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Turbo codes based error correction scheme for dimmable visible light communication systems
CONTENT • INTRODUCTION • VISIBLE LIGHT COMM. SYSTEM • TURBO CODE BASED ERROR CORRECTION SCHEME • ADVANTAGES • SIMULATION RESULTS • CONCLUSION • REFERENCE
INTRODUCTION • Recent use of light-emitting diode (LED) in lighting becomes a prevailing trend. • In accordance with such trends, the IEEE 802.15.7 VLC task group has preceded the standardization of VLC systems. • VLC systems have a constraint that the average intensity adapts to the dimming requirement chosen by a user. • To meet this requirement, various transmission schemes at the modulation level have been presented.
Contd……. • For on-off keying (OOK) that is a widely used simple modulation method in VLC systems. • the ratio of “ON” time to “OFF” time is adjusted in a transmission frame to meet the dimming requirement. • For example, 70% “ON” is needed for a 70% dimming requirement. • Therefore, this letter proposes a dimmable turbo code-based error correction scheme that combines with puncturing and scrambling.
VISIBLE LIGHT COMM. SYSTEM • Visible light communication (VLC) is a data communications medium using visible light between 400 THz (780 nm) and 800 THz (375 nm). • Visible light is not injurious to vision. • The technology uses fluorescent lamps (ordinary lamps, not special communications devices) to transmit signals at 10 kbit/s, or LEDs for up to 500 Mbit/s. • Specially designed electronic devices generally containing a photodiode receive signals from such light sources.
TURBO CODE BASED ERROR CORRECTION SCEHME • We here consider the encoding method of the proposed turbo code-based coding scheme. • Let K be the length of information bits for transmission. Those K information bits are encoded using a trellis-based error correcting code, such as convolutional codes and turbo codes, to generate an N-bit codeword . • We use trellis-based codes for encoding since they resist the decoding performance degradation induced by puncturing .Furthermore, we employ turbo codes, as those outperform convolutional codes. • Subsequently, puncturing is applied to ensure that the resulting N-bit codeword meets the desired dimming rate d. • For puncturing rate p, Np symbols out of N-bit codeword symbols are removed in such an arbitrary pattern known a priori to both the transmitter and the receiver that systematic information bits remain intact.
Contd….. • Thus, N(1− p) bits remain in the codeword after puncturing. To adjust the dimming rate, scrambling is also applied to the N(1 − p)-bit sequence . • If we think scrambled sequence as a binomial random variable of mean N(1−p) /2 and variance N(1−p)/ 4 . Therefore, out of N symbols for transmission, N(1−p)/2 symbols are turned on and the resulting dimming rate is d = (1−p)/ 2 . • Note that if the puncturing rate is too high (p >N−K /N ), the information in a message is indistinguishable. For puncturing rate p ≤ (N−K)/ N , the dimming rate is lower bounded by( K/2N )≤ 1/2 . To satisfy very low dimming requirements, we should lower the code rate K/N . • For the dimming rates greater than 1/ 2 , the binary symbols of the codeword are flipped to obtain those dimming rates. Thus, this approach ensures that an arbitrary dimming rate in [0, 1] is possible.
ADVANTAGES • The iterative decoding algorithm developed for turbo codes allows better decoding performance over existing coding schemes devised for VLC systems. • The robustness to puncturing of a trellis-based encoding method for turbo codes facilitates the support for arbitrary dimming rates. • A single encoding/decoding method for various dimming rates leads to the ease of implementation, while existing schemes require dedicated encoding/decoding methods for different dimming rates. • This coding scheme supports various code rates, while existing schemes can have limited code rate options.
Contd….. • As the dimming rate increases from 50% , the corresponding puncturing rate increases, resulting in the rise of the code rate and the error probability. We see that the performances of high-rate code ‘TC’ are comparable to that of code ‘RM.’ • This demonstrates that the performance of the proposed code is much better than that of existing coding schemes by more than 2dB or has as much as 20-fold gain in the code rate.
Conclusion • This letter proposes a turbo code-based coding scheme for OOK modulation of visible light communication systems. Using puncturing and scrambling in combination with turbo codes, arbitrary dimming and code rate requirements are met. Also compare the performance of the proposed coding scheme with existing codes and show that the proposed code has much better performance.
References • Sang Hyun Lee, Member, IEEE, and Jae Kyun Kwon, ”Turbo Code-Based Error Correction Scheme for Dimmable Visible Light Communication Systems,” IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 17, SEPTEMBER 1, 2012. • T. Komine and M. Nakagawa, “Fundamental analysis for visible-light communication system using LED lights,” IEEE Trans. Consumer Electron., vol. 50, no. 1, pp. 100–107, Feb. 2004. • H. Elgala, R. Mesleh, and H. Haas, “Indoor optical wireless communication: Potential and state-of-the-art,” IEEE Commun. Mag., vol. 49, no. 9, pp. 56–62, Sep. 2011. • Local and Metropolitan Area Networks–Part 15.7, Short-Range Wireless Optical Communication Using Visible Light, IEEE Standard 802.15.7- 2011, Sep. 2011. • H.-C. Kwon, et al. (2008, Jul.). Modulation Categorization of Visible Light Communication [Online]. Available: http://mentor.ieee.org/ 802.15/documents • S. Kaur, W. Liu, and D. Castor. (2009, Sep.). VLC Dimming Proposal [Online]. Available: http://mentor.ieee.org/802.15/documents • J. K. Kwon, “Inverse source coding for dimming in visible light communications using NRZ-OOK on reliable links,” IEEE Photon. Technol. Lett., vol. 22, no. 19, pp. 1455–1457, Oct. 1, 2010.