Ushio Yamamoto

Region division assignment of orthogonal variable-spreading-factor codes in W-CDMA

This work focuses on the problem of efficient assignment of the orthogonal-variable-spreading-factor (OVSF) codes for multimedia communications in W-CDMA systems. Due to the problem of code blocking in OVSF-code assignment considered by Minn and Siu, and the different probability of requests for each data rate, the OVSF-code assignment algorithms proposed so far cannot efficiently serve the high-rate requests when traffic load is high. Even though Minn and Siu proposed the algorithm to completely eliminate the code blocking, a system implemented with such an algorithm has a lower number of simultaneous voice conversations than the system with the code blocking problem. To efficiently provide the services for all supported rates, we propose an OVSF-code assignment scheme called the region division assignment (RDA). The performance of the proposed RDA scheme is evaluated by simulation for voice and video sources based on the multiple access protocol in UMTS/IMT-2000.

Hybrid OVSF code assignment scheme in W-CDMA

This paper focuses on an analysis of an orthogonal-variable-spreading-factor (OVSF) codes assignment scheme called the hybrid OVSF-code assignment (HCA). We propose HCA as an alternative solution for OVSF code assignment in order to mitigate so-called code blocking in OVSF-CDMA and to correspond to each service equally. We present the performance evaluation of HCA by computer simulations.

Data rate improvement with dynamic reassignment of spreading codes for DS-CDMA

We investigate a dynamic reassignment of spreading codes for DS-CDMA systems by reconstructing the code tree, according to the number of users in the system. The system may provide the maximum transmission rate in the homogeneous communication situation, where only delay-tolerant services are supported. The improvement on average data rate when using the reassignment scheme compared to the system without applying it is calculated. The simulation results are also given to demonstrate the effectiveness of the proposed scheme.

Performance evaluation of orthogonal variable-spreading-factor code assignment schemes in W-CDMA

This paper compares the performance of the orthogonal-variable-spreading-factor (OVSF) code assignment among three proposed schemes so far, i.e., the conventional OVSF-code assignment scheme (CONV), the dynamic OVSF-code assignment scheme (DCA), and region division assignment scheme (RDA). In OVSF-CDMA, the maintenance of orthogonality among the assigned OVSF codes causes the code blocking problem which increases the high blocking rate for high-rate requests. DCA is aimed at eliminating the code blocking problem in order to reduce the high blocking rate for high-rate requests, whereas RDA has been proposed to mitigate the code blocking problem in order to efficiently provide services for all supported rates. The computer simulation results show that the blocking probability of RDA is lower than that of DCA under heavy traffic conditions. The simulation results also show that the calculation time of DCA is significantly larger than that of RDA. Furthermore, RDA is shown to be able to simultaneously handle many more voice sources than DCA.

Performance evaluation of orthogonal variable-spreading-factor code assignment schemes based on UMTS/IMT-2000 in W-CDMA

This paper compares the performance of the orthogonal-variable-spreading-factor (OVSF) code assignment schemes based on UMTS/IMT-2000 proposed so far, i.e., the conventional code assignment scheme (CONV), the dynamic code assignment scheme (DCA), and region division assignment scheme (RDA). It was shown that RDA gives better performance than DCA and CONV in terms of the packet dropping probability under heavy traffic conditions. In this paper, we make extensive performance comparisons among the three proposed OVSF code assignment schemes. We focus on two performance measures, i.e., the number of simultaneous voice conversations and the calculation time. The simulation results show that the calculation time of RDA is significantly lower than that of DCA. RDA can simultaneously handle much more voice sources than DCA.

GLGR: Grid Landmark-based Geographic Routing for Ad-hoc Wireless Networks

Geographic routing is one of the most effective routing methods for ad-hoc wireless networks, and each node selects the forwarding node to the destination node by using position information of its neighbor nodes. However, it has a problem that forwarded packets may encounter the void area where no closer node to the destination is. In this paper, we propose a geographic routing method with landmarks between source and destination node, called GLGR (Grid Landmark-based Geographic Routing). To handle node mobility, our proposed method employs grid-based landmarks, not node-based landmarks used in the traditional methods. A grid is a divided region of communication area and its position is static. Therefore, landmarks can keep the appropriate position to forward data packets to the destination, especially in the case that the void is static. Also, to adaptively change the cache interval of landmark information, the node guesses whether the encountered void is static or dynamic by using the node habitability of grids. Network simulation results about packet arrival rate and end-to-end delay showed 30% better performance of our proposed method than the traditional methods in the best case, with suppressing control overhead.

Optimal aggregation factor and clustering under delay constraints in aggregate sequential group paging schemes

This paper considers several optimization problems of sequential paging with aggregation mechanism which has been shown to reduce significantly the paging cost of a wireless communication system. An important problem is to find the optimal aggregation factor subject to a constraint on the average paging delay. Another problem is, given a cost function that depends on both paging cost and paging delay, how to find the optimal aggregation factor to minimize that cost function. We have formulated and shown that these can be solved nicely due to the monotonicity and convexity of the average paging cost function and paging delay function. We demonstrate that the optimization problems of the aggregate factor and subnet clustering are not separable. This leads to joint optimization problems of aggregation factor and clustering that are investigated in this paper. The paper presents different algorithms to solve these joint optimization problems using the monotonicity in the aggregation factor and the number of clusters of the average paging cost and delay with the unconstrained optimal clustering and the structures of the constrained optimal clustering.

Region division assignment: a new OVSF code reservation and assignment scheme for downlink capacity in W-CDMA systems

This work focuses on the efficient management of orthogonal-variable-spreading-factor (OVSF) codes for multimedia communications in the W-CDMA systems. Because these systems must assign only OVSF codes that are mutually orthogonal, even if they have sufficient transmission capacity they block connections for which no orthogonal OVSF codes are available. This code blocking can, with extra overhead, be eliminated by reassigning codes, but in this paper we propose an OVSF code management scheme designed to not reassign codes. This scheme, called Region Division Assignment, cannot eliminate code blocking but can reduce the overhead for code assignment. Computer simulations based on the multiple access protocol in UMTS/IMT-2000 show that the proposed scheme can efficiently provide variable-rate services as well as low-rate services.

Dynamic Allocation of Transmitter Power in a DS-CDMA Cellular System Using Genetic Algorithms

In this paper, we propose an approach to solve the power allocation issues in a DS-CDMA cellular system using genetic algorithms (GAs). The transmitter power control has proven to be an efficient method to control cochannel interference in cellular PCS, increase bandwidth utilization and balance the wireless services. Most of the previous studies have assumed that the transmitter power level is controlled in a constant domain under the assumption of uniform distribution of users in the coverage area or in a continuous domain. In this paper, the optimal Centralized Power Control (CPC) vector is characterized and its optimal solution for CPC is presented using GAs in a large scale DS-CDMA cellular system.

The Feasibility Study on a Spectrum Overlaid System of N-CDMA and W-CDMA

In this paper0, the performance in terms of signal-to-interference ratio (CIR), of a spectrum overlaid system of N-CDMA over W-CDMA is investigated. The radio channel is composed of the inverse fourth-power path loss law with log-normal shadow fading. The perfect power control is assumed in the estimation. In order to suppress the interference, the other important techniques used in the analysis are the ideal notch filter and the signal level clipper for the transmitters and receivers of W-CDMA system. We propose the concepts of the notch filtering depth and signal level clipping depth. The maximum capacity trade-offs between the two systems and the impact of varying transmission rates on the overlaid system are investigated and quantified. Based on the numerical results, the feasibility of the overlaid system is discussed.