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Open AccessArticle

Virtual Space-Time DiversityTurbo Equalization Using Cluster Sparse Proportional Recursive Least Squares Algorithm for Underwater Acoustic Communications

by Zhen Han, Weiliang Tao, Dan Zhang and Peng Jiang

Appl. Sci. 2023, 13(19), 11050; https://doi.org/10.3390/app131911050 - 7 Oct 2023

Cited by 1 | Viewed by 1715

Abstract

The oceanic positioning, navigation and timing (PNT) network requires high-quality underwater acoustic message transmission. Turbo equalization technology has exhibited superior performance for underwater acoustic (UWA) communications compared with conventional channel equalizers. To overcome the performance reduction caused by severe doubly selective UWA channels, the virtual space-time diversity soft direct-adaptation turbo equalization is proposed for UWA communications. The proposed scheme improves the ability of the typical turbo equalizer to deal with both Doppler and multipath effects for time varying channels. We utilize a fractionally spaced soft interference cancellation equalizer (FS-SE) instead of a hard decision to constitute the soft-input soft-output (SISO) equalizer. Combined with another virtual time-reversal mirror equalizer component, we can obtain virtual space and time diversity with only a single receiving transducer and mitigate the error propagation phenomenon of the feedback filter. To satisfy the sparse UWA channel, the

ℓ_{p, q}

-PRLS algorithm is applied to adaptive updates for FS-SE. In the proposed scheme, an adjustable interpolator and digital phase-locked loop are embedded into the equalizer to overcome the residual Doppler frequency shift and recover the timing distortion. Results of simulations and field lake trial show that the proposed scheme achieves better performance than existing ones under the same equalizer order. Full article

(This article belongs to the Section Marine Science and Engineering)

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37 pages, 4365 KB

Open AccessEditor’s ChoiceArticle

Self-Organization in Cold Atoms Mediated by Diffractive Coupling

by Thorsten Ackemann, Guillaume Labeyrie, Giuseppe Baio, Ivor Krešić, Josh G. M. Walker, Adrian Costa Boquete, Paul Griffin, William J. Firth, Robin Kaiser, Gian-Luca Oppo and Gordon R. M. Robb

Atoms 2021, 9(3), 35; https://doi.org/10.3390/atoms9030035 - 23 Jun 2021

Cited by 12 | Viewed by 4970

Abstract

This article discusses self-organization in cold atoms via light-mediated interactions induced by feedback from a single retro-reflecting mirror. Diffractive dephasing between the pump beam and the spontaneous sidebands selects the lattice period. Spontaneous breaking of the rotational and translational symmetry occur in the 2D plane transverse to the pump. We elucidate how diffractive ripples couple sites on the self-induced atomic lattice. The nonlinear phase shift of the atomic cloud imprinted onto the optical beam is the parameter determining coupling strength. The interaction can be tailored to operate either on external degrees of freedom leading to atomic crystallization for thermal atoms and supersolids for a quantum degenerate gas, or on internal degrees of freedom like populations of the excited state or Zeeman sublevels. Using the light polarization degrees of freedom on the Poincaré sphere (helicity and polarization direction), specific irreducible tensor components of the atomic Zeeman states can be coupled leading to spontaneous magnetic ordering of states of dipolar and quadrupolar nature. The requirements for critical interaction strength are compared for the different situations. Connections and extensions to longitudinally pumped cavities, counterpropagating beam schemes and the CARL instability are discussed. Full article

(This article belongs to the Special Issue Collective Atomic and Free-Electron Lasing)

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