中文摘要：

该文基于LDPC码和PPM调制方式构造了一种适用于UWB无线通信系统的低码率的编译码方案LDPC—PPM。本方案在保证系统性能的前提下，通过改变编码比特到调制符号的映射方式，不但避免了一般编码调制系统中译码和解调之间的迭代运算，而且可以应用快速Hadamard变换（FHT）和基于FHT的后验概率译码（APP—FHT）来进一步降低接收端的译码复杂度。可以证明，该方案等价于BPSK调制下的低码率的LDPC．Hadamard码。仿真结果表明，在信息比特长度是65536，该方案可以在-1．18dB处达到误比特率为10^-5的性能，仅比采用BPSK调制的nlrbo-Hadamard码差0．02dB。

英文摘要：

In this paper, a low-rate LDPC-PPM scheme is bring forward which can be applied into UWB communication systems. This scheme can provide good performance with low decoding complexity. By changing the mapping style from coded bits to modulated symbols, not only the iterative operations between decoding and demodulation can be avoided, but also Fast Hadamard Transformation （FHT） and the so called A Posteriori Probability FHT （APP-FHT） algorithm can be adopted to further decrease decoding complexity. It can be proved that this scheme is equivalent to low-rate LDPC-Hadamard codes with BPSK modulation. Simulation results show that this scheme can achieve -1.18dB at bit error rate 10^-5 with data length 65536, which is only worse 0.02dB than Turbo-Hadamard codes with BPSK modulation.