Increasing the information rates of optical communications via coded modulation: a study of transceiver performance

Robert Maher, Alex Alvarado, Domaniç Lavery, Polina Bayvel
2016 Scientific Reports  
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,
more » ... n 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. Optical communications technology has demonstrated unprecedented development over the past three decades and now stands alone as the enabling technology that underpins the global information infrastructure. Over this period, the throughput (measured in bits per second) of lightwave communications systems increased from 100 Mb/s in 1970 to 10 Tb/s in present day commercial systems, which represents an astonishing 100,000-fold increase. The key technologies that fuelled this surge in capacity were wavelength division multiplexing (WDM), improved fibre types, optical amplification and coherent optical networking. Although the introduction of Erbium and Raman fibre amplifiers negated the need for electronic regenerators and enabled dense WDM transmission, the success and performance of these amplifier technologies has effectively constrained the useable fibre bandwidth (BW) to approximately 10-15 THz, which is now beginning to limit the maximum throughput of optical systems. Within this BW, the throughput can only be increased by reducing the spectral guard bands between WDM carriers, increasing the information rate (IR, measured in bits per symbol) of the modulation format or through new forward error correction (FEC) schemes that require lower redundancy without loss in performance. The combination of these techniques aim to increase the spectral efficiency (SE) of each WDM carrier. However, for Nyquist spaced WDM systems, increasing the SE is highly dependent on the IR, which inherently scales with received signal-to-noise ratio (SNR). Currently, the upper limit on the available SNR in a coherent optical transmission system, in the absence of fibre nonlinearity, is bounded by the transceiver subsystems. Coded Modulation (CM), which is the combination of multi-level modulation and FEC, is often coupled with spectral shaping to constrain the bandwidth of optical carriers and is a fundamental methodology applied to maximise the IR of a SNR-limited optical communications system. Figure 1(a) illustrates an example of a generic CM system. The CM transmitter (CM TX) can be seen as a non-binary encoder that maps a sequence of uniformly distributed information bits ( ) i of length N b to sequences of vectors ( ) x of length N s (The information bits are assumed to have already been passed through a source encoder for data compression.). Each vector of
doi:10.1038/srep21278 pmid:26864633 pmcid:PMC4750034 fatcat:xdvkqarb3naivmojm7pgs6h6ha