approach gives superior performance at the low fibre bandwidths as well as the high fibre bandwidths. At first sight, the latter may appear to be a surprising result; however, it should be noted that at the higher fibre bandwidths the received pulse is extremely narrow and so the output voltage is effectively the impulse response of the receiver. Due to the bandwidth of the noise-whitening filter being approximately n ϭ 10 9 rad/s, the output pulse is dispersed and spreads into adjacent time slots leading to ISI. With raised-cosine detection, the latter is eliminated and thus improved receiver sensitivity is achieved. At the higher fibre bandwidths, the raisedcosine approach predicts a sensitivity of Ϫ57.6 dBm, compared to Ϫ51.6 dBm for an equivalent PCM system (calculated using Personick's approach [5]), representing an improvement of 6 dB. Figure 3 shows the receiver sensitivity as a function of coding level M for both the prewhitened matched filter and the raisedcosine filter for a fixed normalised fibre bandwidth of f n ϭ 10. At low coding levels, the slot width, which is given by T s ϭ MT b / 2 M , is large in comparison to the pulse width, so there is no intersymbol interference and both systems exhibit similar performance. As the coding level increases, the slot width becomes narrower and the dispersive effect of the noise-whitening filter leads to ISI; thus, the sensitivity deteriorates. As can be seen, the raised-cosine filter offers greater immunity to ISI and also has the added advantage of being simpler to implement [6] than a filter matched to the optical pulse shape [2].
CONCLUSIONConsideration has been given to minimising intersymbol interference is optical fibre PPM systems by employing central decision detection rather than the conventional threshold-crossing detection technique. A 155.52-Mb/s PIN-FET system was modelled using (i) a prewhitened matched filter and (ii) a raised-cosine filter. It was shown that using central decision detection leads to improvements in excess of 4.5 dB at the lower fibre bandwidths and that moving to the raised-cosine filter offers further enhancements at both the lower and higher fibre bandwidths. The results for a raised-cosine system operating with n ϭ 32 give a sensitivity of Ϫ57.6 dBm, which represents an improvement of 6 dB over an equivalent PCM system. The electrical characteristics of the multilayer dielectric and multiconductor transmission line system are becoming more important factors, dominating the behavior of microwave integration circuits and very-large-scale integration circuits. This has increased the interest in the efficient algorithms for the electromagnetic parameter computation [1][2][3][4][5]. An overla pping domain decomposition method (ODDM) was presented in [6] to calculate the capacitance matrix. This method has achieved good accuracy with savings in memory and computing time. However, the overlapping regions between adjacent subdomains contribute significantly to overall computing time and memory needs. To overcome this problem, a non...
