Channel Spacing, Channel Bandwidth and modulation - RF Cafe Forums

 Post subject: Channel Spacing, Channel Bandwidth and modulation

Posted: Mon Aug 20, 2012 1:46 pm 

Hello,

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?

That 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.

If 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 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?

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 advance