Abstract’Non-contiguous orthogonal frequency division multiplexing
(NC-OFDM) is an effective transmission method for
cognitive radio systems. It can provide high bandwidth utilization
and robustness against time dispersive channels. However, NCOFDM
inherits the problems of high sidelobes and peak-toaverage
power ratio (PAPR) of the OFDM signal. If not accounted
for, this introduces leakage into a primary user (PU) band
and distorts the signal of the secondary user (SU). This paper
proposes a joint sidelobe and PAPR reduction method that can
simultaneously reduce the inherent OFDM out-of-band (OOB)
radiation and the OOB radiation caused by a nonlinear power
amplifier. In this method, selected mapping (SLM) sequences,
which are generated from sequences with a low average sidelobe
power, are used to improve the PAPR performance. The key idea
is to generate multiple representations of the transmit signal and
select a sequence with low OOB radiation and PAPR.We examine
the performance of the proposed method taking into account
the effects of non-ideal (i.e. non-linear) power amplification.
The simulation results show that the proposed technique can
significantly reduce OOB radiation while improving the PAPR
and bit error rate (BER) for NC-OFDM signals.
I. INTRODUCTION
As a multicarrier modulation for cognitive radio systems
[1]-[2], non-contiguous orthogonal frequency division multiplexing
(NC-OFDM) [2] is becoming known as a powerful
tool for spectrum aggregation. This provides cognitive radio
systems with the advantages of OFDM modulation including
efficient bandwidth utilization and robustness against time
dispersive channels. NC-OFDM enables secondary users (SUs)
to deactivate subcarriers occupied by the primary licensed
users (PUs), while employing the subcarriers located in the
unused portion of the spectrum for data transmission.
While NC-OFDM enjoys the advantages of OFDM, it is
also plagued by two significant disadvantages: 1) large spectral
sidelobes, which cause out-of-band (OOB) radiation, and 2)
high peak-to-average power ratio (PAPR), which can lead to
saturation in the power amplifier of SUs and consequently
increases OOB radiation, distorts the signal, and reduces
power amplifier efficiency. The OOB radiation introduces
interference to the adjacent PU frequency bands. Hence, it
is highly desirable to reduce the OOB radiation as much as
possible in NC-OFDM systems.
A number of methods have been proposed in the literature
to reduce the OOB radiation of OFDM [3]-[8], including
multiple-choice sequences (MCS) [9]. MCS can effectively
reduce the sidelobe power levels while requiring only a small
amount of redundancy. With this approach, several sequences
are generated from the original data sequence and the symbol
sequence with the lowest sidelobe power is chosen for transmission.
MCS exploits the fact that different symbol sequences
have different sidelobe power levels.
To reduce the PAPR of NC-OFDM signals, which is the
second source of OOB radiation considering the typically
used non-linear power amplifiers, we must employ PAPR
reduction techniques that can be applied at the transmitter
without regenerating OOB radiation, e.g., the approaches in
[10]-[12]. These techniques also need to provide a low PAPR
with no signal distortion and relatively low complexity. To
achieve such a tradeoff, the technique recently proposed in
[13] employs interleaving [14] in combination with subcarrier
phase adjustment.
All the above-mentioned studies consider either reduction
of the OFDM inherent sidelobes [3]-[8] or PAPR reduction
to avoid spectral regrowth. In this paper, we consider joint
sidelobe and PAPR reduction and propose a method based on
the MCS technique in [9]. The basic idea of MCS originated
from selected mapping (SLM) [10] for the purpose of PAPR
reduction. Our method maps each transmission sequence into
specific sets of sequences that can be exploited by the original
MCS and SLM techniques. Thus, we can simultaneously
reduce the OOB radiation created by high sidelobes and the
PAPR. We evaluate the performance of the proposed technique
in terms of OOB radiation which is generated within an
adjacent PU frequency band. The simulation results show that
OOB radiation is significantly reduced using the proposed
method. We also show that our approach provides considerable
PAPR reduction while effectively reducing the bit error rate
(BER).
The paper is organized as follows. In Section II, an overview
of the MCS and SLM schemes is provided. A description of
978-1-4244-3574-6/10/$25.00 2010 IEEE