An embedded transmission scheme using PPM signaling with symmetric error-correcting codes for optical CDMA

An embedded transmission (ET) scheme is proposed to easily apply error-correcting codes into optical code-division multiple-access (CDMA) systems for immunity from multiple-access interference (MAI). The ET scheme:offers high transmission capability over the traditional scheme using pulse position modulation (PPM) signaling, because a 2(J)-ary symbol of each user is embedded in the signature sequence with 2(J) weighted positions. Furthermore, the ET scheme with 2(J)-ary PPM signaling makes the optical CDMA system J parallel transmission systems, because J bits consisting of 2(J)-ary symbol are separately decided. Since such a separate decision is a comparison decision, the effect of MAI added in the optical channel is converted to symmetric errors in the individual parallel transmission systems. Using the symmetric error-correcting (SEC) code immunizes the individual parallel transmission systems against MAI more easily than the embedded-modulation scheme described in [1] because the ET scheme avoids using the asymmetric error correcting code, which is difficult to implement. We analyze the bit error rate under Poisson Photon counting channel and show that the ET scheme has an advantage of good energy efficiency over the traditional scheme in applying SEC codes.