Simple question regarding FDM (frequency division multiplexing) & using one ANTENNA
I'm a graduate student without much practical knowledge. Here is my question:
in Analog transmission where we use FDM (air is the medium), the frequency spectrum in TX side in front of antenna ,i always thought, is composed of JUST several TX signal spectrums & then we use a BP filter in RX side to separate this spectrums one by one. BUT,I just heard that the frequency spectrum in TX side is composed of several TX &RX signal spectrums because we can have RX signals received by the same TX antenna(there is no problem in tranmitting and receiving signals AT THE SAME TIME).
1-Is that correct?
2- if i want to have just TX band pass signals, should i have 2 medium? in my case two antennas, one for transmission & one for Reception? is it the same case with another medium such as coaxial cable? can we have both TX & RX signals flowing on one coaxial cable and then in RX side just use the BP filter to separate my favorite RX signal?
3- the nyquist theorem for my FDM TX signal (which is a BP signal) is 2*Fhigh/(n+1)<Fsample< 2*Flow/n which means that modulation & demodulation should be in the band of this modulation freq. & then i can use my BP filterin RX side.
If i have RX signals as well in the TX (due to recieving from the same antenna as transmission) does it change this nyquist equation?
Thanks,
sorry that it's a little bit unclear, but that's how it is in my mind right now
Mitra
If TX and RX is differentiated in frequency in a full duplex system must TX signal be blocked from interfere with a assumed weak distant signal.
RX circuit must contain a filter that is blocking TX signal, which can be done in several steps both as RX bandpass in HF and MF filters and TX suppress filter.
TX as well as RX need a certain bandwidth due to TX modulation (in both directions) so RX/TX frequency spacing must be greater then this.
Also a problem is that even good bandpass or notch filters have a certain bandwidth which must be taken in account when calculating needed frequency spacing.
Assume a typical TX level +20 dBm and a distant signal arrives at RX input with a level of -100 dBm, it is a difference of 120 dB. Requires good filtering if not TX signal should block or distor received signal. Also TX output do often have some kind of filter to limit bandwidth and unwanted spurious and harmonics.
A modern type of filtering method is to just filter some of TX signal that tries to interfere with RX LNA and then after filtering add TX signal in anti-phase as a kind of quench. This type of filtering makes it possible to not have any frequency distance at all between TX and RX.
If these filtering problems are solved are TX and RX channels blind to each other and it is no problem for several RX and TX circuits to share coaxial cable or antenna.
Compare with one green and one red laser which can share same optical fiber in any direction without interfering with each other due to the difference in frequency.