Search

Peer-to-peer storage services are a cost-effective alternative for data backup. A basic question that arises in the design of such systems is: In which peers do we store redundant data? Choosing appropmailriate peers for data backup is important at a microscopic level, from an end-user's perspective to guarantee good performance, e.g., quick access, high availability, etc., as well as at a macroscopic level, e.g., for system optimization, fairness, etc. Existing systems apply different techniques, including random selection, based on a distributed hash table (DHT) or based on the peers' past availability pattern. In this paper, we propose as an alternative, a contextual trust based data placement scheme to select suitable data holders. It is originally designed for and applicable to scenarios where there is inadequate historical information about peers, a common scenario in large-scale systems. Specifically, our scheme estimates trustworthiness of a peer based on stereotypes, formed by aggregating information of interactions with other (similar) peers. Simulation experiments show that our placement scheme outperforms not only random selection but also schemes using historical information, in terms of both achieved data availability as well as bandwidth overheads to sustain the system. [ABSTRACT FROM AUTHOR]

This paper demonstrates a power efficient design of high-speed Digital-to-Analog Converters (DACs) for wideband communication systems. For Wireless personal area network applications with a 250 MHz signal bandwidth, a 6 bit DAC capable of two times the Nyquist rate sampling is implemented in a current steering segmented 2 + 4 architecture optimized for power efficiency. Along with a proposed master-slave deglitch circuit, several circuit techniques are investigated to improve dynamic performances such as linearity. Implemented in a 0.18 um CMOS process, our DAC achieved a superior conversion performance over the state-of-the-arts, exhibiting integral nonlinearity of less than 0.27 LSB and differential nonlinearity of less than 0.15 LSB. Measured spurious free dynamic range for 251 MHz output signal is 40.92 dB, with total power consumption at 1 GS/s of 6mW, yielding a figure-of-merits of 78.3 pJ/(conversion step*W). [ABSTRACT FROM AUTHOR]

A new vertical transition between a substrate integrated waveguide in a low-temperature co-fired ceramic substrate and an air-filled standard waveguide is proposed in this paper. A rectangular cavity resonator with closely spaced metallic vias is designed to connect the substrate integrated waveguide to the standard air-filled waveguide. Physical characteristics of an air-filled WR-22 to WR-22 transition are compared with those of the proposed transition. Simulation and experiment demonstrate that the proposed transition shows a -1.3 dB insertion loss and 6.2 GHz bandwidth with a 10 dB return loss for the backto- back module. A 40 GHz low-temperature co-fired ceramic module with the proposed vertical transition is also implemented. The implemented module is very compact, measuring 57 mm × 28 mm × 3.3 mm. [ABSTRACT FROM AUTHOR]

Ethernet ring protection (ERP) technology, which is defined in ITU-T Recommendation G.8032, has been developed to provide carrier grade recovery for Ethernet ring networks. However, the filtering database (FDB) flush method adopted in the current ERP standard has the drawback of introducing a large amount of transient traffic overshoot caused by flooded Ethernet frames right after protection switching. This traffic overshooting is especially critical when a ring provides services to a large number of clients. According to our experimental results, the traditional FDB flush requires a link capacity about sixteen times greater than the steady state traffic bandwidth. This paper introduces four flush optimization schemes to resolve this issue and investigates how the proposed schemes deal with the transient traffic overshoot on a multi-ring network under failure conditions. With a network simulator, we evaluate the performance of the proposed schemes and compare them to the conventional FDB flush scheme. Among the proposed methods, the extended FDB advertisement method shows the fastest and most stable protection switching performance. [ABSTRACT FROM AUTHOR]

In this paper, an ultra-wideband internal antenna for use in mobile applications is proposed. The proposed antenna has symmetrical bi-arm structures printed on the top and bottom of the substrate, and it occupies a compact area of 10 mm × 10 mm × 1 mm. The designed antenna has an impedance bandwidth from 3 GHz to 12 GHz and near omnidirectional radiation patterns over the frequency band of interest. The group delay between two antennas fabricated using the proposed design is less than 0.8 ns, and the maximum gain variation is about 3.16 dB. [ABSTRACT FROM AUTHOR]

Ultra-wideband (UWB) systems have witnessed a debate over whether they may cause interference to other existing and future narrowband systems sharing their band of operation. The detect and avoid (DAA) mechanism was developed as a solution to reduce interference to narrowband systems in order to ease regulatory concerns. It works by adaptively reducing the transmitted power at the overlapping bands upon detecting an active narrowband link. However, employing DAA degrades the performance of UWB transmissions. In this paper, we present the Korean UWB regulations as an example of regulations that require DAA in certain bands. We investigate DAA's impact on performance and propose a method to mitigate it, which provides UWB with the more efficient support of the DAA mechanism and enables it to avoid a larger number of narrowband users while sustaining the data rate. Results show significant improvement in performance with the application of our technique compared to conventional performance. [ABSTRACT FROM AUTHOR]

In the tone reservation (TR) scheme of the orthogonal frequency division multiplexing (OFDM) systems, there exists a trade-off between the peak to average power ratio (PAPR) reduction performance and the peak reduction tone (PRT) set size. In this paper, we propose a multi-stage TR scheme for PAPR reduction, which adaptively selects one of several PRT sets according to the PAPR of OFDM signal while the PRT set is fixed for the conventional TR scheme. It is shown that the PAPR reduction performance of the proposed scheme is better than that of the conventional TR scheme when the tone reservation rate (TRR) is the same. [ABSTRACT FROM AUTHOR]

The iterative exchange of extrinsic information between the K-best sphere detector (SD) and the channel decoder is appealing since it is capable of achieving a near maximum a posteriori (MAP) performance at a moderate complexity. However, the computational complexity imposed by the K-best SD significantly increases when using a large value of K for the sake of maintaining a near-MAP performance in a high-throughput uplink spatial-division multiple access (SDMA) orthogonal frequency-division multiplexing (OFDM) system supporting a large number of users and/or a high number of bits/symbol. This problem is further aggravated when the number of users/mobile stations (MSs) U exceeds that of the receive antennas N at the base station (BS), namely, in the challenging scenario of rank-deficient systems. We demonstrate that the iterative decoding convergence of this two-stage system may be improved by incorporating a unity rate code (URC) having an infinite impulse response, which improves the efficiency of the extrinsic information exchange. Although this results in a slightly more complex three-stage system architecture, it allows us to use a low-complexity SD having a significantly reduced candidate list size Ncand. Alternatively, a reduced signal-to-noise ratio (SNR) is required. For example, given a target bit error ratio (BER) of 10-5 and Ncand = 32 for the SD, the three-stage receiver is capable of achieving a performance gain of 2.5 dB over its two-stage counterpart in a rank-deficient SDMA/OFDM 4-quadratic-amplitude modulation (4-QAM) system supporting U = 8 cochannel users and employing N = 4 receive antennas at the BS, namely, in an (8 x 4) system. For the sake of further enhancing the three-stage concatenated receiver, the proposed iterative center-shifting SI) scheme and the irregular convolutional codes (IrCCs) are intrinsically amalgamated, which leads to an additional performance gain of 2 dB. [ABSTRACT FROM AUTHOR]

We consider the problem of pilot design for channel estimation in multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems in the presence of virtual carriers. The design criterion is the average mean-square error (MSE) of the least square (LS) estimates of the channel frequency response over the data carriers. To maximize bandwidth efficiency, the number of pilots is set to its minimum which is equal to the number of unknown channel coefficients. Both the phase shift orthogonal and disjoint pilot schemes are investigated. For a given pilot placement, a closed-form expression for the pilot power distribution for the disjoint scheme is provided. It is found that the optimal power distribution allocates less power to the pilots that are close to the virtual carriers. Pilot placement is obtained using an exhaustive search. The latter is too complex in systems with large number of subcarriers. A suboptimum pilot placement design for the disjoint pilot scheme is presented. It is shown that, unlike fully loaded systems where the two pilot design schemes have similar performance, in the presence of virtual carriers the optimal disjoint scheme not only has lower complexity but also yields more accurate channel estimation and bit error rate performance. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, closed-form expressions for the capacities per unit bandwidth for Rayleigh fading channels with equal gain combining (EGC) diversity case are derived for power and rate adaptation, constant transmit power, channel inversion with fixed rate, and truncated channel inversion adaptation policies. Channel inversion policies provide the highest capacity over the other adaptation policies with EGC diversity. The constant transmit power policy provides the lowest capacity as compared to the other policies. [Copyright &y& Elsevier]

In modern wireless mesh networks, the transmitted-information signals from the source node arrive at the final destination through relay stations. The introduction of relay stations into metropolitan-area networks allows the provision of ubiquitous broadband access, even in remote places, and increases the scalability potential. Due to the increased demand for bandwidth and the spectral congestion at low frequencies, these wireless mesh networks usually operate at frequencies above 10 GHz, satisfying the line-of-sight condition. In this frequency range, the most dominant fading mechanism degrading the availability of radio links and exposing spatial and temporal stochastic variation is rain attenuation. In this paper, the optimum placement of radio relays in a dual-hop configuration is studied, based on physical prediction models for the total outage probability. The focus is on the spatial inhomogeneity of rain attenuation over the converging microwave paths, which leads to the incorporation of the two-dimensional lognormal distribution. The impact of various operational and geometrical properties of adaptive gain transparent and regenerative radio relays on the outage performance and on the optimum relay position for various locations is investigated through a series of extended numerical results. [ABSTRACT FROM AUTHOR]

This paper presents an analysis framework for estimating spatio-temporal distortion associated with data-gathering algorithms in sensor networks. We assume that the underlying data field being sensed is spatially and temporally correlated and that the sink node performs real-time reconstruction of the field. Unlike existing work, the proposed analysis framework assumes realistic transmission schemes and network/routing parameters, such as congestion and limited transmission range. Our simulations show that, even for a simple collision-free transmission scheme and regular deployment patterns, the distortion analyses yield starkly different results, compared with the analyses performed in the absence of these assumptions. The analytical method presented in this paper could easily be extended to consider the real-time data-reconstruction problem under different transmission schemes, node distributions, and reconstruction methods. [ABSTRACT FROM AUTHOR]

Ultrawideband (UWB) communications involve very sparse channels, because the bandwidth increase results in a better time resolution. This property is used in this paper to propose an efficient algorithm that jointly estimates the channel and the transmitted symbols. More precisely, this paper introduces an expectation-maximization (EM) algorithm within a wavelet- domain Bayesian framework for semiblind channel estimation of multiband orthogonal frequency division multiplexing based UWB communications. A prior distribution is chosen for the wavelet coefficients of the unknown channel impulse response to model a sparseness property of the wavelet representation. This prior yields, in maximum a posteriori estimation, a thresholding rule within the EM algorithm. We particularly focus on reducing the number of estimated parameters by iteratively discarding "insignificant" wavelet coefficients from the estimation process. Simulation results using UWB channels that were issued from both models and measurements show that, under sparseness conditions, the proposed algorithm outperforms pilot-based channel estimation in terms of the mean square error (MSE) and bit error rate (BER). Moreover, the estimation accuracy is improved, whereas the computational complexity is reduced compared with traditional semiblind methods. [ABSTRACT FROM AUTHOR]

The authors present the results of a polygonal patch antenna for ultra-wideband applications covering a frequency band from 4.14 to 13.50 GHz. The fabricated antenna achieved a -10 dB impedance bandwidth in excess of 125% with an antenna size of 0.373λo×0.373λo×0.149λo at its centre frequency. The antenna's impedance bandwidth is 64% higher than what is currently obtainable with state-of-the-art folded-patch techniques. The proposed patch antenna has a polygonal-shape with a rectangular slot and shorting pins. The analysis of this antenna shows that bandwidth broadening is achieved by using a rectangular slot on the patch that is fed from a folded-patch feed, whereas the reduction in antenna size is achieved through the use of two shorting pins strategically located on the radiating patch. [ABSTRACT FROM AUTHOR]

Checkpointing, the process of saving program/application state, usually to a stable storage, has been the most common fault-tolerance methodology for high-performance applications. The rate of checkpointing (how often) is primarily driven by the failure rate of the system. If the checkpointing rate is low, fewer resources are consumed but the chance of high computational loss is increased and vice versa if the checkpointing rate is high. It is important to strike a balance, and an optimum rate of checkpointing is required. In this paper, we analytically model the process of checkpointing in terms of mean-time-between-failure of the system, amount of memory being checkpointed, sustainable I/O bandwidth to the stable storage, and frequency of checkpointing. We identify the optimum frequency of checkpointing to be used on systems with given specifications thereby making way for efficient use of available resources and maximum performance of the system without compromising on the fault-tolerance aspects. Further, we develop discrete-event models simulating the checkpointing process to verify the analytical model for optimum checkpointing. Using the analytical model, we also investigate the optimum rate of checkpointing for systems of varying resource levels ranging from small embedded cluster systems to large supercomputers. [ABSTRACT FROM AUTHOR]

This paper presents a novel generalized transceiver architecture, i.e., Multicarrier Interleave-Division-Multiplexing-aided Interleave Division Multiple Access (MC-LDM-IDMA). A parallel interference cancellation (PIC)-aided iterative multi- user detection (MUD) algorithm is introduced, and the convergence behavior of the system is evaluated with the aid of EXtrinsic Information Transfer (EXIT) charts under the assumption of an equal-power multiuser scenario. Three interesting design tradeoffs of the MC-IDM-IDMA system are analyzed: 1) the multiplexing versus diversity tradeoffs; 2) the coding versus spreading tradeoffs, where the achievable bandwidth efficiency of both MC-IDM and MC-IDMA are characterized, and a general design rule is provided for low-rate codes; and 3) the complexity versus performance tradeoffs and the EXIT characteristics of two different MUD algorithms are investigated, leading to a reduced- complexity hybrid MUD. [ABSTRACT FROM AUTHOR]

This paper presents the design, fabrication, and characterization of a compact narrowband tunable impedance-matching network. A prototype was fabricated in hybrid technology using coplanar waveguide transmission lines and surface-mounted components. The network consists of a H-structure with tunable components made of varactors in series with inductors. Simulations and measurements are in good agreement. A new measurement methodology suitable for tunable impedance-matching networks is proposed. The results show that complex impedances with magnitudes varying from 6 Ω to 1 kΩ can be matched at 1 GHz and tuned in a 50% bandwidth. In addition, nonlinear measurements were done to fully characterize the network. The 1-dB compression point is reached at an input power of 20 dBm, while the input power for the third-order intermodulation intercept point is 21.3 dBm for a 1-kHz offset between the two input frequencies. [ABSTRACT FROM AUTHOR]

This paper addresses when and how to adjust bandwidth allocation on uplink and downlink in a multiservice mobile wireless network under dynamic traffic load conditions. Our design objective is to improve system bandwidth utilization while satisfying call level QoS requirements of various call classes. We develop a new threshold-based multiservice admission control scheme (DMS-AC) as a study base for bandwidth reallocation. When the traffic load brought by some specific classes under dynamic traffic conditions in a system exceeds the control range of DMS-AC, the QoS of some call classes may not be guaranteed. In such a situation, bandwidth reallocation process is activated and the admission control scheme will try to meet the QoS requirements under the adjusted bandwidth allocation. We explore the relationship between admission thresholds and bandwidth allocation by identifying certain constraints for verifying the feasibility of the adjusted bandwidth allocation. We conduct extensive simulation experiments to validate the effectiveness of the proposed bandwidth reallocation scheme. Numerical results show that when traffic pattern with certain bandwidth asymmetry between uplink and downlink changes, the system can reallocate the bandwidth on uplink and downlink adaptively. With the designed bandwidth reallocation scheme in conjunction with the distributed admission control scheme, the QoS requirements of various call classes can be guaranteed under dynamic traffic conditions, and in the meantime, the system bandwidth utilization is improved significantly. [ABSTRACT FROM AUTHOR]

This article proposes a solution methodology for the design of a wide area telecommunication network. This study is motivated by the Alberta SuperNet project, which provides broadband Internet access to 422 communities across Alberta. There are two components to this problem: the network design itself, consisting of selecting which links will be part of the solution and which nodes should house shelters; and the loading problem which consists of determining which signal transport technology should be installed on the selected edges of the network. Mathematical models are described for these two subproblems. A tabu search algorithm heuristic is developed and tested on randomly generated instances and on Alberta SuperNet data. [ABSTRACT FROM AUTHOR]

The focus of this paper is a proposal for a computationally efficient dynamic subcarrier allocation (DSA) algorithm for orthogonal frequency-division multiple access (OFDMA) systems. The proposed DSA algorithm considerably reduces the computational complexity and the amount of channel quality information (CQI) compared to amplitude- craving greedy (ACG) algorithms, which use full CQI. At the same time, the performance of the proposed algorithm closely approaches that achieved by ACG algorithms. Moreover, the authors present a new bandwidth-assignment algorithm produced by modifying bandwidth as- signment based on the signal-to-noise ratio (BABS). This modified BABS algorithm enables the proposed DSA algorithm to produce a strong outage performance gain over the conventional scheme. [ABSTRACT FROM AUTHOR]

A variable fractional delay (VFD) filter is presented that constitutes an efficient alternative to the usual Farrow structure. The filter is constructed by multiplying the signal to interpolate by a window with proper truncation properties, and then applying the Sampling Theorem. The resulting interpolator involves less arithmetic operations than the existing variants of the Farrow structure. Besides, if its coefficients are flumerically optimized, its performance is close to that of the FIR equi-ripple filter. The final part of the correspondence includes a description of the VFD filter implementation corresponding to this interpolator. [ABSTRACT FROM AUTHOR]

Fast sampling is desirable to describe signal transmission through wide-bandwidth systems. The delta-operator provides an ideal discrete-time modeling description for such fast-sampled systems. However, the estimation of delta-domain model parameters is usually biased by directly applying the delta-transformations to a sampled signal corrupted by additive measurement noise. This problem is solved here by expectation-maximization, where the delta-transformations of the true signal are estimated and then used to obtain the model parameters. The method is demonstrated on a numerical example to improve on the accuracy of using a shift operator approach when the sample rate is fast. [ABSTRACT FROM AUTHOR]

We address low-complexity, highly-accurate frequency offset estimation for multi-band orthogonal frequency division multiplexing (MB-OFDM) based ultra-wide band systems in time-invariant as well as time-variant channels. We investigate the unique characteristics of MB-OFDM systems, namely, different frequency offsets, channel responses, received energies, and preamble structures in different frequency bands. Utilizing them, we develop frequency offset estimators based on the best linear unbiased estimation principle. If compared to the reference estimators, our proposed methods achieve significantly better estimation performance (4 to 6.4 dB (5 to 20 dB) estimation mean-square error advantage in the time-invariant (time-variant) channels) for all preamble patterns of the MB-OFDM system in [8]. [ABSTRACT FROM AUTHOR]

Multiparty voice-over-IP (MVoIP) services allow a group of people to freely communicate with each other via the Internet, which have many important applications such as online gaming and teleconferencing. In this paper, we present a peer-to-peer MVoIP system called peerTalk. Compared to traditional approaches such as server-based mixing, peerTalk achieves better scalability and failure resilience by dynamically distributing the stream processing workload among different peers. Particularly, peerTalk decouples the MVoIP service delivery into two phases: mixing phase and distribution phase. The decoupled model allows us to explore the asymmetric property of MVoIP services (for example, distinct speaking/listening activities and unequal inbound/outbound bandwidths) so that the system can better adapt to distinct stream mixing and distribution requirements. To overcome arbitrary peer departures/ failures, peerTalk provides lightweight backup schemes to achieve fast failure recovery. We have implemented a prototype of the peerTalk system and evaluated its performance using both a large-scale simulation testbed and a real Internet environment. Our initial implementation demonstrates the feasibility of our approach and shows promising results: peerTalk can outperform existing approaches such as P2P overlay multicast and coupled distributed processing for providing MVoIP services. [ABSTRACT FROM AUTHOR]

Media distribution through application-layer overlay networks has received considerable attention recently, owing to its flexibility and readily deployable nature. On-demand streaming with asynchronous requests and, in general, with VCR-like interactions nevertheless remains a challenging task in overlay networks. In this paper, we introduce the Dynamic Skip List (DSL), a novel randomized and distributed structure that inherently accommodates dynamic and asynchronous clients. We establish the theoretical foundations of the DSL and demonstrate a practical DSL-based streaming overlay. In this overlay, the costs for typical operations, including join, leave, fast-forward, rewind, and random seek, are all sublinear to the client population. The model also seamlessly integrates a smart data scheduling algorithm using linear network coding, yielding fast and robust downloading from multiple suppliers. Our simulation results show that the DSL-based overlay is highly scalable. It delivers reasonably smooth playback with diverse client interactivities while keeping the computation and bandwidth overheads low. [ABSTRACT FROM AUTHOR]

A new small-size and wideband fractal antenna in the shape of a snowflake is proposed. Various iterations of this fractal antenna with probe feed and capacitively coupled feed are compared and an optimised design is presented. It is shown that, with an air-filled substrate and capacitive feed, an impedance bandwidth >49% and, with a slot-loading technique, a reduction of about 70% in patch surface size compared with an ordinary wideband Koch fractal antenna are achievable. The simulation via a finite-element programme, and measured results on the return loss and the E and H-plane radiation patterns of the proposed antennas are presented and shown to be in good agreement. [ABSTRACT FROM AUTHOR]

We develop a new wideband digital feedback predistortion (WDFBPD) technique for modulated signals with wide bandwidths by combining digital feedback predistortion (DFBPD) linearization and memory-effect compensation techniques. For the experiments, a class-AB amplifier using an LDMOSFET with 90-W peak envelope power is employed. The proposed technique is compared with existing DFBPD and memory polynomial (MP) techniques for a 2.14-GHz forward-link WCDMA 2FA signal with 10-MHz carrier spacing. The experimental results show that the new WDFBPD technique has better linearization performance than conventional DFBPD and lower computational complexity than the MP technique. [ABSTRACT FROM AUTHOR]

We introduce a single-loop PLL that operates in a narrower-bandwidth, integer-N mode during phase lock and in a wider-bandwidth, fractional-N mode during transient. This hybrid PLL, as a generalization of the conventional variable-band-width PLL that shifts only its bandwidth, simultaneously achieves the fast-locking advantage of the fractional-N PLL and design simplicity of the integer-N PLL, and as such, brings benefits in certain important PLL applications. In addition, the frequency division mode switching, unique in the hybrid PLL, enables a new, more digital protocol to execute bandwidth switching. A CMOS IC prototype attests to the validity of the proposed approach. [ABSTRACT FROM AUTHOR]

Online extraction of individual harmonics from repetitive signals using a composite asymptotic observer is presented in this paper. The settling period and the bandwidth of the observer depend on how far the observer poles have been placed from the origin of the s-plane or the z-plane. The errors in the magnitude and phase of the extracted components, due to the deviation of the signal from the central frequency of the observer, are made very small by providing an integrated phase-locking arrangement. Further improvement in the accuracy, particularly in the extracted higher harmonics, is due to the introduction of multirate sampling. [ABSTRACT FROM AUTHOR]

A modification of the background digital calibration procedure for A/D converters by Li and Moon is proposed, based on a method to improve the speed of convergence and the accuracy of the calibration. The procedure exploits a colored random sequence in the calibration algorithm, and can be applied both for narrowband input signals and for baseband signals, with a slight penalty on the analog bandwidth of the converter. By improving the signal-to-calibration-noise ratio of the statistical estimation of the error parameters, our proposed technique can be employed either to improve linearity or to make the calibration procedure faster. A practical method to generate the random sequence with minimum overhead with respect to a simple PRBS is also presented. Simulations have been performed on a 14-bit pipeline A/D converter in which the first 4 stages have been calibrated, showing a 15 dB improvement in THD and SFDR for the same calibration time with respect to the original technique. [ABSTRACT FROM AUTHOR]

Block digital filtering has been suggested to reduce the computational complexity and to increase the parallelism of computation in digital filtering systems. In this letter, a fast algorithm for optimal design of block digital filters (BDFs) is developed. This algorithm, based on circulant, Toeplitz and shift cyclic matrices, not only reduces the computational complexity of the design process but also decreases the memory requirements. [ABSTRACT FROM AUTHOR]

The sum-rate capacity of a cellular system model is analyzed, considering the uplink and downlink channels, while addressing both nonfading and flat-fading channels. The focus is on a simple Wyner-like multicell model, where the system cells are arranged on a circle, and the cell sites are located at the boundaries of the cells. For the uplink channel, analytical expressions of the sum-rate capacities are derived for intra-cell time-division multiple-access (TDMA) scheduling, and a ‘wideband’ (WB) scheme (where all users are active simultaneously utilizing all bandwidths for coding). Assuming individual equal per-cell power constraints, and using the Lagrangian uplink-downlink duality principle, an analytical expression for the sum-rate capacity of the downlink channel is derived for nonfading channels, and shown to coincide with the corresponding uplink result. Introducing flat-fading, lower and upper bounds on the average per-cell ergodic sum-rate capacity are derived. The bounds exhibit an O(loge K) multiuser diversity factor for a number of users per cell K » 1, in addition to the array diversity gain. Joint multicell processing is shown to eliminate out-of-cell interference, which is traditionally considered to be a limiting factor in high-rate reliable communications. [ABSTRACT FROM AUTHOR]

Spartan spatial random fields (SSRFs) are generalized Gibbs random fields, equipped with a coarse-graining kernel that acts as a low-pass filter for the fluctuations. SSRFs are defined by means of physically motivated spatial interactions and a small set of free parameters (interaction couplings). This paper focuses on the fluctuation-gradient-curvature (FGC) SSRF model, henceforth referred to as FGC-SSRF. This model is defined on the Euclidean space Rd by means of interactions proportional to the squares of the field realizations, as well as their gradient and curvature. The permissibility criteria of FGC-SSRFs are extended by considering the impact of a finite-bandwidth kernel. It is proved that the FGC-SSRFs are almost surely differentiable in the case of finite bandwidth. Asymptotic explicit expressions for the Spartan covariance function are derived for d = 1 and d = 3; both known and new covariance functions are obtained depending on the value of the FGC-SSRF shape parameter. Nonlinear dependence of the covariance integral scale on the FGC-SSRF characteristic length is established, and it is shown that the relation becomes linear asymptotically. The results presented in this paper are useful in random field parameter inference, and in spatial interpolation of irregularly spaced samples. [ABSTRACT FROM AUTHOR]

This paper proposes several new schemes which allow a sender to send encrypted messages to multiple recipients more efficiently (in terms of bandwidth and computation) than by using a standard encryption scheme. Most of the proposed schemes explore a new natural technique called randomness reuse. In order to analyze security of our constructions, we introduce a new notion of multirecipient encryption schemes (MRESs) and provide definitions of security for them. We finally show a way to avoid ad hoc analyses by providing a general test that can be applied to a standard encryption scheme to determine whether the associated randomness reusing MRES is secure. The results and applications cover both asymmetric and symmetric encryption. [ABSTRACT FROM AUTHOR]

A symmetrical four-port microstrip coupler has been proposed for crossover application. Models have also been developed to provide insight into the performance of such a coupler, which is based on the double-ring design. Laboratory tests have confirmed that the measured isolation and return loss of the prototype coupler exceed 20 dB over a bandwidth of 20% (from 0.92 to 1.12 GHz). [ABSTRACT FROM AUTHOR]

In this paper, a new power divider concept, which provides high flexibility of transmission line characteristic impedance and port impedance, is proposed. This power divider is implemented on a parallel-strip line, which is a balanced transmission line. By implementing the advantages and uniqueness of the parallel-strip line, the divider outperforms the conventional divider in terms of isolation bandwidths. A swap structure of the two lines of the parallel-strip line is employed in this design, which is critical for the isolation enhancements. A lumped-circuit model of the parallel-strip swap including all parasitic effects has been analyzed. An equal power divider with center frequency of 2 GHz was designed to demonstrate the idea. The experimental results show that the equal power divider has 96.5% - 19-dB impedance bandwidth with more than 25-dB isolation and less than 0.7-dB insertion loss. In order to generalize the concept with an arbitrary power ratio, we also realize unequal power dividers with the same isolation characteristics. The impedance bandwidth of the proposed power divider will increase with the dividing ratio, which is opposite to the conventional Wilkinson power divider. Unequal dividers with dividing ratios of 1 :2 and 1 : 12 are designed and measured. Additionally, a frequency independent 180° power divider has been realized with less than 2° phase errors. [ABSTRACT FROM AUTHOR]

In orthogonal frequency division multiplexing communication systems, the system performance may be degraded by the effect of inter-carrier interference. In this paper, a new scheme is proposed to cancel the effect of ICI. The system performances are evaluated based on carrier-to-interference ratio and bit error rate. The simulation results show that the effect of phase error with the proposed scheme is lighter than that with Zhao's scheme. Hence, the BER performance of proposed scheme is better than that of Zhao's scheme even in a large frequency offset. [ABSTRACT FROM AUTHOR]

Capacity improvement from transmitter and receiver cooperation is investigated in a two-transmitter0 two-receiver network with phase fading and full channel state information (CSI) available at all terminals. The transmitters cooperate by first exchanging messages over an orthogonal transmitter cooperation channel, then encoding jointly with dirty-paper coding. The receivers cooperate by using Wyner-Ziv compress-and-forward over an analogous orthogonal receiver cooperation channel. To account for the cost of cooperation, the allocation of network power and bandwidth among the data and cooperation channels is studied. It is shown that transmitter cooperation outperforms receiver cooperation and improves capacity over noncooperative transmission under most opt, rating conditions when the cooperation channel is strong. However, a weak cooperation channel limits the transmitter cooperation rate; in this case, receiver cooperation is more advantageous. Transmitter-and-receiver cooperation offers sizable additional capacity gain over transmitter-only cooperation at low signal-to-noise ratio (SNR), whereas at high SNR transmitter cooperation alone captures most of the cooperative capacity improvement. [ABSTRACT FROM AUTHOR]

We propose a new class of methods for VLIW code compression using variable-sized branch blocks with self-generating tables. Code compression traditionally works on fixed-sized blocks with its efficiency limited by their small size. A branch block, a series of instructions between two consecutive possible branch targets, provides larger blocks for code compression. We compare three methods for compressing branch blocks: table-based, Lempel-Ziv-Welch (LZW)-based and selective code compression. Our approaches are fully adaptive and generate the coding table on-the-fly during compression and decompression. When encountering a branch target, the coding table is cleared to ensure correctness. Decompression requires a simple table lookup and updates the coding table when necessary. When decoding sequentially, the table-based method produces 4 bytes per iteration while the LZW-based methods provide 8 bytes peak and 1.82 bytes average decompression bandwidth. Compared to Huffman's 1 byte and variable-to-fixed (V2F)'s 13-bit peak performance, our methods have higher decoding bandwidth and a comparable compression ratio. Parallel decompression could also be applied to our methods, which is more suitable for VLIW architectures. [ABSTRACT FROM AUTHOR]

In this paper, ‘Martini,’ a network interface controller chip for our original network called RHiNET, is described. Martini is designed to provide high-bandwidth and low-latency communication with a small overhead. To obtain high-performance communication, protected user-level zero-copy remote direct memory access (RDMA) communication functions are completely implemented by a hardwired logic. Also, to reduce the communication latency efficiently, we have proposed programmed I/O (PIO)-based communication mechanisms called ‘On the fly (OTF)‘ and have implemented them on Martini. The evaluation results show that Martini connected to a 64-bit/66-MHz peripheral component interconnect (PCI) bus achieves a 470-Mbyte maximum bidirectional bandwidth and a 1.74-μsec minimum latency on host-to-host memory copying. [ABSTRACT FROM AUTHOR]

The design of a dual and triple-band planar inverted-F antenna (PIFA) is proposed for WLAN 802.11 a/b/g applications. Dual-band operation is achieved by simply truncating one of the top rectangular radiating plate's corners, and wide bandwidth for the upper-band is obtained by applying an additional small rectangular slit. By further extending the slit length, a triple-band operation can also be achieved with a maximum gain of around 5 dBi. Simulated results are provided to validate the measured results. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2159–2162, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22682 [ABSTRACT FROM AUTHOR]

This paper considers semiblind channel estimation and data detection for orthogonal frequency-division multiplexing (OFDM) over frequency-selective fading channels. We show that the samples of an OFDM symbol are jointly complex Gaussian distributed, where the mean and covariance are determined by the locations and values of fixed pilot symbols. We exploit this distribution to derive a novel maximum-likelihood (ML) semiblind gradient-descent channel estimator. By exploiting the channel impulse response (CIR) statistics, we also derive a semiblind data detector for both Rayleigh and Ricean fading channels. Furthermore, we develop an enhanced data detector, which uses the estimator error statistics to mitigate the effect of channel estimation errors. Efficient implementation of both the semiblind and the improved data detectors is provided via sphere decoding and nulling-canceling detection. We also derive the Cramér-Rao bound (CRB) and design optimal pilots by minimizing the CRB. Our proposed channel estimator and data detector exhibit high bandwidth efficiency (requiring only a few pilot symbols), achieve the CRB, and also nearly reach the performance of an ideal reference receiver. [ABSTRACT FROM AUTHOR]

Abstract: The problem of estimating the direction of arrival (DOA) of wideband sources has always been of interest to the researchers because of its importance in many practical scenarios. The rising demand of communicating at higher data rates can be met by using wideband signals [R.U. Nabar, A. Paulraj, D.A. Gore, H. Bolcskei, An overview of MIMO communications—a key to gigabyte wireless, Proc. IEEE 92 (February 2004) 198–218]. Multipaths, scatterers etc., present near the transmitter render spatial bandwidth to these wideband signals. Traditional DOA estimation techniques, designed assuming point sources, result in serious performance degradation when applied to these wideband sources. This paper talks about the DOA estimation of spatially spread wideband sources i.e. sources having both spatial and temporal bandwidth. The interesting aspect of the approach presented is the usage of “spatial only” observations to estimate the DOA of both wideband point sources and wideband spread sources. Numerical simulation results verify the validity of the proposed method. [Copyright &y& Elsevier]

This paper describes a method for analysis and design of MOS voltage-to-current converters (V–I converters or transconductors) and introduces a novel V–I converter circuit with significantly improved linearity performance. The proposed method uses harmonic compensation for the linearization of the V–I characteristics and introduces a normalized representation of the converter equations. The analysis is applied for several circuit topologies based on MOS differential pairs. The circuits are compared with respect to their current consumption, signal to noise ratio, achievable linearity and bandwidth. The minimum required current consumption for certain linearity and dynamic range is derived. The proposed novel V–I converter circuit uses a combination of local resistive feedback and cross-coupling. In this way, it achieves significant, simultaneous suppression of the third and the fifth order harmonic components in the transconductor output current. The implementation constraints and the performance of the new circuit solution are evaluated via simulations on transistor level. A standard digital 0. 18 micrometer, 1. 8V, CMOS process is used. [Copyright &y& Elsevier]

In this paper we address the emerging application of mobility to DVB-S2/DVB-RCS systems, sometimes also referred to as Satellite-On-The Move (SOTM) communications. Satellite communications have great commercial success in digital audio and video broadcasting as they exploit the inherent wide area coverage for distribution content. It was a natural consequence to extend its use to multimedia applications; DVB-S2/DVB-RCS standards offer a commercially proven solution for fixed terminals in that sense. In addition, communication on the move is becoming a key market sector. Therefore broadband mobile communications represent an additional market for the satellite, addressing primarily transport operators (planes, ships, trains) but also end-users in cars. To ensure economies of scale and fast development and deployment of equipments, the adaptation of current satellite broadcast and interactive broadband standards is foreseen. This paper presents possible techniques applicable to different market segments for both the forward and the return links of the system, with associated performance and impact on the standards. [ABSTRACT FROM AUTHOR]

In this paper, the MoBISAT (Mobile Broadband Interactive Satellite Access Technology) system, the BcSAT (Broadband adaptive satellite communicationS and broadcAsting convergence Technology) system and the STC (Satellite and Terrestrial Convergence) system for interactive service on high speed train environment mainly deal with the information identified as system introduction, detail design description and implementation issues, respectively. First, MoBISAT system which represents a mobile broadband satellite access technology, provides satellite TV broadcasting service in Ku-band and high speed Internet service in Ka band based on DVB-S/DVB-RCS standard to the passengers and crews for land mobile, maritime and aeronautical environments (ETSI EN 301 790). BcSAT system aims at realizing the next generation satellite broadcasting service in Ka-band through DVB-S2 standard (ETSI EN 302 307). It can be implemented through high bandwidth-efficient and flexible transmission mode with provision of high order modulation scheme and robust channel coding technique. Furthermore, we're pursuing the development of prospective convergence system composed with satellite and terrestrial networks for realization of ubiquitous service, briefly. In a result, information of this paper will be useful to figure out the main design features and realistic consideration (IEEE Wireless Commun. 12(5) (2005), 91–97). [ABSTRACT FROM AUTHOR]

For a power limited orthogonal frequency-division multiplexing (OFDM) system transmitting a large number N of subcarriers over a Rayleigh-fading channel, the distribution of the instantaneous capacity is shown to be approximately Gaussian. The mean and variance of the approximating distribution are derived. It is also shown that, in the limit as N → ∞, the capacity approaches a constant value equal to the capacity of the infinite-bandwidth Gaussian channel. [ABSTRACT FROM AUTHOR]