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.     

Multiple Rake Combiners and Performance Improvement in 3G and Beyond WCDMA Systems

We propose an efficient code-assignment scheme utilizing multiple rake combiners to avoid the code-blocking problem in third-generation (3G) and beyond wideband code-division multiple-access (WCDMA) systems based upon orthogonal variable spreading factor (OVSF) channelization codes. The multiple rake combiners are required to be equipped with each base station (BS) and user equipment (UE). The major benefit of the proposed assignment scheme is in handling nonquantized data rates, making internal code fragmentation approximately zero. The task of code assignment to handle quantized data rates is divided into three steps. In the first step, the number of terms satisfying the capacity equation is found. The code combinations for all the terms in step 1 are found in step 2, which can be further subdivided into terms producing unique combinations and multiple combinations of terms. The procedure for finding the optimal code combination according to the number of rake combiners available at the UE and BS is given in step 3. Simulation results show the performance improvement in terms of reduction in blocking probability compared with existing single-code assignment schemes.      

Top Down Code Search to Locate An Optimum Code and Reduction in Code Blocking for CDMA Networks

In this paper, a top down code search scheme is proposed that identify an optimum OVSF code for assignment at the base station of CDMA wireless networks. An optimum vacant code is the one whose usage produces least code blocking compared to other eligible codes. This scheme provides least code blocking compared to existing schemes without reassignments. In addition, the codes searched during locating the optimum code are significantly less than other existing schemes. The call establishment delay which is a significant factor for real time applications is directly proportional to the number of searches and should be low. The design is explained for single code, and extended to multi code assignment to improve code blocking. The multi code assignment is done using four ways. The first and second multi code schemes uses minimum and maximum rakes for a fixed rate system. The third scheme called scattered multi code scheme divide the incoming call into rate fractions equal to number of rakes available in the system, and each rate fraction is handled in a similar way in which the new call is handled in single code scheme. The rate fractions may be scattered or grouped in the code tree. The fourth multi code scheme, namely grouped multi code scheme allocates codes to all the fractions as close as possible. This maximizes future higher rate vacant codes availability by leaving a complete sub tree vacant when call using multi code ends.     

An Efficient Adaptive Grouping for Single Code Assignment in WCDMA Mobile Networks

For achieving high utilization and efficient code management of the OVSF code tree in 3G WCDMA networks, several researches have extensively studied. Based on combining both the code assignment and the reassignment mechanisms, it increases obviously high utilization and reduces completely the code blocking. Nevertheless, the required rate of traffic should be powers of two of the basic rate, i.e. 1R, 2R, 4R, …, etc., which is impractical and results in wasting the system bandwidth while the required rate is not powers of two of the basic rate. Several multi-code assignment mechanisms have proposed to reduce the waste rate. Nevertheless, these methods bring two inevitable drawbacks including, high complexity of handling multiple codes, and increasing the cost of using more rake combiners at both the base stations and mobile nodes. Therefore, we propose an adaptive grouping code assignment herein to provide a single channelization code for any possible rate of traffic, even though the required rate is not powers of two of the basic rate. Based on the dynamic programming algorithm, the adaptive grouping approach forms several calls into a group. Then it allocates a subtree to the group and adaptively shares the subtree codes for these calls in the concept of time-sharing of slots during a group cycle time. Therefore, the waste rate and code blocking are thus reduced obviously while using a single rake combiner. Since the delay problem may be occurred in such a time-sharing approach, we propose two schemes of cycle interleaving methods to reduce delay. Numerical results indicate that the proposed adaptive grouping approach reduces significantly the waste rate and thus increases the system utilization. Moreover, the proposed cycle interleaving scheme reduces data delay significantly. 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. 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 wirless networking, resource management for wireless networks and mobile cellular networks, and performance evaluation of networks. Min-Xiou Chen received the BS degree in computer science and information engineering from Tung Hai University, Tai-Chung, Taiwan, in 1996, and the MS and PhD degrees in computer science and information engineering from National Chung Cheng University, Chia-Yi, Taiwan, in 1998 and 2005, respectively. He is now an assistant professor at the Department of Computer Science and Information Engineering, Chung Hua University, Hsin-Chu, Taiwan. His research interests include wireless communication, SIP, sensor network and resource management in WCDMA systems. He is a member of the IEEE. Kun-Chan Tsai received the BS degree in information engineering and computer science from Feng Chia University, Taichung, Taiwan, in 2001, and the MS degree in computer science and information engineering from National Chung Cheng University, Chia-Yi, Taiwan, in 2003. His research interests include wireless communications and resource management in WCDMA systems.     

A Code Assignment Algorithm for Nonblocking OVSF Codes in WCDMA

OVSF codes are used as channelization codes in WCDMA. Due to code blocking property of OVSF codes, the bandwidth available in the system is severely limited. Code reassignments mitigate the impact of the blocking property at the expense of causing delays and decreasing the throughput of the system. Nonblocking OVSF (NOVSF) codes have been proposed to alleviate the adverse effect of code reassignments. This paper presents a code assignment algorithm for NOVSF codes, which does not require any code reassignments. Simulation results show that NOVSF codes achieve better throughput than OVSF codes, even though code reassignments are allowed in the assignments of OVSF codes.     

Call admission and code allocation strategies for WCDMA systems with multirate traffic

In this paper, call admission and code allocation schemes are proposed to provide service differentiation in the forward link of wideband code-division multiple-access (WCDMA) systems. In particular, this paper proposes multiple leaf code reservation (MLCR) schemes, where different numbers of orthogonal variable spreading factor (OVSF) leaf codes (i.e., codes of the lowest layer of the OVSF code tree) are reserved to differentiate users with different bandwidth requirements. Leaf codes are only reserved for as long as the call admission process lasts. Once the decision of whether a new request is admitted or not has been made, a Code Dereservation procedure is carried out to increase flexibility in the code assignment phase. The performance of these MLCR strategies with/without code reassignments is then evaluated. Analysis shows that MLCR schemes are also useful in improving fair access among different traffic classes. In addition, perfect fair access among requests with different data rates can be achieved when code reassignments are jointly employed with the proposed OVSF-code reservation schemes.     

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     

The Complementary Use of 3G WCDMA and GSM/GPRS Cellular Radio Networks

The traffic performance of integrated 3G wide-band code division multiple access (WCDMA) and GSM/GPRS network is evaluated. This type of network links two cellular radio systems which have different set of frequency bands and the same coverage size. The base station of 3G WCDMA is installed on an existing GSM/GPRS site. Dual-mode mobile terminals use handoff to establish calls on the better system. The soft handoff or inter-frequency hard handoff occurs when mobile terminals of 3G WCDMA or GSM/GPRS move between two adjacent cells, respectively. The inter-system hard handoffs are used between 3G WCDMA and GSM/GPRS systems. The data rate conversions between different systems, soft handoff region size, multiple data rate multimedia services, and the effect of the mobile terminal mobility on the user mean dwell time in each system are considered in the study. The simulation results demonstrate that a great traffic performance improvement on the complementary use of 3G WCDMA and GSM/GPRS cellular radio networks compared with the use of GSM/GPRS cellular radio networks. When high-data rate transmission is chosen for low-mobility subscribers, both the handoff failure probability, and carried traffic rates increase with the new call generation rate. However, both rates decrease conversely with the increasing new call generation rate as soon as the new call generation rate exceeds a critical value. This causes the integrated networks saturation. The higher mean speed for the mobile terminals produces lower new call blocking probabilities and total carried traffic. The new call blocking probabilities and total carried traffic increase with the size of the soft handoff region.     

Modeling and performance analysis of multi‐service wireless CDMA cellular networks using smart antennas

In this paper, we present an analytical model to assess the blocking capacity of multi‐service code division multiple access (CDMA) systems. We include smart antenna systems in our model and show how the capacity of CDMA systems can be improved if smart antennas are employed at the base stations. Applying smart antennas can actually transform CDMA systems from being interference limited to being channel/code limited. To investigate this effect, we extend our model to include the limitation of channelization codes in CDMA‐based universal mobile telecommunication system (UMTS) systems. From the point of view of the call admission control (CAC) in a smart antenna CDMA system, we can either accept the capacity loss due to code limitation, or we can additionally apply space division multiple access (SDMA) techniques to re‐use channelization codes and thus re‐approach the capacity which is obtained if no code limitation is considered. Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

Reducing code wastage in orthogonal variable spreading factor‐based wideband code division multiple access networks

Most third‐generation and beyond wideband code division multiple access networks use the orthogonal variable spreading factor code tree for channelization codes. The codes in this code tree are limited and the performance of a wireless network depends upon the code assignment for new calls. In this paper, we introduce a term called ‘wastage capacity’, which gives us the amount of wastage caused when a code (single or multiple) with a data rate higher than the rate of the incoming call is assigned to it. We suggest two methods to keep wastage capacity below an arbitrary threshold value or zero. In the first method, we devised an algorithm in which wastage up to a certain threshold would be tolerated and the minimum rakes to get this wastage capacity were identified. In the second approach, we reduced the wastage capacity to zero irrespective of the number of rakes at the expense of higher cost and complexity. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

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.     

Enhancing the high speed downlink packet access operation of 3G WCDMA systems

In high‐speed downlink packet access (HSDPA), the combination of adaptive modulation and coding, hybrid automatic repeat request, and an extended multicode operation has replaced two fundamental aspects of wideband code division multiple access systems, namely, variable spreading factor and fast power control. Despite its enhanced characteristics, the HSDPA operation can be problematic when the majority of incoming traffic load consists of low data rate delay‐sensitive services such as mobile voice‐over‐Internet Protocol. To tackle this case, we propose an enhanced HSDPA operation that exploits multiple layers of the channelization code tree through a specifically designed packet scheduling and code allocation process. Enhanced HSDPA, when compared with the conventional HSDPA, provides a more flexible sharing of the available capacity and is able to increase the number of users that can be served with the required quality of service in terms of packet delay. The efficiency of the proposed scheme is confirmed by simulation results. Copyright © 2011 John Wiley & Sons, Ltd.     

Performance analysis of a dynamic handoff scheme in wireless networks with heterogeneous call arrival processes

The Guard Channel Scheme (GCS) and Handoff Queueing Scheme (HQS) are the popular and practical strategies to prioritize handoff calls in wireless cellular networks. A key issue of giving handoff calls the higher priority is how to achieve a tradeoff among the handoff call blocking probability, new call blocking probability and handoff delay. This paper extends GCS and HQS and presents an efficient handoff scheme that dynamically manages the channels reserved for handoff calls depending on the current status of the handoff queue. A three-dimensional Markov model is developed to analyze the performance of this scheme and investigate the desirable performance tradeoff. The Poisson process and Markov-Modulated-Poisson-Process (MMPP) are used to model the arrival processes of new and handoff calls, respectively. The accuracy of this model is evaluated through the extensive comparison of the analytical results to those obtained from discrete-event simulation experiments. Performance measures in terms of the mean number of calls in the system, aggregate response time, aggregate call blocking probability, handoff call blocking probability, new call blocking probability and handoff delay are evaluated. The analytical model is used to investigate the effects of the number of channels originally reserved for handoff calls, the number of dynamic channels, and the ratio of the rate of handover calls to the aggregate arrival rate on the system performance.     

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。     

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.     

Admission control scheme for soft handoff in DS-CDMA cellular systems supporting voice and stream-type data services

The soft handoff call requests of real-time services in third-generation (3G) direct-sequence code-division multiple access (DS-CDMA) and first- and second-generation cellular systems are more important than new call requests from the viewpoint of quality of service (QoS). Rejection of soft handoff requests causes forced termination of an ongoing real-time call, which is a severer problem than blocking of new call attempts. An admission control scheme that can guarantee a higher QoS for the soft handoff requests of real-time services in 3G DS-CDMA systems is proposed for delay-sensitive voice and delay-tolerant stream-type data services. The proposed scheme (P-Scheme) accommodates both voice and data services by utilizing the full bandwidth. However, voice soft handoff call requests are given priority over new voice call and stream-type data packet requests by suppressing interference from stream-type data services according to voice soft handoff requests, and by varying interference levels. Performance of the P-Scheme is evaluated using a Markovian model. Results are compared with a conventional reservation scheme (C-Scheme) that reserves resources exclusively for voice soft handoff requests. Numerical results show that system performance can be significantly improved using the proposed P-Scheme, compared with the conventional C-Scheme, when various types of service are supported in third-generation DS-CDMA systems.     

Optical orthogonal code construction using rejected delays reuse for increasing SubWavelength-switching capacity

Using a mathematical proof, the authors establish that in element-by-element greedy algorithms based on extended set representation of optical orthogonal codes (OOCs), smaller delay elements rejected during a construction step can be accepted in later steps. They design a novel algorithm that exploits this property and call it the rejected delays reuse (RDR) greedy algorithm. They show that employing the RDR method leads to code lengths that are shorter than those achieved for OOCs constructed using the classical greedy algorithm for the same code weight and the same number of simultaneous codes constraints. They then define a quantitative measure (factor) for OOCs efficiency based on its ability to expand subwavelength-switching capacity. They call this factor the expansion efficiency factor. They use this factor to show that reducing the code length, for the same code constraints, enhances the capacity of subwavelength optical code switched networks.     

Uplink capacity improvement through orthogonal code hopping in uplink-synchronized CDMA systems

We propose an orthogonal code hopping multiple access (OCHMA) scheme in order to improve the capacity of an uplink-synchronized code division multiple access (CDMA) systems. When orthogonal codes (OCs) are used for channelization in uplink-synchronized CDMA systems, a finite set of OCs tends to severely limit the capacity gain of the uplink-synchronized CDMA systems. The OCHMA system allows each user to use a different OC for each symbol according to an allocated hopping pattern (HP). It also allows multiple users to use the same OC at a specific symbol time, which is called an HP collision. Thus, the proposed OCHMA scheme can accommodate more users than the number of available OCs. We analyze the capacity of the OCHMA scheme and compare the performance of the OCHMA with that of conventional schemes including the system using multi-scrambling codes (MSC) which have also been proposed to overcome a code-limited situation.