Error Correction Coding for Optical CDMA

The optical fiber is a very attractive communication medium since it offers a large bandwidth and low attenuation and can therefore facilitate demanding services such as high-quality video transmission. As the reach of optical fiber is being extended to the access network it is economically attractive to share fibers between different users without adding active components in the network. The most common multiple access method for such passive optical networks is time division multiple access (TDMA), but lately there has been an increased interest in using wavelength division multiple access (WDMA) and optical code division multiple access (OCDMA). This thesis evaluates forward error correction as a method to improve the performance of passive optical networks, in particular OCDMA networks. Most studies of OCDMA use simple channel models focusing only on the multiple access interference. However, beat noise is the main performance limitation for many implementations of OCDMA. Beat noise occurs when multiple optical fields are incident on a receiver, because of the square-law detection. To make a realistic evaluation of OCDMA, channel models which take interference, beat noise and other noise types into account are studied in this thesis. Both direct sequencing CDMA and fast frequency hopping are considered as spreading methods. An efficient simulation method was developed in order to simulate systems with forward error correction (FEC) and soft decoding. The simulations show that the performance is significantly overestimated when the beat noise is neglected. In order to decrease the error rate without using overly complex equipment the bandwidth has to be increased. Simulation results show that it is beneficial to use error correction codes in addition to spreading codes for the bandwidth expansion. The efficiency can be further improved by using soft decoding; therefore maximum likelihood decoding methods for the OCDMA channels are developed and demonstrate a significant reduction in the error rate. Frequency hopping and direct sequencing are also compared with each other, and the results show that temporally coded OCDMA is more sensitive to beat noise. In addition, the performance of a low complexity soft decoding method for Reed-Solomon codes is evaluated. Soft decoding of Reed Solomon codes has not yet found practical use because the earlier proposed methods do not offer sufficient performance gains to motivate the increased complexity. The bitlevel Chase-decoding algorithm evaluated here can be easily implemented using any algebraic decoder.