I’m wondering if spreading a bitstream (XOR-ing each bit with
SPREAD_FACTOR bits of sequence) before QPSK modulation is equivalent to
operating on the modulated stream, i.e., multiplying the complex output
with -1’s and 1’s from the sequence. So far I can’t get the math to work
out. Any suggestions?
In general you’re right, but it depends on the details. Its usually
straightforward, but a common mistake is to use the wrong bit-to-symbol
mapping. 0 -> +1 and 1 -> -1 means that an XOR applied to bits is
equivalent to multiplication applied to symbols.