04-12-2012, 05:40 PM
Selected Mapping without Side Information for PAPR Reduction in OFDM
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Abstract
Selected mapping (SLM) is a technique used to
reduce the peak-to-average power ratio (PAPR) in orthogonal
frequency-division multiplexing (OFDM) systems. SLM requires
the transmission of several side information bits for each data
block, which results in some data rate loss. These bits must
generally be channel-encoded because they are particularly
critical to the error performance of the system. This increases the
system complexity and transmission delay, and decreases the data
rate even further. In this paper, we propose a novel SLM method
for which no side information needs to be sent. By considering
the example of several OFDM systems using either QPSK or 16-
QAM modulation, we show that the proposed method performs
very well both in terms of PAPR reduction and bit error rate at
the receiver output provided that the number of subcarriers is
large enough.
INTRODUCTION
HIGH peak-to-average power ratio (PAPR) is a wellknown
drawback of orthogonal frequency-division multiplexing
(OFDM) systems. Among all the techniques that
have been proposed to reduce the PAPR (see, e.g., [1] - [7]),
selected mapping (SLM) is one of the most promising ones
because it is simple to implement, introduces no distortion
in the transmitted signal, and can achieve significant PAPR
reduction [2]. The idea in SLM consists of converting the
original data block into several independent signals, and then
transmitting the signal that has the lowest PAPR. The selected
signal index, called side information index (SI index), must
also be transmitted to allow for the recovery of the data
block at the receiver side, which leads to a reduction in data
rate. This index is traditionally transmitted as a set of bits
(the SI bits). The probability of erroneous SI detection has a
significant influence on the error performance of the system
since the whole data block is lost every time the receiver does
not detect the correct SI index. In practice, a channel code
must thus be used to protect the SI bits. This further reduces
the data rate, makes the system more complex, and increases
the transmission delay.
Probability of SI detection error
Fig. 2 shows the probability of SI detection error, Pde, as a
function of the extension factor C, for four different numbers
of subcarriers, N = 65, 125, 255, and 510. We consider two
OFDM systems using either QPSK or 16-QAM modulation.
All results are obtained for Eb/N0 = 10 dB, where Eb
designates the average energy per bit and N0 is the one-sided
power spectral density of white Gaussian noise. The parameter
Pde represents the probability that the receiver cannot recover
the SI index v, i.e. a complete OFDM frame (vector X) is
lost. Note that, for simplicity sake, the numbers of subcarriers
are chosen to be multiples of M = 5 close to the usual powers
of two used in practice. Finally, the suboptimal algorithm
introduced in the previous section is used for recovering the
SI index.
CONCLUSION
We have proposed a simple SLM technique for PAPR reduction
in OFDM that does not require the explicit transmission of
SI bits. Our investigations, performed by considering OFDM
schemes based on QPSK and 16-QAM modulations, have
shown that this technique is particularly attractive for systems
using a large number of subcarriers. In fact, the probability
of SI detection error can be made very small by increasing
the extension factor and/or the number of subcarriers. In
cases where this probability becomes sufficiently low, the
BER performance difference between the proposed technique
and classical SLM using error-free side information has been
shown to be negligible. In addition, the application of our
SLM technique leads to a reduction in PAPR which is identical
to that obtained with classical SLM, for any number
of subcarriers. For OFDM systems with a large number of
subcarriers, the only significant price to pay for employing
the proposed technique instead of classical SLM is a slight
complexity increase at the receiver side due to the use of a SI
detection block.