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.

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.

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.

In this paper, we propose an RTP/UDP/IP header compression method, Multiple-Reference Compression (MRC), which is designed for mobile multimedia communications. MRC is a compression method that calculates differences from the multiple reference headers that have already been sent and inserts them into a compressed header. The receiver can decompress the compressed header as long as at least one of the reference headers is correctly received and decompressed. MRC improves robustness against packet losses compared with CRTP defined in IETF RFC2508, and imposes less overheads and computational burden than robust header compression (ROHC) defined in RFC3095. We also implemented MRC and other header compression algorithms into our mobile testbed, and conducted multimedia streaming experiments over the testbed. The results of the experiments show that MRC offers the same level of packet loss rate as Legacy RTP for both audio and video streams, and provides better media quality than Legacy RTP and CRTP on error-prone radio links. Header compression robust against packet losses is expected as a key technology for VoIP and multimedia streaming services over 3G and future mobile networks.

This paper discusses self-similarity in cell dwell time of a mobile terminal, the discovery of which was described in our previous paper, and its effects on teletraffic of mobile communication networks. We have evaluated various teletraffic statistics, such as cell dwell time and channel occupancy time, of a mobile terminal based on measurements of motion for various types of vehicles. Those results show that cell dwell time follows a long-tailed log-normal distribution rather than the exponential distribution that has been used for modeling. Here, we first elaborate on self-similarity in cell dwell time of various vehicles. We then evaluate self-similarity in channel occupancy time. For future mobile multimedia communication systems employing a micro-cell configuration, it is anticipated that data communication will be the main form of communication and that call holding time will be long. For such cases, we have shown that channel occupancy time will be greatly affected by the cell dwell time of the mobile terminal, and that self-similarity, a characteristic that is not seen in conventional systems, will consequently appear. We have also found that hand-off frequently fails as self-similarity in cell dwell time of a mobile terminal becomes stronger.

In evaluating the teletraffic of mobile communication networks, it is important to model the motion of terminals. In the previous migration model, mobility characteristics of terminals, such as cell dwell time, have been expressed by a single probability distribution. In this paper, we discuss the modeling of the cell dwell time of terminals in each cell. Using measured data we show that cell dwell time differs from cell to cell and follows log-normal distributions rather than conventional exponential distributions.

This paper proposes adaptive antenna array transmit diversity (AAA-TD) in the W-CDMA forward link with frequency division duplexing (FDD), based on adaptively-generated receiver antenna weights in the reverse link, which only track the changes in the average signal-to-interference power ratio (SIR) and direction of arrival (DOA) but with the calibration of the phase/amplitude variations of the parallel RF receiver/transmitter circuits corresponding to the number of array antennas. The laboratory and field experimental results exploiting AAA-TD are presented to show the strong multipath interference (MPI) suppression effect especially from high-rate users with large transmission power. Laboratory experiments elucidate that by using AAA-TD with four antennas, the required transmitted SIR before multiplying the transmitter antenna weights at the average BER of 10-3 is decreased by approximately 13 dB compared to that with one omni-directional antenna transmitter. Field experiments also show that although an error floor above 10-2 is observed with one omni-directional antenna transmitter when the transmitted SIR is -12 dB due to severe MPI, no error floor is observed when employing 4-antenna AAA-TD and the loss of the required received signal power at the average BER of 10-3 from the single-user case is suppressed to below approximately 5 dB. Therefore, we show that AAA-TD is very effective in suppressing severe MPI especially from high rate users with large transmission power due to its adaptive main lobe and null steering.

In this paper, we propose an efficient algorithm of quick routing for private Wireless ATM (Asynchronous Transfer Mode) networks. This algorithm uses hierarchical source routing, which first establishes a connection between the sending node and the home node (which has the terminal location information), and then, partially releases the connection and reroutes it between the sending node and the receiving node. This results in less delay than found in other conventional routing methods, and avoids passing through congested nodes, because this algorithm uses traffic estimation based on queuing theory. We also show the effectiveness of this algorithm with the use of computer simulations.

This paper investigates the optimum adaptive antenna array beam forming (AAA-BF) configuration considering the diversity effect provided by transmit diversity (TD) in a multipath fading channel in the W-CDMA forward link. Computer simulation results show that the required transmit signal energy per bit-to-background noise power spectrum density ratio (Eb/N0) at the average block error rate (BLER) of 10-2 using AAA-BF coupled with TD is decreased by approximately 1.0 dB compared to that of AAA-BF assuming the identical number of total antennas when the capacity, i.e., the number of simultaneously accommodated users with the transmission power proportional to the symbol rate, normalized by processing gain, Pg, is below approximately 20%. However, we find that in an interference-limited channel, when the capacity normalized by Pg is above approximately 30%, AAA-BF employing all antennas accommodates a larger capacity compared to AAA-BF coupled with TD because of a sufficient interference suppression effect due to a much narrower beam width despite the absence of the antenna diversity effect. This paper also elucidates in a multi-cell model that AAA-BF employing all antennas can accommodate approximately 1.5 times more users with the average BLER of 10-2 and with the outage probability of 5%, compared to the case with AAA-BF coupled with TD, when the total number of antennas is 8.

The IETF Mobile IP defines two multicast options: remote subscription (RS) and bi-direction tunneling (BT). In order to synthesize the strong points of these two IETF multicast options, we propose a hybrid approach, mMOM, which selectively uses two IETF multicast options based on the mobility of mobile hosts. Whenever a mobile host requests its first registration to a certain foreign agent, the corresponding foreign agent starts the service using the BT option. Afterwards, if it requests re-registration to the same foreign agent, the foreign agent considers it to be relatively immobile and continues services using the RS option. We propose a new metric to compare heterogeneous algorithms. Simulation results show that our approach outperforms all others.

Transmit beamforming is a promising way to increase the downlink capacity of wireless networks. Since all users are coupled via their radiation patterns, the beamforming vectors must be optimized along with power control. It is necessary to balance the signal-to-interference levels according to individual QoS requirements. This problem leads back to the minimization of the infinity-norm of a certain vector and has first been studied by Gerlach and Paulraj in [1]. It has been assumed that the optimum solution can be obtained by minimizing the 1-norm instead, thereby leading to a new problem, which is generally easier to handle. The analytical and numerical results in this paper, however, indicate that this conjecture is generally not valid. We characterize the case where the 1-norm solution also solves the infinity-norm problem. In particular, it is shown that for the special case of a 2-user scenario, both optimization problems are indeed equivalent and a closed-form solution can be given. The analytical results provide new insights into the problem of coupled downlink beamforming and offer a useful approach to the design of efficient and reliable algorithms.

A closed-form solution of mobile location can be obtained by minimizing the norm of the error vector of a set of linear equations in spherical interpolation (SI) method. However, it assumes that the intermediate variables are independent though they are related by a quadratic equation. To give an improved position estimate, we use this relationship to help SI method and a new location method is proposed based on the approximate linearization of spherical intersecting constraint. Simulation results for line-of-sight and non-line-of-sight situations show that the proposed method performs significantly better than SI and Chan's method, especially for the cases where large bias and large standard deviation exist in range difference measurements.

For ubiquitous computing and communication, a mobile node needs to change upon movements its network- and link-layer points of attachment to the Internet. Conventionally, the network-layer protocol is unaware of the link-layer changes, so IP datagrams to and from a moving node could be lost. Considerable system performance degradation might therefore result. This paper presents a scheme to integrate the handoff procedures of the two layers in the context of Mobile IP with route optimization. Experiment results show that our scheme is promising and can reduce packet loss due to host migrations substantially.

Mobile IP is a solution to support host mobility in the Internet. But, packets can be lost during the movement detection and registration periods of the Mobile IP. Regular TCP interprets theses packet losses as signs of network congestion, so it reduces its transmission rate by reducing its window size and slow start threshold. Besides, the multiple packet losses occurring during handoffs trigger successive retransmission timeouts at the TCP sender, causing a long communication pause even after handoff is completed. These together lead to significant throughput degradation. In this paper, we propose two new TCP schemes to reduce packet losses and to alleviate the effects of handoffs on TCP performance. TCP-MD (Movement Detection) is proposed to reduce packet losses by detecting a handoff earlier, and TCP-R (Registration) is designed to prevent packet losses by freezing data transmission during registration. The proposed schemes maintain end-to-end TCP semantics, making it possible to fully interoperate with the existing infrastructure. Only a small change is required in the mobile host, plus the implementation is simple because some Mobile IP messages are used to notify the handoff, eliminating the need for any additional messages. Simulations confirmed that the proposed schemes can give an excellent performance in an environment where the mobile host experiences frequent handoffs.

Scaled models for an anatomical head model and a simple head model are used to investigate the effects of head size on SAR characteristics for a cellular phone exposure at 835 MHz. From the results, we can see that a larger head produces a higher localized SAR and a lower whole-head averaged SAR.

Transmitter diversity is a powerful technique to improve the transmission quality of downlink in microcellular mobile communications systems. Under cochannel interference (CCI) at the base station (BS), the transmitter diversity is not necessarily effective, because the desired-plus-interference signal power used as a criterion of downlink branch selection is not always relative to the downlink propagation condition. This paper proposes the theoretical derivation of bit error rate (BER) performance in the transmitter diversity under CCI occurring at BS, as parameters of average SIR at BS, normalized Doppler frequency, and so on. It is confirmed from the correspondence of theoretical results with simulation results that the proposed theoretical approach is applicable to the CCI environments at BS.

Implementation of Multi-Input Multi-Output (MIMO) channel sounder is considered, taking hardware cost and realtime measurement into account. A remarkable difference between MIMO and conventional Single-Input Multi-Output (SIMO) channel sounding is that the MIMO sounder needs some kind of multiplexing to distinguish transmitting antennas. We compared three types of multiplexing TDM, FDM, and CDM for the sounding purpose, then we chose FDM based technique to achieve cost effectiveness and realtime measurement. In the framework of FDM, we have proposed an algorithm to estimate MIMO channel parameters. Furthermore the proposed algorithm was implemented into the hardware, and the validity of the proposed algorithm was evaluated through measurements in an anechoic chamber.

This paper deals with security issues in a mobile agent system, especially protecting agent data from malicious servers. For this purpose, one-time key generation system, OKGS in short, is proposed. In OKGS, we integrate notions of an one-way hash function and a coupler. A one-way function plays a major role in ensuring confidentiality and integrity of agent data. And the notion of a coupler is used to establish inter-relationship among consecutive encryption keys for agent data, i.e,. all agent keys form a unidirectional chain. With these two features of OKGS, therefore, only the agent owner, who creates the agent bearing data, can decrypt and protect all agent data which are gathered in its itinerary.

The Fundamental Law of Science and Technology and the Basic Plan of Science and Technology were established in Japan in 1995 and 1996, respectively and the second Basic Plan for Science and Technology R&D was established in April, 2001. In addition, as a part of Japanese government administrative reform, the Council for Science and Technology Policy was organized in the Cabinet Office and the most of the national research laboratories including the Communications Research Laboratory (CRL) have been restructured to the Independent Administrative Institution (IAI). This paper introduces first the relationship between the info-communications R&D and the national science and technology policy in Japan. Second, the R&D of CRL, IAI is introduced. Third, as a typical example of the wireless communications R&D, the research of wireless communication in CRL is shown in more detail in terms of terrestrial wireless/mobile communication system and satellite communication system. Finally the future prospect of such an R&D is shown briefly.

The stateless-based cache invalidation schemes for wireless environments can be categorized into either asynchronous or synchronous cache invalidation according to the broadcasting way of invalidation report. However, if the asynchronous cache invalidation scheme attempts to support local processing of read-only transaction, a critical problem may occur; the asynchronous invalidation reports provide no guarantee of waiting time for mobile transactions requesting commit. To solve this problem, the server in our approaches broadcasts two kind of messages, asynchronous invalidation report to reduce transaction latency and periodic guide message to avoid the uncertainty of waiting time for the next invalidation report. This paper presents a simulation-based analysis on the performance of the suggesting algorithms. The simulation experiments show that the local processing algorithms of read-only transaction based on asynchronous cache invalidation scheme get better response time than the algorithm based on synchronous cache invalidation scheme.