Simulation Model of OFDM with Security Algorithm for Estimation of BER and SNR

Mr. Rahul Gedam¹, Ms. Sanskriti Sharma²

¹Chouksey Engineering College, Bilaspur

²Shri Shankaracharya Institute of Professional Management & Technology, Raipur

*Corresponding Author Email: engg.rahul2801@gmail.com, sanskriti04sharma@gmail.com

ABSTRACT:

Mathematically, the word orthogonal means 90⁰ phase shift. Using the principle of orthogonality we are indulging the range of communication with security algorithms. The research on wireless system is to improve the reliability and performance of wireless radio links. The effects of (multi-path) radio propagation, modulation, and coding and signal processing techniques on the spectrum efficiency and performance of wireless radio networks have to be studied, in particular Orthogonal Frequency Division Multiplexing (OFDM) and related transmission methods .But the multimedia information is not secured in the wireless environment compared to that of wired environment. In this paper, we have carried out a new method of cryptographic algorithm over a future generation (4G and above) wireless system called the OFDM (FFT (Fast Fourier Transform) / IFFT (Inverse FFT)) technique using Walsh Hadamard spreading codes. We have done the simulation in MATLAB programming and the BER (Bit Error Rate) vs. SNR (Signal-to-Noise Ratio) performance is studied for multimedia signals (such as text, audio and image) in an AWGN wireless channel. We have also developed an algorithm for generating the Walsh codes and the RSA secret keys. We found the results of our simulation are very much satisfactory for practical implementation in wireless systems.

In this project is to demonstrate the concept and feasibility of an OFDM system, and investigate how its performance is changed by varying some of its major parameters. This objective is met by developing a MATLAB program to simulate a basic OFDM system. From the process of this development, the mechanism of an OFDM system can be studied; and with a completed MATLAB program, the OFDM Basics.

KEY WORDS: Multimedia, Orthogonal Frequency Division Multiplexing, RSA, Shift Ciphering, Substitution cipher, Walsh Hadamard.

implementation of OFDM and the components which work on high frequencies was not in focus.

Basic principle of OFDM:

Orthogonal Frequency Division Multiplexing (OFDM) is very similar to the well-known and used technique of Frequency Division Multiplexing (FDM). OFDM uses the principles of FDM to allow multiple messages to be sent over a single radio channel. It is however in a much more controlled manner, allowing an improved spectral efficiency.

A simple example of FDM is the use of different frequencies for each FM (Frequency Modulation) radio stations. All stations transmit at the same time but do not interfere with each other because they transmit using different carrier frequencies. Additionally they are bandwidth limited and are spaced sufficiently far apart in frequency so that their transmitted signals do not overlap in the frequency domain. At the receiver, each signal is individually received by using a frequency tuneable band pass filter to selectively remove all the signals except for the station of interest. This filtered signal can then be demodulated to recover the original transmitted information.

Each of the carriers in a FDM transmission can use an analogue or digital modulation scheme. There is no synchronization between the transmission and so one station could transmit using FM and another in digital using FSK. In a single OFDM transmission all the subcarriers are synchronized to each other, restricting the transmission to digital modulation schemes. OFDM is symbol based, and can be thought of as a large number of low bit rate carriers transmitting in parallel. All these carriers transmit in unison using synchronized time and frequency, forming a single block of spectrum. This is to ensure that the orthogonal nature of the structure is maintained. Since these multiple carriers form a single OFDM transmission, they are commonly referred to as ‘subcarriers’, with the term of ‘carrier’ reserved for describing the RF carrier mixing the signal from base band. There are several ways of looking at what make the subcarriers in an OFDM signal orthogonal and why this prevents interference between them.

Precoding:

Precoding is multi-stream beamforming, in the narrowest definition. In more general terms, it is considered to be all spatial processing that occurs at the transmitter. In (single-layer) beamforming, the same signal is emitted from each of the transmit antennas with appropriate phase (and sometimes gain) weighting such that the signal power is maximized at the receiver input. The benefits of beamforming are to increase the received signal gain, by making signals emitted from different antennas add up constructively, and to reduce the multipath fading effect. In the absence of scattering, beamforming results in a well defined directional pattern, but in typical cellular conventional beams are not a good analogy. When the receiver has multiple antennas, the transmit beamforming cannot simultaneously maximize the signal level at all of the receive antennas, and precoding with multiple streams is used. Note that precoding requires knowledge of channel state information (CSI) at the transmitter.

Bit Error Rate (BER):

The BER, or quality of the digital link, is calculated from the number of bits received in error divided by the number of bits transmitted.

BER= (Bits in Error) / (Total bits received):

In digital transmission, the number of bit errors is the number of received bits of a data stream over a communication channel that has been altered due to noise, interference, distortion or bit synchronization errors.

The BER is the number of bit errors divided by the total number of transferred bits during a particular time interval. BER is a unit less performance measure, often expressed as a percentage.

IEEE 802.11 standard has ability to sense the bit error rate (BER) of its link and implemented modulation to data rate and exchange to Forward Error Correction (FEC), which is used to set the BER as low error rate for data applications. BER measurement is the number of bit error or destroys within a second during transmitting from source to destination.

Noise affects the BER performance. Quantization errors also reduce BER performance, through incorrect or ambiguous reconstruction of the digital waveform. The accuracy of the analog modulation process and the effects of the filtering on

signal and noise bandwidth also effect quantization errors.

It is important to evaluate the performance of wireless devices by considering the transmission characteristics, wireless channel parameters and device structure. The performance of data transmission over wireless channels is well captured by observing their BER, which is a function of SNR at the receiver.

In wireless channels, several models have been proposed and investigated to calculate SNR. All the models are a function of the distance between the sender and the receiver, the path loss exponent and the channel gain. Several probability distributed functions are available to model a time-variant parameter i.e. channel gain.

OFDM / RSA / SHIFT ENCRYPTION AND

DECRYPTION

OFDM Transmitter:

Step1 Initialise all network integrated parameter. Encryption

Step 2 Generate the random data and encrypt the data

Step 3: Spread the data with Walsh Hadamard code

Step 4: Convert this serial code stream into parallel

format

Step 5 Select the orthogonal carriers

Step 6: Convert the binary data to DQPSK & add

phase reference

Step 7 Find the required spectrum for this DQPSK

transmission

Step 8: Place the selected carriers into this spectrum

Step 9 Find the symbol time waveform using IFFT

Step10 Add a Guard Period to each symbol time

waveform

Step11 Transmit the signal as frames

Channel:

Step12 Channel is modelled to have Gaussian noise

OFDM Receiver:

Step13 Receive the signal frames

Step14 Find the spectrum of the symbols

Step15 Extract the used carriers from the symbol

spectrum.

Step16 Decode the DQPSK signal to binary

Step17 De-spread the data using the same Walsh

Hadamard code

Decryption:

Step 18 Receive data and decrypt the data

Step 19 Find BER from the received data

Step 20 Plot the results

A. Text encryption/decryption with IFFT/FFT OFDM. The below text files (Input (Transmitter), Output (Receiver) and the Cipher (Wireless Medium)) represent the results of RSA/SHIFT Secured IFFT/FFT Based OFDM Wireless System with the OFDM model and with the RSA/SHIFT Secured method. The Output file was observed at SNR of 10 dB with two characters wrong but for SNR of 12 dB, we got the Output file without a single error.

The Problem:

Since last decade, there has been increasingly demand of wireless communication network. There is a very famous saying "Necessity is the mother of all inventions", but I will rather modify it by saying "Problem is the mother of all inventions". Always new technology raises new problems but best way to find out the solution is to use them without their issue. There are so many WLAN techniques available today but there is still some technical problem remains unsolved such as security, increase in mobility, flexibility, short range, demand of high data rates and data transmission from one destination to another destination.

Mostly IEEE 802.11 standard WLANs used radio waves at the frequency band of 2.4 GHz also known as industrial scientific and medical (ISM) band, sometimes it create interference problem with other communication equipment where two signals are close to each other at the same time and same frequency because one signal may be louder and overcome the other signal, such as Bluetooth and Microwave Ovens. Furthermore there are several problems arises with the different kind of WLANs. There are most common issues regarding Wireless LANs.

Wireless standards are varying more quickly rather than Wired LAN because it require to upgrading for provide higher performace to customers of WLAN, it means also have to replace the wireless equipment such as Wireless network interface cards (NICs) and access points (APs).

The devices are operated at a limited distances from APs. To increase the signal requires more APs which also increases the overall cost. The data rate will be dropped if the user moves further away from APs. The growing of wireless network also growing the risk of security attacks.

The Suggestion:

In today’s world, The WLAN standard is being accepted rapidly in every environment particularly in business and educational institutions. The demand of bandwidth increases because of wire-free technology. Customer satisfaction is fundamental rule for any business. WLAN customers have a right to choose flexible, interoperable solutions from multiple seller and low prices high-speed throughout small enterprise, large enterprise and home market. Following are the basic services which are very important to provide WLANs customers:

· Performance

· Connectivity

· Mobility

· High data rates

There are many more issues in WLAN has to be resolved. Some of them are hidden node, multipath, fading and WLAN configuration.

PROPOSED PLAN OF RESEARCH WORK:

This work presents the synthesis of OFDM for the purpose of security using RSA/SHIFT Cryptography implementation. It is really a good one and it is very difficult for the intruder or interceptor to get the multimedia information. The results are extremely a convincing and coincident with practical existing multimedia wireless applications.

CONCLUSION/ RESULTS/ EXPECTED OUTCOMES OF RESEARCH WORK:

In this paper, the MATLAB simulation and the results of RSA/SHIFT CIPHERED IFFT/FFT Based OFDM multimedia wireless network in an AWGN are presented. IFFT/FFT based OFDM digital communication system has been studied and the results are good for practical implementation and this also provides additional security to the information over wireless medium.

RSA/SHIFT Cryptography implementation is really a good one and it is very difficult for the intruder or interceptor to get the multimedia information.

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Received on 24.02.2013 Accepted on 15.03.2013

Research J. Science and Tech 5(3): July- Sept., 2013 page 323-326