The abstract ray-tracing problem asks, for a given scene consisting of a light source, a light receiver and finitely many obstacles in a space, and a given positive integer ε > 0, whether a ray going out from the light source could reach the light receiver with intensity at least ε. The problem is known to be PSPACE-hard, and it is very unlikely that there exists an efficient algorithm to solve the problem without adding any restriction. In this paper, we show that the problem can be solved in polynomial time under some weak practical restrictions.

We propose a new type of revocation scheme for efficient public-key black-box traitor tracing. Our revocation scheme is flexible and efficient in the sense that (i) any number of subscribers can be revoked in each distribution under an assumption that the number of revoked subscribers who collude in one coalition is limited to a threshold and (ii) both each subscriber's storage and the transmission overhead are independent of n, while (i) the maximum number of revoked ones cannot be changed or (ii) they depend on n in previous schemes, where n is the total number of subscribers. The flexibility in revocation is significant since flexible revocation can be integrated with efficient black-box tracing and this integration can be achieved without a substantial increase in the transmission overhead over the previous schemes. In this paper, we present a concrete construction of an efficient public-key black-box traceable and revocable scheme by combining flexible revocation with a known black-box tracing algorithm which works under the same attack model as assumed in the previous schemes. Our scheme achieves that (i) the transmission overhead remains efficient, especially linear only in k in case of bulk revocation and (ii) the tracing algorithm runs in O(log n) time, while the previous ones cannot satisfy both of these properties, where k is the maximum number of traitors in a coalition.

This letter presents a method to simulate a phase-included UWB channel impulse responses for a given indoor channel. In this method we decompose a UWB pulse into a finite number of spectral components. This enables the received signal to be determined by the sum of the convolutions between each spectral component and a corresponding frequency-dependent UWB channel impulse response. The ray-tracing algorithm is applied to calculate the amplitude and the phase of each frequency-dependent channel impulse response. Based on the calculated results, we finally show the simulation of the UWB channel impulse response.

Fingerprinting is a technique to add identifying marks to each copy of digital contents in order to enhance traceability to a distribution system. Collusion attacks, in which the attackers collect two or more fingerprinted copies and try to generate an untraceable copy, are considered to be a threat for the fingerprinting system. With the aim of enhancing collusion security to the fingerprinting system, several collusion secure codes, such as c-frameproof code, c-secure frameproof code and c-identifiable parent property code, have been proposed. Here, c indicates the maximum number of colluding users. However, a practical construction of the above codes is still an issue because of the tight restrictions originated from their combinatorial properties. In this paper, we introduce an evaluation of frameproof, secure frameproof, and identifiable parent property by the probability that a code has the required property. Then, we focus on random codes. For frameproof and secure frameproof properties, we estimate the average probability that random codes have the required property where the probability is taken over the random construction of codes and random construction of coalitions. For the estimation, we assume the uniform distribution of symbols of random codes and the symbols that the coalitions hold. Therefore, we clarify the adequacy of the assumptions by comparison with numerical results. The estimates and numerical results resemble, which implies the adequacy of the assumption at least in the range of the experiment.

In recent years, interruption of services large-scale business sites and Root Name Servers caused by Denial-of-Service (DoS) attacks or Distributed DoS (DDoS) attacks has become an issue. Techniques for specifying attackers are, thus important. On the other hand, since information on attackers' source IP addresses are generally spoofed, tracing techniques are required for DoS attacks. In this paper, we predict network traffic volume at observation points on the network, and detect DoS attacks by carefully examining the difference between predicted traffic volume and actual traffic volume. Moreover, we assume that the duration time of an attack is the same at every observation point the attack traffic passes, and propose a tracing method that uses attack duration time as a parameter. We show that our proposed method is effective in tracing DDoS attacks.

This paper is concerned with the application of ray tracing method (RTM) to field analysis in bended and branched tunnels. On the line of sight, direct wave from a source and reflected waves at the tunnel walls are dominant compared with diffracted waves, but off the line of sight, diffraction can not be ignored especially beyond an abrupt bending. As a result, a detailed attention should be focused on the RTM analysis when dealing with propagation in the region off the line of sight. In this paper, we take into account of the diffraction rays which are originally reflection rays, next diffracted at a bending edge, and again converted to reflection rays. It is shown that numerical results are in good agreement with the experimental data.

Recently, dielectric lens antennas are paid attentions in ITS applications. Many lens shape designing methods were already developed. And electrical performances were estimated through a ray tracing method. Here, arbitral lens shapes were expressed by a system of power series. In the case of ray tracing, time-consuming three-coordinate root-finder programs were needed to find intersection points of rays on the lens surfaces. In order to calculate complicated structures such as zoned lenses and complicated rays such as multiple reflections between lens surfaces, simple ray tracing methods are requested. In this paper, a simple ray tracing method that utilizes directly designed discrete points of lens surfaces is developed. In this method, a refracted ray is automatically determined for a given incident ray. As for an intersecting point of a lens surface for an outgoing ray, the nearest point to the refracted vector is found out by employing a simple searching procedure. This method is time-saving compared to the previous three-coordinate root-finding program. Through calculated results of focal points and radiation patterns in wide angle beam steering, effectiveness of a developed method is ensured. Application of the developed ray tracing method of complicated multiple reflections are studied. Reflecting points are found out speedily by the same searching procedure. A calculated example of doubly reflected rays is obtained. Through comparing calculated and measured results of wide angle radiation patterns, effectiveness of a developed method is ensured.

A novel parallel acceleration technique is proposed based on intrinsic parallelism characteristics of shooting-and-bouncing ray launching (SBR) algorithm, which has been implemented using the MPI parallel library on common PC cluster instead of dedicated parallel machines. The results reveal that the new technique achieves very large speedup gains and could be the efficient and low-cost propagation prediction solution.

In order to achieve a transponder antenna for intersection collision avoidance systems in Intelligent Transport Systems, a lens horn antenna that generates a cosecant squared beam is developed. This paper clarifies the method for designing the antenna to achieve accurate radiation pattern synthesis. A H-plane sectral horn is selected. The ray tracing method is employed in the design of the lens shape. The aperture of the horn is determined to be seven wavelengths based on a comparison of calculated radiation patterns and the desired cosecant squared beam shape. Accurate electrical performance, such as radiation patterns and electrical fields in the horn, is calculated using Finite Difference Time Domain software. Electrical field disturbances caused by reflected waves at the lens surfaces expanded widely inside the small horn. As a result, sidelobe levels of the radiation patterns are increased. In order to eliminate these disturbances, matching layers are attached to the shaped lens surface. Then, electrical field distributions in the horn are recovered and disturbances disappear. Measured radiation patterns become almost the same as that designed using the ray tracing method. The results show that application of the ray tracing method to radiation pattern synthesis of a small lens horn antenna is effective. We clarify the electrical field disturbances caused by reflections at the lens surfaces and show that eliminating the reflection at the lens surface by attaching matching layers is very important to achieving radiation pattern synthesis.

Propagation modeling and advanced channel characterization techniques represent integral parts of significant impact in advancing progress in enabling next generation wireless communication technology and realizing its much anticipated broader application and economic benefits. In this paper we describe advances in developing computationally efficient ray-tracing channel modeling procedures, and also describe recent results in characterizing challenging propagation environments including transmission through windows and propagation through walls of complex structures. The impact of these realistic propagation environments as well as the antenna mutual coupling effects on the estimation of channel capacity in a MIMO-based communication system is also evaluated. Significant difference between realistic and statistical channel models are identified and quantified for the special cases of the channels modeled in this study.

In this paper, general definitions of collusion secure codes are shown. Previously defined codes such as frameproof code, secure frameproof code, identifiable parent property code, totally c-secure code, traceability code, and (c,g/s)-secure code are redefined under various marking assumptions which are suitable for most of the fingerprinting systems. Then, new relationships among the combined notions of codes and the marking assumptions are revealed. Some (non)existence results are also shown.

The authors have performed a simple computer simulation for a topography that models change in propagation characteristics due to change in traffic volume. The results of this simulation revealed that path loss for a traffic volume of about 2000 vehicles every 30 minutes in a typical urban environment exhibits a Rayleigh distribution. This result agrees well with that of actual measurements demonstrating that even a simple simulation can be a useful tool in system design.

A distributor of digital contents desires to collect users' attributes. On the other hand, the users do not desire to offer the attributes owing to the privacy protection. Previously, an anonymous survey system for attributes statistics is proposed. In this system, asking trusted third parties' helps, a distributor can obtain the correct statistics of users' attributes, such as gender and age, while no information beyond the statistics is revealed. However, the system suffers from the inefficiency of a protocol to generate the statistics, since the cost depends on the number of all the users registering this survey system. This paper proposes an anonymous survey system, where this cost is independent from the number of all the registering users. In this accomplishment, a group signature scheme with attribute tracing is also proposed. A conventional group signature scheme allows a group member to anonymously sign a message on behalf of the group, while only a designated party can identify the signer. The proposed scheme further enables the party to trace signer's attribute.

In this paper, our aim is to extract real-time movement-related potentials, especially readiness-potentials, from EEGs with a small number of scalp electrodes. We proposed a method composed of independent component analysis (ICA), dipole tracing (DT) and scalp Laplacian methods. The proposed method shows a good real-time RP extraction capability from a single-trial of movement by means of the selection of EEGs with high reliability based on the DT and the improvement of the spatial resolution of the scalp potentials based on the scalp Laplacian.

Serious BER performance degradation due to finite numeric precision in VLSI implementation of Log-MAP and MAX-Log-MAP decoders where forward state metric is calculated using reverse tracing method, is analyzed, and two methods are proposed to overcome this problem, the loser storing method for MAX-Log-MAP and the periodic storing method for Log-MAP and MAX-Log-MAP. Both methods can reduce memory storage size effectively by half, but with additional circuit overhead. Our VLSI implementation examples show that, compared with original method, both methods give about 15% improvement in area and power consumption with identical BER performance.

In the field of computer graphics (CG), some adaptive methods have been proposed in order to make CG images more real in relatively low computational cost. As one of such adaptive methods, in this paper, an adaptive method will be proposed for detection of edges and approximation of surfaces in the use of the so-called automatic differentiation. In the proposed method a CG image with high quality can be generated in suitable computational cost. In this paper, three cases will be considered. The first is an adaptive distributed ray tracing which can adaptively generate anti-aliased CG images in suitable computational cost. The second is a high quality triangular meshing, which guarantees accuracy of the generated meshes according to shape of given surface in suitable computational cost. The last case is used in the so-called radiosity method.

A traitor tracing scheme is a broadcast encryption scheme in which a provider can trace malicious authorized users who illegally gave their personal keys to unauthorized users. The conventional schemes have some problems; one of them is that there exists an upper bound on the sizes of keys to certify the security of the scheme against a collusion attack by many traitors, and so that the size of the header increases according to the increase of the bound. We shall propose a new traitor tracing scheme where the header size is independent of the number of traitors.

The number of computer break-ins from the outside of an organization has increased with the rapid growth of the Internet. Since many intruders from the outside of an organization employ stepping stones, it is difficult to trace back where the real origin of the attack is. Some research projects have proposed tracing methods for DoS attacks and detecting method of stepping stones. It is still difficult to locate the origin of an attack that uses stepping stones. We have developed IDA (Intrusion Detection Agent system), which has an intrusion tracing mechanism in a LAN environment. In this paper, we improve the tracing mechanism so that it can trace back stepping stones attack in the Internet. In our method, the information about tracing stepping stone is collected from hosts in a LAN effectively, and the information is made available at the public information server. A pursuer of stepping stone attack can trace back the intrusion based on the information available at the public information server on an intrusion route.

Today software piracy is a major concern to electronic commerce since a digitized product such as software is vulnerable to redistribution and unauthorized use. This paper presents an enhanced electronic software distribution and software protection model. Authentication scheme of the proposed model is based on zero-knowledge (ZK) proof which requires limited computation. The proposed model considers post installation security using authentication agent. It prevents software piracy and illegal copy. It also provides secure and efficient software live-update mechanism based on traitor tracing scheme. Even if software or personal key is copied illegally, a merchant can trace back to its original owner from the electronic license and personal key. The proposed model provides security and reasonable performance and safety.

A proper design and analysis of future wideband wireless communication systems require an accurate radio channel model. This model is claimed to characterize both the spatial and temporal channel characteristics. This paper investigates the spatio-temporal channel modeling based on a ray-tracing approach. The temporal channels are characterized by a delay profile. The statistical median and fading-fluctuation range of delay profiles are predicted from ray tracing by incorporating the random phase approach. A high level of agreement between predicted results and measured ones is observed in the verification. The spatio-temporal channel impulse response (CIR) predicted from ray tracing is also transformed to have limited band-width and limited beam-width characteristics. The applicability of this transformation is also verified by the comparison with measurement. These verifications prepare the ground for the use of ray-tracing approaches to evaluate system performance in real environments.