Explicit Transmission Research Articles

Decentralized Limited‐Feedback Multiuser MIMO for Temporally Correlated Channels

This paper proposes a novel multiuser (MU) multiplexing scheme for temporally correlated multiple‐input, multiple‐output (MIMO) channels, suitable for systems employing low‐rate feedback links. A decentralized solution is obtained by the mobile receivers, which employ interference rejection combiner (IRC) linear filters and command the update of the corresponding per‐user antenna transmit weights through compact feedback messages, thus avoiding explicit transmission of channel information. The proposed limited‐feedback algorithm outperforms existing MU‐MIMO solutions employing quantized matrices, operating at the same feedback overhead. A compensation mechanism is presented, which enables the proposed solution to operate under moderate probabilities of feedback errors, at the expense of a small downlink overhead.

Age-prioritized use of antivirals during an influenza pandemic

BackgroundThe WHO suggested that governments stockpile, as part of preparations for the next influenza pandemic, sufficient influenza antiviral drugs to treat approximately 25% of their populations. Our aim is two-fold: first, since in many countries the antiviral stockpile is well below this level, we search for suboptimal strategies based on treatment provided only to an age-dependent fraction of cases. Second, since in some countries the stockpile exceeds the suggested minimum level, we search for optimal strategies for post-exposure prophylactic treatment of close contacts of cases.MethodsWe used a stochastic, spatially structured individual-based model, considering explicit transmission in households, schools and workplaces, to simulate the spatiotemporal spread of an influenza pandemic in Italy and to evaluate the efficacy of interventions based on age-prioritized use of antivirals.ResultsOur results show that the antiviral stockpile required for treatment of cases ranges from 10% to 35% of the population for R0 in 1.4 – 3. No suboptimal strategies, based on treatment provided to an age-dependent fraction of cases, were found able to remarkably reduce both clinical attack rate and antiviral drugs needs, though they can contribute to largely reduce the excess mortality. Treatment of all cases coupled with prophylaxis provided to younger individuals is the only intervention resulting in a significant reduction of the clinical attack rate and requiring a relatively small stockpile of antivirals.ConclusionOur results strongly suggest that governments stockpile sufficient influenza antiviral drugs to treat approximately 25% of their populations, under the assumption that R0 is not much larger than 2. In countries where the number of antiviral stockpiled exceeds the suggested minimum level, providing prophylaxis to younger individuals is an option that could be taken into account in preparedness plans. In countries where the number of antivirals stockpiled is well below 25% of the population, priority should be decided based on age-specific case fatality rates. However, late detection of cases (administration of antivirals 48 hours after the clinical onset of symptoms) dramatically affects the efficacy of both treatment and prophylaxis.

A multi-queue TCP window control scheme with dynamic buffer allocation

Abstract Explicit transmission control protocol (TCP) window control through the modification of the receiver’s advertised window in ACK packets is one of the ways intermediate network elements can contribute to the end-to-end TCP control. The TCP receiver’s advertised window, which indicates the level of the receive buffer of a TCP connection, limits the maximum window and consequently the throughput that can be achieved by the TCP sender. Thus, appropriate reduction of the advertised window by intermediate network elements can control the number of packets sent from a TCP sender. This paper describes a TCP window control scheme for a shared memory system with multiple queues. A dynamic buffer threshold, computed using a simple recursive algorithm, is used to dynamically allocate buffer space to the queues.

Simulating radiation distribution in a heterogeneous Norway spruce forest on a slope

Simulating the dynamics of heterogeneous forests calls for spatially explicit radiation transmission models at the scale of individual trees and requiring only a short computing time. Such a model was developed for spatially heterogeneous coniferous forest canopies. Based on the interception of light rays by parabolic crowns, it simultaneously calculates the radiation intercepted by each tree and the distribution of irradiance on the ground. For every sky direction, parallel rays aim at ground cell centres and are intercepted by tree crowns on their way. Slope and exposure are taken into account eliminating rays coming from above the slope. Optimisation in computing time is obtained by pre-selecting for each ray direction a rectangular cell neighbourhood around the target cell in such a way that potentially intercepting trees can only be found in this neighbourhood. Model evaluation was done by comparing hemispherical photographs with model predictions in a spatially heterogeneous Norway spruce ( Picea abies, L. Karst) stand at the upper montane level in the Alps. Thanks to a satisfactory fit between model and data and a reasonably short computing time (about 2 min for 0.25 ha), the model can be considered for integration into a forest dynamics simulator. In the reference stand, 42% of incident radiation was intercepted by the highest trees (132 ha −1). Due to the clumped structure, 23% of incident radiation reached the soil. Interception by individual trees varied considerably as a function of tree size and location (from 0 to 60% of the potential interception of a tree growing in isolation). Irradiance at ground level was also extremely variable (between 0 and 50% of irradiance above the canopy), illustrating the need for spatial models in heterogeneous stands. The interception of radiation by trees was not affected by slope and exposure, unlike irradiance at ground level, which increased for southern exposures. Therefore, in terms of dynamics, regeneration should be the most sensitive to changes in these factors. Because the model produces precise radiation values at the ecosystem level, it allows analysis of the links between the structure of a forest stand and its energetic functioning synthetically.

Measuring and modelling the crown and light transmission characteristics of juvenile aspen

Crown dimensions, crown leaf areas, and leaf area densities were determined on individual trees in 96 juvenile Populus tremuloides Michx. stands in central Alberta, Canada. Crown radius, height, leaf area, and leaf area density were well explained by stem diameter at 30 cm (D30). Leaf area index, estimated using the LAI-2000, reached a maximum of 4 m2·m–2 in some stands by age 9 and decreased after age 25. Leaf area indices for the same stands, estimated by allometric relationships, were greater than 7 m2·m–2. Horizontal overlap between adjacent tree crowns declined from 68 to 38% of crown width with increasing average D30 from 1.5 to 15.0 cm. These data were used to calibrate MIXLIGHT, a spatially explicit light transmission model, to predict the range of light conditions in the understory of juvenile stands. Light predictions were validated in 18 additional plots in which light transmission was measured. Measured light transmission ranged from 4 to 68% of above canopy light among sites and from 16 to 53% within a mapped site. MIXLIGHT predicted transmission well for average stand light levels (R2 = 0.80). For individual positions within the mapped stand there was a strong relationship between predicted and observed light (R2 = 0.92), but there was underprediction at high light and over at low light.

Faculty Roles in Professional Socialization

The purpose of this pilot project was to gain insight into the faculty perspectives on professional socialization in physical therapy. We explored the socialization of physical therapist students through the use of focus groups with faculty members and program directors. The study was bounded by, or limited within, the socializing experiences in the academic and the clinical settings in graduate education programs. Data collection occurred over a period of several weeks at conference settings. Faculty members described both implicit and explicit value transmission and believed they were actively engaged in the socializing process. The faculty roles in socializing students were similar to those described in other professions. They were responsible for establishing limits and transmitting the culture and values of the profession. Students were encouraged to remain humanistic, develop collaborative work habits, and respect individuality. Student efforts to attain autonomy were reinforced by the faculty efforts to implement adult education principles and encourage self-discovery. Most faculty members believed that they exerted a positive influence as students entered the profession.

Narratives and explanations during mealtime conversations in Norway and the U.S.

Mealtimes reveal culturally specific ways of talking, and constitute opportunities for socialization of children into those ways. In 22 Norwegian families and 22 American families, matched for age and gender of preschoolaged child and for participant constellation, mealtimes were examined for the occurrence and type of narrative and explanatory talk. All indices suggested that the Norwegian families produced more narrative talk – in particular, talk about minor deviations from social scripts – whereas the American families produced more explanatory talk, particularly talk focused on explanations for physical events or for individual behaviors. When Norwegian families gave explanations, they were likely to be focused on social norms and deviations from them, like their narratives. The results are interpreted in relation to the Norwegian cultural values of mitigated collectivism, egalitarianism, homogeneity, and implicit social rules, in contrast to American values of individualism, diversity, and explicit formulation and transmission of civic values. (Socialization, culture, narrative, explanation, family, mealtimes, Norway, USA.)

Narratives and explanations during mealtime conversations in Norway and the U.S.

ABSTRACTMealtimes reveal culturally specific ways of talking, and constitute opportunities for socialization of children into those ways. In 22 Norwegian families and 22 American families, matched for age and gender of preschoolaged child and for participant constellation, mealtimes were examined for the occurrence and type of narrative and explanatory talk. All indices suggested that the Norwegian families produced more narrative talk – in particular, talk about minor deviations from social scripts – whereas the American families produced more explanatory talk, particularly talk focused on explanations for physical events or for individual behaviors. When Norwegian families gave explanations, they were likely to be focused on social norms and deviations from them, like their narratives. The results are interpreted in relation to the Norwegian cultural values of mitigated collectivism, egalitarianism, homogeneity, and implicit social rules, in contrast to American values of individualism, diversity, and explicit formulation and transmission of civic values. (Socialization, culture, narrative, explanation, family, mealtimes, Norway, USA.)

The low-frequency scattering of Kelvin waves by stepped topography

A straightforward method that yields explicit transmission amplitudes is presented for Kelvin wave scattering by topography whose isobaths are parallel sufficiently far from the vertical, but not necessarily planar, wall supporting the incident wave. These results are obtained by first restricting attention to the low-frequency limit in which the flow splits naturally into three regions: an outer- x region containing the incident and transmitted Kelvin waves, an outer- y region containing outwardly propagating long topographic waves and an inner quasi-steady geostrophic region whose structure follows from earlier time-dependent analyses. The present analysis is further simplified by approximating general smooth features by stepped profiles with no restriction on the size, number or order of steps. Various qualitative results on the transmission amplitudes and flow fields are deduced from the explicit solutions and results are given on orthogonality, completeness and direction of propagation of the scattered long waves.

Data Compression Using Adaptive Coding and Partial String Matching

The recently developed technique of arithmetic coding, in conjunction with a Markov model of the source, is a powerful method of data compression in situations where a linear treatment is inappropriate. Adaptive coding allows the model to be constructed dynamically by both encoder and decoder during the course of the transmission, and has been shown to incur a smaller coding overhead than explicit transmission of the model's statistics. But there is a basic conflict between the desire to use high-order Markov models and the need to have them formed quickly as the initial part of the message is sent. This paper describes how the conflict can be resolved with partial string matching, and reports experimental results which show that mixed-case English text can be coded in as little as 2.2 bits/ character with no prior knowledge of the source.

An investigation of delta modulation of speech for mobile radio

A characteristic of a mobile radio channel is the occurrence of correlated signal fading that results in burst errors. The use of adaptive delta modulation (ADM) based on explicit transmission of the quantizer step size was proposed earlier for speech communication over such a channel. Two other variable step-size delta modulation (VSDM) schemes are presented, and their performance in a mobile radio environment is discussed. One of them is the constant factor delta modulation that uses one-bit memory and produces fast and instantaneous step-size changes. The other is the digitally controlled delta modulation (DCDM) that incorporates a new step-size adaptation strategy using seven bits of memory. In some cases, bit scrambling has been used. This is equivalent to scrambling (spreading out in time) the, clustered errors. Computer simulations providing values of coder parameters for satisfactory signal-to-noise ratios for band-limited speech signals and Gaussian noise are described. New hardware realizations are given that allow those parameters to vary smoothly for a wide range of sampling frequencies. Results of informal listening tests obtained with a mobile radio channel simulator are included. It is shown that for mobile radio, DCDM, as expected, is the better of the two coders. This is because it does not sacrifice its overload performance for the sake of error resistance.

Statistical Block Protection Coding for DPCM-AQF Speech

The statistical block protection coding scheme [2] for protecting DPCM encoded speech signals through noisy channels has been extended to accommodate DPCM-AQF encoded speech, where AQF stands for adaptive quantization with forward (explicit) transmission of step size. Signal-to-noise ratio (SNR) gains, typically 12 dB, have been achieved over a dynamic range <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt; 20</tex> dB for a bit error rate (BER) of 1.4 percent and the SNR improvement is found to increase with BER. Perceptual improvements in decoded speech have a good correspondence with the gains in SNR. The penalty for this substantial enhancement in the performance of the DPCM-AQF system is an increase in transmission bit rate of 3.5 percent and encoding delay of 64 ms.

Step-Size Transmitting Differential Coders for Mobile Telephony

In using digital speech for mobile radio, we encounter the problem of severe bit-error bursts. Error clustering occurs because the bit duration is typically much smaller than that of a signal “fade,” and average bit-error probabilities greater than 1 percent are not uncommon. For speech communication over such channels, this paper proposes variable step-size differential coders based on explicit (and error-protected) transmission of quantizer step size. Specifically, we discuss delta and dpcm coders to be referred to as dm-aqf and dpcm-aqf, where aqf stands for adaptive quantization with forward estimation (and transmission) of step size. (Backward estimation, based on quantized-signal history, has the nice feature that the step-size information does not have to be explicitly transmitted. Furthermore, obtaining this information does not entail any encoding delay. However, due to the dependence of step size on reconstructed signal history, backward estimation is often less reliable in the presence of bit errors than a scheme based on aqf.) The studies reported in this paper cover the problem of step-size determination in aqf, the design of time-invariant first-order predictors for dpcm-aqf, and the performances of aqf encoders with and without burst-error-protecting ploys such as redundant time-diversity coding and bit scrambling. Judging from snr figures and informal listening tests, interesting results are obtained with the following 48-kb/s coders: three-bit dpcm-aqf with redundant error protection, and dm-aqf using bit scrambling.

Effects of Channel Errors on the Signal-to-Noise Performance of Speech-Encoding Systems

The signal-to-noise ratios of different speech-encoding schemes have been measured in the case where the channel contains errors. Those types and probabilities of errors have been considered that are of interest for mobile telephone applications. Most encoding schemes use an adaptive three-bit quantizer with an explicit transmission of the step-size information. A scheme with an adaptive prediction algorithm has also been studied. It has been assumed in all cases that the side information about the quantizer step size and the predictor coefficients is transmitted in an error-protected format. Measurements were made by simulating the coding schemes and the noisy channel on a digital computer. The results include upper bounds of the improvements that can be reached with error protection of the most significant bits.

Signal Coding Using Asynchronous Delta Modulation

A description is given for a delta modulation system wherein the sampling interval is varied as a function of the characteristics of the signal to be coded. The system thus operates asynchronously as compared with the usual delta modulator, and permits the transmission of fewer samples during periods of small signal variation. The algorithm used for sampling interval variation is common to both transmitting and receiving ends of the system, and so signal reconstitution at the receiver is possible without the explicit transmission of interval information. Some results are presented in the form of line printer generated images.