Improvement of bit error rate

Published:

Improvement of Bit Error Rate using Various Modulation Schemes

Abstract-We have proposed a model for reduction of Bit Error Rate (BER) in high data rate wireless communication systems using Orthogonal Frequency Division and Multiplexing (OFDM) on Quadrature Amplitude Modulation (QAM), Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK). In this model we are using techniques of Fast Fourier Transform (FFT), Inverse Fast Fourier Transform (IFFT), Serial to Parallel conversion and Parallel to Serial conversion in a way so that BER could be reduced. We have performed simulations in MATLAB in a Rayleigh fading channel environment to increase the BER so that it could be reduced after application of our model.

Index Terms-BER, BPSK, QAM, QPSK, OFDM.

I. INTRODUCTION

In today's world to achieve high data rates in wireless communication is main goal of all telecommunication organizations. There are many obstacles in achieving this goal, mainly Bit Error Rate (BER). This obstacles reduces quality of the signal received at receiver, after passing through the channel, due to noise, Rayleigh fading etc, hence providing poor data rates. So BER must be reduced in order to achieve the challenging goal of high quality output signal at receiver, thus increasing the data rates. For this purpose we have used Orthogonal Frequency Division and Multiplexing (OFDM) to be applied on Quadrature Amplitude Modulation (QAM), Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK), which will result in reduction of BER. To produce BER in the transmitted signal we have considered a Rayleigh fading channel. When signal passes through a Rayleigh fading channel multiple copies of the signal are received at the receiver with time delay due to reflection, refraction, diffraction and scattering. As a result, when these multiple copies are added to form a final shape of the received signal, bits in these copies overlap hence resulting in incorrect bits received at receiver producing BER. Using OFDM modulated signal will be transmitted in form of separate symbols with separate subcarriers orthogonal to each other hence reducing bit interference resulting in reduction of BER at receiver.

II. Review of the State of Art

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Time Domain Equalizers (TEQs) are used to reduce the additive noise thus reducing Inter Symbol Interference (ISI) power which in turns reduces bit error rate [1].Adjusting the velocities of the nodes to minimum threshold velocities reduces the bit error rate in ad hoc networks [2]. Incremental Redundancy Cooperative coding (IRCC) used in Nakagami channel fading reduces bit error rate more efficiently than the normal IRCC-1 and IRCC-2 channel fading schemes [3].

III. Problem Statement and Main Contribution

How can we reduce BER in high data rate wireless communications using various modulation schemes BPSK, QAM and QPSK?

We will reduce BER by application of OFDM on the mentioned modulation schemes. We shall divide the signal to be transmitted into symbols and we shall provide separate carriers to these symbols in such a way that they do not interfere with each other. This reduces the bit interference that shall result in reduction of bit error rate. At the receiver, the symbols are re- joined to get the original form of the signal.

We shall perform the required simulations for all the above mechanisms using MATLAB. Analyzing the bit error rate on using QAM, BPSK and QPSK modulation schemes

IV. Problem Solution

In this paper we have proposed a model to reduce the BER in QAM, QPSK and BPSK using OFDM, shown in Fig. 1.

A. Block Diagrame:

Model in Fig. 1 shows the complete application of OFDM on the modulations schemes we have taken into account (QAM, QPSK & BPSK) for BER reduction. Explanation of this model is as follows:

First we provide a time domain input signal to the modulation scheme used (QAM, QPSK & BPSK), on which, after modulation, Fast Fourier Transform (FFT) is applied for transformation of the input signal to frequency domain. Then serial stream of the signal is converted into parallel sub streams, before performing its Inverse Fast Fourier Transform (IFFT). Each parallel sub stream consists of no of bits (or bit data) to be transmitted. Now these parallel sub streams, after their IFFT, are provided with separate subcarriers, which are orthogonal to each other, so that these streams do not interfere with each other while passing through the channel. Then parallel to serial block converts this parallel set of data back to serial and then this data is transmitted over the channel.

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Channel that we have considered here is wireless which consists of parameters; Rayleigh fading and noise. These two parameters are selected because they produce a high BER in the signal while it passes through the channel. But the model we have used here will reduce the BER as all the data sub streams have orthogonal subcarriers, so their chances of interference are very less hence reducing the chances of receiving incorrect bits at the receiver.

At receiver data received is converted back to parallel through serial to parallel conversion and its FFT is performed to convert it back to frequency domain then all the sub carriers are removed. This data is then converted back to serial stream trough parallel to serial conversion and its IFFT is performed to convert it to time domain so that it can be demodulated. Then demodulation provides the original signal being transmitted with a low amount of BER.

B. Simulations:

We have performed simulation work in MATLAB. Parameters selected in channel for producing BER are Rayleigh fading and noise. Yet we have performed simulations for OFDM application on QAM for BER reduction.

Simulation result in Fig. 2 shows the original signal to be transmitted, QAM simple signal received at receiver and QAM signal with OFDM applied on it at receiver. We see that the signal with OFDM application is almost same as the original signal. BER in the simple QAM signal is 16.5% and in the in OFDM allied signal is 1 %. This shows that OFDM when applied to QAM reduces BER affectively.

Simulation result in Fig. 3 shows the original signal to be transmitted, QPSK simple signal received at receiver and QPSK signal with OFDM applied on it at receiver. We see that the signal with OFDM application is almost same as the original signal. BER in the simple QPSK signal is 19.8% and in the in OFDM allied signal is 3 %. This shows that OFDM when applied to QPSK reduces BER affectively.

References

[1] Fung, C.C.Chi-Wah, Kok ,"Bit error rate optimized time-domain equalizers for DMT systems,"14th IEEE Proceedings on Personal, Indoor and Mobile Radio Communications, 2003, Volume: 2, pp. 1049 - 1053.

[2] Farhadi, G.Beaulieu, N.C, "The Effects of Mobility on the Bit Error Rate Performance of Ad Hoc Wireless Network." IEEE Global Telecommunications Conference, 2006, pp.1-6.

[3] Feghhi, M.M.Abolhassani, B, "BER Reduction through Soft Information Relaying in Incremental Redundancy Cooperative Coded Schemes," Seventh Annual Communication Networks and Services Research Conference,2009, pp.350-355.