KEYWORDS: Signal to noise ratio, Forward error correction, Quadrature amplitude modulation, Digital signal processing, Receivers, Binary data, Modulation, Nonlinear filtering
We discuss modulation format design techniques used to approach Shannon capacity limit. The techniques include the use of multi-dimensional coded modulation with iterative decoding, probabilistic, and geometric constellation shaping. We also show how the use of coded modulation enables adaptive non-linear compensation techniques to further improve performance in nonlinear transmission systems.
With the exponential growth of global internet traffic, it becomes important to increase spectral efficiency for optical transmission systems in order to meet the challenge of continuing capacity growth. Polarization-division multiplexing (PDM) and multi-level quadrature-amplitude modulation (QAM) such as 16QAM allow spectral efficiencies beyond 4 bits/s/Hz. However, for long-haul optical transmission systems, performance is penalized by amplified spontaneous emission (ASE) noise and nonlinearity interference. Moreover, the decreased Euclidean distance between the signal constellation points in high-level QAM modulation formats reduces tolerance to phase noise and distortions. Such reduction in tolerance causes extra performance degradation and limits the transmission distance. In order to counteract this penalty, coded modulation can be used. In coded modulation, part of the redundancy that is usually assigned to forward-error correction (FEC) is moved into symbol modulation to increase the minimum Euclidean distance of a received sequence of symbols. Demodulation is done by a multi-symbol soft-decoder using either maximum likelihood or maximum a posteriori algorithm. In recent publications, LDPC-based bit-interleaved coded-modulation (BICM) with iterative soft-demapping and decoding is used to approach the capacity of AWGN channel, hence increase achievable transmission distance. We review different multidimensional single-parity check (SPC)-based BICM schemes suitable for high spectral efficiency transoceanic transmission systems, and their experimental verification.
In recent years, we have witnessed an increased demand on optical-networks transmission-capacities due to the growing
popularity of the Internet and multimedia in everyday life. According to industry expert estimates, 1Tb/s-Ethernet should
be standardized by the year 2012-2013. To this end, we propose a non-uniform modulation format that achieves the
channel capacity for SNRs of up to 25dB. The proposed modulation format is optimized for ASE-noise-dominated
channels and can achieve 400Gb/s data rate per polarization utilizing the currently-available components operating at
50-GSymbols/s. One major benefit of the current scheme is that it is an affordable upgrade to the current systems.
KEYWORDS: Digital signal processing, Orthogonal frequency division multiplexing, Modulation, Polarization, Signal detection, Receivers, Optical communications, Multiplexing, Signal processing, Optical fibers
We introduce two important technologies for development of next generation ultra-high-speed optical communications:
(i) polarization multiplexing, phase modulation with digital coherent detection, and (ii) OFDM-based optical fiber
transmission. In both schemes, digital signal processing plays a key role in recovering the signal and mitigating the
detrimental effects from optical signal transmission. We further describe a novel three dimensional low-density parity
check (LDPC) coded modulation scheme, including its principle and system performance.
In high bit-rate return-to-zero (RZ) on-off keying (OOK) systems operating at 40Gb/s and beyond ghost pulses induced by intrachannel four-wave mixing (IFWM), and intrachannel cross-phase modulation (IXPM) are dominant factors affecting the performance of a high-speed optical transmission system. As the creation of ghost pulses is a phase sensitive effect, the common approach to deal with IFWM is to remove short-term phase coherence by employing a proper modulation format. Another approach, known as constrained or line coding, is to identify the most troublesome sequences and to forbid them during encoding process. This paper presents an efficient method for testing the efficiency of a modulation techniques for countering the effects of IFWM by calculating the sum of all the contributors at a given resonance position. For a modulation technique to counter these effects the initial pulses phases must be chosen properly so that different contributors to the ghost pulse creation cancel each other in bit being in "resonance". In addition to this method, we are proposing a modulation technique that shows a significant Q-factor improvement of up to 7.5dB (depending on the number of spans and the phase sequence implemented) over the uncoded RZ-OOK as will be demonstrated. Finally, implementation of this phase modulation technique is very efficient in extending the non-linearity tolerance and enhancing the performance of a high-speed optical transmission system.
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