The Impact of Mismatch on the Performance of Coded Narrow-Band FM with Limiter/Discriminator Detection

The performance of uncoded narrow-band FM with limiter/discriminator (L/D) detection over the additive white Gaussian noise channel has received widespread attention [1]-[14] over the years both from simulation and analytical standpoints. By comparison, little or nothing has appeared in the literature for the performance of this modulation/demodulation technique when used together with coding/decoding. The purpose of this paper is to partially fill this void by considering the potential gain offered by the combination of convolutionally encoded narrow-band FM with L/D detection and Viterbi decoding. Both hard and soft decision decoding cases will be considered with some rather surprising differences between the performances of the two. In particular, it will be shown that a large departure exists from the 2 dB advantage traditionally associated with soft decision decoding which is explained in terms of the mismatch between the coding channel and the decoding metric peculiar to this modulation/demodulation technique.