Performance synchronization enhancement of wireless optical communication over pure seawater channel

Abstract

Synchronization is one of the main challenges at the receiver in detecting the data frame at the right time, especially in very noisy environments. The unipolar OFDM (U-OFDM) has recently been introduced as a new method in optical wireless communication to convert bipolar OFDM signals to unipolar. To increase the accuracy of timing synchronization, we present an efficient method called pre-front-end transmit (PFET) synchronization, carried out based on Schmidl’s, Park’s, and the negative mirror image (NMI) methods. We then propose an algorithm called the selective training sequence (STS) to achieve more reliable time synchronization, which is established by selecting a candidate row from the Hadamard matrix (HM) or Walsh matrix (WM) for use as a training sequence. Another four signals, the m-sequence and chirp signal, random signal, and U-OFDM are applied and compared. Computer simulation results show significantly superior performance to the state-of-the-art method, wherein a 2 to 6-fold enhancement is achieved in timing accuracy in comparison to the former approach depending on underwater optical channel type. Moreover, the mean square error of timing offset is also enhanced clearly. Additionally, the proposed HM and WM row-based algorithm STS is shown to give better performance in terms of timing accuracy and bit error rate among other signals. Further improvement is achieved by increasing both the correlation sequence weight W and TS power of the timing metric, with a bit error rate (BER) enhancement of about 5 dB when the weight changes from 2 to 7.