Attahiru Sule Alfa

University of Manitoba, Winnipeg, Manitoba, Canada

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Publications (207)167.01 Total impact

  • Kamal Darchini, Attahiru S. Alfa
  • 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 03/2014; 42(2). DOI:10.1016/j.orl.2014.01.005 · 0.62 Impact Factor
  • 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 03/2014; 13(3):497-511. DOI:10.1109/TMC.2012.254 · 2.91 Impact Factor
  • Ahmed Ibrahim, Attahiru S. Alfa
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    ABSTRACT: In this paper, we study radio resource allocation (RRA) for multicasting in OFDMA based High Altitude Platforms (HAPs). We formulate an optimization problem for a scenario in which different sessions are multicasted to user terminals (UTs) across HAP service area. We then solve it to find the best allocation of HAP resources such as radio power, sub-channels, and time slots. The objective is to maximize the number of UTs that receive the requested multicast streams in the HAP service area in a given OFDMA frame. The optimization problem comes out to be a Mixed Integer Non-Linear Program (MINLP). Due to the high complexity of the problem and lack of special structures, we believe that breaking it into two easier subproblems and iterating between them to achieve convergence can lead to an acceptable solution. Subproblem 1 turns out to be a Binary Integer Linear Program (BILP) of no explicitly noticeable structure and therefore Lagrangian relaxation is used to dualize some constraints to get a BILP with some special structure that is easy to solve. The subgradient method is used to solve for the dual variables in the dual problem for three proposed methods to get the tightest bound in each. The obtained bounds can be used in a branch and bound (BnB) algorithm as its bounding subroutine at each node. Subproblem 2 turns out to be a simple linear program (LP) for which the simplex algorithm can be used to solve the subproblem to optimality. This paper focuses on subproblem 1 and its proposed solution techniques. In the results section of this paper, we compare the solution goodness for each method versus the well known bounding technique used in BnB which is linear program (LP) relaxation.
    2013 IEEE Globecom Workshops (GC Wkshps); 12/2013
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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). DOI:10.1002/wcm.1178 · 0.86 Impact Factor
  • Source
    Jin-ting Wang, Nan Wang, Attahiru S. Alfa
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    ABSTRACT: A discrete-time GI/G/1 retrial queue with Bernoulli retrials and time-controlled vacation policies is investigated in this paper. By representing the inter-arrival, service and vacation times using a Markov-based approach, we are able to analyze this model as a level-dependent quasi-birth-and-death (LDQBD) process which makes the model algorithmically tractable. Several performance measures such as the stationary probability distribution and the expected number of customers in the orbit have been discussed with two different policies: deterministic time-controlled system and random time-controlled system. To give a comparison with the known vacation policy in the literature, we present the exhaustive vacation policy as a contrast between these policies under the early arrival system (EAS) and the late arrival system with delayed access (LAS-DA). Significant difference between EAS and LAS-DA is illustrated by some numerical examples.
    Acta Mathematicae Applicatae Sinica 10/2013; 29(4). DOI:10.1007/s10255-013-0244-0 · 0.39 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. DOI:10.1016/j.nancom.2013.04.001
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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.
  • 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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    ABSTRACT: A new channel model for binary additive noise communication channel with memory, called weighted queue-based channel (WQBC), is introduced. The proposed WQBC generalizes the conventional queue-based channel (QBC) such that each queue cell has a different contribution to the noise process, i.e. the queue cells are selected with different probabilities. Suitably selecting the modeling function, the generalization introduced by the WQBC does not increase the number of modelling parameters required compared to the QBC. The statistical and information-theoretical properties of the new model are derived. The WQBC and the QBC are compared in terms of capacity and the accuracy in modeling a family of hard decision frequency-shift keying demodulated correlated Rayleigh and Rician fading channels. It is observed that the WQBC requires a much smaller Markovian memory than the QBC to achieve the same capacity, and provides a very good approximation of the fading channels as the QBC for a wide range of channel conditions.
    IEEE Transactions on Communications 11/2011; 59(11):3049-3058. DOI:10.1109/TCOMM.2011.091911.100007A · 1.98 Impact Factor
  • Attahiru Sule Alfa, Haitham Abu Ghazaleh
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    ABSTRACT: We develop an approach for computing the goodput using the idea of terminating Markovian Arrival Process (tMAP) for a finite queueing system in discrete-time with Platoon Arrival Process (PAP). A platoon is said to have been successfully transmitted if all of its packets have been admitted into the queue without any loss. Otherwise, the transmitted packets from the unsuccessful platoon would constitute wasted work. The throughput rate of the non-wasted work is called “goodput.” We apply the idea to modeling a queueing system with packet discarding policies in high speed networks—a system of considerable interest in networking.
    Stochastic Models 10/2011; 27(4):615-628. DOI:10.1080/15326349.2011.614187 · 0.50 Impact Factor
  • Jungong Xue, Attahiru Sule Alfa
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    ABSTRACT: We consider a MAP/PH/1 queue with two priority classes and nonpreemptive discipline, focusing on the asymptotic behavior of the tail probability of queue length of low-priority customers. A sufficient condition under which this tail probability decays asymptotically geometrically is derived. Numerical methods are presented to verify this sufficient condition and to compute the decay rate of the tail probability.
    Queueing Systems 09/2011; 69(1):45-76. DOI:10.1007/s11134-011-9221-6 · 0.60 Impact Factor
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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
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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
  • Yun Han Bae, Attahiru Sule Alfa, Bong Dae Choi
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    ABSTRACT: We investigate a distributed contention-based spectrum access scheme in cognitive radio networks where ON/OFF periods of the channel by primary users follow discrete phase (PH) type distributions. The main motivation for ON/OFF having PH distributions is that the channel activity has a more general behavior depending on the primary users’ traffic. In the past most other researchers assumed that ON/OFF periods of a channel follow a geometric distribution for the purpose of mathematical tractability even though this assumption is restrictive.We propose a distributed medium access control (MAC) scheme for the secondary users (SUs) which is characterized by a constant contention window size and a method to decide whether for each SU to participate in competition or not depending on the queueing delay of a head-of-line (HoL) packet. In order to investigate the performance of our proposed MAC protocol, we construct a two-dimensional Markov chain which incorporates both the proposed MAC scheme and the general channel activity. The resulting one-step transition probability matrix of the Markov chain has a very special structure. With the help of the censored Markov chain method, we provide a computationally efficient method to obtain the stationary distribution of the Markov chain. We then obtain the system capacity, which is defined as the maximum number of SUs that can be accommodated with a quality of service (QoS) guarantee on the packet dropping probability and the packet delay. Numerical examples show that the system capacity considerably depends on the distributions of ON/OFF periods and our proposed MAC scheme achieves a higher capacity than the existing one.
    Performance Evaluation 03/2011; 68:271-289. DOI:10.1016/j.peva.2010.12.003 · 0.89 Impact Factor

Publication Stats

2k Citations
167.01 Total Impact Points


  • 1986–2014
    • University of Manitoba
      • • Department of Electrical and Computer Engineering
      • • Department of Civil Engineering
      Winnipeg, Manitoba, Canada
  • 1987–2011
    • The University of Winnipeg
      • Department of Mathematics and Statistics
      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
  • 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
  • 2003
    • Carleton University
      • School of Mathematical & Statistics
      Ottawa, Ontario, Canada
  • 1995
    • The University of Arizona
      Tucson, Arizona, United States
  • 1993
    • University of Michigan-Flint
      Flint, Michigan, United States
  • 1992
    • University of Regina
      Regina, Saskatchewan, Canada