Shaping codes
In shaping codes are a method of that changes the distribution of signals to improve efficiency.
Contents
Description
Typical digital uses M-() to communicate through an analog channel (specifically a with ). For Higher bit rates(M) the minimum (SNR) required by a QAM system with Error Correcting Codes is about 1.53 higher than minimum SNR required by a Gaussian source(>30% more transmitter power) as given in
where
- C is the in ;
- B is the of the channel in ;
- S is the total signal power over the bandwidth and
- N is the total noise power over the bandwidth.
- S/N is the of the communication signal to the Gaussian noise interference expressed as a straight power ratio (not as ).
This 1.53 dB difference is called the shaping gap. Typically digital system will encode bits with uniform probability to maximize the . 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. Shaping codes are helpful in reducing transmit power and thus reduce the cost of Power amplifier and the interference caused to other users in the vicinity.
Application
Some of the methods used for shaping are described in the trellis shaping paper by Dr.
is used in modems to get a shaping gain of .8 . All the shaping schemes in the literature try to reduce the transmitted signal power. In future this may have find application in where the interference from other nodes are becoming the major issue.