Vertical handoffs in wireless overlay networks

No single wireless network technology simultaneously provides a low latency, high bandwidth, wide area data service to a large number of mobile users. Wireless Overlay Networks - a hierarchical structure of room-size, building-size, and wide area data networks - solve the problem of providing network connectivity to a large number of mobile users in an efficient and scalable way. The specific topology of cells and the wide variety of network technologies that comprise wireless overlay networks present new problems that have not been encountered in previous cellular handoff systems. We have implemented a vertical handoff system that allows users to roam between cells in wireless overlay networks. Our goal is to provide a user with the best possible connectivity for as long as possible with a minimum of disruption during handoff. Results of our initial implementation show that the handoff latency is bounded by the discovery time, the amount of time before the mobile host discovers that it has moved into or out of a new wireless overlay. This discovery time is measured in seconds: large enough to disrupt reliable transport protocols such as TCP and introduce significant disruptions in continuous multimedia transmission. To efficiently support applications that cannot tolerate these disruptions, we present enhancements to the basic scheme that significantly reduce the discovery time without assuming any knowledge about specific channel characteristics. For handoffs between room-size and building-size overlays, these enhancements lead to a best-case handoff latency of approximately 170 ms with a 1.5% overhead in terms of network resources. For handoffs between building-size and wide-area data networks, the best-case handoff latency is approximately 800 ms with a similarly low overhead. This revised version was published online in June 2006 with corrections to the Cover Date.     

Two-state pattern-recognition handoffs for corner-turningsituations

Handoff algorithms are used in wireless cellular systems to decide when and to which base station to handoff. Traditional handoff algorithms generally cannot keep both the average number of unnecessary handoffs and the handoff decision delay low. They do not exploit the relative constancy of path loss and shadow fading effects at any given location around a base station. However, handoff algorithms with both a negligible number of unnecessary handoffs and a negligible decision delay can be realized by exploiting this information. One example is the set of handoff algorithms using pattern-recognition introduced in previous work. In this paper, we describe how pattern-recognition handoff algorithms can be applied to the problem of turning a corner. This can be used as part of an integrated pattern-recognition handoff algorithm or together with a traditional handoff algorithm, in which case the pattern-recognition handles only the special cases like turning a corner     

Optimal scheduling of handoffs in cellular networks

The phenomenon of hard handoffs (as applicable to FDMA- and TDMA-based networks) as well as soft handoffs (as applicable to DS/CDMA-based networks) is formulated as stochastic optimization problems. The signals received by a mobile user are treated as stochastic processes with associated rewards, which are functions of some measurable characteristics of the received signals, while the handoff is associated with a switching penalty. This formulation captures the trade-offs involved in handoffs in a flexible manner and captures many facets of popular cellular communication systems in use currently. Using dynamic programming, necessary and sufficient conditions for determining the optimal base station(s) the mobile should be associated with during each decision epoch are derived. For the cases where the above-mentioned necessary and sufficient conditions fail to determine an optimal decision, “limited lookahead” arguments are used for determining handoff decisions. The decisions are taken in a decentralized manner, which makes its implementation easier compared to centralized algorithms. Simulation results show that for the hard handoffs, performance gain by the proposed algorithm over the simpler threshold algorithms proposed in the literature is small; however, for the case of soft handoffs, the proposed algorithm offers considerable improvement over the algorithm proposed in the IS-95 standard     

Comparing soft and hard handoffs

This paper studies the soft-handoff mechanism and compares its performance with hard handoff. Our study indicates that although a handset may potentially consume extra radio links in soft handoff, the mechanism provides better opportunity to transfer the link successfully in the handoff procedure. Thus, by carefully planning the overlay areas of cells, soft handoff can outperform hard handoff     

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     

Adaptive averaging methodology for handoffs in cellular systems

The purpose of this paper is to show how information in signal strength measurements can be exploited to improve the quality of handoff decisions, for both large and small cells. Averaging of signal strength fluctuations is required. This leads to the following tradeoff problem for the averaging interval for the signal strength measurements. If the interval is too short, the fading fluctuations are not sufficiently smoothed out. If the interval is too long, delay in handoff increases. With this tradeoff in mind, we present a method to adaptively change the averaging interval. The method is based on estimating the maximum Doppler frequency, f_D, as a means to obtain mobile velocity, the key to the tradeoff. A method used for estimating f_D from the squared deviations of the signal envelope is outlined. Exact analysis for the f_D estimate as a function of squared deviations of the logarithmically compressed signal envelope in Rayleigh fading is presented. An extension of the algorithm for robustness in a Rician fading environment is given. Sensitivity issues of the estimates are considered. An adaptive scheme for optimal averaging is outlined     

Hierarchical optimization of microcellular call handoffs

In this paper, we present a novel hierarchical optimization handoff algorithm (HOHA) which has superior performance over the conventional one- and two-threshold handoff algorithms. Our proposed algorithm scans through layers of call handoff options and determines the best candidate based on a heuristic that synchronizes reciprocal handoffs in adjacent cells and maximizes the spare channel capacity in the destination cells. Computer simulation results show that under a unity normalized load condition, the HOHA achieves a gain of about 55% and 11% in the probability of handoff failure as compared to the one- and two-threshold handoff algorithms, respectively. In addition, the total effective load carried is also improved by about 13% and 4%, respectively     

Fade margins for soft and hard handoffs

A particularly attractive feature of CDMA wireless systems proposed in the Telecommunication Industry Association's standard IS-95 [1] is their ability to get into what is known as soft handoff. A mobile in soft handoff maintains simultaneous radio links with multiple base-stations which enables it to make use of the best quality leg most of the time. This is believed to result in a reduction of the fade margin that is needed to provide the desired coverage in a given geographical area in the presence of log-normal shadow fading. This paper presents an analysis of fade margins for systems allowing soft handoffs (e.g. CDMA) and those where only hard handoffs are possible (e.g. FDMA, TDMA). The analysis is based on a model for hard handoffs with hysteresis and connection delay and confirms the belief that soft handoffs result in smaller fade margins.     

Achieving seamless handoffs via backhaul support in Wireless Mesh Networks

In Wireless Mesh Networks (WMNs), mobile clients may experience frequent handoffs due to the relatively small transmission range of the mesh routers. Each handoff may lead to packet delays and/or packet losses, which limits the performance of real-time applications over WMNs. In this work, we propose BASH—a Backhaul-Aided Seamless Handoff scheme. BASH takes advantage of the wireless backhaul feature of WMNs, and allows a mobile station to directly access the backhaul channel to probe the neighboring mesh routers. Our work shows that by utilizing the wireless backhaul, BASH (1) reduces the probing latency and, thus, the Layer-2 handoff latency; (2) allows partial overlap of the Layer-2 and Layer-3 handoffs, reducing the overall handoff latency; and (3) shortens the authentication latency by utilizing the transitivity of trust relationship. The experimental results show that BASH achieves an average Layer-2 handoff of 8.9 ms, which supports real-time applications during the handoff.     

Behavior-based mobility prediction for seamless handoffs in mobile wireless networks

The field of wireless networking has received unprecedented attention from the research community during the last decade due to its great potential to create new horizons for communicating beyond the Internet. Wireless LANs (WLANs) based on the IEEE 802.11 standard have become prevalent in public as well as residential areas, and their importance as an enabling technology will continue to grow for future pervasive computing applications. However, as their scale and complexity continue to grow, reducing handoff latency is particularly important. This paper presents the Behavior-based Mobility Prediction scheme to eliminate the scanning overhead incurred in IEEE 802.11 networks. This is achieved by considering not only location information but also group, time-of-day, and duration characteristics of mobile users. This captures short-term and periodic behavior of mobile users to provide accurate next-cell predictions. Our simulation study of a campus network and a municipal wireless network shows that the proposed method improves the next-cell prediction accuracy by 23~43% compared to location-only based schemes and reduces the average handoff delay down to 24~25 ms.     

Fast and scalable wireless handoffs in support of mobile Internet audio

Future internetworks will include large numbers of portable devices moving among small wireless cells. We propose a hierarchical mobility management scheme for such networks. Our scheme exploits locality in user mobility to restrict handoff processing to the vicinity of a mobile node. It thus reduces handoff latency and the load on the internetwork. Our design is based on the Internet Protocol (IP) and is compatible with the Mobile IP standard. We also present experimental results for the lowest level of the hierarchy. We implemented our local handoff mechanism on Unix-based portable computers and base stations, and evaluated its performance on a WaveLAN network. These experiments show that our handoffs are fast enough to avoid noticeable disruptions in interactive audio traffic. For example, our handoff protocol completes less than 10 milliseconds after a mobile node initiates it. Our mechanism also recovers from packet losses suffered during the transition from one cell to another. This work helps extend Internet telephony and teleconferencing to mobile devices that communicate over wireless networks. This revised version was published online in June 2006 with corrections to the Cover Date.     

Handoff management without intercell hard handoffs in a multifrequency CDMA system

The paper addresses handoff management in a multifrequency code-division multiple-access system with nonuniform traffic loads over cells. We propose a new handoff scheme, adaptive handoff management (AHM), in which the intercell hard handoffs, which not only incur an equipment cost burden but also degrade call quality, are totally removed. Also strengthened in the AHM is the call-control capability of adaptively reflecting a changing traffic environment. Traffic and mobility analysis is first conducted on a single isolated cell with two concentric circle boundaries, thereby rendering two key performance measures. We then formulate a mathematical problem of finding the optimal parameters of the proposed AHM in which intracell hard handoffs are exploited most effectively. After reporting a simple solution process, computational experiments are conducted to illustrate the superiority of AHM in two key performance criteria.     

IDMP-based fast handoffs and paging in IP-based 4G mobile networks

We consider the use of our previously proposed Intra-Domain Mobility Management Protocol (IDMP) in fourth-generation mobile networks. On evaluating the heterogeneous access technologies, cellular layouts, and application characteristics of 4G environments, we realize a need to reduce both handoff latency and the frequency of mobility-related signaling. We first present IDMP's fast intradomain handoff mechanism that uses a duration-limited proactive packet multicasting solution. We quantify the expected buffering requirements of our proposed multicasting scheme for typical 4G network characteristics and compare it with alternative IP-based fast handoff solutions. We also present a paging scheme under IDMP that replicates the current cellular paging structure. Our paging mechanism supports generic paging strategies and can significantly reduce the mobility-related IP signaling load     

Modeling and efficient handling of handoffs in integrated wireless mobile networks

We propose and analyze two handoff schemes without and with preemptive priority procedures for integrated wireless mobile networks. We categorize the service calls into four different types, namely, originating voice calls, originating data calls, voice handoff request calls, and data handoff request calls and we assume two separate queues for two handoff services. A number of channels in each cell are reserved exclusively for handoff request calls. Out of these channels, few are reserved exclusively for voice handoff request calls. The remaining channels are shared by both originating and handoff request calls. In the preemptive priority scheme, higher priority is given to voice handoff request calls over data handoff request calls and can preempt data service to the queue if, upon arrival, a voice handoff request finds no free channels. We model the system by a three-dimensional Markov chain and compute the system performance in terms of blocking probability of originating calls, forced termination probability of voice handoff request calls, and average transmission delay of data calls. It is observed that forced termination probability of voice handoff request calls can be decreased by increasing the number of reserved channels. On the other hand, as a data handoff request can be transferred from a queue of one base station to another, there is no packet loss of data handoff except for a negligibly small blocking probability.     

第四代海事卫星陆用型终端应用中的技术问题与解决措施

本文从目前用户在第四代海事卫星陆用型终端应用中涉及的问题入手．对海事卫星网络结构特点进行了分析．进而提出了具体的解决措施,对用户更好地使用海事卫星业务具有现实的指导意义。     

An overview of scheduling algorithms in MIMO-based fourth-generation wireless systems

In this article an overview of the scheduling algorithms proposed for fourth-generation multiuser wireless networks based on multiple-input multiple-output technology is presented. In MIMO systems a multi-user diversity gain can be extracted by tracking the channel fluctuations between each user and the base station, and scheduling transmission for the "best" user. Based on this idea, several opportunistic scheduling schemes that attempt to improve global capacity or satisfy users with different QoS requirements have been proposed. Transmit beamforming procedures aimed at increasing the channel fluctuations have been proposed. The simultaneous exploitation of both spatial and multi-user diversity is not straightforward; however, it may be achieved by a refined selection of the "best" user. In addition, a multiple access gain can be obtained from a simple SDMA/TOMA system. Finally, several resource allocation schemes are discussed for this hybrid multiple access system.     

Virtually fixed channel assignment in cellular mobile networks with recall and handoffs

A promising approach for implementing channel assignment and control in cellular mobile telephone networks is the virtually fixed channel assignment (VFCA) scheme. In VFCA channels are allocated to cells according to the fixed channel assignment (FCA) scheme, but cells are allowed to borrow channels from one another. As such, VFCA maintains the efficiency of FCA, but adds the flexibility lacking in FCA. One feature of a VFCA network is that, to prevent co-channel interference, it requires several channels to be locked to serve a single call that borrows a channel. This feature raises the concern that VFCA may lead to chain reaction in channel borrowing among cells and cause the network performance to degrade, especially under heavy traffic conditions. In this paper, we propose the virtually fixed channel assignment with recall (VFCAWR) scheme: The network is implemented according to VFCA, but a cell can recall a locked channel to service an arriving handoff call, which occurs when a mobile unit crosses the boundary of its cell. We model the network as a three-dimensional Markov chain and derive its steady-state performance. Through modification of this basic model, we evaluate two dynamic channel assignment strategies, the virtual channel reservation (VCR) strategy and the linear switch-over (LSO) strategy, which exploit the unique borrowing/recall capability of VFCAWR to reduce the weighted cost of blocking fresh and handoff calls by reserving several virtual channels (the channels that may be borrowed from adjacent cells when necessary) for handoff calls. We validate the analytical models by simulation; the simulation test cases show that our models accurately predict the system performance measures of interest. Numerical and simulation results also show that both dynamic strategies outperform conventional channel reservation schemes based on fixed channel assignment and hybrid channel assignment. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of soft and softer handoffs on CDMA system capacity

The effect of soft and softer handoffs on code-division multiple-access (CDMA) system capacity is evaluated for unsectorized and sectorized hexagonal cells according to an average bit energy-to-interference power spectral density, which corresponds to a bit-error rate (BER) of 10^-3. The effect of imperfect sectorization on sectorization efficiency is also considered. On the reverse link, there is no capacity loss as no extra channels are needed to perform soft handoff, while the macrodiversity provided by soft handoff can improve the reverse-link quality and extend the cell coverage. On the forward link, when soft handoff is employed in unsectorized cells, the capacity loss due to two traffic channels assigned to a user in the handoff zone is 0.2% or 1.1% for a voice activity factor of 3/8 or 1/2, respectively. As the forward-link capacity is higher than that of the reverse link, this small capacity loss does not affect the system capacity. For sectorized cells having three sectors per cell, there are overlapping coverage areas between sectors, where mobiles in these areas are subjected to an increase in cochannel interference. For an overlapping angle of 5°, the sectorization efficiency is 0.96 and 0.7 for the reverse-link and forward-link systems, respectively. When soft and softer handoffs are employed, the forward-link sectorization efficiency is improved to 0.97. We find the application of soft and softer handoff improves not only the forward-link capacity, but also the signal-to-interference ratio (SIR) for mobiles near the cell and sector boundaries     

Multicasting with selective delivery: a SafetyNet for vertical handoffs

In the future, mobility support will require handling roaming in heterogeneous access networks. In order to enable seamless roaming it is necessary to minimize the impact of the vertical handoffs. Localized mobility management schemes such as Fast Handovers for Mobile IPv6 (FMIPv6) and Hierarchical Mobile IPv6 do not provide sufficient handoff performance, since they have been designed for horizontal handoffs. In this paper, we propose the SafetyNet protocol, which allows a Mobile Node to perform seamless vertical handoffs. Further, we propose the SafetyNet handoff timing algorithm, to enable a Mobile Node to delay or even completely avoid upward vertical handoffs. We implement the SafetyNet protocol and compare its performance with the FMIPv6 protocol in our wireless test bed and analyze the results. The experimental results indicate that the proposed SafetyNet protocol can provide an improvement of up to 95% for TCP performance in vertical handoffs, when compared with FMIPv6 and an improvement of 64% over FMIPv6 with bicasting. We use numerical analysis of the protocol to show that its over the air signaling and data transmission overhead is comparable to FMIPv6 and significantly smaller than that of FMIPv6 with bicasting.