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Distributed Processing for Multi-Relay Assisted OFDM with Index Modulation(IEEE, 2019-01-16)Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) has become a high-profile modulation scheme for the fifth generation (5G) wireless communications, and thus been extended to multi-hop scenarios in order to improve the network coverage and energy efficiency. However, the extension of OFDM-IM to multi-relay cooperative networks is not trivial, since it is required that a complete OFDM block should be received and decoded as an entity in one node. This requirement prevents the employment of multiple relays to forward fragmented OFDM blocks on individual subcarriers. In this regard, we propose a distributed processing scheme for multirelay assisted OFDM-IM, by which multiple relays are selected to forward signals in a per-subcarrier manner to provide optimal error performance for two-hop decode-and-forward (DF) OFDMIM systems. Specifically, a single selected relay only needs to decode partial information carried on certain active subcarriers and forward just as for traditional OFDM systems without IM. After receiving all signals on active subcarriers forwarded by different relays, the destination can reconstruct the complete OFDM block and retrieve the full information. We analyze the average block error rate (BLER) and modulation capacity of the two-hop OFDM-IM system employing the proposed distributed DF protocol and verify the analysis by numerical simulations.
OFDM with Index Modulation Assisted by Multiple Amplify-and-Forward Relays(IEEE, 2019-01-15)A multi-relay assisted orthogonal frequency division multiplexing (OFDM) system with index modulation (OFDM-IM) is proposed in this letter, assuming that amplify-and-forward (AF) relaying protocol is adopted at relays. Two commonly used AF protocols are considered: fixed-gain (FG) AF and variable-gain (VG) AF relaying protocols. To utilize relays in an efficient manner, we also employ two multi-carrier relay selection schemes termed bulk and per-subcarrier (PS) relay selection in the proposed system. We analyze the outage performance of the proposed system and derive closed-form expressions of the average outage probabilities (AOPs) for all cases with different AF relaying protocols and multi-carrier relay selection schemes. In addition, we obtain the asymptotic expressions of AOPs in the high signal-to-noise ratio (SNR) region, by which it is proven that the full diversity gain equaling the number of relays is attainable.
Projected Changes in Photosynthetic Picoplankton in a Warmer Subtropical Ocean(Frontiers Media SA, 2019-01-15)The oligotrophic subtropical gyres are the largest biome on Earth, where picoplankton constitute the dominant autotrophs. The trend for autotrophic picoplankton to increase with sea temperature has led to predictions that picophytoplankton abundance will increase with warming. Here we conducted a global survey in the open subtropical-tropical ocean to resolve the functional relationships between picophytoplankton abundance and oceanic properties (water temperature, chlorophyll a concentration, nutrient concentrations, and underwater visible and ultraviolet B radiation). We then used these relationships to build models projecting the future changes of Synechococcus, Prochlorococcus, and eukaryotic picoautotrophs populations in the subtropical gyres with warming. Our goal is to refine the forecasts for this large biome and implement the analysis by including the picoeukaryotes, absent in previous models, but a relevant component of picophytoplankton. The data obtained and the relationships found in our global survey of the subtropical-tropical ocean between picophytoplankton abundance and ocean properties differed from previous global studies including colder (temperate, subpolar, and polar) and coastal waters. These differences included a lower abundance of Synechococcus populations, significant negative relationships between Prochlorococcus abundance and nutrient concentrations, and positive relationships for picoeukaryotes and no relationship for Synechococcus abundance, and, a moderate response to temperature in the warm waters of the tropical-subtropical open ocean. A model based on temperature increase alone forecasts a general increase in picoautotrophs by year 2100, although minimal for picoeukaryotes, and much more moderate for Synechococcus than previously forecasted. However, a global change model linking the thermal increase with the associated decline in chlorophyll a, and increased underwater solar radiation penetration, projected a decline in the abundance of autotrophic picoplankton. The decline was larger at the surface layer and partially compensated by the increased importance of deep picophytoplankton blooms, especially those of Prochlorococcus. The global change model predicted an increased dominance of Prochlorococcus sp. in the subtropical-tropical ocean with future warming. Our results, based on current patterns of picophytoplankton distribution, help improve existing projections by considering feedbacks affecting picophytoplankton abundance in the future subtropical and tropical ocean, the larger biome on Earth.
Experimental and Numerical Investigation of the Response of a Swirled Flame to Flow Modulations in a Non-Adiabatic Combustor(Springer Nature, 2019-01-11)Turbulent combustion models for Large Eddy Simulation (LES) aims at predicting the flame dynamics. So far, they have been proven to predict correctly the mean flow and flame properties in a wide range of configurations. A way to challenge these models in unsteady situations is to test their ability to recover turbulent flames submitted to harmonic flow modulations. In this study, the Flame Transfer Function (FTF) of a CH4/H2/air premixed swirled-stabilized flame submitted to harmonic flowrate modulations in a non-adiabatic combustor is compared to the response computed using the Filtered TAbulated Chemistry for LES (F-TACLES) formalism. Phase averaged analysis of the perturbed flow field and flame response reveal that the velocity field determined with Particle Image Velocimetry measurements, the heat release distribution inferred from OH* images and the probability of presence of burnt gases deduced from OH-Planar Laser Induced Fluorescence measurements are qualitatively well reproduced by the simulations. However, noticeable differences between experiments and simulations are also observed in a narrow frequency range. A detailed close-up view of the flow field highlight differences in experimental OH* and numerical volumetric heat release rate distributions which are at the origin of the differences observed between the numerical and experimental FTF. These differences mainly originate from the outer shear layer of the swirling jet where a residual reaction layer takes place in the simulations which is absent in the experiments. Consequences for turbulent combustion modeling are suggested by examining the evolution of the perturbed flame brush envelope along the downstream distance of the perturbed flames. It is shown that changing the grid resolution and the flame subgrid scale wrinkling factor in these regions does not alter the numerical results. It is finally concluded that the combined effects of strain rate and enthalpy defect due to heat losses are the main factors leading to small but sizable differences of the flame response to coherent structures synchronized by the acoustic forcing in the outer shear layer of the swirling flow. These small differences in flame response lead in turn to a misprediction of the FTF at specific forcing frequencies.