receiver question - how to separate interfering source which has the sam frequency?

Superheterodyne receiver is known to be able to separate the image frequency from the wanted signal.

However, if the interfering sources occupy exactly the same frequency as the wanted signal, how does the receiver separate them?

Thank you for your comments.

The receiver can't separate them.

However, if it is an f.m. system and the receiver and the two signals on the same frequency are f.m., the receiver will demodulate only the stronger signal - supposing there is a difference in strength. It is known as "capture effect"'.

An a.m. receiver, presented with two signals on the same frequency, will produce an output due to both signals. Although the weaker one may be unreadable the listener will know that an additional signal is there.

This is why the aeronautical v.h.f. band uses a.m. - so that a station in distress will have it's signal at least heard and therefore alert the listener to the presence of it - and ask the stronger station to cease transmission so the other may be read.

Wonderful. Thank you for the great explanation!

Separating or distinguish between two received signals on the same frequency is a qualification not only of the superheterodine receivers, but of all types of receivers.

The name of the test performance is Co-channel Rejection, and is the ability of any receiver to decode or demodulate a useful modulated signal in the presence of an interferer, having the same characteristics (frequency, modulation, bandwidth, etc).

Generally digital modulated signals behave better than analog modulated signals in the presence of a co-channel interferer (due to error correction codes used by digital systems).

I agree that it is a qualifcation for all receivers. Thank you for pointing this out!

At the beginning, I thought receiver is just so "passive". It really has no control at all over what is out there: even if the interferer lands on the same frequency or band as the desired signal.

You are very right, the digital systems or maybe digital signal processing techniques (DSP) can help to improve on the co-channel rejection. So the receive itself does have room to do something. Not so "passive" as I first thought.

Say WCDMA systems, all users transmitt in the same freq and same bandwidth , the receiver can work well because of the codes repectively. For one special code, the other users is noise. SO digital systems surely can work.
And for analog systems, there is still some ways to solve the problem, such as SDMA, seperate the signal in the space. You can use line-of-sight telecomm systems to win.

Very interesting comment. There are lots of smart methods (and people) out there!

When you say "line of sight", I guess you mean using antenna beam steering to avoid receiving the interferer. This may not always be possible though.

The line-of-sight means point-to-point telecomm, use parabolic antenna, such as in microwave telecomm, it has very narrow beamwidth. In microwave you can also use one-hop or many hop if the interference angle is fixed. There is a specification named as "same-channel-interference", that is used for your situations. Say you signal is 20dB higher than interference, then it's OK.
Also you can use satellite telecomm, tx signal to satellite, then receive from it. That is one hop.

agreed...high gain, needle beam, aperture antenna can be used for this purpose. This is a great technique at the physical layer of antenna.