  In this paper, we propose using a generic protograph to design joint
source-channel codes (JSCCs). We present a generalized algorithm, called
protograph extrinsic information transfer for JSCC algorithm (PEXIT-JSCC
algorithm), for analyzing the channel threshold of the proposed JSCC. We also
propose a source generic protograph EXIT (SGP-EXIT) algorithm, which is more
appropriate than the generalized source protograph extrinsic information
transfer (GSP-EXIT) algorithm, for evaluating the source threshold of a generic
protograph. Moreover, a collaborative optimization method based on the SGP-EXIT
and PEXIT-JSCC algorithms is proposed to construct generic-protograph JSCCs
with good source and channel thresholds. Finally, we construct
generic-protograph JSCCs, analyze their decoding thresholds, and compare their
theoretical and error performance with JSCC systems based on optimized
double-protographs. Results show that our proposed codes can attain channel
thresholds within 1 dB from the Shannon limit and outperform
double-protograph-based JSCCs.
