Stable Throughput and Delay Analysis of a Random Access Network With Queue-Aware Transmission

In this work we consider a two-user and a three-user slotted ALOHA network with multi-packet reception (MPR) capabilities. The nodes can adapt their transmission probabilities and their transmission parameters based on the status of the other nodes. Each user has external bursty arrivals that are stored in their infinite capacity queues. For the two- and the three-user cases we obtain the stability region of the system. For the two-user case we provide the conditions where the stability region is a convex set. We perform a detailed mathematical analysis in order to study the queueing delay by formulating two boundary value problems (a Dirichlet and a Riemann-Hilbert boundary value problem), the solution of which provides the generating function of the joint stationary probability distribution of the queue size at user nodes. Furthermore, for the two-user symmetric case with MPR we obtain a lower and an upper bound for the average delay without explicitly computing the generating function for the stationary joint queue length distribution. The bounds as it is seen in the numerical results appear to be tight. Explicit expressions for the average delay are obtained for the symmetrical model with capture effect which is a subclass of MPR models. We also provide the optimal transmission probability in closed form expression that minimizes the average delay in the symmetric capture case. Finally, we evaluate numerically the presented theoretical results.Comment: Submitted for journal publicatio

Performance and sustainability of short-rotation energy crops treated with municipal and industrial residues

The sustainability of short-rotation willow coppice (SRWC) as a multifunctional system for phytoremediation—the use of plants for treatment of contaminated air, soil or water—and for producing energy biomass, was studied. SRWC is grown commercially in Sweden to produce energy biomass, nutrient-rich residues being applied as cost-efficient fertiliser to increase production. The principal residues used are municipal wastewater, landfill leachate, industrial wastewater (e.g. log-yard runoff), sewage sludge and wood-ash. Small- and large-scale experiments with residues aimed to quantify the extent of potential hazards and the performance of SRWC in reducing them. Lysimeter experiments with willow plants, intensively irrigated with N-rich municipal wastewater, showed that N-leaching is a potential threat when high N loads are applied. Experimental data from SRWC fields irrigated with municipal wastewater in central Sweden suggest that in practice, N-leaching is significantly lower, even when the N load applied is greater than the N requirements of SRWC. Growth of willow plants of five different clones in pot experiments irrigated with landfill leachate was reduced by comparison with that of control plants. The reduction was attributed to saline stress or P deficiency, and indicates that, when hazardous compounds are present in wastewater, irrigation rates should be adjusted to avoid growth reduction. Genetic differences were observed between willow clones in salt tolerance and growth performance. The careful selection of clones to suit specific situations is therefore recommended. Leaf length can be used for rapid diagnosis of stress, to permit adjustment of the irrigation rate, and thus to avoid growth reduction. Phytoremedial efficiency of SRWC is satisfactory when the concentration of hazardous compounds in wastewaters is low, as in log-yard runoff, and depends on irrigation intensity. Application of sludge–ash mixtures to SRWC is not a substantial threat to sustainability, in terms of heavy metals. Total Cd in the soil is expected to decrease after harvest, but other metals and P loads, must also be considered. Any decision concerning residue application must take into account factors such as residue composition, soil type, climate, and species or clone characteristics, if both a high growth rate of SRWC and sustainability are to be attained

Impact of Populus Plantations on Water and Soil Quality

Trees of genus Populus (in our context primarily poplars) are predominantly grown in Sweden in small plantations on arable land in southern and central parts of the country to produce biomass for energy and other purposes. This study evaluated the effects (i) of poplar plantations on groundwater quality, by determining differences in leaching of nitrogen and phosphorus to groundwater, and (ii) of poplar and hybrid aspen plantations on soil quality in terms of carbon in the top- and subsoil. The study was conducted comparing Populus plantations in Sweden with adjacent fields with cereals and grasslands. The experiment concerning the groundwater leaching was conducted in eight poplar plantations along three growing seasons (2012–2015). For the soil carbon experiments, 19 poplar and two hybrid aspen plantations and the respective reference fields were sampled. NO3-N leaching from poplar plantations was significantly lower than that from reference fields with cereals, but not when compared with grasslands. Spring NO3-N leaching was significantly lower in poplars than in the reference fields, whereas leaching of NO3-N in autumn did not differ. Concentrations of PO4-P in the groundwater of poplar plantations were lower compared to the respective ones of the reference fields. There were no clear trends observed when comparing carbon concentrations in the topsoil of the poplar and hybrid aspen plantations compared to the respective adjacent reference fields. For the subsoil, the average carbon concentrations in the poplar and hybrid aspen plantations were equal to the respective ones of cereals, but were higher when compared to grassland.publishedVersio