BER Performance Comparison of DCO-OFDM and Convolutional Coded DCO-OFDM in IM/DD Systems

BER Performance Comparison of DCO-OFDM and Convolutional Coded DCO-OFDM in IM/DD Systems

Jayasudha Koti (Dept. Electronics and Telecommunication, Thakur College of Engineering and Technology, Mumbai, India) and Braj Kishore Mishra (Dept. Electronics and Telecommunication, Thakur College of Engineering and Technology, Mumbai, India)
DOI: 10.4018/IJECME.2019070102

Abstract

Optical wireless communication (OWC) is an alternative technology to meet the demands of the exponentially-growing high data rate applications run by broadband users. The implementation of single carrier modulation techniques in OWC is an age old technology, but for the last few years research is focused towards the multicarrier modulation techniques in OWC. In OWC, information is carried using intensity modulation and retrieved using direct detection. To perform intensity modulation, the baseband signal should be a unipolar signal. To obtain a unipolar signal, various techniques such as DC-biased orthogonal frequency division multiplexing (DCO-OFDM), Asymmetrically clipped orthogonal frequency division multiplexing (ACO-OFDM), Flip orthogonal frequency division multiplexing (Flip-OFDM) and Unipolar orthogonal frequency division multiplexing (U-OFDM) techniques are reported in the literature. Though the DCO-OFDM is spectrally efficient compared to other techniques it requires more power to achieve the targeted BER. In this article, a convolutional Coded DCO-OFDM (CDCO-OFDM) has been introduced by applying channel coding. A convolutional encoder and a hard-decision Viterbi decoder are considered in CDCO-OFDM. It has been observed that CDCO-OFDM requires less transmitted power than DCO-OFDM to attain the targeted BER. The performance of DCO-OFDM and CDCO-OFDM is evaluated for 4, 16, 64 QAM- 7dB and 13dB bias, in the presence of the additive white Gaussian noise (AWGN) channel. It is observed that CDCO requires less power to transmit than DCO-OFDM for the BER 10-4.
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1. Introduction

Rapid utilization of RF spectrum may lead to congestion of the spectrum to meet the demands of the exponentially growing users. This laid a path to use the infrared (IR) region of electromagnetic spectrum where optical radiation is used as a mode to communicate. This focused research towards optical wireless communication (OWC). In OWC information is send wirelessly in the form of optical radiations using IR region as a medium. Light emitting diodes (LED’s) and photo diodes are used as optical transmitters and detectors for indoor applications. In OWC intensity modulation (IM) and direct detection (DD) is used. Initially single carrier modulation (SCM) is used in OWC where entire bandwidth is used to transmit one symbol. Time taken to transmit one symbol is 1/B seconds. SCM is ideal for low data rate applications and it results in inter symbol interference when used for high data rate applications. Concept of dividing entire bandwidth in to different sub bands and transmitting different data streams in parallel over different subcarriers made OFDM a well-known broadband wireless technology. Resilience to ISI and its ability to tune the transmitted signal to channel frequency made it as a radical broad band wireless technology. Concept of Fast Fourier algorithms made the implementation of OFDM modulators and demodulators easy by replacing bank of modulators and demodulators. This technology forms a basis for 4G technologies like LTE, WiMAX and supports high data rate greater than 100Mbps.This made possible of utilization of high data rate applications like HDTV, online gaming, mobile videophones and soon (Weinstein & Ebert, 1971; Armstrong, 2009; Kahn & Barry, 1997; Elgala, Meshleh, & Haas, 2011).

OFDM was widely used in RF domain for the last few decades. But OFDM in OWC is a state of art technology. In conventional systems, OFDM generates a bipolar signal. To perform IM unipolar signal is needed. Unipolar signal should be real and positive. These signals are generated by various unipolar techniques and they differ in the subcarrier assignment and conversion of bipolar real signal to positive real signal. Various techniques like DC-biased orthogonal frequency division multiplexing, asymmetrically clipped orthogonal frequency division multiplexing, Flip orthogonal frequency division multiplexing and Unipolar orthogonal frequency division multiplexing have been described in the literature. Among these techniques DCO is spectrally efficient but it requires more power to transmit. The average transmitted power governs the eye safety as well as the electrical power consumption by the transmitter. Thus, power efficiency is not only important for eye safety but also to enhance life of battery-operated handheld devices. Channel coding techniques are introduced to improve the performance of the system (Viterbi, 1971). This Coded DCO (CDCO) has been implemented for 4-QAM, 16-QAM,64-QAM at 7dB and 13dB bias in the presence of AWGN channel. It is noticed that CDCO-OFDM requires less power to transmit for the targeted BER than DCO-OFDM.

In this paper we the authors present a novel technique CDCO-OFDM and compare it with the DCO-OFDM CDCO-OFDM has been proposed to improve the power efficiency of DCO-OFDM. A literature review on unipolar communication techniques is presented in section II, Implementation of DCO-OFDM and CDCO is presented in Section III. Simulation results are discussed in section IV. Conclusions are drawn in Section V.

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