Spectrally efficient transmit diversity scheme for differentially modulated multicarrier transmissions

Cyclic delay diversity is a simple, yet effective, transmit diversity scheme for multicarrier based transmissions employing coherent digital linear modulation schemes. It is shown that, for satisfactory operation, the scheme requires additional channel estimation overhead compared to single antenna and traditional space–time coded transmissions owing to the inherent increase in frequency selective fading. The authors analyse the additional channel estimation overhead requirement for a Hiperlan #2 style system with two transmit antennas operating in a NLOS indoor environment. The analysis shows that an additional overhead of 500% is required for the candidate system compared to a single antenna system. It is also shown that by employing differential modulation the channel estimation overhead can be eliminated with significant performance improvement compared to a system employing a practical channel estimation scheme. This novel combination, termed ‘differentially modulated cyclic delay diversity, is shown to yield a highly spectral efficient, yet simple transmit diversity solution for multi-carrier transmissions

Fluctuations and oscillations in a simple epidemic model

We show that the simplest stochastic epidemiological models with spatial correlations exhibit two types of oscillatory behaviour in the endemic phase. In a large parameter range, the oscillations are due to resonant amplification of stochastic fluctuations, a general mechanism first reported for predator-prey dynamics. In a narrow range of parameters that includes many infectious diseases which confer long lasting immunity the oscillations persist for infinite populations. This effect is apparent in simulations of the stochastic process in systems of variable size, and can be understood from the phase diagram of the deterministic pair approximation equations. The two mechanisms combined play a central role in explaining the ubiquity of oscillatory behaviour in real data and in simulation results of epidemic and other related models.Comment: acknowledgments added; a typo in the discussion that follows Eq. (3) is corrected

Freshwater balance and the sources of deep and bottom waters in the Arctic Ocean inferred from the distribution of H218O

Data from sections across the Eurasian Basin of the Arctic Ocean occupied in 1987 and 1991 are used to derive information on the freshwater balance of the Arctic Ocean and on sources of the deep waters of the Nansen, Amundsen and Makarov basins. Using salinity, H218O, and mass balances we estimate the river-runoff and the sea-ice melt water fractions contained in the upper waters of the Arctic Ocean and infer pathways of the river-runoff signal from the shelf seas across the central Arctic Ocean to Fram Strait. The average mean residence time of the river-runoff fraction contained in the Arctic Ocean halocline is determined to be about 11 to 14 years. Pacific water entering through Bering Strait is traced using silicate and its influence on the halocline waters of the Canadian Basin is estimated. Water column inventories of river-runoff and sea-ice melt water are calculated for a section just north of Fram Strait and implications of these inventories for sea-ice export through Fram Strait are discussed. Comparison of the ratios of shelf water, Atlantic water and the deep waters of the Arctic Ocean indicate that the sources of the deep and bottom waters of the Eurasian Basin are located in the Barents and Kara seas

Study on the wood anatomy, annual wood increment and intra-annual growth dynamics of Podocarpus oleifolius var. macrostachyus from Costa Rica

Tropical countries in the future will have an increasing demand for softwoods which favours mixed plantations possibly with minor portions of native conifer species. In this context numerous tropical species of the Podocarpaceae can be of ecological and economical interest. In Costa Rica the native species Podocarpus oleifolius var. macrostachyus (Parl.) Buchholz & Gray could be attractive for the establishment of manmade forests. However, profound knowledge on growth characteristics and wood properties is missing. In particular, information on the annual wood increment and intra-annual growth dynamics under natural site conditions at higher altitudes where P. oleifolius var. macrostachyus competes with hardwood species is not available and therefore the objective of this study. At the Cordillera de Talamanca (approx. 2,700 m a.s.l.), Costa Rica, an old-growth stand was chosen from which in total 5 trees (40 to 80 cm diameter at DBH) were sampled by taking stem sections and discs (2 trees)or increment cores (3 trees). During the period from October 1998 to December 2000, two trees of the site were pinned monthly for exact determination of the annual wood increment and the intra-annual growth dynamics in relationship to climate. The results of one of these pinned trees are demonstrated. It turned out that at high altitude the annual wood increment of old growth trees amounts to 1-2 mm (diameter) only. The tracheids show a rather constant cell wall thickness (2.5-4.5 μm) throughout the year, but a very variable radial cell diameter from 29 to 61 μm. The exact age of the tree cannot be determined anatomically, as there are no distinct tree-ring boundaries, but only very moderately developed terminal bands of flattened tracheids which do not circle the entire circumference of the stem. The monthly pin-labelling documents that during the dry season from about January until March in 1999 and 2000, virtually no cells were formed. However, with the beginning of the rainy season, about 59 % of the wood increment resulted from the months April to June. This increment rate already decreased from July to September to 32 %. Due to a distinct decrease in precipitation towards the end of the year, only 9 % of the wood increment were formed in the last quarter of the year, mainly consisting of the hardly visible terminal and flattened tracheids. The wood of this species is of very homogeneous structure and certainly attractive for highquality wood production

Dynamic parity recovery in a strongly driven Cooper-pair box

We study a superconducting charge qubit coupled to an intensive electromagnetic field and probe changes in the resonance frequency of the formed dressed states. At large driving strengths, exceeding the qubit energy-level splitting, this reveals the well known Landau-Zener-Stuckelberg (LZS) interference structure of a longitudinally driven two-level system. For even stronger drives we observe a significant change in the LZS pattern and contrast. We attribute this to photon-assisted quasiparticle tunneling in the qubit. This results in the recovery of the qubit parity, eliminating effects of quasiparticle poisoning and leads to an enhanced interferometric response. The interference pattern becomes robust to quasiparticle poisoning and has a good potential for accurate charge sensing.Comment: 5 pages, 4 figure

The Late Pleistocene history of the Pechora Sea

More reliable reconstructions of the Late Quaternary glacial history of the Pechora Sea have been carried out due to new radiocarbon datings. The bulk of evidence favors the view that complete deglaciation of the Pechora Sea occurred in the middle Valdai epoch, about 35-40 ka. After a short interstadial period with normal marine conditions, sea-level fall gave rise to establishment of continental environments. In the late Valdai, the Novaya Zemlya ice sheet occupied only the northernmost Pechora Sea and did not reach the Pechora Lowland. In the Course of the subsequent Holocene transgression, the shelf was abraded. Modern lithodynamic conditions in the Pechora Sea determine accumulation of sandy-silty deposits

Fast tuning of superconducting microwave cavities

Photons are fundamental excitations of the electromagnetic field and can be captured in cavities. For a given cavity with a certain size, the fundamental mode has a fixed frequency {\it f} which gives the photons a specific "color". The cavity also has a typical lifetime $\tau$, which results in a finite linewidth $\delta${\it f}. If the size of the cavity is changed fast compared to $\tau$, and so that the frequency change $\Delta${\it f} $\gg \delta${\it f}, then it is possible to change the "color" of the captured photons. Here we demonstrate superconducting microwave cavities, with tunable effective lengths. The tuning is obtained by varying a Josephson inductance at one end of the cavity. We show data on four different samples and demonstrate tuning by several hundred linewidths in a time $\Delta t \ll \tau$. Working in the few photon limit, we show that photons stored in the cavity at one frequency will leak out from the cavity with the new frequency after the detuning. The characteristics of the measured devices make them suitable for different applications such as dynamic coupling of qubits and parametric amplification.Comment: 2nd International Workshop on Solid-State Quantum Computing, June 2008, Taipei, Taiwa

Adaptability of the fine root system of Ceiba pentandra (L.) Gaertn. to various sites of central Amazônia, Brazil

Ceiba pentandra (L.) Gaertn. (sumaúma), an economically important tree species in central Amazônia, stands out on account of its considerable ecological capacity of adapting to different site conditions. It can survive under seasonally dry conditions on terra firme as well as in the seasonally inundated várzea. Particularly the fine roots can structurally adapt to different soil conditions. The adaptation of the fine roots to terra firme condition results in the formation of an aerenchyma consisting of only small intercellular canals. During the dry months the walls of the epidermis cells increase in thickness and moreover they lignify and the exodermis suberize to prevent drainage. On the other hand the fine roots growing on várzea sites form lysigenously an aerenchyma consisting of extended and large intercellulars both within the cortex and the central cylinder. This large intercellular system supplies the plant with water and mineral elements and maintains the oxygen supply enabling photosynthesis throughout the year. In addition, above the soil surface numerous adventitious roots grow during the inundation period developing only small intercellular spaces within the cortex. Suberization is missing and only the primary xylem lignifies.A simulation of the fine root study under similar growth conditions in a tropical greenhouse completely confirmed the structural adaptations of the fine roots to dry and seasonally inundated conditions. This ecological amplitude is important for the selection of Ceiba pentandra for sustainably managed forestry systems

Adaptability of the fine root system of Ceiba pentandra (L.) Gaertn. to various sites of central Amazônia, Brazil.

The objective of the following study of this commercially important species was to find out in how far dynamic adaptation of the growth and structure of the fine roots can contribute to that outstanding ecological amplitude. The structural study of the fine roots from trees from terra firme and várzea in different seasons in combination with a simulation of drought and inundation conditions in the greenhouse with young plants were expected to help explain the species extensive ecological amplitude