RF help needed

EdTheLad

I'm trying to understand the theory behind how data is sent over RF channels.
I have data i want to transmit from site A to site B. I use a carrier wave at 50Mhz, phase shift modulation is being used.I am given a frequency channel of 8Mhz.

So the center frequency in my channel is the 50Mhz, which means my frequency spectrum range is between 46 and 54Mhz.
Since i have a frequency range does this mean i can send multiple waves with differing frequencies simultaneously, as long as they are within my range?
Or is the frequency channel range given in case my original wave changes frequency during transmission?

The problem i see is that waves with similar frequencies interfere with each other, if i sent multiple waves in the one channel surely they will cause interference.Since all these wave should have high power the signal to noise ratio will hit the floor.

Also what about the next 8Mhz channel between 55 and 62, how is the interference avoided between channels.

Is there a rule which states there should be a certain amount of frequency space between waves within the same channel to avoid overlap?
Is it typical for a device transmitting between two end stations to use multiple waves with slightly different frequencies to transmit the data or is a single wave used.

Hope someone can help, i cant find anything on the net that explains this.

dynamik

The frequencies are different, but the same concept that applies to 802.11b/g networks should apply here.



While there are 11 usable channels, you can (should) only use three to avoid overlap. You can use closer ones, but your interference will increase the closer you get.

As far as multiple waves go, I would say it depends on the frequency you're using (i.e. 1x8mhz, 2x4mhz, etc.). If you can avoid (or at least minimize) overlap, you should be able to use more than one.

I'm by no means a wireless expert, but I believe that's correct.

EdTheLad

I'm working on a cable project and all the 8Mhz channels are adjacent without any gaps.

I've worked it out, its amazing how asking a question helps get the brain working.
Ok, within each channel there is one carrier, phase modulation actually varies both the phase and the frequency.So the number of resultant frequencies depends on the type of modulation used.We are using QAM 256 which uses both phase and amplitude modulation simultaneously, this will result in at least 8 different frequencies.Amplitude modulation also produces 2 sideband frequencies so i suppose we could see up to 24 different frequencies.Once all these frequencies fall within the channel we are golden.

I'll need to get clarification on this but i'm feeling happier about it now.

Phase modulation will actually create an infinite number of sideband frequencies,the number of significant side bands is dependent on the modulating freq and oscillator deviation.The higher the modulating frequency the bigger the sideband spread.

So every adjacent channel will interfere with each other, the amount of interference can be controlled by the modulating frequency and the signal amplitude.

JDMurray

Also realize that a 22MHz channel width is not guaranteed by the laws of physics. Depending on several factors (e.g., position and type of antenna, power output, proximity of transmitter and receiver) the channel width can be much greater. In some configurations, you could have overlapping channels 1 and 6 or 6 and 11.

ITdude

Wouldn't whether you are using a Time Division methodology such as TDMA, versus a Code Division type like CDMA be a factor too?

Time Division having time slices where everyone waits their turn to use the same channel/frequency versus everyone transmitting at the same with Code Division where the modulated signals are spread across multiple frequencies simultaneously as in "spread spectrum"