Stratospheric platforms have been recently proposed as a new wireless infrastructure for realizing the next generation of communication systems. To provide high quality services, an investigation of the wireless stratospheric platform channel is essential. This paper proposes a definition and describes an analysis of the wireless channel for the link between stratospheric platforms and terrestrial mobile users based on an experiment in a semi-urban environment. Narrowband channel characteristics are presented in terms of Ricean factor (K factor) and local mean received power over a wide range of elevation angles ranging from 10to 90. Finally, we evaluated average bit error probability based on the proposed channel model to examine the channel performance. For the environment in which the measurements were conducted, we find that elevation angles greater than 40yield better performance.

This paper proposes a new kind of communication system for air traffic control over the oceans; it is particularly effective at handling high air traffic loads due to many oceanic flights. In this system, each aircraft position report is sent to its relevant ground station by forwardly relaying them via a multi hop ad-hoc network that is formed by the aircraft between this aircraft and the ground station. In addition, an effective multiple access scheme with optimal values is also proposed. This scheme enables the various aircraft involved in relaying the signal to operate autonomously in a flight-route airspace. Furthermore, two useful schemes are proposed for efficient timeslot reuse and timeslot assignment in cases of low aircraft densities: the position aided timeslot reuse (PATR) and distance based timeslot assignment (DBTA), respectively. Finally, another scheme is proposed to improve the achievable relayed packet rate under low aircraft densities, which is called interference-based node selection (IB-NS). In all, the proposed system combined with those three schemes show the availability to utilize this system for air traffic control communications, specifically on high traffic ocean routes.

This paper presents a practical implementation scheme of the variable gain amplifier (VGA) using a Cds photo coupler (Cds PC) as a variable resister at the feedback loop. The fundamental design policies of IF amplifier stage in superheterodyne receiver were described. We demonstrated the VGA's experimental results. The results indicated the excellent IIP3 of +25 dBm achieved by a gain of 15 dB, and the reasonable thermal stability and variable gain range. Third-order intermodulation distortion (IMD3) comparison between the proposed VGA and conventional PIN diode attenuation type VGA was evaluated and the result indicated that the proposed VGA surpassed the PIN VGA. The proposed VGA was practically fabricated in 455 kHz IF amplifier stage for an airborne VHF communication receiver in order to improve the large signal handling capability to eliminate numerous interferences resulting from the collocated airborne VHF communication systems on the aircraft.

We propose FuzzyCoop, a cooperative MAC layer protocol employing a fuzzy logic partner selection algorithm. The protocol is based on the Distributed Coordination Function (DCF) protocol used in the IEEE 802.11 standard. There are three inputs to the fuzzy system: the Signal to Noise Ratio (SNR), the error ratio between two neighbors and the time the most recent packet was received from a neighbor. The fuzzy output is the partnership probability of a neighboring terminal. Besides, we introduce a cooperation incentive to the stations by providing them with the right to transmit their own data without contention after a successful cooperation. The protocol is evaluated through extensive simulations in different scenarios and is compared to the DCF protocol and a previously proposed cooperative protocol. Simulation results show that FuzzyCoop improves the performances of a wireless network and provides a more robust partner selection scheme.

As the fundamental component of dynamic spectrum access, implementing spectrum sensing is one of the most important goals in cognitive radio networks due to its key functions of protecting licensed primary users from harmful interference and identifying spectrum holes for the improvement of spectrum utilization. However, its performance is generally compromised by the interference from adjacent primary channels. To cope with such interference and improve detection performance, this paper proposes a non-coherent power decomposition-based energy detection method for cooperative spectrum sensing. Due to its use of power decomposition, interference cancellation can be applied in energy detection. The proposed power decomposition does not require any prior knowledge of the primary signals. The power detection with its interference cancellation can be implemented indirectly by solving a non-homogeneous linear equation set with a coefficient matrix that involves only the distances between primary transmitters and cognitive secondary users (SUs). The optimal number of SUs for sensing a single channel and the number of channels that can be sensed simultaneously are also derived. The simulation results show that the proposed method is able to cope with the expected interference variation and achieve higher probability of detection and lower probability of false alarm than the conventional method in both hard combining and soft combining scenarios.

In this paper, we propose a cross-layer design of packet level cooperation for wireless LANs that support rate adaptation. While keeping compatibility with legacy wireless LANs, distributed control of multi-hop packet transmission is enabled without pre-negotiation of routing or pairing. These features are provided by prioritization scheme based on IEEE 802.11e EDCF in which we set the parameters according to the measured link condition at each terminal. Relaying packets with high transmission rate makes much efficient use of radio resource, and it leads not only to improve performance of the total system, but also to overcome the fairness issue known in rate adaptation, where the terminals with good link conditions cannot gain enough resource because of the time consumed by low data rate transmission. These advantages are confirmed through computer simulations considering packet error rate at each transmission which is assumed by receiving power calculated from the distance between source, relay, and destination terminals. Furthermore, we also discuss about the fairness between cooperation-enabled and legacy terminals when they coexist in the same system, in order to make gradual deployment feasible.

In recent years, several global network systems using non-stationary satellites have been proposed. Some of them are announced to start services within years. We also have several experimental systems with stratospheric aircrafts. In the future, the radio communication system using stratospheric aircrafts will be one of the promising media for personal communications. The question of how to establish the optimal communication under such circumstance seems to be still open. In this paper, performance evaluations of wireless communication systems using LEO satellites and stratospheric aircrafts are proposed. We will show some proper communication parameters to improve competence of mobile communication in the such systems as well.

Future wireless communication systems aim at very high data rates. As the medium access control (MAC) protocol plays the central role in determining the overall performance of the wireless system, designing a suitable MAC protocol is critical to fully exploit the benefit of high speed transmission that the physical layer (PHY) offers. In the latest 802.11n standard [2], the problem of long overhead has been addressed adequately but the issue of excessive colliding transmissions, especially in congested situation, remains untouched. The procedure of setting the backoff value is the heart of the 802.11 distributed coordination function (DCF) to avoid collision in which each station makes its own decision on how to avoid collision in the next transmission. However, collision avoidance is a problem that can not be solved by a single station. In this paper, we introduce a new MAC protocol called Intelligent Local Avoided Collision (iLAC) that redefines individual rationality in choosing the backoff counter value to avoid a colliding transmission. The distinguishing feature of iLAC is that it fundamentally changes this decision making process from collision avoidance to collaborative collision prevention. As a result, stations can avoid colliding transmissions with much greater precision. Analytical solution confirms the validity of this proposal and simulation results show that the proposed algorithm outperforms the conventional algorithms by a large margin.

Satellite clusters have been satisfactorily implemented in a number of applications, such as positioning and sensor networks, with the purpose of improving communication system capabilities. However, because the use of clusters requires good management of the resources, those solutions imply new challenges for communication systems. This paper focuses on improving the data management between network elements by considering a network formed by satellite clusters. Satellite clusters work in cooperation to provide real-time and non-real-time services in different footprint areas. This study proposes the adjustable energy consumption access scheme (AECS) as one possible solution response to particular necessities of communication and at the same time, as a way of decreasing the system energy consumption. Energy consumption is a key issue that concerns green network operations and it is directly linked to the cooperation and coordination between network elements. On the other hand, we support the implementation of Optical Inter-Satellite Links (OISL) for communication between cluster elements. The analysis involves the study of energy consumption, transmission delay, specific link margins, bit error rate (BER) and QoS.

This paper is a study on the behavioral aspects of the input buffer limiting scheme whose basic feature is to award priority to the transit messages over the input messages so that congestion does not develop in the network. The numerical method employed in the analysis is that proposed in Ref.(7). The performance aspects are studied for different buffer capacities, different message handling capacities and different levels of reservation for transit traffic. The numerical method indicates that thrashing occurs at low levels of reservation for the transit messages, irrespective of the buffer size or the processor capacities of the node. This observation is supported by simulation results. With reference to the state-space of the model of our study, the congestion aspects are related to two Liapunov functions. Under the domain of one of the Liapunov functions, the evolution of the perturbed system is towards a congested state whereas, under the domain of the other Liapunov function, the evolution is towards a congestion-free state. Regardless of the configuration, it is found that the fundamental characteristic of the congestion under the input buffer limiting scheme is the characteristic of a fold catastrophe. In the systems with insufficient level of reservation for the transit traffic, the performance degradation appears to be inevitable, irrespective of the capacities of the nodal processor and output channel processor, and the size of the buffer pool. Given such an inevitability, the active life of a node under a typical node configuration is studied by simulation. A suitable performance index is suggested to assess the performance of deadlock-prone nodes.

Sensor networks are often used to understand underlying phenomena that are reflected through sensing data. In real world applications, this understanding supports decision makers attempting to access a disaster area or monitor a certain event regularly and thus necessary actions can be triggered in response to the problems. Practitioners designing such systems must overcome difficulties due to the practical limitations of the data and the fidelity of a network condition. This paper explores the design of a network solution for the data acquisition domain with the goal of increasing the efficiency of data gathering efforts. An unmanned aerial vehicle (UAV) is introduced to address various real-world sensor network challenges such as limited resources, lack of real-time representative data, and mobility of a relay station. Towards this goal, we introduce a novel cooperative path selection framework to effectively collect data from multiple sensor sources. The framework consists of six main parts ranging from the system initialization to the UAV data acquisition. The UAV data acquisition is useful to increase situational awareness or used as inputs for data manipulation that support response efforts. We develop a system-based simulation that creates the representative sensor networks and uses the UAV for collecting data packets. Results using our proposed framework are analyzed and compared to existing approaches to show the efficiency of the scheme.

For the connection request procedure in mobile communication systems, a previous study had shown that the 3-channel systems provide the haighest maximum of stable per channel throughput. In this paper, we propose and study a new algorithm, called the Parallel Collision Resolution Algorithm, which can be implemented in a Q-channel connection request environment, where Q3. For the implementation, the channels are arranged in R groups, where R is a positive integer. The collision resolution scheme distributes the collided messages over all the groups so that throughput and delay measures can be improved. At any point in time, there can be a maximum of R collision resolution schemes operational irrespective of the channel or the group number over which collisions occurred. The performance measures are estimated by computer simulation. Under the new algorithm, almost the same level of the perchannel stable throughput measure of a 3-channel network can be achieved in networks for which Q3. This feature allows freedom to the network designer to employ a higher number of connection request channels without forfeiting high channel utilization rates. When Q is an integral multiple of 3, the maximum stable per channel throughput level achieved can be the same as that achieved by the 3 channel system, if the grouping of channels is such that each group consists of 3 channels. When Q is not an integral multiple of 3, the intuitive strategy of organizing the channels in such a way that Q/3 groups consist of 3 channels each and one group consists of (Q mod 3) channels, may result in much degraded performance. It is found that, if the channels are so organised that no group is composed of (Q mod 3) channels, the performance levels can be substantially enhanced. Also, under the new algorithm, the delay measure is significantly improved, particularly in schemes like the mobile satellite systems with high propagation delays. We conclude that the new scheme presents a promising collision resolution methodology for connection request procedures.

This paper presents a new architecture of a copy network which employs the principle of recursive generation of copy cells. The proposed architecture achieves high utilization of the links and buffers of the copy network, and preserves the cell sequence. The architecture lends itself modularity so that large multicast ATM switches can be fabricated by employing the proposed copy network. Two different modular structures - one for reduced latency of the unicast cell and the master cell from which copies are made, and the other for reduced hardware overhead - for realizing large multicast ATM switches are configured. The hardware of functional elements of the copy network are the same as those of the functional elements of a modular point-to-point switch proposed earlier, thereby resulting in the modularity of functional elements as well. Simulation studies show that the proposed copy network achieves high throughput and low cell loss probability, and the required buffer sizes are small. The delay of cells is found to be very small for traffic loads up to 90%.

In this paper, we present a new mathematical framework based on point process theory for a type of four-way handshake-based medium access control schemes which have so far only been studied by simulation. The theoretical model we present takes into account time-varying channel impairments, the interference inherent to large networks, different decoding requirements for each packet and the influence of the routing protocol. Moreover, in contrast with the majority of the literature, the influence of imperfect feedback is also considered in the analysis. Throughout the paper, we derive in closed forms the average link outage probability as well as the Average Spatial Density of Transport (ASDT) in a mobile multi-hop ad hoc network. All results are confirmed by comparison to simulated data and lead to two general conclusions. In the presence of fading uncorrelated between traffic and control handshakes, we observe the following. 1) Optimal contention is achieved by designing control packets that are decodable even in the presence of strong interference. 2) The impact of imperfect feedback on performance in an interference-limited mobile ad hoc network is not negligible.

We have been investigating an orthogonal frequency division multiple access (OFDMA) based cellular system that is called "dynamic parameter controlled orthogonal frequency and time division multiple access (DPC-OF/TDMA)" for the development of beyond third generation (B3G) mobile communication systems. Moreover, we have already proposed a time alignment control (TAC) to compensate propagation delays that induce a multiple-access interference (MAI) in the uplink OFDMA. However, that TAC includes a large amount of computations. This means that it is quite difficult for the OFDMA systems to implement TAC into volume-limited hardware devices such as field programmable gate array (FPGA). Thus, we propose a new complexity-reduced TAC (CRTAC) in this paper. CRTAC can be implemented into such devices easily. In this paper, we show some computer simulation results, and then evaluate the error rate performances of DPC-OF/TDMA employing CRTAC. Moreover, we also show the benefit of the reasonable level of the implementation complexity made by CRTAC.

Vehicles' speed is one of the key factors in vehicle travel efficiency, as speed is related to vehicle travel time, travel safety, fuel consumption, and exhaust gas emissions (e.g., CO2 emissions). Therefore, to improve the travel efficiency, a recommended speed calculation scheme is proposed to assist driving in Vehicle Ad hoc networks (VANETs) circumstances. In the proposed scheme, vehicles' current speed and space headway are obtained by Vehicle-to-Roadside unit (V2R) communication and Vehicle-to-Vehicle (V2V) communication. Based on the vehicles' current speed and adjacent vehicles' space headway, a recommended speed is calculated by on-board units installed in the vehicles, and then this recommended speed is provided to drivers. The drivers can change their speed to the recommended speed. At the recommended speed, vehicle travel efficiency can be improved: vehicles can arrive at destinations in a shorter travel time with fewer stop times, lower fuel consumption, and less CO2 emission. In particular, when approaching intersections, vehicles can pass through the intersections with less red light waiting time and a higher non-stop passing rate.

We investigate the multipacket message transmissions and variable length message transmissions in slotted ALOHA systems with capture effect. First, we propose an approach that the transmission power level is controlled probabilistically depending on message length for multipacket messages. We consider the multipacket messages model with capture. We derive explicit equations of the effective channel utilization of the model. It is demonstrated that if we increase the numbar of power levels, we can get more effective channel utilization of the system. Secondly, we propose how to assign the slot size and show that the effective utilization of the channel is improved for variable length messages using the approach proposed for multipacket messages. Channel design issue about length of the slot depending on the number of power levels used for transmission is discussed. Thirdly, we propose the multiple messages per slot model with capture. The analytical results show that the multiple messages per slot model can achieve the highest channel utilization among the models discussed in this paper.

Secure access is one of the key concerns of wireless sensor networks (WSNs). In WSNs, because there are many dynamically mutable attributes, continuous access decisions and dynamic attribute updates should be important properties of access control. In addition, WSNs need low-complexity authentication protocols because of the constrained resources. However, the authentication protocols of most current security access schemes have relatively high complexity. More importantly, the access control models of existing schemes cannot provide attribute mutability and continuous decisions dynamically. To address above issues, we propose a dynamic secure access mechanism for WSNs. Firstly, we design a lightweight secure authentication protocol and dynamic access control based on security token and usage control (UCON), respectively. Then, the agent technology is adopted to implement the proposed secure access scheme. Secondly, we analyze the probability of the dynamic attribute update and decisions. Thirdly, we implement an instance of UCON. The implementation results indicate the feasibility of using UCON in WSNs. Finally, by evaluating and comparing with current schemes, the authentication protocol in our scheme presents several advantages including the low expenses in calculation, storage and communication. To our best knowledge, this paper is the first to realize next generation dynamic access control with attribute mutability and continuous decisions in WSNs.

This paper describes a proposed propagation estimation method and TCP/IP-based evaluations for mobile communications employing a stratospheric platform. To estimate a wireless channel, a realistic and detailed description of its physical environment must be accurately defined. Therefore, a building distribution model characterizing the physical environment in areas in Japan is presented. The analyses of the propagation estimation method are based on the "ray-tracing" model. The results from the proposed method are derived depending on elevation and azimuth angles. In order to validate our results, comparisons between the proposed method and our previous measurement are made for a typical semi-urban area in Japan. The comparisons show close agreement between the estimation results and the measurement results. Finally and interestingly, we present communication performance evaluations based on TCP/IP protocol by using the results achieved from our channel estimation with semi-analytical and simulation approach.