A synchronization method deselecting candidate positions with chip level detection for optical CDMA

In order to reduce the number of preamble bits required for synchronization at correct positions, a method of synchronization by deselecting the candidate positions for synchronization is proposed. As decision methods for each preamble bit, a method using the threshold detection and a method using chip level detection are discussed. Both methods make use of the fact that the preamble bit is the bit "1": any synchronization position judged not to be a "1" bit even once is deselected as a candidate. In this way, the only position eventually remaining without being deselected is judged to be the synchronization position. As a result, it is possible to carry out synchronization with fewer preamble bits than the conventional method in which bit decision is based on the logarithmic likelihood ratio of the continuously transmitted preamble bits. A theoretical analysis is made under the assumption that the output of the photodetector follows a Poisson distribution. It is found that the proposed method can perform synchronization within a shorter time than the conventional synchronization method. Further, in the method using chip level detection, it is shown that synchronization is achieved in a shorter time even if the bit decision threshold is fixed.