International Journal of Scientific & Engineering Research, Volume 4, Issue 8, August-2013 208

ISSN 2229-5518

Performance analysis of PAPR reduction techninques for MC-CDMA system

Jaswinder Kaur, Komal Arora

Abstract- MC-CDMA and OFDM systems are widely used in the existing 3rd and 4th generation wireless networks. They are also good candidates for the future generation networks for broadband and personal communications. But, high peak to average power ratios (PAPR) is serious problem in MC- CDMA which can increase complexity in the analog to digital and digital to analog converter, reduce the system efficiency and increase the overall sys- tem cost. The major scope of this paper is to reduce PAPR in MC-CDMA using PTS, SLM and Hybrid techniques. According to the simulation results presented in this paper, using PTS technique, PAPR is reduced up to 2.9 dB than original PAPR of the signal for different number of users and iterations. By analyzing the plots of Complementary Cumulative Distribution Function (CCDF) versus probability (PAPR 0 ), it has been shown that SLM technique lowers PAPR up to 2.2 dB and Hybrid technique is reduced PAPR upto1.5 dB than original PAPR of the signal.

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Index Terms— MC-CDMA, OFDM, PAPR, CCDF, SLM, PTS.

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1 INTRODUCTION

Multi-Carrier Code Division Multiple Access (MC-CDMA) is fusion of OFDM (Orthogonal Frequency Division Multiplex) modulation and the CDMA multiple access technique. This multiple access technique invented in 1993 [1] and further re-

viewed for development in 1997 [2].
• Code-division multiple access (CDMA) schemes have
been considered as attractive multiple access schemes
in both second-generation (2G) and third-generation
(3G) wireless systems.
• The evolution from 2G to 3G corresponds to adapting
a new air interface i.e. changes of focus from voice to
multimedia.
• MC-CDMA scheme has become a most likely tech-
nique for 4G air interface.
• MC-CDMA is a direct sequence CDMA, but in this
applied Fourier Transform (FFT) after spreading of
the signal.
• MC-CDMA is also called “CDMA-OFDM” because it
is a form of orthogonal frequency division multiplex-
ing (OFDM) but we first apply an orthogonal matrix
operation to the user bits.
• MC-CDMA is also a form of frequency diversity be-
cause each bit is transmitted simultaneously on many
different subcarriers.
MC-CDMA and OFDM system are widely used in the exiting
3rd and 4th generation of wireless networks. It is the most
promising technique for high bit rate and high speed data
transmission in mobile communications.

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Jaswinder kaur is currently pursuing masters degree program in electronic and coomunication engineering in Lovely Professional University, Punjab (India). E-mail: jaswinderkaur.lpu@gmail.com

Komal Arora is Asst. Prof. in electronic and communication department in

Lovely Professional University, Punjab (India). E-mail: ko-

mal.15703@lpu.co.in
One of the major negative aspects with MC-CDMA is high Peak-to-Average Power Ratio (PAPR). Due to high PAPR re- sults in nonlinear distortion at a high power amplifier (HPA) and degradation of the bit error rate (BER).
To handle these occasional large peaks, a high-power amplifier with linear characteristics is required. But this increased the complexity of the ADC and DAC converters and cost of the system. MC-CDMA spreads the signal in frequency domain. MC-CDMA performs better in downlink level, but it performs poor in uplink level. MC-CDMA has gained much regard be- cause the signal can be easily transmitted and received using FFT device without increase the transmitter and receiver com- plexity and it is potential robust to channel frequency selectiv- ity with a good frequency use efficiency. MC- CDMA system is widely used for Long Term Evaluation (LTE), WIMAX, and Digital TV transmission, Digital data Transmission over the Telephone system, Digital audio broadcasting, Digital Televi- sion, and Wireless Local Area Networks [3].
To reduce the PAPR, several techniques have been invented [3-
6], which basically can be divided in three categories. First of
all, signal distortion techniques which reduce the peak ampli-
tudes simply by nonlinearly distorting the MC-CDMA signal
at or around the peaks e.g. clipping [7]. Second, there are cod-
ing techniques that use DWT, DCT and DHT to lower PAPR
[8]. The third technique scrambles each MC-CDMA symbol
with different scrambling sequences and selecting the se-
quence that gives the smallest PAPR [9].
In this paper, we use SLM, PTS techniques for PAPR reduction in MC-CDMA system. We investigate the Hybrid technique which is combination of PTS and SLM technique and then MC-CDMA siganl is obtained and PAPR parameter will be calaulated.

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2 SYSTEM DISCRIPTION

2.1 MC-CDMA System

MC-CDMA is a multiple access scheme which allowing the system to support multiple users at the same time. The block diagram of MC-CDMA transmitter is shown in Figure 1. The working of each block describes as:

K user data: The source of information can be symbol of K user. It transmits data symbol of K user simulta- neously on several narrow band sub channels.
Spreader: The most important purpose of the spread- ing codes is to help preserve orthogonality among dif- ferent physical channels of the uplink user. Walsh- Hadamard codes, also known as OVSF (Orthogonal Variable Spreading Factor) codes, are used for the fre- quency spread coding to achieve the orthogonality among users [4].

Figure 1: MC-CDMA Transmitter

Walsh Hadamard spreading code: In MC CDMA scheme, Walsh-Hadamard codes are used for the fre- quency spread coding to achieve the orthogonality among users. This code is simply obtained by select- ing as codewords, the rows of Hadamard matrix. +1 and –1 denote the elements of the Hadamard matrix. The Hadamard matrix Hn is an n´n matrix such that n is an even integer with the property that any row dif- fers from any other rows in exactly n/2 positions. Usually, the first row of the matrix contains all +1s. The other rows contain +1s of n/2 and -1s of n/2. The rows of the Hadamard matrix are then mutually or- thogonal. To generate the code, the fundamental unit of Hadamard matrix is given as

The following recursive matrix operation is employed to produce the Walsh-Hadamard codes of length 2n.

where the matrix, H2n , of the size 2n*2n is formed by using the matrix, Hn, of size n*n with H2 given above. These codes fulfill completely the orthogonality be- tween each other when synchronization is regularly held. [4]
Modulator: Modulation is the technique by which the signal wave is transformed in over the communica- tion channel in order to minimize the effect of noise. QPSK modulation is used.
Serial to parallel converter
Data to be transmitted is in the form of a serial data
stream. So a serial to parallel conversion stage is

needed to convert the input serial bit stream to the da-
ta to be transmitted in each OFDM symbol.
IFFT: By working with MC-CDMA in frequency do-
main the modulated QPSK data symbols are fed onto
the orthogonal sub-carriers. But transfer of signal over
a channel is only possible in its time-domain. For
which we implement IFFT which converts the MC-
CDMA signal in from frequency domain to time do-
main. [10]
Parallel to serial converter: The parallel to serial
converter is used to converted data back into serial
data form.
Digital to Analog Converter and high power ampli-

fier: After the parallel to serial conversion of the data

stream send to the digital to analog converter fol-
lowed by high power amplifier and up convertor for
transmission.
Channel: This is the channel through which the data
is transferred. Presence of noise in this medium af-
fects the signal and causes distortion in its data con-
tent.

2.2 PAPR of signal

The ratio of the peak to average power value is termed asPeak- to-Average Power Ratio [1-6].

where E[.] denotes expected value.

2.2.1 Disadvantages of high PAPR

• Increased complexity in the analog to digital and dig- tal to analog converter.
• Reduction is efficiency of RF amplifiers.
• Increased cost of the system

2.3 Cumulative Distribution Function

The cumulative distribution function (CDF) parameter is used to measure the efficiency on any PAPR technique. Normally, the Complementary CDF (CCDF) is used instead of CDF, which helps us to measure the probability that the PAPR of a certain data block exceeds the given threshold [1-17]. The CDF of the amplitude of a signal sample is given by


The CCDF of the PAPR of the data block is desired is our case to compare outputs of various reduction techniques. This is given by

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3 PAPR REDUCTION TECHNIQUES

PAPR reduction techniques [1-17] vary according needs of the system and dependent on various factors such as PAPR reduction capacity, increase in power in transmit signal, loss in data rate, complexity of computation and increase in the bit-error rate at the receiver end etc.

3.1 Selective Mapping

Selective Mapping Technique is called signal scrambling technique with explicit side information. The MC-CDMA with SLM is shown in Figure 2. In this technique the input data frame is divided into non-overlapping sub-blocks and each sub-block is phase shifted by constant factor. After it the block with minimum PAPR is select for transmission. The PAPR value is also depend upon the phase shift factor. [18-19]

Figure 2: MC-CDMA with SLM

3.2 Partial Transmit Sequence

Partial Transmit Sequence is called signal scrambling with explicit side information or distortion less technique. The MC- CDMA with PTS is shown in Figure 3. The basic idea of sym- bol scrambling is that, for each MC-CDMA symbol, the input sequence is divided by a certain number of sub-blocks. The output signal with the smallest PAPR is transmitted. In this technique, data of varying sub-carrier is only transmitted which covers all the information to be sent in the signal as a whole is called Partial Transmit Sequence Technique. [19-20]

Figure 3: MC-CDMA with PTS

3.3 Hybrid Technique

Hybrid technique is combination of Partial Transmit Sequence and Selective Mapping Technique. The MC- CDMA with Hybrid technique is shown in Figure 4. This technique is signal scrambling technique. In this technique, first input signal is applied to SLM and after it; output of SLM is applied to PTS. The lowest PAPR is selected by each optimization block of PTS. [21]

Figure 4: MC-CDMA with Hybrid technique

4 SIMULATION RESULTS

This section is devoted to the graphical and simulation results of SLM, PTS and Hybrid techniques. The system parameters for different techniques, used in thesis are listed in Table 1.

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TABLE 1

Simulation Parameter

4.1 The PAPR Reduction Performance for SLM:

The PAPR reduction performance for SLM shows in Figure 5,
6 and 7 for 8, 16 and 32 input users with 100 iterations respec-

tively. The graph is plotted between CCDF [PAPR>PAPR0] versus PAPR0 [dB]. The improvement in PAPR reduction at CCDF= 0.1 is listed in Table 2.

Figure 5: PAPR reduction performance of Selective Mapping for 8 input users


Figure 6: PAPR reduction performance of Selective Mapping for 16 input users


Figure 7: PAPR reduction performance of Selective Mapping for 32 input users

TABLE 2

Improvement in PAPR reduction using SLM for different

number of input users


Table 2 shows that using Selective Mapping, PAPR maximum reduced up to 2.2 dB for 8 input users with 100 itreations.

4.2 The PAPR Reduction Performance for PTS:

The Figure 8, 9 and 10 illustrate the PAPR reduction perfor- mance for PTS for 8, 16, 32 input users with 100 iterations re- spectively. The improvement in PAPR reduction at CCDF= 0.1 is listed in Table 3.

Figure 8: PAPR reduction performance of Partial

Transmit Sequence for 8 input users

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Figure 9: PAPR reduction performance of Partial Transmit

Sequence for 16 input users

Figure 10: PAPR reduction performance of Hybrid technique for 32 input users

TABLE 3

Improvement in PAPR reduction using PTS for different

number of input users

Figure 11: PAPR reduction performance of Hybrid technique for 8 input users


Figure12: PAPR reduction performance of Hybrid technique for 16 input users


Table 3 shows that PAPR is reduced maximum up to 2.6 dB for
8 and 16 input users respectively.

4.3 The PAPR Reduction Performance for Hybrid

Technique:

The Figure 11, 12 and 13 shows the PAPR reduction perfor- mance for and Hybrid technique for 8, 16 and 32 input users with 100 iterations respectively. The improvement in PAPR reduction at CCDF= 0.1 is listed in Table 4.

Figure 13: PAPR reduction performance of Hybrid technique for 32 input users

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TABLE 4

Improvement in PAPR reduction using Hybrid technique for

different number of input users with 100 iterations


Table 4 shows that PAPR reduction using Hybrid techniques remained approximately constant for different number of in- put users with 100 iterations

5 CONCLUSION

MC-CDMA gained a lot of attention for future generation of wireless communication systems. Different methods SLM, PTS have been investigated to lower the PAPR in MC-CDMA sys- tems. With the reduction of PAPR, MC-CDMA has applica- tions in the area of wireless communications such as Broad- band Multi-User Communications, WLANs and Broadcasting. In this paper, three PAPR reduction techniques have been dis- cussed. The Performance of PAPR reduction techniques has been evaluated using CCDF [PAPR>PAPR0] versus PAPR0 [dB] plots.
The PTS technique reduces the PAPR up to 2.6 dB than origi- nal PAPR of the signal for different number of users with 100 iterations. With 100 iterations, SLM technique reduced PAPR up to 2.2 dB than original PAPR of the signal. It has been ob- served that PAPR of the hybrid technique lies always between
1.1-1.5 dB for different number of users. In the case of hybrid technique, the PAPR is reduced maximum up to 1.5 dB than original PAPR.

ACKNOWLEDGMENT

This work is supported by my guide, Miss. Komal Arora, As- sistant Professor, Electronics and Communication Department, Lovely Professional University, Jalandhar.

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