This letter shows the performance comparisons of several different rate scheduling schemes for non-real time data service over the uplink of burst switching-based direct sequence code division multiple access (DS/CDMA) system to support the integrated voice/data service. The closed-form solution of optimal scheduling formulation, which minimizes average transmission delay when all of the active data users are transmitting simultaneously, is presented and mathematical analyses with other rate scheduling schemes, which provide efficiency criterion of transmission delay for rate scheduling schemes, are performed. Numerical results show the analyses explicitly.

In this paper, an adaptive transmission scheme considering the stay time in a spot mobile access system is proposed. The proposed adaptive transmission scheme selects the modulation format according to the user's stay time in the spot communication zone and the types of data requested by each user. In the proposed system, when the stay time of a user is short, high-speed modulation is selected for this user. When the stay time of a user is long, a more reliable modulation format is selected. The computer simulation results show that the proposed transmission scheme without any channel estimation can achieve the same or better performance than when using the modulation format fixedly when the carrier-to-noise ratio changes rapidly.

Quality of Service (QoS) provisioning is a new but challenging research area in the field of Mobile Ad hoc Network (MANET) to support multimedia data communication. However, the existing QoS routing protocols in ad hoc network did not consider a major aspect of wireless environment, i.e., mutual interference. Interference between nodes belonging to two or more routes within the proximity of one another causes Route Coupling. This can be avoided by using zone-disjoint routes. Two routes are said to be zone disjoint if data communication over one path does not interfere with the data communication along the other path. In this paper, we have proposed a scheme for supporting priority-based QoS in MANET by classifying the traffic flows in the network into different priority classes and giving different treatment to the flows belonging to different classes during routing so that the high priority flows will achieve best possible throughput. Our objective is to reduce the effect of coupling between routes used by high and low priority traffic by reserving zone of communication. The part of the network, used for high priority data communication, i.e, high priority zone, will be avoided by low priority data through the selection of a different route that is maximally zone-disjoint with respect to high priority zones and which consequently allows contention-free transmission of high priority traffic. The suggested protocol in our paper selects shortest path for high priority traffic and diverse routes for low priority traffic that will minimally interfere with high priority flows, thus reducing the effect of coupling between high and low priority routes. This adaptive, priority-based routing protocol is implemented on Qualnet Simulator using directional antenna to prove the effectiveness of our proposal. The use of directional antenna in our protocol largely reduces the probability of radio interference between communicating hosts compared to omni-directional antenna and improves the overall utilization of the wireless medium in the context of ad hoc wireless network through Space Division Multiple Access (SDMA).

In order to benefit from the advantages of soft handoff (SHO), it is important that the SHO parameters (the SHO thresholds; T_ADD and T_DROP are well assigned. T_ADD is the threshold used for triggering a pilot with high strength to be added to the Active Set (AS) list. The AS means the pilots associated with the forward traffic channels assigned to mobile station. In contrast, T_DROP is the threshold used for triggering a pilot with low strength to be dropped from the AS list. This paper analyzes the effects of varying SHO thresholds in a cellular code division multiple access (CDMA) system on the blocking probability based on traffic load and geometrical distances in hexagonal layout of base stations (BSs). In addition, the previously proposed traffic load equation is applied to the proposed SHO model for balancing the numbers of new and handoff calls on the forward link capacity in case of uniform traffic load. The results show that the blocking probability is more sensitive to T_DROP than to T_ADD variations.

We propose a method for analyzing trade-off between an environment where a Java mobile code application is running and requirements for the application. In particular, we focus on the security-related problems that originate in low-level security policy of the code-centric style of the access control in Java runtime. As the result of this method, we get feasible requirements with respect to security issues of mobile codes. This method will help requirements analysts to compromise the differences between customers' goals and realizable solutions. Customers will agree to the results of the analysis by this method because they can clearly trace the reasons why some goals are achieved but others are not. We can clarify which functions can be performed under the environment systematically. We also clarify which functions in mobile codes are needed so as to meet the goals of users by goal oriented requirements analysis(GORA). By comparing functions derived from the environment and functions from the goals, we can find conflicts between the environments and the goals, and also find vagueness of the requirements. By resolving the conflicts and by clarifying the vagueness, we can develop bases for the requirements specification.

This paper proposes an SoC test architecture generation framework. It contains a database, which stores the test cost information of several DFTs for every core, and a DFT selection part which performs DFT selection for minimizing the test application time using this database in the early phase of the design flow. Moreover, the DFT selection problem is formulated and the algorithm that solves this problem is proposed. Experimental results show that bottlenecks in test application time when using a single DFT method for all cores in an SoC is reduced by performing DFT selection from two types of DFTs. As a result, the whole test application time is drastically shortened.

In this paper, we propose a preemptive test scheduling technique (a test can be interrupted and later resumed) for core-based systems with the objective to minimize the test application time. We make use of reconfigurable core test wrappers in order to increase the flexibility in the scheduling process. The advantage with such a wrapper is that it is not limited to a single TAM (test access mechanism) bandwidth (wrapper chain configuration) at each core. We model the scheduling problem as a Bin-packing problem, and we discuss the transformation: number of TAM wires (wrapper-chains) versus test time in combination with preemption, as well as the possibilities and the limitations to achieve an optimal solution in respect to test application time. We have implemented the proposed preemptive test scheduling algorithm, and we have through experiments demonstrated its efficiency.

In wireless ad hoc networks, various medium access control (MAC) protocols have been developed to avoid data packet collision and improve channel utilization in the presence of hidden terminals. For distributed MAC protocols, exchange of messages (handshake), are usually required before data transmission. Based on the initiator of the handshake, MAC protocols can be categorized as sender-initiated or receiver-initiated protocols. Theoretically, the latter outperforms the former since less control overhead is required. However, fundamental assumptions made in the receiver-initiated protocols are very vulnerable. In this paper, we propose a new MAC protocol known as multiple access with reduced handshake (MARCH). MARCH utilizes the broadcast characteristics of an omni-directional antenna to reduce the number of handshakes required to initiate a data transmission. Simulation results demonstrate that this low-overhead MAC protocol results in a 65% increase in network throughput.

This letter introduces a novel multi-user detection method, successive interference cancellation based on the order of log-likelihood-ratio(LLR-SIC), for code division multiple access (CDMA) systems. Unlike the conventional successive interference cancellation (SIC) based on the order of correlation, LLR-SIC operates on the fact that the user with the largest absolute value of log-likelihood ratio (LLR) should be first detected and cancelled from received signal. Simulation results show that LLR-SIC significantly outperforms the conventional SIC and partial parallel interference cancellation (P-PIC) over Rayleigh fading channels, and that LLR-SIC performance is not sensitive to channel estimation error at medium Eb/N0.

An Ad Hoc network is a collection of wireless mobile nodes dynamically forming a temporary network without the use of any existing network infrastructure or centralized administration. The choice of medium access is difficult in Ad Hoc networks due to the time-varying network topology and the lack of centralized control. In this paper, we propose a novel multichannel schedule-based Medium Access Control (MAC) protocol for Ad Hoc networks named Multichannel Reservation Protocol for TDMA-based networks (MRPT). MRPT ensures collision free in successfully reserved data links, even when hidden terminals exist. The reservation of MRPT is based a control channel and in order to improve throughput we propose Four-Phase-Two-Division (FPTD) as a media access scheme of the control channel for broadcasting control or reservation messages. In FPTD, the collision can be solved rapidly with an efficient backoff algorithm which results in that system block is avoided in case of high traffic. In this paper, we also present the throughput performance of MRPT, which shows a high value and no system block even in case of high traffic load.

As an access network to the Internet, CATV/HFC network has been widespread recently. Such a network employs a reservation access method under which reservation and data transmission periods appear by turns. Before data transmission, a station must send a request in a random access manner during the reservation period called a request cluster. If the cluster size is large, the probability of request collision occurrence becomes small. A large cluster size however increases the packet transmission delay. Moreover the throughput decreases since vacant duration of reservation period increases. DOCSIS, a de facto standard for the networks, employs the binary back-off method for request cluster allocation. Since that method normally allocates unnecessary large request cluster, the transmission delay increases under heavy load conditions. In this paper, we propose a request cluster allocation method which dynamically changes the cluster size according to the load conditions. In order to evaluate performance of the proposed method, we build a queuing model and execute computer simulation. Simulation result shows that the proposed method provides smaller delay than the binary back-off method.

IEEE 802.11 standard and the enhanced amendments have defined several transmission rates for mobile stations to transmit and receive data frames. With the characteristic of modulation schemes, a higher network throughput can be expected by using higher level modulation scheme, but frame error probability will also become higher. Doubtlessly, it is an open issue of selecting a proper modulation scheme for a pair of mobile stations in time-varying indoor environment. This paper proposes a safe multiple access-rates transmission (SMART) scheme to enhance the reliability of data transmission in IEEE 802.11 multi-rate infrastructure wireless networks. SMART scheme provides reliable transmission by reserving a retransmission period which immediately follows the transmitted frame and is estimated by a lower transmission rate according to the transmitted frame. The performance of SMART scheme is evaluated by analytical model and simulation experiment, which show that the proposed scheme is significantly better than the IEEE 802.11 standard under the real environment with asymmetric traffic load. An enhanced SMART (ESMART) scheme is also proposed especially to improve bandwidth waste for long packet length at access point (AP). The performance measurements, such as goodput, throughput, average access delay, and service rate, are obtained and compared for the different schemes.

This paper describes feasibility of a proposed fixed wireless access system with CDMA technology. The system adopts a primary modulation of 16 QAM and the same frequency allocation in all cells to improve spectral efficiency. The system capacity is 1 Gbps per cell within 120 MHz bandwidth. The number of available orthogonal codes corresponds to the orthogonal code length in the system. All subscribers can attain an error free condition with output power control in the presence of inter-cell interference. The following two items are considered to examine the proposed system feasibility. 1) A test modem is fabricated, and a back-to-back modem BER performance is measured. An inter-symbol interference (ISI) level of the modem is estimated with the measured performance. 2) A computer simulation of down-link power control is carried out considering inter-cell interference and impairment factors of the power control such as intra-sector interference caused by the ISI and limited ranges of total and relative output power controls. The simulation results show that the proposed system would be feasible because the obtained power penalties caused by the above impairment factors are negligible.

Synchronous Gaussian code-division multiple access (CDMA) systems employing group-orthogonal signature waveforms are proposed and analyzed. All users in the system are divided into groups of users. The signature waveforms are constructed such that all the signature waveforms in one group are orthogonal to all the signature waveforms used in all other groups. This construction of signature waveforms ensures that there is no inter-group interference (i.e., among users in different groups), but at the expense of having intra-group interference (i.e., among users in the same group). However, by choosing a small size for each group, the intra-group interference can be effectively handled by a low-complexity, optimal (or suboptimal) multiuser detector. It is shown that a significant improvement in the system capacity can be achieved by the proposed technique over the conventional one that uses signature waveforms constructed from Welch-bound-equality (WBE) sequences. In particular, it is demonstrated that, while the conventional system's error performance is very sensitive to even small amount of overload, the proposed system with an appropriate design of signature waveforms can achieve a much higher overload (up to 300% as shown in the paper) with an excellent error performance.

The safety problem in access matrix models determines whether a given subject can eventually obtain access privilege to a given object. Generally speaking, the safety problem is, unfortunately undecidable. Not much is known about protection systems for which the safety problem is decidable, except for strongly constrained systems (e.g., monotonic systems). Therefore, we propose the Dynamic-Typed Access Matrix (DTAM) Model, which extends the Typed Access Matrix model of Sandhu by allowing the type of an object to change dynamically. The DTAM model has an advantage that it can describe non-monotonic protection systems for which the safety problem is decidable. In particular, with further restrictions, we can show that the problem becomes NP-hard. In this paper, we formally define the DTAM model and then discuss various aspects of it thoroughly.

This letter examines a very simple iterative chip-by-chip multiuser detection strategy for spread spectrum communication systems. An interleaving-based multiple-access transmission technique is employed to facilitate detection. The proposed scheme can achieve near single-user performance in situations with very large numbers of users while maintaining very low receiver complexity.

Many carriers are introducing multi-media services to satisfy customer demands for these services. In order to provide such services, carrier must increase their system capacity. It is well known that space division multiple access (SDMA) improves system capacity and is compatible with existing access systems. In order to evaluate the performance of SDMA, we developed an SDMA test bed. The test bed maintains the personal handy phone systems (PHS). The PHS adopts time division multiple access (TDMA). Aiming to compare the performance of SDMA and TDMA using the same analog hardware, the SDMA test bed employs a software-defined radio (SDR) technique. This paper shows the outline and performance of the test bed. The results of laboratory tests indicate that the bit error rate (BER) of the test bed operated in the SDMA mode at under 10-3 when the carrier-tointerference ratio (CIR) was larger than approximately -22 dB. Antenna patterns measured in an anechoic chamber show that the SDMA test bed produces correct antenna patterns when there are three desired signals and one interference signal. The results of the four field tests confirm that the test bed operated while two-multiplex SDMA mode doubled of the traffic and decreased the interference level as compared with the TDMA mode. Furthermore, the test bed operated while threemultiplex SDMA mode improves the traffic about 2.4 to 2.7 times. The SDMA test bed decreased the impact of the adjusted TDMA base station (BS). Therefore, we confirmed that the SDMA improves system capacity without any degradation.

Mining traversal patterns on the Internet is one of critical issues for exploring the user access behaviors. In this paper, we propose a new data mining scheme for mining frequent trip traversal patterns on the Internet. First, we define a trip traversal as a historical contiguous sequence of web sites or web pages, which were surfed or visited on an information-providing system by one user. Next, we derive all of the maximal trip traversals by analyzing and filtering these collected trip traversals. For mining the large trip traversals from the maximal trip traversals, we present a data mining scheme integrated with the schemes presented in. Here, the extracted large trip traversals can be thought of as the realistic frequent browsed behaviors for most of users either on a web site or on an information-providing system, such as a proxy server. Finally, we implement and design a data mining system to explore the large trip traversal patterns in order to capture user access patterns to some proxy server.

In this paper, a new method is proposed to construct a visual secret sharing scheme with a general access structure for plural secret images. Although the proposed scheme can be considered as an extension of Droste's method that can encode only black-white images, it can encode plural gray-scale and/or color secret images.

A unified linear multiuser receiver that minimizes a weighted sum of the multiple access interference (MAI) and the background noise is introduced. The proposed receiver includes the three popular linear receivers, namely the matched-filter, the decorrelating and the minimum mean-square-error receivers as special cases. Furthermore, by tuning a single weighting factor, it is possible to improve the performance of the proposed receiver over that of any of the above mentioned linear receivers.