Look up table based Demodulation
The Look-up table based demodulator scheme is optimized towards use with MGFSK modulation scheme. The complete demodulation scheme consists of an FM Demodulator, Filtering and Symbol Detector as shown in figure below. The filter is designed carefully to band-limit the noise and preserve a well-behaved impulse response; i.e. it does not add significantly to the inter-symbol interference.
Symbol Detector
The most critical component in this scheme is the Symbol Detector. This detector makes use of look-up templates that store expected waveforms for every possible symbol transition (256 (16*16) templates for 16 level MGFSK).
The initial set of templates can be built from the unit impulse response and margins of noise due to common sources. Since FM is used over the channel, linearity can be assumed, even though the channel is highly non-linear. The set of templates is adaptively updated in run-time.
Because of gaussian filtering at MGSFK modulator, the error patterns are very different from QAM schemes. The errors that the symbol detector makes here frequently appear in pairs. and detected symbols are wrong only by +1/-1 symbol. These error characteristics together with the adaptive updation scheme, can be used to design an efficient Forward-Error-Correction scheme.
