Handoff Problem Research Articles

A scalable multicast key management scheme for heterogeneous wireless networks

Secure multicast applications require key management that provides access control. In wireless networks, where the error rate is high and the bandwidth is limited, the design of key management schemes should place emphasis on reducing the communication burden associated with key updating. A communication-efficient class of key management schemes is those that employ a tree hierarchy. However, these tree-based key management schemes do not exploit issues related to the delivery of keying information that provide opportunities to further reduce the communication burden of rekeying. In this paper, we propose a method for designing multicast key management trees that match the network topology. The proposed key management scheme localizes the transmission of keying information and significantly reduces the communication burden of rekeying. Further, in mobile wireless applications, the issue of user handoff between base stations may cause user relocation on the key management tree. We address the problem of user handoff by proposing an efficient handoff scheme for our topology-matching key management trees. The proposed scheme also addresses the heterogeneity of the network. For multicast applications containing several thousands of users, simulations indicate a 55%-80% reduction in the communication cost compared to key trees that are independent of the network topology. Analysis and simulations also show that the communication cost of the proposed topology-matching key management tree scales better than topology-independent trees as the size of multicast group grows.

Migration toward 4G wireless communications - Vertical handoffs in fourth-generation multinetwork environments

Revolutionary drivers for 4G include a push toward universal wireless access and ubiquitous computing through seamless personal and terminal mobility. One of the major challenges for seamless mobility is the criterion of a vertical handoff protocol: a handoff protocol for users that move between different types of networks. Traditional operations for handoff detection policies, decision metrics, and radio link transfer are not able to adapt to dynamic handoff criteria or react to user inputs and changing network availabilities. Nor are they able to deliver context-aware services or ensure network interoperability. Thus, new techniques are needed to manage user mobility between different types of networks. This article presents a tutorial on the design and performance issues for vertical hand-off in an envisioned multinetwork fourth-generation environment. Various network architectures and technologies for 3G and beyond are described, including wireless LANs, cellular, satellite, and Mobile IP. Then the problem of vertical handoff is defined in the context of such a diverse network environment. Finally, research efforts to resolve the open problems are explored, including new techniques for dynamic handoff decision and detection algorithms and context-aware radio link transfer.

Multistation data fusion for code-division multiple access wireless communications

Data fusion is the study of optimal information processing in distributed multisensor environments with the benefits of more reliable detection and higher immunity to noise and sensor failures. We investi- gate applications of the data fusion principle to the handoff problem in popular code-division multiple access (CDMA) cellular wireless commu- nication networks. When a mobile station traverses a cell boundary, cell handoff is required to make the transition of a mobile between cells smoother without degrading performance. CDMA multiple access allows soft handoff, in which the signal strength of the mobile is detected and tracked by the searchers from more than one base station. The cell receiving the strongest signal strength handles signal processing. We demonstrate that the signal detection quality in the handoff region can be further improved by fusing the detected results from multiple base sta- tions. A new fusion handoff strategy is designed, and its error perfor- mance measures are derived analytically in the presence of log-normal shadowing. Theoretical fading margin analysis shows that the fusion handoff approach offers better network performance over conventional hard and soft handoffs. © 2003 Society of Photo-Optical Instrumentation Engineers.

Variations on optimal and suboptimal handoff control for wireless communication systems

The design of handoff algorithms for cellular communication systems based on signal-strength measurements is addressed. The system is modeled using a hybrid framework: a mixture of continuous state and discrete event systems. The handoff problem is formulated as an optimization problem to control the switchings within the discrete event system. Performance is evaluated as a function of the expected number of handoffs, the expected handoff delay, and the expected number of signal degradations. A signal degradation occurs when the signal level falls below a threshold. The cost of handoff delay is explicitly specified, in contrast to prior work. Various optimization problems are posed to trade off between these quantities. Based on the optimal solutions which are obtained through dynamic programming, suboptimal versions are proposed for ease of implementation. The performance of the suboptimal algorithm which trades off between the expected number of handoffs and the expected number of signal degradations is improved through the use of signal averaging; however, this algorithm suffers from excessive handoff delay. Therefore, the tradeoff between handoff delay and number of handoffs is considered. The corresponding suboptimal algorithm provides nearly one handoff and almost no delay, which is ideal if call quality is also good. Finally, an algorithm which is a combination of the two previous algorithms is explored.

An efficient scheme to reduce handoff dropping in LEO satellite systems

The problem of handoffs in cellular networks is compounded in a low earth orbit (LEO) satellite-based cellular network due to the relative motion of the satellites with respect to a stationary observer on earth. Typically, the velocity of motion of mobiles can be ignored when compared to the very high velocity of the footprints of satellites. We exploit this property of LEO satellite systems and propose a handoff scheme based on a channel sharing approach that results in a substantial decrease in handoff dropping. For the same handoff dropping performance, our scheme has significantly lower new call blocking probability than the conventional reservation scheme. We also present an analytical approximation that is in very good accord with simulation results.

Channel carrying: a novel handoff scheme for mobile cellular networks

We present a new scheme that addresses the call handoff problem in mobile cellular networks. Efficiently solving the handoff problem is important for guaranteeing quality of service to already admitted calls in the network. Our scheme is based on a new approach called channel carrying: when a mobile user moves from one cell to another, render certain mobility conditions, the user is allowed to carry its current channel into the new cell. We propose a new channel assignment scheme to ensure that this movement of channels will not lead to any extra co-channel interference or channel locking. In our scheme, the mobility of channels relies entirely on localized information, and no global coordination is required. Therefore, the scheme is simple and easy to implement. We further develop a hybrid channel carrying scheme that allows us to maximize performance under various constraints.

Optimal policies for handoff and channel assignment in networks of LEO satellites using CDMA

In this paper, we formulate the combined handoff and channel assignment problems in a CDMA LEO satellite network as a reward/cost optimization problem. The probabilistic properties of signals (channel fading as function of satellite elevation angle) and of the traffic in the footprints are used to formulate a finite-horizon Markov decision process. The optimal policy is obtained by minimizing a cost function consisting of the weighted sum of the switching costs and blocking costs of traffics subject to a bit-error-rate or outage probability constraint. A backward induction algorithm is applied to derive the optimal policy. Performance of the optimal policy and the direct threshold policy are compared.

Wireless ATM: a perspective on issues and prospects

In this article the author examines several key issues in wireless ATM and even offers possible resolutions for some. The primary differences that distinguish wireless from wireline communications are presented, together with a short summary of so-called second-generation digital cellular system concepts. The operation of a wireline ATM network, especially with regard to those aspects bearing on the feasibility of wireless extension, is briefly reviewed, as well as some key technologies which may be very important enablers for wireless ATM, and some possible approaches for enabling bandwidth on demand (the media access problem) and maintaining service quality guarantees (the cell handoff problem), respectively. Finally, a possible format for signaling and synchronization, needed to tie it all together, is presented.

Stochastic control of handoffs in cellular networks

A dynamic programming formulation is used to obtain an optimal strategy for the handoff problem in cellular radio systems. The formulation includes the modeling of the underlying randomness in received signal strengths as well as the movements of the mobile. The cost function is designed such that there is a cost associated with switching and a reward for improving the quality of the call. The optimum decision is characterized by a threshold on the difference between the measured power that the mobile receives from the base stations. Also we study the problem of choosing the best fixed threshold that minimizes the cost function. The performance of the optimal and suboptimal strategies are compared. >

Multiple call hand-off problem with queued hand-offs and mixed platform types

Cellular communication systems that support a mixture of platform types and queuing of hand-off calls are considered. In pure loss systems, if there are no channels available at the target gateway, the hand-off attempt fails and the call is forced-to terminate. In delay systems however, hand-off calls can be held in queue while the supporting mobile is within a transition region where acceptable performance can be provided by at least two different gateways (base stations). When only a finite number of hand-off calls can be queued, mixed delay/loss configurations arise. An analytically tractable model is developed to predict communications traffic performance for such systems. Theoretical performance characteristics obtained show trade-offs among blocking and forced termination probabilities and carried traffic.

Understanding Patient-Centered Care in the Context of Total Quality Management and Continuous Quality Improvement

Implementing patient-centered care (PCC) requires a fundamental shift in thinking-from how to best provide a wide variety of independent services to how to effectively combine individual service components into an integrated health care experience that meets patient needs and preferences. PCC attempts to improve patient care by organizationally and physically moving selected service functions such as basic laboratory, pharmacy, admitting/discharge, medical records, housekeeping, and material support services to patient care areas, thus effecting an organizational restructuring. PCC creates teams composed of multiskilled or cross-trained individuals capable of providing more of the services directly on the patient care unit. Extensive redesign of the basic work processes as proposed by PCC advocates may result in significant changes in employee job scope, task responsibilities, professional autonomy, and reporting relationships. From the employee's perspective such changes may be neither warranted nor welcomed. Therefore, critical PCC implementation issues include obtaining employee buy-in and establishing appropriate incentive structures to facilitate the desired changes. How does PCC fit in with the popular improvement philosophies of total quality management (TQM) and continuous quality improvement (CQI)? Inherent within TQM and CQI is the belief that it is wiser to maximize efforts to design a product or process to be right the first time and to minimize resources devoted to inspection and repair caused by poor processes. PCC builds upon previous TQM/CQI health care efforts by focusing on ways to reduce the white space handoff problem by examining what, if any, changes in underlying structures and processes may be required. In the PCC hospital, TQM/CQI can function as intended, as a methodology for examining and improving the process of care and patient-care outcomes, regardless of internal departmental or profession-based organizational boundaries. For hospitals to remain competitive in today's rapidly changing environment, it is becoming necessary to reevaluate both how they are organized and how their work processes have been designed and controlled. The groundwork already laid by TQM/CQI initiatives will facilitate the more fundamental and long-lasting improvements derived from the redesign of the patient-care unit as prescribed by the goals of PCC.

The multiple-call hand-off problem in high-capacity cellular communications systems

The author presents a preliminary analytical model that characterizes the central issues of the hand-off problem when vehicles can support multiple calls simultaneously. In such cases a cell boundary crossing by a single vehicle can generate multiple hand-off attempts. A suitable vector state representation is identified which casts the problem as a multidimensional birth-death process. An iterative method is used to find implicit hand-off parameters for systems in statistical equilibrium. Theoretical performance characteristics that show blocking, hand-off failure, and forced termination probabilities as functions of communication traffic are determined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Distributed Antennas for Indoor Radio Communications

The idea of implementing an indoor radio communications system serving an entire building from a single central antenna appears to be an attractive proposition. However, based on various indoor propagation measurements of the signal attenuation and the multipath delay spread, such a centralized approach appears to be limited to small buildings and to narrow-band FDMA-type systems with limited reliability and flexibility. In this paper, we present the results of indoor radio propagation measurements of two signal distribution approaches that improve the picture dramatically. In the first, the building is divided into many small cells, each served from an antenna located in its own center, and with adjacent cells operating in different frequency bands. In the second approach, the building is divided into one or more large cells, each served from a distributed antenna system or a leaky feeder that winds its way through the hallways. This approach eliminates the frequency cell handoff problem that is bound to exist in the first approach, while still preserving the dramatic reductions in multipath delay spread and signal attenuation compared to a centralized system. For example, the measurements show that, with either approach, the signal attenuation can be reduced by as much as a few tens of decibels and the rms delay spread becomes limited to 20 to 50 us, even in large buildings. This can make possible the implementation of sophisticated broad-band TDMA-type systems that are flexible, robust, and virtually building-independent.