Question

Compare and contrast DSSS and FHSS. Which one is more suitable than the other, and for what scenario?

Answer 1

Hey,

Below is the answer to this question

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Spread spectrum is basically the variety of techniques which employ a bigger bandwidth in sending information which actually covers a bit of bandwidth. It is employed to get a specific effect. There are 2 types of spread spectrum, FHSS and DSSS where FHSS is Frequency Hopping Spread Spectrum and DSSS is Direct Sequence Spread Spectrum. The difference is about how the data is spread over its bandwidth. FHSS employs hoping of frequency while DSSS modifies the signal by the use of pseudo-noise.

In frequency hopping, smaller channels are formed out of a large bandwidth in order to fit the data and then the signal is sent to those channels in a random behaviour. As only one channel can be used at a given time, it is actually wasting the bandwidth equal to one less than the number of channels multiplied by the data bandwidth. In contrast, DSSS uses a different approach to spread the information on the band. It actually uses pseudo-random-noise into the signal to vary the phase. It creates an output which basically is a static noise and would appear to others just like that. But, using de-spreading process, the actual signal can be found from the noise, if the pseudo-random sequence in known.

To decode the sent information, it should be in the synchronization to the transmitter. In FHSS, it is quite easy as transmitter actually stays on the channel and waits for the decoded transmission. After which it follows the sequence which adopted to follow the transmitter which hops on the variety of channels. But for DSSS, a timed search algorithm should be used to get synchronization.

There is a secondary effect of despreading is to establish a relation of timing between the transmitter and receiver. The relative distances of the receiver from each transmitter can be established using relative timings which are known to have multiple transmitters in known locations. One of the important application of this principle is in GPS. In GPS, the receiver can calculate its relative distance from all transmitting satellites and the finds the location. To the very contrast, this function is not present in FHSS.

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