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首页Poisson PPM通道上的BICM-ID优化:新型PEXIT算法与高效原型编码
本文主要探讨了在Poisson脉冲位置调制(PPM)信道上利用原型图(Protograph)技术优化比特交织编码调制(BICM)与迭代解码(BICM-ID)方案的性能。在传统的通信系统中,基于原型的LDPC编码在加性白高斯噪声(AWGN)信道上已经展现出接近香农极限的优秀性能。然而,当应用于Poisson PPM通道时,这种结合并未得到充分的研究。 Poisson PPM是一种非线性信道,其特性对编码策略有着显著的影响。文章的核心贡献在于提出了一个改进的原型外在信息传输(PEXIT)算法,这是一种用于迭代解码的高效工具,它能够更精确地处理信道状态信息,从而提升编码效率。此外,作者还设计并优化了一个针对BICM-ID方案的原型代码,旨在更好地适应Poisson PPM信道的特点。 通过细致的阈值分析和仿真实验,研究结果显示,与现有的原型图编码相比,提出的优化方案在BICM-ID情况下表现出明显优势。新的编码能够在0.6分贝的范围内逼近信道容量,这表明了优化后的BICM-ID方案在Poisson PPM信道上的性能提升是实质性的。这一成果对于提高在实际通信系统中,尤其是在对数据速率和可靠性有较高要求的应用中的信号传输性能具有重要意义。 这项工作填补了Poisson PPM信道上基于Protograph的BICM-ID编码研究的空白,为在该特殊信道环境下实现高效、接近最优性能的编码设计提供了新的思路和方法。对于那些依赖于BICM-ID技术,并且需要考虑非线性信道特性的系统设计者来说,这篇论文提供的理论和技术成果将极具参考价值。
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2344 IEEE COMMUNICATIONS LETTERS, VOL. 17, NO. 12, DECEMBER 2013
Optimization of Protograph-Based LDPC Coded BICM-ID for the
Poisson PPM Channel
Huang Zhou, Ming Jiang, Member, IEEE, Chunming Zhao, Member, IEEE, and Jiaheng Wang, Member, IEEE
Abstract—Protograph-based LDPC coded bit-interleaved
coded modulation (BICM) schemes are able to achieve near
Shannon-limit performance over the additive white Gaussian
noise (AWGN) channel. However, the performance of protograph-
based BICM and BICM with iterative decoding (BICM-ID)
schemes over the Poisson pulse-position modulation (PPM) chan-
nel has not been adequately studied. In this work, we investigate
and optimize the performance of protograph-based BICM-ID
schemes for the Poisson PPM channel. We propose a modified
protograph extrinsic information transfer (PEXIT) algorithm
and construct an optimized protograph code for the BICM-ID
schemes. Threshold analysis and simulation results demonstrate
that, for the case of BICM-ID, our proposed code is superior to
existing protograph codes, achieving a threshold within a gap of
0.6dBtothecapacity.
Index Terms—Protograph codes, BICM-ID, Poisson PPM
channel, PEXIT algorithm, threshold analysis.
I. INTRODUCTION
T
HE huge power loss as a resu lt of beam spreading makes
communication over d eep-space distance extremely dif-
ficult [1]. For this reason, the joint design of power-efficient
pulse-position modulation (PPM) and channel coding is prefer-
able in deep-space optical co mmunications [2]. Low-density
parity-check (LDPC) codes have been applied in optical PPM
communication systems due to their superior error-correctio n
capability [3], [4]. Bit-interleaved coded m odulation with
iterative decoding (BICM-ID), has been shown to provide
excellent performance over both additive white Gaussian noise
(AWGN) channels and Rayleigh-fading channels [5].
Protograph-based LDPC codes are a subclass of multi-edge-
type (MET) LDPC codes [6] that can be constructed from a
projected graph or a simple protograph. It was shown in [7]
that protograph codes can achieve near Shannon-limit perfor-
mance with low complexity over the AWGN channel. Several
protograph codes, including repeat-accumulate (RA) codes,
irregular repeat-accumulate (IRA) codes, accumulate-repeat-
accumulate (ARA) codes and accumulate-repeat-jagged-
accumulate (ARJA) codes, have been proposed [8], [9].
The serially concatenated pulse-position modulation
(SCPPM) [10] and the LDPC coded PPM (LDPC-PPM)
[4] have exhibited good performance over the Poisson PPM
channel. Although superior in performance, the SCPPM
Manuscript received August 13, 2013. The associate editor coordinating
the review of this letter and approving it for publication was G. Liva.
This work was supported in part by the 973 Program under 2013CB329204,
and by the Research Fund of NCRL, Southeast Unive rsity (No. 2013A05).
The authors are with the National Mobile Communications Research
Laboratory, Southeast University, Nanjing 210096, P. R. China (e-mail:
{zhouhuang, jiang
ming, cmzhao, jhwang}@seu.edu.cn).
Digital Object Identifier 10.1109/LCOMM.2013.111113.131847
may be less suitable for parallel processing [4]. Even if
window-based decoding is used for the inner soft-input-soft-
output (SISO) and the outer SISO, the forward and backward
recursion of the Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm
[11] within each block have to be performed in a serial
manner [4]. Therefore, the LDPC-PPM has received much
attention. However, simulation results in [4] demonstrated
that a protograph code optimized for a binary phase shift
keying (BPSK) AWGN channel, may not maintain its superior
performance over the Poisson PPM channel, when BICM-ID
is considered. Moreover, the performance of other protograph-
based BICM-ID schemes over this channel remains unknown
so far. Hence, there are still some substantial problems about
LDPC coded BICM-ID with PPM worth discussing.
In this paper, we investigate the optimization of the
protograph-based LDPC coded BICM-ID schemes over the
Poisso n PPM channel. Although the protograph extrinsic
information transfer (PEXIT) algorithm proposed by Liva et
al. in [12] can effectively evaluate the iterative decoding
thresholds in AWGN channels, it cannot be applied to the
threshold evaluation of protograph-based BICM-ID schemes
in the Poisson PPM channel. To solve this problem, a modified
PEXIT algorithm is proposed, which provides a specific ex-
trinsic mutual information (MI) exchange scheme for the PPM
demodulator and the LDPC decoder. Additionally, u sing our
modified PEXIT algorithm, we proposed an effective approach
to optimize the protograph with necessary constraints. An
optimized rate-1/2 protograph code is given, which noticeably
outperforms the irregular LDPC code proposed in [4].
Note that the Gaussian-assumption (GA) based density
evolution (DE) analysis [13], which tracks the mean rather
than the MI, has b een used in [14] to determine the thresholds
of protograph-based (spatially coupled) BICM-ID schemes.
However, as noted in [15], the MI generally gives more
accurate threshold prediction results than the m ean. Further-
more, Barsoum et al. have pointed out that it would be
less accurate when the extrinsic log-likelihood ratios (LLRs)
of the PPM demodulator are assumed to be Gaussian [4].
Consequently, our modified PEXIT algorithm may give more
accurate threshold evaluation analysis than the modified GA-
based DE analysis for this specific channel model.
II. S
YSTEM MODEL
The block diagram of a protograph-based LDPC coded
BICM-ID system is illustrated in Fig. 1, where the information
bits are firstly encoded by a protograph-based LDPC encoder
to yield a coded sequence u. The sequence u is then bit-
interleaved as a and sent to the PPM modulator. For M-ary
1089-7798/13$31.00
c
2013 IEEE
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