I have seen a few examples of link budgets, and they all assume that the total noise at the Rx is just going to be -174 dBm + 10 log BW. However, this seems to me to assume that the transmit signal is perfect and the SNR is the transmit power (dB) minus the noise floor (followed by the noise figure of the Rx chain etc).
I'm having a bit of trouble reasoning through what the effect of a finite SNR on the transmit side would do to the receive noise. Clearly, the signal level is much higher, but if the Tx SNR is relatively bad to begin with (e.g., 10 dB), what is the received SNR going to be? I assume that the finite Tx SNR should have some effect, right?
To make things a bit more concrete, let's say radiated Tx power is 30 dBm, path loss is 100 dB, and we have a 100 MHz wide signal with no antenna gain (or loss). The analyses that I've seen would say the SNR coming off the antenna is 30 dBm - 100 dB - (-174 + 10log 100 Mhz) = 24 dB. But how can I calculate the SNR if we further assume the Tx SNR was only 10 dB (or 20 dB, or 50 dB)?