PPT Slide
- Note that Channel includes the Modem and its effect
- For a Gaussian channel, the optimum stand-alone modem for binary phase-shift keyed (BPSK) modulation, is a correlator or matched filter which decides “0” or “1” based on the sign of the correlator output “HARD DECISION.” This destroys information useful for the decoder; thus degrades channel performance.
- Optimum strategy is to pass to the decoder the actual value of correlator output -- usually digitized to 8 or more levels (3 or more bits) - called a “SOFT DECISION.”
- For the AWGN channel, soft decisions extend C and increase E(R). Effect is to reduce required Eb/N0, for a given PE , by about 2dB.