The design of application-specific integrated circuits (ASIC) is at the core of modern ultra-high-speed transponders employing advanced digital signal processing (DSP) algorithms. This manuscript discusses the motivations for jointly utilizing transmission techniques such as probabilistic shaping and digital sub-carrier multiplexing in digital coherent optical transmissions systems. First, we describe the key-building blocks of modern high-speed DSP-based transponders working at up to 800G per wave. Second, we show the benefits of these transmission methods in terms of system level performance. Finally, we report, to the best of our knowledge, the first long-haul experimental transmission – e.g., over 1000 km – with a real-time 7 nm DSP ASIC and digital coherent optics (DCO) capable of data rates up to 1.6 Tb/s using two waves (2 × 800G).
Probabilistically-shaped constellations are found to have larger than expected Kerr nonlinearity induced penalty in long-haul transmission. In this paper, a novel nonlinear tolerant super-Gaussian distribution has been proposed which outperforms the Maxwell-Boltzmann distribution. The simulation and experimental results both show a significant improvement.
The state-of-the-art in realtime DSP for long haul coherent optical communication systems is described. We discuss how emerging applications for coherent transceivers may influence DSP design choices.
The information rate (IR) of a digital coherent transceiver is constrained by the inherent practical signal-to-noise ratio (SNR) limit. Coded modulation, which is the combination of multi-level modulation and forward error correction, aims to maximize the IR within this SNR envelope. While probabilistic constellation shaping has enhanced this methodology by providing an increase in IR over conventionally employed square quadrature amplitude modulation (QAM) formats, it is the ability to eloquently tune the per wavelength IR by varying the symbol probabilities that has gained this scheme significant traction within optical communications in recent years. As commercial line cards continue their evolution towards 100 GBd and to modulation formats beyond 64QAM, we discuss the merits of probabilistic shaping for high symbol rate digital coherent transceivers in the presence of a practical SNR limit.
A constant three-user interference channel wherein users are not symbol synchronous is considered. A novel practical interference alignment scheme for this channel is proposed wherein aligning interfering signals at each receiver is performed with the help of asynchronous delays which inherently exist among the received signals at each receiver node. Due to the existence of asynchronism among the users, the underlying quasi-static flat fading links are converted into frequency selective and accordingly into time varying channels with correlated coefficients across the time. This solves the lack of channel variation required for interference alignment in quasi-static scenarios. It is proved that the proposed scheme achieves the total degrees of freedom equal to 3/2 of this channel almost surely in the limit of codeword's length. Application of the proposed scheme is not limited to high signal to noise ratio (SNR) regime and it can be used in any range of SNR.
A constant K-user interference channel in which the users are not symbol-synchronous is considered. It is shown that the asynchronism among the users facilitates aligning interfering signals at each receiver node while it does not affect the total number of degrees of freedom (DoF) of the channel. To achieve the total K/2 DoF of the channel when single antenna nodes are used, a novel interference alignment scheme is proposed wherein the alignment task is performed with the help of asynchronous delays which inherently exist among the received signals at each receiver node. When each node is equipped with M >; 1 antennas, it is argued that the same alignment scheme is sufficient to achieve the total MK/2 DoF of the medium when all links between collocated antennas experience the same asynchronous delay. The proposed alignment scheme is also extended to achieve the total DoF of X networks at the presence of the asynchronism among the nodes.
We consider causal coding of three jointly Gaussian correlated sources, X 1 , X 2 , X 3 , with a given covariance matrix and determine an analytic closed-form formula for its total rate distortion function subject to Mean Square Error (MSE) distortion constraints when all sources need a positive rate to be represented. It is first shown that the optimal reproduction random variables are jointly Gaussian with the sources. A novel causal coding scheme is then proposed to achieve the total rate distortion function, in which each source is first whitened with respect to all previous original sources and then encoded via encoding a proper linear combination of the residues of the previous sources with respect to all available encoded sources and the residue of the current source with respect to all previous original sources. The more-and-less coding theorem in causal coding of correlated sources - when sources do not form a Markov chain as X 1 → X 2 → X 3 , under some conditions on sources and distortion, the more sources need to be encoded, the less total rate is required - is also investigated for Gaussian sources. For the underlying scenario in which all sources need a positive rate to be represented, it is proved that the more-and-less coding is always revealed for non-Markov chain Gaussian sources.
We report on the first real-time operation of coherent point-to-multipoint in high-speed fiber-optic communications. The broadcast and aggregation network consists of a 400 Gb/s hub transceiver achieving post-FEC error-free communication with 4 × 100 Gb/s leaf nodes, 5 – 50 km away.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)