Soft-Output Demapper and Viterbi Decoder for Software-Defined Radio

Viterbi algorithm is commonly used in communication systems to decode convolutional codes. Soft decision demapping can be used to further improve Viterbi decoder performance. This paper presents implementation of soft-decision demapping and Viterbi decoder for software-defined radio (SDR). Fast simplified algorithms for soft demapping of four modulations common in satellite communications systems (BPSK, QPSK, 8-PSK and 16-APSK) were implemented. To increase software processing speed SIMD (single instruction multiple data) instructions were used.