Data bits and spread chips
At the antenna — spread signal + tone jammer + noise
Correlator output — received × selected code, summed over one bit, slid along the stream
correlator output y[n]
true bits
locked, bit right
locked, bit wrong
no lock
Link
Chips / bit
Jammer J/S
10 dB
Receiver
Right code (A)
Wrong code (B)
A
B
Two different m-sequences; the transmitter sends with A.
Timing error τ
0.000 chip
New bits
—
Processing gain
G
p
—
—
Post-despread
J/S
—
(J/S − G
p
)
Peak correlation
—
ideal peak = 1.000
Bits recovered
—
—
—
Same code, same time, and the bits come back. Change either and the link is gone — that is why the key is the key.