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.

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.

A new traffic forecasting method using state space representation is proposed. By means of a state space model, the forecasting value is sequentially calculated by applying the Kalman filter. However the true traffic structure is not easy to grasp as changes in traffic are largely due to social activities. In addition, experience has shown that economic trends in society also have an influence on traffic. For this reason, the traffic structure becomes too complex to describe changes in traffic by using a single state space model. In this paper a multiple state space model is proposed. The multiple state space models is composed of several state space models calls sub-models. This model is more easily adaptable to change in the traffic structure than a single state space model. The Bayesian forecasting value is given by the weighted summation for each sub-model forecasting value. The Bayesian posterior probability, which is calculated from the likelihood, is used as the weight of the sub-model. A good fitting sub-model posterior probability increases as the number of observations increases. In this paper the initial state and noise variances of each sub-model are estimated by numerical maximization of the likelihood. Examples of how this method may be applied to monthly telephone revenue data and trunk group load data are given, demonstrating the possibility of adapting exceptional data and structural changes in traffic. Parameter estimation using a multiple state space model is also shown.

This paper describes IP traffic, especially the control of VoIP traffic, on the carrier-scale, and proposes algorithms for it. It examines a case that has already been introduced in the United States and discusses the trend of standardization for this control. Control techniques that will be introduced into the IP network in the future are considered from the viewpoints of both "quality" that users receive and the "control" that carriers perform.

In a cellular system for mobile communications, every service area is divided into a number of cells for utilizing the frequency spectrum efficiently. Service areas for such systems are two dimensional, however, the analysis of the characteristics of the communication traffic for the areas are quite complicated, since the motion of the vehicles in the area can not be predicted precisely. For making the analysis easily, the areas are assumed to be band-shaped like a highway. Furthermore, in the analysis, the traffic offered to a cell is assumed to be stationary. In actual systems, the density of vehicles and the offered communication traffic is not stationary, so that many differences exist between the analysis and the actual systems. This paper presents an analysis method using state equations. The equations represent the transient characteristics of mobile communication traffic when a band-shaped service area is assumed. The transition is made by accidents or congestion, and causes the rapid offered traffic change in a communication system. In the method, numerical analysis is made under the consideration of "handoff" operation. The operation consists of surrendering the channel used in the previous cell and reassigning a new channel when the vehicle crosses the cell boundary. The analytical results are compared with the simulations, and the two results show good agreement. The method presented in this paper can be used for designing the switching system when the offered traffic changes rapidly due to accidents or congestion.

A novel algorithm, as an advanced Hybrid Channel Assignment strategy, for channel assignment problem in a cellular system is proposed. A difference from the conventional Hybrid Channel Assignment method is that flexible fixed channel allocations which are variable through the channel assignment can be performed in order to cope with varying traffic. This strategy utilizes the Channel Rearrangement technique using the artificial neural network algorithm in order to enhance channel occupancy on the fixed channels. The strategy is applied to two simulation models which are the spatial homogeneous and inhomogeneous systems in traffic. The simulation results show that the strategy can effectively improve blocking probability in comparison with pure dynamic channel assignment strategy only with the Channel Rearrangement.

In a cellular mobile system, assigning a channel for a call in a cell so as to achieve high spectral efficiency is an important problem. In usual channel assignment for a cellular mobile system, a channel can be simultaneously assigned to some cells with a constant separation distance. This usual model of a cellular mobile 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, a new channel assignment scheme has been proposed. This scheme takes the degree of interference into consideration. In the scheme, a channel is simultaneously assigned if the CIR (carrier-to-interference ratio) is more than the desired value. In this paper, we formulate this new model using a network and a new coloring problem of networks. The new coloring problem of networks is a generalization of the usual coloring problem of graphs. One of the merits of this formulation is that the degree of cochannel interference between cells can be represented. In the usual formulation using a graph, the degree of cochannel interference between cells can not be represented. Therefore, spectral efficiency in the formulation using a network is higher than spectral efficiency in the formulation using a graph. In this paper, we show that the new coloring problem is an NP-hard problem. Subsequently, we rewrite the new coloring problem of networks to a coloring problem of graphs on some assumptions and consider the relation between the results on the new coloring and the results on the usual coloring.

Location theory on networks is concerned with the problem of selecting the best location in a specified network for facilities. Many studies for the theory have been done. However, few studies treat location problems on networks from the standpoint of measuring the closeness between two vertices by the capacity (maximum flow value) between two vertices. This paper concerns location problems, called covering problems on flow networks. We define two types of covering problems on flow networks. We show that covering problems on undirected flow networks and a covering problem on directed flow networks are solved in polynomial times.

In mobile communication systems using Dynamic Channel Assignment, channels are possible to be rearranged so that blocking probability can be made low. The smaller the number of cells where channels are rearranged, the smaller the load on the base stations in the cells. Also, we can reduce the deterioration of communication quality caused by reassingning a new channel to a call instead of the channel already assigned. In this paper, we consider not only how to rearrange channels but also which channel should be rearranged and assigned to a new call in rearrangement, and propose very simple but effective methods for rearrangement. The ways to select a candidate channel to be rearranged and assigned to a new call in the new methods make the number of cells where a channel is rearranged smaller. We also examine the relations between characteristics and the number of cells where a channel is rearranged. Using computer simulation results, the properties of the new rearrangement methods are compared with those of the traditional methods.

This paper describes a forward subchannel control of multi-carrier scheme intended to compensate for phase/amplitude distortions under frequency selective fading. The forward subchannel control scheme is used for a Time Division Duplex (TDD) multi-carrier system on up-link. The forward subchannel control scheme provides forward subchannel control of phase/amplitude variation and subchannel assignment control. These controls are applied before transmission of an up-link signal. The forward control parameters are estimated by a preamble down-link signal. Simulation results clarify that the BER performance with the forward subchannel control scheme shows a superiority of more than one order at the condition of 22 dB of Eb/N0 and 400 Hz of fading frequency.

We introduce the distance between two edges in a graph (nondirected graph) as the minimum number of edges in a tieset with the two edges. Using the distance between edges we define the eccentricity ετ (ej) of an edge ej. A finite nonempty set J of positive integers (no repetitions) is an eccentric set if there exists a graph G with edge set E such that ετ (ej) J for all ei E and each positive integer in J is ετ (ej) for some ej E. In this paper, we give necessary and sufficient conditions for a set J to be eccentric.

A recent trend in routing research is the use of overlay routing to improve end-to-end QoS without changing the network-level architecture. The key of this technology is to find an alternative route that can avoid congested routes, using an overlay network. Developing cost-efficient overlay routing in terms of calculation cost and information distribution cost needed to find an alternative route is important for deploying QoS-aware overlay routing. Thus, this paper evaluates how effective overlay routing can be when the number of alternative route candidates is limited to reduce costs. Evaluation results using actual measurement data indicate that overlay routing is still effective even if alternative route candidates are limited to 1/4 of all possible alternative routes. We also discuss an overlay routing algorithm to enable us to find an appropriate route under the constraint that the number of alternative route candidates is limited.

This paper describes communication traffic characteristics in cellular systems employing the concept of reuse partitioning and Dynamic Channel Assignment. Such systems hava a problem of the spatial unbalance of blocking probability. The objective of this paper is overcoming this problem. To accomplish this objective, we use a method for analyzing communication traffic characteristics. We also show results on traffic characteristics in the systems.

This paper describes two dimensional (2D) equalization scheme of orthogonal coding multi-carrier CDMA for reverse link of mobile communication systems. The purpose of the 2D equalization is the reduction of Multiple Access Interference (MAI) which is caused by the random access and the different propagation path from each mobile station. The orthogonal coding multi-carrier CDMA multiplexes all mobile stations' data by Code Division Multiplexing (CDM). The 2D coding scheme spreads a preamble signal at time (in subchannel signals) and frequency (between subchannel signals) domains. The 2D decoding scheme estimates transmission delay time and instantaneous fading frequency from preamble signal for individual mobile stations and compensate the received data using these estimation values to reduce MAI.

In location problems, the outtransmission and intransmission numbers are important indices to evaluate a directed network. We formulate and consider a new problem of fault diagnosis in a system modeled by a directed network in which to each edge a positive real number called the length of edge is assigned and to each vertex a positive real number called the weight of vertex is assigned. By a fault in a directed network, we mean any increase in the length of an edge with respect to its nominal length. A theory and an algorithm for detecting a fault edge in a directed network in which the above indices, i.e. outtransmission and/or intransmission numbers, are measurable, are presented.

We propose a method of dimensioning and managing the bandwidth of a link on which flows with heterogeneous access-link bandwidths are aggregated. We use a processor-sharing queue model to develop a formula approximating the mean TCP file-transfer time of flows on an access link in such a situation. This only requires the bandwidth of the access link carrying the flows on which we are focusing and the bandwidth and utilization of the aggregation link, each of which is easy to set or measure. We then extend the approximation to handle various factors affecting actual TCP behavior, such as the round-trip time and restrictions other than the access-link bandwidth and the congestion of the aggregation link. To do this, we define the virtual access-link bandwidth as the file-transfer speed of a flow when the utilization of the aggregation link is negligibly small. We apply the virtual access-link bandwidth in our approximation to estimate the TCP performance of a flow with increasing utilization of the aggregation link. This method of estimation is used as the basis for a method of dimensioning the bandwidth of a link such that the TCP performance is maintained, and for a method of managing the bandwidth by comparing the measured link utilization with an estimated threshold indicating degradation of the TCP performance. The accuracy of the estimates produced by our method is estimated through both computer simulation and actual measurement.

Location theory on networks is concerned with the problem of selecting the best location in a specified network for facilities. In networks, the distance is an important measure to quantify how strongly related two vertices are. Mereover, the capacity between two vertices is also an important measure. In this paper, we define the location problems called the p-center problem, the r-cover problem and the p-median problem on undirected flow networks. We propose polynomial time algorithms to solve these problems.

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.