Showing papers in "IEEE Communications Letters in 2005"
TL;DR: A medium access control protocol is proposed that exploits both the channel state information and the residual energy information of individual sensors and maximizes the minimum residual energy across the network in each data collection.
Abstract: We derive a general formula for the lifetime-of wireless sensor networks which holds independently of the underlying network model including network architecture and protocol, data collection initiation, lifetime definition, channel fading characteristics, and energy consumption model. This formula identifies two key parameters at the physical layer that affect the network lifetime: the channel state and the residual energy of sensors. As a result, it provides not only a gauge for performance evaluation of sensor networks but also a guideline for the design of network protocols. Based on this formula, we propose a medium access control protocol that exploits both the channel state information and the residual energy information of individual sensors. Referred to as the max-min approach, this protocol maximizes the minimum residual energy across the network in each data collection.
713 citations
TL;DR: Simulation results show that the proposed scheme enhances the system capacity, providing almost near optimal solutions with low computational burden.
Abstract: In this letter, we focus on joint subcarrier and power allocation in the uplink of an OFDMA system Our goal is to maximize the rate-sum capacity in the uplink For the purpose, we formulate an optimization problem subject to subcarrier and power constraints and draw necessary conditions for optimality, from which we derive joint subcarrier and power allocation algorithms Simulation results show that our proposed scheme enhances the system capacity, providing almost near optimal solutions with low computational burden
372 citations
TL;DR: A simple system is presented, which proves its superiority over other schemes for multicarrier transmission, e.g. extended round robin and the PF scheduling scheme for the HDR system.
Abstract: This letter extends the proportional fair (PF) scheduling proposed in the high data rate (HDR) system to multicarrier transmission systems. It is known that the PF allocation (F. P. Kelly et al. (1998)) results in the maximization of the sum of logarithmic average user rates. We propose a PF scheduling that assigns users to each carrier while maximizing the sum of logarithmic average user rates.
366 citations
TL;DR: This letter presents a new approach to evaluate the throughput/delay performance of the 802.11 distributed coordination function (DCF) that relies on elementary conditional probability arguments rather than bidimensional Markov chains and can be easily extended to account for backoff operation more general than DCF's one.
Abstract: This letter presents a new approach to evaluate the throughput/delay performance of the 802.11 distributed coordination function (DCF). Our approach relies on elementary conditional probability arguments rather than bidimensional Markov chains (as proposed in previous models) and can be easily extended to account for backoff operation more general than DCF's one.
296 citations
TL;DR: Energy consumption for the IEEE 802.16e broadband wireless access (BWA) network is studied and the sleep-mode scheme is analytically model and validated with simulations.
Abstract: In this paper, we study energy consumption for the IEEE 802.16e broadband wireless access (BWA) network, A mobile subscriber station goes to sleep-mode after negations with the base station and temporarily wake-up periodically for a short interval to check whether there is downlink traffic to it to decide whether it goes to wake-mode or continues to be in the sleep-mode. The sleep interval is increased exponentially upon no arrival traffic. We analytically model the sleep-mode scheme and validate the model with simulations.
292 citations
TL;DR: This paper presents an extension of Bianchi's model to a non-saturated environment and its predictions are validated against simulation and are found to accurately capture many interesting features of non-Saturated operation.
Abstract: Analysis of the 802.11 CSMA/CA mechanism has received considerable attention recently. Bianchi (2000) presents an analytic model under a saturated traffic assumption. Bianchi's model is accurate, but typical network conditions are non-saturated. This paper presents an extension of Bianchi's model to a non-saturated environment. Its predictions are validated against simulation and are found to accurately capture many interesting features of non-saturated operation.
267 citations
TL;DR: Power allocation strategies are developed for amplify-and-forward cooperative relaying networks in fading channels by maximizing the sum and product of the average signal-to-noise ratio and an upper bound on the SNR of the relay link.
Abstract: Power allocation strategies are developed for amplify-and-forward cooperative relaying networks in fading channels. The average signal-to-noise ratio (SNR) and outage performances are optimized in some sense by maximizing the sum and product, respectively, of the average SNR of the direct link and an upper bound on the SNR of the relay link. The power allocation strategies require knowledge of only the mean strengths of the channels.
250 citations
TL;DR: A block minimum mean-squared error (MMSE) equalizer for orthogonal frequency-division multiplexing (OFDM) systems over time-varying multipath channels is presented and turns out to be smaller than a serial MMSE equalizer characterized by a similar performance.
Abstract: We present a block minimum mean-squared error (MMSE) equalizer for orthogonal frequency-division multiplexing (OFDM) systems over time-varying multipath channels. The equalization algorithm exploits the band structure of the frequency-domain channel matrix by means of a band LDL/sup H/ factorization. The complexity of the proposed algorithm is linear in the number of subcarriers and turns out to be smaller with respect to a serial MMSE equalizer characterized by a similar performance.
249 citations
TL;DR: A new strategy for cooperative transmit diversity based on superposition modulation and multiuser detection is proposed that can outperform "classical" cooperative Diversity based on decode-and-forward by about 1.5-2 dB in the SNR range of interest and at the same computational complexity.
Abstract: We propose a new strategy for cooperative transmit diversity based on superposition modulation and multiuser detection. The new scheme can outperform "classical" cooperative diversity based on decode-and-forward by about 1.5-2 dB in the SNR range of interest and at the same computational complexity.
242 citations
TL;DR: An outage probability analysis for a decode-and-forward system, valid at all signal to noise ratios (SNR), and a closed form solution is obtained for independent and identically distributed (i.i.d.) channels.
Abstract: Cooperative diversity improves the performance of wireless networks by having several nodes transmit the same information. We present an outage probability analysis for a decode-and-forward system, valid at all signal to noise ratios (SNR). A closed form solution is obtained for independent and identically distributed (i.i.d.) channels, and two tight lower bounds are presented for correlated channels.
226 citations
TL;DR: A robust and scalable algorithm that can find the optimal sensor placement under the minimum cost limitation for small sensor fields and can also find a placement with minimum distance error for large sensor fields under the cost limitation is developed.
Abstract: In this letter, we develop a robust and scalable algorithm to cope with the sensor placement problem for target location under constraints of the cost limitation and the complete coverage. The problem is NP-complete for arbitrary sensor fields. The grid-based placement scenario is adopted and the sensor placement problem formulated as a combinatorial optimization problem for minimizing the maximum distance error in a sensor field under the constraints. The proposed algorithm is based on the simulated annealing approach. The experimental results reveal that, for small sensor fields, the algorithm can find the optimal sensor placement under the minimum cost limitation. Moreover, it can also find a placement with minimum distance error for large sensor fields under the cost limitation.
TL;DR: This paper proposes a random access based feedback protocol for achieving multiuser diversity with a fixed number of feedback slots that is jointly designed to maximize the average sum rate of all users.
Abstract: This paper proposes a random access based feedback protocol for achieving multiuser diversity with a fixed number of feedback slots. Active users send a feedback message with some probability if their channel power is above a threshold. The threshold and random access probability are jointly designed to maximize the average sum rate of all users.
TL;DR: An algorithm that performs joint subcarrier and power allocation in multiuser OFDM is proposed and simulation results are shown to compare the performance of the proposed algorithm with that of existing algorithms.
Abstract: This paper investigates subcarrier and power allocation in multiuser OFDM. The aim is to maximize the overall rate while achieving proportional fairness amongst users under a total power constraint. Achieving the optimal solution is computationally demanding thereby necessitating the use of sub-optimal techniques. Existing sub-optimal techniques either use fixed power allocation and perform only subcarrier allocation or handle subcarrier and power allocation separately. In this paper, we propose an algorithm that performs joint subcarrier and power allocation. Simulation results are shown to compare the performance of the proposed algorithm with that of existing algorithms.
TL;DR: In this letter, formulas for the calculation of the detection and false alarm probability are derived for the general case of an arbitrary measurement covariance matrix, allowing for a maximum exploitation of the proposed distributed detection approach.
Abstract: The innovative new strategy of spectrum pooling enables public access to spectral ranges of already licensed yet rarely used frequency bands by overlaying a secondary mobile radio system (the rental system, RS) to an existing one (the licensed system, LS). Coexistence of both systems is realized by filling the idle time-frequency gaps of the LS. A key issue in spectrum pooling is the reliable and efficient detection of those spectral ranges that are currently accessed by the LS as those ranges have to be spared from the RS's transmission power. In this letter, formulas for the calculation of the detection and false alarm probability are derived for the general case of an arbitrary measurement covariance matrix, allowing for a maximum exploitation of the proposed distributed detection approach.
TL;DR: This letter addresses the problem of designing jointly the linear transmitter and receiver for the downlink of Multiuser MIMO system, using minimum total mean square error criterion (T-MMSE), subject to a total transmit power constraint.
Abstract: In this letter, we address the problem of designing jointly the linear transmitter and receiver for the downlink of Multiuser MIMO system, using minimum total mean square error criterion (T-MMSE), subject to a total transmit power constraint. We show that transmitter and receiver under such criterion could be realized through a joint iterative algorithm. The convergence of the proposed algorithm is proved. Simulation results have also been provided to demonstrate the feasibility of this new method.
TL;DR: This letter presents an accurate model for the performance analysis of the IEEE 802.11 saturation throughput with freezing of the backoff counter that corrects the existing model presented by Ziouva and Antonakopoulos and shows the accuracy of the new model.
Abstract: This letter presents an accurate model for the performance analysis of the IEEE 802.11 saturation throughput with freezing of the backoff counter. The model corrects the existing model presented by Ziouva and Antonakopoulos which assumes that the channel access probability and station collision probability are independent of channel status. Simulation results show the accuracy of the new model.
TL;DR: The modified weighted bit flipping decoding algorithm for low density parity-check (LDPC) codes is improved by introducing a more efficient method for computing the reliability of the parity checks and an approximate theoretical expression for the weighting factor is derived.
Abstract: We present a major improvement to the modified weighted bit flipping (MWBF) decoding algorithm for low density parity-check (LDPC) codes. We improve the flipping criterion by introducing a more efficient method for computing the reliability of the parity checks. We derive an approximate theoretical expression for the weighting factor and demonstrate its consistency with simulation results. The improved MWBF algorithm achieves a noticeable gain over the MWBF algorithm with only a modest increase in computational complexity.
TL;DR: A generalized sphere decoder for rank-deficient multiple input multiple output (MIMO) systems using N transmit antennas and M receive antennas is derived and a method to handle non-constant modulus constellations is developed.
Abstract: We derive a generalized sphere decoder (GSD) for rank-deficient multiple input multiple output (MIMO) systems using N transmit antennas and M receive antennas. This problem arises when N>M or when the channel gains are strongly correlated. The upper triangular factorization of the Grammian yields an under-determined system and the standard sphere decoding (SD) fails. For constant modulus constellations, we modify the maximum likelihood (ML) cost metric so that the equivalent Grammian is rank N. The resulting GSD algorithm has significantly lower complexity than previous algorithms. A method to handle nonconstant modulus constellations is also developed.
TL;DR: A simple solution is developed that uses the 801.11e AIFS, TXOP and CW/sub min/ parameters to ensure fairness between competing TCP uploads and downloads to address transport layer unfairness in WLANs.
Abstract: We investigate the use of the 8.02.11e MAC EDCF to address transport layer unfairness in WLANs. A simple solution is developed that uses the 802.11e AIFS, TXOP and CW/sub min/ parameters to ensure fairness between competing TCP uploads and downloads.
TL;DR: In this letter, the system throughput and the access delay of the proposed algorithm are analyzed, and it is shown that the throughput of the algorithm is superior to that of conventional algorithms in IEEE 802.16d/e system.
Abstract: An efficient uplink scheduling algorithm that can be used in IEEE 802.16d/e system for VoIP services is proposed to support as many voice users as possible. Our proposed algorithm requires no modification of the framework of IEEE 802.16d/e system. In this letter, we analyze the system throughput and the access delay of our proposed algorithm, and show, via OPNET simulation, that the throughput of our algorithm is superior to that of conventional algorithms in IEEE 802.16d/e system.
TL;DR: The proposed normalized threshold selection technique exploits the kurtosis of the received signal samples and yields efficient threshold selection, has low complexity and sampling rate requirements, and accounts both the signal to noise ratio and the statistics of individual channel realizations.
Abstract: In this letter, a normalized threshold selection technique for time of arrival estimation of ultra-wideband signals is proposed. It exploits the kurtosis of the received signal samples. The dependency between the kurtosis and optimal normalized threshold are investigated via simulations. The proposed technique yields efficient threshold selection, has low complexity and sampling rate requirements, and accounts both the signal to noise ratio and the statistics of individual channel realizations.
TL;DR: Highly accurate, simple closed-form expressions for the sum distributions and densities are presented and are valid for a wide range of argument values and number of summands.
Abstract: Sums of Rayleigh random variables occur extensively in wireless communications. A closed-form expression does not exist for the sum distribution and consequently, it is often evaluated numerically or approximated. A widely used small argument approximation for the density is shown to be inaccurate for medium and large values of the argument. Highly accurate, simple closed-form expressions for the sum distributions and densities are presented. These approximations are valid for a wide range of argument values and number of summands.
TL;DR: It is shown that the maximum a posteriori (MAP) symbol detection strategy, usually implemented by using the Forney observation model, can be equivalently implemented based on the samples at the output of a filter matched to the received pulse, i.e.,based on the Ungerboeck observation model.
Abstract: In this letter, the well-known problem of a transmission over an additive white Gaussian noise channel affected by known intersymbol interference is considered. We show that the maximum a posteriori (MAP) symbol detection strategy, usually implemented by using the Forney observation model, can be equivalently implemented based on the samples at the output of a filter matched to the received pulse, i.e., based on the Ungerboeck observation model. Although interesting from a conceptual viewpoint, the derived algorithm has a practical relevance in turbo equalization schemes for partial response signalling, where the implementation of a whitening filter can be avoided.
TL;DR: This letter introduces an efficient algorithm for computing the optimal PTS weights that has lower complexity than exhaustive search.
Abstract: List-based algorithms for. decoding block turbo Codes (BTC) have gained popularity due to their low computational complexity. The normal way to calculate the soft outputs involves searching for a decision code word D and a competing codeword B. In addition, a scaling factor /spl alpha/ and an estimated reliability value /spl beta/ are used. In this letter, we present a new approach that does not require /spl alpha/ and /spl beta/. Soft outputs are generated based on the Euclidean distance property of decision code words. By using the new algorithm, we achieve better error performance with even less complexity-for certain BTCs.
TL;DR: Simulation results show that low rate TCP denial-of-service attacks can be detected by a novel approach, which does not require any modification to TCP congestion control algorithms like randomizing the fixed minimum RTO.
Abstract: Low rate TCP denial-of-service attacks are a new type of DoS attacks that are carefully orchestrated to exploit the fixed minimum TCP RTO property and thereby deny services to legitimate users. This type of attacks is different from traditional flood-based attacks and hence conventional solutions to detect these attacks are not applicable. We propose a novel approach to detect these attack flows at edge routers. A flow exhibiting a periodic pattern is marked malicious if its burst length is greater than or equal to RTTs of other connections with the same server and its time period is equal to the fixed minimum RTO. A carefully designed light weight data structure is proposed to store the necessary flow history at edge routers. Simulation results show that such flows can be detected by our proposed approach, which does not require any modification to TCP congestion control algorithms like randomizing the fixed minimum RTO.
TL;DR: It is shown that if, among other factors, the power efficiency-PAPR relationship of the power amplifier is also taken into account, then there exists a PAPR level for which the BER reaches a minimal value.
Abstract: A high PAPR is a main drawback of OFDM. There are many methods for reducing the PAPR with an ultimate goal of reducing the PAPR as much as possible. It is shown that if, among other factors, the power efficiency-PAPR relationship of the power amplifier is also taken into account, then there exists a PAPR level for which the BER reaches a minimal value. This PAPR may be used as a definition of an efficient PAPR. The efficient PAPR is not necessary the lowest possible value of PAPR.
TL;DR: The cumulative distribution function, the probability density function and the moment generating function of the MRT output signal-to-noise ratio (SNR) with imperfect CSI are derived, enabling the evaluation of some useful performance metrics such as the average error rate and the outage performance.
Abstract: Maximal ratio transmission (MRT) is designed assuming the availability of perfect channel state information (CSI) at both the transmitter and the receiver. However, perfect CSI is not available in practice. This paper investigates the impact of Gaussian estimation errors on the MRT performance in independently and identically distributed (i.i.d.) Rayleigh fading channels. We derive the cumulative distribution function (cdf), the probability density function (pdf) and the moment generating function (mgf) of the MRT output signal-to-noise ratio (SNR) with imperfect CSI, enabling the evaluation of some useful performance metrics such as the average error rate and the outage performance. Numerical and simulation results are provided to show the impact of imperfect CSI on the MRT performance.
TL;DR: A computationally efficient recursive least squares (RLS) type algorithm for jointly estimating the parameters of the channel and the receiver is developed in order to suppress multiaccess (MAI) and inter-symbol interference (ISI).
Abstract: A code-constrained constant modulus (CCM) design criterion for linear receivers is investigated for direct sequence code division multiple access (DS-CDMA) in multipath channels based on constrained optimization techniques. A computationally efficient recursive least squares (RLS) type algorithm for jointly estimating the parameters of the channel and the receiver is developed in order to suppress multiaccess (MAI) and inter-symbol interference (ISI). An analysis of the method examines its convergence properties and simulations under nonstationary environments show that the novel algorithms outperform existent techniques.
TL;DR: It is demonstrated that the use of TR in wireless communication has a promising potential in mitigating the effect of channel dispersion and especially in reducing the cochannel interference where a margin of 18 dB interference reduction has been obtained.
Abstract: Based on outdoor measurements we study the feasibility of applying the time reversal techniques (TR) in multiple element antenna (MEA) wireless communication systems. It is demonstrated that the use of TR in wireless communication has a promising potential in mitigating the effect of channel dispersion and especially in reducing the cochannel interference where a margin of 18 dB interference reduction has been obtained.
TL;DR: Novel, closed-form expressions for the average Shannon capacity of single-branch receivers, operating over generalized fading channels (Nakagami-m, Rice and Weibull), are derived and the optimum switching threshold for maximizing the data transmission rate of switched and stay combining receivers is obtained.
Abstract: Novel, closed-form expressions for the average Shannon capacity of single-branch receivers, operating over generalized fading channels (Nakagami-m, Rice and Weibull), are derived. As an application, the optimum switching threshold for maximizing the data transmission rate of switched and stay combining receivers is obtained and several numerical results are presented.