Manchester encoding is not immune from noise but does have built-in error detection.
You haven't indicated just which RF modules you are using but most of the low cost modules use OOK (On-Off Keying) and a superregenerative receiver. The range spec given is usually based on free air line-of-sight. Typical range indoors will be about 20-25% of that if you are using an efficient antenna on the receiving end.
In the absence of a strong signal it is the nature of superregenerative receivers to output continuous noise.
For small amounts of data, I prefer to use the NEC protocol which has a fairly long initial pulse to set the receiver AGC and then transmits two payload bytes with built-in error checking.
I have posted example code for transmitting and receiving in the Code Examples forum http://www.picbasic.co.uk/forum/showthread.php?t=6261





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