Digital coherent combining (DCC) technique can increase the free space optical signal collection area by combining the signals received by an array of small apertures in a coherent manner. To realize DCC the different versions of signals must be aligned in phase by the digital phase alignment algorithm (PAA). Low computation complexity is imperative for the PAA because the main obstacle to implement the PAA and DCC in a real-time manner is the availability of digital signal processing (DSP) circuits offering very high gate density and processing speed. In this paper we investigate the relationship between the computation complexity, optical phase offset estimation error and the combining loss for the equal gain combining technique. Analytical expressions are deduced allowing easy minimization of the computation complexity at an arbitrary input OSNR and acceptable combining loss. Extensive numerical simulations are carried out to validate the analytical expressions.
Urban multi-energy systems (UMESs) are integrated energy systems (IESs) which can alleviate the current energy crisis, improve energy utilization and realize multi-energy complementarity of modern. Various subsystems involved in the coupling of UMESs, including power grids, gas pipeline networks, cold/heat networks, transportation networks, and energy cyber-physical system, exhibit coupling characteristics such as multi-energy source modeling, complex uncertainty factor modeling, mutual influence of information and physical coupling. At present, the modeling and reliability assessments of UMESs are the most urgent tasks. In this paper, the latest research results on the reliability modeling and evaluation of UMESs are analyzed by considering their coupling components. In addition, the reliability modeling methods and the evaluation indexes of UMESs, including power-gas systems, power-thermal systems, powertraffic systems, and energy cyber-physical systems, are presented in this paper, with specific UMES modeling methods divided into model-driven modeling and data-driven modeling. Finally, future challenges for the reliability modeling and evaluation of UMESs are proposed, such as the dispatch strategy of the coupling components needs to be developed and continuously optimized to improve the reliability of UMESs, and the resilience of UMESs under the extreme scenarios needs to be further studied.INDEX TERMS Urban multi-energy system, reliability indexes, reliability modeling, coupling components, resilience.
In free space optical communication systems, multi-aperture coherent optical receivers based on digital coherent beam combining (D-CBC) technique can provide exceptionally high sensitivity and are more robust to the atmosphere turbulence compared with the single aperture receivers with the same collection area. D-CBC relies on the digital phase alignment algorithm (PAA) to align the different versions of signals in phase. However, due to the limited working frequency and space of the digital signal processing (DSP) circuits, the main obstacle to realizing real-time phase alignment of multiple high-speed optical signals is the computation complexity. Therefore, we need to minimize the computation complexity while guaranteeing a satisfactory performance. In this paper, we investigate the relationship between the computation complexity and the combining loss (CL) for both maximum ratio combining (MRC) and equal gain combining (EGC) based D-CBC. Universal analytical expressions are deduced that allow easy minimization of the computation complexity for both MRC and EGC based receivers according to the prescribed CL and input optical signal-to-noise ratios (OSNRs). The analytical expressions are validated by extensive numerical simulations. It is demonstrated that the computation complexity is mainly determined by the quality of the signal with a larger OSNR in MRC, while it is determined by the overall quality of the signals in EGC. When EGC is replaced with MRC, the computation complexity can be reduced by more than 55% at the same CL when the OSNR difference between the signals to be combined is above 10dB. The maximum computation complexity increases exponentially with decreasing input OSNR lower limit and the smaller the CL, the steeper the slope. Furthermore, when the prescribed CL is relaxed from 0.1 to 0.5dB, the maximum computation complexity can be reduced by about 80%. The results provide useful guidelines toward practical phase alignment on a real-time platform.INDEX TERMS Multi-aperture receiver, free-space optical communication, phase alignment, digital coherent combining, equal gain combing, maximum ratio combining, combining loss
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