These days, cheap and intelligent sensors, networked through wireless links and deployed in large numbers, provide unprecedented opportunities for monitoring and controlling homes, cities and the environment. Networked sensors also offer a broad range of applications. Localization capability is essential in most wireless sensor networks applications; for instance in environmental monitoring applications such as animal habitat monitoring, bush fire surveillance, water quality monitoring and precision agriculture, the measurement data are meaningless without accurate knowledge of where they are obtained. Localization techniques are used to determine location information by estimating the location of each sensor node. Distance measurement errors are commonly known to affect the accuracy of the estimated location; resulting in errors that may be due to inherent or environmental factors. Trilateration [1] is a well-known method for localizing nodes by using the distances to three anchor nodes; yet it performs poorly when they are many distance measurement errors. Therefore, we propose the LRD (Localization with Ratio-Distance) algorithm, which performs strongly even in the presence of many measurement errors associated with the estimated distance to anchor nodes. Simulations using the OPNET Modeler show that LRD is more accurate than trilateration.

In this paper, we present Greedy Routing for Maximum Lifetime (GRMax) [1],[2] which can use the limited energy available to nodes in a Wireless Sensor Network (WSN) in order to delay the dropping of packets, thus extend the network lifetime. We define network lifetime as the time period until a source node starts to drop packets because it has no more paths to the destination [3]. We introduce the new concept of Network Connectivity Aiming (NCA) node. The primary goal of NCA nodes is to maintain network connectivity and avoid network partition. To evaluate GRMax, we compare its performance with Geographic and Energy Aware Routing (GEAR) [4], which is an energy efficient geographic routing protocol and Greedy Perimeter Stateless Routing (GPSR) [5], which is a milestone among geographic routing protocol. We evaluate and compare the performance of GPSR, GEAR, and GRMax using OPNET Modeler version 15. The results show that GRMax performs better than GEAR and GPSR with respect to the number of successfully delivered packets and the time period before the nodes begin to drop packets. Moreover, with GRMax, there are fewer dead nodes in the system and less energy is required to deliver packets to destination node (sink).

The demand for mobile communications is continuing to grow, but there is a limit on the radio frequency resources. Micro cellular systems are a strong solution to this problem. However, Forced Call Termination (FCT) and Channel Changing (CC) occur frequently in these systems because of their small cell size. This paper proposes a new Dynamic Channel Assignment (DCA) strategy which uses information of moving direction of Mobile Stations (MSs) to reduce FCT and CC. This strategy, the MD (Moving Direction) strategy, is compared with other major DCA strategies by simulating a one-dimensional service area covering a road, such as an expressway. The simulation shows that the MD strategy performs better than the other strategies with regard to FCT, CC, and carried load. FCT is an especially important factor in the quality of service. The MD strategy reduces FCT and has the largest carried load of the strategies, which means that it has the most efficient channel usage. This is an attractive characteristic of the MD strategy for micro cellular systems.

In estimating the performances of Distributed control Dynamic Channel Assignment (DDCA) strategies in sector cell layout systems, we find that sector cell layout systems with DDCA achieved a large system capacity. Moreover, we also indicate the problem, which is the increase of occurrences of cochannel interference, raised by using DDCA in sector cell layout systems. The new channel assignment algorithm, which is called Channel Searching on Direction of Sector (CSDS), is proposed to cope with the problem. CSDS assigns nominal channels to each sector according to their direction so that the same frequency channel tends to be used in sectors having the same direction. We show, by simulations, that CSDS is an adequate algorithm for sector cell layout systems because it significantly improves performance on co-channel interference while only slightly decreasing system capacity.

During devastating natural disasters, numerous people want to make calls to check on their families and friends in the stricken areas, but many call attempts on mobile cellular systems are blocked due to limited radio frequency resources. To reduce call blocking and enable as many people as possible to access mobile cellular systems, placing a limit on the holding time for each call has been studied [1],[2]. However, during a catastrophe, emergency calls, e.g., calls to fire, ambulance, or police services are also highly likely to increase and it is important that the holding time for these calls is not limited. A method of limiting call holding time to make provision for emergency calls while considering the needs of ordinary callers is proposed. In this method, called the HTL-E method, all calls are classified as emergency calls or other according to the numbers that are dialed or the terminal numbers that are given in advance to the particular terminals making emergency calls, and only the holding time of other calls is limited. The performance characteristics of the HTL-E method were evaluated using computer simulations. The results showed that it reduced the rates of blocking and forced call termination at handover considerably, without reducing the holding time for emergency calls. The blocking rate was almost equal for emergency and other calls. In addition, the HTL-E method handles fluctuations in the demand for emergency calls flexibly. A simple method of estimating the holding-time limit for other calls, which reduces the blocking rate for emergency and other calls to the normal rate for periods of increased call demand is also presented. The calculated results produced by this method agreed well with the simulation results.

We compared--for the same propagation conditions and parameters--the performances of distributed dynamic channel assignment (DDCA) strategies and the performance of fixed channel assignment (FCA). This comparison quantitatively showed the effects of DDCA strategies in increasing spectrum efficiency. It also showed that using DDCA with transmitter power control (TPC) increases the system capacity to 3 4 times what it is with FCA and to 1.4 1.8 times what it is when using DDCA without TPC. We also evaluated the blocking rate and the interference probability for the inside of a cell and found that these are generally much higher close to the cell border than they are near the base station.

The Satellite/Terrestrial Integrated mobile Communication System (STICS), which allows terrestrial mobile phones to communicate directly through a satellite, has been studied [1]. Satellites are unaffected by the seismic activity that causes terrestrial damage, and therefore, the STICS can be expected to be a measure that ensures emergency call connection. This paper first describes the basic characteristics of call blocking rates of terrestrial mobile phone systems in areas where non-functional base stations are geographically clustered, as investigated through computer simulations that showed an increased call blocking rate as the number of non-functional base stations increased. Further simulations showed that restricting the use of the satellite system for emergency calls only ensures the STICS's capacity to transmit emergency communications; however, these simulations also revealed a weakness in the low channel utilization rate of the satellite system [2]. Therefore, in this paper, we propose increasing the channel utilization rate with a priority channel framework that divides the satellite channels between priority channels for emergency calls and non-priority channels that can be available for emergency or general use. Simulations of this priority channel framework showed that it increased the satellite system's channel utilization rate, while continuing to ensure emergency call connection [3]. These simulations showed that the STICS with a priority channel framework can provide efficient channel utilization and still be expected to provide a valuable secondary measure to ensure emergency communications in areas with clustered non-functional base stations during large-scale disasters.

Call demand suddenly and greatly increases in the aftermath of a major disaster, because people want to check on their families and friends in the stricken area. Many call attempts in mobile cellular systems are blocked due to the limited radio frequency resources. In this paper, as a solution to this problem, limiting the holding time of calls is investigated and a dynamic holding time limit (DHTL) method, which varies the holding time limit dynamically based on the number of call attempts, is proposed. The effect of limiting the holding time is investigated first using a computer simulation with a constant and heavy traffic load model. This simulation shows that the average holding time of calls is decreased as the holding time limit is reduced. But it also shows limiting the holding time decreases the number of calls blocked and forced call terminations at handover considerably. Next, a simple estimation method for the holding time limit, which reduces the blocking rate to the normal rate for increasing call demand, is described. Finally, results are given of a simulation, which show that the DHTL method keeps good performance for a sudden and great traffic load fluctuation condition.

The rising demand for mobile communication is increasing the importance of efficient use of limited radio frequency resources. The assignment of radio channels to the cells of current cellular mobile radio systems, specifically, to each base station, has been much studied to increase efficiency in radio frequency use. Dynamic Channel Assignment (DCA) is one approach to this problem. This paper compares the basic characteristics of DCA with Fixed Channel Assignment (FCA) and describes the main DCA strategies. The most important current research topics on DCA are discussed, focusing on micro-cellular systems, which are considered indispensable in meeting the huge demand for future mobile communications.

The dynamic channel assignment (DCA) strategy proposed here uses information on the mobile station speed and direction of motion to reduce the number of forced call terminations and channel changes in micro cellular systems. This SMD (speed and moving direction) strategy is compared with the main DCA strategies by simulating a one-dimensional service area covering a road on which there are high-speed mobile stations (HSMSs) and low-speed mobile stations (LSMSs).The simulation results show that the SMD strategy has the best performance in terms of forced call termination and channel change. The performance difference between the SMD strategy and the other DCA strategies increases as cell size decreases and as HSMS speed increases. While the SMD strategy does not yield the best total call blocking rate, its total carried load is the best when cells are small and HSMS speed is high. Also, the SMD performance improves when the HSMS offered load is small and the LSMS offered load is large. Although the SMD strategy requires information on the speed and direction of each mobile station and it increases call blockings somewhat, it reduces the number of forced call terminations and channel changes considerably, which is important in micro cellular systems.