KEYWORDS: Modulation, Oscillators, Signal to noise ratio, Receivers, Digital signal processing, Polarization, Signal detection, Sensors, Quadrature amplitude modulation, Wavelength division multiplexing
To meet the growing demands of data traffic in long haul communication, it is necessary to efficiently use the
low-loss region(C-band) of the optical spectrum, by increasing the no. of optical channels and increasing the
bit rate on each channel .But narrow pulses occupy higher spectral bandwidth. To circumvent this problem,
higher order modulation schemes such as QPSK and QAM can be used to modulate the bits, which increases
the spectral efficiency without demanding any extra spectral bandwidth. On the receiver side, to meet a satisfy,
a given BER, the received optical signal requires to have minimum OSNR. In our study in this paper, we
analyses for different modulation schemes, the OSNR required with and without preamplifier. The theoretical
limit of OSNR requirement for a modulation scheme is compared for a given link length by varying the local
oscillator (LO) power. Our analysis shows that as we increase the local oscillator (LO) power, the OSNR
requirement decreases for a given BER. Also a combination of preamplifier and local oscillator (LO) gives the
OSNR closest to theoretical limit.
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