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The title pretty much says it all. What technique can be used to demodulate a signal with SNR less than 1?

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The necessary condition is that the techniques transmit at the rate less than channel capacity.

For example in AWGN channel, for complex (two dimensions of freedom) signal

$$\mathrm{C} = \log_2(1+\mathrm{SNR}) \leq_{(SNR < 1)} 1 \quad \textrm{bit per channel use}$$

A channel use is, in most cases, the generalized term of modulation symbol.

In fading channels, channel capacity (if it exists) formula may be different but the idea is the same.

Thus other than the techniques to increase SNR such as beamforming, there are two principal technique families to deal with the low SNR condition

  • Channel coding (the usage of pilot and equalizer, diversity MIMO, etc, can be considered as a form of coding)
  • Spead spectrum : this can also be considered as coding by taking advantage of multipath diversity
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