Using Raised Cosine Filter to Reduce Inter Symbol Interference in OFDM with BPSK Technique Khalid Aslam 1,*, Bodiuzzaman Molla 2, Md. Jashim uddin 3,Prof. Wlodek Kulesza 4 1 MSc EE Manager / Kundekonsulent Syvstjerners Renhold Transport og Flyttebyrå, Norway. 2,3 Student of BTH, Sweden 4 Prof BTH, Sweden. khalid.aslam@yahoo.com, wlodek.kulesza@bth.se Abstract: This paper aims to investigate the reduction of Inter Symbol Interference (ISI) using Raised Cosine Filter (RCF) in Orthogonal Frequency Division Multiplexing (OFDM) channel with Binary Phase Shift Keying (BPSK) technique. In order to reduce ISI in wireless communication channel, there is a demand of a pulse shaping filter such as RCF which is one of the most familiar pulse-shaping filter to reshape the data symbols in order to reduce the effect of ISI errors. The implementation of RCF shows the decrease in Bit Error Rate (BER) versus Signal to Noise Ratio (SNR) for the transmitted data symbols. A comparison is made, graph is drawn between BER and SNR when RCF is implemented and when RCF is not implemented in OFDM channel. The decrease in BER clearly indicates the reduction in ISI when it is implemented with RCF. Keywords: BER, BPSK, ISI, OFDM, and RCF. Received: April 2011, Published: August 2011 *Corresponding Author: Khalid Aslam, khalid.aslam@yahoo.com 1. Introduction The paper shows how to reduce Inter Symbol Interference (ISI) of Binary Phase Shift Keying (BPSK) signal which passes through complex noise channel. Raised Cosine Filters (RCFs) are implemented in Orthogonal Frequency Division Multiplexing (OFDM) channel which is applied to symbols that leads to reduce ISI. The obtained result is plotting by the graph between BER and SNR. There are two important demands of wireless communication system; one is reduction in ISI and another one is band limitation of the channels. These demands can be fulfilled by pulse shaping filter applied to each symbol [1].The OFDM system is attract for future suitable modulation scheme for commercial high-speed broadband wireless communication systems because of its resistance to multipath fading. OFDM can provide large data rates however ISI cannot be avoided in this system [3].The paper describe the main problems occur in OFDM system during wireless transmission and we explain the basic idea and benefit of the 115
Khalid et al: Using Raised Cosine Filter to Reduce Inter Symbol Interference in OFDM with BPSK Technique proposed scheme. We proposed a solution using Raised Cosine Filter to reduce ISI and at receiver we found good result when we compare the data symbols transmitted without using RCFs. In order to implement RCFs, one RCF is placed at transmitter while other RCF is placed in receiver [5]. 2. Related work Cyclic Prefix length can be adaptively controlled by adjusting sampling rate and ISI can be reduced [2] and bit errors caused by ISI cannot be recovered by increasing signal power. The multipath distortion creates ISI interference which can be overcome by cutting the starting and ending parts of a symbol period by pulse shaping filter such as RCF [1]. Data transmission over band limited channel requires pulse shaping to eliminate or control ISI-free transmission [4].ISI can be prevented by using RCF, performance is measured in terms of operating frequency and bandwidth [5]. 3. Problem Statement and Main Contribution The research question is how we can reduce the ISI from the OFDM system by implementing RCF and using BPSK as a modulation scheme. The main contributions of this paper are: Implementation of RCF which can be used to reduce ISI errors in OFDM system with BPSK Techniques. Simulation of transmission channel using RCF which will reduce BER. Validating the BER versus SNR in OFDM system by MATLAB. 4. Problem Solution 4.1 Implementation For ISI transmission in OFDM, we used the following model. Figure 1: Block Diagram of Transmission and Receiver Channel in OFDM 116
Random data are generated and modulated. After modulation data is converted from serial to parallel and IFFT is applied over it. IFFT coverts frequency domain data to time domain and IFFT generates samples of a waveform with frequency components orthogonal to each other and parallel to serial conversion takes place and digital to analog converter is applied. RCF is applied which reshapes the incoming symbols and cuts the starting and ending points of symbol period. This point is tending to creating ISI from multipath distortion. Now data is transmitted [5]. Noise is introduced in the channel blocks and transmitted through OFDM channel. At the receiver again RCF is applied which reduces noise and reshapes the symbols as similar to original symbols. The receiver performs serial to parallel conversion, converting the OFDM data, being transmitted in form of serial stream, to parallel data. Then FFT converts the time domain data again to a frequency domain and then passes it to parallel to serial converter which converts again parallel data into a serial to recover BPSK modulated data, then it is demodulated to recover the original signal. Now errors are calculated in the received data by comparison with transmitted data by using method of maximum likelihood. The input data symbols with ISI at transmitter can be described below. (1) Output signal can be written as (2) Basic criteria for Nyquist s method for zero ISI can be eliminated by using an equivalent transfer function He(f), such that the impulse response satisfies the condition: (3) Where k is the integer, is the sample symbol (sample) period, is the offset in the receiver sampling clock times and is a nonzero constant. Since such impulse response is not possible due to infinite time duration of sinc function and sharp transmission band in frequency band thus ultimate choice RCF which is defined as Where is the 6-dB bandwidth of the filter, roll of factor and bandwidth 117
Khalid et al: Using Raised Cosine Filter to Reduce Inter Symbol Interference in OFDM with BPSK Technique Figure 2: Bit Error Rate Vs Signal-Noise-Ratio in Db 4.2 Results Analysis We use two scenarios one is implemented with RCF and other is without RCF and result is shown in the Fig.2. It is observed that by applying RCF to OFDM modulated signal sharp transitions are smooth which results in a signal with specific band and as a result ISI between OFDM symbols is minimized. This improves the performance of the system. The simulation is done using BPSK technique and the BER for different values of SNR is noted. It is found that by using RCF, BER reduces significantly as compared 118
to without implementation of RCF. The parameters used in simulation are given in Table 1.The MATLAB tools are used to perform simulation. This paper shows that using RCF ISI can be reduced. 5. Conclusion In this paper, we found that ISI is reducing in OFDM system. When we used the RCFs with BPSK technique. Most of the OFDM system, the guard interval, e.g. cyclic prefix, powerful coding are used to reduce ISI which requires complexities but using our scheme only two RCFs are used. So, it is concluded that by applying our scheme in OFDM system BER can be reduced by 4.7 % and ISI is decreased. We consider the BER in OFDM system to reduce ISI the work can be extend under other parameters like ICI. This scheme can be implemented through other modulation schemes for different data rates instead of different higher data rates. 6. Acknowledgement We would like to special thanks to Prof. Wlodek Kulesza, for his great ideas and support. References 1. C. Ong, Use pulse-shape filtering for smooth transitions Electronic Engineering, Times-Asia, Dec. 2006. 2. Chiwoo Lim, Youngbin Chang, Jaeweon Cho, Panyuh Joo, and Hyeonwoo Lee Novel OFDM Transmission Scheme to Overcome ISI Caused by Multipath Delay Longer than Cyclic Prefix Samsung Electronics Co., Ltd, IEEE 0-7803-8887-2005. 3. R.kumar, and Dr.S.Malarvizhi, Member, IEEE Reducation of Inter carrier Interference in OFDM Dept. of Electronics and Comm. Engg., SRM University, Chennai, India- 603203. 4. N. S. Alagha, and Peter Kabal, Member, IEEE Generalized Raised-Cosine Filters IEEE Tansaction on Communications, vol. 47, pp. 989-999, 1999. 5. Tutorial 14 Inter Symbol Interference (ISI) and Raised cosine filtering (Electronic) Available: http://www.complextoreal.com/chapters/isi.pdf (2008-01-29). 119