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I don't know a lot about comm system design.

I have an AWGN channel and low SNR (-3dB). Bandwidth is constrained to some maximum, B. I would like to know good choices of modulation and coding to maximize throughput.

I've read that restriction to binary modulation at low SNR does not degrade capacity wrt to log_2(1 + SNR), but that coding is necessary and that B needs to be big.

I've also read that low-rate linear binary codes can achieve capacity. Is there a particular code family I should use for my scenario?

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  • $\begingroup$ The normalized achievable rate (technically speaking, when a practical modulation is used, it's called achievable rate not capacity) of BPSK is 1 bit per channel use at best. With coding, this rate is less than 1. Capacity is achieved when Gaussian symbols are used. Not with practical modulations. It's a theoretical limit. $\endgroup$ – BlackMath Aug 22 at 2:32
  • $\begingroup$ Let me try to explain more fully. Fig 1 of www-classes.usc.edu/engr/ee-s/568/org/Forney-Ung.pdf shows that for asymptotically low SNR the "channel capacity" associated with a binary input (2-PAM) in AWGN approaches that of a Gaussian input in AWGN. In Section IIIA, the paper further states that at low SNR linear binary codes (with soft decoding) are "optimum" in the sense that they can achieve rates at the capacity bound. These are theoretical results. In my problem SNR is very low (<0dB) and BW is restricted. I'm wondering what mod & coding would maximize throughput... $\endgroup$ – rhz Aug 22 at 3:05

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