An Efficient Joint Network Coding Modulation
Scheme
Fan Yang
∗
, Zhanji Wu
∗
, Xiang Gao
∗
∗
School of Information and Communication Engineering
Beijing University of Posts & Telecommunications
Beijing, China, 100876
Email: bupt07yangfan@gmail.com
Abstract—To improve diversity gain and error-control perfor-
mance in the uplink multiple-access relay systems, an efficient
joint network coding scheme based on bit-interleaved coded
modulation (BICM) with iterative decoding (ID) is proposed.
Based on Extrinsic Information Transfer (EXIT) chart analysis,
different 16-QAM mapping schemes are compared to match
with the proposed network coding modulation scheme, and it is
unveiled that modified set partitioning (MSP) mapping is optimal
for ID scheme. Simulation results turn out that MSP mapping
obtains 5.8 dB signal-to-noise-ratio (SNR) gain as compared with
the conventional Gray mapping, which coincides with our EXIT
chart analysis.
Index Terms—Network Coding, Bit-Interleaved Coded Modu-
lation, Iterative Decoding, Extrinsic Information Transfer Chart
I. INTRODUCTION
Although the idea of network coding was first developed
in [1] to improve the multicast capacity of the wired network
with error-free transmission, the application of network coding
in wireless networks is being extensively studied [2][3]. In
this paper, we consider a basic uplink multiple-access wireless
system which is known as a multiple-access relay channel
(MARC) as illustrated in Fig. 1, in which two users are sharing
one relay. The relay combines the information received from
these two users, which is the essence of network coding,
and then sends the combined information to the base station
(BS). Conventional network coding based on XOR-operation
is adopted at the relay to improve the system throughput and
provide additional error protection capability.
The idea of bit-interleaved coded modulation (BICM) was
first proposed to increase diversity order by using bitwise inter-
leaving with a fixed minimum Hamming distance of the code
[4]. Diversity order and system performance can be further
improved by concatenating a larger constellation with a lower
rate code [5]-[7]. Although one pitfall of BICM is degradation
over Gaussian channels due to the random modulation caused
by bitwise interleaving [4], iterative decoding can resolve this
problem.
In [8] and [9], it shows that iterative decoding can increase
the minimum intersignal Euclidean distance of BICM by
exchanging extrinsic information between the demodulator and
the channel decoder while retaining desirable Hamming dis-
tance. Therefore, with soft-decision feedback, the performance
of BICM can be further improved by iterative decoding. This
User
User
Base station
Relay
Fig. 1. Multiple-access relay channel model
improved scheme is usually referred to as BICM with iterative
decoding (BICM-ID).
As aforementioned, in order to obtain higher diversity gain
provided by network coding, BICM-ID is a promising option
in MARC systems over Rayleigh fading channel. In this paper,
the BER performance of the network coded cooperation is
discussed in MARC scenario over Rayleigh fading channel.
The rest of this paper is organized as follows. In section II,
system model of BICM-ID based network coded cooperation
scheme in MARC systems is introduced. In section III,
the principle of iterative demodulation and decoding of the
proposed scheme is described in details. In section IV, EXIT
chart is used to analyze the iterative decoding convergence
performance of the system. In section V, simulation results
for Rayleigh fading channel is presented. Finally, conclusions
are summarized in section VI.
II. SYSTEM MODEL
The block diagram of a time-division uplink MARC system
based on BICM-ID is shown in Fig. 2, in which two users
communicate with BS with the help of one relay.
In Fig. 2, we consider such a system with one half-
duplex relay and both users are close to the relay. Thus, one
transmission frame consists of three time slots. Each user
sends its symbols to the relay and the BS during the first
two time slots. In the third time slot, relay decodes and re-
encodes the information received from these two users, then
combines the coded bits and sends them to the BS. The BS