Abstract:

The idea of fountain code is,the transmitter transmits unlimited numbers of coded packets until the receiver correctly receive enough number of coded packets to decode the original uncoded information.This characteristic implies that during the decoding of original information,almost all the coded packets have equally importance.In this paper,we propose a novel hybrid asymmetric BPSK/QPSK mapping modulation scheme for fountain codes.This scheme can increase system spectrum efficiency sharply,especially when the channels have relatively low signal-to-noise ratios（SNR）.Firstly,we calculated the symbol error probability of different mapping methods in AWGN channels,and used different simulation results to prove our novel hybrid asymmetric BPSK/QPSK mapping modulation scheme is better than the classic QPSK modulation in low SNR scenarios.From the results above we can be inspired that,when the channels are fading or affected by Doppler effect,our novel scheme can have better performance because they will most likely to have high probability of relatively low instant SNR values.

Key words: fountain code, hybrid mapping, LT code