01-03-2013, 02:12 PM
SC-FDMA with Iterative Multiuser Detection: Improvements on Power/Spectral Efficiency
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ABSTRACT
This article outlines the basic principles of
single-carrier frequency division multiple access
with iterative multiuser detection, called grouped
frequency division multiple access. GFDMA
allows multiple users to share a common set of
subcarriers and separates the signals of users by
employing distinct interleavers and frequencydomain
multiuser detection. Some attractive features
of GFDMA are explained, including
low-cost iterative multiuser detection, multiuser
and frequency diversity gains, and flexibility in
resource allocation. We also discuss the applications
of GFDMA to spatial diversity schemes
and to intercell interference mitigation in the
cell edge. It is shown that GFDMA offers advantages
over SC-FDMA with respect to
power/spectral efficiency. Such power/spectral
efficiency can provide considerable performance
improvements in future wireless communication
networks.
INTRODUCTION
In various fora and organizations for wireless
research, such as the Wireless World Research
Forum (WWRF) and International Telecommunication
Union (ITU), there have been active
discussions about 4G or beyond 3G systems to
be deployed around 2010 [1]. Third-Generation
Partnership Project Long Term Evolution (3GPP
LTE) represents a major advance in cellular
technology [2]. The overall target of 3GPP LTE
is to provide improved coverage and system
capacity as well as increased data rates and
reduced latency. Single-carrier frequency division
multiple access (SC-FDMA) has been considered
to be the most promising multiple access
techniques for the uplink of 3GPP LTE. SCFDMA
is a combination of FDMA and singlecarrier
modulation with frequency-domain
equalization (SC-FDE), which has similar structure
and performance as orthogonal frequency
division multiple access (OFDMA) [3–5].
SC-FDMA PRINCIPLES
Figure 1a describes the transceiver block diagram
of SC-FDMA. SC-FDMA has much in
common with OFDMA except for additional discrete
Fourier transform (DFT) and inverse DFT
(IDFT) blocks at the transmitter and receiver,
respectively. For this reason, SC-FDMA is sometimes
referred to as DFT-spread or DFT-precoded
OFDMA. The transmitter of SC-FDMA uses
different subcarriers to transmit information
data, as in OFDMA. However, SC-FDMA transmits
the subcarriers sequentially, rather than in
parallel. This approach has the advantage in
enabling low PAPR which is important to
increase cell coverage and to prolong the battery
lifetime of mobile terminals. At the transmitter,
input data sequence of the uth user is encoded
by an RC-rate forward error correction (FEC)
encoder. The coded bits are mapped to multilevel
symbols in one of modulation formats such
as BPSK (Q = 1), QPSK ( Q = 2), and16QAM
(Q = 4), where Q is the modulation level. The
system adapts transmission data rate to match
the channel condition of each user.
SUBCARRIER MAPPING FOR SC-FDMA
SC-FDMA has two apportionment methods for
allocating subcarriers to multiple users (Fig. 2a).
In localized FDMA (LFDMA), each user uses a
set of adjacent subcarriers to transmit the signals.
LFDMA can achieve multiuser diversity in frequency-
selective fading, if it assigns the symbols
of each user to subcarriers where the user has
high channel gain. It requires the use of channeldependent
scheduling (CDS). The system using
CDS should monitor the quality of channel frequency
response, and adaptively assign subcarriers
to users.
GFDMA PRINCIPLES:
SC-FDMA WITH IMD-FDE
GFDMA is a SC-FDMA incorporating the IMDFDE,
which allows more than one user to share a
common set of subcarriers (chunk). GFDMA with
one user in a chunk is the same as SC-FDMA.
Each transmitter of GFDMA consists of two main
parts: channel coding and interleaving with different
patterns for different users. For basic explanation
of decoding in GFDMA, let us assume that
information data of users 1 and 2 are interleaved
by interleaving patterns 1 and 2, respectively, and
are transmitted to the receiver. At the receiver,
after deinterleaving with pattern 1, the desired signal
from user 1 is of its original order and can be
decoded successfully, while the interference from
user 2 is of pseudo-random order and thus acts as
independent and uncorrelated interference. Channel
decoder, which treats the whitened interference
as noise, makes reliable detection possible.
Figure 1b shows the transmitter and receiver
structures of GFDMA. The coded bits are first
interleaved by user-specific interleavers, and
then converted to multilevel symbols. As in SCFDMA,
P symbols are grouped, followed by a Ppoint
DFT. Each of the P DFT outputs is
assigned to one of the N subcarriers by using a
subcarrier mapping mode which depends on the
channel condition of each user.
APPLICATIONS OF GFDMA
In this section, several applications of GFDMA
are discussed. First, we apply transmit diversity
schemes to GFDMA. Second, GFDMA is
employed to achieve spatial diversity without
sacrificing spectral efficiency in half-duplex constrained
relay networks. Lastly, GFDMA can be
utilized to mitigate ICI in the cell edge.
GFDMA WITH TRANSMIT DIVERSITY SCHEMES
Spatial diversity schemes have been shown to
provide large performance improvement for
wireless communication channels. Diversity gain
can be achieved by transmitting from multiple
spatially separated antennas. Alamouti proposed
space-time block code (STBC) and space-frequency
block code (SFBC) schemes, which are
simple and powerful diversity techniques. The
signal vectors of each transmit antenna are
orthogonal in the STBC and SFBC schemes.
CONCLUSIONS
In this article, the basic principles of SC-FDMA
with IMD-FDE, called GFDMA, were presented.
GFDMA allows more than one user to share
a common set of subcarriers. The interference
caused by the other users is cancelled with userspecific
interleavers and frequency-domain multiuser
detection. Simulation results show that
GFDMA can increase the power/spectral efficiency
of SC-FDMA. Such power/spectral efficiency
can provide considerable performance
improvements in future wireless communication
systems. Due to the ability of multiuser detection
and interference randomization, GFDMA
can be utilized to increase capacity in the cooperative
network or the cell edge where the same
frequency resource is used. Moreover, GFDMA
can offer high flexibility for resource allocation
and achieve both multiuser and frequency diversity
gains with channel-dependent resource
scheduling schemes.