We describe in this paper our experience of developing a large-scale, highly distributed multi-agent system using wireless-networked sensors. We provide solutions to the problems of localization (position estimation) and dynamic, real-time mobile object tracking, which we call PET problems for short, using wireless sensor networks. We propose system architectures and a set of distributed algorithms for organizing and scheduling cooperative computation in distributed environments, as well as distributed algorithms for localization and real-time object tracking. Based on these distributed algorithms, we develop and implement a hardware system and software simulator for the PET problems. Finally, we present some experimental results on distance measurement accuracy using radio signal strengths of the wireless sensors and discuss future work.

DS-CDMA provides a flexible support for the low-to-high bit rate of multimedia services upon a specific user's request. A simple capacity expression is derived for a power-controlled reverse link of orthogonal multi-code DS-CDMA with multiple connections. It is found that an orthogonal multi-code user having multiple connections is equivalent to a single connection user, but with a spreading factor reduced by a factor of the total number of parallel codes and a required signal energy per symbol-to-interference plus noise power spectrum density ratio which is the average taken over multiple connections. Furthermore, the use of antenna diversity is found equivalent to the use of higher spreading factor increased by a factor of the number of antennas.

Today, many audiovisual delivery systems, including video streaming and video conferencing, are being developed for use over a range of networking technologies, the differing characteristics of which pose problems for service level interoperability. Multimedia transcoding is one means to provide interoperability between different types of audiovisual terminals and between terminals that connect to different networks. In this paper, we will present a multimedia transcoder system, which provides interoperability between video conferencing terminals on IP networks and mobile terminals on mobile networks.

The mobile system has been taking an important role as the infrastructure of the advanced information society. First, as the telecommunication infrastructure, it enabled us to communicate with anybody, anytime, and anywhere. Next, as the multimedia infrastructure, it enabled us to send, receive, and retrieve data. Now, as the IT infrastructure, it enables us to purchase commodities, transact business, and enjoy entertainment on-line. The mobile system has morphed into the third generation in almost two decades. Several mobile services have been introduced in each generation to feature the above infrastructures. This paper describes the authors' views on how mobile services have contributed to the advance of the information society in the past three generations. It also discusses what role the future mobile system should take in realizing the wealthier information society.

Hierarchical Mobile IPv6 (HMIPv6) has been proposed to accommodate frequent mobility of terminals within the Internet. It utilizes a router, named Mobility Anchor Point (MAP), so that networks can manage mobile terminals without floods of signaling message. Note here that, particularly in a wide area network, such as a mobile communication network, it is efficient to distribute several MAPs within the same network and make the MAP domains cover overlapped areas. This enables the network to manage the terminals in a flexible manner according to their different mobility scenarios. The method described in the Internet-Draft at the IETF, however, lets mobile terminals select its MAP. This may cause load concentration at some particular MAPs and/or floods of signaling messages, because the terminals may not select a feasible MAP in a desirable manner. In this paper, a MAP selection method in distributed-MAPs environment is proposed. It reduces signaling messages to/from outside networks without load concentration at any particular MAPs. Finally, we show that the proposed method works effectively by simulation experiments.

It is very important to accurately and effectively track the movement of each mobile terminal in the personal communications networks. Accordingly, various dynamic location update schemes reflecting the mobile terminal's moving pattern are proposed in the literature. Important schemes include time-based, movement-based, distance-based, and direction-based location update schemes. In this paper, we intend to improve the performance of the movement-based location update scheme which is simple to implement and shows good performance. The proposed location update scheme reflects each mobile terminal's movement characteristics and reduces the number of unnecessary location updates. The performance of the proposed scheme is evaluated and compared with the previously proposed movement-based location update scheme through simulation. The proposed location update scheme requires three memory elements to maintain the movement history in the mobile terminal. The simulation results show that the location management cost is significantly reduced especially for the case with small call-to-mobility ratio by minimizing the unnecessary location updates intrinsic in the movement-based location update scheme.

Mobile terminals with Internet services such as i-mode are in wide use, and communication bandwidths are growing even further under 3G technology. However, displays of mobile terminals will remain small in view of their portable size and power consumption. In this paper, we propose a "ubiquitous display" that can be used in combination with mobile terminals. The user operates the mobile terminal and the ubiquitous display shows any content that requires a large screen space.

Multiple access interference (MAI) due to many simultaneous users is the main factor that limits the performance of DS-CDMA system. Multiuser detection is a method to avoid performance degradation due to MAI. We propose a blind multiuser detection method based on the algorithm consisting of two-stage decoding process, i.e., linearly constrained constant modulus (LCCM) and iterative least squares (ILS). The computer simulations confirmed that the algorithm is near-far resistant and that the proposed method is effective in the application to the slow fading channels.

We developed a priority queueing method that can keep the loss ratio of high-priority packets at a target value. It regulates the number of input low-priority packets when the queue length exceeds the threshold that is adjusted so as to minimize the Lyapunov function of the system. We showed that the number of calculations for the optimum threshold is sufficiently small and derived a sufficient condition for the proposed method to be robust against unknown load fluctuations. From the viewpoint of guaranteeing the loss ratio of high-priority packets, we showed, by computer simulation and theoretical analysis, that the proposed method is superior to conventional methods such as the fixed-threshold method and the pushout method.

During TCP/IP communications, MobileIP routing optimization functions causes out-of-order TCP packet sequences. To solve this problem, we propose a dynamic delayed ACK control scheme in which the wireless link-state management part notifies the upper TCP/IP layer of base-station hand-over, and at this time the TCP/IP layer sends dynamic delayed ACKs in response by using two-level-timer (i.e., hard-timer and soft-timer) processing. Simulation results confirm that applying dynamic delayed ACK control to MobileIP networks improves average throughput.

This paper investigates the effect of fast cell selection (FCS) associated with fast packet scheduling methods and hybrid automatic repeat request (HARQ) with Chase combining, in which the optimum cell (or sector) transmitting a slot-assigned downlink shared channel (DSCH) is selected based on the received signal-to-interference power ratio (SIR), in high-speed downlink packet access (HSDPA). The Round robin (RR), Proportional fairness (PF) and Maximum carrier-to-interference power ratio (CIR) schedulers are used as the scheduling algorithm. The simulation results elucidate that although almost no additional diversity gain through FCS is obtained for the PF and Maximum CIR schedulers, the improvement in throughput by FCS coupled with the RR scheduler is achieved. Furthermore, we elucidate that the effect of FCS is small when only inter-sector FCS is performed; however, inter-cell FCS is effective in improving the radio link throughput for the access users with a lower received SIR near the cell edge. The radio link throughput at the cumulative distribution of 20% of soft handover users when both inter-sector and inter-cell FCS are performed is increased by approximately 20% and 60% for PF and RR schedulers, respectively, compared to that without FCS, i.e. with hard handover. We also show that when a traffic model such as the modified ETSI WWW browsing model is taken into account, the effect of FCS associated with the decreasing effect of fast packet scheduling is greater than that assuming continuous packet transmission. The user throughput at the cumulative distribution of 20% employing both inter-sector and inter-cell FCS is increased by approximately 60% compared to that without FCS.

The plan of High Altitude Platform Station (HAPS) is considered as a revolutionary wireless system plan with several economic and technological advantages over both space- or ground-based counterparts. In this paper, we propose a joint system of terrestrial and HAPS cellular for Wideband-CDMA mobile communication. This system makes the conventional terrestrial W-CDMA cellular area smaller and the remainder area covered by HAPS to increase the total capacity. Furthermore in down link channel, we introduce the polarized wave and doughnut-like radiation. However, in the proposed system, the performance would be dependent on the terminal position especially near the boundary of doughnut-like cell zone. To overcome this, site diversity that uses both signals from terrestrial Base Station and HAPS Base Station is also introduced. To confirm the availability of the proposed system, we evaluate the system performance by computer simulation.

In the Mobile IP handoff procedure, mobile node movement is detected from advertisements of foreign agents that differ from previously received advertisement and the new "care-of" address is registered with the home agent. However, user packets are not forwarded to the new foreign agent until a registration is completed and this interruption may degrade the quality of service especially in real-time applications such as audio and video, or may lower the TCP throughput due to retransmission timeout. To tackle these issues, we propose a new low latency handoff method, where access points used in a wireless LAN environment and a dedicated MAC bridge are jointly used to alleviate packet loss without altering the Mobile IP specifications. In this paper, we present design architecture of the proposed method and evaluate its performance in an actual network environment to verify the effectiveness of our approach.

In this letter, we investigate the blind channel estimation in MIMO-OFDM systems based on the second-order statistics of the channel outputs only. We exploit the cyclostationarity induced by OFDM with cyclic prefix, establish the sufficient identifiability conditions, and develop a subspace algorithm. Finally, we demonstrate the validity of the algorithm by computer simulations.

This paper addresses a high-order sliding mode control strategy for output tracking of nonholonomic mobile robots. First, we introduce the dynamic model of robots, driving motors and nonslipping kinematics constraint conditions. Second, we decompose the system into linear and nonlinear components via diffeomorphism and nonlinear input transformation. Also we consider parameter variations of robots and deduce the uncertain model of robots. Third, we design a high order sliding mode controller for output tracking of known and uncertain systems, respectively. Finally, we perform numerical simulations, demonstrating that the proposed high-order sliding mode control not only reduces the chattering problem of sliding mode systems, but also has certain robustness properties with respect to uncertainties of robots.

This paper assesses the performance of the handoff algorithm based on distance and RSSI measurements in a multi-cellular environment by computer simulation. The algorithm performs a handoff if handoff initiation conditions, handoff possible conditions, and handoff selective conditions are met. The performance criteria are based on the average number of handoffs, the crossover points and the average number of outages. Numerical results are presented to demonstrate the feasibility of the algorithm. The performance of the distance-assisted handoff algorithm is compared with that of a conventional algorithm that utilizes signal strength alone. Overall, the distance-assisted algorithm exhibits higher performance in average number of handoffs and the crossover points, yet exhibits a higher number of outages on average than the conventional algorithm.

The handoff of in Mobile IP causes packet sequence disruption during packet forwarding procedure, which may result in degradation of network performance in higher layer protocol. In this paper, we investigate the impact of handoff on out-of-sequence packets in Mobile IP with Route-Optimization extension. To evaluate the handoff procedure, we use the unstable time period (UTP) when the packet sequence could be mis-ordered. The results show that the UTP mainly depends on queuing delay of old Foreign Agent (FA) and link capacity between old FA and new FA. Further, we propose that the priority queuing for the astray packets in the old FA can increase Mobile IP performance during handoff procedure by reducing the UTP.

This paper proposes a multi-hop wireless link system for radio access networks (RANs) of new generation mobile communication systems. The performance of the multi-hop wireless link system is evaluated from the viewpoints of total output power, co-frequency interference characteristics, and the system frequency bandwidth based on a comparison with that of the single-hop wireless link system, which is currently used as a RAN. The proposed system is effective in realizing an enormous approach link capacity from both the total output power and the co-frequency interference viewpoints. From the system frequency bandwidth viewpoint, the optimum number of relays in the multi-hop connection is determined to be three hops in a line-of-sight propagation environment in order to minimize the frequency bandwidth for transferring traffic. We conclude that the multi-hop wireless link system is suitable for new generation mobile communication systems.

Caching of frequently accessed data has been shown to be a useful technique for reducing congestion on the narrow bandwidth of wireless channels. However, traditional client/server strategies for supporting transactional cache consistency, which require extensive communications between a client and a server, are not appropriate in a wireless mobile database. This paper proposes two, simple but effective, transactional cache consistency protocols for mobile read-only transactions by utilizing the broadcast-based solutions for the problem of invalidating caches. The novelty of our approach is that the consistency check on accessed data and the commitment protocol are implemented in a truly distributed fashion as an integral part of cache invalidation process. The applicability of proposed techniques is also examined by an analytical study.

Future wireless communication systems will provide mobile terminals with high bit-rate transmissions for accessing broadband wired networks. In this paper, we envisage a Time Division Multiple Access - Time Division Duplexing (TDMA-TDD) air interface and we propose a Medium Access Control (MAC) protocol, named Dynamic Scheduling - TDD (DS-TDD), that allows guaranteeing the QoS of different traffic classes and efficiently supports uplink/downlink traffic asymmetries. The DS-TDD performance is theoretically analyzed. Moreover, the DS-TDD protocol is compared with another scheme proposed in the literature. Finally, the impact of packet errors on the DS-TDD performance is evaluated.