How to evaluate quality of signal? I'm trying to evaluate received signals from a transceiver. I want to make a threshold of received signals so that we can say some signals are meaningless.

I have learnt something about Signal-to-Noise-Ratio (SNR) and Received-Signal-Strength (RSS). But I found that symbols can still be decoded even in low SNR ($\le 0$ dB).

Here are my questions:


If I choose SNR as my metrics, will my threshold be relevant to modulation or coding schemes?
Will RSS be suitable for theoretic analysis as metrics?
Indeed, in practical systems there is usually a cut-off SNR above which communication is possible with a given amount of reliability. This is usually determined according to a design goal, expressed in terms of a bit error rate (BER) or a packet error rate (PER). Typically, the BER should not exceed $10^{-4}$ or $10^{-5}$.
According to your signal parameters (modulation and coding), sometimes it is possible to derive the BER as a function of the SNR. This depends on the complexity of your transmitter-receiver architecture and the channel model you use.
So:


Yes.


Now, regarding the RSS, I would say that it is not a standard metric. Some equipments give RSS metrics, according to an unknown criteria. Sometimes, it is just a table where a given range of received power is indexed. Hence:


It depends. In general, we don't use the RSS to directly determine design parameters of a demodulator (in this case, a threshold). But if you know exactly what it means, you may derive a relation SNR-RSS. But since RSS measures the total received signal power--including noise--I suppose you will have to estimate the noise power also.