• Quantifying code orthogonality factor for UTRA-FDD down-links

      Hunukumbure, Mythri; Beach, Mark; Allen, Ben (IET, 2002-05)
      The degradation of code orthogonality due to multi-path propagation has a direct impact on DS-CDMA system performance. A novel approach in deriving the orthogonality factor, which quantifies this degradation, is presented. Code orthogonality factors for realistic radio channels are calculated and the results analysed. The extension of the derivation to cover down-link beamforming investigates its effectiveness in preserving code orthogonality.
    • Randomly shifted certification authority authentication protocol for MANETs

      Safdar, Ghazanfar Ali; McLoone, Maire (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2007)
    • A review of pulse design for impulse radio

      Allen, Ben; Ghorashi, S. Ali; Ghavarm, M. (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2004-07)
      “Impulse radio” is currently being considered as a low cost, power efficient, medium range (400m) low data rate (400MBis) wireless solution. This paper reviews the design of pulses for impulse radio. Rectangular, Rayleigh, Laplacian, cubic monocycles and Gaussian pulses are first reviewed. Modified Hermite Polynomials, wavelets and prolate spheroidal wavefunctions are then introduced as candidate waveforms for pulse shaped modulation. Finally, challenges relating to the implementation of pulse shape modulation in a wireless environment are discussed.
    • Robust DoA estimation by extension of the SAGE algorithm: algorithm description and trails results

      Tyler, Neville; Allen, Ben; Aghvami, A.Hamid; King's College London; University of Oxford (IEE, 2006-12)
      By extending the space-alternating generalised expectation-maximisation (SAGE) parameter estimation algorithm to include a novel cost variant process, the robustness of direction of arrival (DoA) estimation from the SAGE algorithm is shown to improve. A novel extension to the standard SAGE algorithm that enhances the parameter estimation process is first introduced. Field trials that have been conducted using an adaptive antenna to verify the performance of this algorithm are presented. Results show that the extended SAGE algorithm improves DoA performance compared to standard SAGE by 5°, when operating in an urban environment.
    • Scalable feature-based video retrieval for mobile devices

      Morbee, Marleen; Velisavljević, Vladan; Mrak, Marta; Philips, Wilfried (ACM, 2009)
      The authors present a novel method for video retrieval on mobile devices. The target scenario is the following: features are extracted from a captured query video at the user side and transmitted to a remote server; then, video retrieval is applied within a stored database at the server side and relevant information about the query is returned to the user. In particular, we focus on the user side and propose a scalable method for video feature extraction and encoding taking into consideration the processing capabilities of the device and available bandwidth. Despite these constraints, the first results show a promising accuracy of retrieval.
    • Scalable user-adaptive multiview video coder

      Velisavljević, Vladan; Chakareski, Jacob; Stankovic, Vladimir; University of Bedfordshire; University of Alabama; Strathclyde University (IEEE, 2013-07)
      We derive an optimization framework for joint view and rate scalable coding of multi-view video content represented in the texture plus depth format. The optimization enables the sender to select the subset of coded views and their encoding rates such that the aggregate distortion over a continuum of synthesized views is minimized. We construct the view-rate scalable bitstream such that it delivers optimal performance simultaneously over a discrete set of transmission rates. In conjunction, we develop a user interaction model that characterizes the view selection actions of the client as a Markov chain over a discrete state-space. Our scheme outperforms the state-of-the-art H.264 SVC codec as well as a multi-view wavelet-based coder equipped with a uniform rate allocation strategy, across all scenarios studied. Finally, we observed that the interactivity-aware coding delivers superior performance over conventional allocation techniques that do not anticipate the client's view selection actions in their operation.
    • A secure MAC protocol for Cognitive Radio Networks (SMCRN)

      Alhakami, Wajdi; Mansour, Ali; Safdar, Ghazanfar Ali; Albermany, Salah A.; University of Bedfordshire (IEEE, 2014-10)
      In addition to standard authentication and data confidentiality requirements, Cognitive Radio Networks (CRNs) face distinct security issues such as primary user emulation and spectrum management attacks. A compromise of these will result in a denial of service, eavesdropping, forgery, or replay attack. These attacks must be considered while designing a secure media access control (MAC) protocol for CR networks. This paper presents a novel secure CR MAC protocol: the presented protocol is analysed for these security measures using formal logic methods such as Burrows-Abadi-Needham (BAN) logic. It is shown that the proposed protocol functions effectively to provide strong authentication and detection against malicious users leading to subsequent secure communication.
    • Severely fading MIMO channels: models and mutual information

      Choi, Seung Ho; Smith, Peter; Allen, Ben; Malik, Wasim Q.; Shafi, M. (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2007-06)
      In most wireless communications research, the channel models considered experience less severe fading than the classic Rayleigh fading case. In this work, however, we investigate MIMO channels where the fading is more severe. In these environments, we show that the coefficient of variation of the channel amplitudes is a good predictor of the link mutual information, for a variety of models. We propose a novel channel model for severely fading channels based on the complex multivariate t distribution. For this model, we are able to compute exact results for the ergodic mutual information and approximations to the outage probabilities for the mutual information. Applications of this work include wireless sensors, RF tagging, land-mobile, indoor-mobile, ground-penetrating radar, and ionospheric radio links. Finally, we point out that the methodology can also be extended to evaluate the mutual information of a cellular MIMO link and the performance of various MIMO receivers in a cellular scenario. In these cellular applications, the channel itself is not severely fading but the multivariate t distribution can be applied to model the effects of inter-cell interference.
    • Shared-key based secure MAC protocol for CRNS

      Alhakami, Wajdi; Mansour, Ali; Safdar, Ghazanfar Ali; University of Bedfordshire (IEEE, 2014)
      The unique characteristics and parameters of Cognitive Radio Networks (CRNs) present several possible threats such as selfish misbehaviours, primary user emulation and attacks on spectrum managers. A secure MAC protocol is required to maintain the CRNs and operate against the possible threats. This paper is a second generation of the Secure MAC protocol for CRNs (SMCRN) to address the security requirements in CRNs and introduces a novel method based on Shared-key cryptography. The paper mainly presents performance analysis of shared key based SMCRN protocol. The paper also discusses and compares SMCRN for Symmetric and Asymmetric cryptography Techniques. The proposed protocol is analysed and described using the formal Burrows-Abadi-Needham (BAN) logic to prove the protocol operation for addressing the security requirements. Moreover, the implementation of current protocol is provided to compute the overall time over both Common Control Channel (CCC) and data transmission channel.
    • Short‐range wireless communications

      Fettweis, Gerhard; Zimmermann, Ernesto; Allen, Ben (John Wiley & Sons, 2008)
      Probably the largest portion of the practical applications of short-range communication takes the form of Wireless Local Area Network (WLAN), Wireless Personal Area Network (WPAN) and Wireless Body Area Networks (WBAN), covering ranges from tens of meters down to sub-meter communications.
    • A simple adaptive beamformer for ultrawideband wireless systems

      Malik, Wasim Q.; Allen, Ben; Edwards, David J. (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2004-09)
      This paper introduces a simple beamformer for ultrawideband (UWB) wireless networks. The architecture consists of a two-element antenna array, a phase shifter and a signal combiner. The performance of the proposed beamformer is analyzed in terms of radiation pattern characteristics over the FCC's defined operating band for UWB communications devices. It is shown that this simple architecture can provide useful interference rejection and range extension capabilities for high data-rate UWB wireless networks. In particular, it is shown that the radiation pattern characteristics of the proposed sub-optimal beamformer remain beneficial over the allocated UWB band, even given its low complexity and implementation cost
    • Simple closed-form channel capacity formulas for the SIMO Nakagami channel

      Vagenas, Efstathios D.; Karadimas, Petros; Kotsopoulos, Stavros A. (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2008)
    • A small scale fading model with sectored and three dimensional diffuse scattering

      Karadimas, Petros; Vagenas, Efstathios D.; Kotsopoulos, Stavros A. (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2008)
    • Smart antenna calibration: requirements and techniques

      Tyler, Neville; Allen, Ben; Aghvami, A.Hamid; King's College, London (IET, 2005)
      This paper presents a detailed analysis of the classes of distortion that degrade the performance of Smart Antennas. This uses the results of a Smart Antenna test bed to illustrate the impact of temperature on performance. A design technique that aide's calibration is then described.
    • Smart antennas for high data rate FDD wireless links

      Allen, Ben (University of Bristol, 2001)
    • A software definable MIMO testbed: architecture and functionality

      Hong, Xuemin; Delannoy, Eric; Allen, Ben; Dohler, Mischa; King's College London (2009)
      Following the intensive theoretical studies of recently emerged MIMO technology, a variety of performance measures become important to investigate the challenges and trade-offs at various levels throughout MIMO system design process. This paper presents a review of the MIMO testbed recently set up at King’s College London. The architecture that distinguishes the testbed as a flexible and reconfigurable system is first preseneted. This includes both the hardware and software aspects, and is followed by a discussion of implementation methods and evaluation of system research capabilities.
    • Space-frequency quantization for image compression with directionlets

      Velisavljević, Vladan; Beferull-Lozano, Baltasar; Vetterli, Martin (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2007)
      The standard separable 2-D wavelet transform (WT) has recently achieved a great success in image processing because it provides a sparse representation of smooth images. However, it fails to efficiently capture 1-D discontinuities, like edges or contours. These features, being elongated and characterized by geometrical regularity along different directions, intersect and generate many large magnitude wavelet coefficients. Since contours are very important elements in the visual perception of images, to provide a good visual quality of compressed images, it is fundamental to preserve good reconstruction of these directional features. In our previous work, we proposed a construction of critically sampled perfect reconstruction transforms with directional vanishing moments imposed in the corresponding basis functions along different directions, called directionlets. In this paper, we show how to design and implement a novel efficient space-frequency quantization (SFQ) compression algorithm using directionlets. Our new compression method outperforms the standard SFQ in a rate-distortion sense, both in terms of mean-square error and visual quality, especially in the low-rate compression regime. We also show that our compression method, does not increase the order of computational complexity as compared to the standard SFQ algorithm.
    • Space-frequency quantization using directionlets

      Velisavljević, Vladan; Beferull-Lozano, Baltasar; Vetterli, Martin (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2007)
      The authors combined the directionlets with the space-frequency quantization (SFQ) image compression method, originally based on the standard two-dimensional (2-D) wavelet transform (WT). We show that our new compression method outperforms the standard SFQ as well as the state-of-the-art compression methods, like SPIHT and JPEG-2000, in terms of the quality of compressed images, especially in a low-rate compression regime. We also show that the order of computational complexity remains the same, as compared to the complexity of the standard SFQ algorithm.
    • Sparse image representation by directionlets

      Velisavljević, Vladan; Vetterli, Martin; Beferull-Lozano, Baltasar; Dragotti, Pier Luigi (Elsevier, 2010)
      Despite the success of the standard wavelet transform (WT) in image processing in recent years, the efficiency and sparsity of its representation are limited by the spatial symmetry and separability of its basis functions built in the horizontal and vertical directions. One-dimensional discontinuities in images (edges or contours), which are important elements in visual perception, intersect too many wavelet basis functions and lead to a non-sparse representation. To capture efficiently these elongated structures characterized by geometrical regularity along different directions (not only the horizontal and vertical), a more complex multidirectional (M-DIR) and asymmetric transform is required. We present a lattice-based perfect reconstruction and critically sampled asymmetric M-DIR WT. The transform retains the separable filtering and subsampling and the simplicity of computations and filter design from the standard two-dimensional (2D) WT, unlike the case for some other directional transform constructions (e.g., curvelets, contourlets, or edgelets). The corresponding asymmetric basis functions, called direction-lets, have directional vanishing moments along any two directions with rational slopes, which allows for a sparser representation of elongated and oriented features. As a consequence of the improved sparsity, directionlets provide an efficient tool for nonlinear approximation of images, significantly outperforming the standard 2D WT. Furthermore, directionlets combined with wavelet-based image compression methods lead to a gain in performance in terms of both the mean square error and visual quality, especially at low bit-rate compression, while retaining the same complexity. Finally, a shift-invariant non-subsampled version of directionlets is successfully implemented in image interpolation, where critical sampling is not a key requirement.
    • Spatial and polarisation correlation characteristics for UWB impulse radio

      Liu, Junsheng; Allen, Ben; Malik, Wasim Q.; Edwards, David J.; King's College, London; University of Bedfordshire (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2005-09)
      Correlation between diversity branches is the key parameter in determining the diversity performance of wireless systems. In this paper, measurements in a temporally and spatially stationary indoor environment are carried out to obtain the channel impulse responses for the UWB impulse radio using receivers with different positions or different polarisation states. A two-dimension (2D) RAKE receiver with post-detection equal gain combining (EGC) diversity is assumed to be used. Three definitions of correlation coefficients of the diversity branches are considered: correlation coefficient, normalized correlation and channel profile correlation, are analysed in this paper. The results show that the normalised correlation is more effective to ascertain the degree of similarity of the correlated diversity branches where medium scale fading dominates.