IS-LDPC码优化OFDM-WLAN系统的PAPR与频谱多样性
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本文探讨了在OFDM-WLAN系统中降低峰值信噪比(Peak-to-Average Power Ratio, PAPR)的一种新型低密度奇偶校验码（Low Density Parity Check, LDPC）技术——可逆子集LDPC (IS-LDPC) 码。IS-LDPC码是一种最近提出的新编码方法，它以较低的复杂度实现了高效的PAPR降低效果。PAPR是OFDM系统中的一个重要挑战，尤其是在无线局域网络(WLAN)如IEEE 802.11标准的OFDM-WLAN系统中，高PAPR可能导致信号失真和功率效率下降。
本文首先介绍了IS-LDPC码的基本原理，这种编码方式利用子集的特性来设计，其编码过程不仅保持了信息的有效传递，还能够在编码过程中实现某种程度的反转性，有助于平滑OFDM信号的幅度波动，从而减少PAPR。作者关注的是将其应用到实际的OFDM-WLAN系统中，以满足IEEE 802.11标准对系统性能的要求。
为了进一步提升系统的性能，文中提出了一种交错子集映射方案。这种方案旨在通过频域上的多样化映射，增强信号的空间多样性，从而提高系统的抗干扰能力和数据传输的可靠性。这样做的目的是在降低PAPR的同时，不牺牲系统的其他关键指标。
通过仿真结果，作者证明了IS-LDPC码对于IEEE 802.11 OFDM-WLAN系统具有出色的纠错能力，并显著降低了PAPR。IS-LDPC编码方案的优势明显，包括高效的PAPR降低、良好的错误纠正性能以及对系统整体性能的积极影响。因此，基于IS-LDPC码的PAPR降低方案对于OFDM-WLAN系统来说是一个有前景的技术解决方案，值得在实际应用中进一步研究和优化。
Invertible Subset LDPC Codes for PAPR Reduction
in OFDM-WLAN Systems
Si Shu and Daiming Qu
School of Electronic Information and Communications
Huazhong University of Science and Technology
Wuhan 430074, China
shusihoon@gmail.com
qudaiming@hust.edu.cn
Xuebing Pei
China Ship Development and Design Center
Wuhan 430064, China
peixbhust@163.com
Abstract—A new type of low density parity-check (LDPC)
codes, called as invertible subset LDPC (IS-LDPC) codes, was
proposed to reduce the peak-to-average power ratio (PAPR)
for orthogonal frequency-division multiplexing (OFDM) systems
with low complexity recently. In this paper, we apply IS-
LDPC codes to OFDM-wireless local area networks (OFDM-
WLAN) systems and design parameters of IS-LDPC codes to
satisfy requirements of the IEEE 802.11 OFDM-WLAN system.
Moreover, we propose interleaved subset mapping scheme to
guarantee the frequency diversity gain. The simulation results
show that the IS-LDPC codes for the IEEE 802.11 OFDM-
WLAN system exhibit good error-correcting performance and
significantly reduce the PAPR. With all mentioned advantages,
the PAPR reduction scheme based on IS-LDPC codes could serve
as an attractive PAPR reduction solution for the IEEE 802.11
OFDM-WLAN systems.
Keywords—orthogonal frequency-division multiplexing (OFD-
M); wireless local area networks (WLAN); peak-to-average
power ratio (PAPR); invertible subset low density parity-check
codes (IS-LDPC); IEEE 802.11
I. INTRODUCTION
The IEEE 802.11 Wireless Local-Area Networks (WLAN)
[1] is the dominant technology adopted in many local broad-
band wireless communication scenarios. The IEEE 802.11
adopts orthogonal frequency-division multiplexing (OFDM)
as its physical layer specifications due to its immunity to
frequency selective channels. However, one major drawback
of OFDM-WLAN systems is high peak-to-average power ratio
(PAPR). High PAPR results in the in-band distortion and
out-of-band radiation when the OFDM signal is fed into a
nonlinear power amplifier [2].
Recently, a new kind of low density parity-check (LDPC)
codes, called as invertible subset LDPC (IS-LDPC) codes,
was proposed in [3], of which each invertible subset can be
independently inverted to generate other valid codewords of
the LDPC code. The main idea of PAPR reduction scheme
with IS-LDPC codes is to generate candidate codewords and
select the candidate with low PAPR for transmission. It is ob-
vious that IS-LDPC codes belong to the type of coding-based
candidate generation schemes [3], [4]. It has been concluded
This work was supported in part by the National Science Foundation of
China with Grants 61271228.
that the PAPR reduction scheme with IS-LDPC codes has good
error-correcting and significant PAPR reduction performance.
Moreover, it supports multiple-OFDM-symbol frames very
well, by dramatically reducing the searching complexity [3].
In this paper, we apply IS-LDPC codes to the IEEE 802.11
OFDM-WLAN system. We design parameters of IS-LDPC
codes to satisfy requirements of the IEEE 802.11 OFDM-
WLAN systems and propose a novel subset mapping scheme
that distributes symbols of invertible subsets over the sub-
carriers in an interleaved manner. Moreover, we verify their
error-correcting and PAPR reduction performance by simula-
tions.
II. B
RIEF REVIEW OF IS-LDPC CODES
A. Definitions of IS-LDPC Codes
Definition 1 (Invertible Subset): Let vector A =
[a
1
,a
2
, ··· ,a
n
] denote a codeword of binary linear
block code A, and subset S = {i
1
,i
2
, ··· ,i
L
} denote
a subset of indexes of the coded bits, i.e., {i
1
,i
2
, ··· ,i
L
}
⊆{1, 2, ··· ,n}. Subset S is an invertible subset if, for any
valid codeword A of A, codeword
A =[a
1
, a
2
, ··· , a
n
] is a
valid codeword of A, where
a
i
=
a
i
,i∈S
a
i
, otherwise
. (1)
Definition 2 (IS-LDPC codes of inversion freedom U): An
invertible subset LDPC (IS-LDPC) code of inversion freedom
U is an LDPC code with U invertible subset, and there is no
intersection among different invertible subsets.
According to the above definitions, if an IS-LDPC code
has multiple disjoint invertible subsets, these subsets can be
inverted independently, and all generated codewords are valid
codewords.
B. PAPR Reduction Based on IS-LDPC Codes
Fig. 1 illustrates the transmitter of the OFDM system
with an IS-LDPC code for PAPR reduction. After the IS-
LDPC encoding, the coded bits can be represented as A =
[A
0
, A
1
, ··· , A
U
], where A
u
(1 ≤ u ≤ U ) represents the
coded bits of the uth invertible subset S
u
, and A
0
represents
____________________________________
978-1-4799--5 /15/$31.00 ©2015 IEEE
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