Showing papers in "Wireless Networks in 1999"

A unified framework and algorithm for channel assignment in wireless networks

Abstract: Channel assignment problems in the time, frequency and code domains have thus far been studied separately. Exploiting the similarity of constraints that characterize assignments within and across these domains, we introduce the first unified framework for the study of assignment problems. Our framework identifies eleven atomic constraints underlying most current and potential assignment problems, and characterizes a problem as a combination of these constraints. Based on this framework, we present a unified algorithm for efficient (T/F/C)DMA channel assignments to network nodes or to inter-nodal links in a (multihop) wireless network. The algorithm is parametrized to allow for tradeoff-selectable use as three different variants called RAND, MNF, and PMNF. We provide comprehensive theoretical analysis characterizing the worst-case performance of our algorithm for several classes of problems. In particular, we show that the assignments produced by the PMNF variant are proportional to the thickness of the network. For most typical multihop networks, the thickness can be bounded by a small constant, and hence this represents a significant theoretical result. We also experimentally study the relative performance of the variants for one node and one link assignment problem. We observe that the PMNF variant performs the best, and that a large percentage of unidirectional links is detrimental to the performance in general.

Scheduling data broadcast in asymmetric communication environments

Abstract: With the increasing popularity of portable wireless computers, mechanisms to efficiently transmit information to wireless clients are of significant interest. The environment under consideration is asymmetric in that the information server has much more bandwidth available, as compared to the clients. In such environments, often it is not possible (or not desirable) for the clients to send explicit requests to the server. It has been proposed that in such systems the server should broadcast the data periodically. One challenge in implementing this solution is to determine the schedule for broadcasting the data, such that the wait encountered by the clients is minimized. A broadcast schedule determines what is broadcast by the server and when. In this paper, we present algorithms for determining broadcast schedules that minimize the wait time. Broadcast scheduling algorithms for environments subject to errors, and systems where different clients may listen to different number of broadcast channels are also considered. Performance evaluation results are presented to demonstrate that our algorithms perform well.

An adaptive algorithm for optimizing the packet size used in wireless ARQ protocols

Abstract: We develop an algorithm that allows an ARQ protocol to dynamically optimize the packet size based on estimates of the channel bit-error-rate. Our algorithm is particularly useful for wireless and satellite channels where the bit-error-rates tend to be relatively high and time variable. Our algorithm uses the acknowledgment history to make estimates of the channel bit-error-rate, based on which the optimal packet size can be chosen. We develop a Markov chain model for the analysis of the system, under static channel conditions, and show that the algorithm can achieve close to optimal performance using a history of just 10,000 bits. We also use the Gilbert–Elliott two-state Markov channel to model dynamic channel conditions. We show, through simulation, that the algorithm performs well even under rapidly changing channel conditions. Finally, we discuss a maximum likelihood approach for choosing the packet size, which performs almost optimally but is much easier to implement.

Efficient algorithms for scheduling data broadcast

Abstract: With the increasing acceptance of wireless technology, mechanisms to efficiently transmit information to wireless clients are of interest. The environment under consideration is asymmetric in that the information server has much more bandwidth available, as compared to the clients. It has been proposed that in such systems the server should broadcast the information periodically. A broadcast schedule determines what is broadcast by the server and when. This paper makes the simple, yet useful, observation that the problem of broadcast scheduling is related to the problem of fair queueing. Based on this observation, we present a log-time algorithm for scheduling broadcast, derived from an existing fair queueing algorithm. This algorithm significantly improves the time-complexity over previously proposed broadcast scheduling algorithms. Modification of this algorithm for transmissions that are subject to errors is considered. Also, for environments where different users may be listening to different number of broadcast channels, we present an algorithm to coordinate broadcasts over different channels. Simulation results are presented for proposed algorithms.

Composable ad hoc location-based services for heterogeneous mobile clients

Abstract: This paper introduces a comprehensive architecture that supports adapting a client device's functionality to new services it discovers as it moves into a new environment. Users wish to invoke services – such as controlling the lights, printing locally, gaining access to application‐specific proxies, or reconfiguring the location of DNS servers – from their mobile devices. But a priori standardization of interfaces and methods for service invocation is infeasible. Thus, the challenge is to develop a new service architecture that supports heterogeneity in client devices and controlled objects while making minimal assumptions about standard interfaces and control protocols. Four capabilities are needed for a comprehensive solution to this problem: (1) allowing device mobility, (2) augmenting controllable objects to make them network‐accessible, (3) building an underlying discovery architecture, and (4) mapping between exported object interfaces and client device controls. We motivate the need for these capabilities by using an example scenario to derive the design requirements for our mobile services architecture. We then present a prototype implementation of elements of the architecture and some example services using it, including controls to audio/visual equipment, extensible mapping, server autoconfiguration, location tracking, and local printer access.

A new approach to the design and analysis of peer-to-peer mobile networks

Abstract: This paper introduces a new model and methodological approach for dealing with the probabilistic nature of mobile networks based on the theory of random graphs. Probabilistic dependence between the random links prevents the direct application of the theory of random graphs to communication networks. The new model, termed Random Network Model, generalizes conventional random graph models to allow for the inclusion of link dependencies in a mobile network. The new Random Network Model is obtained through the superposition of Kolmogorov complexity and random graph theory, making in this way random graph theory applicable to mobile networks. To the best of the authors' knowledge, it is the first application of random graphs to the field of mobile networks and a first general modeling framework for dealing with ad-hoc network mobility. The application of this methodology makes it possible to derive results with proven properties. The theory is demonstrated by addressing the issue of the establishment of a connected virtual backbone among mobile clusterheads in a peer-to-peer mobile wireless network. Using the Random Network Model, we show that it is possible to construct a randomized distributed algorithm which provides connectivity with high probability, requiring exponentially fewer connections (peer-to-peer logical links) per clusterhead than the number of connections needed for an algorithm with a worst case deterministic guarantee.

A selective location update strategy for PCS users

Abstract: A new location update strategy for personal communication services (PCS) networks and its implementation using a genetic algorithm are proposed. Most of the practical cellular mobile systems partition a geographical region into location areas (LAs) and users are made to update on entering a new LA. The main drawback of this scheme is that it does not consider the individual user mobility and call arrival patterns. Combining these factors with the LA‐based approach, we propose an optimal update strategy which determines whether or not a user should update in each LA, and minimizes the average location management cost derived from a user‐specific mobility model and call generation pattern. The location management cost optimization problem is also elegantly solved using a genetic algorithm. Detailed simulation experiments are conducted to capture the effects of mobility and call‐arrival patterns on the location update strategy. The conclusion from this work is that skipping location updates in certain LAs leads to the minimization of the overall location management cost for a user with a specific mobility pattern and even with moderately high call arrival rate.

Broadcast scheduling for information distribution

Abstract: Broadcast data delivery is encountered in many applications where there is a need to disseminate information to a large user community in a wireless asymmetric communication environment. In this paper, we consider the problem of scheduling the data broadcast such that average response time experienced by the users is low. In a push-based system, where the users cannot place requests directly to the server and the broadcast schedule should be determined based solely on the access probabilities, we formulate a deterministic dynamic optimization problem, the solution of which provides the optimal broadcast schedule. Properties of the optimal solution are obtained and then we propose a suboptimal dynamic policy which achieves average response time close to the lower bound. The policy has low complexity, it is adaptive to changing access statistics, and is easily generalizable to multiple broadcast channels. In a pull-based system where the users may place requests about information items directly to the server, the scheduling can be based on the number of pending requests for each item. Suboptimal policies with good performance are obtained in this case as well. Finally, it is demonstrated by a numerical study that as the request generation rate increases, the achievable performance of the pull- and push-based systems becomes almost identical.

A general purpose proxy filtering mechanism applied to the mobile environment

Abstract: Host mobility complicates the standard networking model in unexpected ways. It increases network heterogeneity, causing difficulty within applications that expect a high or constant amount of network bandwidth. Mobility increases cost when a user switches from a relatively inexpensive high bandwidth network to a low bandwidth pay‐by‐call or pay‐by‐byte network. Mobility may also reduce security when a user moves from a “safe” network onto a foreign one. This situation can be improved by introducing a proxy between the mobile client and its peer. The purpose of this intermediary is to process data moving between the client and peer. The type of processing performed depends on the desired result: the proxy can hide network heterogeneity via data filtering, reduce cost by using compression, and increase security through the use of secure protocols between the client and proxy. This paper presents a general purpose proxy system and how it has been applied to the mobile environment. The overall architecture is described, examples of its use given, and a study on its feasibility and performance is presented.

Enhanced privacy and authentication for the global system for mobile communications

Abstract: The Global System for Mobile Communications (GSM) is widely recognized as the modern digital mobile network architecture. Increasing market demands point toward the relevancy of security-related issues in communications. The security requirements of mobile communications for the mobile users include: (1) the authentication of the mobile user and Visitor Location Register/Home Location Register; (2) the data confidentiality between mobile station and Visitor Location Register, and the data confidentiality between Visitor Location Register and Visitor Location Register/Home Location Register (VLR/HLR); (3) the location privacy of mobile user. However, GSM does not provide enough security functions to meet these requirements. We propose three improved methods to enhance the security, to reduce the storage space, to eliminate the sensitive information stored in VLR, and consequently to improve the performance of the system. Proposed methods include an improved authentication protocol for the mobile station, a data confidentiality protocol, and a location privacy protocol. The merit of the proposed methods is to improve but not to alter the existing architecture of the system. Furthermore, this study also performs computational and capacity analyses to evaluate the original GSM system and proposed approaches on a comparative basis.

Real-time support in multihop wireless networks

Abstract: Personal communications and mobile computing will require a wireless network infrastructure which is fast deployable, possibly multihop, and capable of multimedia service support. The first infrastructure of this type was the Packet Radio Network (PRNET), developed in the 70's to address the battlefield and disaster recovery communication requirements. PRNET was totally asynchronous and was based on a completely distributed architecture. It handled datagram traffic reasonably well, but did not offer efficient multimedia support. Recently, under the WAMIS (Wireless Adaptive Mobile Information Systems) and Glomo ARPA programs several mobile, multimedia, multihop (M3) wireless network architectures have been developed, which assume some form of synchronous, time division infrastructure. The synchronous time frame leads to efficient multimedia support implementations. However, it introduces more complexity and is less robust in the face of mobility and channel fading. In this paper, we examine the impact of synchronization on wireless M3 network performance. First, we introduce MACA/PR, an asynchronous network based on the collision avoidance MAC scheme employed in the IEEE 802.11 standard. Then, we evaluate and compare several wireless packet networks ranging from the totally asynchronous PRNET to the synchronized cluster TDMA network. We examine the tradeoffs between time synchronization and performance in various traffic and mobility environments.

Integrated services packet networks with mobile hosts: architecture and performance

Abstract: This paper considers the support of real-time services to mobile users in an Integrated Services Packet Network. In the currently existing architectures, the service guarantees provided to the mobile hosts are mobility dependent, i.e., mobile hosts experience wide variation in the quality of service and often service disruption when hosts move from one location to another. The network performance degrades significantly when mobile hosts are provided with mobility independent service guarantees. In this paper we have proposed a service model for mobile hosts that can support adaptive applications which can withstand service degradation and disruption, as well as applications which require mobility independent service guarantees. We describe an admission control scheme for implementing this service model and evaluate its performance by simulation experiments. Simulation results show that, if sufficient degree of multiplexing of the mobility dependent and independent services are allowed, the network does not suffer any significant performance degradation and in particular our admission control scheme achieves high utilization of network resources.

Performance comparison of battery power consumption in wireless multiple access protocols

Abstract: Energy efficiency is an important issue in mobile wireless networks since the battery life of mobile terminals is limited. Conservation of battery power has been addressed using many techniques such as variable speed CPUs, flash memory, disk spindowns, and so on. We believe that energy conservation should be an important factor in the design of networking protocols for mobile wireless networks. In particular, this paper addresses energy efficiency in medium access control (MAC) protocols for wireless networks. The paper develops a framework to study the energy consumption of a MAC protocol from the transceiver usage perspective. This framework is then applied to compare the performance of a set of protocols that includes IEEE 802.11, EC‐MAC, PRMA, MDR‐TDMA, and DQRUMA*. The performance metrics considered are transmitter and receiver usage times for packet transmission and reception. The time estimates are then combined with power ratings for a Proxim RangeLAN2 radio card to obtain an estimate of the energy consumed for MAC related activities. The analysis here shows that protocols that aim to reduce the number of contentions perform better from an energy consumption perspective. The receiver usage time, however, tends to be higher for protocols that require the mobile to sense the medium before attempting transmission. The paper also provides a set of principles that could be applied when designing access protocols for wireless networks.

Assigning codes in wireless networks: bounds and scaling properties

Abstract: In the Code Division Multiple Access (CDMA) framework, collisions that can occur in wireless networks are eliminated by assigning orthogonal codes to stations, a problem equivalent to that of coloring graphs associated to the physical network. In this paper we present new upper and lower bounds for two versions of the problem (hidden and primary collision avoidance – HP-CA – or hidden collision avoidance only – H-CA). In particular, optimal assignments for special topologies and heuristics for general topologies are proposed. The schemes show better average results with respect to existing alternatives. Furthermore, the gaps between the upper bound given by the heuristic solution, the lower bound obtained from the maximum-clique problem, and the optimal solution obtained by branch and bound are investigated in the different settings. A scaling law is then proposed to explain the relations between the number of codes needed in Euclidean networks with different station densities and connection distances. The substantial difference between the two versions HP-CA and H-CA of the problem is investigated by studying the probabilistic distribution of connections as a function of the distance, and the asymptotic size of the maximum cliques.

A public-key based secure mobile IP

Abstract: The need of scaleable key management support for Mobile IP, especially the route‐optimized Mobile IP, is well known. In this paper, we present the design and the implementation of a public key management system that can be used with IETF basic and route optimized Mobile IP. The system, known as the Mobile IP Security (MoIPS) system, was built upon a DNS based X.509 Public Key Infrastructure and the innovation in cross certification and zero‐message key generation. The system can supply cryptographic keys for authenticating Mobile IPv.4 location management messages and establishing IPSec tunnels for Mobile IP redirected packets. It can also be used to augment firewall traversal of Mobile IP datagrams. A FreeBSD UNIX implementation of the MoIPS prototype is available for non‐commercial uses.

Adaptive link layer strategies for energy efficient wireless networking

Abstract: Low power consumption is a key design metric for portable wireless network devices where battery energy is a limited resource. The resultant energy efficient design problem can be addressed at various levels of system design, and indeed much research has been done for hardware power optimization and power management within a wireless device. However, with the increasing trend towards thin client type wireless devices that rely more and more on network based services, a high fraction of power consumption is being accounted for by the transport of packet data over wireless links [28]. This offers an opportunity to optimize for low power in higher layer network protocols responsible for data communication among multiple wireless devices. Consider the data link protocols that transport bits across the wireless link. While traditionally designed around the conventional metrics of throughput and latency, a proper design offers many opportunities for optimizing the metric most relevant to battery operated devices: the amount of battery energy consumed per useful user level bit transmitted across the wireless link. This includes energy spent in the physical radio transmission process, as well as in computation such as signal processing and error coding. This paper describes how energy efficiency in the wireless data link can be enhanced via adaptive frame length control in concert with adaptive error control based on hybrid FEC (forward error correction) and ARQ (automatic repeat request). Key to this approach is a high degree of adaptivity. The length and error coding of the atomic data unit (frame) going over the air, and the retransmission protocol are (a) selected for each application stream (ATM virtual circuit or IP/RSVP flow) based on quality of service (QoS) requirements, and (b) continually adapted as a function of varying radio channel conditions due to fading and other impairments. We present analysis and simulation results on the battery energy efficiency achieved for user traffic of different QoS requirements, and describe hardware and software implementations.

Improving call admission policies in wireless networks

Abstract: It is well known that the call admission policy can have a big impact on the performance of a wireless network. However, the nonlinear dependence of new calls and handoff calls makes the search for a better call admission policy – in terms of effective utilization – a difficult task. Many studies on optimal policies have not taken the correct dependence into consideration. As a result, the reported gains in those studies cannot be confirmed in a real network. In this paper we develop a solution to the problem of finding better call admission policies. The technique consists of three components. First, we search for the policy in an approximate reduced-complexity model. Second, we modify the Linear Programming technique for the inherently nonlinear policy-search problem. Third, we verify the performance of the found policy in the exact, high-complexity, analytical model. The results shown in the paper clearly demonstrate the effectiveness of the proposed technique.

Footprint handover rerouting protocol for low earth orbit satellite networks

Abstract: Low Earth Orbit (LEO) satellite networks will be an integral part of the next generation telecommunications infrastructures. In a LEO satellite network, satellites and their individual coverage areas move relative to a fixed observer on Earth. To ensure that ongoing calls are not disrupted as a result of satellite movement, calls should be transferred or handed over to new satellites. Since two satellites are involved in a satellite handover, connection route should be modified to include the new satellite into the connection route. The route change can be achieved by augmenting the existing route with the new satellite or by completely rerouting the connection. Route augmentation is simple to implement, however the resulting route is not optimal. Complete rerouting achieves optimal routes at the expense of signaling overhead. In this paper, we introduce a handover rerouting protocol that maintains the optimality of the initial route without performing a routing algorithm after intersatellite handovers. The FHRP makes use of the footprints of the satellites in the initial route as the reference for rerouting. More specifically, after an optimum route has been determined during the call establishment process, the FHRP ensures that the new route due to handover is also optimum. The FHRP demands easy processing, signaling, and storage costs. The performance results show that the FHRP performs similar to a network without any handovers in terms of call blocking probability.

Cost reduction in location management using semi-realtime movement information

Abstract: This paper introduces a dynamic paging scheme based on the semi-realtime movement information of an individual user, which allows a more accurate predication of the user location at the time of paging. In general, a realtime location tracking scheme may require complex control schemes and incur unacceptably high computation and messaging cost. Our proposed approach, namely the velocity paging scheme, relaxes the realtime constraints to semi-realtime to provide a good combination of cost reduction and ease of implementation. The proposed velocity paging scheme utilizes semi-realtime velocity information, namely velocity classes, of individual mobile terminals and dynamically calculates a paging zone (a list of cells to be paged) for an incoming call. Therefore, the total paging cost can be reduced due to the paging area reduction. Much consideration also has been given to reduce the complexity of the proposed scheme. As a result, it only requires minimal extra overhead and is feasible to implement in current cellular/PCS networks. The velocity paging can be combined with the movement-based registration or other registration schemes. Analytical and simulation results of the velocity paging and movement-based registration combination are provided to demonstrate the cost effectiveness of the scheme under various parameters in comparison with the location area scheme.

Energy-efficient selective cache invalidation

Abstract: In a mobile environment, users of portable computers can query databases over the wireless communication channels To reduce contention on the limited bandwidth of the wireless channels, frequently accessed data are cached at the mobile clients However, maintenance of the cache consistency is complicated by the mobile clients' frequent disconnection to conserve energy One promising approach in the literature is for the server to periodically broadcast invalidation reports from which clients can salvage their cache content that are still valid This mechanism is, however, not energy efficient as clients are expected to examine the entire invalidation report In this paper, we reexamine the cache coherency problem and propose three novel cache coherency schemes While these schemes are based on periodic broadcast of invalidation reports, they allow clients to selectively tune to the portions of the invalidation report that are of interest to them This allows the clients to minimize the power consumption when invalidating their cache content We conducted extensive studies based on a simulation model Our study shows that the proposed schemes are not only effective in salvaging the cache content that are still valid (and hence result in lower access time), but are also efficient in energy utilization While none of the proposed algorithms is superior in both the access time and energy efficiency, one of the schemes, Selective Cache Invalidation, provides the best overall performance

Equal resource sharing scheduling for PCS data services

Abstract: For high speed mobile communication applications, the data rate can be increased by using multiple channels (or time slots) instead of one channel. To reduce the high blocking rate of multiple channels assignment, flexible resource allocation strategies have been proposed. This paper proposes the Equal Resource Sharing Allocation scheme (ERSA scheme) for flexible resource allocation. The ERSA scheme dynamically averages the allocated resource to the call requests based on the number of calls in a base station. The scheme accommodates the maximum number of requests while providing acceptable quality to the admitted requests. We developed an analytic model to investigate the performance of ERSA, and conducted simulation experiments to validate the analytic model. We define satisfaction indication SI as the performance measurement of the resource allocation algorithm. The experiment results indicate that the ERSA scheme outperforms other resource allocation algorithms proposed in our previous study.

A cost effective distributed location management strategy for wireless networks

Abstract: The mobility feature of mobile stations (MSs) imposes a large burden on network traffic control as a result of location management. Design issues of location management include MS registration (updating) and call set-up (paging). Previous approaches introduced several network topologies for updating and paging procedures, but most of them focused on a single problem: either updating optimization or paging optimization. In this paper, we design and integrate two mechanisms, distributed temporary location caches (TLCs) and distributed home location registers (HLRs), to reduce database access delay and to decrease network signaling traffic in both updating and paging for low power, low tier micro cellular systems. By using TLCs, our approach can improve the performance of updating and paging in comparison with previous approaches. Experimental results based on our analytic model show that our location management procedures have lower HLR access rate, lower registration cost, and lower call set-up cost than other approaches.

Signature caching techniques for information filtering in mobile enviroments

Abstract: This paper discusses signature caching strategies to reduce power consumption for wireless broadcast and filtering services. The two-level signature scheme is used for indexing the information frames. A signature is considered as the basic caching entity in this paper. Four caching policies are compared in terms of tune-in time and access time. With reasonable access time delay, all of the caching policies reduce the tune-in time for the two-level signature scheme. Moreover, two cache replacement policies are presented and compared by simulation. The result shows that, when the cache size is small, caching only the integrated signatures is recommended. When the size of cache is greater than that of the integrated signatures, caching both of the integrated and simple signatures is better.

Collision avoidance and resolution multiple access with transmission queues

Abstract: We introduce a stable multiple access protocol for broadcast channels shared by bursty stations, which we call CARMA-NTQ (for collision avoidance and resolution multiple access with non-persistence and transmission queues). Like previous efficient MAC protocols based on tree-splitting algorithms (e.g., DQRAP), CARMA-NTQ maintains a distributed queue for the transmission of data packets and a stack for the transmission of control packets used in collision resolution. However, CARMA-NTQ does not require the mini-slots commonly used in protocols based on collision resolution. CARMA-NTQ dynamically divides the channel into cycles of variable length; each cycle consists of a contention period and a queue-transmission period. The queue-transmission period is a variable-length train of packets, which are transmitted by stations that have been added to the distributed transmission queue by successfully completing a collision-resolution round in a previous contention period. During the contention period, stations with packets to send compete for the right to be added to the data-transmission queue using a deterministic first-success tree-splitting algorithm, so that a new station is added to the transmission queue. A lower bound is derived for the average throughput achieved with CARMA-NTQ as a function of the size of the transmission queue and the number of queue-addition requests that need to be resolved. This bound is based on the upper bound on the average number of collision resolution steps needed to resolve a given number of queue-add requests.

Power-induced time division on asynchronous channels

Abstract: Time division multiple access offers certain well-known advantages over methods such as spread spectrum code division. Foremost is the interference immunity provided by dedicated time slots. Partly offsetting this is TDMA's need for network-wide synchronization. Viewing arbitrary time intervals as potential TDMA time slots, we ask whether it is possible to obtain some of the benefit of time division without incurring the synchronization cost. In particular, we address the question of whether a TDMA-like state can be induced on asynchronous channels in such a way as to reduce interference and energy consumption. Through analysis and simulation we find conditions under which it is beneficial to use time division, and then show how autonomous power management may be used as a mechanism to induce a form of time division. In this context a backlog-sensitive power management system is presented.

Active tracking: locating mobile users in personal communication service networks

Abstract: The problem of tracking mobile users in Personal Communication Service (PCS) networks is discussed. We propose a novel approach for reducing the wireless cost of tracking users. The basic idea is to use non-utilized system resources for initiating queries about the location of mobile users, in addition to the process of user registration. Queries are applied at each cell, independently of the other cells, whenever the load on the local control channel drops below a pre-defined threshold. Our study focuses on two issues: (1) proposing the initiated queries approach and an algorithm for its application, and (2) studying and quantifying the value of location information and evaluating the parameters affecting it. Our analysis shows that the expected benefit due to location knowledge in a Markovian motion model depends, among other things, on the determinant of the transition matrix and on the variability of the location distribution function. The active tracking approach, as opposed to other dynamic strategies, does not require any modification of user equipment. The importance of this property is in its practicality: An implementation of a new registration strategy in current systems would require a modification of the users equipment. Moreover, the proposed method can be easily implemented in addition to any known tracking strategy, to reduce further the tracking cost. The performance of the active tracking method is evaluated under two registration strategies: The geographic-based strategy, currently used in cellular networks, and the profile-based strategy, suggested elsewhere. Under both strategies, it significantly reduces the tracking cost.

A new localized channel sharing scheme for cellular networks

Abstract: Enhancing system capacity while maintaining quality of service is an important issue in wireless cellular networks. In this paper, we present a new localized channel sharing scheme to address this problem. Our basic idea is to allow channels to be shared between adjacent cells. We further propose a fixed channel assignment scheme to maximize channel reuse efficiency while allowing channel sharing. We show that our sharing scheme can also facilitate handoff processing. An important feature of our sharing scheme is that channel management is localized between adjacent cells, and no global coordination or optimization is required, thus making it suitable for implementation. We provide simulation results comparing our scheme with the conventional channel assignment and handoff techniques. We find that our scheme improves system capacity over a broad range of traffic parameters and a variety of quality of service requirements.

Dealing with server corruption in weakly consistent replicated data systems

Abstract: Providing high availability and the ability to share data despite the weak connectivity of mobile computing raises the problem of trusting replicated data servers that may be corrupt. This is because servers must be run on portable computers, and these machines are less secure and thus less trustworthy than those traditionally used to run servers. We describe the kinds of problems one must be prepared to deal with, noting that even users of secured, non‐portable computers are at risk if servers trust all authorized peers. We show that high availability through data replication on portable computers need not be mutually exclusive with various levels of data security one might want. We give three solutions to this trust problem for a simple example architecture, achieving progressively higher levels of security with progressively higher costs. We then show how to solve this trust problem for the more complex architecture of Bayou, a weakly consistent replicated data system we built at Xerox PARC.

Reuse partitioning in cellular networks in dynamic channel allocation

Abstract: Great interest in recent years has been devoted to mobile communications. The research effort has been directed to increasing the capacity of radio systems by applying space reuse techniques. Higher efficiency in the usage of the available frequency spectrum can be obtained either by reducing the cell size, thus requiring the provision of new base stations, or by reusing the available spectrum more efficiently without cell size reduction. In this paper we present a dynamic frequency allocation algorithm for cellular networks that exploits a given reuse pattern. The performance of the proposed scheme, in terms of blocking probability, is evaluated by means of computer simulations both when the position of the mobiles remains unchanged and when mobility is taken into account, under both uniform and hot-spot traffic. The numerical results show that the capacity of the proposed scheme is sensibly higher than that of a dynamic channel allocation without reuse partitioning. The effects of both user mobility and reuse partitioning on the signalling load are also considered.

Multibeam cellular communication systems with dynamic channel assignment across multiple sectors

Abstract: In cellular communication systems, directional multibeam antennas at cell sites can be used to reduce co-channel interference, increase frequency reuse and improve system capacity. When combined with dynamic channel assignment (DCA), additional improvement is possible. We propose a multibeam scheme using dynamic channel assignment across multiple sectors. A cell is divided into several sectors, each of which is covered by several directional beams. Specific channels are allocated to each sector as in fixed channel assignment (FCA). A channel of a sector is dynamically assigned to a wireless user who communicates through one of the several beams of the sector. The assignment is made so that constraints on the allowable co-channel interference are satisfied. Limitations due to co-channel interference are analyzed. A tractable analytical model for the proposed scheme is developed using multidimensional birth–death processes. Theoretical traffic performance characteristics such as call blocking probability, forced termination probability, hand-off activity, carried traffic and channel rearrangement rate are determined. With the proposed scheme, call blocking probability can be reduced significantly for a fixed offered traffic. Alternatively, system capacity can be increased while blocking probability is maintained below the required level. Smaller forced termination probability is obtainable in comparison with corresponding FCA schemes.