Abstract: Information security is a prime concern especially when it comes to business and corporate sectors where off the record information should be given utmost importance. Very cooperative especially to business enterprise, modern day expertise has contributed a lot towards sharing a surreptitious data. The field of wireless communication has surmounted many hurdles in the past few years and is now finally capable of supporting higher data rates thanks to the development of Orthogonal Frequency Division Multiplexing (OFDM). It is the most ideal format for broadband communication to meet its need for higher data rates. This study further augment the usefulness of OFDM communication by making it secure to avert data piracy. This study propose an idea of embedding the secret message at the real part of Inverse Fast Fourier Transform (IFFT) output of OFDM system and performance of the system is examined for various modulations. Thus, this study is a conjunction of wireless and data security which serves the purpose of secret sharing and can be applicable and suitable to entrepreneurial activities.
INTRODUCTION
Check in order to deal with the increasing demand for higher data rate and channel capacity, the concept of multiplexing was introduced (Van Nee and Prasad, 2000). Frequency Division Multiplexing (FDM) which was the first type of multiplexing scheme, enhanced the bandwidth efficiency of the channel, by dividing the entire channel into many sub carriers (Peled and Ruiz, 1980; Saltzberg, 1967; Scholtz, 1982). With advancements in wireless technology and their several smart applications, the demands of the users increased manifold calling for new and improved technology. Thus, the FDM paved way for OFDM (Chang, 1970) which is a multicarrier broadband system in which usable bandwidth is separated into narrow bands by which serial data is transmitted in parallel (Akansu et al., 1998; Akay and Ayanoglu, 2004; Hussain et al., 2011; Pickholtz et al., 1984; Thenmozhi et al., 2012; Sun et al., 2013).
Each sub carrier has a unique frequency which is an integral multiple of cycles in symbol period (Chang and Gibby, 1968; Van Nee and Prasad, 2000). Adding cyclic prefix, larger than or almost equal to the channel order, during transmission is primarily used to avoid IBI. The effectuation of FFT in this technique has allowed the use of a set of harmonically related functions as sub carriers, each of which is fed to the single tap equaliser at the receiver end using scalar division (Amirtharajan and Balaguru, 2011; Kumar et al., 2011; Peled and Ruiz, 1980; Thenmozhi et al., 2011, 2012; Praveenkumar et al., 2012a-c, 2013a-l). Thus, OFDM, to-day, is widely preferred in high speed internet and proposed to be used in 4G technology (Al-Kebsi, 2008).
Wireless communication has revolutionized the world by overcoming the inconveniences caused by wired communication. Even though there is no trouble of wires, wireless communication is prone to bit errors while transmission. The packets received are often undetectable or have a very high impact of noise that renders it un-usable. Hence, Forward Error Correction (FEC) is vital in wireless communication systems. These FEC codes add error correction information to the code prior to the transmission that helps in correcting the errors in the signal, if any.
The methods of encoding and decoding data, used to achieve privacy, leak the message out to the intruders. To prevent this, a technology was developed to embed a message or a cipher text inside an image or a multimedia file called steganography (Al-Frajat et al., 2010; Ahmed et al., 2010; Amirtharajan et al., 2013a-e; Amirtharajan and Rayappan, 2012a-d; Ramalingam et al., 2014; Janakiraman et al., 2012a, b; Marvel et al., 1999; Padmaa et al., 2011; Thanikaiselvan et al., 2011, 2012a, b, 2013) and the file is called stego image.
The simplest LSB steganography technique a spatial domain, changes the LSB of any of the layers of the RGB colour pattern of an image. The palette based technique hides the message in one of the colour palettes of the image. The transform based techniques employ an alteration in the coefficients of the frequency domain representation of the image. Another classification related to secure communication called spread communication (Pickholtz et al., 1982; 1984; Pickholtz et al., 1991). Covert communication using CDMA was proposed and carried out in (Amirtharajan and Balaguru, 2011; Praveenkumar et al., 2012a-c, 2013a-j) and a detailed survey on image steganography was illustrated in (Amirtharajan et al., 2012a; Cheddad et al., 2010; Karzenbeisser and Perircolas, 2000; Rajagopalan et al., 2012).
The LSB, though primitive, is the easiest way to implement but also easily detectable. Cryptography and watermarking are two techniques that are very strongly correlated to steganography. While watermarking is another type of technology, Cryptography uses keys for protecting the messages (Karzenbeisser and Perircolas, 2000; Schneier, 2007). Watermarking is used to protect the ownership of a file or a message or simply to copyright a file. The creator’s name is embedded in the file that cannot be detected by steg analysis thus preventing any one else claiming the ownership of that file. Cryptography generates keys that are acknowledged merely to the sender and the receiver alone during each transmission to secure the message, without whose knowledge one cannot open the message. Cheddad et al. (2010) presented a detailed survey on image steganography and steg analysis and there are several information hiding schemes for secret data embedding.
After reviewing the available literature carefully, this study proposes the data embedding scheme after the IFFT block of OFDM to ensure secure data transmission and reception over wireless medium. The next section explains the proposed method followed by results and discussion. The final section conclude that BPSK modulation is preferred compared to other two QPSK and QAM modulation schemes.
METHODOLOGY
The OFDM block diagram with data embedding and extraction is shown in the Fig. 1. In this system, the symbols are produced with symbol rate and these serial data streams are converted in to parallel data’s with symbol rate of where N denotes the sub carriers and TS represents the symbol period. Modulation schemes like Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) schemes are used to produce the constellation points. The complex parallel data symbols are modulated by an Inverse Fast Fourier Transform (IFFT) for converting the symbols from frequency to time domain waveforms. Then separating the real and imaginary parts of the IFFT outputs and then embedding the secret data in the real part of the IFFT output as in Fig. 2. Because embedding in the phase value of IFFT degrades the output performance. This increases the security level of the hidden information and increases the complexity in extracting the information by the intruder.
The FFT and IFFT equations are given in Eq. 1 and 2, respectively:
| (1) |
| (2) |
X(m) ⇒real
then X(m) can be represented as Eq. 3:
| (3) |
| Fig. 1: | Proposed block diagram |
| Fig. 2: | Embedding in real part of IFFT |
| Fig. 3: | IFFT output using BPSK modulation scheme before embedding the secret data bits |
Then one fourth of the total symbol is added to the actual OFDM symbol which is termed as cyclic prefix which is maintained to eradicate Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI). By maintaining the orthogonality condition between the sub carriers of the OFDM system more number of subcarriers can be accommodated without interference. Then finally the orthogonal symbols are converted in to equivalent. As final stage in transmitter section, the digital signal is transformed into its equivalent analog form and then transmitted over the Additive White Gaussian Noise (AWGN) channel.
In Receiver, analog signal is converted into its digital form. After the cyclic prefix removal, the data’s are passed over FFT block, where by knowing the exact key value the secret data has been extracted. Then the data’s are demodulated to get the desired output. If the key value is known the secret data can be extracted, if unknown secret remains secret and the original data alone can be extracted.
RESULTS AND DISUSSION
The various output waveforms of the time domain signal outputs at the IFFT of the OFDM system previous to and behind embedding the secret data using modulation schemes like BPSK, QPSK and QAM are shown below.
Figure 3 and 4, shows the variation of the IFFT output using BPSK modulation before and after embedding are shown.
Figure 5 and 6, shows the variation of the IFFT output using QPSK modulation before and after embedding are shown.
Figure 7 and 8, shows the variation of the IFFT output using QAM modulation before and after embedding is shown.
From the output plots, BPSK is preferred compared to the other two modulation schemes, because the number of modulated output bits are larger compared to the other two modulation schemes and the number of IFFT output points is also more. So, the numbers of confidential bits that is being fixed and extracted are more. Figure 9 provides the comparison between various modulation schemes like BPSK, QPSK, 8, 16, 32 and 64 QAM, respectively.
| Fig. 4: | Embedding done before and after IFFT block using BPSK modulation |
| Fig. 5: | IFFT output using QPSK modulation scheme before embedding secret data bits |
| Fig. 6: | Embedding done before and after IFFT block using QPSK modulation |
| Fig. 7: | IFFT output using QAM modulation scheme before embedding secret data bits |
| Fig. 8: | Embedding done after IFFT block using QAM modulation |
| Fig. 9: | BER comparison between various modulation schemes before and after embedding secret data bits at the output of IFFT |
From the figure, there is no change in BER graph before and after embedding the secret data bits till 20 dB for all modulation schemes.
CONCLUSION
In this study, the performance of secure communication in OFDM system is analyzed for various modulation schemes like BPSK, QPSK and QAM. From the IFFT output plots, embedding secret data in the real part of the IFFT output shows better results even after embedding. The input bits are meager and the IFFT output points are also lesser for QAM modulation compared to QPSK and BPSK modulation schemes. So, the number of confidential bits that can be entrenched in QAM should be less compared to the other two schemes, otherwise it deteriorates the output performance by reducing the orthogonality between subcarriers. So, for better embedding, BPSK modulation is preferred compared to other two modulation schemes. This study has all the practical implementation possibilities which lend a helping hand in maintaining and safeguarding important testimonials and credentials.