IJRIT International Journal of Research in Information Technology, Volume 2, Issue 3, March 2014, Pg: 424-430

International Journal of Research in Information Technology (IJRIT) www.ijrit.com

ISSN 2001-5569

Precoding and clipping for Space Time Block Coded (STBC) OFDM Systems Gursharan Singh1, Kamaljit Singh Bhatia2, Kulwinder Singh3, Harsimrat Kaur4 1& 3 Department 2

of Electronics and communication , BMSIET, Muktsar

Department of Electronics Engineering, Sri Guru Granth Sahib World University, Fatehgarh Sahib, Punjab, India 4

Department of Electronics and communication Engineering, CTIEMT, Jalandhar city

Abstract In this paper, performance analysis of intensity modulation with direct detection (IM/DD) optical orthogonal frequency division multiplexing (OFDM) system is done with collective approach coding and clipping to maintain the threshold level of Peak to average ratio. For 9.875 Gb/s data rate OFDM signal within BER of 10-3 , sudden improvement in PAPR is observed with this approach.

1. INTRODUCTION Space-time block coded orthogonal frequency-division multiplexing (STBC-OFDM) schemes have garnered much attention as a simple transmit diversity technique in combating frequency selective fading channels [1], [2]. Although STBC-OFDM using Alamouti code can improve the performance of OFDM, its second-order diversity gain is still insufficient for communication requiring high reliability over fading channels. Thus, recent works have considered STBCOFDM schemes aimed at achieving fourth-order diversity gain [3], [4].STBC-OFDM using G4 code with 1/2 rate and H4 code with 3/4 rate are the conventional schemes being employed to provide fourth-order diversity gain [5]–[7]. However, problems arise when the conventional schemes described in [6] and [7] are used in spatially transmit correlated fading channels. These conventional schemes have been designed without taking spatial transmit correlation in consideration, although it occurs in many practical applications. The spatial transmit correlation causes a deficiency of randomness and independence in channel [8], [9]. Therefore, increasing the spatial transmit correlation results in bit-error-probability (BEP) performance degradation for STBC-OFDM schemes targeting a fourth-order diversity gain [10].Space-frequency coded OFDM (SFC-OFDM) schemes have been proposed to exploit both spatial and frequency diversity [11]–[13]. The authors of [12] proposed a type of SFC-

Gursharan Singh, IJRIT

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 3, March 2014, Pg: 424-430

OFDM without permutation from a simple mapping process using STBC. As with STBC-OFDM schemes, however, the SFC-OFDM design described in [12] fails to consider spatial transmit correlation, so that it suffers from spatially correlated channels. By precoding the incoming information symbol across OFDM subcarriers, the SFC-OFDM with permutation proposed in [13] attains both full transmission rate and frequency diversity over independent channels. However, exploiting the frequency diversity with this approach also limits the BEP performance in spatially correlated channels and requires high decoding complexity. This is because the SFC-OFDM described in [13] uses the precoding method instead of STBC for the full transmission rate. In the literature [11], the authors have shown that clipping can improve the overall communication system performance, under the peak power constraint. Inspired by the concept in [10] and [11], we proposed a combined STBC precoding and clipping scheme in the IM/DD optical OFDM systems. We studied the BER performance of the STBC-precoded and clipped optical IM/DD OFDM system using our experimental optical IM/DD OFDM transmission platform. The experimental results show that the BER performance of the system can be greatly improved with the PAPR reduction with a CR of 6 dB. Therefore, a new STBC-based scheme using frequency diversity that can improve the performance in spatially correlated channels is still needed. Later on STBC-based transmit diversity scheme using frequency diversity was introduced that is more robust against spatial correlation than the STBC-OFDM schemes and the SFCOFDM schemes, and which achieves fourth-order diversity gain. From the upper bound of BEP performance over spatially transmit-correlated channels, it was shown that performance can be improved by increasing the determinant of the channel correlation matrix. In order to exploit both frequency and spatial diversity and thereby increase the determinant, attempt was made to design an STBC-based transmit diversity scheme. The merged determinant condition for robustness is derived from detailed analysis of the determinant.

2. SYSTEM PRINCIPLE Fig. 1 illustrates the block diagram of an optical IM/DD OFDM system using precoding and clipping. In this study, a discrete-time baseband optical IM/DD OFDM system with N subcarriers has been considered. The main difference between the proposed system and the conventional IM/DD optical OFDM systems is that there are the precoding and clipping block at the transmitter and inverse precoding block at the receiver in the proposed scheme. The OFDM system consists of transmitter, channel and receiver blocks, which are described below.

Gursharan Singh, IJRIT

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 3, March 2014, Pg: 424-430

Fig1. Conceptual diagram of Clipped and STBC coded Optical-OFDM System In general, the clipping technique can cause impairment to a communication system. When the CR is maintained at a small value, the signal is effectively clipped. In our work, a slight clipping scheme is employed for the generation of a STBC-precoded OFDM signal. The results of the measured BER in the following experiment show that the soft clipping can improve the BER of the system when the value of the CR is set at 6 dB. In this work, STBC precoding is first employed before the IFFT operation; therefore, the PAPR of the OFDM signal at the output of the IFFT block has been reduced. For the STBC-precoded OFDM signal, soft clipping is used in order to reduce the PAPR furthermore .

3. RESULTS AND DISCUSSION PAPR is defined as the ratio between the maximum peak power and the average power of the transmitted OFDM signals. The performance of an OFDM system can be evaluated using the complementary cumulative distribution function (CCDF) to measure the PAPR

Gursharan Singh, IJRIT

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 3, March 2014, Pg: 424-430

Fig. 3. Comparison of the PAPR of the STBC-precoded and clipped OFDM signals, the STBC-precoded OFDM signals, and the original OFDM signals.

We have evaluated the PAPR of the STBC-precoded with clipped OFDM signal that is produced by the MATLAB program. In the experimental setup, the value of CR is set at 6 dB [10]. Fig. 3 shows the CCDF comparisons of a QPSK signal of 50000 OFDM frames. We observe that at CCDF = 10-3, the STBC precoding with clipping (CR = 6 dB) scheme can reduce the PAPR by approximately 3.5 dB, as compared with the original OFDM signal and by 1.2 dB, as compared with the STBC-precoded scheme

Fig. 4. Temporal waveform of the STBC-precoded and clipped QPSK OFDM signals.

Gursharan Singh, IJRIT

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 3, March 2014, Pg: 424-430

Fig. 5. Temporal waveform of the STBC-precoded QPSK OFDM signals.

Fig. 6. Temporal waveform of the original QPSK OFDM signals Gursharan Singh, IJRIT

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 3, March 2014, Pg: 424-430

Figs. 4, 5, and 6 show the temporal waveforms of the STBC-precoded with clipped OFDM signals, the STBCprecoded OFDM signals, and original OFDM signals, respectively. We observe that the STBC-precoded with clipped OFDM signals fluctuate less than the STBC-precoded and the original OFDM signals Fig. 7 shows the measured BER performance of the proposed STBC-precoded and clipped optical OFDM transmission system. In the experimental setup, the value of CR is set at 6 dB. For the 2.5 Gs/s STBC-precoded and clipped QPSK OFDM signals, the received sensitivity at a BER of 10-4 after 100-km SMF transmission is improved by approximately 4 dBm as compared with the original OFDM signals, and by 1 dBm, as compared with the STBC-precoded OFDM signals.

Fig. 7. BER comparison

4. CONCLUSION We proposed a coding with clipping scheme for the optical IM/DD OFDM system. The BER performance of the proposed scheme was studied using the optical IM/DD OFDM transmission experimental platform. For the 9.875 Gb/s STBC-precoded and clipped QPSK OFDM signal, the received sensitivity of the systems is improved by approximately 4 dBm as compared with the original OFDM systems. The experimental results show that STBC precoding with clipping (CR = 6 dB) scheme can greatly reduce the PAPR by approximately 3.5 dB, as compared with the original OFDM systems.

REFERENCES [1] J. Armstrong, “OFDM for optical communications,” J. Lightw. Technol., vol. 27, no. 3, pp. 189-204, Feb.2009. [2] E. Vanin, “Performance evaluation of intensity modulated optical OFDM system with digital baseband distortion,” Opt. Express, vol. 19, no. 5, pp. 4280-4293, Feb. 2011. Gursharan Singh, IJRIT

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 3, March 2014, Pg: 424-430

[3] D. Wulich, “Definition of efficient PAPR in OFDM,” IEEE Commun. Lett., vol. 9, no. 9, pp. 832-834, Sep. 2005. [4] Y. Gao, et al., “Direct-detection optical OFDM transmission system with pre-emphasis technique,” J. of Lightw. Technol., vol. 29, no. 14, pp. 2138-2145, May 2011. [5] F. Li, et al., “Reducing the peak-to-average power ratio with companding transform coding in 60 GHz OFDMROF systems,” IEEE J. Opt. Commun. Netw., vol. 4, no. 3, pp. 202-209, Mar. 2012. [6] B. Ranjha and M. Kavehrad, “Precoding techniques for PAPR reduction in asymmetrically clipped OFDM based optical wireless system,” SPIE OPTO. Int. Soc. Optics and Photonics, 2013: 86450R-86450R-9. [7] Z. Wang, et al., “Hadamard precoding for PAPR reduction in optical direct detection OFDM systems,” Optoelectr. Lett., vol. 7, no. 5, pp. 0363-0366, Sep. 2011. [8] Q. Yang, et al., “Coherent optical STBC-Spread OFDM transmission using orthogonal band multiplexing,” Opt. Express, vol. 20, no. 3, pp. 2379-2385, Jan. 2012. [9] J. Xiao, et al., “Hadamard transform combined with companding transform technique for PAPR reduction in an optical direct-detection OFDM system,” IEEE J. Opt. Commun. Netw., vol. 4, no. 10, pp. 709-714, Oct. 2012. [10] K. R. Panta and J. Armstrong, “Effect of clipping on the error performance of OFDM in frequency selective fading channels,” IEEE Trans. Wireless Commun., vol. 3, no. 2, pp. 668-671, Mar. 2004. [11] H. Qian, R. Raich, and G. T. Zhou, “On the benefits of deliberately introduced baseband nonlinearities in communication systems,” ICASSP2004 (II), pp. 905-908, 2004. [12] J. G. Proakis, Digital Communications, New York: McGraw-Hill, 1995. [13] Y.-P. Lin and S.-M. Phoong, “BER minimized OFDM systems with channel independent precoders,” IEEE Trans. Signal Precoding, vol. 51, no. 9, pp. 2369-2380, Sep. 2003 [14] E K. Singh Bhatia ,T.S. Kamal, R.S. Kaler, “Peak-to-average power ratio reduction using coded signal in optical-orthogonal frequency division multiplexing systems” IET Optoelectron., Elsevier Science. (2012), Vol. 6, Iss. 5, pp. 250–254, doi: 10.1049/iet-opt.2011.0089, Impact Factor-1.201 Available online at www.ieeexplore.ieee.org. [15] Kamaljit Singh Bhatia, R.S. Kaler, T.S. Kamal, “DESIGN AND SIMULATION OF OPTICAL-OFDM SYSTEMS” Journal of Russian Laser Research, Springer Science+ Business Media (2012), Volume 33, Number 5, pp. 202-208, September, 2012, Impact Factor-0.9, Available online at www.springerlink.com. [16] K.S. Bhatia, T.S. Kamal, R.S. Kaler, “An adaptive compensation scheme-based coded direct detection optical– orthogonal frequency division multiplex (OFDM) system” Computers and Electrical Engineering 38, Elsevier Science (2012) 1573–1578, http://dx.doi.org/10.1016/j.compeleceng.2012.06.007, Impact Factor-0.7, Available online at www.sciencedirect.com.

[17] Kamaljit Singh Bhatia , T.S. Kamal, “Modeling and simulative performance analysis of OADM for hybrid multiplexed

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(STBC) OFDM Systems - International Journal of Research in ...

2 Department of Electronics Engineering, Sri Guru Granth Sahib World University, Fatehgarh Sahib, Punjab,. India. 4 Department of Electronics and communication Engineering, CTIEMT, Jalandhar city. Abstract. In this paper, performance analysis of intensity modulation with direct detection (IM/DD) optical orthogonal ...

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