FHSS vs DSSS

Spread spectrum is a group of techniques that utilizes a much larger bandwidth in transmitting information than would otherwise occupy a fraction of the bandwidth used. This is done to achieve a certain effect. FHSS and DSSS, which stand for Frequency Hopping Spread Spectrum and Direct Sequence Spread Spectrum, are two spread spectrum techniques. The main difference is in how they spread the data into the wider bandwidth. FHSS utilizes frequency hopping while DSSS utilizes pseudo noise to modify the phase of the signal.

Frequency hopping is achieved by dividing the large bandwidth into smaller channels that would fit the data. The signal would then be sent pseudo-randomly into a different channel. Because only one of the channels is in use at any given time, you are actually wasting bandwidth equivalent to the data bandwidth multiplied by the number of channels minus one. DSSS spreads the information across the band in a very different manner. It does so by introducing pseudo-random noise into the signal to change its phase at any given time. This results in an output that closely resembles static noise and would appear as just that to others. But with a process called “de-spreading,” the original signal can be extracted from the noise as long as the pseudo-random sequence is known.

In order for the receiver to decode the transmitted information, it must be synchronized with the transmitter. For FHSS it is relatively easy as the transmitter simply waits on one of the channels and waits for a decodable transmission. Once it finds that out, it can then follow the sequence being used to follow the transmitter which jumps across the different channels. With DSSS, it is not as simple. A timing search algorithm needs to be employed for the receiver to correctly establish synchronization.

A side effect of “de-spreading” is its ability to establish relative timing between the receiver and transmitter. With multiple transmitters that are in known locations, the relative timing can be used to establish the relative distances of the receiver from each transmitter. This is the working principle behind positioning systems like GPS. Since the receiver can calculate how far it is from each transmitting satellite, it is then able to triangulate its location. This ability is not present in FHSS.

Summary:

1.FHSS changes the frequency being used while DSSS changes the phase.
2.FHSS is easier to synchronize than DSSS.
3.DSSS is used in positioning systems while FHSS is not.

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