Basic Question on DSSS

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Jan 16th, 2009
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I have searched this forum, and am currently refering to 3 books on WiFi, but still cant find the answer to this very basic question :-

How does DSSS "spread the signal" when sending data over the air ?

I am not talking about the barker code...

I understand that concept where 11 chips are used to represent 1 data bit.

My question is if a single frequency is used with phase modulation, then

does that not imply using only a SINGLE frequency ?

However all the docs say that the frequency is spread across 22 Mhz.

So are there multiple carrier frequencies used to transmit the data ?

I am confused because frequency = numer of ocilations in a fixed time = Hz.

So how can you "spread" a frequency over a bandwidth which is a range of frequencies ?

Hope my question makes sense.

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ericgarnel Thu, 01/22/2009 - 13:48
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This may shed some light on it.

It does not use a "single" frequency, it spreads it across a few

An important concept to note regarding channel assignments is that the channel actually represents the center frequency that the transceiver within the radio and access point uses (e.g., 2.412 GHz for channel 1 and 2.417 GHz for channel 2). There is only 5 MHz separation between the center frequencies, and an 802.11b signal occupies approximately 30 MHz of the frequency spectrum. The signal falls within about 15 MHz of each side of the center frequency. As a result, an 802.11b signal overlaps with several adjacent channel frequencies. This leaves you with only three channels (channels 1, 6, and 11 for the U.S.) that you can use without causing interference between access points.

shahedvoicerite Thu, 01/22/2009 - 14:02
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Thanks for the reply. One of the books I am refering to is actually the first link you posted :-)

I guess my main confusion is :-

If the signal is spread over a range of frequencies, then when is the "phase shift"

demodulated/interpreted ?

Does the reciever first mix all the recieved signals in the frequency range to form a single waveform and THEN analyse / intreperet the waveform for phase shifts ?

i.e Is the flow something like :-

Signal is DBPSK modulated --> Resulting sine wave is spread across frequency range ---> Rx then joins all the frequencies back to produce a single sinewave ---> Phase shift in resulting sine wave is converted to binary 1/0


ericgarnel Thu, 01/22/2009 - 16:56
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I'm not sure, but I found an excellent link worth sharing:

Lots of good info in it

I believe the correlator on the receiver end functions to accept the incoming frequencies and adjust accordingly, so yes, the receiver does "massage" signals before it demodulates them


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