FDM of AM signals having trouble

hello i am trying to use FDM with AM signals for 2,4 and 6 users
on what equation or calculation should i choose deltaf the carrier difference
and regarding FDM

i've seen some examples but not all the same
should FDM be done as:

1- get message signals m1 m2 m3 and multiply with the a subcarrying signals seperated by deltaf c1(lowF) c2(lowF+deltaf) c3(lowF + 2*deltaf)
i am using lowF as 50*pi
2- add the resulting signals
3- multiply the resulting signal with a Fc of high value lets say 200*pi*t

or

1- get the message signals m1 m2 m3
2- multiply with the initial carrier c1, c2(frequency+deltaf), c3(frequency+2*deltaf)
3- send the sum of all signals

can i get some help understading which is correct?

ghattas.akkad said:

hello i am trying to use FDM with AM signals for 2,4 and 6 users
on what equation or calculation should i choose deltaf the carrier difference

Click to expand...

The spectra of the modulated signals must not overlap. So the separation must be greater than the sum of the two correspondent bandwidths.

and regarding FDM
...
should FDM be done as:

1- get message signals m1 m2 m3 and multiply with the a subcarrying signals seperated by deltaf c1(lowF) c2(lowF+deltaf) c3(lowF + 2*deltaf)
i am using lowF as 50*pi
2- add the resulting signals
3- multiply the resulting signal with a Fc of high value lets say 200*pi*t

or

1- get the message signals m1 m2 m3
2- multiply with the initial carrier c1, c2(frequency+deltaf), c3(frequency+2*deltaf)
3- send the sum of all signals

can i get some help understading which is correct?

Click to expand...

The two forms can get the same result.
The first one does it in two steps; the second one in only one step.
For some reasons the first one is more practical.
regards

Z

First solution is better because there isn't any extra carrier to discriminate the information signals. Deltaf should be at least bandwidth of the information signal to prevent from "interference inter-symbol".For instance the 2B bandwidth of the information signal is 100kHz, deltaf must be at least 100kHz or more to get a guard-band.But a subscriber can receive one signal at a time.
Second solution needs an extra carrier to discriminate the information signal.But a subscriber can receive all information signals at a time by adjusting sub-carrier frequency only because you will use one carrier only.
Both method has advantages and disadvantages...

great thank you i have developed two matlab files of both methods but does not seem to get the same results

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zorro said:

The spectra of the modulated signals must not overlap. So the separation must be greater than the sum of the two correspondent bandwidths.



The two forms can get the same result.
The first one does it in two steps; the second one in only one step.
For some reasons the first one is more practical.
regards

Z

Click to expand...

how should i choose the sub carrying frequencies for each message ? and with a correct deltaf

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BigBoss said:

First solution is better because there isn't any extra carrier to discriminate the information signals. Deltaf should be at least bandwidth of the information signal to prevent from "interference inter-symbol".For instance the 2B bandwidth of the information signal is 100kHz, deltaf must be at least 100kHz or more to get a guard-band.But a subscriber can receive one signal at a time.
Second solution needs an extra carrier to discriminate the information signal.But a subscriber can receive all information signals at a time by adjusting sub-carrier frequency only because you will use one carrier only.
Both method has advantages and disadvantages...

Click to expand...

ok i will be using a 5Khz bandwith where i have to calculate how many users can send the same message without overlapping in this case i will choose my deltaf to be 5Khz and a carrier frequency of 100MHz then the number of users to be able to send will be deduced from a mathmatical calculation correct?

BigBoss said:

First solution is better because there isn't any extra carrier to discriminate the information signals.
......

Click to expand...

Note that the transmitted signals are the same in the two cases provided that in the first method the undesired sideband is filtered out and the carrier frequencies are the needed.

ghattas.akkad said:

how should i choose the sub carrying frequencies for each message ? and with a correct deltaf

Click to expand...

If you plot the spectra of the modulated signals (with paper and pencil, no simulation) at each step it becomes clear. DSB spectra that are added must not overlap.
And don't forget to filter out the undesired sideband after final modulation in the first method.

Regards

Z

thank youu