Design of Unequal Error Protection LDPC Code in
PPM and LDPC Optical Communication System
Jingsong Xiang
School of Communication
and Information
Engineering
Chongqing University of
Posts and
Telecommunications
Chongqing, China
xiangjs@cqupy.edu.cn
Yimiao Wu
School of Communication
and Information
Engineering
Chongqing University of
Posts and
Telecommunications
Chongqing, China
wuyimiao0415@163.com
Xiaohui Lu
School of Communication
and Information
Engineering
Chongqing University of
Posts and
Telecommunications
Chongqing, China
2197284809@qq.com
Gaomeng Cha
School of Communication
and Information
Engineering
Chongqing University of
Posts and
Telecommunications
Chongqing, China
2078745248@qq.com
Abstract—In the space optical communication system, the
link power loss is large, and the unequal error protection (UEP)
scheme can effectively protect more important data in the
transmission process and save transmission power. However,
the performance of existing unequal error protection LDPC
codes used directly in space optical communication systems is
not ideal. In this paper, a design scheme of LDPC codes with
multilevel unequal error protection performance is proposed
for pulse position modulation (PPM) and Low-density parity-
check (LDPC) codes iterative demodulation system. The
scheme is aimed at maximizing the average variable node
degree of each protection level to improve the unequal error
protection characteristics of LDPC codes while combining the
improved external information in the BICM-ID system
extrinsic information transfer(EXIT) method to analyze the
convergence threshold of degree distribution and optimize the
degree distribution of UEP-LDPC code by iterative linear
programming to obtain the minimum threshold and satisfy the
unequal error protection characteristics. The simulation
results show that the UEP-LDPC code constructed in this
paper has improved performance compared with the existing
UEP-LDPC code.
Keywords—UEP-LDPC, iterative demodulation, degree
distribution optimization, EXIT
I. INTRODUCTION
With the advantages of high energy efficiency, strong
anti-interference ability and high detection frequency, pulse
position modulation (PPM) is widely applied in deep-space
optical communication. Low-density parity-check (LDPC)
codes have performance close to Shannon limit, which
makes LDPC and PPM iterative demodulation systems
widely used in deep space communication.
In the field of channel coding, UEP transmission schemes
are considered to take into account the different importance
of the data. Some methods for an efficient design of such
UEP schemes are proposed in recent years. Design of UEP
schemes by using partial superposition transmission of
LDPC codes
[1]
, optimizing encoder structure
[2]
, combining
with modulation constellation
[3]
. In [4], a criterion for
constructing UEP QC-LDPC codes via the masking
technique is proposed. However, the LDPC code constructed
in[4] does not take into account the characteristics of the
deep space channel and the characteristics of the iterative
demodulation system. The performance is not very good to
use it directly in PPM and LDPC optical communication
systems.
In this paper, we design the UEP-LDPC codes from the
perspective of variable node degree distribution based on
PPM and LDPC iterative demodulation system, and
increases the error performance gap between different
protection levels between LDPC codes by designing the
degree distribution. In the convergence analysis of the degree
distribution, the output mutual information(MI) of the PPM
demodulator is used as the channel information to participate
in the update of the variable node information, so as to
accurately analyze the decoding threshold of the LDPC code.
The UEP-LDPC codes are constructed with the objective of
maximizing the average node degree of variables and
minimizing the convergence threshold.
II. SYSTEM MODEL
The iterative demodulation system of PPM and LDPC is
mainly composed of a transmitting end and a receiving end,
and the transmitting end includes an LDPC encoding module,
an interleaving module and a PPM modulation module. PPM
demodulation module and the LDPC decoding module in the
receiving end are connected by an inter-leaver and a
deinterleaver, and the PPM demodulator receives the symbol
sequence demodulation to generate bit likelihood
information, and after deinterleaving, acts as a priori
information of the decoder. The decoder decodes the a priori
information to obtain a posteriori information that is re-
interleaved and fed back to the demodulator as priori
information. The information is iteratively transmit until the
decoding is complete. The iterative demodulation method
makes full use of the posterior information and obtains good
error performance.
LDPC
Encoder
Interleaver
PPM
Modulator
DeInter-
leaver
Interleaver
LDPC
Decoder
PPM
Demodu-
lator
Information
bits
Poisson
channel
Decoded
bits
-
-
+
LLR
+
LLR
Fig. 1. Block diagram of PPM and LDPC iterative demodulation system.
2019 IEEE International Conference on Artificial Intelligence and Computer Applications
978-1-7281-1223-7/19/$31.00 ©2019
Dalian, China
March 29-31,