Spatial diversity and combination technology using amplitude and phase weighting method for phase-coherent underwater acoustic communications

The UWA channel is characterized as a time-dispersive rapidly fading channel, which in addition exhibits Doppler instabilities and limited bandwidth. To eliminate inter- symbol interference caused by multipath propagation, spatial diversity equalization is the main technical means. The paper combines the passive phase conjugation and spatial processing to maximize the output array gain. It uses signal-to-noise-plus-interference to evaluate the quality of signals received at different channels. The amplitude of signal is weighted using Sigmoid function. Second order PLL can trace the phase variation caused by channel, so the signal can be accumulated in the same phase. The signals received at different channels need to be normal- ized. It adopts fractional-decision feedback diversity equalizer （FDFDE） and achieves diversity equalization by using different channel weighted coefficients. The simulation and lake trial data processing results show that, the optimized diversity receiving equalization algorithm can im- prove communication system＇s ability in tracking the change of underwater acoustic channel, offset the impact of multipath and noise and improve the performance of communication system. The performance of the communication receiving system is better than that of the equal gain combination. At the same time, the bit error rate （BER） reduces 1.8%.

Variational Approach for the Bose-Hubbard Model

The phase diagram of the one-dimensional Bose-Hubbard model describing interacting bosons in optical lattice is investigated with the variational approach. This method can also be generalized to the two-dimensional case.

Generation of visible Raman operation laser by a fiber electro-optical modulator feedback loop

Phase-coherent multi-tone lasers play a critical role in atomic,molecular,and optical physics.Among them,the Raman opeartion laser for manipulating atomic hyperfine qubits requires gigahertz bandwidth and low phase noise to retain long-term coherence.Raman operation lasers generated by directly modulated and frequency-multipled infrared lasers are compact and stable but lack feedback control to actively suppress the phase noise,which limits their performance in practical applications.In this work,we employ a fiber electro-optical modulator driven by a voltage-controlled oscillator(VCO)to modulate a monochromatic laser and employ a second-harmonic generation process to convert it to the visible domain,where the beat note of the Raman operation laser is stabilized by controlling the output frequency of VCO with a digital phase-locked loop(PLL).The low-frequency phase noise is effectively suppressed compared to the scheme without active feedback and it reaches-80 d Bc/Hz@5 k Hz with a 20 k Hz loop bandwidth.Furthermore,this compact and robust scheme effectively reduces the system's complexity and cost,which is promising for extensive application in atomic,molecular,and optical physics.