Comparitive Analysis of ACO-OFDM and CACO-OFDM for Indoor Optical Wireless Communication

Abstract

A comparative analysis of asymmetrically clipped orthogonal frequency division multiplexing (ACO-OFDM) and proposed coded ACO-OFDM (CACO-OFDM) for intensity modulation (IM) and direct detection (DD) indoor optical wireless communication (OWC) is presented. CACO-OFDM is an extended version of ACO-OFDM. Existing OWC-OFDM techniques are basically classified based on the data assigned to subcarriers and the approach of converting bipolar real signal to unipolar real positive signal. In ACO-OFDM subcarrier assignment is performed by assigning information only on the odd subcarriers and setting the even subcarriers to zeros. Conversion of bipolar real signal to unipolar real positive signal is performed by clipping the amplitude of the negative samples at zero. In general clipping results in clipping noise which may degrade the performance. But the performance of ACO-OFDM is not degraded due to its design feature and the clipping noise is present only in the even subcarriers which are orthogonal to the data present in the odd subcarriers. In proposed approach convolutional coding a class of channel coding is introduced to for better performance of the system by attaining the targeted BER at the lower value of the transmitted power. Application of convolutional coding and hard decision Viterbi decoder results in a power efficient system. ACO-OFDM and CACO-OFDM are analysed and simulated for 4, 16, 64 and 256-QAM in the presence of AWGN channel. Performance is investigated by plotting E _b(elec) /N _o versus BER and E _b(opt) /N _o for BER of 10 ^-4 versus normalized bandwidth. It has been observed that CACO-OFDM is more power efficient as compared to ACO-OFDM. Therefore power efficiency of the ACO-OFDM can be further improved by using coding techniques.