well, the signal is corrupted, if it could not be well filtered. by the way, there is a lot of methods to overcome this (i mean, where the signal is concentrate in a certain narrow-band). for example, the chanel noise could be estimated with a smart cicuit (such as neural network), this way, the chanel charasteristics is estimated, consequently the noise output is estimated, and thus the main signal could be well reproduced. i check the method with a noise value more than 3times biger than the signal. (by the way, the most critical charasteristic of noise is that its average is zero). hope it will help.
marti