
Research Article


Reversible Steganography on OFDM ChannelA Role of RS Coding


Padmapriya Praveenkumar,
K Thenmozhi,
J.B.B. Rayappan
and
Rengarajan Amirtharajan


ABSTRACT

The kinetics of higher data requirements in the upcoming wireless
generation is engendered by the fact that the usage of high quality multimedia
applications in the form of data, audio, SMS and video is on the rise. In order
to meet the challenging needs of the increasing data rate requirements, the
wireless system employs Orthogonal Frequency Division Multiplexing (OFDM). Security
is the need of the hour in today’s wireless technological advancement.
In this study, OFDM with Reed Solomon codes (RS) as Forward Error correction
codes (FEC) has been proposed to achieve lower Bit Error Rate (BER) and data
embedding has been carried out to ensure security in wireless communication.
Further RS codes are used to restore the cover object.





Received:
March 05, 2013; Accepted: April 15, 2013;
Published: April 18, 2014 

INTRODUCTION
In today’s era of internet and wireless media, the rise in use of high definition multimedia applicationsds calls for higher data rates. In order to accomplish the requirement of increased data rates, Orthogonal Frequency Division Multiplexing (OFDM) is being recognized as a format for modulation for the future wireless communication such as Digital Multicarrier Multiplexing/Modulation (DMCMM) (Kumar et al., 2008; Praveenkumar et al., 2012c). It is an orthogonal mode of Frequency Division Multiplexing (FDM) in which the orthogonal subcarriers make the system spectrally efficient (Liu et al., 2006; Salari et al., 2008). Here, the cyclic prefix consists of the onefourth of the OFDM symbol that is being inserted in the guard interval which is followed by the actual OFDM symbol which annihilates Inter Symbol Interference (ISI) (Joshi and Saini, 2011; Thenmozhi et al., 2011). It is proves immune to Multipath propagation and frequency selective fading channels (Van Nee and Prasad, 2000; Praveenkumar et al., 2012a, b).
Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) pairs make the system design less complex and the Reed Solomon codes (RS) are called as Maximum Distance Separable (MDS) codes (Hussain et al., 2011). The redundant bits added to the encoder are very minimal for error correction and detection but it has better performance when Channel State Information (CSI) is known (Van Meerbergen et al., 2006). It outperforms the other Forward Error Correction codes (FEC) especially when the channel introduces burst errors. The minimum distance between the codes can be arrived by the largest possible way (Van Meerbergen et al., 2009). When the number of the inputs to the system is large, then RS codes are preferred which works efficiently on the channel. Steganography literally means covered writing that is security through obscurity in any digital media (AlAzawi and Fadhil, 2010; AlFrajat et al., 2010; Bender et al., 1996). There are three types of steganography called pure, symmetric and public key steganography, in pure steganography no key is used, where as symmetric key uses single key for concealing and revealing the information and public key steganography uses two keys one for embedding and another for extraction (Amirtharajan et al., 2012). Cryptography, Steganography and Watermarking are multifarious in secret data communication (Amirtharajan et al., 2012; Zanganeh and Ibrahim, 2011; Zhu et al., 2011).
Cryptography scramble the confidential information, Steganography conceals the very existence of the confidential information and watermarking is for authorization or to prove the ownership (Amirtharajan and Rayappan, 2012a; Zhao and Luo, 2012). AlFrajat et al. (2010) describes the data concealment for videos as cover object by identifying the redundant bits embed the encrypted data. However, while embedding the confidential information on the cover object, there always exists trade off between capacity and imperceptibility (Amirtharajan and Rayappan, 2012ad; Kumar et al., 2011; Padmaa et al., 2011). A review on various random data embedding methods have been analysed by Amirtharajan et al. (2012), Thenmozhi et al. (2012) and Rajagopalan et al. (2012).
 Fig. 1:  OFDM transceiver model with RS codes 
After carefully reviewing the existing literature on OFDM and steganography, this study has been proposed to implement Reed Solomon codes (RS) as Forward Error correction codes (FEC) to achieve lower Bit Error Rate (BER) and data embedding has been carried out to ensure security in wireless communication. Further RS codes revert the cover also. PROPOSED METHODOLOGY In OFDM technique, higher data rate bit streams are converted in to parallel slower data rates and then, their own digital modulation schemes can be chosen based on the requirement of power and spectral efficiency as shown in Fig. 1. It can either be BPSK or QPSK which gives out two phase values for binary digits and results in four phase output values. To accommodate more data bits in the pixel, Mary QAM is preferred which will cause changes in both amplitude and phase of the subcarriers. With Mary spectrum, efficiency can be increased and this supports variety of applications and provides large capacity at the cost of Eb/No. In order to avoid this, equalization is done in set of subcarriers rather than single carrier making the symbol period longer and applying Cyclic prefix wipes out Inter Symbol Interference (ISI). As the data rate increases, probability of multipath propagation will results in Inter Symbol Interference (ISI). Thus, to minify Bit Error rate (BER), the reduction in data rate should be done and a solution to this can be arrived at by using OFDM which reduces the symbol rate without compromising the data rate. Since, it broadcasts larger number of subcarriers, which are very closely spaced and maintains orthogonality, the symbol rate of the subcarriers slow down and higher data rate can be maintained because of the huge number of orthogonal subcarriers. Further, each subcarrier can be modulated individually by IFFT to reduce cross talk between the adjacent users. One fourth of each OFDM symbol is then copied and the redundant value can be added to the symbol to avoid ISI.
Work on RS code was initially based on Mathematics and was invented by French mathematician Evariste Galois in 1830’s, then known as the finite field or Galois field.
 Fig. 2:  RS code representation 
It was further carried out by Irving Reed and Gustave Solomon at MIT laboratory in 1960. RS codes are systematic linear block codes and can also be called as non binary cyclic codes where the parity bits are appended to the original information. They add redundancy to the original information and thus, the extra information added to the original information decides the errors that can be corrected and detected. It is specified as RS (n, k) and is given in Fig. 2 where n refers to the encoded bits and K refers to the original bits. nk = 2t refers to the parity bits that has to be appended to the original information in the form of encoding. nk/2 symbols can be corrected by the RS decoder. The minimum distance of RS(n, k) code is nk+1 which are often termed as the maximum distance separable codes. RS code(n, k) derives its symbols from Galois field called as GF(2m). They are intended for burst errors and is very effective for the systems considering fading channels with memory.
In this study, the input data bits are encoded using RS codes because they provide better results in case of burst errors especially when transmission channel information is known. Then, the covert data embedding is carried out in the redundant bits of the input information bits which is followed by the modulation of subcarriers with secret data using BPSK/QPSK/QAM which in turn depends on the required spectral efficiency. Further, the frequency to time domain conversion is carried out using IFFT which maintains the orthogonality between subcarriers because they are closely spaced. Then, CP is inserted and analog to digital conversion is carried out to transmit over AWGN channel. At the receiver end, the reverse operation of modulation is carried out and the input and the embedded secret data is retrieved and BER graphs are plotted.
 Fig. 3:  Comparison among BPSK, QPSK and QAM in OFDM with RS codes 
 Fig. 4:  Comparison between BPSK, QPSK and QAM after data embedding in OFDM with RS codes 
 Fig. 5:  Comparison between Uncoded, RS coded and RS coded with data embedding using BPSK 
RESULTS AND DISCUSSION
The comparative results among BPSK, QPSK and QAM in OFDM using RS codes were analysed. (Van Meerbergen et al., 2006). Then, BER was plotted after embedding the secret data. Figure 3 depicts the comparison results between BPSK, QPSK and QAM with RS codes. From the graph, it is proved that BPSK provides better BER and at Eb/No = 14.2 db, it approaches to zero.
 Fig. 6:  Comparison between Uncoded, RS coded and RS coded with data embedding using QPSK 
 Fig. 7:  Comparison between Uncoded, RS coded and RS coded with data embedding using QAM 
Figure 4 gives the comparison results among BPSK, QPSK and QAM with RS codes after data embedding. From the graph, it can be illustrated that BPSK provides better BER as mentioned by Praveenkumar et al. (2012a, b) and at Eb/No = 14 db it approaches to zero. Figure 57 gives the comparison between RS coded, uncoded and data embedding using BPSK, QPSK and QAM, respectively.
RS coded in all the three modulations provides better results. After the embedding of data, BER of OFDM system employing QAM system provides better error performance as discussed by Kumar et al. (2011).
CONCLUSION OFDM has attracted the Wireless environment by adapting higher data rate with improved spectrum efficiency by utilising orthogonal subcarriers. In this study, BPSK, QPSK and QAM has been adapted as a base modulation prior to IFFT block along with RS codes to control channel errors and the BER performance has been plotted. This result confirms that all three modulations are good with RS codes, even after additional data embedding. In addition, this study also proves that QAM out performs the remaining two (QPSK, BPSK) as proved by many researchers, even with additional data through information hiding. Furthermore, this study also confirms that the BER performance is excellent for RS coded and good for RS coded with data embedding also to get back the cover object and then decent for Uncoded data.

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