Use of optical orthogonal codes with 2-PPM and BCH codes for time-hopping optical CDMA system

In this paper, we propose the use of 2-PPM for time-hopping optical code division multiple access (CDMA) where the transmission bits are embedded in the weighted positions of signature sequences. To use 2-PPM, each chip is composed of two slots, and an optical pulse is transmitted in either slot, according to a bit transmitted in the corresponding weighted position. In the receiver, threshold detection is used, which makes bits that have suffered multiple access interference (MAI) erasure bits. By using codeword bits of the BCH codes for the transmitted bits, the effect of MAI is removed in the decoding of BCH codes. Since the effect of MAI is treated as erasure bits, the proposed method reduces the effect of MAI more than the decoding procedures treating them as error bits. In addition, the use of extended BCH (eBCH) codes allows us to reduce the effect of MAI more efficiently. We analyze the bit error rate (BER) of the proposed system, assuming that the photodetector outputs follow the Poisson distribution. The numerical results show that the proposed system achieves better BER performance than the system without Manchester signaling.