International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 11, November 2013
ISSN 2319 - 4847
Reduction of Cubic Metric and Block Error
Rate in OFDM system using Network Coding
with Selective Mapping
Pallavi Chandake¹, Dr. S.V.Sankpal²
¹Student, D.Y.Patil College of Engineering and Technology, Kolhapur, Maharashtra, India.
²Associate Professor, D.Y.Patil College of Engineering and Technology, Kolhapur, Maharashtra, India.
Abstract
High PAPR is a major issue in OFDM system. According to 3GPP standards, it is observed that peak to average power ratio
(PAPR) does not predict power amplifier power de-rating as accurately as Cubic Metric (CM). As the value of CM decreases the
efficiency of power amplifier increases. In this paper we introduce the new scheme which combines coding and selective mapping
to reduce CM and BLER. Cubic Metric (CM) and Block Error Rate (BLER) of Network Coding Selective Mapping (NC-SLM)
system is compared with Conventional Selective Mapping (C-SLM) system. The transmitted signal is found to have received
effectively under additive white gaussian noise and multipath fading with implementation of NC-SLM algorithm.
Keywords: Cubic metric, BLER, Block Code, Selective Mapping, OFDM.
1. INTRODUCTION
New access technology used by 3GPP LTE downlink is OFDM [1]. OFDM is multiple access scheme where data is
transmitted to different users on different subcarriers. It provides high data rate transmission in wireless systems because
of its spectral efficiency and better immunity to multipath fading. But like any other systems, it has to many issues, one of
which is high peak to average power ratio. High PAPR increases the power inefficiency of the system. But there is one
more parameter which predicts this power efficiency better than PAPR and it is known as Cubic Metric (CM). The CM is
introduced and adopted by 3GPP standard [2],[3]. The reduction of CM is critical issue in wireless systems where power
is important factor.
Another major issue in wireless system is to reduce transmission error rate to achieve original signal. Forward error
correction (FEC) technique is used to solve this problem and to achieve secure communication [4]. Recently, in wireless
communication, the data stream is divided into number of block. Each block is passed through FEC process. Now output
of this process gives coded blocks which reduce channel errors. Thus the proposed system uses these coded blocks to
minimize BLER.
Along with the network coding, selective mapping scheme is proposed to reduce CM in this paper. Originally, selective
mapping (SLM) is distortionless technique used for PAPR reduction [5]. In SLM, Reduction of PAPR is achieved by
multiplication of independent phase sequences to the original data and determination of the PAPR of each phase
sequence. The sequence with the lowest PAPR is transmitted [6]-[8].SLM has many advantages like less implementation
complexities and better PAPR reduction capacity. However, it can be used directly to other parameters related with
OFDM frame.
2. CUBIC METRIC
A cubic metric (CM) has been adopted by the 3GPP standard as a parameter to determine PA power de-rating because of
its accuracy over a wide range of devices and signals.
The CM of a signal is defined as [3]
(1)
Where,
KCM is an empirical slope factor,
RCM is a raw CM, and
ref is the raw CM of WCDMA reference signal.
is defined for a signal () as
(2)
Where, RMS is the root mean square value of signal.
Volume 2, Issue 11, November 2013
Page 278
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 11, November 2013
ISSN 2319 - 4847
As RCMref and KCM are having constant values, RCM can be used as a performance metric.
3. PROPOSED MODEL
Figure shows block diagram of the proposed CM and BLER reduction scheme. Figure 1 - Block diagram of transmitter.
Figure 2 - Block diagram of receiver.
The transmitter divides the input bit stream into blocks with fixed size. Then encoding is done and these encoded blocks
are a linear combination of the original data blocks. Each generated encoded block is modulated or mapped to one or
more modulation symbols. The mapper output is applied to IFFT, which is the most important block in OFDM system.
The output of this stage is orthogonal signals on its sub channels. From these signals, the one which has minimum CM is
selected.
At the receiver, the process described above is reversed. The received baseband signal is passing through several blocks to
eventually obtain the transmitted data sequence.
Figure 1. Block diagram of proposed OFDM transmitter
Figure 2. Block diagram of proposed OFDM transmitter
4. RESULTS AND ANALYSIS
In this section, we will discuss about the block coded OFDM system with QAM modulation technique. The performance
of NC-SLM is compared with C-SLM. It is observed that network coding and selective mapping jointly reduce the block
error rate and cubic metric.
Table 1. Simulation Parameter
Data source
512
Modulation
QAM
FFT Size
32
Coding technique
Block Code
Message Length
4
Codeword Length
7
Channel
AWGN
To overcome the drawback of the PAPR, as discussed in Section I, we evaluate our proposed system using the CM. With
the given simulation setting, the is computed as described in Section II. The performance evaluation results of
are shown in Fig. 3. We can see that the proposed scheme achieves a good performance when evaluated using the
CM.
Figure 3. CM performance of C-SLM and NC-SLM
Volume 2, Issue 11, November 2013
Page 279
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 11, November 2013
ISSN 2319 - 4847
Fig. 4 shows the BLER performance of C-SLM and NC-SLM under the AWGN and multipath fading. It is observed that
NC-SLM is able to reduce BLER by employing network coding [9].
Figure 4. BLER performance of C-SLM and NC-SLM
5. CONCLUSION
In this paper we have proposed a cubic metric and block error rate reduction scheme to improve the performance of
OFDM system. The BLER can be reduced by applying block coding technique and CM can be reduced by applying
selective mapping technique. We can expect further improvement with implementation of different channels and coding
for OFDM system.
REFERENCES
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[2] 3GPP TSG RAN WG4 #31,3GPP TSG RAN WG1 #37, Motorola Tdoc R4-040367, R1-040522 and R1-040642
Comparison of PAR and cubic metric for power derating2004, May.
[3] 3GPP TSG RAN WG1 LTE, Motorola Tdoc R1-060023 Cubic metric in 3GPP-LTE2006, Jan..
[4] S. Lin and D. J. Costello, Error Control Coding: Fundamentals and Applications. Englewood Cliffs, NJ: PrenticeHall, 1983.
[5] R. J. Baxley, “Analyzing selected mapping for peak-to-average power reduction in OFDM,” Ph.D. dissertation,
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[6] P. Eevelt, M. Wade, and M. Tomlinson, “Peak to average power reduction for OFDM schemes by selective
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[7] R. J. Baxley, “Analyzing selected mapping for peak-to-average power reduction in OFDM,” Ph.D. dissertation,
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[8] [13] A. D. S. Jayalath and C. Tellambura, “A blind SLM receiver for PAR reduced OFDM,” in Proc IEEE VTC, Sep.
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[9] A. A. Yazdi, S. Sorour, S. Valaee, and R. Y. Kim, “Optimum network coding for delay sensitive applications in
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