Throughput analysis of OFDM based wireless system with FrFT

Abstract

Fourier Transform (FT) is a well defined transform which has seen wide publicity and advantageous applications in the field of signal processing, optics and image processing. However, the real revolution in signal processing is given by its generalized form Fractional Fourier transform (FrFT) and has been utilized in this research work for wireless systems. The fractional variable given the freedom to transform the signal into time-frequency plane and hence FrFT is able to analyze the signal performance more precisely in fast time varying frequency selective channel. The main problem in the existing Orthogonal frequency division multiplexing (OFDM) systems is its inability to detect the change in frequency of signal with time under wireless channel. This non-detection of exact frequency of the signal at any instance of time in a wireless channel increases the bit error rate (BER) of the system. The aim of this research work is to use the FrFT in OFDM wireless system which is based on IEEE 802.11g protocol and capable of delivering 54 Mbps of bandwidth. Here, the throughput analysis of FrFT based OFDM system has been carried out in terms of BER, Signal to Noise Ratio (SNR), Inter Carrier Interference (ICI), Carrier Frequency Offset (CFO) and Peak-to-Average Power Ratio (PAPR) as the performance metrics. Hence, the contribution of this research work is develop an FrFT based OFDM system by using various modulation techniques for better performance and security of said system using the benefits of FrFT. The security of the signal transmitted on the wireless channel is not taken care in case of conventional OFDM system. Security of OFDM modulated signal in wireless domain has been enhanced as compared to existing OFDM system. In this proposed technique the FrFT is appended with the traditional OFDM system for encrypting the signal before transmitting. Now, instead of transmitting the plain OFDM signal in wireless domain; the OFDM modulated signal is encrypted with fractional variable using FrFT before transmission. At the receiver side the FrFT appended OFDM signal must be first of all decrypted with exactly the same value of fractional variable as was used for encrypting the signal at the transmitter. For all other values of the demodulator intercepts the signal as noise. Further, the results achieved from the FrFT appended OFDM system in terms of BER vs. SNR are much better as compared to the traditional OFDM system. The proposed system has been able to improve the BER as compared to the traditional OFDM system under all the wireless fading channels. Next, a FrFT based OFDM system has been proposed for higher order modulation techniques namely 1024-PSK and 1024-QAM, where, the IFFT/FFT block of existing OFDM system are replaced with an IFrFT / FrFT block at transmitter and receiver respectively. The said model has been able to give a improved performance in terms of BER as compared to existing FFT based OFDM system for higher order modulation techniques. In addition, the performance of this model has been simulated with various wireless fading channels and compared with existing FFT based OFDM system. The optimum value of fractional variable is obtained to get the best performance of the given model. 1024-PSK and 1024-QAM modulation techniques have been proposed for the FrFT based OFDM model; henceforth the proposed model is considered to be very useful in the higher order transmission of data in wireless channels. The recovery of source data from OFDM modulated signal at the receiver is mainly dependent on the orthogonality between the subcarriers used for modulating OFDM symbols. However, due to Doppler shift caused by mobility of source and destination; there is a change in the carrier frequency of subcarriers and generates CFO. This introduces Inter Carrier Interference (ICI) in OFDM and effects the channel estimation and symbol recovery at the receiver. Ultimately, it will increase in the BER of OFDM system. Another contribution of this thesis is to reduce the effect of CFO with the help of FrFT for further enhancing the performance of the existing OFDM system. The effect of CFO has been mitigated by proposing an FrFT based OFDM system with Differential Quadrature Phase-Shift Keying ( DQPSK) modulation technique. The BER of DQPSK modulated FrFT based OFDM system has been derived mathematically with CFO under the frequency selective Rayleigh channel. The BER calculated theoretically is compared with simulated results and found to be same. It has been established that the proposed model outperforms than FFT based OFDM for the optimized value of . Gaussian minimum shift keying (GMSK) has also been used as a modulation technique for evaluating the performance of FrFT based OFDM. The problem of ISI and low spectral efficiency of GMSK has been solved by proposing the FrFT based OFDM system using GMSK. The BER is calculated mathematically and also simulated for the proposed model under the presence of CFO. Results obtained has shown that proposed GMSK based FrFT based OFDM system outperforms the existing FFT based OFDM under all wireless frequency selective fading channels. Finally, the everlasting problem of PAPR in OFDM systems has also been reduced and is a contribution to literature. The PAPR analysis of the FrFT based OFDM system has been carried out for different type of modulation techniques like 1024-PSK, 1024-QAM, DQPSK and GMSK. It can be concluded from the results achieved that the improved OFDM system has been proposed without any additional burden of PAPR. The PAPR can further be improved by using Selective Mapping (SLM) clipping technique. Results achieved in this research work have shown that the FrFT based OFDM system has shown improvement in PAPR for all the modulation techniques discussed above as compared to existing OFDM system.