Showing papers presented at "European Wireless Conference in 2005"

Analysis and performance evaluation of the OFDM-based metropolitan area network IEEE 802.16

Abstract: Wireless last mile technology is becoming a challenging competitor to conventional wired last mile access systems like DSL and cable modems or even fiber-optic cables. The Institute of Electrical and Electronics Engineers has developed a standard for fixed broadband wireless access systems namely IEEE 802.16. Its OFDM mode targets frequency bands below 11GHz. This paper gives an overview of the OFDM-based transmission mode of the IEEE 802.16 standard. The medium access control (MAC) and the physical layer are described in detail. Especially the MAC frame structure is elaborated. An analytical performance evaluation of an example scenario is performed which results in overall system performance measures. Especially the interaction of fragmentation and padding of OFDM symbols and its effect on the system capacity is evaluated. Furthermore, different MAC layer configurations with different levels of robustness are analyzed. Optional features to resist challenging channel conditions are outlined. Their trade off, i.e., a reduced MAC layer capacity is pointed out. It is shown that the system can be optimized while maintaining the necessary robustness against environmental challenges. A prototypical IEEE 802.16 protocol stack including a sophisticated channel model has been implemented. By means of this stochastic event-driven computer simulator, downlink and uplink delay as well as throughput evaluation is performed. Thus, performance results based on meaningful MAC configuration examples are provided. Simulative and analytical results are compared.

Cooperative multi-hop transmission in wireless networks

Abstract: We consider various relaying strategies for wireless networks by comparatively examining direct transmission, conventional relaying, and the novel concepts of cooperative relaying. The latter build on two inherent benefits of relaying systems: the spatial diversity offered by the relay channel, and the ability to exploit the broadcast nature of the wireless medium. Studied cooperative protocols include adaptive decode-and-forward schemes as a simple extension of conventional store-and-forward relaying systems, and more complex decode-and-reencoding schemes that realize distributed coding strategies. We provide a unifying analysis for the tractable two-hop case, before extending the consideration to multi-hop scenarios. The analysis is conducted from the perspective of communication over fading channels under limited bandwidth, energy, and end-to-end delay; main parameters include propagation loss, network geometry, and targeted end-to-end spectral efficiency. Main results indicate that (i) cooperative relaying provides attractive benefits for wireless systems whenever temporal and frequency diversity are scarce or not exploited, (ii) using just two hops is reasonable for many practical scenarios, and (iii) the advantages of the studied relaying schemes decrease for higher desired end-to-end spectral efficiency.

Inter-cell scheduling in wireless data networks

Abstract: Over the past few years, the design and performance of channel-aware scheduling strategies have attracted huge interest. In the present paper we examine a different notion of scheduling, namely coordination of transmissions among base stations, which has received little attention so far. The inter-cell coordination comprises two key elements: (i) interference avoidance; and (ii) load balancing. The interference avoidance involves coordinating the activity phases of interfering base stations so as to increase transmission rates. The load balancing aims at diverting traffic from heavily-loaded cells to lightly-loaded cells. We consider a dynamic scenario where users come and go over time as governed by the arrival and completion of random data transfers, and evaluate the potential capacity gains from inter-cell coordination in terms of the maximum amount of traffic that can be supported for a given spatial traffic pattern. Numerical experiments demonstrate that inter-cell scheduling may provide significant capacity gains, the relative contribution from interference avoidance vs. load balancing depending on the configuration and the degree of load imbalance in the network.

Characterizing the duration and association patterns of wireless access in a campus

Abstract: Our goal is to characterize the access pat- terns in a IEEE802.11 infrastructure. This can be bene- ficial in many domains, including coverage planning, re- source reservation, supporting location-dependent applica- tions and applications with real-time constraints, and pro- ducing models for simulations. We conducted an exten- sive measurement study of wireless users and their asso- ciation patterns on a major university campus using the IEEE802.11 wireless infrastructure. We characterized and analyzed the wireless access pattern based on several pa- rameters such as mobility, session and visit durations. We show that the mobility and building type affect the session and visit durations. As the mobility increases, the visit du- ration tends to decrease stochastically. The opposite hap- pens in the case of the session duration. Moreover, there exist different stochastic orders among visit durations of different building types when conditioning on session mo- bility. A family of BiPareto distributions can model the visit and session duration.

Realization and Performance Analysis of a SOAP Server for Mobile Devices

Abstract: A middleware platform for mobile devices to efficiently build distributed applications in a heterogenous mobile environment is essential. Especially for mobile Peer-to-Peer (P2P) applications and applications which have to call back a mobile device, server functionalities for mobile devices are required. In this paper the server building block of our mobile Web Service based middleware is introduced. This server is integrated in the wireless Simple Object Access Protocol (SOAP) in terms of a HTTP server binding to SOAP. Thus, the mobile application can define services which will be published and used by other devices. This paper presents a lightweight SOAP server architecture for Java2 Micro Edition (J2ME) devices as well as the implementation in Java. Finally, the results of a performance analysis are given in consideration of criteria like latency and memory usage.

Performance evaluation of SIP-based multimedia services in UMTS

Abstract: With an ever increasing penetration of IP technologies and the tremendous growth in wireless data traffic, the wireless industry is evolving the mobile core networks towards IP technology. The third Generation Partnership Project (3GPP) has specified an IP multimedia sub-system (IMS) in UMTS Release 5/6, which is adjunct to the UMTS packet-switched (PS) GPRS core network. This IP-based network provides full packet call control capabilities by using the text-based Session Initiation Protocol (SIP). Initial indications, as to the signalling delay associated with SIP messages, have concerned mobile operators about the viability of SIP services over the UMTS air interface. This article provides an insight into the UMTS system performance, focusing on selected UMTS SIP-based services. Typical services with real-time requirements such as voice as well as delay-sensitive and non-sensitive applications, such as real-time chat and instant messaging services are investigated. Furthermore, the paper discusses and analyses the requirements and possible solutions for improving efficiency of SIP usage in a wireless environment through signalling protocol message compression. Results of a performance evaluation of SIP signalling scenarios are presented in terms of time delay and message overload in the system. Results show that message compression can considerably reduce SIP message transmission time on the radio access network while core network delay contributions are found to be still high.

Securing OLSR Using Node Locations

Abstract: In this paper we examine security issues related to the Optimized Link State Routing protocol, a proactive routing protocol for MANETs. We enumerate a number of possible attacks against the integrity of the OLSR routing infrastructure, and present a technique for securing the network. In particular, we concentrate on the remaining attacks when a mechanism of digitally signed routing messages is deployed and an attacker may have taken control over trusted nodes. Our solution is based on inclusion of the geographical position of the sending node in control messages, and on evaluation of verisimilitude of links; this is accomplished using a GPS device and a directional antenna embedded in each node.

Throughput and Delay Performance of IEEE 802.11e Wireless LAN with Block Acknowledgments

Abstract: The IEEE 802.11e standard extension for medium access control protocol in wireless local area networks enhances the existing IEEE 802.11 protocol specification towards support for quality of service and improved spectrum management. We look in this paper specifically into an 802.11e concept that is referred to as block acknowledgment. Unlike the original 802.11, where every frame reception must be acknowledged independently, block acknowledgments offer the possibility to acknowledge several consecutive data frames in one response. This obviously may help to decrease the protocol overhead, but is expected to introduce higher end-to end delays for the data frame transmissions. We present a closed-form analytical analysis of the block acknowledgment protocol, and compare our findings to simulation results. Our results show that whereas throughputs improve considerably when block acknowledgments are applied, care has to be taken not to increase the delays above unacceptable durations.

ADOPT. A Distributed OCSP for Trust Establishment in MANETs

Abstract: The establishment of trust in mobile ad hoc networks (MANET) has been receiving increasing attention during the last few years. Due to the infrastructure-less nature of MANETs, the design and implementation of adequate security and trust mechanisms is imperative. Many trust establishment solutions, including recent proposals for secure routing protocols, rely on public key certificates. The use of certificates in ad hoc networks must be accompanied by an efficient mechanism for certificate revocation and validation. In this paper we address this open issue, and a distributed, on-demand, OCSP-based scheme is proposed. This scheme, called Ad-hoc Distributed OCSP for Trust (ADOPT), uses caches of OCSP responses that are distributed and stored on intermediate nodes, avoiding the exchange of extended certificate status lists among the ad-hoc nodes. The method takes into account the status of intermediate nodes, such as network topology, energy thresholds, and cellular connectivity, to materialise the caching of OCSP responses. The paper discusses alternative design approaches that enable the revision and distribution of up-to-date OCSP responses.

Forced Convergence Decoding of LDPC Codes - EXIT Chart Analysis and Combination with Node Complexity Reduction Techniques

Abstract: Recently, the concept of forced convergence decoding for Low-Density Parity-Check Codes has been introduced. Restricting the message passing in the iterative process to the nodes that still significantly contribute to the decoding result, this approach allows for substantial reduction in decoding complexity at negligible deterioration in performance. We analyze this novel technique using EXIT charts and show how it compares to and can be combined with other complexity reduction techniques. Our findings imply that forced convergence works effectively in conjunction with other complexity reduction techniques while retaining its attractiveness in terms of the complexity-performance trade-off.

IEEE 802.15.3a Wireless Personal Area Networks - The MBOA Approach

Abstract: A new generation of Wireless Personal Area Networks (WPANs) aims at high data rates of several hundred Mb/s. Based on Ultrawideband (UWB) technology theses WPANs make use of a decision of the Federal Communications Commission (FCC) which decided to allow low power emissions in the spectrum from 3 to 10GHz. Among direct sequence spread spectrum proposals the IEEE 802.15.3a Task Group (TG) received an OFDM based proposal from the Multi Band OFDM Alliance (MBOA). Unlike competing technologies, the MBOA approach provides both, a new Physical Layer (PHY) proposal and a new, decentralized Medium Access Control (MAC) protocol, which differs from the legacy IEEE 802.15.3 MAC. In this paper we give technical description of the new MBOA MAC and PHY layer and provide insight to their performance by means of simulation.

High speed downlink packet access (HSDPA): enhanced data rates for UMTS evolution

Abstract: The increasing demand for capacity in order to provide high data-rate multimedia services in wireless environments necessitates enhanced radio transmission techniques and network protocol functionality. Such techniques have to be added to already existing mobile cellular networks, e.g., as provided by Enhanced Data Rates for GSM Evolution (EDGE). For 3rd generation UMTS networks based on WCDMA, High Speed Downlink Packet Access (HSDPA) is being introduced to meet this demand and to improve spectral efficiency. This technique is based on well-known Link Adaptation (LA) by using various modulation and coding schemes and with this realising several different data rates for downlink transmission. Moreover, fast scheduling techniques based on shortened radio frames enable efficient and flexible sharing of the radio resources among different users and services. This article gives a detailed overview on the integration of HSDPA in UMTS networks and provides a detailed performance evaluation for a realistic scenario comprising a multi-cellular environment with various types of applications.

Spectrum Agile Radio: A Society of Machines with Value-Orientation

Abstract: Our radio spectrum is not efficiently utilized because of the complicated and time-consuming radio regulation process In this paper, technologies to help overcoming this barrier, namely, spectrum agile radios, are discussed Spectrum agile radios operate in the radio spectrum that was originally licensed to other (incumbent, primary) radio services A spectrum agile radio seeks unused radio resources and communicates using these opportunities, without interfering with the operation of licensed radios We discuss two main problems: protection of incumbents, and coexistence of agile radios Spectrum sharing among different radio systems can be understood as a scenario forming a society of independent decision-makers Therefore, basic concepts to classify social action that are taken from social science are applied to define system strategy rules The rules represent algorithms for decision-making entities (referred to as actors) that reside in the radio systems For a simple scenario of spectrum sharing, we investigate the need for regulation as opposed to voluntary rules

Reconfigurable Multi-Mode Protocol Reference Model Facilitating Modes Convergence

Abstract: The vision for the next generation of wireless communications is a ubiquitous radio system concept, providing wireless access from short-range to wide-area, with one single adaptive system for all envisaged radio environments. To achieve this, a future radio system is envisaged to efficiently adapt to multiple scenarios by using different modes of a common technology basis. By introducing a reference model for multi-mode protocols, we target a harmonization of the various current research efforts in approaching multi-mode capability and protocol adaptability. Based on the idea of identification of commonalities between multiple envisaged modes and a separation of the common and the mode-specific parts of the protocol, this paper defines a reference model for a multi-mode protocol incorporating the ideas of functional partitioning and re-configuration of layers to achieve both horizontal and vertical convergence between multiple modes.

Mobile Web Services based Middleware for Context-Aware Applications

Abstract: Mobile devices, communication systems and applications increasingly become more complex. In order to manage mobile distributed applications a middleware is essential which simplifies and speeds up the application development. A mobile environment with its heterogeneity and mobility has different requirements compared to a fixed environment. Thus, new concepts and mechanisms for a middleware have to be developed to cope with the challenge of these heterogeneous distributed systems. In this article, we introduce a Web Service based middleware for mobile devices, adapted to a mobile environment with special focus on context-awareness. The contextawareness will be realized by a rule based data monitoring in order to minimize the communication frequency over the mobile links and, thus, to minimize the runtime costs of the mobile application. Within this middleware mobile context providers are able to publish their context-data. This data can be remotely monitored by subscribing to a service of the context-provider and receiving notifications from the context provider. The Web Service based mobile middleware will be described in general, special focus is given to the rule based data monitoring for context-aware applications. In addition a discussion of the expected application runtime costs regarding the communication is given.

Experimental Analysis of TCP and UDP Traffic Performance over Infra-structured 802.11b WLANs

Abstract: In this paper we present the results of experiments carried out in an infra-structured 802.11b WLAN comprising a single Access Point (AP) and a number of user terminals. The aim of this work is twofold: by the one hand, we validate well-known ?TCP (UDP) over 802.11? analytic models by means of experimental results; by the other hand, we focus our analysis on TCP behavior over 802.11b. We present results about flow fairness, about interaction between WLAN link layer parameters (ARQ) and transport protocols and about TCP flows traffic characteristics. With respect to earlier related works, an added value of our analysis is given by a more accurate knowledge and control of key parameters that highly affect the system performance (e.g., mobile station transmission bitrate, maximum number of retransmission attempts, TCP version, etc.) that guarantees the reliability of numerical results. This is a key issue in order to confidently explain observed experimental phenomena.

Configurable Ultra Low Noise Ultra Wideband Power Efficient VCOs

Abstract: Voltage-controlled oscillators with wide tuning range (more than octave-band) and low phase noise are essential building blocks for next-generation wireless communication systems. The coexistence of second and third generation wireless system require multi-mode, multi-band and multi-standard mobile communication systems, therefore, requiring a wideband source that may replace narrow band voltage-controlled oscillator (VCOs) modules by a single low noise wideband VCO. Phase noise can vary dramatically over the tuning range of the VCOs, but the published literature often obscures this fact by reporting the typical phase noise measurement at selected tuning voltages. The VCO design approach demonstrated in this work enables wideband tunability, lower noise performance over the tuning range, power efficient, compact size, configurable, and easily amenable for integration in chip form. The measured phase noise is better than ?108 dBc at 100 kHz offset over the tuning range (1500-6000 MHz), and featuring uniform phase noise within 4 dB variations over the tuning range.

A Table-driven AOA Estimation Algorithm for Switched-beam Antennas in Wireless Networks

Abstract: Accurate Angle of Arrival (AOA) estimation is crucial in two aspects: adaptive beam-forming in smart antenna systems and object positioning in ubiquitous computing applications. The AOA information is essential for smart antenna systems in order to improve efficiency (e.g. throughput enhancement) in transmission and reception. The AOA information assists the system to automatically change the direction of the radiation/reception beam pattern to adapt to the position of the receiver/sender, respectively. This adaptive beam forming technique can maximize the signal-to-interference/noise-ratio (SINR). In ubiquitous computing applications, AOA information is utilized for the purpose of positioning of the object with a wireless interface. However, the existing AOA estimation algorithms incur large computational complexity or exhibit antenna design-dependent performance. In this paper, we propose a simple table-driven AOA estimation (TAE) algorithm for a switched-beam antenna system. Due to its relatively low hardware complexity and cost, the switched beam antenna system is most widely implemented type in smart antenna systems. Simulation results show that the proposed table-driven approach yields a low estimation error (half of angular granularity) with O(1) computational complexity. We also demonstrated that the performance of our TAE algorithm is independent of beam-coverage, beam-width and side-lobe level.

A Performance Comparison of QoS Approaches for Ad Hoc Networks: 802.11e versus Distributed Resource Allocation

Abstract: We compare two approaches for Quality of Service support in WLAN-based ad hoc networks. The rst approach is to use per-packet priorities, according to the IEEE 802.11e standard. The second approach is to allo- cate radio resources on the path between source and des- tination, according to our protocol 'Distributed end-to-end Allocation of time slots for REal-time trafc' (DARE). Performance simulations show the following results: In case of low load, IEEE 802.11e has slightly lower end-to- end delay and higher packet loss rate, since it does not use any coordination among nodes for real-time packets. In case of medium load, DARE is superior in terms of jitter, delay, and packet loss. In case of high load, DARE clearly outperforms 802.11e. The results still hold if DARE has to repair the resource reservation path due to node failures.

Limits of Phase Noise Suppression in OFDM

Abstract: We introduce two independent approaches for phase noise suppression. The dominant effects, responsible for the degradation of an OFDM system performance, if phase noise is present, are identified. We found out, that the system performance is strongly influenced by certain phase noise realizations, which cause burst errors, resulting in the performance error floor. Consideration of the system capacity in the presence of phase noise, motivates the idea, that bit interleaving can significantly improve the system performance. For systems where interleaving delay is a critical issue, we propose, as a second approach, an iterative phase noise suppression algorithm. Simulation results in terms of packet error rate (PER) show, that both bit interleaving and iterative algorithm are capable to significantly suppress the phase noise.

System performance and adaptive configuration of link adaptation techniques in packet-switched cellular radio networks

Abstract: Link Adaptation is an adaptive radio resource management technique that selects a transport mode from a set of predefined modes of varying robustness, depending on the experienced channel quality conditions and dynamics. Previous work has shown the need to adapt the configuration of Link Adaptation to certain operating conditions affecting the channel quality dynamics. In particular, it was shown that the system load can considerably influence the performance and configuration of Link Adaptation techniques. The user mobile speed is another key factor influencing the channel quality dynamics. As a result, the aim of this paper is to investigate whether the user mobile speed should also be taken into account when configuring and designing the operation of Link Adaptation techniques.

Application of SQRD Algorithm for Efficient MIMO-OFDM Systems

Abstract: High data rate techniques in wireless communication systems have gained considerable significance in recent years. A strong candidate for broadband wireless communication systems for achieving high data rate and combating mutipath fading is OFDM (Orthogonal Frequency Division Multiplexing). Combining OFDM with multiple antenna systems has been shown to provide significant increase in the capacity in rich-scattering environments. In this paper, we combine a MIMO (Multiple Input Multiple Output) system based on the V-BLAST architecture with OFDM multicarrier transmission. The receiver is implemented using previously proposed sorted QR decomposition (SQRD) for the successive interference cancellation (SIC) between the different transmitted data streams. We will show a significant performance improvement of this technique over linear detection algorithms. In addition, we evaluate the influence of the incorporation of a soft output detector on the bit error rate (BER) performance.

A comparision of OFDM and non-linear SC/FDE signals: Non-linear amplification

Abstract: One of the most influential parameters in today's wireless communications market is the battery lifetime, especially for mobile devices. In contrast to OFDM (Orthogonal Frequency Division Multiplexing), SC/FDE (single carrier transmission with frequency domain equalization) allows to apply signal formats with constant envelope, such as CPM (Continuous Phase Modulations) or at least significantly lower Peak to Average Power Ratios (PAPRs) such as Offset-QPSK. This allows the signal to drive the power amplifier into its maximum power efficient region. As a consequence the batteries lifetime is extended and its size is reduced. In this work, we compare non-linear modulated, block-oriented SC/FDE signals with OFDM signals in terms of bit error rate and spectrum disturbances at the non-linear region of the power amplifier.

Provisioning and performance of mobility-aware personalized push services in wireless broadband hotspots

Abstract: In this work we investigate the deployment of so-called ''media points'' to facilitate the provisioning of personalized push services to mobile users within WLAN hotspots. We present the feasible usage scenarios with stationary and moving user terminals for some envisioned services and outline the requirements characteristic for a media point system. A hierarchical architecture with centralized service control for small-scale media point networks is proposed and the employment of Session Initiation Protocol (SIP) for handling the mobility and session management of media point services is described. A demonstrator system has been developed in order to show the technical feasibility of the concept by means of state-of-the-art technologies and to allow an experimental performance evaluation of the proposed protocols and mechanisms for a typical service-provisioning scenario. We observed that system performance is strongly dependent on the level of interworking between the various protocols and software modules, e.g., SIP modules, DHCP modules, WLAN device driver, etc. In particular the signaling mechanism within the network by means of SIP doesn't cause any significant delay. The overall system performance is found as acceptable when assuming that the dwell time of mobile users within WLAN hotspots is in the order of minutes or longer.

A generic IP paging architecture and protocol

Abstract: Mobile communication systems are increasingly adopting Internet Protocol solutions for mobility management. Taking into account that an increasing number of mobile devices communicate actively for only a fraction of the time, location tracking costs must be appropriate for active and idle devices. Compelling all devices to maintain detailed location information in the network causes unnecessary signaling overhead and mobile battery drainage. Contemporary cellular mobile communication systems deploy technology-specific paging, which supports the maintenance of a mobile terminal's coarse location information and to request its exact location when a call comes in. IP paging goes beyond that by targeting heterogeneous access in IP-based mobile communication networks, and by contributing towards meeting IP mobility management requirements on the scalability of future systems. This paper describes a generic IP paging architecture and protocol for heterogeneous mobile communication systems that integrates modularly into various IP-based mobility protocols. At the same time efficient integration of various access technologies and associated utilization of technology specific power save mechanisms is supported. The concept facilitates flexible paging area design and the deployment of enhanced paging strategies aimed at optimizing overall signaling costs. The concept is analyzed with regard to its characteristics to save signaling costs and compared with a different paging concept (P-MIP). Design and associated performance characteristics are evaluated based on the topological structure of a domain's access network. To enhance power saving efficiency in Wireless-LAN-enabled networks, the integration of the IEEE802.11 access technology with the proposed paging protocol including the efficient utilization of the standard's Power Save Mode is described. The complete paging system's delay characteristics are evaluated analytically to estimate dependencies of the session blocking probability on the network's link delay and IEEE802.11 specific power saving parameter settings.

New Algorithm for Distributed Frequency Assignments in IEEE 802.11 Wireless Networks

Abstract: The continuous growth of IEEE 802.11 wireless local area networks (WLAN) brings the benefit of a high speed wireless access to packet networks, such as Internet. But it also entails the successive appearance of new unresolved problems. One of these problems consists in the degradation of the performance observed by the users when there is a great number of radio networks coexisting in the same area. In densely populated zones it is not strange to find WLANs of different nature (private, public, etc.) sharing a scarce resource as it is the radio spectrum. This problem can be mitigated with an appropriate channel allocation. In this paper we present an algorithm for the frequency assignment problem (FAP) in IEEE 802.11, based on classical graph colouring but adding the objective of reducing interferences among overlapping cells, and thus improving the global throughput performance. An accurate evaluation of interferences is obtained not only measuring power levels, but also being aware of traffic load. Important performance improvements have been observed on a real scenario.

CLAMP: Active queue management at wireless access points

Abstract: This paper investigates the performance of CLAMP, a distributed algorithm to enhance the performance of TCP connections that terminate in a wireless access network. CLAMP works at a receiver to control a TCP sender by setting the TCP receiver’s advertised window limit. CLAMP provides explicit control over wireless link utilisation and queueing delay at the access point buffer, and can drastically increase the throughput of socalled short TCP flows with negligible loss in long TCP flow throughput.

Performance of TCP and HTTP Proxies in UMTS Networks

Abstract: It is well known that the large round trip time and the highly variable delay in a cellular network may degrade the performance of TCP. Many concepts have been proposed to improve this situation, including performance enhancing proxies (PEP). One important class of PEPs are split connection proxies, which terminate a connection from a server in the Internet in a host close to the Radio Access Network (RAN) and establish a second connection towards the mobile User Equipment (UE). This connection splitting can be done either purely on the transport layer (TCP proxy) or on the application layer (HTTP proxy). While it is clear that an application layer proxy also infers the splitting of an underlying transport layer connection, the performance of web applications may be essentially different for both approaches. This paper first investigates the TCP connection behavior of the Mozilla web browser. Subsequently, the performance of TCP and HTTP proxies in UMTS networks is studied under different scenarios and for different HTTP configurations by means of emulation.

Delay-Limited Capacity for Broadcast Channels

Abstract: In this paper, we consider an OFDM downlink situation, where the information is sent from only one transmitter to several receivers. For such a scenario the delay-limited capacity is investigated. We show that the delay-limited capacity is equal to zero when one Rayleigh channel is assigned to each user, and that it is greater than zero in case of the broadcast model where n users share n independent channels simultaneously. Furthermore, the case of more than one channel per user is considered. For an OFDM situation with many channels, it turns out that sharing all channels by all users outperforms the exclusive assignment of channels to individual users. However, the shared channel use is worse in case of very few channels.

A Low Complexity Algorithm for Sensor Localization

Abstract: Locationing in sensor networks is an area that has received much attention over the last few years. In most applications of sensor networks, position-awareness at individual sensors is crucial for a successful operation of the sensor network. Estimation of relative node coordinates, based on measured distances between nodes in the network, has been suggested as a means to provide position-awareness in sensor networks. Localization algorithms, based on distance measurements, may be realized using complex optimization algorithms, estimating not only node coordinates, but also individual node clock-offsets. As sensor sizes decrease and power becomes a scarce resource, such complex locationing algorithms may not be feasible. In this paper, we present a distributed locationing algorithm, called the kernel algorithm. The kernel algorithm handles unknown clock-offsets through a time-difference-of- arrival approach and does not assume any synchronization among the nodes in the network. The algorithm offers robustness and accuracy while keeping complexity low by means of a divide and conquer approach. Numerical simulations verifies the performance and robustness of the proposed algorithm.