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https://doi.org/10.22214/ijraset.2020.31551

September 2020

Transmit Diversity based OFDM-IM and Adaptive Selection of Subcarriers Fayis A1, Dr. D. Jeyakumari2, R. Kannan3 1

(PG Student, Dept. of ECE, RVS College of Engineering & Technology Coimbatore, India) 2 (Professor, Dept. of ECE, RVS College of Engineering & Technology Coimbatore, India) 3 (Assistant Professor, Dept. of ECE, RVS College of Engineering & Technology Coimbatore, India) Abstract: The index modulation is a novel technique. This index modulation concept firstly introduced in MIMO system. So it is possible to adopt this concept in case of OFDM. Orthogonal frequency division multiplexing (OFDM) with index modulation (OFDM-IM) is a recently proposed multicarrier transmission technique. In which the indices of the active subcarriers are used to carry additional information bits. In order to achieve a diversity gain for index detection a simple transmit diversity scheme is used with OFDM-IM. Convolution encoder is used in the OFDM-IM with transmit diversity(OFDM-IM-TD) to achieve better BER performance under Rayleigh fading channel. But OFDM-IM with transmit diversityc has poor spectral efficiency. Because only 50% of active subcarrier is used for the transmission of information in the existing work. So here propose adaptive selection of active subcarrier. We can choose 75% active subcarrier, 50% active subcarrier and 25% active subcarrier out of total subcarrier adaptively. In this adaptive selection we can select 25% active subcarrier, when channel in high fading condition. When low fading condition, can choose 75% of active subcarrier. Phase Shift Keying is the most simple modulation method and this technique is more immune to the noise. We can improve BER performance by using PSK. Keywords: OFDM-IM, OFDM-IM, transmit diversity, MIMO, Rayleigh fading channel I. INTRODUCTION MIMO is an antenna technology consist of number of antennas at receiver and transmitter sides and makes variety of signal path to carry the data. A new technique index modulation has been introduced for multiple input multiple output (Başar,2015a). In MIMO index modulation technique is used to transmitthe data by indices of the active transmit antenna (Mesleh et al., 2008). This type of modulation scheme is called spatial modulation (SM) and only one antenna is active during the transmission in order to avoid inter channel interference. In the Spatial modulation, which make use of spatial domain to convey information to the classical signal constellation. SM technique has been extended with channel coding and various modification (Choi,2014)(Jeganathan,2008). In (Başar et al.,2013) (Abu-Alhiga and Haas,2009) orthogonal frequency division multiplexing is also adopted the notion of index modulation. Some of total subcarriers are used for the transmission. This subcarriers are called active subcarrier and others are called inactive subcarriers. The inactive subcarriers does not carry any information. But indices of active subcarrier are used to convey the additional information bits. But OFDM-IM has poor performance Rayleigh fading channel. So in order to improve performance a simple transmit diversity scheme is introduced in the OFDM-IM block diagram. In OFDM-IM with transmit diversity, the block of subcarriers are equally divided into sub blocks. The each sub block again divided into clusters. Coordinated interleaving OFDM-IM is a another method to increase the diversity gain. Coordinated interleaving OFDM-IM [CI-OFDM-IM] has better performance than OFDM-IM. In coordinated interleaving OFDM-IM (Başar,2015b) the real and imaginary parts of complex signal are carried by different subcarrier. In (Choi,2017) OFDM-IM with transmit diversity can achieve better performance than CI-OFDM-IM and conventional OFDM-IM in terms of index error rate and bit error rate for large number of cluster value.It is important that channel coding plays an important role in the frequency selective fading channel to improving the performance of OFDM-IM systems. Achieve path diversity gain is a better in case of fading channel, So we can achieve using interleaving and channel coding in coded OFDM and a good performance can be obtained. However, it is clearly shown in (Choi,2017) that the good result can not obtain due to the direct use of channel coding to OFDM-IM unless a transmit diversity scheme is used. The ML approach is used in the receiver side for index detection. If only 50 percentage of subcarrier (active subcarrier) of active subcarriers are used for the entire transmission, the spectral effiency of OFDM-IM-TD is very poor. In this scheme we can adaptively choose 75% and 25% of active subcarrier selection depending on the channel condition.At the time of low fading we can select 75 percentage of active subcarrier and in case of high fading 25 percentage of active subcarrier can be selected to reduce bit error rate. The modulation technique used in the OFDM-IM-TD is QAM. QAM is an amplitude modulation scheme. In case of amplitude modulation, which is more susceptible to the noise. For better performance BPSK modulation technique is used. In BPSK phases of the transmitted signal varied to convey information bits.

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International Journal for Research in Applied Science & Engineering Technology (IJRASET) ISSN: 2321-9653; IC Value: 45.98; SJ Impact Factor: 7.429 Volume 8 Issue IX Sep 2020- Available at www.ijraset.com The main objectives of proposed work are as follows: 1) To improve BER performance of OFDM-IM with transmit diversity using BPSK mod ulation scheme. 2) To adaptively select the active subcarrier to reduce information loss. II. PROPOSED SYSTEM In existing work the modulation scheme used is quadrature amplitude modulation. The QAM uses both phase and amplitude component and it also provide high spectral efficiency. QAM is a signal ,it uses two carriers are 90 degree phase shift each other. Because of this it called qaudrture. So in QAM consist of two signal one signal is I signal and other is Q signal. I and Q signal also called In-phase and quadrature respectively. The resultant overall signal consisting of the combination of both I and Q carriers contains of both amplitude and phase variations. It is able to provide a highly effective form of modulation for data and as such it is used in everything from cellular phones to Wi-Fi and high speed data communications system. The constellation diagram consist constellation points. Each points set with equal vertical and horizontal spacing on square grid. In QAM number of constellation points is equal to power of 2. The common form of QAM are 2 4 8 16..etc.Fig.6.1shows the 4-QAM constellation.

Fig. 6.1 4-QAM constellation(Hanzo et al.,2004) QAM has many diadvantages,that are providing below 1) The states of QAM is more close together, it is susceptible to the noises. The noise must be lower level in oder to move the signal to different decision point. 2) The amplitude component of QAM must be maintained in linearly. In order to occur this linear amplifiers are needed. The main disadvantages of linear amplifier is less efficient and consume more power. Due to this fact it is less suitable for mobile communication. 3) Requires coherent Demodulation with exact phase and frequency. We can use phase shift keying modulation scheme, in order to get better result than QAM 4) Phase-shift keying (PSK) is a method of digital communication in which the phase of a transmitted signal is varied to convey information. 5) Binary phase shift keying (BPSK) is the simplest form of phase shift keying. It also known as 2PSK or phase reversal keying. 6) It uses two phases and these phases are separated by 180 degree. 7) BPSK is more robust when compare with other PSK types. Since to make the demodulator reach an incorrect decision, it use high level of noise and distorsion. 8) BPSK means 1 bit per symbol, data rate is poor, so does not use for high data rate application. 9) Less bandwidth 10) Minimum error rate 11) Good noise immunity

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International Journal for Research in Applied Science & Engineering Technology (IJRASET) ISSN: 2321-9653; IC Value: 45.98; SJ Impact Factor: 7.429 Volume 8 Issue IX Sep 2020- Available at www.ijraset.com A. Adaptive Selection of Active Subcarrier In existing work only 50% of active subcarriers are used from the cluster for the transmission of information bits. Because of 50% inactive spectral efficiency is poor.In this proposed work not only the 50% of subcarrier but also use 75% and 25% of subcarrier from the cluster. There is some condition to select one of these, it is based on the fading of channel. Channel may be high fading or low fading condition . If high fading condition we can select 25% subcarrier to carry information bits. We can select 75% subcarrier from the cluster with the low fading . The Table6.1 shows the relative increment of number of bits when 75% subcarriers are used instead of 50% subcarriers. Table 6.1 Relative percetage of increment in data rate when 75% subcarrier are used Subcarriers 1024 2048 4096 8192

Number of bits(50%) 1072 2152 4328 8672

Number of bits(75%) 1568 3152 6328 12672

Relative percentage of increment 46.268 46.4684 46.2107 46.1285

The Table6.2shows the relative increment of number of bits when 25% subcarriers are used instead of 50% subcarriers. Table 6.2 Relative percetage of decrement in data rate when 25% subcarrier are used Subcarriers Number of bits(50%) Number of bits(75%) Relative percentage of decrement 1024 1072 544 46.85 2048 2152 1104 48.69 4096 4328 2232 48.42 8192 8672 4480 48.33 III. RESULTS MATLAB tool used for this simulation in this work. The toatl system ia implemented and the performance analysis is carried out in MATLAB. The results obtained from the simulation is discussed in this chapter. In this system frequency selective fading channel is implemented. Here number of bits per subblock with respect to increasing the number of cluster G when L=128 and M=4 is plotted. The different techniques such as OFDM-IM with transmit diversity and OFDM-IM is used for the evaluation. In both techniques, number of active subcarrier Q per cluster take as 2 and N/2. So number of bits per subblock with respect to different the number of cluster value shown in Fig.7.1. From this graph, we can clearly shown that number of bits per subblock increases with increasing the number of cluster value when active subcarrier is fixed. Here takes number of active subcarrier per cluster 2 as a fixed value.

Fig. 7.1 The number of bits per subblock for different values of G From Eq.5.5and Eq.5.9, we can clearly shown that number of bits per subblock less in case of OFDM-IM-TD when compare to OFDM-IM. Hence OFDM-IM-TD can provide better performance than OFDM-IM and CI-OFDM-IM. Active index detection can also enhanced for large cluster value G.

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A. Performance Comparison Between OFDM-IM-TD and CI-OFDM-IM Using QAM Here presents the simulation results of comparison between the OFDM-IM-TD and CI- OFDM-IM. SNR vs bit error rate(BER) index error rate(IER) curve is plotted. The parameters, number of subcarrier per cluster N takes as 4, active subcarrier Q is 2 and Mary signal constellation is 4. The Fig.7.2shows the comparison of these two technique for various SNR values with cluster value G=1. The OFDM-IM-TD perform better than CI-OFDM-IM in terms of IER.

Fig. 7.2 IER and BER in terms of SNR when G=1 The Fig.7.3shows the comparison of these two technique for various SNR values with cluster value G=2. In this case obtains high diversity gain for IER. So we can conclude that when cluter value increases ,can achieve high diversity gain for IER.

Fig. 7.3 IER and BER in terms of SNR when G=2

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International Journal for Research in Applied Science & Engineering Technology (IJRASET) ISSN: 2321-9653; IC Value: 45.98; SJ Impact Factor: 7.429 Volume 8 Issue IX Sep 2020- Available at www.ijraset.com B. Simulation Results FOR Coded Signal In order to get better performance channel coding is used for transmitting information.The convolution encoder is used for this purpose. The Fig7.5shows the simulation results in terms of IER and coded BER with index error and coded BER without index error when cluster value G=4. Here takes various SNR value with active subacarrier Q=8 B=2048 and M=4. Here IER is high.

Fig. 7.5 IER and coded BER of OFDM-IM-TD when G=4

Fig. 7.6 IER and coded BER of OFDM-IM-TD when G=8

The The Fig7.6shows the simulation results in terms of IER and coded BER with index error and coded BER without index error with cluster value G=8, Q=8, B=2048 and M=4. From these two graph, we can conclude that,when G=8 the IER is low so obtains better results in both cases.

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International Journal for Research in Applied Science & Engineering Technology (IJRASET) ISSN: 2321-9653; IC Value: 45.98; SJ Impact Factor: 7.429 Volume 8 Issue IX Sep 2020- Available at www.ijraset.com C. Adaptive Selection of Number of Active Subcarrier For OFDM-IM-TD The Fig.7.10shows the IER and BER of OFDM-IM-TD for different active subcarrier selection. Here takes 25%, 50% and 75% active subcarrier. From this graph, obtain better result for in case of 25%. But compare with others has low data rate. So we can select this for channel with high fading. Data rate is high in case of 75% subcarrier selection. Hence it is better to choose when channel with low fading. Through this way we can reduce information loss due to fading.

Fig. 7.10 Active subcarrier selection for OFDM-IM-TD IV. CONCLUSION OFDM is a area of high attention today. It is very useful in present day communication. The technique used in 4G is OFDM. Index modulation is an up and coming concept for spectrum and energy-efficient next generation wireless communications systems to be employed in 5G wireless networks. The OFDM-IM can provide better result in terms of BER than OFDM. The new system for wireless communication by using adaptive selection of subcarriers and also better modulation scheme is better idea for OFDM-IM with transmit diversity. The results of existing method is done and compare their results with proposed work. It is clear from the simulation result that the proposed work can provide better results than existing work. BER can enhanced through the prosed work because of PSK modulation. In order to reduce information loss adaptive selection of subcarriers also introduced through the proposed work by considering fading in channel. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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