Spread Spectrum, Wider Bandwidth Question.

walid97 · July 2005

Hi guys

I read that Spread Spectrum is a technique that spreads a narrowband communication signal over a wide range of frenquencies for transmission. A signal is a spread spectrum when the bandwidth is much wider that what is required.

in the case of FHSS, does the term "wider bandwidth" means the range from 2.402 --> 2.480 band (79 MHz)
OR
the carrier frenquency bandwidth (1 MHz to 5 MHz in the case of FHSS) ???

I understood it in 2 ways:

THE FIRST:

For FHSS, instead of using just one frequency, FHSS changes frequency when dwell time expires. The frequencies pattern is predefined. So, instead of using 1 fixed frequency, FHSS can use up to 75 frequencies and restart the pattern again. This is the meaning of wider bandwidth here=the use of a number of frequencies, not just 1.

For DSSS, wider bandwidth means that 1 bit is presented by 11 chips (by XORing the initial bit with the Pseudorandom Number). So we are sending 11 times the desired data. This is the meaning of wider bandwidth here.

THE SECOND:

wider bandwidth means, wider carrier bandwidth.. not 2.402 to 2.480, but carrier frequency bandwidth for 5 MHz of these 79 MHz (not the 79 MHz by themselves, but the sub carriers that are composed of 5MHz each, with 15 different hop (carrier) note that 15*5=75 ).

For examle, for FHSS, at t=5, lets say the frequency is at 2.417, this means that it can accept signals from 2.415 to 2.419 = 5 MHz of frequency bandwidth, which means less power is required because hitting anything from 2.414 to 2.419 is good, unlike narrowband, where u HAVE TO hit the exact frequency, which means more accurate, which means more power.

Same applies for DSSS and OFDM, which are both Spread Spectrum, but they don't hop at all, they stay on a given frequency band (about 5 MHz) centered on a given frequency (ex 2.422). So, these 5 MHz "acceptance" is the Spread Spectrum, where systems can accept anything between 2.420 and 2.424...

I just wantto know, how is Spread Spectrum in FHSS and DSSS and OFDM.. I mean, where is it applied in every1 of these technologies?

I am a bit confused of the meaning of "increase in bandwidth means less power required for the same transmission of info" ?

Can you please tell me where I was right?
Thanks a lot.

rossonieri#1 · July 2005

hello,

i'm not good at explaining this but let me try by example in everyday life :
do you play musical instrumen? like electric guitar/piano?

FHSS and DSSS is more like analog and digital sound of your guitar.
Digital(in this is DSSS) sound capable of delivering a wide range of sound variety with small a mount of energy but with a bigger disruption (you can not hear any manouver at specific distance) , but on the other hand Analog (FHSS) deliver only a small amount of sound variety but the manouver is very clear and require a bigger energy.

JDMurray · August 2005

First of all, ODFM is not a spread spectrum technology. It is a modulation technique used by DSSS.

FHSS and DSSS are wide-band transmission technologies. Because they break up the data into chunks and send it out over different frequency channels (like the data in a parallel printer cable), there is no reliance in a single frequency for transmitting all of the data. If several frequencies in the spectrum are useless due to noise, other clearer frequencies can be used. The data is send using a lower power because it isn't necessary to transmit using a high power over a clear channel (unless you are going for a great distance). Narrow-band transmission technologies, however, have no choice in which frequencies to use. They are fixed on one frequency channel, and if that channel is not clear, they must blast through the noise floor using a lot of transmission power.

walid97 · August 2005

jdmurray wrote:

FHSS and DSSS are wide-band transmission technologies. Because they break up the data into chunks and send it out over different frequency channels (like the data in a parallel printer cable), there is no reliance in a single frequency for transmitting all of the data.

This is true for FHSS, because the frequency keeps on changing and FHSS uses the whole 2.4 GHz ISM band.

For DSSS, the channel used does not change, for example, if I choose channel 1, I will send and receive on this channel only which is 22 MHz long as I remember.

So, as I understandm FHSS kepps on changing frequencies every dwell time. But for DSSS, the channel used does not change at all. So, where is Spread Spectrum in DSSS ??

Thank you !

JDMurray · August 2005

walid97 wrote:

jdmurray wrote:

So, as I understandm FHSS kepps on changing frequencies every dwell time. But for DSSS, the channel used does not change at all. So, where is Spread Spectrum in DSSS ??

Using fixed (direct sequence) or dynamic (frequency hopping) channels is not part of what "spread spectrum" technology really is.

In 802.11, each DSSS channel is 22MHz in width, and the center frequency of each channel is 5MHz apart so channels overlap. That is very wide compared to more common narrowband channels, such as AM radio, where the channels are only 5kHz wide and the center frequencies are spaced 10kHz apart (no overlap). The DSSS (and FHSS) channels are much wider than they need to be to transmit the data (i.e., frequency content) that they carry. Narrowband transmissions (e.g., AM and FM radio) transmit on channels only wide enough to carry their total frequency content.

Transmitting over excessively wide frequency bands using low power is basically what spread spectrum is. The use of fixed or dynamic (or both) channels are simply variations of how spread spectrum is used.

walid97 · August 2005

Thanks a lot for your answer.
So, my 2nd explanation is the right one here??

walid97 wrote:

wider bandwidth means, wider channel.. not 2.402 to 2.480, but carrier frequency bandwidth of 5 MHz of these 79 MHz (not the 79 MHz by themselves, but the channels that are composed of 5MHz each, with 15 different channels. note that 15*5=75 ).

For examle, for FHSS, at t=5, lets say the frequency is at 2.417, this means that it can accept signals from 2.415 to 2.419 = 5 MHz of frequency bandwidth channel, which means less power is required because hitting anything from 2.414 to 2.419 is good, unlike narrowband, where u HAVE TO hit the exact frequency, which means more accurate, which means more power.

Same applies for DSSS and OFDM, which are both Spread Spectrum, but they don't hop at all, they stay on a given frequency band centered on a given frequency (ex 2.422). So, the 20 MHz channel is the "Spread Spectrum", where systems can accept anything between 2.412 and 2.432...

I just want to know what does it mean physically that if I send data on wider channel it would require less power:
"The DSSS (and FHSS) channels are much wider than they need to be to transmit the data (i.e., frequency content) that they carry". Whats frequency content?
Is it the reason I mentioned above?

Thanks a lot

JDMurray · August 2005

Signal energy is used to not only gain distance, but also to over come noise in the signal band. You don't need to use less power on a spread spectrum signal, but not as much energy is required because the signal is spread over a wide broadcast area, and is therefore much less affected by narrowband noise.

walid97 · August 2005

jdmurray wrote:

but not as much energy is required because the signal is spread over a wide broadcast area, and is therefore much less affected by narrowband noise.

What does "the signal is spread over a wide broadcast area" mean ?
how does this happens physically ?? because at any instant, a signal is sent on a specific frequency (which is the number of cycles per second), so how can it be that a signal is sent over a "wider broadcast area" ??

Thanks again.

JDMurray · August 2005

All radio signals are transmitted over a contiguous range of frequencies called "a channel" or "a band." An AM radio station on the 600kHz band is actually transmitting its signal in the frequency range of 600-605kHz. The band is therefore 5kHz wide. The FCC defines channel 1 used by IEEE 802.11 as in the frequency range of 2.401 to 2.423MHz. Channel 1 is therefore 22MHz wide.

Any more details would require a lesson in radio wave propagation. Have a look at web pages like: http://en.wikipedia.org/wiki/Radio_propagation

Spread Spectrum, Wider Bandwidth Question.

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