Articles

Compact UWB Antenna With I-Shaped Band-Notch Parasitic Element for Laptop Applications

Nakchung Choi et al. — Thu, 06 Aug 2009 08:28:21 PDT

A compact ultrawideband (UWB) antenna with a band-notch function is proposed for laptop applications. The band-notch function is realized by a half-wavelength parasitic element printed on the rear side of the substrate. The impedance bandwidth (VSWR < 2) of the antenna is 3.1 ~ 11.4 GHz (114%) with a notched frequency from 5.05 to 5.90 GHz. The antenna has a fairly good omnidirectional pattern, and the average gain is -3.0 ~ -1.2 dBi over the UWB frequency band except for the notched frequency band. The performance of the proposed antenna is confirmed by simulation and measurement results.

Enabling Location-Based Services in Data Centers

Krishna Kant et al. — Fri, 05 Jun 2009 07:11:50 PDT

In this article, we explore services and capabilities that can be enabled by the localization of various assets in a data center or IT environment. We also describe the underlying location estimation method and the protocol to enable localization. Finally, we present a management framework for these services and present a few case studies to assess benefits of location-based services in data centers.

Impact of Channel Errors on Decentralized Detection Performance of Wireless Sensor Networks: A Study of Binary Modulations, Rayleigh-Fading and Nonfading Channels, and Fusion-Combiners

Venkateshwara R. Kanchumarthy et al. — Fri, 05 Jun 2009 07:03:54 PDT

We provide new results on the performance of wireless sensor networks in which a number of identical sensor nodes transmit their binary decisions, regarding a binary hypothesis, to a fusion center (FC) by means of a modulation scheme. Each link between a sensor and the fusion center is modeled independent and identically distributed (i.i.d.) either as slow Rayleigh-fading or as nonfading. The FC employs a counting rule (CR) or another combining scheme to make a final decision. Main results obtained are the following: 1) in slow fading, a) the correctness of using an average bit error rate of a link, averaged with respect to the fading distribution, for assessing the performance of a CR and b) with proper choice of threshold, ON/OFF keying (OOK), in addition to energy saving, exhibits asymptotic (large number of sensors) performance comparable to that of FSK; and 2) for a large number of sensors, a) for slow fading and a counting rule, given a minimum sensor-to-fusion link SNR, we determine a minimum sensor decision quality, in order to achieve zero asymptotic errors and b) for Rayleigh-fading and nonfading channels and PSK (FSK) modulation, using a large deviation theory, we derive asymptotic error exponents of counting rule, maximal ratio (square law), and equal gain combiners.

Throughput Performance of an Adaptive ARQ Scheme in Rayleigh Fading Channels

A. Mehta et al. — Fri, 05 Jun 2009 07:01:13 PDT

Using a simulation study we analyze the throughput performance of Yao’s adaptive ARQ scheme in time-varying channels. The simulation takes into account the Rayleigh amplitude and the fast or the slow fading characteristics of a wireless channel, under a representative M-FSK modulation and Reed-Solomon coding scheme. We show that, for a specific set of design parameters, Yao’s adaptive procedure works well for all channel fading rates, except for moderately slow rates. By observing variations of packet error rates at a specified SNR we provide an explanation for these varied behaviors under different channel fading rates.

On the Autocorrelation of Complex Envelope of White Noise

R. Viswanathan — Fri, 05 Jun 2009 07:01:12 PDT

About four decades ago, in an article in this transactions, Thomas Kailath pointed out that the autocorrelation of the complex envelope of white noise is not strictly an impulse function, even though when treated as an impulse in practical problems, it does lead to correct results. However, it is commonly assumed that by simply letting the bandwidth of a flat-bandlimited noise process to go to infinity, one obtains the result that the autocorrelation of the complex envelope of white noise equals an impulse function. In this correspondence, we show that 1) the limit operation has to be done carefully and 2) when done properly, it leads to the result in the Kailath’s paper, which is different from a pure impulse function.

Performance Analysis of Optimum Receivers for Differentially Encoded M-PSK in Low SNR

Venkateshwara R. Kanchumarthy et al. — Fri, 05 Jun 2009 06:57:32 PDT

A recent paper has proved that the classical receiver for coherent detection of differentially encoded M-PSK in AWGN is optimum for the MAP sequence detection criterion. In this letter, we show that asymptotically, as SNR tends to zero, the MAP symbol detection criterion receiver is equivalent to the classical differentially coherent receiver, for M greater than two. An asymptotic relative efficiency figure of merit is defined in order to compare the performances of the classical coherent receiver and the classical differentially coherent receiver.

Two Rank Order Tests for M-ary Detection

Viswanath Annampedu et al. — Fri, 05 Jun 2009 06:54:57 PDT

We consider a general M-ary detection problem where, given M groups of L samples each, the problem is to identify which unique group of L samples have come from the signal hypothesis. The optimal likelihood ratio test is unrealizable, when the joint distribution of ML samples is not completely known. In this paper we consider two rank order types of tests termed as the modified rank test (MRT) and the modified rank test with row sort (MRTRS). We examine through simulation, the small sample probability of error performances of MRT and MRTRS for detecting a signal among contaminants. Numerically computable closed –form error expressions are derived for some special cases. Asymptotic (large sample) error rate of MRT is also derived. The results indicate that MRTRS provides improved performance over other previously known rank tests.

Performance of Distributed CFAR Test Under Various Clutter Amplitudes

Chandrakanth H. Gowda et al. — Fri, 05 Jun 2009 06:52:24 PDT

We evaluate the performances of several distributed constant false-alarm rate (CFAR) tests operating in different background clutter conditions. The analysis considers the detection of Rayleigh target in various clutters with the possibility of differing clutter power levels in the test cells of distributed radars. Numerical results studied for a two-radar system show how the false-alarm rate of the maximum order statistic (MOS) test changes with differences in the clutter power levels of the test cells. The analysis for the detection of Rayleigh target in Rayleigh clutter indicates that, with the power levels of differing test cells, the OR fusion rule can be quite competitive with the new normalized test statistic (NTS). However, for the detection of Rayleigh target in Weibull or K-distributed clutter, the results show that NTS outperforms both the OR and the AND rules under the condition of large signal-to-clutter power ratio and moderate shape parameter values.

Diversity Combining in Antenna Array Base Station Receiver for DS/CDMA System

C. H. Gowda et al. — Fri, 05 Jun 2009 06:50:44 PDT

We evaluate using simulation studies the performance of several schemes for combining base station antenna array signals in wireless direct sequence code-division multiple access (DS/CDMA). The results indicate that under certain assumptions, on multiple access interference statistics, the probability of error of modified rank test (MRT) is lower than that of equal gain combining (EGC), if a few high power interfering users are present along with a low power user of interest. If there are a moderately large number of users, and if the received power of all the users are nearly the same, then EGC out performs MRT. In fact, under this condition, the performance of EGC is close to that of the optimal likelihood ratio test (LRT).

A New Distributed Constant False Alarm Rate Detector

Hamid Amirmehrabi et al. — Fri, 05 Jun 2009 06:47:10 PDT

A new constant false alarm rate (CFAR) test termed signal-plus-order statistic CFAR (S+OS) using distributed sensors is developed. The sensor modeling assumes that the returns of the test cells of different sensors are all independent and identically distributed. In the S+OS scheme, each sensor transmits its test sample and a designated order statistic of its surrounding observations to the fusion center. At the fusion center, the sum of the samples of the test cells is compared with a constant multiplied by a function of the order statistics. For a two-sensor network, the functions considered are the minimum of the order statistics (mOS) and the maximum of the order statistics (MOS). For detecting a Rayleigh fluctuating target in Gaussian noise, closed-form expressions for the false alarm and detection probabilities are obtained. The numerical results indicate that the performance of the MOS detector is very close to that of a centralized OS-CFAR, and it performs considerably better than the OS-CFAR detector with the AND or the OR fusion rule. Extension to an N-sensor network is also considered, and general equations for the false alarm probabilities under homogeneous and nonhomogeneous background noise are presented.

Distributed Detection With Multiple Sensors: Part I—Fundamentals

Ramanarayanan Viswanathan et al. — Fri, 05 Jun 2009 06:45:36 PDT

In this paper, basic results on distributed detection are reviewed. In particular, we consider the parallel and the serial architectures in some detail and discuss the decision rules obtained from their optimization based on the Neyman–Pearson (NP) criterion and the Bayes formulation. For conditionally independent sensor observations, the optimality of the likelihood ratio test (LRT) at the sensors is established. General comments on several important issues are made including the computational complexity of obtaining the optimal solutions, the design of detection networks with more general topologies, and applications to different areas.

On SNR as a Measure of Performance for Narrowband Interference Rejection in Direct Sequence Spread Spectrum Systems

Arif Ansari et al. — Fri, 05 Jun 2009 06:44:07 PDT

The usefulness of SNR as a figure of merit to quantify the narrowband interference rejection capability of a DS receiver is examined. The interference considered is a peaked autoregressive Gaussian process. The probability of error and SNR estimates of a Kalman, a modified Kalman, and a nonlinear filter proposed in [2] are obtained by simulation. Based on this simulation study and the available theoretical error rate analysis of transversal filters, we can conclude that SNR is a useful measure if the processing gain, PG, of the DS system is moderately large. When the PG is small, such as 7, and if thermal noise is negligible compared to the signal, the SNR is not a reliable measure of performance.

Robustness of Decentralized Tests with ε-Contamination Prior

Chandrakanth H. Gowda et al. — Fri, 05 Jun 2009 06:42:39 PDT

We consider a decentralized detection problem where the prior density is not completely known, but is assumed to belong to an ε-contamination class. The expressions for the infimum and the supremum of the posterior probability that the parameter under question is in a given region, as the prior varies over the ε-contamination class, are derived. Numerical results are obtained for a specific case of an exponentially distributed observation and an exponentially distributed nominal prior. Asymptotic (as number of sensors tends to a large value) results are also obtained. The results illustrate the degree of robustness achieved with quantized observations as compared to unquantized observations.

Performance Study of Maximum Likelihood Receivers and Transversal Filters for the Detection of Direct Sequence Spread Spectrum Signal in Narrowband Interference

Arif Ansari et al. — Fri, 05 Jun 2009 06:40:48 PDT

Linear least squares estimation techniques can be used to enhance suppression of narrowband interference in direct sequence spread spectrum systems. Nonlinear techniques for this purpose have also been investigated recently. In this paper, we derive maximum likelihood receivers for direct sequence signal in Gaussian interference with known second order characteristics. It is shown that if the receiver uses samples from outside the bit interval, then the receiver structure is nonlinear. The bit error rate performances of these receivers are compared to those of linear receivers employing one-sided and two-sided least squares estimation filters, for the case of Gaussian autoregressive interference. The results in this paper show that intersymbol interference due to filter taps extending beyond the bit interval cannot be ignored for small processing gains. In some cases, not accounting for intersymbol interference yields too optimistic error estimates, very much away from the true error rates.

A Note on Distributed Estimation and Sufficiency

R. Viswanathan — Fri, 05 Jun 2009 06:38:35 PDT

In relation to distributed parameter estimation, the notion of local and global sufficient statistics is introduced. It is shown that when a sufficiency condition is satisfied by the probability distribution of a random sample, a global sufficient statistic is obtainable as a function of local sufficient statistics. Several standard distributions satisfy the said sufficiency condition.

Application of Expectation-Maximization Algorithm to the Detection of a Direct-Sequence Signal in Pulsed Noise Jamming

Arif Ansari et al. — Fri, 05 Jun 2009 06:37:54 PDT

We consider the detection of a direct-sequence spread-spectrum signal received in a pulsed noise jamming environment. The expectation-maximization algorithm is used to estimate the unknown jammer parameters and hence obtain a decision on the binary signal based on the estimated likelihood functions. The probability of error performance of the algorithm is simulated for a repeat code and a (7,4) block code. Simulation results show that at low signal-to-thermal noise ratio and high jammer power, the EM detector performs significantly better than the hard limiter and somewhat better than the soft limiter. Also, at low SNR, there is little degradation as compared to the maximum-likelihood detector with true jammer parameters. At high SNR, the soft limiter outperforms the EM detector.

A Selection and Estimation Test for Multiple Target Detection

R. Viswanathan et al. — Fri, 05 Jun 2009 06:35:23 PDT

We apply a selection and estimation procedure for the detection of multiple targets in clutter. The selection and estimation (SE) test perfoms better than a fixed order statistic (OS) detector. The SE test has some similarity to a variable trimmed mean (VTM) test and has a slight performance advantage. Unlike the VTM, whose performance is evaluated only by simulation studies, the performance of the SE test, for Rayleigh fluctuating target model, can be evaluated analytically. Further improvement in the performance is possible if a better selection procedure can be found.

Asymptotic Performance of a Distributed Detection System in Correlated Gaussian Noise

V. Aalo et al. — Fri, 05 Jun 2009 06:33:58 PDT

In this correspondence we consider the detection of a constant signal in noise with a large set of geographically dispersed sensors. The noise at the sensors are correlated Gaussian. Two correlation models are considered: one where the correlation coefficient between any two sensors decreases geometrically as the sensor separation increases, and the other where the correlation coefficient between any two sensors is a constant. For both correlation models, the asymptotic (as the number of sensors becomes large) performances of a distributed detection system and a central system are examined.

Optimal Distributed Decision Fusion

S. C. A. Thomopoulos et al. — Fri, 05 Jun 2009 06:30:33 PDT

The problem of decision fusion in distributed sensor system is considered. Distributed sensors pass their decisions about the same hypotheses to a fusion center that combines them into a final decision. Assuming that the semor decisions are independent from each other conditioned on each hypothesis, we provide a general proof that the optimal decision scheme that maximizes the probability of detection at the fusion for fixed false alarm probability comists of a Neyman-Pearson test (or a randomized N-P test) at the fusion and likelihood-ratio tests at the sensors.

Distributed Detection of a Signal in Generalized Gaussian Noise

R. Viswanathan et al. — Fri, 05 Jun 2009 06:29:10 PDT

The problem of distributed detection of a signal in incompletely specified noise is considered. The noise assumed belongs to the generalized Gaussian family and the sensors in the distributed network employ the Wilcoxon test. The sensors pass the test statistics to a fusion center, where a hypothesis testing results in a decision regarding the presence or the absence of a signal. Three monotone and admissible fusion center tests are formulated. Restricted numerical evaluation over a certain parameter range of the noise distribution and the range of signal level indicates that these tests yield performances at comparable levels.