Channel Spacing, Channel Bandwidth and modulation - RF Cafe Forums
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Post subject: Channel Spacing, Channel Bandwidth and modulation
Posted: Mon Aug 20, 2012 1:46 pm
Joined: Mon Aug 20, 2012
I've got a few questions about wiresless
communication. For a lot of you out there this maybe
some kind of basic knowledge but i'm trying to visualize
some things for myself so i can explain some things
in a easy way to people who are not into technical
ereas. Al lot of information is out there on the
internet but not quit easily explained. Maybe you
can help me out?
Regarding the channel spacing
of a wireless communcation channel i've figured
out that a transmit frequency must be that far out
of reach of the next transmit frequency to avoid
So if it's to close, it
wil disturb somebody else or yourself?
automaticly comes to my next question. The channel
bandwith. Let's take my WIFI connection e.g. When
i use the Inssider tool, i can see that my modem
is using channel 4 but actually occupy channel 2
to 6. It has a range of 4 channels. I've noticed
that WIFI uses 20 MHz or 40 MHz so i can come up
with the theory that my WIFI connection uses 4 times
5MHz channels to transfer the data, right?
Which leads to my next question, the modem itself.
Something refered as radiomodem but they are two
different things right? The modem is responsible
for the data and the radio (transmittor and receiver)
for getting the data from A to B.
says, i've got a QAM modem. Alright, i've figured
out that QAM modulation uses sine and cosine of
a datasignal to "store" the original data and after
processing, this data is summed in a singel signal
which is ready for transmission. This sine and cosine
result in different positions as in a combination
of phase and amplitude.
So if my WIFI modem
does 64QAM to send data over 4 x 5MHz channels,
it needs probably 20 MHz to transmit the data, stored
in this 64QAM package. (64 posible positions to
store the data, right?)
Now, what actually
does this 64QAM signal to the actual radio signal?.
The radio signal itself, (the 2.4 GHz) must be modulated
also in some form, right? Does this mean that there
is also a form of frequency modulation in terms
of the radio path? I think that the modulated signal
needs to vary the 2.4 GHz carrier signal to tell
the remote receiver what's coming?
somebody is speaking about a 20MHz channel, it actually
means that the modem uses 20.000 different frequencies
to transmit the modulated signal (data)
and thanks in advance