A.S. Madhukumar

Nanyang Technological University, Tumasik, Singapore

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Publications (176)89.21 Total impact

  • Ponnu Jacob, Ashish James, A. S. Madhukumar
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    ABSTRACT: With the introduction of third generation mobile services, femtocells are considered as an economically feasible solution for combining mobile and internet technologies, thereby giving fast and reliable access to data with a better coverage. However, it is well-known that the femtocells and macrocells sharing the same licensed frequency spectrum results in heavy cross-tier interference which degrades the downlink performance considerably. In this paper, we investigate a novel frequency–division duplex allocation strategy which eliminates the downlink cross-tier interference to the femtocell network from the macrocell base station throughout its coverage area. The proposed scheme seamlessly embed the femtocells within a macrocell resource network to create a heterogeneous two-tier system. It makes use of a cross-tier complementary spectrum sharing technique known as reverse frequency allocation (RFA) where the frequency carriers used in the macrocell transmission are reversed and allocated to femtocells. As a result, it better balances the requirement of greater inter-cell orthogonality and reduced inter-cell interference since macrocell and femtocell operates on different bands in uplink and downlink. It also assures enhanced spectral efficiency and the well-known benefit of reduced outage probability, especially for cell-edge users. This work further analytically quantifies and highlights through simulation results that RFA guarantees greater overall network throughput in the downlink and reduced cross-tier interference regardless of the positioning of the femtocell with respect to the macrocell base station. Also it is to be noted that, with recent academic surveys illuminating that the benefit of femtocells is reflected more in downlink, the focus of the current work is on downlink transmission where the traffic is high and the deployment is more beneficial.
    Wireless Networks 12/2014; · 0.74 Impact Factor
  • S. Barman Roy, A. S. Madhukumar
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    ABSTRACT: For a multiple access channel, each user has its own power constraint. However, when a multiple access channel is being considered as the dual of a broadcast channel, it must exploit the additional freedom in power allocation and the constraint should be a sum power constraint. Duality helps us to transform the non-convex problems in a broadcast channel to convex problems in a multiple access channel. This helps to solve sum-rate optimisation problems with linear power constraints. However, maximising the sum-rate does not completely characterise the entire rate region boundary, formally called as the Pareto frontier. This work first reviews some of the existing results of polymatroid formulation from the Pareto optimality perspective and then proposes a complete characterisation of the Pareto frontier to show its relationship with the sum-rate optimisation problem. The significance of decoding order on the achievable rate region is considered. The work also shows some of the decoding orders to be suboptimal in Pareto sense and proposes an algorithm to find the correct decoding order based on power allocation. Simulation results are presented to support the theoretical arguments.
    Wireless Networks 12/2014; · 0.74 Impact Factor
  • N.D. Duong, AS Madhukumar
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    ABSTRACT: This study addresses the problem of spectrum trading in a cognitive radio network with multiple primary users (PUs) competing to sell spectrum to a secondary user (SU). The spectrum trading process is modelled using a 'Cournot game model' of competition by which the PUs set the size of spectrum to sell. In this study, the spectrum requirements for the PUs?? services are not fixed but time varying, and the spectrum trading process is carried out before the realisation of these values. If the spectrum retained for a PU after selling is less than the spectrum requirement for the PU's service, a cost must be charged to the PU. The Nash equilibrium (NE) for a static game when the PUs have complete knowledge on the utility functions of other PUs is studied first. A dynamic game, in which the players adaptively change their strategies to reach the NE, is discussed subsequently. Finally, the trading problem is extended to a scenario which involves multiple SUs.
    IET Signal Processing 06/2014; 8(4):410-420. · 0.71 Impact Factor
  • Ashish James, A. S. Madhukumar
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    ABSTRACT: Rateless code properties ideally suit multi-node transmissions with decode-and-forward based relaying strategy, and have been extensively employed in delay tolerant networks. The overall latency suffered by such networks forces to conserve the end-to-end delay, especially for real-time applications, by limiting the number of rateless coded transmissions. However, the performance of rateless codes deteriorates in such circumstances due to the lack of sufficient mutual information for successfully recovering the entire source packets. This performance degradation can be gauged by the average packet recovery and cooperation among nodes results in enhanced performance. The exact closed-form equations for the average packets retrieved based on the channel usage for such delay constrained networks (DCNs) is derived in this paper. These are verified and supported with results from extensive simulation studies.
    2014 ICC - 2014 IEEE International Conference on Communication Workshop (ICC); 06/2014
  • Ponnu Jacob, A.S. Madhukumar, A. Alphones
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    ABSTRACT: Multi-tier networks comprising of macro-cellular network overlaid with less power, short range, home-base station like femtocells provide an economically feasible solution for meeting the unrelenting traffic demands. However, femtocells that use co-channel allocation with macrocells result in cross-tier interference which eventually degrades the system performance. It is for this reason that, cross-polarized data transmission is proposed in this paper as a potential approach towards improving the spectral efficiency of cellular systems and at the same time permitting co-channel allocation. Here two independent information channels occupying the same frequency band can be transmitted over a single link. The paper evaluates a scenario where femtocell network makes use of right hand circular polarization (RHCP) and macrocell network makes use of left hand circular polarization (LHCP) for signal transmission. The polarizations being orthogonal to each other due to their sense of rotation ensure isolation between the networks and enable both of them to use the same spectral resources simultaneously. Analytical and simulation results prove that this opens the scope for an easily implementable, remarkable opportunity in the context of two-tier femto-macro network that can increase the system capacity. The paper closes by discussing the technical challenges involved in the implementation as well as the possible solutions to overcome the same.
    Computer Communications. 01/2014;
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    ABSTRACT: Cognitive Radio (CR) aims to provide efficient spectrum utilization in spectrum scarce wireless environments. One of the key CR functionalities is the spectrum sensing, which allows CRs to monitor the electromagnetic spectrum and detect unused bands of spectrum. Wideband spectrum sensing needs to be employed for better spectrum opportunity detection and interference avoidance both in the case of commercial and military applications. Accurate sensing needs to be employed for blocker detection in commercial systems such as LTE for the design of transmit/receive path. In military radios, the challenge lies in the robust detection of the location of the center frequencies and bandwidths of individual radio channels in the wideband input signal. In this paper, an energy detector based on tree-structured discrete Fourier transform based filter bank (TDFTFB) is proposed for detecting the edges of the channels in the spectrum. The proposed method is compared with the conventional wavelets based method for complexity and performance. The design example and simulations show that the gate count resource utilization of the proposed detection scheme is 22.9% lesser than the wavelets method at the cost of a slight degradation (0.5%) in detection accuracy. Over-the-air tests performed using Universal Software Radio Peripheral 2 (USRP2) and MATLAB/SIMULINK showed that the present method is not input specific whereas the conventional wavelet based approach depends on the spectral location of the input.
    Physical Communication 12/2013; 9:45–60.
  • Ashish James, A.S. Madhukumar, Fumiyuki Adachi
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    ABSTRACT: Multihop cooperative communication is emerging as a key concept to extend the coverage area of the network and potentially increase the capacity. The spectral efficiency of such networks can be improved by adapting the transmission to time-varying channel conditions, referred to as incremental relaying. Although such incremental relaying concepts are progressively being studied, many challenges, such as erroneous transmissions by intermediate nodes and end-to-end delay of the network, limit its practical use due to lack of an efficient implementation. This paper proposes an efficient multihop incremental relaying technique. In this method, erroneous relay forwarding is mitigated, and the overhead for coordination among nodes is reduced by exploiting the implicit feedback channel available due to the broadcast nature of wireless transmissions. The proposed scheme fully leverages the benefit of overhearing and eliminates the additional feedback slots required for validation. Further, it ensures reliable forwarding of information, which optimizes the throughput of multihop networks. Thorough analysis of the proposed scheme is performed under different deployment environments, and the theoretical analyses presented in this paper are supported with results from extensive simulation studies. Copyright © 2013 John Wiley & Sons, Ltd.
    Wireless Communications and Mobile Computing 06/2013; · 0.86 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Two key tasks in the development of cognitive radio networks in commercial and military applications are spectrum sensing and automatic modulation classification (AMC). These tasks become even more difficult when the cognitive radio receiver has no information about the channel or the modulation type. An integrated scheme which includes both these aspects is proposed in this paper. Spectrum sensing is done using cumulants derived from fractional lower order statistics. It is shown through simulations that the proposed sensing method has improved performance, especially in low SNR environments in Gaussian and non-Gaussian noise when compared with the conventional higher-order statistics (HOS) based method. The performance of the automatic modulation classifier is presented in the form of conditional probability of classification, probability of correct classification and confusion matrix under noisy and under fading conditions. Simulations in our previous work showed that the proposed method achieved better classification accuracy when compared to cumulant based AMC method in noise conditions that are highly impulsive than Gaussian. In this paper, simulations show significant improvement in the performance of AMC in the presence of AWGN and under multipath fading, for a known frequency band of interest when compared with the conventional AMC methods available.
    AEU - International Journal of Electronics and Communications 06/2013; 67(6):479–490. · 0.55 Impact Factor
  • A. James, A.S. Madhukumar, E. Kurniawan, F. Adachi
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    ABSTRACT: Fountain codes have been extensively employed in delay-tolerant networks (DTNs) due to their near-capacity performance with very low encoding/decoding complexity. A decode-and-forward-based relaying strategy is ideally suited for fountain codes in such networks due to its ability to recover the source message from any subset of encoded packets with sufficient mutual information. However, the unreliable nature of the channel may lead to the starvation of some subsequent nodes with good channel conditions. By cooperation among the forwarding nodes, the overall latency of such networks can be alleviated. This paper analytically quantifies the latency of both cooperative and conventional fountain-coded delay-tolerant multihop networks by deriving the exact closed-form equations for the channel usage. The overall latency suffered by such networks forces conservation of the end-to-end delay, particularly for real-time applications. However, by constraining the total delay (the number of encoded transmissions), the performance of fountain codes deteriorates due to the lack of encoded packets for retrieving the entire source message. This degradation can be gauged by the average packet loss experienced with partial decoding of fountain codes. The exact closed-form equation for the average packet loss based on the channel usage for such delay-constrained networks (DCNs) is derived in this paper. The tradeoff between average delay and the channel usage required for successful decoding is also analyzed. It is observed that the average packet loss can be minimized by optimizing the total delay based on the performance across each link. Finally, the pros and cons of using DCNs and DTNs employing fountain codes are evaluated, and theoretical grounding to the simulated results is provided.
    IEEE Transactions on Vehicular Technology 01/2013; 62(9):4379-4391. · 2.06 Impact Factor
  • Xionghu Zhong, A.B. Premkumar, A.S. Madhukumar
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    ABSTRACT: NonGaussian impulsive noises distort the source signal and cause problems for direction of arrival (DOA) estimation of an acoustic source. In this paper, a Bayesian framework and its particle filtering (PF) implementation for DOA tracking in the presence of complex symmetric alpha-stable noise process are developed. A constant velocity model is employed to model the source dynamics, and spatial spectra are exploited to formulate a pseudo likelihood of particles. Since the second-order statistics of alpha-stable processes do not exist, the fractional lower order moment matrix of the received data is used to replace the covariance matrix in calculating the spatial spectra. The noise usually spreads and distorts the mainlobe of the likelihood function and the particles cannot be weighted accurately. Hence, the likelihood function is exponentially weighted to emphasize the particles in a high likelihood area and thus enhance the resampling efficiency. The performance of the proposed tracking algorithm is extensively studied under simulated alpha-stable noise environments. The results show that the proposed algorithm significantly outperforms the existing PF tracking approach and the traditional localization approaches in DOA estimation.
    IEEE Sensors Journal 01/2013; 13(2):589-600. · 1.85 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Rateless codes have been found to be particularly attractive for decode and forward based relaying strategy in delay tolerant multihop networks. The latency performance of such networks is dependent on the worst links that results in the starvation of subsequent nodes with good channel conditions. The total delay suffered by such networks can be constrained by limiting the number of rateless coded transmissions, especially for applications with critical latency requirements. However, the performance of rateless codes deteriorates in such circumstances due to the lack of sufficient mutual information for successfully recovering the entire source packets. The fraction of source packets that can be recovered will depend on the encoded packets received across the transmission channel. This paper investigates the degradation in the performance of such rateless coded networks by deriving the average packet recovery rate. In order to improve the reliability in such delay constrained networks, a novel spectrally efficient transmission scheme for reliable multihop data transfer is proposed. The proposed scheme exploits the broadcast nature of wireless transmissions, which provides an inherent implicit feedback channel, to ensure the reliable delivery of information packets to the nodes in the network. Rather than allocating dedicated channels to feedback the packet recovery information, the implicit feedback channel determines such information which enhances the spectral efficiency. Further, the optimum number of packets recoverable within the specified delay constraint to reduce the reprocessing of lost packets across hops is analytically analysed in this paper.
    IEEE Transactions on Communications 01/2013; 61(11):4462-4474. · 1.75 Impact Factor
  • P. Jacob, A.S. Madhukumar, A. Alphones
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    ABSTRACT: Femtocells use co-channel allocation with macrocells resulting in cross-tier interference which eventually degrades the system performance. Cross-polarized data transmission is proposed as a potential approach towards improving the spectral efficiency and at the same time permitting co-channel allocation in cellular systems. This letter evaluates a scenario where femtocell network makes use of right-hand circular polarization (RHCP) and macrocell network makes use of left-hand circular polarization (LHCP) for simultaneous transmissions even when using the same frequency spectrum. The polarizations being orthogonal to each other due to their sense of rotation ensure isolation between the networks and hence promotes interference-free independent operation with better coverage and higher throughput.
    IEEE Communications Letters 01/2013; 17(10):1940-1943. · 1.16 Impact Factor
  • Ashish James, A.S. Madhukumar, Fumiyuki Adachi
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    ABSTRACT: Multihop cooperative communication is emerging as a key concept to extend the coverage area and potentially increase the capacity of wireless networks. With multiple hops, processing at the forwarding nodes is a major source of error propagation. In this context, a novel spectrally efficient transmission scheme with error protection for such networks is proposed in this paper. The proposed scheme retains the spatial diversity and enhances the bandwidth efficiency by integrating incremental redundancy concepts into such cooperative multihop systems. The guard against error propagation and the overhead for coordination among multiple nodes in such networks is mitigated by the implicit feedback channel available due to broadcast nature of wireless transmissions. The proposed scheme fully leverages the benefit of overhearing and eliminates the additional feedback slots required for validation. Further, it ensures reliable forwarding of information which optimizes the throughput of multihop networks. Theoretical framework is also developed for the proposed system and supported with results from extensive simulation studies.
    Personal Indoor and Mobile Radio Communications (PIMRC), 2013 IEEE 24th International Symposium on; 01/2013
  • Ponnu Jacob, A.S. Madhukumar, A. Alphones
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    ABSTRACT: Interference issues arising from spectrum reuse in femto-macro networks can result in significant performance degradation in the system. As part of addressing these challenges, this paper proposes cross-polarized data transmission as a potential solution for permitting interference free spectrum reuse in cellular systems. Different from prior art techniques, the paper evaluates a scenario where femtocell network makes use of right hand circular polarization (RHCP) and macrocell network makes use of left hand circular polarization (LHCP) for data transmission so as to aid co-channel allocation. The polarizations being orthogonal to each other due to their sense of rotation ensure isolation between the networks and hence enable both of them to efficiently reuse the same spectral resources simultaneously in the same location. Analytical and simulation results prove that this technology opens the scope for an easily implementable, remarkable opportunity in the context of two-tier femto-macro network that will increase system capacity, spectral efficiency and the probability of successful transmission.
    Vehicular Technology Conference (VTC Fall), 2013 IEEE 78th; 01/2013
  • Ponnu Jacob, Ashish James, A.S Madhukumar
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    ABSTRACT: Multi-tier networks comprising of macro-cellular network overlaid with less power, short range, home base stations like femtocells provide an economically feasible method for increasing the cellular capacity. Also the benefit of femtocells is found more in downlink since the traffic in downlink is several times more than in uplink. However, the femtocells share the same licensed frequency spectrum as that of the macrocell resulting in cross tier interference which degrades the downlink traffic considerably. This paper investigates a novel method for seamlessly embedding femtocells within a macrocell resource distribution framework to create a two-tier system, which incorporates a cross-tier complementary spectrum sharing known as Reverse Frequency Allocation (RFA) along with Soft Frequency Reuse (SFR) strategy. The proposed scheme guarantees inter-cell orthogonality with reduced interference. It also assures a reasonably high system spectral efficiency, while providing better performance for the users especially in the downlink. Simulation results prove that this scheme can bring about higher system throughput and lower co-channel interference irrespective of the distance of the femtocell from the macrocell base station.
    2012 IEEE International Conference on Communication Systems (ICCS); 11/2012
  • Surya Dharma Tio, Ashish James, A.S. Madhukumar
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    ABSTRACT: Cooperative communication is a promising paradigm that could address many of the challenges encountered in the development of future mobile communication systems. With multiple wirelessly communicating devices that cooperate with each other, a network of wireless nodes with virtual antenna arrays is created. This emulates a MIMO system and mimics most of its benefits. However bandwidth efficiency is a major issue in cooperative communication. Incremental-redundancy based techniques can be employed in cooperating nodes to minimize the bandwidth penalty but lacks error protection capability against unreliable decoding. Motivated by such shortcomings, a technique that combines the concept of incremental redundancy with space–time block coding (STBC) is proposed in this paper. The proposed scheme retains spatial diversity and bandwidth efficiency advantage offered by normal incremental redundancy based systems, in addition to offering guard against error propagation due to imperfect decoding at the relay. The theoretical framework is also developed to provide an understanding on the fundamental performance characteristics of the system. Two parameters such as Error Propagation Quotient (EPQ) and Spectral Efficiency Gain (SEG) are proposed to gauge the benefits offered by the proposed technique.
    Physical Communication 09/2012; 5(3):240–252.
  • Manju Mathew, A. B. Premkumar, A. S. Madhukumar
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    ABSTRACT: Cognitive radio (CR) transceiver that can offer adequate data rate and multiuser support for future wireless networks is a promising technology for reliable and spectrum-efficient mobile communication. Orthogonal frequency division multiplexing (OFDM) and scalar wavelet based schemes have been proposed as physical layer techniques for CR. This paper proposes multiwavelet packet-based multicarrier multiple-access scheme as an equally promising candidate for multi-user CR networks and using existing orthonormal multiwavelets, the performance of the proposed system is evaluated. It is shown that the error performance of the proposed system under frequency and phase offset conditions is comparable with existing schemes.
    ISRN Communications and Networking. 08/2012; 2012.
  • V.N. Hari, G.V. Anand, A.B. Premkumar, A.S. Madhukumar
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    ABSTRACT: This paper presents the design and performance analysis of a detector based on suprathreshold stochastic resonance (SSR) for the detection of deterministic signals in heavy-tailed non-Gaussian noise. The detector consists of a matched filter preceded by an SSR system which acts as a preprocessor. The SSR system is composed of an array of 2-level quantizers with independent and identically distributed (i.i.d) noise added to the input of each quantizer. The standard deviation σ of quantizer noise is chosen to maximize the detection probability for a given false alarm probability. In the case of a weak signal, the optimum σ also minimizes the mean-square difference between the output of the quantizer array and the output of the nonlinear transformation of the locally optimum detector. The optimum σ depends only on the probability density functions (pdfs) of input noise and quantizer noise for weak signals, and also on the signal amplitude and the false alarm probability for non-weak signals. Improvement in detector performance stems primarily from quantization and to a lesser extent from the optimization of quantizer noise. For most input noise pdfs, the performance of the SSR detector is very close to that of the optimum detector.
    Signal Processing 07/2012; 92(7):1745–1757. · 2.24 Impact Factor
  • Xionghu Zhong, A. B. Premkumar, A. S. Madhukumar
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    ABSTRACT: Acoustic vector sensor (AVS) measures acoustic pressure as well as particle velocity, and therefore AVS signal contains 2-D (azimuth and elevation) DOA information of an acoustic source. Existing DOA estimation techniques assume that the source is static and extensively rely on the localization methods. In this paper, a particle filtering (PF) tracking approach is developed to estimate the 2-D DOA from signals collected by an AVS. A constant velocity model is employed to model the source dynamics and the likelihood function is derived based on a maximum likelihood estimation of the source amplitude and the noise variance. The posterior Cramér-Rao bound (PCRB) is also derived to provide a lower performance bound for AVS signal based tracking problem. Since PCRB incorporates the information from the source dynamics and measurement models, it is usually lower than traditional Cramér-Rao bound which only employs measurement model information. Experiments show that the proposed PF tracking algorithm significantly outperforms Capon beamforming based localization method and is much closer to the PCRB even in a challenging environment (e.g., ${\rm SNR}=-10\ {\rm dB}$).
    IEEE Sensors Journal 01/2012; 12(2):363-377. · 1.85 Impact Factor
  • R.P. Sirigina, A.S. Madhukumar, Qian Li
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    ABSTRACT: Interference forwarding in an interference channel makes the interference stronger which in turn enables the usage of simple detectors like the successive interference canceler (SIC) at the destinations. In this paper, detect and forward (DF), amplify and forward (AF) based interference forwarding protocols (also called as AF-SIC and DF-SIC protocols) are analyzed. Symbol error rate (SER) performance of the SIC, AF-SIC and DF-SIC protocols using M-PAM modulation is studied. Also the relay channel gains required to restore full diversity for the AF-SIC and the DF-SIC protocols are obtained. Impact of the relay power and relay location on the SER performance is also studied. The proposed protocols are proved to provide the same SER performance as the orthogonal transmission scheme while improving the spectral efficiency.
    Wireless Communications and Networking Conference (WCNC), 2012 IEEE; 01/2012

Publication Stats

431 Citations
89.21 Total Impact Points

Institutions

  • 1997–2013
    • Nanyang Technological University
      • • School of Computer Engineering
      • • Center for Multimedia and Network Technology
      Tumasik, Singapore
  • 2008
    • University of Michigan-Dearborn
      • Department of Electrical and Computer Engineering
      Dearborn, MI, United States
  • 2003–2007
    • Institute for Infocomm Research
      Tumasik, Singapore
  • 1997–2007
    • Nanyang Normal University
      Nan-yang-shih, Henan Sheng, China
  • 2003–2004
    • National University of Singapore
      • • Centre for Remote Imaging, Sensing and Processing (CRISP)
      • • Department of Electrical & Computer Engineering
      Singapore, Singapore
    • Singapore-MIT Alliance
      Cambridge, Massachusetts, United States