to a particular hop-sequence, which it chooses automatically or is user-configured. Because the hop-sequences of different transmitters are different, the hopping of a “foreign” transmitter exhibits statistical randomness (though not truly random).
FHSS hopping sequences are pseudo-random, in that the probability of a foreign transmitter hopping to a particular channel appears to be random.If more than one system hops onto the same channel, a “hop-clash” event, then those radio messages are corrupted. However when the transmitters hop again, the probability that both transmitters will hop to the same frequency a second time is very remote.
Direct Sequence Spread Spectrum (DSSS) differs from FHSS in that the transmitted data packet is “spread” across a wide-channel, effectively transmitting on multiple narrow channels simultaneously. When a data packet is transmitted, the data packet is modulated with a pseudorandom generated key, normally referred to as a “chipping-key”, which spreads the transmission across the wide-band channel.
The receiver decodes and recombines the message using the same chipping key to return the data packet to its original state.
FHSS focuses the transmission power into one frequency channel at any one time. The first diagram shows a time-delayed spectrum-analysis of FHSS transmission – it shows three successive data packets transmitted on three channels – it does not indicate which of the three transmissions occurred first. The signal received is extremely large relative to the base background noise.
DSSS spreads the power of the transmitter across each channel. The result, as illustrated in the second diagram, is a wide span of low power transmissions. As a result of the low power transmissions, the distance capabilities as well as the penetrating power of the DSSS are greatly reduced when compared to FHSS, however the possible data rates for DSSS are much higher.
The ability to withstand interference is different between the two techniques.
The average power level of DSSS transmissions is much lower than FHSS – DSSS can not tolerate the same level general background noise that FHSS can. DSSS also has more problems with “multipath fading”, a phenomena caused by the mixing of direct and out-of-phase reflected signals.
FHSS systems can transmit further because of the higher effective RF power. However DSSS can interleave a large number of systems that FHSS, which eventually breaks down when there are too many hop-clashes.