Noise limits due to light mixing in optical code-division multiple-access systems. (1998)
AuthorsSmith, Elwyn D. J.show all
This thesis is concerned with the performance limits arising in optical code-division multiple-access (OCDMA) networks due to the mixing of light from the independent sources of each user. The scheme of spectral-amplitude OCDMA is shown here to be significantly limited by the phase-induced intensity noise arising from such mixing, and the corresponding signal-to- noise ratio and network capacity limits are of the same order as those arising in coherence-multiplexing systems, also due to such noise. Mixing can only occur between spatially coherent light, and this typically takes place through the combination of the signals into a single-mode fibre. The use of multimode fibre instead of single-mode can thus significantly reduce the levels and effects of phase-induced intensity noise, and this is experimentally demonstrated. It is shown that in general, assuming independent sources, there are only four possible ways in which to alleviate or eliminate noise limits of the form found for spectral-amplitude OCDMA. These are to separate the signals from each user in either the temporal, spectral, or spatial domains, or else to coherently despread the received signal. Neither spectral separation nor coherent de spreading are practical for spectral-amplitude OCDMA, but spatial separation via multimode fibre may be applied. The use of pulse-position modulation (PPM) with spectral-amplitude OCDMA is shown to be able to improve the performance beyond the limits found earlier, and this is because of the temporal separation it introduces. However, unlike the direct reduction of source duty cycle, PPM signalling can be applied without increasing, relative to the bit rate, the modulation or detection bandwidths, nor the dispersion sensitivity. Such PPM signalling and the associated decoding can also be applied to other similarly limited systems, including those based upon coherence multiplexing. If an OCDMA system is incoherent, has independent sources of the same spectrum, and has only a single-mode fibre to and from each user, then it can only avoid the significant noise limits found for spectral-amplitude OCDMA by the temporal separation of the signals from each user. This is the case for incoherent unipolar OCDMA systems, since the sparse codes of these systems rely on such temporal separation. Bipolar codes are not sparse, and in bipolar systems there is ordinarily no significant temporal separation between the signals from each user. Consequently, assuming sole single-mode-fibre paths and independent sources with identical spectra, every incoherent bipolar OCDMA system must encounter performance limits at least as bad as those found for spectral-amplitude OCDMA. These worst-case limits are identified for each of the incoherent bipolar OCDMA proposals to date.