In multi-hop wireless networks, since source and destination nodes usually have some candidate paths between them, communication quality depends on the selection of a path from these candidates. For network design, characterizing the best path is important. To do this, in [1], [2] we used expected transmission count (ETX) as a metric of communication quality and showed that the best path for ETX is modeled by a path that consists of links whose lengths are close to each other in static one-dimensional multi-hop networks with a condition that the ETX function of a link is a convex monotonically increasing function. By using the results of this characterization, a minimum route ETX can be approximately computed in a one-dimensional random network. However, other metrics fail to satisfy the above condition, like medium time metric (MTM). In this paper, we use MTM as a metric of communication quality and show that we cannot directly apply the results of to the characterization of the best path for MTM and the computation of minimum route MTM. In this paper, we characterize the path that minimizes route MTM in a different manner from [1] [2] and propose a new approximate method suitable for the computation of minimum route MTM.

In general, dynamic channel assignment has a better performance than fixed channel assignment in a cellular mobile communication system. However, it is complex to control the system and a lot of equipments are required in each cell when dynamic channel assignment is applied to a large service area. Therefore, it is effective to limit the size of the service area in order to correct the defects of dynamic channel assignment. So, we propose an application of dynamic channel assignment to a part of a service area when fixed channel assignment is applied to the remaining part of the area. In the system, the efficiency of channel usage in some cells sometimes becomes terribly low. The system has such a problem to be improved. We show that the rearrangement of the channel allocation is effective on the problem.

Huynen has already provided a method to decompose a Mueller matrix in order to retrieve detailed target information in a polarimetric radar system. However, this decomposition sometimes fails in the presence of small error or noise in the elements of a Mueller matrix. This paper attempts to improve Huynen's decomposition method. First, we give the definition of stable decomposition and present an example, showing a problem of Huynen's approach. Then two methods are proposed to carry out stable decompositions, based on the nonlinear least square method and the Newton's method. Stability means the decomposition is not sensitive to noise. The proposed methods overcomes the problems on the unstable decomposition of Mueller matrix, and provides correct information of a target.

An FM-CW radar system for the detection of objects buried in sandy ground is explored and is applied to a field measurement. The key factors for underground FM-CW radar performance are the center frequency and bandwidth determining the depth at which the radar can detect targets and the resolution in the range direction. In order for FM-CW radar sounding, two ridged horn antennas are employed in the system, which are operative in the frequency range of 250-1000MHz. The impedance matching to the ground is optimized by measuring the echo strength from a fixed target as a function of the spacing interval between the antenna aperture and the ground surface. It is shown that the radar with an output power of 18dBm could detect a metallic plate (30100cm) and a pipe (10cmφ) buried at the depth of 1.2m. Also the synthetic aperture technique together with an averaging and subtracting method produced fine image in shallow region up to 100cm in the sandy ground.

In multi-hop wireless networks, communication quality depends on the route from a source to a destination. In this paper, we consider a one-dimensional multi-hop wireless network where nodes are distributed randomly and theoretically analyze the relation between communication quality and routing policy using a measure called the Expected Transmission Count (ETX), which is the predicted number of data transmissions required to send a packet over that link, including retransmissions. First, we theoretically analyze the mean length of links, the mean number of hops, and the mean route ETX, which is the sum of the ETXs of all links in a route, of Longest Path Routing (LPR), and Shortest Path Routing (SPR). Second, we propose Adjustable Routing (AR), an approximation to Optimum Routing (OR), which minimizes route ETX. We theoretically compute the above characteristic values of AR. We also theoretically compute a lower bound of the mean route ETX of OR. We compare LPR, SPR, and OR using the results of analyses and show differences between these algorithms in the route ETX.

This paper describes a delay control scheme for synchronous detection of an orthogonal coding multi-carrier CDMA (Code Division Multiple Access) system. The delay control scheme estimates transmission timing of data from each mobile station. At a base station, delay time is obtained by detecting phase shift value of the preamble signal from each mobile station. The estimated transmission timing information is sent from base station to each mobile station and the mobile station then adjusts its transmission timing. Simulation results clarified that Bit Error Rate (BER) is 2.510-3 at 19dB of Eb/No under conditions of 29.4 msec initial delay time, 32kbit/sec data rate, 16 subchannels and 100Hz of fading frequency.

This paper presents an experimental result of polarimetric detection of objects buried in a sandy ground by a synthetic aperture FM-CW radar. Emphasis is placed on the reduction of surface clutter by the polarimetric radar, which takes account of full polarimetric scattering characteristics. First, the principle of full polarimetric imaging methodology is outlined based on the characteristic polarization states for a specific target together with a polarimetric enhancement factor which discriminates desired and undesired target echo. Then, the polarimetric filtering technique which minimizes a surface reflection is applied to detect a thin metallic plate embedded in a sandy ground, demonstrating the potential capability of reducing surface clutter which leads to an improvement of underground radar performance, and validating the usefulness of FM-CW radar polarimetry.

This paper presents novel flooding schemes for wireless mobile ad hoc networks. Clustering of nodes is assumed as a basic ad hoc network structure. GWF (Gateway Forwarding) and SGF (Selected Gateway Forwarding) are presented based on clustering. A new protocol, termed FGS (Flooding Gateway Selection) protocol, between a cluster head and its gateways to realize SGF is presented. It is shown that SGF significantly improves the packet delivery performance in ad hoc networks by reducing flooding traffic.

This paper applied the polarimetric filtering principle to Synthetic Aperture Radar (SAR) image sets in three possible polarimetric radar channels and compared the resultant imagery. The polarimetric radar channels in consideration here are Co-Pol, Cross (X)-pol, and Matched (M)-pol channels. Each channel has its own polarimetric characteristics for imaging. Using the formulation of the contrast enhancement factors based on the Stokes vector formalism, polarimetric enhanced images for three channels are shown using NASA JPL DC-8 AIRSAR data sets (CC0045L, Bonanza Creek, AK/USA). It is shown that the optimally enhanced Co- and X-Pol channel images play a decisive role in imaging in a complex featured background.

Delay Tolerant Network (DTN) has been emerged to support the network connectivity of the disruptive networks. A variety of routing methods have been proposed to reduce the latency for message delivery. PROPHET was proposed as a probabilistic routing that utilizes history of encounters and transitivity of nodes, which is computed as contact probability. While PROPHET improves the performance of DTN due to contact probability, contact probability is just one parameter reflecting the mobility pattern of nodes, and further study on utilizing contacting information of mobility pattern is still an important problem. Hence, in this paper, we try to improve routing for DTN by using a novel metric other than contact probability as mobility information. We propose the routing protocol to use mean residual contact time that describes the contact period for a given pair of nodes. The simulation results show that using the mean residual contact time can improve the performance of routing protocols for DTN. In addition, we also show in what situations the proposed method provides more efficient data delivery service. We characterize these situations using a parameter called Variation Metric.

In cellular mobile systems, an alternative approach for a Dynamic Channel Assignment problem is presented. It adaptively assigns the channels considering the cochannel interference level. The Dynamic Channel Assignment problem is modeled on the different cellular system from the conventional one. In this paper, we formulate the rearrangement problem in the Dynamic Channel Assignment and propose a novel strategy for the problem. The proposed algorithm is based on an artificial neural network as a specific dynamical system, and is successfully applied to the cellular system models. The computer simulation results show that the algorithm utilized for the rearrangement is an effective strategy to improve the traffic characteristics.

The demand for mobile communication services is rapidly increasing, because the mobile communication service is synonymy of an ideal communication style realizing communication in anytime, anywhere and with anyone. The development of economic and social activities is a primary factor of the increasing demand for mobile communication services. The demand stimulates the development of technology in mobile communication including personal communication services. Thus mobile communication has been one of the most active research in communications in the last several years. There exist various problems to which graph & network theory is applicable in mobile communication services (for example, channel assignment algorithm in cellular system, protocol in modile communication networks and traffic control in mobile communication ). A model of a cellular system has been formulated using a graph and it is known that the channel assignment problem is equivalent to the coloring problem of graph theory. Recently, two types of coloring problems on graphs or networks related to the channel assignment problem were proposed. Mainly, we introduce these coloring problems and show some results on these problems in this paper.

Analysis of waiting time to deliver a message M from a source S to a destination D is deeply related to connectivity analysis, which is an important issue in fundamental studies of mobile multi-hop networks. In [1], we compared the mean waiting times of two methods to deliver M with the mean value of the minimum waiting time. The mean minimum waiting time was obtained by computer simulation because theoretical analysis of this mean is not easy, although another two methods were analyzed theoretically. In this paper, we propose an approximate method to theoretically analyze the mean minimum waiting time in a one-dimensional street network, and show that this method gives a good approximation of the mean minimum waiting time. Also, we consider shadowing and change of directions of mobile nodes at intersections as negative factors arising in two-dimensional street networks. We extend the above method to compute the mean minimum waiting time considering these factors, and discuss how the mean minimum waiting time is affected by these factors.

In multi-hop wireless networks, communication quality depends on the selection of a path between source and destination nodes from several candidate paths. Exploring how path selection affects communication quality is important to characterize the best path. To do this, in [1], we used expected transmission count (ETX) as a metric of communication quality and theoretically characterized minimum route ETX, which is the ETX of the best path, in a static one-dimensional random multi-hop network. In this paper, we characterize minimum route ETX in static two-dimensional multi-hop networks. We give the exact formula of minimum route ETX in a two-dimensional network, assuming that nodes are located with lattice structure and that the ETX function satisfies three conditions for simplifying analysis. This formula can be used as an upper bound of minimum route ETX without two of the three conditions. We show that this upper bound is close to minimum route ETX by comparing it with simulation results. Before deriving the formula, we also give the formula for a one-dimensional network where nodes are located at constant intervals. We also show that minimum route ETX in the lattice network is close to that in a two-dimensional random network if the node density is large, based on a comparison between the numerical and simulation results.

When a mobile station with a call in progress moves across cell boundary in a cellular mobile communications system, the system must switch the circuit to the base station in the destination cell to enable uninterrupted communications in a process called "handoff. " However, if a circuit to the destination base station cannot be secured when a handoff is attempted, the call is forcibly terminated. Studies have therefore been performed on methods of decreasing the percentage of forcibly terminated calls by giving handoff calls priority. With the aim of simplifying system design, we propose a system for automatically setting the number of circuits reserved for handoff based on the handoff block rate. In this paper, we describe this system and evaluate static traffic characteristics taking into account reattempt calls, the occurrence of which can have a major effect on system performance. We also consider the effects of the proposed system on service quality since giving priority to handoff calls and decreasing the rate of forced terminations results in a tradeoff with the blocking rate of new call attempts. Finally, we evaluate the traffic characteristics associated with the number of control requests, an important element in estimating the processing capacity required by control equipment at the time of system design.

The applicability of Dynamic Channel Assignment methods to a Reuse Partitioning system in cellular radio systems is investigated in this paper. The investigations indicate that such a system has a tendency to increase the difference between blocking probability for the partitioning two coverage areas in comparison with the conventional Reuse Partitioning system employing Fixed Channel Assignment method. Two schemes using new Channel Rearrangement algorithms are also proposed in order to alleviate the difference as a disadvantage which gives unequal service to the system. The simulation results show that the proposed schemes are able to reduce the difference significantly while increasing the carried traffic by 10% as compared with the conventional system.