Stego in Multicarrier: A Phase Hidden Communication

INTRODUCTION

The present day processors are so fast that the data that need to be processed upon needs to be communicated should be able to synchronize with the pace. Wireless networks are widely used nowadays since they are simpler to implement and involve less complexity and power consumption. Hence, there is a need for improving the data rates through the system without affecting the message integrity. With the recent trends in the field of communication preferring wireless systems over the primitive wired systems, there is an utter need of security. The issue of security needs to be addressed to prevent the integrity and privacy of the message.

It has to be addressed seriously since any tapping or tampering of data can’t be detected by any physical means as there is no physical channel is involved. MC-CDMA scheme couples CDMA and OFDM. This system is robust to frequency selective fading and provides efficient frequency usage. It has high spectral efficiency and facilitates the accommodation of more number of users than CDMA system. In OFDM, the serial data streams are converted into parallel bit streams utilizing orthogonal subcarriers. The entire bandwidth is subdivided into several lower rate narrow band channels thus reducing Inter Symbol Interference [ISI] (Praveenkumar et al., 2012a-c). The MC-CDMA system uses user specific PN sequence code that allows the data stream to be spread over the multiple sub carriers.

The near far problem occurring in the conventional CDMA systems hinder its performance efficiency. Also the use of rake receivers in multipath environment increases its complexity. But the MC-CDMA system uses orthogonal narrow band sub carriers and thus the channel is flat fading at each sub carrier (Hara and Prasad, 1996). Thus, the complexity of the receiver in an MC-CDMA system can be reduced through the efficient use of matched filters. IFFT and FFT. The OFDM principle is not popularly used because of the difficulties in generating, receiving and retrieving an OFDM signal (Kumar et al., 2011). But this problem can be easily solved if the signal is represented in frequency domain and the Inverse Fourier and Fourier transforms are used. IFFT and FFT are mainly used to spread the signal in time domain (Van Nee and Prasad, 2000; Thenmozhi et al., 2012). They reduce the number of multiplications and summations and make the implementation of OFDM easier and simpler. IFFT is used in the transmitter and FFT is used in the receiver side. In OFDM, the orthogonality of sub carriers can be maintained and the individual carriers can be effectively separated using FFT when there is no ISI, but that is not possible because there is a considerable amount of noise added in the channel.

From time immemorial, ways have been found out to protect the privacy of the message. Steganography is one such a technique that allows embedding a message in an image or such a multimedia file. Over the years focus has drifted from encoding the messages to suspicion free transmission i.e., the intruder is given no concrete information regarding the availability of the message. This is achieved by embedding the message in the file, called stego image, with very little alteration in the original file that is almost imperceptible to the hacker. To achieve this various techniques are employed (Amirtharajan and Balaguru, 2011, 2012a-d; Janakiraman et al., 2012).

One of the most primitive and simplest spatial domain methods is the Least Significant Bit (LSB) technique (Amirtharajan and Rayappan, 2012a-d; Janakiraman et al., 2012). Then the more efficient transform based techniques, distortion based techniques and Spread Spectrum Image Steganography (SSIS) (Marvel et al., 1998) were developed further that increased the imperceptibility of the hidden message. Amirtharajan and Balaguru (2011) proposed information hiding model using CDMA system. The process of detecting the presence of a message in a stego image is called stego analysis. The message needs to be absolutely imperceptible to the stego analysis failing which the whole system is assumed to have failed.

Steganography is often confused with cryptography and watermarking. Cryptography is the art of encoding a message using the sender or receiver’s key generated each time the communication takes place (Schneier, 2007). The key prevents the intruder of reading the contents of the message. Watermarking is a form of steganography that protects the copyright of a file or prevents any modification to the file (Amirtharajan and Rayappan, 2012d, Amirtharajan et al., 2012). To detect and nullify these watermarks or message various algorithms have been put to use. Advanced techniques focus on distorting the message or watermark in the stego image (Karzenbeisser and Perircolas, 2000; Petitcolas et al., 1999) rendering it unusable without altering its basic attributes. To make the system completely fool proof various techniques and algorithms are being developed to conceal data. In this study, after reviewing the literature, the proposed method is focusing for transmitting data using CDMA and MCCDMA. The proposed Information embedding schemes employing CDMA and MCCDMA are studied in the presence of AWGN channel for BPSK, QPSK and QAM.