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Interscience Research Network Interscience Research Network Conference Proceedings - Full Volumes IRNet Conference Proceedings 12-9-2012 Proceedings of International Conference on Electrical, Electronics & Computer Science Prof.Srikanta Patnaik Mentor IRNet India, [email protected] Follow this and additional works at: https://www.interscience.in/conf_proc_volumes Part of the Electrical and Electronics Commons, and the Systems and Communications Commons Recommended Citation Patnaik, Prof.Srikanta Mentor, "Proceedings of International Conference on Electrical, Electronics & Computer Science" (2012). Conference Proceedings - Full Volumes. 61. https://www.interscience.in/conf_proc_volumes/61 This Book is brought to you for free and open access by the IRNet Conference Proceedings at Interscience Research Network. It has been accepted for inclusion in Conference Proceedings - Full Volumes by an authorized administrator of Interscience Research Network. For more information, please contact [email protected]. Proceedings of International Conference on Electrical, Electronics & Computer Science (ICEECS-2012) 9th December, 2012 BHOPAL, India Interscience Research Network (IRNet) Bhubaneswar, India Rate Compatible Punctured Turbo-Coded Hybrid Arq For Ofdm System RATE COMPATIBLE PUNCTURED TURBO-CODED HYBRID ARQ FOR OFDM SYSTEM DIPALI P. DHAMOLE1& ACHALA M. DESHMUKH2 1,2Department of Electronics and Telecommunications, Sinhgad College of Engineering, Pune 411041, Maharashtra Abstract-Now a day’s orthogonal frequency division multiplexing (OFDM) is under intense research for broadband wireless transmission because of its robustness against multipath fading. A major concern in data communication such as OFDM is to control transmission errors caused by the channel noise so that error free data can be delivered to the user. Rate compatible punctured turbo (RCPT) coded hybrid ARQ (RCPT HARQ) has been found to give improved throughput performance in a OFDM system. However the extent to which the RCPT HARQ improves the throughput performance of the OFDM system has not been fully understood. HARQ has been suggested as a means of providing high data rate and high throughput communication in next generation systems through diversity combining transmit attempts at the receiver. The combination of RCPT HARQ with OFDM provides significant bandwidth at close to capacity rates of channel. In this paper, we evaluate by computer simulations the throughput performance of RCPT code HARQ for OFDM system as compared to that of conventional OFDM. Keywords: OFDM, hybrid ARQ, rate compatible punctured turbo codes, mobile communication, multipath fading I. INTRODUCTION with each transmission would result in the highest throughput as unnecessary redundancy is avoided. Orthogonal frequency division multiplexing has been However in a practical system, [4] the number of considered as one of the strong standard candidates retransmissions allowed is limited to avoid for the next generation mobile radio communication unacceptable time delay before the successful systems. Multiplexing a symbol data serial stream transmission. When the number of retransmissions is into a large number of orthogonal sub channels makes limited, the residual bit error is produced. the OFDM [1] signals spectral bandwidth efficient. Recently, high speed and high quality packet data II. RCPT ENCODER/DECODER AND RCPT communication is gaining more importance. A major HARQ concern in wireless data communication is to control transmission errors caused by the channel noise so The RCPT encoder and decoder are included as a part that error free data can be delivered to the user. For of the transmission system model shown in fig.1. successful packet data communication, hybrid RCPT encoder consists of a turbo encoder, a puncture automatic repeat request (HARQ) is the most reliable and a buffer. Turbo encoder /decoder parameters are error control technique for the OFDM systems as in shown in Table 1. HARQ schemes, channel coding is used for the error correction which is not applied in the Simple ARQ Table 1. Turbo encoder/ decoder parameters schemes. In HARQ, the advantage of obtaining high Rate 1/3 reliability in ARQ system is coupled with advantage of forward error correction system to provide a good Component RSC Encoder throughput even in poor channel conditions. Turbo encoder codes, introduced in 1993 by Berrou et al., have been Interleaver S-random intensively studied as the error correction code for (S = K ½) mobile radio communications. Rate compatible Component Log-MAP punctured turbo (RCPT) coded HARQ (RCPT decoder Decoder HARQ) scheme was proposed in [2] and shown to Number of 8 achieve enhanced throughput performance over an iterations additive white Gaussian noise (AWGN) channel. In [3] it is shown that the throughput of the RCPT Turbo encoder considered in this paper is a rate 1/3 hybrid ARQ scheme outperforms other ARQ schemes encoder. Turbo encoded sequences are punctured by over fading and shadowing channels. the puncturer and the different sequences obtained are stored in the transmission buffer for possible In this paper, we evaluate the throughput performance retransmissions. With each retransmission request, a of RCPT coded HARQ for OFDM system over new sequence (previously unsent sequence) is additive white Gaussian noise channel. When the transmitted. Turbo decoder consists of a depuncturer, number of allowable retransmissions is unlimited, buffer and a turbo decoder. At the RCPT decoder, a transmitting minimum amount of redundancy bits newly received punctured sequence is combined with International Conference on Electrical, Electronics Engineering 9th December 2012, Bhopal, ISBN: 978-93-82208-47-1 1 Rate Compatible Punctured Turbo-Coded Hybrid Arq For Ofdm System the previously received sequences stored in the adopted HARQ scheme. The punctured sequences are received buffer. The depuncturer inserts a channel of different length for different puncturing periods. value of 0 for those bits that are not yet received and The sequence to be transmitted is block interleaved 3 sequences, each equal to the information sequence by a channel interleaver and then transformed into length, are input to the turbo decoder where decoding PSK symbol sequence. The Nc data modulated is performed as if all 3 sequences are received. In this symbols are mapped onto Nc orthogonal subcarriers paper the type I HARQ is considered. For type I to obtain the OFDM signal waveform. This is done HARQ, the two parity bit sequences are punctured by applying the inverse fast Fourier transform (IFFT). with P = 2 and the punctured bit sequence is After the insertion of a guard interval (GI), the transmitted along with the systematic bit sequence. If OFDM signal is transmitted over Additive white the receiver detects errors in the decoded sequence, a Gaussion noise channel and received by multiple retransmission of that packet is requested. The antennas at the receiver. The OFDM signal received retransmitted packet uses the same puncturing matrix on each antenna is decomposed into the Nc as the previous packet. Instead of discarding the orthogonal subcarrier components by applying the erroneous packet, it is stored and combined with the fast Fourier transform (FFT) and each subcarrier retransmitted packet utilizing the packet combining or component coherently detected to be combined with time diversity (TD) combing effect. those from other antennas based on maximal ratio combining. The MRC combined soft decision sample III. TRANSMISSION SYSTEM MODEL [5] sequence is de-interleaved and input to the RCPT decoder which consists of a de-puncturer, a buffer The OFDM system model with RCPT coded HARQ and a turbo decoder. Error detection is performed by is shown in Fig 1. At the transmitter a CRC coded the CRC decoder which generates the ACK/NAK sequence of length K bits is input to the RCPT command and recovers the information in case of no encoder where it is coded, punctured and stored in the errors. buffer for possible retransmissions. The parity sequences are punctured before transmission based on Fig. 1. OFDM with HARQ system model IV. SIMULATION RESULTS N= 512 sub-channels, number of pilots P= N/8, total number of data sub-channels S=N-P and guard The turbo encoder/decoder parameters are as shown interval length GI=N/4. In our simulation, we assume in Table 1. The computer simulation conditions are channel length L=16, pilot position interval 8 and the summarized in Table 2. number of iterations in each evaluation are 500. A In this paper, the information sequence length K = rate 1/3 turbo encoder is assumed and a log-map 1024 bits is assumed. The turbo encoded sequence is decoding is carried out the receiver. The numbers of interleaved with a random interleaver. The data frame errors to count as a stop criterion are limited to modulation scheme considered is BPSK modulation 15. and the ideal channel estimation is assumed for data demodulation at receiver. We assume OFDM using International Conference on Electrical, Electronics Engineering 9th December 2012, Bhopal, ISBN: 978-93-82208-47-1 2 Rate Compatible Punctured Turbo-Coded Hybrid Arq For Ofdm System Table 2. Simulation Conditions Information sequence length K = 210 214 bits Channel interleaver Block interleaver Modulation / demodulation BPSK No. Of subcarreiers Nc = 256 OFDM Subcarrier spacing 1/Ts Guard interval Tg = 8/Ts Type Type I ARQ Max. No. Of tx. Ω Forward Additive white gaussion noise Propagation channel Reverse Ideal for a given range of signal-to noise ratio (SNR) while The throughput in bps/Hz is defined as the red line shows the bit error rate (BER) for turbo coded type I HARQ for the same range of given SNR. From graph (fig. 2) below it can be seen that the bit error rate of turbo coded HARQ decreases .....(1) with the increase in SNR however the BER for the conventional OFDM remains as it is for the entire Fig. 2 shows the bit error rate as a function of average range of SNR, hence we can get improved throughput bit energy- to- additive white Gaussian noise for turbo coded HARQ as compared to that of (AWGN) power spectrum density ratio. The conventional OFDM.
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