Shigeto Tajima

An Internet Gateway Access-Point Selection Problem forWireless Infrastructure Mesh Networks

This paper presents a study of the gateway access-point selection problem for the wireless infrastructure mesh network (WIMNET). WIMNET is a wireless mesh network composed of multiple access-points (APs) that communicate mutually using radio transmissions, and all the traffics to/from the Internet go through the limited number of gateway APs. Thus, the selection of these gateway APs often determines the whole performance of WIMNET, because heavy congestions at gateway APs and their peripheries can drastically increase the delay and the link failure due to radio interferences. In this paper, we define a cost function to minimize the total traffic in WIMNET in order to reduce congestions and present a simple algorithm. Through simulations using the newly developed WIMNET simulator, we show the effectiveness of our approach.

A Proposal of Fixed Backoff-time Switching Method by Link Activation Rate for Wireless Mesh Networks

As an inexpensive and scalable access network to the Internet, we have extensively studied the Wireless Internet-access Mesh Network (WIMNET) that is composed of multiple access points (APs) connected through wireless links. Because most traffic in WIMNET passes through the gateway (GW) to the Internet, the links around the GW are much crowded so that they should be activated with higher priorities than others. In this paper, we propose the Fixed Back off-time Switching (CSMA-FBS) method for WIMNET using the CSMA/CA protocol to improve the performance by giving necessary activation chances to such links. Before starting communications, the target link activation rate and the active/passive back off-times for collision avoidance are calculated from the expected traffic load for each link. During communications, these back off-times are switched by comparing the actual link activation rate with the target one. By assigning different fixed values to these back off-times, the conflicts among the interfered links can be avoided. Through extensive simulations using two types of network topologies, we verify the effectiveness of our proposal.

An extension of clustering algorithm for considering link speed in wireless mesh networks

As a scalable Internet-access network, we have studied Wireless Internet-access Mesh Network (WIMNET) using multiple access point (APs) that are connected through wireless communications. In previous studies, we proposed a clustering algorithm for efficiently composing a large scale WIMNET by partitioning the APs into a set of clusters. This algorithm assumes that any link speed is constant for simplicity, whereas our preliminary experiments found that it is greatly changed as the link distance increases due to interferences when adopting the new high-speed IEEE802.11n protocol. In this paper, we extend our clustering algorithm to consider this link speed change to introduce IEEE802.11n into WIMNET. We verify the effectiveness of the extended algorithm through simulations in a grid topology instance using the WIMNET simulator.

A minimal-state processing search algorithm for satisfiability problems

The satisfiability problem (SAT) is a typical NP-complete problem where a wide range of applications has been studied. Given a set of variables U and a set of clauses C, the goal of SAT is to find a truth assignment to variables in U such that every clause in C is satisfied if it exits, or to derive the infeasibility otherwise. This paper presents an approximation algorithm, called a minimal-state processing search algorithm for SAT (MIPS-SAT). MIPS-SAT repeatedly transits minimal states in terms of the cost function for searching a solution through a construction stage and a refinement stage. The first stage greedily generates an initial state composed of as many satisfied clauses as possible. The second stage iteratively seeks a solution while keeping state minimality. The performance of MIPS-SAT is verified through solving DIMACS benchmark instances.

An Extension of Wireless Internet-Access Mesh Network Simulator for Use of Smart Antenna

As an inexpensive, flexible, and scalable Internet-access network, we have studied architecture, protocols, and design optimizations of the Wireless Internet-access Mesh NETwork (WIMNET). WIMNET consists of multiple access-points (APs) as wireless routers that are connected through wireless links. To improve the performance by realizing high-speed and simultaneous plural link communications, we have considered use of the smart antenna as a generalized technology of the Multiple-Input-Multiple-Output (MIMO) channel and the Adaptive Array Antenna (AAA). In this paper, we present an extension of the WIMNET simulator for use of the smart antenna, which has been developed by our group to estimate the throughput in WIMNET. Through simulations in six simple topologies, we evaluate impacts of the smart antenna use strategy and of the interval between adjacent wireless nodes including APs and hosts, into the performance of WIMNET. Their results show that the best strategy depends on a network topology, and the throughput becomes peak when the interval is 30 m or 50 m.

A proposal of a minimal-state processing search algorithm for link scheduling problems in packet radio networks

In a large scale communication network, packets are transmitted from source nodes to destinations by going through several nodes on routes connecting them, because each node usually does not have a direct link to every node in the network. To maximize the usability in the network, the problems of selecting a transmission route for each connection request and of scheduling the link activations such that the total transmission time is minimized. This paper presents a four-stage approximation algorithm for the link scheduling problem (LSP) in a packet radio network, called MIPS-LSP, a MInimal-state Processing Search algorithm for LSP It is assumed that every link activation in the network is synchronously controlled by a single clock with a fixed time interval called a time slot. The first stage of MIPS-LSP computes the lower bound on the number of time slots for a given LSP instance. The second stage generates a link compatibility graph to represent the conflicts between links to be activated simultaneously. The third stage gives the initial state for search by a greedy method. The last stage iteratively searches a solution by a minimal-state evolution method The performance of the algorithm is verified through extensive simulations using benchmarks in literature.

A Proposal of a Quasi-Solution State Evolution Algorithm for Channel Assignment Problems

The channel assignment problem (CAP) in a cellular network requires finding a channel assignment to the call requests from cells such that three types of interference constraints are not only satisfied, but also the number of channels (channel span) is minimized. This paper presents a three-stage iterative algorithm, called the Quasi-solution state evolution algorithm for CAP (QCAP). QCAP evolutes quasi-solution states where a subset of call requests is assigned channels and no more request can be satisfied without violating the constraint. The first stage computes the lower bound on the channel span. After the second stage greedily generates an initial quasi-solution state, the third stage evolutes them for a feasible solution by iteratively generating best neighborhoods, with help of the dynamic state jump and the gradual span expansion for global convergence. The performance is evaluated through solving benchmark instances in literature, where QCAP always finds the optimum or near-optimum solution in very short time.

Cross-layer selective routing for cost and delay minimization in IEEE 802.11ac wireless mesh network

A Wireless Internet-access Mesh NETwork (WIMNET) provides scalable and reliable internet access through the deployment of multiple access points (APs) and gateways (GWs). In this work, we propose a selective routing algorithm aiming at a hierarchical minimization of the operational cost and the maximal end-to-end delay. In particular, by deploying redundant APs/GWs in the network field, the WIMNET becomes robust to the link or AP/GW failure. However, these redundant APs/GWs increase the operational cost like the power consumption. By using Dijkstra algorithm and 2-opt algorithm, the proposed algorithm iteratively deactivates the deployed APs/GWs and performs the routing that reduces the maximal end-to-end delay based on the APs/GWs remaining active. The generated route meets the real-world constraints like fairness criterion. We further propose a cross-layer design to enhance the routing performance by exploiting the MAC-layer frame aggregation technique. The selective routing algorithm is then implemented in the WIMNET simulator proposed by our group. The numerical experiments demonstrate that in both indoor and open space environments, the proposed selective routing greatly reduces the operational cost, i.e., up to

An extension of clustering algorithm for smart-antenna use in Wireless Mesh Networks

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