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We propose a "synchronous" wavelength-time optical code-division multiple access (CDMA) scheme using 2D Walsh-based bipolar-bipolar code, in which Walsh code sequences are used for time spreading, in place of Barker sequences used in our previously proposed ldquoasynchronousrdquo bipolar-bipolar code. The new synchronous coding scheme supports larger cardinality and better performance than the asynchronous scheme because Walsh code has a larger cardinality than Barker sequences and has a zero cross-correlational property at the expense of system synchronization. One potential application is future CDMA-based passive optical networks, in which synchronous downstream traffic uses the Walsh-based bipolar-bipolar code and asynchronous upstream traffic uses the Barker-based counterpart. Since the new code is modified from the original code, the same hardware can be used for both directions of traffic.