145 research outputs found
Characterization and modeling of link loss for an outdoor free-space optics transmission system
In this paper we propose three low-complexity algorithms to estimate the time-varying loss of an outdoor 1550-nm free-space optics (FSO) link with 55-m transmission length. Longterm experimental measurements taken for different weather conditions demonstrate that the link loss can be predicted accurately while still using low-complexity algorithms
Wildfire patterns and landscape changes in Mediterranean oak woodlands
Fire is infrequent in the oak woodlands of southern Portugal (montado) but large and severe fires affected these
agro-forestry systems in 2003–2005. We hypothesised transition from forest to shrubland as a fire-driven process
and investigated the links between fire incidence and montado change to other land cover types, particularly
those related with the presence of pioneer communities (generically designed in this context as “transitions
to early-successional communities”). We present a landscape-scale framework for assessing the probability of
transition from montado to pioneer communities, considering three sets of explanatory variables: montado
patterns in 1990 and prior changes from montado to early-successional communities (occurred between 1960
and 1990), fire patterns, and spatial factors. These three sets of factors captured 78.2% of the observed variability
in the transitions from montado to pioneer vegetation. The contributions of fire patterns and spatial factors were
high, respectively 60.6% and 43.4%, the influence of montado patterns and former changes in montado being lower
(34.4%). The highest amount of explained variation in the occurrence of transitions from montado to earlysuccessional
communities was related to the pure effect of fire patterns (19.9%). Low spatial connectedness in
montado landscape can increase vulnerability to changes, namely to pioneer vegetation, but the observed changes
were mostly explained by fire characteristics and spatial factors. Among all metrics used to characterize fire patterns
and extent, effective mesh size provided the best modelling results. Transitions from montado to pioneer
communities are more likely in the presence of high values of the effective mesh size of total burned area. This cross-boundary metric is an indicator of the influence of large fires in the distribution of the identified transitions
and, therefore, we conclude that the occurrence of large fires inmontado increases its probability of transition to
shrubland
Experimental Demonstration of a Frequency-Domain Volterra Series Nonlinear Equalizer in Polarization-Multiplexed Transmission
Experimental demonstration of a dual-polarization Volterra series nonlinear equalizer applied in frequency-domain is carried out for 100G polarization-multiplexed QPSK test signals. We were able to reduce the BER by a factor of ~2.5x relatively to the single-polarization approach, with a 1 dB increase in the optimum power
Adaptive Stokes-Based Polarization Demultiplexing for Long-Haul Multi-Subcarrier Systems
We experimentally evaluate the performance of the adaptive Stokes polarization demultiplexing (PolDemux) algorithm over a long-haul optical link considering the propagation of 31 × 192 Gb/s channels modulated as PM-16QAM multisubcarrier (MSC) signals. Initially, we consider 1 × 24 Gbaud channel under test, and subsequently, we assess the performance of the algorithm on an increasing number of subcarriers (up to
12 × 2 Gbaud) while keeping the same aggregate symbol rate. Taking advantage of the higher robustness of MSC signals toward chromatic dispersion (CD), we demonstrate that the memoryless Stokes-based PolDemux algorithm, originally designed for short reach links, can also be used for low-complexity and modulation transparent polarization demultiplexing in long-haul systems. In addition, we demonstrate that the PolDemux rotation matrix for the MSC signals can be estimated over a restricted group of Nref subcarriers and seamlessly applied to all Nsc subcarriers, thereby significantly reducing the overall complexity by a factor of ∼Nsc/Nref
Adaptive probabilistic shaped modulation for high-capacity free-space optical links
Infrared free-space optics (FSO) provide an attractive solution for ultra-high-capacity wireless communications. However, the full potential of FSO is still being hindered by the apparent random fluctuations on the received optical power, which can be triggered by external factors such as atmospheric turbulence, weather instability, and pointing errors. Through the analysis of long-term experimental measurements, we identify the existence of significant time-domain memory in outdoor FSO links, which is found to be particularly strong under rainy weather conditions. Following this observation, we demonstrate that these memory effects can be effectively utilized to design accurate FSO channel estimation algorithms. Taking advantage of the arbitrary bit-rate granularity provided by probabilistic constellation shaping (PCS), and resorting to a simple moving average channel estimator, we demonstrate 400G+ transmission over a seamless fiber-FSO 55-m link with enhanced resilience towards adverse weather conditions. Comparing with unsupervised fixed modulation, we demonstrate a significant increase in average bit-rate (>35 Gbps) after continuous measurement over 3 hours, including raining periods
The Portuguese Large Wildfire Spread database (PT-FireSprd)
Wildfire behaviour depends on complex interactions between fuels,
topography, and weather over a wide range of scales, being important for
fire research and management applications. To allow for significant
progress towards better fire management, the operational and research
communities require detailed open data on observed wildfire behaviour. Here,
we present the Portuguese Large Wildfire Spread database (PT-FireSprd) that
includes the reconstruction of the spread of 80 large wildfires that
occurred in Portugal between 2015 and 2021. It includes a detailed set of
fire behaviour descriptors, such as rate of spread (ROS), fire growth rate
(FGR), and fire radiative energy (FRE). The wildfires were reconstructed by
converging evidence from complementary data sources, such as satellite
imagery and products, airborne and ground data collected by fire personnel, and
official fire data and information in external reports. We then implemented
a digraph-based algorithm to estimate the fire behaviour descriptors and
combined it with the Meteosat Second Generation (MSG) Spinning Enhanced Visible and Infrared Imager (SEVIRI) fire radiative power estimates. A total of 1197
ROS and FGR estimates were calculated along with 609 FRE estimates. The
extreme fires of 2017 were responsible for the maximum observed values of
ROS (8900 m h−1) and FGR (4400 ha h−1). Combining both descriptors, we describe
the fire behaviour distribution using six percentile intervals that can be
easily communicated to both research and management communities. Analysis of
the database showed that burned extent is mostly determined by FGR rather
than by ROS. Finally, we explored a practical example to show how the
PT-FireSprd database can be used to study the dynamics of individual
wildfires and to build robust case studies for training and capacity
building.
The PT-FireSprd is the first open-access fire progression and behaviour
database in Mediterranean Europe, dramatically expanding the extant
information. Updating the PT-FireSprd database will require a continuous
joint effort by researchers and fire personnel. PT-FireSprd data are
publicly available through https://doi.org/10.5281/zenodo.7495506 (Benali
et al., 2022) and have large potential to improve current
knowledge on wildfire behaviour and to support better decision making.</p
- …