Browsing by Subject "Location"
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Item Open Access Accurate positioning in ultra-wideband systems(Institute of Electrical and Electronics Engineers, 2011) Soganci, H.; Gezici, SinanAccurate positioning systems can be realized via ultra-wideband signals due to their high time resolution. In this article, position estimation is studied for UWB systems. After a brief introduction to UWB signals and their positioning applications, two-step positioning systems are investigated from a UWB perspective. It is observed that time-based positioning is well suited for UWB systems. Then time-based UWB ranging is studied in detail, and the main challenges, theoretical limits, and range estimation algorithms are presented. Performance of some practical time-based ranging algorithms is investigated and compared against the maximum likelihood estimator and the theoretical limits. The trade-off between complexity and accuracy is observed.Item Open Access Allocation Strategies in Hub Networks(Elsevier, 2011-06-11) Yaman, H.In this paper, we study allocation strategies and their effects on total routing costs in hub networks. Given a set of nodes with pairwise traffic demands, the p-hub median problem is the problem of choosing p nodes as hub locations and routing traffic through these hubs at minimum cost. This problem has two versions; in single allocation problems, each node can send and receive traffic through a single hub, whereas in multiple allocation problems, there is no such restriction and a node may send and receive its traffic through all p hubs. This results in high fixed costs and complicated networks. In this study, we introduce the r-allocation p-hub median problem, where each node can be connected to at most r hubs. This new problem generalizes the two versions of the p-hub median problem. We derive mixed-integer programming formulations for this problem and perform a computational study using well-known datasets. For these datasets, we conclude that single allocation solutions are considerably more expensive than multiple allocation solutions, but significant savings can be achieved by allowing nodes to be allocated to two or three hubs rather than one. We also present models for variations of this problem with service quality considerations, flow thresholds, and non-stop service.Item Open Access Applied Materials uses operations research to design its service and parts network(Institute for Operations Research and the Management Sciences (INFORMS), 2010) Şen, A.; Bhatia, D.; Doǧan, K.Applied Materials Inc. is the global leader in nanomanufacturing technology solutions. It has a broad portfolio of innovative equipment, service, and software products and supports its customers worldwide with an extensive service and parts network with more than 100 locations. At the end of 2006, Applied Materials decided to evaluate and rationalize the design of its North American network. It set up a detailed optimization model (including 50,000 parts) to develop a network and distribution strategy. To our knowledge, this is the first large-scale multiechelon network-design model that incorporates safety stock inventory costs while considering the effects of lead time and risk pooling. The company used the model's recommendations to reduce costs while maintaining or improving its service to customers. The recommendations included simplifying the distribution network by consolidating depot locations for specific customers and skipping an echelon for others, thus leading to a projected inventory reduction of $10 million. The company is currently implementing these recommendations and has already eliminated five depots. Applied Materials estimates that during the first year of implementation, inventory reductions of $5.24 million and total savings of $1.1 million can be attributed to these network changes.Item Open Access A Benders decomposition approach for the charging station location problem with plug-in hybrid electric vehicles(Elsevier, 2016) Arslan, O.; Karaşan, O. E.The flow refueling location problem (FRLP) locates p stations in order to maximize the flow volume that can be accommodated in a road network respecting the range limitations of the vehicles. This paper introduces the charging station location problem with plug-in hybrid electric vehicles (CSLP-PHEV) as a generalization of the FRLP. We consider not only the electric vehicles but also the plug-in hybrid electric vehicles when locating the stations. Furthermore, we accommodate multiple types of these vehicles with different ranges. Our objective is to maximize the vehicle-miles-traveled using electricity and thereby minimize the total cost of transportation under the existing cost structure between electricity and gasoline. This is also indirectly equivalent to maximizing the environmental benefits. We present an arc-cover formulation and a Benders decomposition algorithm as exact solution methodologies to solve the CSLP-PHEV. The decomposition algorithm is accelerated using Pareto-optimal cut generation schemes. The structure of the formulation allows us to construct the subproblem solutions, dual solutions and nondominated Pareto-optimal cuts as closed form expressions without having to solve any linear programs. This increases the efficiency of the decomposition algorithm by orders of magnitude and the results of the computational studies show that the proposed algorithm both accelerates the solution process and effectively handles instances of realistic size for both CSLP-PHEV and FRLP.Item Open Access A bilevel uncapacitated location/pricing problem with Hotelling access costs in one-dimensional space(International Conference on Information Systems, Logistics and Supply Chain, 2016) Arbib, C.; Pınar, Mustafa Ç.; Tonelli, M.We formulate a spatial pricing problem as bilevel non-capacitated location: a leader first decides which facilities to open and sets service prices taking competing offers into account; then, customers make individual decisions minimizing individual costs that include access charges in the spirit of Hotelling. Both leader and customers are assumed to be risk-neutral. For non-metric costs (i.e., when access costs do not satisfy the triangle inequality), the problem is NP-hard even if facilities can be opened at no fixed cost. We describe an algorithm for solving the Euclidean 1-dimensional case (i.e., with access cost defined by the Euclidean norm on a line) with fixed opening costs and a single competing facility.Item Open Access A branch and price approach for routing and refueling station location model(Elsevier, 2016) Yıldız, B.; Arslan, O.; Karaşan, O. E.The deviation flow refueling location problem is to locate p refueling stations in order to maximize the flow volume that can be refueled respecting the range limitations of the alternative fuel vehicles and the shortest path deviation tolerances of the drivers. We first provide an enhanced compact model based on a combination of existing models in the literature for this relatively new operations research problem. We then extend this problem and introduce the refueling station location problem which adds the routing aspect of the individual drivers. Our proposed branch and price algorithm relaxes the simple path assumption generally adopted in the existing studies and implicitly takes into account deviation tolerances without the pregeneration of the routes. Therefore, the decrease in solution times with respect to existing models is significant and our algorithm scales very efficiently to more realistic network dimensions.Item Open Access A branch-and-cut algorithm for the alternative fuel refueling station location problem with routing(INFORMS, 2019) Arslan, O.; Karaşan, Oya Ekin; Mahjoub, A. R.; Yaman, HandeBecause of the limited range of alternative fuel vehicles (AFVs) and the sparsity of the available alternative refueling stations (AFSs), AFV drivers cooperatively deviate from their paths to refuel. This deviation is bounded by the drivers’ tolerance. Taking this behavior into account, the refueling station location problem with routing (RSLP-R) is defined as maximizing the AFV flow that can be accommodated in a road network by locating a given number of AFSs while respecting the range limitation of the vehicles and the deviation tolerance of the drivers. In this study, we develop a natural model for the RSLP-R based on the notion of length-bounded cuts, analyze the polyhedral properties of this model, and develop a branch-and-cut algorithm as an exact solution approach. Extensive computational experiments show that the algorithm significantly improves the solution times with respect to previously developed exact solution methods and extends the size of the instances solved to optimality. Using our methodology, we investigate the tradeoffs between covered vehicle flow and deviation tolerance of the drivers and present insights on deviation characteristics of drivers in a case study in California.Item Open Access A branch-and-price algorithm for the vehicle routing problem with roaming delivery locations(Elsevier Ltd, 2017) Ozbaygin G.; Ekin Karasan O.; Savelsbergh M.; Yaman, H.We study the vehicle routing problem with roaming delivery locations in which the goal is to find a least-cost set of delivery routes for a fleet of capacitated vehicles and in which a customer order has to be delivered to the trunk of the customer's car during the time that the car is parked at one of the locations in the (known) customer's travel itinerary. We formulate the problem as a set-covering problem and develop a branch-and-price algorithm for its solution. The algorithm can also be used for solving a more general variant in which a hybrid delivery strategy is considered that allows a delivery to either a customer's home or to the trunk of the customer's car. We evaluate the effectiveness of the many algorithmic features incorporated in the algorithm in an extensive computational study and analyze the benefits of these innovative delivery strategies. The computational results show that employing the hybrid delivery strategy results in average cost savings of nearly 20% for the instances in our test set. © 2017 Elsevier LtdItem Open Access A capacitated hub location problem under hose demand uncertainty(Elsevier, 2017) Meraklı, M.; Yaman, H.In this study, we consider a capacitated multiple allocation hub location problem with hose demand uncertainty. Since the routing cost is a function of demand and capacity constraints are imposed on hubs, demand uncertainty has an impact on both the total cost and the feasibility of the solutions. We present a mathematical formulation of the problem and devise two different Benders decomposition algorithms. We develop an algorithm to solve the dual subproblem using complementary slackness. In our computational experiments, we test the efficiency of our approaches and we analyze the effects of uncertainty. The results show that we obtain robust solutions with significant cost savings by incorporating uncertainty into our problem.Item Open Access Clean water network design for refugee camps(Springer, 2021-03) Karsu, Özlem; Yetiş Kara, Bahar; Akkaya, Elif; Ozel, AysuMotivated by the recent rise in need for refugee camps, we address one of the key infrastructural problems in the establishment process: The clean water network design problem. We formulate the problem as a biobjective integer programming problem and determine the locations of the water source, water distribution units and the overall network design (pipelines), considering the objectives of minimizing cost (total network length) and maximizing accessibility (total walking distance) simultaneously. We solve the resulting model using exact and heuristic approaches that find the set (or a subset) of Pareto solutions and a set of approximate Pareto solutions, respectively. We demonstrate the applicability of our approach on a real-life problem in Gaziantep refugee camp and provide a detailed comparison of the solution approaches. The novel biobjective approach we propose will help the decision makers to make more informed design decisions in refugee camps, considering the trade-off between the two key criteria of cost and accessibility.Item Open Access Competitive location and pricing on a line with metric transportation costs(Elsevier, 2020-04-01) Arbib, C.; Pınar, Mustafa Ç.; Tonelli, M.Consider a three-level non-capacitated location/pricing problem: a firm first decides which facilities to open, out of a finite set of candidate sites, and sets service prices with the aim of revenue maximization; then a second firm makes the same decisions after checking competing offers; finally, customers make individual decisions trying to minimize costs that include both purchase and transportation. A restricted two-level problem can be defined to model an optimal reaction of the second firm to known decision of the first. For non-metric costs, the two-level problem corresponds to Envy-free Pricing or to a special Net- work Pricing problem, and is APX -complete even if facilities can be opened at no fixed cost. Our focus is on the metric 1-dimensional case, a model where customers are distributed on a main communica- tion road and transportation cost is proportional to distance. We describe polynomial-time algorithms that solve two- and three-level problems with opening costs and single 1 st level facility. Quite surpris- ingly, however, even the two-level problem with no opening costs becomes N P -hard when two 1 st level facilities are considered.Item Open Access A distance-limited continuous location-allocation problem for spatial planning of decentralized systems(Elsevier, 2017) Gokbayrak, K.; Kocaman, A. S.We introduce a new continuous location-allocation problem where the facilities have both a fixed opening cost and a coverage distance limitation. The problem has wide applications especially in the spatial planning of water and/or energy access networks where the coverage distance might be associated with the physical loss constraints. We formulate a mixed integer quadratically constrained problem (MIQCP) under the Euclidean distance setting and present a three-stage heuristic algorithm for its solution: In the first stage, we solve a planar set covering problem (PSCP) under the distance limitation. In the second stage, we solve a discrete version of the proposed problem where the set of candidate locations for the facilities is formed by the union of the set of demand points and the set of locations in the PSCP solution. Finally, in the third stage, we apply a modified Weiszfeld's algorithm with projections that we propose to incorporate the coverage distance component of our problem for fine-tuning the discrete space solutions in the continuous space. We perform numerical experiments on three example data sets from the literature to demonstrate the performance of the suggested heuristic method.Item Open Access Exact and heuristic approaches based on noninterfering transmissions for joint gateway selection, time slot allocation, routing and power control for wireless mesh networks(Elsevier, 2017) Gokbayrak, K.; Yıldırım, E. A.Wireless mesh networks (WMNs) provide cost-effective alternatives for extending wireless communication over larger geographical areas. In this paper, given a WMN with its nodes and possible wireless links, we consider the problem of gateway node selection for connecting the network to the Internet along with operational problems such as routing, wireless transmission capacity allocation, and transmission power control for efficient use of wired and wireless resources. Under the assumption that each node of the WMN has a fixed traffic rate, our goal is to allocate capacities to the nodes in proportion to their traffic rates so as to maximize the minimum capacity-to-demand ratio, referred to as the service level. We adopt a time division multiple access (TDMA) scheme, in which a time frame on the same frequency channel is divided into several time slots and each node can transmit in one or more time slots. We propose two mixed integer linear programming formulations. The first formulation, which is based on individual transmissions in each time slot, is a straightforward extension of a previous formulation developed by the authors for a related problem under a different set of assumptions. The alternative formulation, on the other hand, is based on sets of noninterfering wireless transmissions. In contrast with the first formulation, the size of the alternative formulation is independent of the number of time slots in a frame. We identify simple necessary and sufficient conditions for simultaneous transmissions on different links of the network in the same time slot without any significant interference. Our characterization, as a byproduct, prescribes a power level for each of the transmitting nodes. Motivated by this characterization, we propose a simple scheme to enumerate all sets of noninterfering transmissions, which is used as an input for the alternative formulation. We also introduce a set of valid inequalities for both formulations. For large instances, we propose a three-stage heuristic approach. In the first stage, we solve a partial relaxation of our alternative optimization model and determine the gateway locations. This stage also provides an upper bound on the optimal service level. In the second stage, a routing tree is constructed for each gateway node computed in the first stage. Finally, in the third stage, the alternative optimization model is solved by fixing the resulting gateway locations and the routing trees from the previous two stages. For even larger networks, we propose a heuristic approach for solving the partial relaxation in the first stage using a neighborhood search on gateway locations. Our computational results demonstrate the promising performance of our exact and heuristic approaches and the valid inequalitiesItem Embargo Facility location decisions for drone delivery: A literature review(Elsevier BV, 2023-10-30) Dükkancı, Okan; Campbell, James F.; Yetiş Kara, BaharThis study presents a comprehensive literature survey on facility location problems for drone (uncrewed vehicle) delivery, where either (i) drones are the only vehicles, or (ii) drones and other vehicles (e.g., trucks) work together for delivery, but drones do not ride in or on the other vehicles. The main goals of this review are to identify and categorize fundamental facility location problems associated with drone delivery, to document the large volume of research in this area, to provide a connection between the studies from different research fields that consider similar location problems, and to highlight promising areas for future research. We first discuss and classify the functions of drones and the various types of facilities used for drone and hybrid vehicle-drone (e.g., truck and drone, or transit and drone) delivery systems, including drone bases (fixed or temporary), other vehicle bases, recharging stations, (re)supply points, and platooning points. The literature is reviewed and categorized based on the types of vehicles involved and their interactions, the types of facilities located, the types of drones and the location space (discrete or continuous). Each category is analyzed in terms of the modeling approach, decision(s), the objective function(s), constraint(s) and additional feature(s). The paper concludes with some promising future research directions.Item Open Access Fiber optical network design problems: a case for Turkey(Elsevier, 2016-09) Yazar, B.; Arslan, O.; Karaşan, O. E.; Kara, B. Y.In this paper, we consider problems originating from one of the largest Internet service providers operating in Turkey. The company mainly faces two different design problems: the green field design (area with no Internet access) and the copper field re-design (area with limited access over copper networks). In the green field design problem, the aim is to design a least cost fiber optical network that will provide high bandwidth Internet access from a given central station to a set of aggregated demand nodes. Such an access can be provided either directly by installing fibers or indirectly by utilizing passive splitters. Insertion loss, bandwidth level and distance limitations should simultaneously be considered in order to provide a least cost design to enable the required service level. In the re-design of the copper field application, the aim is to improve the current service level by augmenting the network with fiber optical wires, specifically by adding cabinets to copper rings in the existing infrastructure and by constructing direct fiber links from cabinets to distant demand nodes. Mathematical models are constructed for both problem specifications. Extensive computational results based on realistic data from Kartal (45 nodes) and Bakırköy (74 nodes) districts in Istanbul show that the proposed models are viable exact solution methodologies for moderate dimensions.Item Open Access A finite dominating set of cardinality O(k) and a witness set of cardinality O(n) for 1.5D terrain guarding problem(Springer New York LLC, 2017) Eliş, Haluk1.5 dimensional (1.5D) terrain is characterized by a piecewise linear curve. Locating minimum number of guards on the terrain (T) to cover/guard the whole terrain is known as 1.5D terrain guarding problem. Approximation algorithms and a polynomial-time approximation scheme have been presented for the problem. The problem has been shown to be NP-Hard. In the problem, the set of possible guard locations and the set of points to be guarded are uncountable. To solve the problem to optimality, a finite dominating set (FDS) of size O (n2) and a witness set of size O (n3) have been presented, where n is the number of vertices on T. We show that there exists an even smaller FDS of cardinality O (k) and a witness set of cardinality O(n), where k is the number of convex points. Convex points are vertices with the additional property that between any two convex points the piecewise linear curve representing the terrain is convex. Since it is always true that k≤ n for n≥ 2 and since it is possible to construct terrains such that n= 2 k, the existence of an FDS with cardinality O(k) and a witness set of cardinality of O (n) leads to the reduction of decision variables and constraints respectively in the zero-one integer programming formulation of the problem. © 2017, Springer Science+Business Media New York.Item Open Access Green location and routing problems with conventional vehicles and drones(Bilkent University, 2019-05) Dükkancı, OkanGreen Location and Routing Problems extend the network design problems that consider location and routing decisions by explicitly accounting environmental impacts such as CO2 emissions caused by fuel or energy consumption of delivery vehicles. These environmental impacts estimated by fuel or energy consumption models are a ected by several factors including payload and speed of delivery vehicles. We present four new green location and routing problems where we consider these factors while calculating the environmental impacts. We rst introduce the Green Location-Routing Problem, in which vehicle payload and speed decisions are incorporated to a location-routing problem and the fuel consumption of trucks is estimated and minimized. Second, we study the Green Hub Location Problem, where we minimize the fuel consumption by optimizing truck payload and speed decisions on a hub network. Third, we present a freight transportation problem called the Drone Delivery Problem, where the integration of trucks and drones is used to make deliveries. Drone speed is considered as a decision of the problem in order to minimize energy consumption of drones while not exceeding the drone range. Fourth, we study an extension of the Drone Delivery Problem, called the Stochastic Drone Delivery Problem, where uncertainty of wind speed and its impact on the drone speed are considered.Item Open Access A hub covering network design problem for cargo applications in Turkey(Palgrave Macmillan, 2009) Alumur, S.; Kara, B. Y.Hub location problems involve locating hub facilities and allocating demand nodes to hubs in order to provide service between origin-destination pairs. In this study, we focus on cargo applications of the hub location problem. Through observations from the Turkish cargo sector, we propose a new mathematical model for the hub location problem that relaxes the complete hub network assumption. Our model minimizes the cost of establishing hubs and hub links, while designing a network that services each origin-destination pair within a time bound. We formulate a single-allocation hub covering model that permits visiting at most three hubs on a route. The model is then applied to the realistic instances of the Turkish network and to the Civil Aeronautics Board data set.Item Open Access Hub location under competition(Elsevier, 2016) Mahmutogullari, A. I.; Kara, B. Y.Hubs are consolidation and dissemination points in many-to-many flow networks. Hub location problem is to locate hubs among available nodes and allocate non-hub nodes to these hubs. The mainstream hub location studies focus on optimal decisions of one decision-maker with respect to some objective(s) even though the markets that benefit hubbing are oligopolies. Therefore, in this paper, we propose a competitive hub location problem where the market is assumed to be a duopoly. Two decision-makers (or firms) sequentially decide locations of their hubs and then customers choose one firm with respect to provided service levels. Each decision-maker aims to maximize his/her own market share. We propose two problems for the leader (former decision-maker) and follower (latter decision-maker): (r|Xp)hub − medianoid and (r|p)hub − centroid problems, respectively. Both problems are proven to be NP-complete. Linear programming models are presented for these problems as well as exact solution algorithms for the (r|p)hub − centroid problem. The performance of models and algorithms are tested by computational analysis conducted on CAB and TR data sets.Item Open Access Integrating social features into mobile local search(Elsevier Inc., 2016) Kahveci, B.; Altıngövde, İ. S.; Ulusoy, ÖzgürAs availability of Internet access on mobile devices develops year after year, users have been able to make use of search services while on the go. Location information on these devices has enabled mobile users to use local search services to access various types of location-related information easily. Mobile local search is inherently different from general web search. Namely, it focuses on local businesses and points of interest instead of general web pages, and finds relevant search results by evaluating different ranking features. It also strongly depends on several contextual factors, such as time, weather, location etc. In previous studies, rankings and mobile user context have been investigated with a small set of features. We developed a mobile local search application, Gezinio, and collected a data set of local search queries with novice social features. We also built ranking models to re-rank search results. We reveal that social features can improve performance of the machine-learned ranking models with respect to a baseline that solely ranks the results based on their distance to user. Furthermore, we find out that a feature that is important for ranking results of a certain query category may not be so useful for other categories.