Abstract
Optical fibre underpins the global communications infrastructure and has experienced an astonishing evolution over the past four decades, with current commercial systems transmitting data rates in excess of 10 Tb/s over a single fibre core. The continuation of this dramatic growth in throughput has become constrained due to a power dependent nonlinear distortion arising from a phenomenon known as the Kerr effect. The mitigation of fibre nonlinearities is an area of intense research. However, even in the absence of nonlinear distortion, the practical limit on the transmission throughput of a single fibre core is dominated by the finite signal-to-noise ratio (SNR) afforded by current state-of-the-art coherent optical transceivers. Therefore, the key to maximising the number of information bits that can be reliably transmitted over a fibre channel hinges on the simultaneous optimisation of the modulation format and code rate, based on the SNR achieved at the receiver. In this work, we use an information theoretic approach based on the mutual information and the generalised mutual information to characterise a state-of-the-art dual polarisation m-ary quadrature amplitude modulation transceiver and subsequently apply this methodology to a 15-carrier super-channel to achieve the highest throughput (1.125 Tb/s) ever recorded using a single coherent receiver.
Highlights
Complex symbols belongs to the codebook, B = {x1, x2, ..., x2Nb }, where 2Nb is the number of possible transmitted messages
The mutual information (MI) between the transmitted and received symbols is a key quantity in information theory, as it represents the largest achievable IR (A rate is said to be achievable if there exist an encoder operating at that rate and a decoder giving a vanishing error probability as the block length tends to infinity.) for a CM system based on the optimal maximum likelihood (ML) sequence decoder
Experimental demonstrations have verified the use of MI to predict the post soft decision forward error correction (FEC) (SD-FEC) bit error ratio (BERpost) in optical systems based on the dual polarisation (DP) quadrature phase shift keyed (QPSK) format[7], while the MI has been used as a figure of merit for wavelength division multiplexing (WDM) transmission systems based on the m-ary quadrature amplitude modulation (QAM) format[8,9]
Summary
A more feasible (albeit sup-optimal) receiver is to use a simple two-step decoding process that decouples the decoder and demapper into two distinct processes The generalised mutual information (GMI), which is calculated using the coded bits and the LLRs, is an achievable IR for a BICM system and provides a more accurate prediction of the performance of commercial optical systems that employ bit-wise decoders. We show that the modulation format and code rate of a practical BICM optical system must be simultaneously optimised to match the specific SNR achieved after digital coherent detection This experimental characterisation shows that the industry standard DP-QPSK modulation format should only be employed in a transmission system if the received SNR is < 2.5 dB. It is shown that the variable rate LPDC scheme provides a 15.3% increase in throughput, achieving a net bit rate of 1.125 Tb/s, which is the largest throughput ever recorded using a single receiver
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