Error Correction Coding for Optical CDMA

Abstract: The optical fiber is a very attractive communication mediumsince it offers a large bandwidth and low attenuation and cantherefore facilitate demanding services such as high-qualityvideo transmission. As the reach of optical fiber is beingextended to the access network it is economically attractive toshare fibers between different users without adding activecomponents in the network. The most common multiple accessmethod for such passive optical networks is time divisionmultiple access (TDMA), but lately there has been an increasedinterest in using wavelength division multiple access (WDMA)and optical code division multiple access (OCDMA). This thesisevaluates forward error correction as a method to improve theperformance of passive optical networks, in particular OCDMAnetworks.Most studies of OCDMA use simple channel models focusingonly on the multiple access interference. However, beat noiseis the main performance limitation for many implementations ofOCDMA. Beat noise occurs when multiple optical fields areincident on a receiver, because of the square-law detection. Tomake a realistic evaluation of OCDMA, channel models which takeinterference, beat noise and other noise types into account arestudied in this thesis. Both direct sequencing CDMA and fastfrequency hopping are considered as spreading methods. Anefficient simulation method was developed in order to simulatesystems with forward error correction (FEC) and soft decoding.The simulations show that the performance is significantlyoverestimated when the beat noise is neglected. In order todecrease the error rate without using overly complex equipmentthe bandwidth has to be increased. Simulation results show thatit is beneficial to use error correction codes in addition tospreading codes for the bandwidth expansion. The efficiency canbe further improved by using soft decoding; therefore maximumlikelihood decoding methods for the OCDMA channels aredeveloped and demonstrate a significant reduction in the errorrate. Frequency hopping and direct sequencing are also comparedwith each other, and the results show that temporally codedOCDMA is more sensitive to beat noise.In addition, the performance of a low complexity softdecoding method for Reed-Solomon codes is evaluated. Softdecoding of Reed Solomon codes has not yet found practical usebecause the earlier proposed methods do not offer sufficientperformance gains to motivate the increased complexity. Thebit-level Chase-decoding algorithm evaluated here can be easilyimplemented using any algebraic decoder.