Performance Analysis of Unipolar CommunicationTechniques for Optical Wireless Communication

A Comparative analysis of Asymmetrically clipped orthogonal frequency division multiplexing (ACO-OFDM), Flip orthogonal frequency division multiplexing (Flip-OFDM) and Unipolar orthogonal frequency division multiplexing (U-OFDM) for intensity modulation (IM) and direct detection (DD) indoor optical wireless communication (OWC) is presented. These techniques are differentiated based on the higher order modulated information symbols assigned to the subcarriers and conversion of real bipolar signal to non negative unipolar signal. In ACO higher order QAM symbols are assigned only to the odd subcarriers where as in Flip-OFDM and U-OFDM data is assigned to all the subcarriers. In ACO-OFDM bipolar signals are converted to unipolar by clipping the negative real time domain samples, in Flip-OFDM by separation of bipolar signals in to positive and negative frames and then flipping is incorporated where as in U-OFDM concept of representing each positive and negative time domain sample by a pair of samples is incorporated. Analytical approach is discussed and the implementation of these techniques is performed Performance of ACO-OFDM and Flip-OFDM is evaluated for various higher order modulation techniques in terms of bit error rate (BER) as a function of electrical energy per bit to noise ratio for additive white Gaussian noise (AWGN) channel. A comparative analysis of ACO, Flip and U-OFDM for 4-QAM is illustrated. It is observed that the BER performance is same but they vary in spectral efficiency, transmit power, bandwidth and clipping noise