Shaping codes
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Typical digital communication systems uses M-Quadrature Amplitude Modulation(QAM) to communicate thorough an analog channel (Specifically Channel with Gaussian noise). For Higher bit rates(M) the minimum Signal to Noise ratio(SNR) required by a QAM system with Error Correcting Codes is about 1.53 dB higher than minimum SNR required by a Gaussian source as given in Shannon–Hartley theorem
where
- C is the channel capacity in bits per second;
- B is the bandwidth of the channel in hertz;
- S is the total signal power over the bandwidth and
- N is the total noise power over the bandwidth.
- S/N is the signal-to-noise ratio of the communication signal to the Gaussian noise interference expressed as a straight power ratio (not as decibels).
This 1.53 dB difference is called shaping gap. Shaping codes are used to fill this gap (ie. Save energy). Typically Digital system will bits with uniform probability to maximize the Entropy. Shaping code act as buffer between Digital sources and Modulator communication system. They will receive uniformly distributed data and convert it to Gaussian like distribution before presenting to the modulator.
