Channel Spacing, Channel Bandwidth
and modulation - RF Cafe Forums
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Post subject: Channel Spacing, Channel Bandwidth
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 interference, right?
So if it's to close, it wil
disturb somebody else or yourself?
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?
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.
somebody 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
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?)
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?
So when somebody is
speaking about a 20MHz channel, it actually means that the
modem uses 20.000 different frequencies to transmit the
modulated signal (data)
Regards and thanks in