Attahiru Sule Alfa

University of Pretoria, Πρετόρια/Πόλη του Ακρωτηρίου, Gauteng, South Africa

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Publications (218)179.42 Total impact

  • Chamara N Devanarayana · Attahiru S Alfa
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    ABSTRACT: Inclusion of statistical knowledge of the primary user (PU) channel usage had shown to be beneficial in dynamic spectrum access. Motivated by this fact, this paper investigated the importance of collecting and using statistics on neighboring secondary users (SUs) in selecting channels in addition to the knowledge of PU channel usage. The paper assumed that PU traffic characteristics of the channels are included in the radio environment map in the form of probabilistic suffix trees, which is a sequence predictor based on Markov property. In the proposed method, an intelligent sequence hopping-based common control channel and a carrier sense multiple access (CSMA)/collision avoidance (CA)-based medium access control (MAC) protocol were introduced. As shown in the paper, selecting channels using statistics of both the neighboring SUs and PUs reduced the number of packet collisions compared to a scheme which only uses PU statistics. Furthermore, the simulation results showed that the scheme proposed had better throughput performance with respect to both the random channel selection scheme and the scheme which only uses PU statistics while having less training complexity.
    EURASIP Journal on Wireless Communications and Networking 12/2015; 2015(1). DOI:10.1186/s13638-015-0309-2 · 0.72 Impact Factor
  • Chamara N. Devanarayana · Attahiru S. Alfa
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    ABSTRACT: The issue of channel selection in a decentralized cognitive radio network having a spectrum broker is discussed in this paper. In the proposed method we take into account both the Primary user (PU) channel usage statistics and, Secondary user (SU) channel usage statistics as perceived by an SU of interest. In the marked Markov process based method proposed, each SU calculates the reward of using each channel and selects the channel set with the highest reward, which will in turn be used for proactive spectrum access and hand-off. We compare our scheme with two other schemes, where the channel set is randomly selected in one and selected based only on PU statistics in the other. By comparison of the simulation results, we show that our scheme causes less interference to the PUs and has comparable throughput performance which compensates for the increased computational complexity.
    Cognitive Cellular Systems (CCS), 2014 1st International Workshop on; 09/2015
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    ABSTRACT: We investigate optimal call admission control (CAC) policy for multicast streaming services (MSS) in 3rd generation (3G) and beyond wireless mobile networks. Several MSS sessions are supported simultaneously in a bandwidth-limited network. Active sessions are those that are currently serving some users, and inactive sessions are those that are currently not serving any users. An admission decision in MSS is required only when an inactive session is requested, unlike in unicasting. For this reason, if a user request for an inactive MSS session arrives, we should make an admission decision in anticipation of (i) the possible reward earned based on users served during a session active time generated by accepting it, and (ii) the influence of the session active time upon the future status of network bandwidth and admission decisions. Our objective is to determine when to admit or block a user asking an inactive MSS session to achieve the optimality in rewards. We formulate this problem as a semi-Markov decision process (SMDP), and a value iteration algorithm is used to obtain an optimal stationary deterministic policy. We also derive the user blocking probability of the optimal policy by analyzing an embedded Markov chain induced by it.
    IEEE Transactions on Communications 06/2015; 63(6):1-1. DOI:10.1109/TCOMM.2015.2421896 · 1.99 Impact Factor
  • Yalda Farazmand · Attahiru S. Alfa
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    ABSTRACT: In this paper, a game theoretic relay load balancing and power allocation scheme is proposed for downlink transmission in a decode-and-forward orthogonal frequency division multiple access-based cellular relay network. A system with a base station communicating with multiple users via multiple relays is considered. The relays have limited power, which must be divided among the users they support. In traditional scheme, each relay simply divides its transmit power equally among all its users. Moreover, each user selects the relay with the highest channel gain. In this work, we do not apply the traditional relay scheme. It is because the users are distributed randomly, and by applying the traditional relay selection scheme, it may happen that some relays have more users connected to them than other relays, which results in having unbalanced load among the relays. In order to avoid performance degradation, achieve relay load balancing, and maximize the total data rate of the network, a game theoretic approach is proposed, which efficiently assigns the users to relays. The power of each relay is wisely distributed among users by the efficient power allocation scheme. Simulation results indicate that the proposed game-based scheme can considerably improve the average sum-spectral efficiency. Moreover, it shows that by applying the game, users who can connect to uncongested relays join them as opposed to connecting to congested relays. Copyright © 2015 John Wiley & Sons, Ltd.
    Wireless Communications and Mobile Computing 05/2015; DOI:10.1002/wcm.2597 · 0.86 Impact Factor
  • Miaomiao Yu · Attahiru Sule Alfa
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    ABSTRACT: This paper presents a discrete-time single-server finite-buffer queue with Markovian arrival process and generally distributed batch-size-dependent service time. Given that infinite service time is not commonly encountered in practical situations, we suppose that the distribution of the service time has a finite support. Recently, a similar continuous-time system with Poisson input process was discussed by Banerjee and Gupta (2012). But unfortunately, their method is hard to apply in the analysis of discrete-time case with versatile Markovian point process due to the fact that the difference equation governing the boundary state probabilities is more complex than the continuous one. If we follow their ideas, we will eventually find that some important joint queue length distributions cannot be computed and thus some key performance measures cannot be derived. In this paper, replacing the finite support renewal distribution with an appropriate phase-type distribution, the joint state probabilities at various time epochs (arbitrary, pre-arrival and departure) have been obtained by using matrix analytic method and embedded Markov chain technique. Furthermore, UL-type RG-factorization is employed in numerical computation of block-structured Markov chains with finitely-many levels. Some numerical examples are presented to demonstrate the feasibility of the proposed algorithm for several service time distributions. Moreover, the impact of the correlation factor on loss probability and mean sojourn time are also investigated.
    European Journal of Operational Research 02/2015; 244(1). DOI:10.1016/j.ejor.2015.01.056 · 2.36 Impact Factor
  • Yalda Farazmand · Attahiru S. Alfa
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    ABSTRACT: In this paper, a framework for power allocation of downlink transmissions in orthogonal frequency division multiple access-based decode-and-forward cellular relay networks is investigated. We consider a system with a single base station communicating with multiple users assisted by multiple relays. The relays have limited power which must be divided among the users they support in order to maximize the data rate of the whole network. Advanced power allocation schemes are crucial for such networks. The optimal relay power allocation which maximizes the data rate is proposed as an upper bound, by finding the optimal power requirement for each user based on knapsack problem formulation. Then by considering the fairness, a new relay power allocation scheme, called weighted-based scheme, is proposed. Finally, an efficient power reallocation scheme is proposed to efficiently utilize the power and improve the data rate of the network. Simulation results demonstrate that the proposed power allocation schemes can significantly improve the data rate of the network compared to the traditional scheme.
    Journal of Communications and Networks 10/2014; 16(5):559-567. DOI:10.1109/JCN.2014.000094 · 1.01 Impact Factor
  • Kamal Darchini · Attahiru S. Alfa
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    ABSTRACT: Nanocommunications are communication techniques used in nanonetworks. Molecular communication is a type of nanocommunication which uses molecules to encode messages. In this paper, we consider a hybrid of molecular communication using microtubules and free diffusion in a bounded channel and show that in the proposed scenario the molecules can be kept in a desired region. This is a property needed in several applications. We use a Markov model to analyze molecular propagation in the channel. The Markov model is an approximation for Brownian motion and jump diffusion process, the two processes which explain molecular movement in the considered scenario. The scenario in this paper considers a two dimensional channel. Future work can generalize the model to three dimensions. We solve the Markov model using a matrix analytic method, and find the stationary probability distribution for final position of molecules. We will show that the probability distribution of final position of molecules is mostly concentrated in the region desired to keep the molecules in. Finally, the model is used to investigate performance of the system.
  • Behzad Kasiri · Jun Cai · Attahiru S. Alfa
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    ABSTRACT: In this paper, a secure scheme is purposed for coalitional game in multi-channel cognitive radio networks. First, 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. Then, a hierarchical ID-based key management scheme is proposed, where cognitive radios can only play on a certain number of requested channels and channel access for sensing is limited to the honest cognitive radios selected in the coalitional game. Simulation results show that the proposed attack can considerably decrease the number of potential channels with a low attack cost when security scheme is not considered, in addition to the energy efficiency of the proposed key management scheme. Finally, formal verification of the proposed key management scheme guarantees its immunity and effectiveness.
    Electronic Commerce Research 03/2014; 15(1):121-146. DOI:10.1007/s10660-014-9158-z · 0.97 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 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 · 2.54 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
  • Weiwei Wang · Jun Cai · Attahiru S. Alfa · Anthony C.K. Soong · Simin Li
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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 · 0.38 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
  • T.M.N. Ngatched · Shuo Dong · A.S. Alfa
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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
  • Y. Farazmand · A.S. Alfa
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    ABSTRACT: In this paper, a framework for power allocation is presented for downlink transmissions in an OFDMA-based cellular relay network. We consider a system with a single base station communicating with multiple users assisted by multiple relays. The relays have limited power which must be divided among the users they support in order to maximize the data rate of the whole network. By finding the optimal power requirement of each user, and based on knapsack problem the optimal power allocation which maximizes the data rate is proposed as an upper bound. Another power allocation scheme, weighted-based scheme, is proposed considering the fairness issue. Finally, to utilize the power wisely and improve the data rate of the network, an efficient power reallocation scheme is proposed. Simulation results demonstrate the efficacy of the proposed schemes.
    Consumer Communications and Networking Conference (CCNC), 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. Alvarenga 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
  • Yalda Farazmand · Attahiru S. Alfa
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    ABSTRACT: In this paper, a game theoretic power allocation and relay load balancing scheme for downlink transmission in an OFDMA-based cellular relay network is proposed. In the cell, a BS is located at cell center communicating with multiple users assisted by multiple relays. The relays adopt the decode-and-forward protocol and can cooperatively assist the transmission from BS to users. In order to assign the users to relays, achieve the load balancing among relays and distribute the power of relays among users while considering the total data rate of the network, a game theoretic scheme is introduced to solve the problem. Simulation results indicate that, the proposed game-based power allocation scheme can improve the average sum- spectral efficiency approximately 20% compared to the traditional scheme.
    Vehicular Technology Conference (VTC Fall), 2013 IEEE 78th; 01/2013

Publication Stats

2k Citations
179.42 Total Impact Points


  • 2015
    • University of Pretoria
      • Department of Electrical, Electronic and Computer Engineering
      Πρετόρια/Πόλη του Ακρωτηρίου, Gauteng, South Africa
  • 1986–2015
    • 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
  • 1992
    • University of Regina
      Regina, Saskatchewan, Canada