Receiving multiple signal

Question

Maybe this is a really silly question even from me, a completely amateur...

But I like to know about how's the antenna output when its receive multiple signal

1. Is it possible different signals on same frequency exist? And can they resonate to antenna at the same time?

2. If it's possible how's the antenna response? Is it will sum up or superposition all the signals?

I am confuse with a wideband signal or wideband antenna fundamental. I learned about FM and AM modulation and these question appears.

Thanks!! Sry bad english

Answer 1

Transmission between antennas is "two port in to two port out".

But: in between there is a transmission path with reflections. Some frequencies are attenuated and some other frequencies are even stronger than average. That is called Rayleigh path (sorry, not complete for the moment). The transfer is frequency-dependent AND antenna position dependent.

A different receiving antenna position has different effect from that "selective fading". The frequency transfer between transmitter antenna and receiver antenna can be completely different. That is why MIMO works.

Back to your questions: different signal on same frequency can. From different TX-positions. Same TX position can also but same frequency and same position makes separation impossible (....other options...).

Question 2 can't be answered: the propagation path between TX and RX is a not-used variable in your imagination. Position of (TX and) RX antenna in a Rayleigh path, and also frequency offset give new opportunities.

Answer 2

There's multiple way that multiple signals can be sent at the same time on the same frequency.

One is to have multiple antennas, where even though the antennas themselves can be omnidirectional, they can be used to "beam form" by carefully offsetting the signal. And likewise the receiver can do this trick. Also see this article on MIMO. Wifi can get faster using these spacial streams, using the same amount of spectrum. See this table on 802.11ac.

Then there's CDMA, where multiple transmitters are sending different signals on the same frequency. The individual transmissions can be differentiated because they all have different "codes".

CDMA can be more efficient than separating by frequency or in time, since separation by frequency requires a quiet border between channels, and in time (TDMA) requires a guard interval, and transmitters that are well synchronized and able to turn on and off very quickly. In this way CDMA is more efficient, since the transmitter just blasts a continuous spread signal, and the receiver has to extract the desired signal by processing.

A single transmitter could transmit two different CDMA channels "on top of" each other. There's nothing particularly strange about it. From the antenna (and before that, the Digital-To-Analog converter) perspective two signals is just an oscillating electric signal, just like one signal is.

For FM the other comment about "capture effect" is what practically happens. But FM is just one mode. E.g. in AM if two transmit on the same frequency at the same time receivers will hear both, as if it were two people in a room talking at the same time. AM doesn't have capture effect. Nor, of course, does LSB or USB, so same there.

Answer 3

Yes, multiple signals can exist on the same frequency at once. In a net, this could happen if two people tried to talk at the same time. This is called a "double". When this happens, the signals mix, or add together.

If the two signals are AM, the amplitudes of the radio waves add, which corresponds to the amplitudes of the audio, so you get pretty much the same effect if two people were talking at once -- you hear them both.

For FM, when the radio amplitudes add, it doesn't help the audio at all. Instead, the FM receiver will try to lock on to one signal ("the capture effect") and if one signal is stronger than the other, it will lock on. You'll still hear buzzing interference from the other signal. If the two signals are close in power, the receiver will alternately lock on to one and then the other, possibly getting nothing intelligible from either, or maybe getting short pieces of each alternately.

With FM, it is possible to capture both signals using software defined radio, and then subtract the stronger signal and recover some of the weaker signal, but this is more theoretical and not commonly done.

To the antenna, when it gets the energy from both signals; it doesn't see them as separate signals. It also gets the energy from all the other frequencies around it, some of which it absorbs and these all mix and appear as voltages and currents on the antenna feed line. This signal is then fed into your radio, which has to amplify and discriminate the desired frequencies from, and then demodulate those into the information -- possibly audio -- that you want.