Ken'ichi Kawanishi

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.

Performance analysis of call centers with abandonment, retrial and after-call work

This paper considers a multiserver queueing model with abandonment, retrial and after-call work for call centers. Upon a phone call, customers that find a free call line occupy the line immediately while those who see all the call lines busy are blocked and join an orbit. Customers holding a call line are served according to the first-come first-served discipline. After completing a call, the customer leaves the system while the server must start an after-call work and the call line is released for a newly arrived customer. Waiting customers may abandon after some waiting time and then either join the orbit or leave forever. Customers in the orbit retry to hold a free call line after some time. We formulate the queueing system using a continuous-time level-dependent quasi-birth-and-death process for which a sufficient condition for the ergodicity is derived. We obtain a numerical solution for the stationary distribution based on which performance measures such as the waiting time distribution and the blocking probability are derived. Using Littles law, we obtain explicit formulae which verify the accuracy of the numerical solution. We compare our model with some simpler models which do not fully take into account some human behaviors. The comparison shows significant differences implying the importance of our model. Numerical results show various insights into the performance of call centers.

On the Counting Process for a Class of Markovian Arrival Processes with an Application to a Queueing System

In this paper, we consider the counting process for a class of Markovian arrival processes (MAPs). We assume that the representing matrices in such MAPs are expanded in terms of matrix representations of the standard generators in the Lie algebra of the special linear group. The primary purpose of this paper is to construct an explicit solution of the time-dependent distribution and factorial moments of the number of arrival events in (0,t] of the counting process for this class of MAPs. Our construction relies on the Baker–Hausdorff lemma and the specific structure of the representing matrices. To investigate the efficiency of CPU usage with the explicit solution, we have conducted numerical experiments on computing the time-dependent distribution of the counting process through the explicit solution and uniformization-based method. We show that the CPU times required to compute the time-dependent distribution of the number of arrival events in (0,t] through the explicit solution have little sensitivity to t, while the consumption of CPU times with the uniformization-based method becomes greater as t increases. For illustrative purposes, we present a system performance analysis of a queueing system for possible use in automatic call distribution (ACD) systems. As an application of the explicit solution, we use it to express the waiting time distribution of the queueing system. Some numerical examples are also given with comparisons to computer simulations.

AN EFFICIENT METHOD FOR PERFORMANCE ANALYSIS OF BLENDED CALL CENTERS WITH REDIAL

This paper considers a multiserver retrial queue with two-way communication for blended call centers. Primary incoming calls arrive at the servers according to a Poisson process and request an exponentially distributed service time. Incoming calls that find all the servers fully occupied join the orbit and retry to occupy a server again after some random time. A retrial incoming call behaves the same as a primary incoming call. A server not only serves as an incoming call but also makes an outgoing call after some random idle time. We assume that the distribution of the duration of outgoing calls is different from that of incoming calls. Artalejo and Phung-Duc (2012) have extensively studied the single server case and have obtained some preliminary results for the multiserver model. In this paper, we present an extensive analysis for the multiserver model in which, we propose a new formulation by a level-dependent quasi birth-and-death (QBD) process, whose block matrices have some special block structure. Based on a matrix continued fraction approach and a censoring technique, we develop an efficient algorithm utilizing the special structure for computing the stationary distribution. Furthermore, we derive explicit formulae for the mean number of incoming calls and that of outgoing calls in the servers. Through various numerical results, we study the characteristics of the queueing system and find some insights into the optimal outgoing call rate.

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.

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.

Simulation-Based Optimization on MPEG-4 over SCTP Multi-streaming with Differentiated Retransmission Policy in Lossy Link

The stream control transmission protocol (SCTP) is a reliable transport layer protocol standardized by IETF as RFC 2960 in 2000. Because of its attractive features such as multi-homing and multi-streaming, SCTP has received much attention from researchers in worldwide with an expectation that would address limitations of TCP performance encountered in wired-cum-wireless networks. In this paper, we focus on the multi-streaming feature of SCTP. Making use of the partial reliability extension in SCTP, we compare performance of MPEG-4 video quality under the policy with/without differentiated retransmission among streams. Differentiated policy allows packets belonging to a specific stream to be retransmitted, while packets are transmitted exactly one time (no retransmissions) in homogeneous policy. We show by simulation, that video object traffic streams encoded by MPEG-4 can be efficiently conveyed under lossy link by SCTP with differentiated policy among streams than with homogeneous policy. We further analyze and show, also by simulation, that SCTP with optimally tuned retransmission policy can minimize the play- out delay time as well as keep high quality video decoding of MPEG-4 traffic stream.

Achieving robust fairness of SCTP extension for MPEG-4 streaming

In this paper, we introduce an extension of Stream Control transmission Protocol (SCTP) aiming for improving the efficiency of delay-sensitive transmission of multimedia data such MPEG-4 traffic. Briefly, our SCTP extension exploits one-way delay between SCTP endpoints, which allows the SCTP extension to retransmit lost packets selectively depending on whether the lost packets would still arrive before the schedule time. By computer simulations, we show that our extension provides the better performance than a simple retransmission method based on lifetime under both wired and wireless access links. Specifically, we confirm that our extension maintains the fair sharing of bandwidth among the legacy TCP flows, irrelevant to wired and wireless access links.

MPEG-4 optimal transmission over SCTP Multi-streaming in 802.11 wireless access media

Stream control transmission protocol (SCTP) is a transport layer protocol for the next-generation Internet. In this paper, we investigate optimally condition for MPEG 4 video communication with SCTP in 802.11 wireless access media. We focus on the multi-streaming feature and the partial reliability extension in SCTP and compare performance of MPEG4 video quality under the policy w th/without differentiated retransmission among streams. Differentiated policy allows packets belonging to a specific stream to be retransmitted, while packets are transmitted exactly one time (no retransmissions) in homogeneous policy. By computer simulation, we show that SCTP can (1) improve the video quality by exploiting the multi-streaming and partial reliability features, and (2) enhance the video transmission quality can by adjusting SCTP fast retransmit threshold.

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.