The pros and cons of DSS over FHSS

[Maya Laxman Sawant]

PLEASE SEND ME THE DIFFERENCES AND ALSO THE ADVANTAGES OF DSSS OVER FHSS.............
PLEASE...........
SENT IT AS SOON AS POSSIBLE

 

[sachit]

dsss below noise floor

FHSS:- Frequency Hopping Spread Spectrum. In this , the user trasmitted signal changes the frequency randomly. That means, suppose user is on frequency f1 when it starts the transmission. Then after some random time, it switches to f2 then f3 the f6 and then f9 and so on. These frequencies are in the same band of frequencies. Thus this transmission is not easy to track or intercept because it is not possible to know the frequecy since it is always changing.

DSSS:- Direct Sequence Spread Spectrum. In this , the user does not change the frequency but it changes the Pseudorandom code. This code has the property of never repeating itself.This code is unique. That means, user encrypts its signal using this code and code is changed time to time. Thus it is not possible to intercept the users signal.

Both techniques are used for signal security. This techniques are used by military for secure communication.

FHSS is easy to implement but the security is lesser than DSSS. DSSS is a great technique and provides maximum security but little bit difficult to implement.

Tula answer milale ka?
Sachit

Added after 36 seconds:

FHSS:- Frequency Hopping Spread Spectrum. In this , the user trasmitted signal changes the frequency randomly. That means, suppose user is on frequency f1 when it starts the transmission. Then after some random time, it switches to f2 then f3 the f6 and then f9 and so on. These frequencies are in the same band of frequencies. Thus this transmission is not easy to track or intercept because it is not possible to know the frequecy since it is always changing.

DSSS:- Direct Sequence Spread Spectrum. In this , the user does not change the frequency but it changes the Pseudorandom code. This code has the property of never repeating itself.This code is unique. That means, user encrypts its signal using this code and code is changed time to time. Thus it is not possible to intercept the users signal.

Both techniques are used for signal security. This techniques are used by military for secure communication.

FHSS is easy to implement but the security is lesser than DSSS. DSSS is a great technique and provides maximum security but little bit difficult to implement.

Tula answer milale ka?
Sachit

 

[jiveshgovil]

spreading code of fhss

PLEASE SEND ME THE DIFFERENCES AND ALSO THE ADVANTAGES OF DSSS OVER FHSS.............
PLEASE...........
SENT IT AS SOON AS POSSIBLE

Hi

Please find the attached document on the comparison of DSSS with FHSS. It uses illustrations to express the difference in lucid manner.

Hope that helps. Please mark "helped me" if it is of some use to you.

Regards,

 

[drabos]

advantages of fhss

Hello,

I think the setting up time is the main disadvantage of the FH SS.
And beacuse of it is a narrow band system, so it can disturb other narrow band systems (it is true, that it isn't a permanent disturbance, but if my television receiving is damaged in every 10 minutes, it isn't very felicific)

 

[Buriedcode]

fhss military

hi,

Essentially, sachit outlined the basics, but he only mentioned security. Thats not really what FHSS and DSSS is used for in Wifi terms. Both a FHSS and a DSSS can be tracked, and decoded, its actually not that difficult, but requires some clever kit.

Both FHSS and DSSS are excellent at avoiding, or operating in the presence of noise and other signals, namely 'narrow-band interference'. FHSS can avoid other channels in a band, by simply not hopping to them, and DSSS has the clever ability to attenuate everything (all narrow band interference) apart form the desired signal. Because of this, DSSS can in fact, operate below the noise floor.

Yet another implication of this, is that they can both be used, in the same band, with other devices. For wifi/bluetooth the 2.4Ghz band has approximately 72 channels for FHSS, 1Mhz (bluetooth?) and 11 channels for DSSS (22MHz, overlaps.). Now, there is enough room in the band for 3 22Mhz channels, but using DSSS, this can go up to 30, providing all the transcievers in question have a different spreading code. FHSS on the other hand, switches channels, therefore, a transciever will only take up one of the 72 channels for a brief period of time. Meaning MANY bluetooth devices can be used in the same band, in relatively close proximity.

This is why they are used so extensively, on ISM bands (no licence required). It means devices can be sold, that are wireless, cheap (no licence for spectrum) and can be used virtually anywhere, even in the presence of many other similar devices. They are excellent at avoiding interference, and DSSS even has the advantage of not generating much interference, and they provide a certain level of security over standard narrow band methods.

Back to answering your question:

Differences?
-FHSS uses a 'narrow band carrier' that is periodically changed usings a pseudo-random code. (pseudo-random because it DOES actually repeat itself, for blue tooth, this means a hop sequence of 72 hops) The spreading occurs in the RF band.

-DSSS is wideband. It 'spreads' the data using a pseudo-random spreading code (11-bit barker code for 802.11b?). The 'spreading' effect occurs in the baseband.

-FHSS is easier to implement because its narrow band. (simple amplifiers, filters)

-DSSS is more difficult, due to its wideband nature. Also more complicated syncronising circuitry is needed to 'extract' the data from the code sequence.

-FHSS can avoid interference, but ultimately, if a channel it uses is jammed, the information transmitted in that channel will be lost. Because of this, as the FHSS signal gets weaker, more channels are lost, and it degrades gracefully. That is, the more interference, the more errors in the data, bit its linear.

-DSSS on the otherhand, 'spread' interference, when it is decoded in the reciever, it attentuates any jamming signals, and noise in the baseband, whilst 'extracting' the desired signal. However, when conditions worsen, the channel will colapse, and NO data can be sent. Unlike the FHSS, DSSS will work fine up until a point, then simple stop working.

Advantages of DSSS over FHSS:

Doesn't use narrow band carriers, so, in effect, a much wider bandwidth is allocated, allowing much more throughput (up to 11MB/s I believe).

Can operate below the noise floor, therefore giving it greater range and better stability in slightly bad conditions.

Provides a certain level of security, since to intercept it, one must know the spreading code, and syncronise to it.

Also, for military apps, DSSS appears as 'noise' on a spectrum. Narrow band interferers, and signals can clearly be seen on a scope, as a 'spike'. DSSS simply increases the noise level by a small amount, meaning the 'enemy' does not even know someone is transmitting.


I hope this helps, you didn't mention what exactly you were after, wifi? Bluetooth? 802.11x? Or were you simply interested in the basic theory behind such methods?

Regards,
BuriedCode.

 

[koppolu1981]

+wifi +fhss +dsss

The FHSS is very complex becoz the frequency hopping will result in a complex circuitry which needs to switch between different frequencies.
If we take DSSS only it's a matter of multiplier.

 

[rostam63]

fhss dsss comparison

some of the differences is mentioned in my attachment.