Performance Analysis of Circular 8-QAM Constellation with MMSE Equalizer for OFDM System Using USRP

EMITTER International Journal of Engineering Technology Vol.4, No.2, December 2016 ISSN:2443-1168 Performance Analysis of Circular 8-QAM Constellation with MMSE Equalizer for OFDM System Using USRP Muh. Alfan Taufiqurrahman, I Gede Puja Astawa, Amang Sudarsono Graduate School of Engineering Technology Department of Electrical Engineering Politeknik Elektronika Negeri Surabaya PENS Campus, Jalan Raya ITS, Sukolilo 60111, Indonesia E-mail: , {puja, amang}@pens.ac.id Abstract Bandwidth is very important in communication system, and it is a limited resource. In order to save the limited bandwidth resource, high order M-ary modulation is widely employed in modern communication and broadcasting systems. In Orthogonal Frequency Division Multiplexing (OFDM), fading environment lead to a loss of orthogonality between the subcarriers. In this paper, we present the performance analysis of circular 8-Quadrature Ampilutude Modulation (QAM) constellation for Orthogonal Frequency Division Multiplexing (OFDM) system. We also combine the system with Minimum Mean Square Error (MMSE) equalizer to mitigate the effect of Inter Symbol Interference (ISI). Then, all of this system is implemented practically using Universal Software Radio Peripheral (USRP). The performance of circular 8-QAM and MMSE equalizer is evaluated by comparing with other 8-QAM modulation models such as circular-Zero Forcing (ZF), star-ZF, Square-ZF, Star-MMSE, and Square-MMSE. The performance of circular 8-QAM with MMSE equalizer is better than the other combinations. Bit Error Rate (BER) graph shows that the performance of circular-MMSE is better than star-MMSE. The performance improvement using circular-MMSE is about 1.6%. The performance will decrease when the distance is increased. The performance of this system is greatly affected by the distance between transmitter and receiver. Keywords: OFDM, Circular-8QAM, USRP, MMSE Equalizer 1. INTRODUCTION Currently, the development of wireless communication technology is very fast, many users need higher data rate communication, so that it needs large bandwidth to support the development of wireless communication. Bandwidth is determined by the symbol rate rather than bit rate [1]. Bandwidth is very important thing in wireless communication system, and it is very limited, so that it must be used efficiently. For saving the limited bandwidth resource, high order M-ary modulation is widely employed in Copyright © 2016 EMITTER International Journal of Engineering Technology - Published by EEPIS 259 260 Volume 4, No. 2, December 2016 modern communication and broadcasting systems. In M-ary modulation, several bits can be represented in one symbol, by this way it can increase the data rate communication. However, the higher index of modulation is very sensitive to noise effect, not only White Gaussian Noise, but also Phase Noise which is caused by many factors in transmitter and receiver side. To combat this problem, many researchers research about the best constellation diagram to mitigate noise variation. Nowadays, square constellation diagram, is widely used in wireless communication system. The square type is used because the square type’s symbol distances are same for all constellation points. Several types of M-ary modulation are widely assigned to different subcarriers based to the bits. Among the candidates, 8-QAM is the one that presents many possible signal constellations with different performance when distinct symbol distances and amplitudes are considered. Therefore, it needs an investigation and research to determine the best type or model of constellation diagram for 8QAM. There are several types of constellation model for QAM, such as circular and star. The circular constellation has been widely used in telecommunication, especially in broadcasting system. The circular constellation is used because circular QAM has smaller amplitude fluctuation compared with rectangular or square QAM. [2-3]. Nadal et al have conducted research about several types of constellation diagram for 8-QAM such as star, circular, rectangular and square. The paper compare the performance of four types of constellation diagram for 8-QAM. The circular constellation has a better performance than the square constellation. However, the potential of circular constellation to combat phase noise effect is overlooked. Based on that fact, the higher index of circular constellation can be effectively used to improve bandwidth efficiency in high speed communication. These have not been applied widely because higher index of modulation is very sensitive to disturbance of noise. In wireless communication, the Gaussian noise, Doppler effects, multipath fading cause bad affect in transmission. Therefore, today OFDM is widely applied on wireless communication systems, because OFDM can combat these problem [4]. The performance of OFDM depends on modulation technique which used on the system. One of problem that degrades the performance of modulation is phase noise. Phase noise can turn the distribution symbols in constellation diagram to be not accurate, so that it makes the demodulation process cannot work very well. Another solution for this bad effect is by applying the equalizer technique. The equalizer technique in OFDM is a digital filter that provides approximate inverse of channel frequency response. The purpose of this equalizer is to mitigate the effect of ISI (Inter symbol Interference) and to decrease the probability of error that occurs without suppression of ISI. There are several equalizer techniques which is commonly used in OFDM system, such as ZF and MMSE. The potential of these equalizers have been EMITTER International Journal of Engineering Technology, ISSN: 2443-1168 Volume 4, No. 2, December 2016 261 widely research. The previous research about different equalization technique in OFDM system is shown that Bit Error Rate (BER) can be improved by using equalizer [5]. Furthermore, the MMSE outperforms than ZF equalizer because MMSE not only mitigate the effect of ISI, but also minimize the noise power. In this paper, we propose the combination of circular 8-QAM with MMSE equalizer technique in OFDM system. The performance of this combination is evaluated by bit error rate percentage. The performance analysis has been done in real time communication between transmitter and receiver by using Software Defined Radio (SDR) and USRP as front end. 2. RELATED WORKS Several researches about OFDM system have been taken. An OFDM system which is using Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK) modulation and implemented in USRP by Pollet et al [4]. The result of that research is a good performance achieved by using QPSK modulation rather than using BPSK modulation. An experimental research of several 8-QAM constellation on OFDM system are investigated by Nadal et al and Li et al [2,3]. The research investigate 8-QAM constellation model such as square, ci (...truncated)