Modulation Comparison Of Ofdm Stego Ifft Embedding

Published Date: 02 Nov 2017

Abstract -- Communication through wireless is tricky and it is a rapidly emerging field because of its reward in safety measures, increased ability and it has an ability to cope with different data rates. OFDM has discovered into a fashionable scheme for broadband digital communication process and plays a major role in radio communication due to its self secured communication schemes. Generally, we use the OFDM system for this secure purpose, in this paper, we propose an additional secure step to make a new process in OFDM SECURE COMMUNICATION by embedding the information to the IFFT output of OFDM system with BPSK, QPSK and QAM modulations and performance of the system for achieving better desired output of the system with these modulations is examined.

Keywords – OFDM, IFFT EMBEDDING, MOD. – BPSK, QPSK and QAM, CYCLIC PREFIX

INTRODUCTION

OFDM is the projected scheme for the 4G system. The problem related by means of the wireless channel is fading effect by the frequency selection and the multipath propagation. OFDM scheme can be used capably for the process of communication in such argumentative environments. For the effectual use of frequency band width OFDM method has received the significant interest in wired and in the wireless communication. For the errors taken place in the system respective correction codes been used in OFDM model. OFDM is mainly used for the transmission of data at high rate.

Generally, OFDM proceeds the transmission method with the simple principle that the entire transmitted channel is splitted into several sub channels in the available Bandwidth and then they are transmitted parallely with increased symbol duration to avoid the cause of affecting the Inter Symbol Interference (ISI) and the another possible Inter Carrier Interference (ICI) is avoided by maintaining the orthogonality condition as from the name it’s a Orthogonal FDM technology. Thus these two causes meliorates OFDM technology among the other.

The performance of the OFDM can be increased by the usage of different modulation scheme and it results in poor performance for the sub carriers in the case of fixed modulation. Interference arises during the data transmission, self-cancellation method is been used for the reduction of ICI. OFDM is the talented applicant to moderate the ISI. Insertion of guard period helps to prevent against ISI .The band width of the OFDM signal is been separated into number of sub-channels which been transmitted in parallel form.

The channel equalizer in the OFDM system is very easy to execute and the signals are denoted by frequency domain. Efficiency has been loss due to the cyclic prefix in the system. Single carrier OFDM model can be able to perform better in BER and also in power ratio. OFDM is the model which is been adopted for the audio and video broad casting and also in wireless LAN. A different method of modulation system and the interference reduction in the system helps to achieve a better performance of signals.

Security system can be achieved by steganography or water marking a survey on different types of information hiding techniques and by the cryptography. The secret information is been scrambled and to provide privacy is been deals with cryptography. The data has been encrypted along with the public key in the encryption system and the data been decrypted by the private key at the other end and the data been taken by the cryptosystem. By the usage of keys in the cryptography the data been secure in the cryptosystem. The intention of the cryptography is mainly for the protection of data. Encryption allows protected communication require a solution to read the information. An aggressor cannot eliminate the encryption but it is comparatively easy to adapt the file, making it badly written for the intended beneficiary.

With the improvement of processor and escalating it’s utilize in various areas of existence and effort, the problem of message sequence safety measures has become progressively more significant. One of the grounds discuss in message sequence safety measures is the swap over of message sequence through the cover up media. To this last part, different methods such as cryptography, steganography, coding, etc have been used. The technique of steganography is among the schemes that have arriving awareness in current years. The key objective of steganography is to screen message sequence in the other cover up media so that other person will not observe the occurrence of the message sequence.

This is a most important peculiarity between this technique and the other techniques of covert swap over of message sequence because, for illustration in cryptography, the individuals become aware of the message sequence by allowing for the coded message sequence but they will not be able to realize the message sequence. However, in steganography, the survival of the message sequence in the foundation will not be noticed at all. Most steganography jobs have been accepted out on pictures, video clip, text, tune and sound. Now-a-days, using a grouping of steganography and the other methods, message sequence safety measures has enhanced significantly. In adding together to being used in the hidden swap over of message sequence, steganography is used in other grounds such as patent, prevents e-document forging. Steganography is imitative from the Greek for enclosed lettering and basically means "to bury in plain view".

Steganography is the skill of unremarkably hiding information within information. The main goal of steganography is to hide information well sufficient such that the accidental recipient do not suspect the steganographic standard of containing hidden information Simple steganographic technique have been in employ for hundreds of years, but with the ever-increasing use of records in an electronic arrangement advance techniques for information thrashing have become possible. Most steganography works have been accepted out on various storage space cover media like text, image, sound or video. Steganography and encryption are equally used to make sure information privacy. However, the most important dissimilarity between them is that with encryption anyone can observe that both party are exchanging in top secret. Steganography is used for secret messaging and in best case unknown can see that both party are exchanging top secret. This makes steganography appropriate for some responsibilities for which encryption aren’t; such as patent marking.

MODULATION

BPSK

BPSK is the simplest form of phase shift keying (PSK). It uses two phases which are separated by 180° and so can also be termed 2-PSK. It does not particularly matter exactly where the constellation points are positioned, and in this figure they are shown on the real axis, at 0° and 180°. This modulation is the most robust of all the PSKs since it takes the highest level of noise or distortion to make the demodulator reach an incorrect decision. It is, however, only able to modulate at 1 bit/symbol (as seen in the figure) and so is unsuitable for high data-rate applications.

QPSK

QPSK uses four points on the constellation diagram, equi-spaced around a circle. With four phases, QPSK can encode two bits per symbol, shown in the diagram with gray coding to minimize the bit error rate (BER) — sometimes misperceived as twice the BER of BPSK.

The mathematical analysis shows that QPSK can be used either to double the data rate compared with a BPSK system while maintaining the same bandwidth of the signal, or to maintain the data-rate of BPSK but halving the bandwidth needed. In this latter case, the BER of QPSK is exactly the same as the BER of BPSK - and deciding differently is a common confusion when considering or describing QPSK.

Given that radio communication channels are allocated by agencies such as the Federal Communication Commission giving a prescribed (maximum) bandwidth, the advantage of QPSK over BPSK becomes evident: QPSK transmits twice the data rate in a given bandwidth compared to BPSK - at the same BER. 

QAM

Quadrature amplitude modulation (QAM) is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. The two carrier waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers or quadrature components — hence the name of the scheme. The modulated waves are summed, and the resulting waveform is a combination of both phase-shift keying (PSK) and amplitude-shift keying (ASK)

PROPOSED METHODOLOGY

The Proposed OFDM block diagram is shown in the figure 1. It is a usual block, but in this proposed methodology, a modulator is added next to the IFFT block to embed the secret information to the output of IFFT symbols. By this method of embedding the secret information, we can increase the level of security to the hidden information and also this method increases the complexity of extracting the information by unknown.

In the OFDM system, symbols are produced with symbol rate and converted from serial to parallel with symbol rate of N sub carriers represented from X0 – XN-1 with the expression represented as

(1)

Where, fn is the frequency of the nth subcarrier, n ranges from X0 – XN-1

For the signals to be orthogonal, the following condition should be satisfied.

(2)

Otherwise 0 < t ≤ Ts

1.JPG

Figure 1. Block Diagram of an

OFDM Transceiver

Modulation schemes BPSK, QPSK and QAM are used to indicate the constellation then complex parallel data symbols X k,m are modulated by an Inverse Fast Fourier Transform (IFFT) for converting the domain of the symbols from frequency to time waveforms represented as s(n).

To the output of IFFT, the secret data is embedded individually to the time domain symbols and the parallel data is converted to serial with the converter which operates in reverse manner to the previously used one to convert the symbols as parallel. At the final stage of the transmitter, the cyclic prefix is added to eliminate ISI and ICI by maintaining the orthogonality condition between the sub carriers of the OFDM system by adding the cyclic prefix as a preamble to the OFDM symbols and also to convert the convolution type from linear to circular. Another advantage of using guard interval is that it act as a guard space between the symbols to vanish Inter frame Interference. As final stage in transmitter section, the digital signal is converted into analog with digital to analog converter to tune the signal, capable of transmitting over the channel with symbol rate .

At the receiver part, initially the received analog signal is converted into digital form. Since it is an OFDM scheme, it can overcome the channel noise effects easily. After the added cyclic prefix is removed, again the signal is converted into parallel stream then the embedded data at the transmitter section is extracted at this stage with respect to the real and imaginary values and then the signal is converted back into frequency domain with Fast Fourier Transform (FFT) and demodulate with the used modulation scheme then converted serially with converter and the desired output is obtained.

RESULTS

The various waveforms output of time domain signal from IFFT block of the OFDM system before and after embedding the secret data of modulation schemes BPSK, QPSK and QAM are shown below in the plots

2bpsk.JPG

Fig.2 IFFT Plot for BPSK Mod. Before Embedding

1bpsk.JPG

Fig.3 IFFT Plot for BPSK Mod. Before Embedding

From Fig. 2 & 3, the variation of the output of IFFT with BPSK modulation before and after embedding are shown

4qam.JPG

Fig.4 IFFT Plot for QAM Mod. Before Embedding

3qam.JPG

Fig.5 IFFT Plot for QAM Mod. Before Embedding

From Fig. 4 & 5, the variation of the output of IFFT with QAM modulation before and after embedding are shown

6qpsk.JPG

Fig.6 IFFT Plot for QPSK Mod. Before Embedding

5qpsk.JPG

Fig.7 IFFT Plot for QPSK Mod. Before Embedding

From Fig. 6 & 7, the variation of the output of IFFT with QPSK modulation before and after embedding is shown.

CONCLUSION

In this paper, the performance of secure communication in OFDM technology is analyzed for better results by embedding the secret information to the output of IFFT symbols in time domain by modulating the signals either by BPSK, QPSK or QAM modulations and the performance of the system by embedding with these modulation schemes are also examined and results are plotted in the above graphs. From the above graphs, it shows that BPSK modulation produces better results in transmission of the signal in OFDM system even after embedding.