A reduced complexity decoding scheme for wireless applications

Cellular transmission standards such as GSM and IS-54 produce data streams with varying degrees of significance at the source encoder level. We propose an unequal error protection scheme based on a non-uniform signal set which provides the more important data with a preferential Euclidean distance. The unequal error protection is partly accomplished by the modulator in contrast to the conventional systems where the channel encoder is solely responsible. The coding gain resulting from the asymmetric modulation can be translated into a reduction in the complexity of the channel encoder; specifically a reduction by more than half in the number of encoder states can be expected. We study the interaction between the code complexity and the modulation asymmetry in quantitative terms. For differentially coherent systems, a π/6-shifted differential embedded QPSK is proposed. Decentralizing the bit protection culminates in an extra degree of freedom which in turn introduces more flexibility into the system design