I think it's to do with the auto gain int he receiver, you should send a preamble of 2 or 3 bytes so the receiver sets the gain right, then send the data bytes.
With the data bytes you can still use the serial UART data but split one data byte into two 4bit chunks, then send each 4 bit chunk as one byte, by inverting each bit;
so if the 4bit chunk is 0001
send; 01 01 01 10 as a byte
that is a like a cheat form of "manchester encoding" that ensures every byte sent has an equal number of 0 and 1 bits, that keeps the biasing and auto gain in the receiver working right.