International Journal of Applied Information Systems (IJAIS) – ISSN : 2249-0868 Foundation of Computer Science FCS, New York, USA Volume 3– No.4, July 2012 – www.ijais.org 6 Performance of Linear Block Coded OFDM system in BER and PAPR under different channels V.B.MALODE Department of E&TC J.N.E.C.,N-6 Cidco Aurangabad, Maharashtra,India B.P.PATIL Department of E&TC Army Institute of Technology Dighi, Pune,Maharashtra, India ABSTRACT Orthogonal Frequency Division Multiplexing (OFDM) plays a prominent role in wireless communication technology as multicarrier transmission scheme. The IEEE 802.11a standard uses coded OFDM scheme. This paper is to evaluate the performance of Coded OFDM system in different channels. Linear Block Code (LBC) with Extended Hamming is used to check the improvement of the system performance in BER (bit error rate). The results are obtained for different channels Like Additive White Gaussion Noise (AWGN), Rayleigh and Rician. The DQPSK modulation is used for scheme are comparison purpose. The Differential detection technique is used to avoid the need of a complex equalization process. PAPR for QPSK and DQPSK with conventional and SLM is computed for the comparison purpose. Keyword OFDM, Linear Block Code, BER, PAPR, SLM, AWGN , Rayleigh and Rician Channel. 1. INTRODUCTION OFDM is a very efficient technique for high-speed data transmission used in mobile communication, Digital Audio Broadcasting (DAB), Digital terrestrial mobile communication, Digital Video Broadcasting terrestrial (DVB-T), wireless asynchronous transfer mode (WATM), Modem/ADSL[1,2]. OFDM has many advantages such as High spectral efficiency, robustness in frequency selective fading channels, immunity to inter-symbol interference and capability of handling very strong multipath fading.[3] Recently the advance system such as 3GPP LTE and terrestrial digital TV Broadcasting have been sophisticatedly developed using OFDM technology. Besides the above advantages, however, the bit error occurs in the presence of Doppler frequency shift because of the inter-carrier interference (ICI) in OFDM. OFDM is having major drawback of a high Peak-to-Average Power ratio (PAPR) [4, 5].This high PAPR increases complexity in the Digital Analog converter as well as affects the efficiency of power amplifier. In [6] the sub-block coding scheme for reducing the PAPR of OFDM has low complexity and is found that more than 3dB reduction in PAPR can be achieved when the code rate is ¾. The Hadamard transform sequence to reduce the PAPR problem and it requires no side information to be transmitted to the receiver. Swngwoo et al [8] in their paper proposed new SLM method which rotates the phase of input data after IFFT by using Walsh Hadamard Code. This method has 0.5dB lower PAPR reduced than common SLM and also reduced a computational burden more largely. OFDM is easy to scheme [7] is to reduce the autocorrelation of the input implement due to efficient FFT (Fast Fourier Transform) algorithm. The implementation of FFT engine mainly depends on the number of input bit size. However the BER performance according to the input bit resolutions of FFT in AWGN channel. Orlandos Grigoriadis, H. Shrikanta Kamat have given the results of the BER as a function of Multi propagation for each set of carriers.[10] The effects of channel estimation error on the BER performance of BPSK and16-QAM modulated OFDM signal in multipath Rayleigh fading channels respectively has been shown. In [11] the high-order modulation like 16-QAM needs more accurate CFO and channel estimation to avoid performance loss. Whereas the Improvement of performance elevation criteria of RS coded OFDM transmission over noisy channel analyzed using simulink. Performance of a flat fading channel using Convolution Interleave that cuts down time delay and memory space increases considerably [12]. In this era the demand of mobile radio communication systems are increasing to provide a variety of high quality services, more features, compactness and other such variety of services to mobile users. Hence modern mobile radio transceiver systems must be able to provide high capacity and variable bit rate information with high bandwidth efficiency, efficient signal power as such[13,14]. Linear block coding is employed to improve the system performance in terms of BER. Here in this paper Modified SLM method using the standard arrays of linear block codes and DQPSK modulation technique. In earlier paper [9] the same method was used with QPSK modulation whereas we have used DQPSK modulation, our method has shown better performance in PAPR reduction. 2. FADING CHANNEL A signal in mobile communication does not necessarily reach the receiving antenna directly. It gets reflected, diffracted or scattered due to various natural or manmade obstacles in the signal path. Thus the signal undergoes a multipath propagation. This results in infinite number of replicas of the transmitted signal. At receiving antenna the signal reaches as superimposition of the signal which may be constructive or destructive. This affects the amplitude and other such parameters of the signal. Some of the frequencies even get erased due to the reflections, diffractions and other such effects called as selective frequency fading. The signal along with its replicas is received at receiving antenna in a scattered manner. A time spread occurs between the first and last of the multipath signal at the receiver called as delay spread. In condition when the transmitter and receiver are in motion relatively in a radio