Abstract
In order to improve the performance of optically multiplexed multicarrier systems
with channel spacing equal to the symbol rate per carrier, we propose and systematically
investigate an electronic signal processing technique to achieve near-interchannel
crosstalk free and intersymbol-interference (ISI) free operation. We theoretically show
that achieving perfect orthogonality between channels in these systems, together with
ISI free operation as needed in generic communication systems, requires the shaping of
the spectral profiles of not only the demultiplexing filter, but also the signal of each
channel before demultiplexing. We develop a novel semianalytical method to
quantitatively analyze the levels of residual crosstalk and ISI arising from nonideal
system response in these systems. We show that by prefiltering the signal to ensure that
the system impulse response before channel demultiplexing approaches the targeted
condition, the residual crosstalk due to imperfect orthogonality can be significantly
mitigated and the necessity for carrier phase control in single-quadrature format-based
system can be relaxed. Further combining prefiltering and receiver-side postfiltering to
adaptively trim the demultiplexing filter enhances the performance. The use of the
combined digital signal processing (DSP) in coherent-detection quadrature phase-shifted
keying (QPSK)-based optically multiplexed multicarrier system shows that this method
outperforms conventional QPSK-based multicarrier system without DSP or with only
receiver-side DSP, especially when the responses of the transmitter and the
demultiplexing filter are not precisely designed and the sampling rate of the
analogue-to-digital converter is not sufficiently high. In addition, the inclusion of
ISI free operation, with this aspect similar to the reshaping method in conventional
wavelength-division-multiplexing systems, allows the relaxation of the modulation
bandwidth and chromatic dispersion compensation.
© 2010 IEEE
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