3,220 research outputs found
Approaching the ultimate capacity limit in deep-space optical communication
The information capacity of an optical channel under power constraints is
ultimately limited by the quantum nature of transmitted signals. We discuss
currently available and emerging photonic technologies whose combination can be
shown theoretically to enable nearly quantum-limited operation of a noisy
optical communication link in the photon-starved regime, with the information
rate scaling linearly in the detected signal power. The key ingredients are
quantum pulse gating to facilitate mode selectivity, photon-number-resolved
direct detection, and a photon-efficient high-order modulation format such as
pulse position modulation, frequency shift keying, or binary phase shift keyed
Hadamard words decoded optically using structured receivers.Comment: 9 pages, 4 figures. Presented at Free-Space Laser Communications
XXXI, 4-6 February 2019, San Francisco, C
A refined analysis of the Poisson channel in the high-photon-efficiency regime
We study the discrete-time Poisson channel under the constraint that its
average input power (in photons per channel use) must not exceed some constant
E. We consider the wideband, high-photon-efficiency extreme where E approaches
zero, and where the channel's "dark current" approaches zero proportionally
with E. Improving over a previously obtained first-order capacity
approximation, we derive a refined approximation, which includes the exact
characterization of the second-order term, as well as an asymptotic
characterization of the third-order term with respect to the dark current. We
also show that pulse-position modulation is nearly optimal in this regime.Comment: Revised version to appear in IEEE Transactions on Information Theor
MC-CDMA aided multi-user space-time shift keying in wideband channels
In this paper, we propose multi-carrier code division multiple access (MC-CDMA)-aided space-time shift keying (STSK) for mitigating the performance erosion of the classic STSK scheme in dispersive channels, while supporting multiple users. The codewords generated by the STSK scheme are appropriately spread in frequency-domain (FD) and transmitted over a number of parallel frequency-?at subchannels. We propose a new receiver architecture amalgamating the single-stream maximum-likelihood (ML) detector of the STSK system and the multiuser detector (MUD) of the MC-CDMA system. The performance of the proposed scheme is evaluated for transmission over frequency-selective channels in both uncoded and channel-coded scenarios. The results of our simulations demonstrate that the proposed scheme overcomes the channel impairments imposed by wideband channels and exhibits near-capacity performance in a channel-coded scenario
Numerical analysis of suction embedded plate anchors in structured clay
As offshore energy developments move towards deeper water, moored floating production facilities are increasingly preferred to fixed structures. Anchoring systems are therefore of great interest to engineers working on deep water developments. Suction embedded plate anchors (SEPLAs) are rapidly becoming a popular solution, possessing a more accurate and predictable installation process compared to traditional alternatives. In this paper, finite element analysis has been conducted to evaluate the ultimate pullout capacity of SEPLAs in a range of post-keying configurations. Previous numerical studies of anchor pullout capacity have generally treated the soil as an elastic-perfectly plastic medium. However, the mechanical behaviour of natural clays is affected by inter-particle bonding, or structure, which cannot be accounted for using simple elasto-plastic models. Here, an advanced constitutive model formulated within the kinematic hardening framework is used to accurately predict the degradation of structure as an anchor embedded in a natural soft clay deposit is loaded to its pullout capacity. In comparison with an idealised, non-softening clay, the degradation of clay structure due to plastic strains in the soil mass results in a lower pullout capacity factor, a quantity commonly used in design, and a more complex load–displacement relationship. It can be concluded that clay structure has an important effect on the pullout behaviour of plate anchors.Peer ReviewedPostprint (author's final draft
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