Towards Efficient Design of Fixed-point Iterative
Receiver for Coded MIMO-OFDM Systems
Rida El Chall
1
, Fabienne Nouvel
1
, Maryline H
´
elard
1
, Yvan Kokar
1
, and Ming Liu
2
1
INSA, IETR, CNRS UMR 6164, F-35708 Rennes, France
2
Beijing Key Lab of Transportation Data Analysis and Mining, Beijing Jiaotong University, Beijing, China
Email:{relchall, fabienne.nouvel, maryline.helard, yvan.kokar}@insa-rennes.fr, mingliu@bjtu.edu.cn
Abstract—In recent years, iterative receiver has been widely
considered since it is able to achieve high data rates and reliable
communications for coded MIMO-OFDM systems. However,
such a receiver is extremely challenging in practical implementa-
tion. In this paper, we present an efficient fixed-point arithmetic
of iterative receiver based on low-complexity K-Best decoder in
order to reduce the hardware costs in terms of area and execution
time. The performance of fixed-point receiver is then evaluated
with different modulation orders and channel models. Moreover,
a testbed for MIMO-OFDM system using wireless open access
research platform (WARP) is introduced in order to evaluate the
performance of the receiver under real system conditions.
Index Terms—Iterative receiver; MIMO; K-Best decoder;
Turbo decoder; Fixed-point arithmetic; WARP platform.
I. INTRODUCTION
In future wireless communication system, the demand for
high rate data and reliable transmission is rapidly increasing.
Modern techniques have been adopted into the emerging stan-
dards such as IEEE 802.11, 802.16, DVB-NGH, 3GPP long
term evolution (LTE) and LTE-Advanced (LTE-A). Among
these techniques, multiple-input multiple-output (MIMO) tech-
nology which utilizes multiple antennas at transmitter and re-
ceiver offers an increase of channel capacity and link reliabil-
ity [1]. Orthogonal frequency division multiplexing (OFDM)
technique is generally combined with MIMO technique to
combat the inter-symbol interference in multi-path propagation
and to achieve better spectral efficiency. Meanwhile, advanced
forward correction codes such as turbo codes are able to
achieve performance close to theoretical limits [2].
In such a coded MIMO-OFDM system, iterative receiver
is used to cancel the co-antenna interference and to achieve
near capacity [3]. Such a receiver consists of soft-input soft-
output (SISO) detector and channel decoder that exchange
soft reliable information. Several sub-optimal detectors have
been proposed in the literature to reduce the complexity
of maximum a posteriori probability (MAP) detector. These
solutions include linear equalization, interference cancellation
and tree search-based detection. Examples include minimum
mean square error-interference cancellation (MMSE-IC) [4],
single tree search sphere decoder (STS-SD) [5], K-Best de-
coder [6] and fixed sphere decoder (FSD) [7]. K-Best and FSD
Ming Liu is supported by the NSFC Grant (61501022), BJTU Founds
(15RC00040), and Fundamental Research Funds for the Central Universities
(2014JBZ003).
algorithms perform a breadth-first search and are hence more
attractive in practical implementation than STS-SD as they
offer a best trade-off between performance and complexity [8].
Theoretically, floating-point representation is typically used
to evaluate the performance of the system. However, when
considering practical systems, fixed-point design must be
considered for an efficient use in terms of area and execution
time. The conversion into a fixed-point format requires to
specify a finite word length with a specific number of bits for
integer parts and for fractional parts. However, this conversion
has a great impact on the performance of the system due
to the reduced dynamic range and the precision inaccuracy.
Additionally, in real transmission several aspects have to be
considered in the receiver design such as synchronization,
channel estimation and front-end radio-frequency (RF) mod-
ules (e.g. amplifier, filter, ADC, DAC).
In this context, the purpose of this paper is to investigate
the impact of real aspects on the performance of the iterative
receiver. The rest of this paper is organized as follows. Section
II provides the MIMO-OFDM system model and the principle
of iterative detection and decoding. Low-complexity K-Best
based MIMO detector is introduced in section III. Section
IV illustrates the process of the conversion of the receiver
into fixed-point format. The BER performance of the iterative
receiver is then evaluated in section V. Section VI presents
the testbed for MIMO-OFDM system using wireless open
access research platform (WARP) and the results of wireless
measurements. We conclude the paper in section VII.
II. MIMO-OFDM S
YSTEM MODEL
We consider a MIMO-OFDM system with N
t
transmit
antennas and N
r
receive antennas (N
r
≥ N
t
) as depicted
in Figure 1. At the transmitter, the information bits u are
first encoded. The coded bits c are then interleaved and gray-
mapped into complex symbols of 2
Q
quadrature amplitude
modulation (QAM) constellation, where Q is the number of
bits per symbol. The symbols are then spatially multiplexed
over N
t
transmit antennas. IFFT is applied to N
c
parallel
symbols to obtain the time domain OFDM symbols, where
N
c
is the number of sub-carriers. The symbols are then sent
though the radio channel after the insertion of the cyclic prefix
(CP) which is assumed larger than the maximum excess delay
of the channel. At the receiver, after the removal of the CP,
2016 23rd International Conference on Telecommunications (ICT)
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