J.W. Wallace

Jacobs University, Bremen, Bremen, Germany

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Publications (80)63.48 Total impact

  • Source
    J.W. Wallace, R. Mehmood
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    ABSTRACT: The equivalent circuit model of a general PEC antenna array is derived, based on a rigorous method of moments (MOM) formulation, indicating that network analysis is exact from the standpoint of electromagnetic wave theory. It is found that the network parameters (Z, Y, or S-parameters) for the transmit mode can be used for exact prediction of the receive-mode array response. Numerical and experimental examples illustrate the validity of the analytical results.
    IEEE Transactions on Antennas and Propagation 03/2012; · 2.33 Impact Factor
  • R. Mehmood, J.W. Wallace
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    ABSTRACT: A possible solution to allow high-rate MIMO communications in interference-limited scenarios is to employ reconfigurable antennas that optimize antenna patterns to avoid and suppress interference as well as enhance useful multipath directions. The reconfigurable aperture (RECAP) antenna [1] is a dense array of reconfigurable elements (REs), which can be manipulated in order to support many important applications. As shown in [2], a RECAP with multiple feeds can be used in a MIMO system to null the effect of interfering users and provide many fold increase in capacity compared to traditional fixed antennas. However, these findings were based solely on simulations.
    Antennas and Propagation Society International Symposium (APSURSI), 2012 IEEE; 01/2012
  • R. Mehmood, J.W. Wallace
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    ABSTRACT: The capacity of multiple-input multiple-output (MIMO) systems employing reconfigurable apertures (RECAPs) is carefully analyzed with a realistic thermal noise model for three different power constraints: average receive signal-to-noise ratio (SNR), maximum effective isotropic radiated power (EIRP), and average transmit power. Performance is studied not only for a noise-limited single link, but also in the presence of interference and multiple RECAP-equipped users. The impact of loss and finite bandwidth on the operation of the RECAP is also considered. For the practical EIRP constraint, results show that a compact MIMO RECAP (1λ×1λ) provides 30%-50% capacity improvement for a single link. It is also found that RECAPs are even more beneficial in interference-limited and multiuser scenarios, where capacity is increased by 50% to 800% depending on the severity of the interference, indicating that RECAPs are an attractive solution for future wireless systems employing aggressive spectral reuse.
    IEEE Transactions on Antennas and Propagation 01/2012; 60(2):665-673. · 2.33 Impact Factor
  • R. Mehmood, J.W. Wallace
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    ABSTRACT: The potential of automatically generating secret keys by randomly changing the state of a reconfigurable antenna is investigated through direct measurement. Measurements are performed with a 5×5 parasitic reconfigurable aperture (RECAP) antenna at 2.54 GHz in an indoor laboratory environment, illustrating that keys can be generated with physical-layer reciprocal channel key generation (RCKG) even in static or line-of-sight (LOS) conditions. Details of the parasitic array, varactor-diode reconfigurable elements, and network-analyzer based measurement setup are presented. Analysis of the data shows that secure key bits is lower when an eavesdropper is in the LOS path between legitimate nodes as opposed to a direction perpendicular to that path. It is also illustrated that the statistics of fading induced with the reconfigurable array are not strictly Gaussian for LOS channels. Encouragingly, measurements show that around 80% of generated key bits are secure even in the worst case conditions.
    Antennas and Propagation (EUCAP), 2012 6th European Conference on; 01/2012
  • R. Mehmood, J.W. Wallace
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    ABSTRACT: The potential of capacity enhancement with a parasitic reconfigurable aperture (RECAP) antenna is investigated through a measurement campaign in line-of-sight (LOS) and non-LOS conditions in an indoor laboratory environment. Measurements are performed using a 5×5 parasitic RECAP for a bandwidth of 70 MHz centered at 2.55 GHz. Both noise-limited as well as interference-limited cases are considered with a varying level of interference under two different realistic power constraints: average receive signal-to-noise ratio (SNR) and fixed total transmit power. For RECAP optimization a simple genetic algorithm (GA) is implemented and its performance is compared with that of a random search. Measurements confirm that a large increase in capacity is possible especially for the case when there is high interference present.
    Smart Antennas (WSA), 2012 International ITG Workshop on; 01/2012
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    A.M. Eid, J.W. Wallace
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    ABSTRACT: An accurate method for modeling body shadowing in body area networks (BANs) is presented, based on an efficient surface-based method-of-moments (MOM) solution. The method allows the fields radiated by a transmit line current with arbitrary orientation (polarization) in the presence of a lossy dielectric cylinder of arbitrary cross section to be computed with high accuracy. The fields due to a point source are then found using Fourier transform techniques. The utility of the method for BAN modeling is demonstrated by comparing with BAN measurements on a human subject performed in a compact chamber and in an open field.
    IEEE Transactions on Antennas and Propagation 09/2011; · 2.33 Impact Factor
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    R.K. Sharma, J.W. Wallace
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    ABSTRACT: Minimal missed detection rate of primary users is critical for adoption of cognitive radio networks, underlining the need for robust collaborative sensing combined with near-optimal single-node detection methods. Although correlation-based detection methods potentially provide needed per-node performance improvements for correlated signals, their performance for realistic blind sensing is unclear since the type and extent of correlation may be unknown in practice. Although standard Neymon-Pearson (NP) based detection can be applied when correlation is perfectly known, difficulty arises when the correlation is random, which is the focus of this paper. A tighter bound for the performance of correlation-based methods is developed herein based on a signal with random correlation and NP detection under the assumption of correlation distribution information (CDI). Simulations of existing ad-hoc correlation-based detectors are compared to the upperbound using a simple uniform random correlation model (RCM). Additionally, a measurement campaign is presented where radio-frequency (RF) spectra in many bands of interest are measured throughout a large sub-urban environment, generating realistic models for the random signal correlation. The measurement-based model indicates limits on performance gains possible with correlation-based detection and how well existing ad-hoc techniques can be expected to perform in practice.
    IEEE Sensors Journal 04/2011; · 1.48 Impact Factor
  • R.K. Sharma, J.W. Wallace
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    ABSTRACT: Two methods are developed that can be applied to generate secret keys automatically from fading propagation channels that are reciprocal but have arbitrary (i.e. non Gaussian) statistics. Such methods may be necessary for physical-layer key generation in cases where the line-of-sight (LOS) component produces Rician channel statistics, not only because channel quantization based on Gaussian statistics will not provide equally probable symbols, but also because the symbol error rate (SER) and efficiency analysis based on Gaussian channels does not reflect true performance. An improved channel quantization method compared to is developed, where the empirical cumulative distribution function (cdf) of the channel is used directly to ensure equal probability of the key symbols. The results show that LOS channels can have slightly better SER performance than strictly Gaussian channels, especially at low SNR. Second, the idea of positional coding is developed, where a secret key can be transmitted by dividing empirical channel observations into multiple codewords and conveying a secret message from Alice to Bob in the sequence of channels fed forward from Alice to Bob. Analysis of the method illustrates that key mismatch rate can be made arbitrarily low by properly selecting the codeword length.
    Antennas and Propagation (APSURSI), 2011 IEEE International Symposium on; 01/2011
  • R. Mehmood, J.W. Wallace
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    ABSTRACT: The ability of parasitic reconfigurable aperture (RECAP) antennas to generate secret keys is investigated, which allows physical-layer reciprocal channel key generation (RCKG) methods to be employed even in the case of static and line-of-sight channels. Since the artificial channel fluctuations created by RECAP structures are not necessarily Gaussian, a numerical procedure for computing available and secure key bits is developed that is applicable to channels with arbitrary fading. It is identified that for limited RECAP complexity, a reduced-complexity brute-force attack is possible, and a lower bound on the required RECAP complexity to avoid this possibility is developed. Numerical examples of a 9×9 parasitic RECAP with varying levels of complexity illustrate the importance of controlling the reflection coefficient to ensure Gaussian statistics, the need for sufficient complexity to attain maximum secure key bits, and the importance of placing the RECAP on the node near the eavesdropper.
    Antennas and Propagation (EUCAP), Proceedings of the 5th European Conference on; 01/2011
  • R. Mehmood, J.W. Wallace
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    ABSTRACT: Most of today's practical wireless systems are interference limited, meaning that the degree of spectral reuse possible is dictated by the size of the coverage area (or cell) and the degree of tolerable interference experienced at the cell edges. Much higher spectral reuse is possible with nodes that can control their radiation patterns to suppress and avoid interference, which is also the basis of spatial division multiple access (SDMA) techniques. A typical goal in interference-limited systems is to maximize the signal-to-interference ratio (SIR) of the nodes in the network. Although SIR minimization is possible with array signal processing, a drawback is the need for additional radio-frequency (RF) chains and digital signal processing (DSP) resources.
    Antenna Technology (iWAT), 2011 International Workshop on; 01/2011
  • R.K. Sharma, J.W. Wallace
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    ABSTRACT: This paper has computed the bit error rate (BER) performance of two practical key generation methods CQG and CQA considering Gray coded mapping. An improved efficiency metric was also presented that gives a more realistic indication of key generation performance.
    Wireless Information Technology and Systems (ICWITS), 2010 IEEE International Conference on; 10/2010
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    A.M. Eid, N. Murtaza, J.W. Wallace
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    ABSTRACT: This work investigates using the spatially varying Green's function of an idealized body to provide a model that is independent of the antennas and scattering environment. This paper considers a lossy infinite circular cylinder model for the body, but generalized to include arbitrary source and observation polarization. Additionally, measurements are performed in a compact anechoic chamber with non-resonant short monopoles, better approximating an ideal point current source and field sensor. The results indicate that the simple model is adequate for certain polarizations, but insufficient for others. Also, this effort reveals some practical limitations of BAN measurements in compact chambers.
    Wireless Information Technology and Systems (ICWITS), 2010 IEEE International Conference on; 10/2010
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    J.W. Wallace, R.K. Sharma
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    ABSTRACT: Information theoretic limits for random key generation in multiple-input multiple-output (MIMO) wireless systems exhibiting a reciprocal channel response are investigated experimentally with a new three-node MIMO measurement campaign. As background, simple expressions are presented for the number of available key bits, as well as the number of bits that are secure from a close eavesdropper. Two methods for generating secret keys are analyzed in the context of MIMO channels and their mismatch rate and efficiency are derived. A new wideband indoor MIMO measurement campaign in the 2.51- to 2.59-GHz band is presented, whose purpose is to study the number of available key bits in both line-of-sight and nonline-of-sight environments. Application of the key generation methods to measured propagation channels indicates key generation rates that can be obtained in practice for four-element arrays.
    IEEE Transactions on Information Forensics and Security 10/2010; · 1.90 Impact Factor
  • R. Mehmood, J.W. Wallace
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    ABSTRACT: A 1λ x 1λ circular planar RECAP structure was analyzed for beamforming performance with respect to constrained complexity, in terms of the number of reconfigurable ports N<sub>RP</sub> and allowed reconfigurable states per port N<sub>RS</sub>. It was found that diminishing returns of the structure occurred near N<sub>RP</sub> = 64 reconfigurable ports. Also, increasing the reconfigurable states per port appears to partially compensate for a limited number of ports. Although the structure considered in this work is somewhat idealized, future work will consider more practical RECAP structures that include loss and biasing effects, as well as other applications, such as null-steering and three-dimensional pattern synthesis.
    Antennas and Propagation Society International Symposium (APSURSI), 2010 IEEE; 08/2010
  • J.W. Wallace, R.K. Sharma
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    ABSTRACT: There is growing interest in physical-layer key generation schemes that provide very strong or even perfect security in wireless communication systems. One such scheme is reciprocal channel key generation, where two nodes quantize reciprocal channel state information to generate keys. Although the use of multiple-input multiple-output (MIMO) techniques is interesting in this case since the number of random parameters available for rapid key generation is increased, MIMO techniques may also provide more information to an eavesdropper. This work presents a new MIMO measurement campaign performed in LOS and NLOS indoor environments that studies the correlation of the channel between legitimate users with the eavesdropper channel, revealing what key generation rates can be attained in practice and what fraction of generated key bits are safe from eavesdroppers. The effect of eavesdropper separation, number of antennas, eavesdropper advantage, and covariance separability are studied.
    Communications (ICC), 2010 IEEE International Conference on; 06/2010
  • R.K. Sharma, J.W. Wallace
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    ABSTRACT: Accurate and efficient spectrum sensing is a critical component of cognitive radio, which is a technology poised to improve dynamic resource management in future wireless networks. Autocorrelation exploitation has been shown to provide improvements in sensing performance over simple methods like energy detection. A new upper bound on the performance of autocorrelation-based detectors based on an NP test under the assumption of correlation distribution information (CDI) is presented, where random parameters of the signal autocorrelation are not known, but their distribution is assumed to be known. Not only does this reveal how close existing ad-hoc autocorrelation-based detectors are to optimal performance, but also it indicates how much performance improvement over energy detection is theoretically possible.
    Smart Antennas (WSA), 2010 International ITG Workshop on; 03/2010
  • Source
    R. Mehmood, J.W. Wallace
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    ABSTRACT: Reconfigurable aperture (RECAP) antennas consist of a regular array of reconfigurable elements confined to an aperture, representing a generalization of the reconfigurable antenna concept. RECAPs have the potential of supporting operations like beamforming, null steering, interference suppression, adaptive matching, and frequency and bandwidth agility in a single aperture. Although very complex RECAP structures theoretically have performance that is only limited by the physical extent of the aperture, high complexity is impractical due to increased loss, biasing difficulties, and system cost. This work reviews the RECAP antenna concept and focuses on the role of performance bounds imposed by limited complexity in practical RECAP structures. This initial effort studies empirical bounds on beamforming performance observed from detailed simulation of two idealized RECAP structures: (i) a 5 × 5 parasitic dipole array, and (ii) a 8 × 8 planar configuration of reactively connected patches. Performance bounds for limited complexity are identified, indicating the number of elements per wavelength and the states per reconfigurable element that are needed to capture most of the available performance.
    Smart Antennas (WSA), 2010 International ITG Workshop on; 03/2010
  • Source
    R. Mehmood, J.W. Wallace
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    ABSTRACT: Reconfigurable aperture (RECAP) antennas hold the promise of allowing nearly arbitrary antennas to be synthesized dynamically. An important outstanding question regards the level of RECAP complexity required to capture most of the performance gains in terms of the number of reconfigurable elements (REs) per wavelength and the number of reconfiguration bits (RBs) per reconfigurable element. Diminishing improvement of beamforming performance with increasing RECAP complexity is investigated by considering a square parasitic dipole array with 9 ?? 9 elements with dimensions 1?? ?? 1?? ?? 0.5??. For a fixed RECAP complexity of N<sub>RE</sub>N<sub>RB</sub>, it is observed that having higher N<sub>RE</sub> is more beneficial than high N<sub>RB</sub> for dynamic beam steering, and that approximately eight REs per wavelength are required to reach diminishing performance returns.
    IEEE Antennas and Wireless Propagation Letters 02/2010; · 1.67 Impact Factor
  • R.K. Sharma, J.W. Wallace
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    ABSTRACT: There is growing interest in physical-layer security methods that exploit the random nature of the physical propagation channel to strengthen existing crypto-systems. In reciprocal channel key generation (RCKG), legitimate nodes (Alice and Bob) observe a common fluctuating channel to generate keys that are safe from the eavesdropper (Eve). Previous work derived expressions for the available key bits and those safe from an eavesdropper for MIMO systems with correlated complex Gaussian statistics. This paper applies these expressions to new MIMO measurements in indoor LOS and NLOS environments, indicating how propagation effects limit secure key generation and what key generation rates can be expected in practice.
    Antennas and Propagation Society International Symposium (APSURSI), 2010 IEEE; 01/2010
  • Source
    J.W. Wallace, M.A. Jensen
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    ABSTRACT: A novel method for estimating the power angle spectrum (PAS) is presented that decomposes the true PAS into a small set of basis functions. The basis coefficients for this sparse representation are found by enforcing equality to the covariance or Bartlett PAS subject to a minimum lscr<sub>1</sub>-norm constraint. The method, referred to as sparse PAS estimation (SPASE), can be implemented conveniently using existing linear-programming (LP) solvers. Further, because only a few clusters are required in the representation, the method enables reduced-complexity stochastic models for the channel and possibly allows reduced overhead in channel feedback schemes. Application of the method to simulated channels and multiple-input multiple-output (MIMO) propagation data demonstrates the utility of the method.
    IEEE Transactions on Antennas and Propagation 09/2009; · 2.33 Impact Factor

Publication Stats

1k Citations
63.48 Total Impact Points

Institutions

  • 2007–2012
    • Jacobs University
      • SES - School of Engineering & Science
      Bremen, Bremen, Germany
    • Universität Bremen
      Bremen, Bremen, Germany
    • New Jersey Institute of Technology
      Newark, New Jersey, United States
  • 2007–2008
    • Università degli studi di Cassino e del Lazio Meridionale
      Cassino, Latium, Italy
  • 2000–2008
    • Brigham Young University - Provo Main Campus
      • Department of Electrical and Computer Engineering
      Provo, UT, United States
  • 2005–2007
    • University of Pretoria
      • Department of Electrical, Electronic and Computer Engineering
      Pretoria, Gauteng, South Africa