Code placement and replacement strategies for wideband CDMA OVSF code tree management

The use of OVSF codes in WCDMA systems has offered opportunities to provide variable data rates to flexibly support applications with different bandwidth requirements. Two important issues in such an environment are the code placement problem and code replacement problem. The former may have significant impact on code utilization and, thus, code blocking probability, while the latter may affect the code reassignment cost if dynamic code assignment is to be conducted. The general objective is to make the OVSF code tree as compact as possible so as to support more new calls by incurring less blocking probability and less reassignment costs. Earlier studies about these two problems either do not consider the structure of the OVSF code tree or cannot utilize the OVSF codes efficiently. To reduce the call blocking probability and the code reassignment cost, we propose two simple yet efficient strategies that can be adopted by both code placement and code replacement: leftmost and crowded-first. Numerical analyses on call blocking probability and bandwidth utilization of OVSF code trees when code reassignment is supported are provided. Our simulation results show that the crowded-first strategy can significantly reduce, for example, the code blocking probability by 77 percent and the number of reassignments by 81 percent, as opposed to the random strategy when the system is 80 percent fully loaded and the max SF = 256.     

Dynamic Channel Allocation for Real-Time Connections in Highway Macrocellular Networks

The wide deployment of multimedia services in third generation wireless networks will require handoff designs that can simultaneously reduce the blocking probability of handoff requests and decrease the handoff delay. Reducing the handoff blocking probability is needed to prevent frequent call dropping of real-time VBR/VCR connections and decreasing the delay associated with handoff is needed to prevent QoS degradation for multimedia traffic. In this paper, we present a channel assignment/reassignment scheme for highway cellular networks that achieves both requirements. The scheme can be used to deliver real-time data to a large segment of global highways, namely, highways in which the radio channels used in a given cell cannot be simultaneously used in the two neighboring cells to its left and to its right. The scheme possesses the desirable features of real-time algorithms: the execution time per handoff request has a constant time complexity, the number of transmitted messages per request is small, and the space overhead is also O(1). The scheme uses a non-compact initial assignment of nominal channels to neighboring cells and utilizes a set of pointers in each base station to implement an efficient channel assignment and reassignment strategy. The resulting approach greatly simplifies the selection process and avoids the expensive computation and message exchanges typically needed by dynamic channel allocation schemes. The low communication overhead of the scheme can be further reduced via control thresholds. Performance simulation results show that the scheme achieves low blocking probability and is therefore suitable for handling handoffs of real-time connections in highway cellular networks.     

A tree partitioning dynamic policy for OVSF codes assignment in wideband CDMA

This paper proposes some novel techniques to accommodate users with different rate requirements in a wideband code-division multiple-access system employing orthogonal variable spreading factor codes. Two simple static code assignment strategies are first considered, and an improvement based on multicode assignment. Then the new idea of tree partitioning is introduced and used to devise a dynamic code reassignment algorithm. The behavior of these different techniques is experimentally investigated, in terms of call blocking probability and number of required reassignments. The tree partitioning method exhibits very good performances.     

Markov-Based OVSF Code Assignment Scheme and Call Admission Control for Wideband-CDMA Communication Systems

For the reason of the orthogonal characteristic of the Orthogonal Variable Spreading Factor (OVSF) code tree in Wideband CDMA (WCDMA) systems, code blocking increases as traffic load (i.e. Erlang load) or the required rate increases. This causes inefficient utilization of channelization codes. Hence, how to efficiently manage the resource of channelization codes of the OVSF code tree in WCDMA systems is an important issue and has been studied extensively. There are two aspects to achieve efficiency including code assignment and code reassignment. In the aspect of code assignment, an efficient code assignment scheme reduces code blocking probability significantly. In the aspect of code reassignment, code reassignment results in several drawbacks, such as large overhead of computation, high complexity of codes moving, and long call setup time for a new request call, etc. Therefore, in this paper we focus on the first aspect of how to efficiently assign the channelization codes. Additionally, most researches did not consider the analysis of tree state with dynamic traffic load and their analysis lack of systematic call admission control (CAC) mechanism. Therefore, in this paper, we first propose the Markov decision process (MDP) based analysis to assign channelization codes efficiently. Next, we extend the MDP-based approach as the call admission control mechanism to maximize the system revenue while reducing blocking probability. Furthermore, a bit string masking algorithm is proposed to reduce the time complexity of tree managing and searching for available channelization codes. Numerical results indicate that the proposed MDP approach yields the best fractional reward loss, code blocking reward loss, and code blocking ratio as compared to that of other schemes, including the random, left most, and crowded first schemes. Ben-Jye Chang received his M.S. degree in computer engineering from University of Massachusetts, Lowell, in 1991 and the Ph.D. degree in computer science and information engineering from National Chung-Cheng University, Taiwan, in 2001. He joined the Department of Computer Science and Information Engineering faculty at Chaoyang University of Technology, Taiwan, in 2002, where he is currently an associate professor. His research interests include QoS-based networks, QoS wireless networking, resource management for wireless networks and mobile cellular networks, and performance evaluation of networks. Min-Xiou Chen received the B.S. and M.S. degrees in computer science and information engineering from Tung Hai University and National Chung Cheng University in 1996, and 1998, respectively. He is currently a Ph.D. candidate in the Department of Computer Science and Information Engineering, National Chung Cheng University. His research interests include wireless communication, SIP, and resource management in WCDMA systems. Ren-Hung Hwang received his M.S. and Ph.D. degrees in computer science from University of Massachusetts, Amherst, Massachusetts, USA, in 1989 and 1993, respectively. He joined the Department of Computer Science and Information Engineering, National Chung Cheng University, Chia-Yi, Taiwan, in 1993, where he is now a full professor and the Chair of the Department of Communication Engineering. His research interests include Internet QoS, peer-to-peer infrastructure design, and 3G QoS. Chun-Huan Chuang received the B.S. and M.S. degrees in computer science and information engineering from National Chung Cheng University, Taiwan, in 2001 and 2003, respectively. His research interests include wireless communication and resource management in WCDMA systems.     

低重分配概率的OVSF码重分配算法

在采用正交可变长扩频因子(OVSF)码作为信道化码的直接序列扩频码分多址系统中,提出用重分配概率作为重分配算法的一个新的评价指标,重分配概率越小,系统的计算复杂度越低。进而提出一种低重分配概率的、基于空码容量的重分配算法,在解决本次码阻塞的同时,兼顾对未来高数据速率的呼叫的支持能力,减少未来码阻塞发生。仿真证实,重分配概率比已有2种重分配算法都小。     

Markov Decision Process Based Multiple Codes Assignment in UMTS WCDMA Mobile Networks

For achieving high transmission rate in mobile multimedia communications, 3G WCDMA systems adopt the Orthogonal Variable Spreading Factor (OVSF) code tree to assign a single channelization code for each accepted connection. Based on the orthogonal characteristic of an OVSF code tree, an allocated code blocks the channelization codes that are on the descendant branches and the ancestral codes of the allocated code. Several researches have been proposed to overcome the code-blocking problem for maximizing system utilization. By using both the code assignment and reassignment mechanisms, the system utilization and code blocking can be improved. Nevertheless, the data rate of traffic classes in such single code assignment system should be powers of two of the basic rate, which is impractical and wastes the system capacity when the required rate is not powers of two of the basic rate. A good solution is to assign multiple codes to a new connection, but causes two drawbacks: high complexity of handling multiple codes and high cost from using more number of rake combiners. Consequently, there is a trade-off between waste rate and complexity of handling multiple codes assignments. In previous researches, high computation complexity of assigning multiple codes for a connection and large number of reassignment codes suppressed the advantage of reducing waste rate. Therefore, we propose an adaptive efficient partition algorithm with the Markov Decision Process (MDP) analysis approach to reduce the large number of reassignment codes while improving waste rate. There are two main motivations in the proposed approach. First, we propose an adaptive efficient partition algorithm to determine multiple codes based on the current state of the OVSF code tree for the new incoming connection. Second, after determining the multiple codes, we adopt the MDP analysis to assign the least-cost code for each determined code. Numerical results demonstrate that the proposed MDP approach yields the least number of reassignments and the least number of codes per connection while reducing waste rate significantly, as compared to other approaches.     

Dynamic assignment of orthogonal variable-spreading-factor codes inW-CDMA

This paper presents an optimal dynamic code assignment (DCA) scheme using orthogonal variable-spreading-factor (OVSF) codes. The objective of dynamic code assignment is to enhance statistical multiplexing and spectral efficiency of W-CDMA systems supporting variable user data rates. Our scheme is optimal in the sense that it minimizes the number of OVSF codes that must be reassigned to support a new call. By admitting calls that would normally be blocked without code reassignments, the spectral efficiency of the system is also maximized. Simulation results are presented to show the performance gain of dynamic code assignment compared to a static assignment scheme in terms of call blocking rate and spectral efficiency. We also discuss various signaling techniques of implementing our proposed DCA scheme in third-generation wideband CDMA systems     

OVSF code channel assignment with dynamic code set and buffering adjustment for UMTS

The Universal Mobile Telecommunications System (UMTS) provides users variable data rate services, which adopts wide-band code-division multiple-access (WCDMA) as the radio access technology. In WCDMA, orthogonal variable spreading factor (OVSF) codes are assigned to different users to preserve the orthogonality between users' physical channels. The data rate supported by an OVSF code depends on its spreading factor (SF). An OVSF code with smaller SF can support higher data rate services than that with larger SFs. Randomly assigning the OVSF code with a large SF to a user may preclude a larger number of OVSF codes with small SFs, which may cause lots of high data rate call requests to be blocked. Therefore the OVSF code assignment affects the performance of the UMTS network significantly. In this paper, we propose two OVSF code assignment schemes CADPB1 and CADPB2 for UMTS. Both schemes are simple and with low system overhead. The simulation experiments are conducted to evaluate the performances for our schemes. Our study indicates that our proposed schemes outperform previously proposed schemes in terms of the weighted blocking probability and fairness index. Our schemes improve the call acceptance rate by slightly introducing call waiting time.     

An interference avoidance code assignment strategy for downlink multi-rate mc-ds-cdma with tf-domain spreading

Frequency selective fading may affect the orthogonality of the spreading codes in the multi-carrier direct sequence code division multiple access (MC-DS-CDMA) systems. In this paper, we define a new performance metric called the multiple access interference (MAI) coefficient for the MC-DS-CDMA system to quantitatively predict the impact of inter-code interference with the time- and frequency-domain spreading in a frequency selective fading channel. With the help of MAI coefficient, a novel interference avoidance code assignment strategy is proposed. By jointly considering the incurred MAI effect and the blocking probability in the code tree structure, the proposed interference avoidance code assignment method can effectively reduce the MAI for the multi-rate MC-DS-CDMA system, while maintaining very good call blocking rate performance.     

Design and analysis of time-based code allocation schemes in W-CDMA systems

Efficient resource allocation for requests is an important issue in the radio resource management. In the third generation mobile communication systems, Orthogonal Variable Spreading Factor (OVSF) codes are used for spreading codes. In this paper, we consider the OVSF code allocation problem for supporting real-time services in which the service time of a request can be obtained a priori. The impact of the remaining time factor on the OVSF code allocation in W-CDMA systems is investigated. Two time-based allocation schemes are proposed for code assignment and reassignment. This paper represents the first attempt on addressing the remaining time impact on the OVSF code allocation. Simulation results show that the time-based allocation schemes have better performance on reducing the blocking probability and the reassignment cost.     

Scalable Path Computation Flooding Approach for PCE‐Based Multi‐domain Networks

In this letter, we assess the scalability of a path computation flooding (PCF) approach to compute optimal end‐to‐end inter‐domain paths in a path computation element‐based multi‐domain network. PCF yields a drastically reduced network blocking probability compared to a blind per‐domain path computation but introduces significant network control overhead and path computation complexity. In view of this, we introduce and compare an alternative low overhead PCF (LoPCF) solution. From the obtained results, LoPCF leads to similar blocking probabilities to PCF while exhibiting around 50% path computation complexity and network control overhead reduction.     

A new frequency channel assignment algorithm in high capacity mobile communication systems

A new algorithm for frequency channel assignment in mobile radio communication is proposed. The algorithm uses flexible fixed channel assignment which enables the calls having all their nominal channels busy to borrow channels from the neighboring cells provided that co-channel interference will not result. The borrowed channel cannot be used in three interfering cells; therefore reassignment strategy is used when a call is terminated on a nominal channel to switch a call in progress in a borrowed channel to that nominal one and set free the borrowed channel in the three interfering cells. Thus the traffic carried on borrowed channels is minimized. Also different call switching strategies are proposed to give priority to some channels to be used by the nominal cells and other channels to be borrowed by neighboring cells. These strategies considerably reduce the blocking probability. A simulation study of the algorithm was carried out on 49 hexagonal zones having uniform traffic density. Also a combined telephone and dispatch traffic was included in the simulation study. The results showed that the new algorithm is better than the other channel assignment schemes. This can be applied in high capacity mobile communications systems to utilize the spectrum efficiently.     

Code placement and replacement schemes for WCDMA Rotated-OVSF code tree management

Orthogonal variable spreading factor (OVSF) channelization codes are widely used to provide variable data rates for supporting different bandwidth requirements in wideband code division multiple access (WCDMA) systems. Many novel works in the literature have intensively investigated code placement and replacement schemes in OVSF code trees to reduce the code blocking probability and the code reassignment cost. In this paper, we introduce a new code tree structure, namely, a rotated-orthogonal variable spreading factor (ROVSF) code tree, whose code capacity is the same as that of the traditional OVSF code tree. This work addresses both code placement and replacement schemes in the ROVSF code tree system, where ROVSF codes can be used at the forward link of WCDMA systems. Some valuable properties of the developed ROVSF code tree are presented to develop code placement/replacement schemes with lower code blocking probability and less code reassignment cost. The main contribution of our scheme is to identify linear-code chains (LCCs) and nonlinear-code trees (NCTs) in the ROVSF code tree. This work exploits the unsequence property of linear-code chains to design a new code placement and replacement mechanism. Our code placement/ replacement schemes initially attempt to allocate request codes on LCCs and, then, to allocate them to NCTs. Using LCCs with the unsequence property allows us to efficiently reduce the code blocking probability and the code reassignment cost. Extensive simulations are conducted to illustrate that our code placement/replacement results based on the ROVSF code tree actually improve the code blocking probability and the code reassignment cost.     

Integrated cellular and ad hoc relaying systems: iCAR

Integrated cellular and ad hoc relaying systems (iCAR) is a new wireless system architecture based on the integration of cellular and modern ad hoc relaying technologies. It addresses the congestion problem due to unbalanced traffic in a cellular system and provides interoperability for heterogeneous networks. The iCAR system can efficiently balance traffic loads between cells by using ad hoc relaying stations (ARS) to relay traffic from one cell to another dynamically. This not only increases the system's capacity cost effectively, but also reduces the transmission power for mobile hosts and extends system coverage. We compare the performance of the iCAR system with conventional cellular systems in terms of the call blocking/dropping probability, throughput, and signaling overhead via analysis and simulation. Our results show that with a limited number of ARSs and some increase in the signaling overhead (as well as hardware complexity), the call blocking/dropping probability in a congested cell and the overall system can be reduced     

Asymmetric FCA for Downlink and Uplink Transmission in Clustered Multihop Cellular Network

A clustered multihop cellular network (CMCN) with virtual cells has been proposed to achieve the characteristics of macrocell/microcell hierarchically overlaid architecture by applying clustering techniques. As a complement to the traditional cellular networks, CMCN is able to incorporate the flexibility of ad hoc networks by allowing multihop transmission. In this paper, we first propose to use dedicated information ports (DIPs) as clusterheads for CMCN; then we analyze the performance of fixed channel assignment (FCA) for downlink transmission in CMCN. Two multi-dimensional Markov chain models are developed to study the call blocking probability. Due to the nature of multihop transmission in CMCN, channel assignment for uplink and downlink transmission is different and unbalanced. We then propose an asymmetric FCA (AFCA) for uplink and downlink transmission in CMCN. By making use of the proposed AFCA for uplink and downlink transmission, we can reduce the call blocking probability significantly as compared with the FCA for traditional single-hop cellular networks. The salient contribution is that the proposed CMCN with AFCA scheme can increase the spectrum efficiency and the system capacity by introducing the structure of CMCN with DIPs for virtual microcells.     

Dynamic Routing and Wavelength Assignment in Optical Networks by Means of Genetic Algorithms

We propose a novel genetic algorithm for solving the dynamic routing and wavelength assignment (DRWA) problem in wavelength-routed optical networks. The algorithm not only obtains low call blocking probability, but it also employs a very short computation time. Moreover, it is capable of providing fairness among connections, that is, to offer approximately the same quality of service (in terms of blocking probability) for all source-destination node pairs. Since requirements on optical network availability are highly severe, we also propose an extension of the algorithm to provide fault-tolerance capability at the optical layer. It is achieved by means of protection, where each optical connection request is provided with a pair of lightpaths (a primary and a backup lightpath). Again, the genetic algorithm proves to be highly efficient, in this case, at performing routing and wavelength assignment of pairs of lightpaths.     

A Network-Based Dynamic Channel Assignment Scheme for TDMA Cellular Systems

Network-based dynamic channel assignment (DCA) schemes can be used to increase the capacity of TDMA cellular systems. In this paper, a new distributed network-based DCA scheme, known as DCA with interference information, DCA-WI, is proposed and its performance is studied. In this scheme, a base station (BS) assigns a channel in such a way as to minimize the effect on the availability of channels for use in its interfering cells. To accomplish this, each BS maintains an interference information table which contains information about the local cell and its interfering cells. DCA-WI does not require system-wide information. Channel reassignment for new and completed calls are used to further reduce the call blocking probability. Simulation results show that DCA-WI provides a lower call blocking probability compared to other existing schemes in both uniform and nonuniform traffic distributions.     

Vacant codes grouping and fast OVSF code assignment scheme for WCDMA networks

Channelization codes used in WCDMA are Orthogonal Variable Spreading Factor (OVSF) codes. These codes suffer from code blocking limitation. Many designs are proposed to avoid this limitation but most of them do not consider number of codes searched, which affects call establishment delay prior to handling a call. We propose a fast OVSF code assignment design which aims to reduce number of codes searched with optimal/suboptimal code blocking. The code assignment scheme aims to use those vacant codes whose parents are already blocked. This leads to occurrence of more vacant codes in groups, which ultimately leads to less code blocking for higher rate calls. The number of codes searched increases linearly in our design compare to most of other novel proposed single code methods like crowded first assignment, where it increases exponentially with increase in user rates. Also the calculation of vacant codes at one layer will be sufficient to identify the vacant code adjacency for all the layers which reduces complexity. Simulation results are presented to verify the superiority of the design.     

基于QoS优化的WCDMA上行接入控制算法

文章研究QoS优化下WCDMA系统的上行接入控制算法。从QoS分层结构,可度量参数出发,结合新用户呼叫到达率和用户切换到达率,提出WCDMA上行信号呼叫接入控制算法,考虑和平衡新接入用户阻塞率和已接入用户中断率。通过满足不同业务需要的QoS优化策略来控制上行允许接入用户,从而保证系统有稳定的通信质量。仿真分析结果表明,文中给出算法使系统阻塞率和掉话率下降,系统性能得到改善。