Attahiru Sule Alfa

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

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Publications (228)188.31 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.
    No preview · Article · Dec 2015 · EURASIP Journal on Wireless Communications and Networking
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    ABSTRACT: Sensing is one of the most challenging issues in cognitive radio networks. Selection of sensing parameters raises several tradeoffs between spectral efficiency, energy efficiency and interference caused to primary users (PUs). In this paper we provide representative mathematical models that can be used to analyze sensing strategies under a wide range of conditions. The activity of PUs in a licensed channel is modeled as a sequence of busy and idle periods, which is represented as an alternating Markov phase renewal process. The representation of the secondary users (SUs) behavior is also largely general: the duration of transmissions, sensing periods and the intervals between consecutive sensing periods are modeled by phase type distributions, which constitute a very versatile class of distributions. Expressions for several key performance measures in cognitive radio networks are obtained from the analysis of the model. Most notably, we derive the distribution of the length of an effective white space; the distributions of the waiting times until the SU transmits a given amount of data, through several transmission epochs uninterruptedly; and the goodput when an interrupted SU transmission has to be restarted from the beginning due to the presence of a PU.
    No preview · Article · Dec 2015
  • Q.-M. He · Attahiru Sule Alfa
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    ABSTRACT: Two GI/M/1 type Markov chains associated with the queue length are often used in analyzing the discrete time MAP/PH/K queue. The first Markov chain is introduced by tracking service phases for servers; a method we call TPFS. The transition probability matrix of the Markov chain can be constructed in a straightforward manner. The second Markov chain is introduced by counting servers for phases; which we call CSFP. An algorithm is developed for the construction of the transition probability matrix of the second Markov chain, which is the main contribution of this paper. Whereas the construction of the matrices for the case of continuous time is available in the literature, it is not available for the discrete time case. The effort in constructing the matrices for the discrete time case is extensively more involved than for the continuous time case. Some basic properties of the constructed transition blocks are shown. We demonstrate that for queueing systems with a large number of servers and many service phases, there is a considerable saving in the matrix sizes. For example, when those values are 30 and 2, respectively, the block size for TPFS is more than 3×107 times that of CSFP; a major saving.
    No preview · Article · Nov 2015 · Performance Evaluation
  • Kamal Darchini · Attahiru S Alfa
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    ABSTRACT: Molecular communication is a nanoscale communication method in which information is encoded in molecules. Molecular communication using microtubules in free space is one of the realistic scenarios proposed for this type of nanocommunication. Based on this technique, molecular communication via filaments using nano-relays is proposed in this paper to improve the performance of the system in terms of delay and bit error probability. An analytical model using jump diffusion processes is proposed for molecular propagation in this scenario. It is shown that the proposed mathematical model is capable of tracking molecular propagation in molecular communication via filaments using nano-relays. Also, the model is used to investigate effects of different parameters on delay and bit error probability.
    No preview · Article · Nov 2015 · IEEE transactions on nanobioscience
  • Ahmed Ibrahim · Attahiru S. Alfa
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    ABSTRACT: In this paper, we study radio resource allocation for multicasting in OFDMA based High Altitude Platforms (HAPs). We formulate and solve an optimization problem that finds the best allocation of HAP resources such as radio power, subchannels, and time slots. The problem also finds the best possible frequency reuse across the cells that constitute the service area of the HAP. The objective is to maximize the number of user terminals that receive the requested multicast streams in the HAP service area in a given OFDMA frame. A bounding subroutine in a branch and bound algorithm can be obtained by decomposing it into two easier subproblems, due to its high complexity, and solving them iteratively. Subproblem 1 turns out to be a Binary Integer Linear Program of no explicitly noticeable structure and therefore Lagrangian relaxation is used to dualize some constraints to get a structure that is easy to solve. Subproblem 2 turns out to be a linear program with a Continuous Knapsack problem structure. Hence a greedy algorithm is proposed to solve subproblem 2 to optimality. The subgradient method is used to solve for the dual variables in the dual problem to get the tightest bounds.
    No preview · Article · Oct 2015 · IEEE Transactions on Wireless Communications
  • 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.
    No preview · Conference Paper · Sep 2015
  • Miaomiao Yu · Attahiru Sule Alfa
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    ABSTRACT: The prime objective of this paper is to give some analysis results concerning the discrete-time finite-buffer NT-policy queue, which can be utilized to determine the optimal threshold values. By recording the waiting time of the leading customer in server’s vacation period, the model is successfully described as a vector-valued Markov chain. Meanwhile, depending on the special block structure of the one-step transition probability matrix, the equilibrium queue length distribution is calculated through a more effective UL-type RG-factorization. Due to the number of customers served in the busy period does not have the structure of a Galton-Watson branching process, analysis of the regeneration cycle is regarded as a difficult problem in establishing the cost structure of the queueing system. However, employing the concept of i-busy period and some difference equation solving skills, the explicit expression for the expected length of the regeneration cycle is easily derived, and the stochastic decomposition structure of the busy period is also demonstrated. Finally, numerical results are offered to illustrate how the direct search method can be implemented to obtain the optimal management policy.
    No preview · Article · Jul 2015 · Operational Research
  • Miaomiao Yu · Attahiru Sule Alfa
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    ABSTRACT: In this paper, some tips and tricks for algebraic manipulations are utilized to explicitly get the mean and variance of the duration of the busy period in a discrete-time finite-buffer vacation queue. Applying the law of total expectation, the closed-form expressions for the first two moments of the busy period initiated with an arbitrary number of customers are firstly derived. Then, by employing the queue length distribution at vacation termination and the quantities that mentioned above, we give the stochastic decomposition structure of the busy period. Finally, in order to ensure the reliability of the analytical approach, an effective way to validate the correctness of our results along with a numerical example is also provided. We may find that these simple tips and tricks can greatly reduce the difficulty of problem solving.
    No preview · Article · Jul 2015 · 4OR quarterly journal of the Belgian, French and Italian Operations Research Societies
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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.
    No preview · Article · Jun 2015 · IEEE Transactions on Communications
  • 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.
    No preview · Article · May 2015 · Wireless Communications and Mobile Computing
  • 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.
    No preview · Article · Feb 2015 · European Journal of Operational Research
  • 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.
    No preview · Article · Oct 2014 · Journal of Communications and Networks
  • 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.
    No preview · Article · May 2014
  • 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.
    No preview · Article · Mar 2014 · Electronic Commerce Research
  • 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.
    No preview · Article · Mar 2014 · Operations Research Letters
  • 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.
    No preview · Article · Mar 2014 · IEEE Transactions on Mobile Computing
  • 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.
    No preview · Conference Paper · Dec 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.
    No preview · Article · Oct 2013 · Wireless Communications and Mobile Computing
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    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.
    Full-text · Article · Oct 2013 · Acta Mathematicae Applicatae Sinica
  • 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.
    No preview · Conference Paper · Sep 2013

Publication Stats

2k Citations
188.31 Total Impact Points

Institutions

  • 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
  • 2013
    • Memorial University of Newfoundland
      Saint John's, Newfoundland and Labrador, Canada
  • 1987-2011
    • The University of Winnipeg
      • Department of Mathematics and Statistics
      Winnipeg, Manitoba, Canada
  • 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
  • 1992
    • University of Regina
      Regina, Saskatchewan, Canada