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1 To Reduce Bit Error Rate in Turbo Coded OFDM with using different Modulation Techniques Shivangi #1, Manoj Sindhwani *2 #1 Department of Electronics & Communication, Research Scholar, Lovely Professional University, Punjab, India #2 Department of Electronics & Communication, Assistant Professor, Lovely Professional University, Punjab, India Abstract OFDM(Orthogonal Frequency Division Multiplexing) has gathered increased interest due to its high spectrum efficiency and robustness against multipath interference, which makes the efficient use of spectrum by allowing overlap. OFDM is a popular modulation method for the transmission of high data rates over the wireless channels. There is a growing demand for the transmission of information quickly without the formation of errors. In this paper, the use of Turbo codes increases the reliability of OFDM system to achieve high data rates. In this project the system throughput of a working OFDM system has been enhanced by adding turbo coding and with using different modulation techniques. The use of turbo coding and power allocation in OFDM is useful to the desired performance at higher data rates. The purpose of this project is to increase the system throughput while maintaining system performance under a desired bit error rate (BER). To improve the performance of the uncoded OFDM signal by convolution coding. Simulation is to be done over additive white Gaussian noise (AWGN) channel. Keywords Bit error rate, Orthogonal frequency division multiplexing, Turbo codes, FEC, Quadrature Amplitude Modulation. I. INTRODUCTION Orthogonal Frequency Division Multiplexing (OFDM) is a Multi-Carrier Modulation technique in which a single high rate data-stream is divided into multiple low rate data-streams and is modulated using sub-carriers which are orthogonal to each other [1]. Some of the main advantages of OFDM are its multi-path delay spread tolerance and efficient spectral usage by allowing overlapping in the frequency domain. Also one other significant advantage is that the modulation and demodulation can be done using inverse fast fourier transmission (IFFT) and fast fourier transmission (FFT) operations, which are totally efficient. In a single OFDM transmission all the subcarriers are synchronized to each other, controlling the transmission to digital modulation schemes [1, 2]. OFDM is symbol based, and can be thought of as a large number of low bit rate carriers transmitting in parallel. These all carriers transmitted using same time and frequency, which forms a single block of spectrum. This is to established that the orthogonal nature of the structure is maintained [3, 4]. Since these multiple carriers form a single OFDM transmission, they are commonly referred to as subcarriers. There are several ways of looking at what make the subcarriers in an OFDM signal orthogonal and why this prevents interference between them. II. TURBO CODES The combination of turbo codes with the OFDM transmission is so called Turbo Coded OFDM (TC- OFDM) can yield significant improvements in terms of lower energy needed to transmit data, a very improvement matter is in personal communication devices [5, 6]. Turbo codes were first presented at the International Conference on Communications in Until then, it was generally believed that to achieve near Shannon s bound performance, one would ought to implement a decoder with infinite complexity. Parallel concatenated codes, as they are also known, can be implemented by using either convolutional codes (PCCC) or block codes (PCBC). PCCC resulted from the combination of three ideas that were known to all in the coding community: The transformation of commonly used nonsystematic convolutional codes into systematic convolutional codes. ISSN: Page 320

2 The utilization of soft input soft output decoding. Preferably of using hard decisions, the decoder uses the feasibilities of the received data to generate soft output which also contain information about the degree of certainty of the output bits. This is achieved by using an interleaver. Encoders and decoders working on permeated versions of the same information. The mixture of turbo codes with the OFDM transmission is so called Turbo Coded OFDM (TCOFDM) can yield significant improvements in terms of lower energy needed to transmit data, a very improvement issue is in personnel Fig 1: Structure of a Turbo Encoder communication devices. A. Turbo Encoding A Turbo encoded frame is processed by multiple convolutional encoders and interleavers, hence the principle of these sub-blocks are presented in the following, to support the understanding of a complete Turbo Coding system. The encoder is a parallel concatenated convolutional code. The binary input data sequence is represented by, the input sequence is passed into the input of a convolutional encoder [7], and a coded bit stream, is generated. The data sequence is then interleaved. So, the bits are loaded into a matrix and read out in a way so as to spread the positions of the input bits. The bits are usually read out in a pseudo random manner. The interleaved data sequence is proceed to a second convolutional encoder, and a second coded bit stream, is generated. and are Recursive Systematic Convolutional (RSC) codes that is, convolutional codes which use feedback (they are recursive ) and in which the uncoded data bits appear in the transmitted code bit sequence. The code sequence that is passed to the modulator for transmission is a multiplexed stream consisting of systematic code bits and parity bits from both the first encoder and the second encoder. B. Turbo Decoding A block diagram of a turbo decoder is shown in Figure below. The input to the turbo decoder is a sequence of received code values = { } from the demodulator [8, 9,10]. The turbo decoder consists of two component decoders to decode sequences from, and to decode sequences from. takes as its input the received sequence of systematic values and the received sequence of parity values belonging to the first encoder. The output of is a sequence of soft estimates of the transmitted data its. is called external data, that does not contain any information which was given to by. This information is interleaved, and then passed to the second decoder takes as its input the (interleaved) systematic received values s k y and the sequence of received parity values from the second encoder, along with the interleaved form of the extrinsic information provided by the first decoder. outputs a set of values, when de-interleaved using an converse form of the interleaver, constitute soft estimates of the transmitted data sequence. This extrinsic data, formed without the aid of parity bits from the 1 st code, is feedback This procedure is replicated in a iterative manner. The decoding process adds greatly to the BER performance of turbo codes. However, after many iterations, the two decoders estimates of k d will tend to meet. At this point, outputs a ISSN: Page 321

3 value Λ( ); a log-likelihood representation of the estimate of Λ( ) This log likelihood value takes into account the probability of a transmitted 0 or 1 based on systematic information and parity information from both component codes. Many negative values of Λ( ) represent a strong likelihood that the transmitted bit was a 0 and more positive values represent a strong likelihood that a 1 was transmitted. Λ( ) is de-interleaved so that its sequence coincides with that of the systematic and first parity streams. Then a simple threshold operation is performed on the result, to Fig3: Turbo Coded OFDM System produce hard decision estimates, Λ( ), for the transmitted bits. The decoding estimates and IV. SIMULATION do not necessarily converge to a correct bit A. Simulation Algorithm decision. If a set of corrupted code bits form a pair The performance of the turbo coded OFDM has of error sequences that neither of the decoders is been measured through MATLAB simulation. The able to correct, then and may either simulation follows the procedure given below: concide or deviate to an incorrect soft value. 1. Generate the information bits randomly. 2. Encode the information bits using a turbo encoder with the specified generator matrix. 3. Performed serial to parallel conversion. 4. Use BPSK, QPSK and 16 QAM modulations to Fig 2: Structure of Turbo Decoder III.TURBO CODED OFDM SYSTEM The merging of turbo codes with the OFDM transmission is so called Turbo Coded OFDM (TC- OFDM) can yield significant improvements in terms of lower energy needed to transmit data, a very improvement issue in personal communication devices [5, 6]. convert the binary bits 0 and 1 into complex signals. 5. Use IFFT to generate OFDM signals, zero padding is just done before IFFT. 6. Use parallel to serial convertor to transmit signal serially. 7. Introduce noise to simulate channel errors. We imagine that the signals are transmitted over an AWGN channel. 8. At the receiver side, perform reverse operations to decode the received sequence. 9. Count the number of erroneous bits by comparing the decoded bit sequence with the original one. 10. Calculate the BER and plot it. ISSN: Page 322

4 B. Simulation Parameters Error Correcting Code TABLE I Turbo Codes Digital Modulation BPSK, QPSK, 16-QAM, 64- QAM Channel AWGN Code Generator {111, 101} Inter-leaver Pseudo Random Inter-leaver Table 1 shows the various simulation parameters used in MATLAB. During the simulations, in order to compare the results, some random messages were generated. For that radiant function is in MATLAB. V. SIMULATION RESULTS First the development of an OFDM system model then try to improve the performance by applying forward error correcting codes to the uncoded system. From the study of the system, it can be concluded that we are able to improve the performance of uncoded OFDM by convolutional coding scheme. A. Implementation For QPSK Fig 4: BER vs SNR plot for Turbo Coded OFDM using QPSK In this section we consider the case of the higher order modulation techniques of QPSK. We follow an approach of turbo coded OFDM. That is, we begin with the values for each subcarrier. As with the base paper case, bp is evaluated many times, only this time Equation is evaluated instead. We plot the results for QPSK in Fig4. Again, Figure appears to demonstrate the same sort of results we found with QPSK, and this is predictable as the only thing changing is based on the difference between BPSK and 16-QAM performance in AWGN. So the task at hand now becomes simply a matter of plotting the results of 16-QAM we drastically decrease the range of Bit error rate. B. Implementation For BPSK Fig 5: BER vs SNR plot for Turbo Coded OFDM using BPSK In Fig5. we plot the results for BPSK. For some perspective, we plot the results of our evaluation given by Code results against the two endpoints in the uniform distribution. The results are interesting. What we find is that for low values of 0 / b E N, the performance trend for the average of all subcarriers appears to be near the midpoint However, as the SNR per bit increases, the average BER decreases.the more at low SNR the average ISSN: Page 323

5 appears to closely split the difference between the VI. IMPROVED RESULTS two boundaries. At high SNR, the average tends toward the higher value. TABLE 2 C. Implementation For 16-QAM S. No Applied Value of X(Eb/No) Base Paper Bit Error Rate(y) TCOFD M QPSK TCOFD M BPSK TCOFD M 16- QAM Table 2: Comparison between BER of uncoded OFDM and TCOFDM Table given above shows the comparison between the bit error rates of the base paper and the different modulation techniques. Value of the bit error rate is less than the previous results which verifies the improvement of the results. Fig 6: BER vs SNR plot for Turbo Coded OFDM using 16 QAM In this section we consider the case of the higher order modulation techniques of 16-QAM. We follow an approach similar to that for BPSK. That is, we begin with the values for each subcarrier. As with the BPSK case, bp is evaluated many times, only this time Equation is evaluated instead. We plot the results for 16-QAM in Fig 6. Again, Figure appears to demonstrate the same sort of results we found with 16-QAM, and this is predictable as the only thing changing is based on the difference between BPSK/QPSK and 16-QAM performance in AWGN. So the task at hand now becomes simply a matter of plotting the results of 16-QAM we drastically decrease the range of Bit error rate. VII. CONCLUSIONS Then we tied concepts of OFDM and turbo coding with a modulation scheme. To solve this problem we have proposed a method to use the stronger convolutional turbo codes. We focused our attention to improve the performance of OFDM by eliminating its shortcomings. It is shown that bit error rate (BER) is improved by adding convolutional turbo codes with OFDM and by using different modulation schemes than uncoded OFDM. By the study, we jump to the conclusion that better performance of un-coded OFDM is achieved by using convolutional turbo codes. On the other hand, Turbo codes can eliminate the residual inter symbol interference ( ISI ) and inter channel interference ( ICI ) and therefore reduce the length of the required Cyclic prefix in an OFDM system. This decreases the overhead combined with the Cyclic Prefix. The Turbo codes which are used in OFDM system for high data rate transmission in wireless LANs, results in a extenisve improvement in terms of bit error rate performance and bandwidth efficiency. ISSN: Page 324

6 To conclude, this major project gives the detail FUTURE WORK knowledge of a current key issue in the field of The following are the some of the interesting communications named Orthogonal Frequency extensions of the present work: Division Multiplexing (OFDM). We have An interesting topic for future research is to investigated the orthogonal frequency division perform more extensive performance comparisons multiplexing used for high data rate transmission. A between FFT based BTC OFDM and DCT based wide investigation showed that the OFDM BTC OFDM systems under additional real-world inherently suffers from high Inter symbol channel impairments, such as multipath fading, interference (ISI) (caused by a dispersive channel), time dispersion which leads to inter symbol Inter channel interference (ICI) (caused by interference ( ISI ). frequency offset). In order to combat ISI and ICI we have proposed a cyclic prefix and guard time ACKNOWLEDGEMENT insertion between OFDM symbols, in which the length of the guard time is made longer than the The authors also would like to thank the length of delay spread. We have also investigated anonymous reviewers for their comments and that in 48 OFDM bit errors occur in burst form suggestions to improve this paper. The authors also rather than independent, and burst errors would like to thanks Mr. Manoj Sindhwani. extensively degrade the performance of the system. Then we tied concepts of OFDM and REFERENCES turbo coding with a modulation scheme. To solve [1 ] Communications systems, Artech House Publishers, [2] L. Hanzo, M. Munster, B.J. Choi, T. Keller, OFDM & MC-CDMA for this problem we have proposed a method to use the Broadband Multiuser Communications, WLANs and Broadcasting John stronger convolutional turbo codes. We focused our Wiley Publishers, [3] John G. Proakis, Masoud Salehi, communication system using attention to improve the performance of OFDM by MATLAB Thomson Asia Pvt. Ltd., Singapore, 2003 eliminating its shortcomings. It is shown that bit [4] Anibal Luis Intini, orthogonal FrequencyDivision Multiplexing For Wireless Networks Standard IEEE a, University Of California, Santa error rate (BER) is improved by adding Barbara. convolutional turbo codes with OFDM and by using [5] J. Erfanian, S. Pasupathy, and G. Gulak, Reduced Complexity Symbol Detectors with Parallel Structures for ISI Channels, IEEE Trans. different modulation schemes than uncoded OFDM. Communications, vol. 42, pp , Feb. Mar. Apr By the study, we jump to the conclusion that better [6] O. G. Hooijen, On the channel capacity of the residential power circuit used as a digital communications medium, IEEE Commun. Lett., vol. 2, no. performance of un-coded OFDM is achieved by 10, pp , Oct using convolutional turbo codes. On the other hand, [7] Jun Zheng, Analysis Of Coded OFDM System Over Frequency- Selective Fading Channels Phd thesis Submitted to the Office of Graduate Turbo codes can eliminate the residual inter symbol Studies of Texas A&M University interference ( ISI ) and inter channel interference ( [8] T. A. Summers and S. G. Wilson, SNR Mismatch and Online Estimation in Turbo Decoding, IEEE Trans. On Communications, vol. 46, no. 4, pp. ICI ) and therefore reduce the length of the required , April Cyclic prefix in an OFDM system. This decreases [9] A. G. Burr, G. P. White, Performance of Turbo-coded OFDM in IEE Trans. Of International Conference on Universal Personal Communications, the overhead associated with the Cyclic Prefix. The use of Turbo codes in OFDM system for high data [10] Niji Kuriakose, PAPR Reduction in OFDM Systems using PTS Reduction Technique, IJETT-Volume 22 Number 2- April rate transmission in wireless LANs, results in a [11] Nidhi Jain, Vinod Sonkar PAPR Reduction for OFDM System using considerable improvement in terms of bit error rate iterative clipping and filtering and performance improvement using different performance and bandwidth efficiency. FEC code, IJETT-Volume 22 Number 8-April ISSN: Page 325