Attahiru Sule Alfa

University of Manitoba, Winnipeg, Manitoba, Canada

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Publications (196)119.44 Total impact

  • Kamal Darchini, Attahiru S. Alfa
    05/2014;
  • Yun Han Bae, Bong Dae Choi, Attahiru S. Alfa
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    ABSTRACT: This paper considers random access protocols with multipacket reception (MPR), which include both slotted-Aloha and slotted $(tau)$-persistent CSMA protocols. For both protocols, each node makes a transmission attempt in a slot with a given probability. The goals of this paper are to derive the optimal transmission probability maximizing a system throughput for both protocols and to develop a simple random access protocol with MPR, which achieves a system throughput close to the maximum value. To this end, we first obtain the optimal transmission probability of a node in the slotted-Aloha protocol. The result provides a useful guideline to help us develop a simple distributed algorithm for estimating the number of active nodes. We then obtain the optimal transmission probability in the $(tau)$-persistent CSMA protocol. An in-depth study on the relation between the optimal transmission probabilities in both protocols shows that under certain conditions the optimal transmission probability in the slotted-Aloha protocol is a good approximation for the $(tau)$-persistent CSMA protocol. Based on this result, we propose a simple $(tau)$-persistent CSMA protocol with MPR which dynamically adjusts the transmission probability $(tau)$ depending on the estimated number of active nodes, and thus can achieve a system throughput close to the maximum value.
    IEEE Transactions on Mobile Computing 01/2014; 13(3):497-511. · 2.40 Impact Factor
  • Attahiru Sule Alfa
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    ABSTRACT: We analyze the MAP/PH/1 vacation system at arbitrary times using matrix-analytic method, and obtain decomposition results for the RR and GG matrices. The decomposition results reduce the amount of computational effort needed to obtain these matrices. The results for the GG matrix are extended to the BMAP/PH/1 system. We also show that in the case of the Geo/PH/1 and M/PH/1 systems with PH vacations both the GG and the RR matrices can be obtained explicitly.
    Operations Research Letters 01/2014; · 0.52 Impact Factor
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    ABSTRACT: In this paper, a novel spectrum-sensing scheme, called adaptive dual-radio spectrum-sensing scheme (ADRSS), is proposed for cognitive radio networks. In ADRSS, each secondary user (SU) is equipped with a dual radio. During the data transmission, with the received signal-to-noise ratio of primary user (PU) signal, the SU transmitter (SUT) and the SU receiver (SUR) are selected adaptively to sense one channel by one radio while communicating with each other by the other one. The sensing results of the SUR are sent to the SUT through feedback channels (e.g., ACK). After that, with the sensing results from the SUT or the SUR, the SUT can decide whether the channel switching should be carried out. The theoretical analysis and simulation results indicate that the normalized channel efficiency, defined as the expected ratio of time duration without interference to PUs in data transmission to the whole frame length, can be improved while satisfying the interference constraint to PUs. After that, an enhanced ADRSS is designed by integrating ADRSS with cooperative spectrum sensing, and the performance of ADRSS under imperfect feedback channel is also discussed. Copyright © 2011 John Wiley & Sons, Ltd.
    Wireless Communications and Mobile Computing 10/2013; 13(14). · 0.86 Impact Factor
  • Kamal Darchini, Attahiru S. Alfa
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    ABSTRACT: Advancements in nanotechnology have promised the building of nanomachines capable of carrying out simple tasks. Final goals of using these nanomachines in applications such as monitoring body tissues, drug delivery or other complicated applications need them to do complex tasks. A feasible way to have them cooperate and do complex tasks is to interconnect them by building a nanonetwork. As nanomachines can only carry out simple tasks, nanonetworks bring up new issues in networking and communication. There have been many papers addressing solutions for communication challenges in nanonetworks and analysing their characteristics. At this stage, going towards higher goals needs a comprehensive look at the literature and state of the art in the field. In this paper, we present an inclusive look at issues introduced and analysed in nanonetworks. We focus on two methods which provide properties needed in nanonetworks efficiently, namely communication via microtubules and physical contact.
    Nano Communication Networks 06/2013; 4(2):73–85.
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    ABSTRACT: We presents a queuing theoretic approach to model a packet-oriented data network which links a set of phasor measurement units (PMU) to a phasor data concentrator (PDC) in a wide area monitoring, protection and control system (WAMPaCS). The PMU-PDC communication network is approximated as a cyclic polling system and the associated Markov chain is set up. Based on this model, closed-form expressions are derived for important reliability measures such as the packet loss probability and the communication delay. We then demonstrate how the proposed model can be used to predict the impact of the number of data sources on the network, as well as the buffer capacity of the network switches on the overall reliability of the communication link. An important property of the proposed model is that it's computational complexity is only linear in the number of data sources connected, making it suitable for the study of large systems.
    Power and Energy Society General Meeting (PES), 2013 IEEE; 01/2013
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    ABSTRACT: In this paper, the performance of a multichannel cognitive radio network (CRN) where each secondary user (SU) can assemble a constant number of primary channels is investigated. The effects of imperfect spectrum sensing (with false alarms and mis-detections) are taken into account and it is assumed that spectrum handover is enabled in the secondary network. A buffering mechanism is introduced to save blocked SU requests and interrupted SU services. An analytical model based on Continuous-time Markov chains (CTMC) is developed, and the system performance is evaluated in terms of throughput, blocking probability, and forced termination probability. Numerical results show that the buffer is able to significantly reduce the SU blocking probability while increasing its throughput.
    Wireless Communications and Networking Conference (WCNC), 2013 IEEE; 01/2013
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    ABSTRACT: In this paper, we consider a scenario in which two users communicate via a single access point with two buffers using network coding. We focus on the particular situation when there are no packets in one of the buffers for network coding to proceed. In this case, there is a trade-off between the delay due to waiting for a coding opportunity and the increased efficiency of spectrum access due to network coding. In order to analyze this situation, we develop an analytical model for the system using a discrete time Markov chain (DTMC). The packet arrivals for any general arrival distribution are modeled as a discrete time Markovian arrival process (DMAP). We then find the age distribution of the waiting packets and hence determine the waiting-time which achieves the optimal trades-off between spectrum access efficiency and packet delay.
    Computing, Networking and Communications (ICNC), 2013 International Conference on; 01/2013
  • S.C.A. Chu, A.S. Alfa, Jun Cai
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    ABSTRACT: In this paper, we investigate the impact of channels that have a bursty nature in a cognitive radio network scenario. Our goal is to design a general statistical model that can handle bursty primary user (PU) channel usage. The proposed model describes idle periods with a discrete platoon arrival process (PAP) and describes busy periods with a discrete phase type (PH) distribution. This channel model is referred to as a PAP-PH process. We further introduce a proactive access scheme as the potential application of the proposed channel model and use it to compare the performance of the proposed model, in terms of spectrum utilization and interference probability, with two traditionally encountered channel usage models, i.e., the geometrically distributed idle-busy period model and the phase type distributed idle-busy period model, under both bursty and non-bursty channel scenarios. Numerical results show that with the proposed model, the proactive access scheme can guarantee the interference threshold to the PU and can be used for both bursty and non-bursty spectrum use patterns.
    Wireless Communications and Mobile Computing Conference (IWCMC), 2013 9th International; 01/2013
  • B. Kasiri, Jun Cai, A.S. Alfa
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    ABSTRACT: In this paper, lightweight identity-based and certificate-based distributed key management algorithms are proposed for self-organized multi-channel cognitive radio networks. Both key management schemes use threshold secret sharing. In identity-based scheme, after system initialization, each cognitive radio sends its channel key request to distributed private key generators. Then, it can construct the channel key based on its received shares. In certificate-based scheme, channel certificate can be constructed by receiving certificate authority shares at the cognitive radio. Simulation results show that the proposed identity-based and certificate-based schemes significantly increase the average battery life of cognitive radios and decrease the traffic overhead in multi-channel cognitive radio networks.
    MILITARY COMMUNICATIONS CONFERENCE, 2012 - MILCOM 2012; 01/2012
  • S. Wijedasa, A.S. Alfa
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    ABSTRACT: To analyze scenarios like spectrum handoff in overlay cognitive radio networks we need to capture the status change of the target channels in a more realistic manner. Most of the existing cognitive radio channel models assume target channels to be busy only when occupied by primary users. However, in reality the channel could be occupied by another secondary user in absence of a primary user and hence seen as busy by a secondary user that senses the channel. Actual busy channel consists of either a primary or other secondary user activity as observed by a potential incoming secondary user to the channel. Here, we present a channel model that captures a more realistic behavior of channel occupancy.
    Communications (ICC), 2012 IEEE International Conference on; 01/2012
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    ABSTRACT: This paper investigates the performance of a wideband cognitive radio network where each cognitive user can assemble multiple primary channels. Two channel assembling schemes are considered: a constant channel assembling (CCA) and a variable channel assembling (VCA). In the variable channel assembling scheme, cognitive users assemble their channels on the basis of the number of detected residual channels that are unoccupied by primary users or cognitive users. The effects of imperfect spectrum sensing (with false alarms and misdetections) are taken into account and it is assumed that spectrum handover is implemented in the secondary network. These channel assembling schemes are analyzed by using Continuous-time Markov chains (CTMC), and the system performance is evaluated in terms of throughput, blocking probability, and forced termination probability. Numerical results show that channel assembling achieves lower forced termination probability, but does not increase achieved system throughput and leads to higher blocking probability. They also show that VCA outperforms CCA in terms of throughput and forced termination probability.
    Communications (ICC), 2012 IEEE International Conference on; 01/2012
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    T.M.N. Ngatched, A.S. Alfa, Jun Cai
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    ABSTRACT: A reliability-based iterative majority-logic decoding algorithm for regular low-density parity-check (LDPC) codes was recently proposed by Huang et al. In this paper we present an improved version of that algorithm by introducing a different reliability measure for each check-sum of the parity-check matrix, and taking it into account in the computation of the extrinsic information that is used to update the reliability measure of each received bit in each iteration. Some simulations results are given, which show that the new algorithm, while requiring very little additional computational complexity, not only achieves a considerable error performance gain over the standard one, but also, importantly, outperforms the iterative decoding based on belief propagation (IDBP), especially for short and medium block length finite-geometry (FG) LDPC codes.
    Communications (ICC), 2011 IEEE International Conference on; 07/2011
  • Source
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    ABSTRACT: In this paper, channel assignment in cooperative spectrum sensing is studied for multi-channel cognitive radio networks. Based on the information from each secondary user, e.g., primary signal-to-noise ratios over all channels, a centralized scheme is proposed, which assigns channels to different secondary users for sensing so that the number of available channels, which meet the sensing performance requirements in terms of miss detection and false alarm probabilities, can be greatly increased. By further taking the communication overhead into account, a greedy scheme is proposed to reduce the reporting information from the secondary users to the base station. The simulation results demonstrate that both schemes can significantly increase the number of available channels, while the latter also shows advantages in reducing the signaling overhead.
    Communications (ICC), 2011 IEEE International Conference on; 07/2011
  • Source
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    ABSTRACT: In this paper, a distributed cooperative multi-channel spectrum sensing scheme is proposed for the non-infrastructure based cognitive radio networks. The proposed scheme has iterative property and is carried out round-by-round. In each round, each secondary user selects a few primary channels as the candidates for sensing based on primary signal-to-noise ratio. Then, the users with the same selected channel collaboratively form coalitions through coalitional game and thereby multiple games are played concurrently over multiple channels. After generating stable coalitional structure, the best coalition on each channel is chosen to perform the cooperative spectrum sensing. The simulation results show that the proposed scheme can significantly increase the number of available channels, which can be sensed with predefined miss detection and false alarm probabilities.
    Global Telecommunications Conference (GLOBECOM 2010), 2010 IEEE; 01/2011
  • Source
    T.M.N. Ngatched, A.S. Alfa, J. Cai
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    ABSTRACT: This paper presents an improvement of the reliability-based iterative majority-logic decoding algorithms for regular low-density parity-check (LDPC) codes proposed by Huang et al. We improve the computation of the extrinsic information that is used to update the reliability measure of each received bit in each iteration with some kind of reliability measures of the check-sums that are orthogonal on the considered bit. The improved algorithm achieves a significant gain over the standard one with only a modest increase in computational complexity.
    Global Telecommunications Conference (GLOBECOM 2010), 2010 IEEE; 01/2011
  • T. M. N. Ngatched, Attahiru S. Alfa, Jun Cai
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    ABSTRACT: This paper considers a cognitive radio network where the cognitive user has a constant bandwidth which is K times that of a primary user. The primary system consists of constant M primary channels and the cognitive system consists of N = M/K channels, where x is the largest integer not greater than x. The effects of imperfect spectrum sensing (with false alarms and misdetections) are analyzed using a Markov chain. Explicit expressions for state dependent transition rates are derived for the case M =6 and K =2 ,a nd the system performance is evaluated in terms of throughput, blocking probability, and forced termination probability. I. INTRODUCTION
    01/2011;
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    ABSTRACT: This paper presents a hybrid decoding algorithm for low-density parity-check (LDPC) codes based on the interior point method with barrier function for linear programming (LP) decoding introduced by Wadayama . First, an efficient implementation of Wadayama's algorithm is presented. The main idea behind the modification is to approximate the barrier function for the fundamental polytope defining the code so that it contains only one linear constraint for each of the parity-check constraints. A two-stage hybrid decoding which combines the interior point decoding and a low-complexity decoding algorithm for LDPC codes is then proposed. Simulation results show that the approximations introduced in the proposed algorithms do not result in any performance degradation, while considerably reducing the decoding complexity and latency.
    IEEE Transactions on Communications 01/2011; 59:740-749. · 1.75 Impact Factor
  • Behzad Kasiri, Jun Cai, Attahiru Sule Alfa, Weiwei Wang
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    ABSTRACT: In this paper, a distributed cooperative attack on multi-channel cooperative spectrum sensing is proposed. In the proposed attack, attackers play two coalitional games to maximize the number of invaded channels in a distributed manner. In the first game, attackers play with their fellow ones to allocate an optimal number of attackers for each channel, while in the second game, they play in the coalitions with other honest cognitive radios so as to conquer as many channels as possible. Simulation results show that the proposed attack can considerably decrease the number of available channels with a low attack cost.
    Proceedings of the Global Communications Conference, GLOBECOM 2011, 5-9 December 2011, Houston, Texas, USA; 01/2011
  • Source
    Chamara Devanarayana, Attahiru Sule Alfa
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    ABSTRACT: The concept of CR enables the unlicensed users to share the spectrum with licensed users, on the condition that the licensed users have preemptive priority. The use of the channel by unlicensed users should not result in more than acceptable interference level to the licensed users, if interference occurs. The sense and react strategy by unlicensed users sometimes does not lead to acceptable level of interference while maintaining an acceptable data transfer rate for the unlicensed users. In this paper we introduce a predictive channel usage scheme which is capable of reducing the interference caused by the unlicensed users. Furthermore our scheme is capable of increasing the data rates the unlicensed users experience through the reduction of the idle channel identification delay. In our scheme no assumptions are made about the distribution of licensed user channel usage. We learn the traffic characteristics of the channels using a learning scheme called Probabilistic Suffix Tree (PST) algorithm.
    Proceedings of the Global Communications Conference, GLOBECOM 2011, 5-9 December 2011, Houston, Texas, USA; 01/2011

Publication Stats

1k Citations
119.44 Total Impact Points

Institutions

  • 1986–2014
    • University of Manitoba
      • Department of Electrical and Computer Engineering
      Winnipeg, Manitoba, Canada
  • 2010
    • Korea University
      • Department of Mathematics
      Seoul, Seoul, South Korea
  • 2008
    • University of British Columbia - Vancouver
      • Department of Electrical and Computer Engineering
      Vancouver, British Columbia, Canada
  • 2003–2008
    • Dalhousie University
      Halifax, Nova Scotia, Canada
    • Carleton University
      • School of Mathematical & Statistics
      Ottawa, Ontario, Canada
  • 2007
    • University of Waterloo
      • Department of Electrical & Computer Engineering
      Waterloo, Quebec, Canada
  • 2000–2007
    • University of Windsor
      • Department of Industrial and Manufacturing Systems Engineering
      Windsor, Ontario, Canada
    • University of Louisiana at Lafayette
      • Department of Electrical & Computer Engineering
      Lafayette, LA, United States
  • 2006
    • Columbia University
      • Department of Electrical Engineering
      New York City, NY, United States
  • 2005
    • University of Antwerp
      • Department of Mathematics and Computer Science
      Antwerpen, Flanders, Belgium
  • 1987–1995
    • The University of Winnipeg
      • Department of Mathematics and Statistics
      Winnipeg, Manitoba, Canada
  • 1993
    • University of Michigan-Flint
      Flint, Michigan, United States
  • 1992
    • Rensselaer Polytechnic Institute
      Troy, New York, United States