Majid Khabbazian

Secure Cooperative Sensing Techniques for Cognitive Radio Systems

The most important task for a cognitive radio (CR) system is to identify the primary licensed users over a wide range of spectrum. Cooperation among spectrum sensing devices has been shown to offer various benefits including decrease in sensitivity requirements of the individual sensing devices. However, it has been shown in the literature that the performance of cooperative sensing schemes can be severely degraded due to presence of malicious users sending false sensing data. In this paper, we present techniques to identify such malicious users and mitigate their harmful effect on the performance of the cooperative sensing system.

Severity analysis and countermeasure for the wormhole attack in wireless ad hoc networks

In this paper, we analyze the effect of the wormhole attack on shortest-path routing protocols for wireless ad hoc networks. Using analytical and simulation results, we show that a strategic placement of the wormhole when the nodes are uniformly distributed can disrupt/control on average 32% of all communications across the network. We also analyze a scenario in which several attackers make wormholes between each other and a case where two malicious nodes attack a target node in the network. We show how to evaluate the maximum effect of the wormhole attack on a given network topology. Then, we compute the maximum effect of the wormhole attack on grid topology networks and show that the attackers can disrupt/control around 40% to 50% of all communications when the wormhole is strategically placed in the network. Finally, to defend against the wormhole attack, we propose a timing-based countermeasure that avoids the deficiencies of existing timing-based solutions. Using the proposed countermeasure, the nodes do not need synchronized clocks, nor are they required to predict the sending time or to be capable of fast switching between the receive and send modes. Moreover, the nodes do not need one-to-one communication with all their neighbors and do not require to compute a signature while having to timestamp the message with its transmission time.

Efficient Broadcasting in Mobile Ad Hoc Networks

This paper presents two efficient flooding algorithms based on 1-hop neighbor information. In the first part of the paper, we consider sender-based flooding algorithms, specifically the algorithm proposed by Liu et al. In their paper, Liu et al. propose a sender-based flooding algorithm that can achieve local optimality by selecting the minimum number of forwarding nodes in the lowest computational time complexity O(n logn), where n is the number of neighbors. We show that this optimality only holds for a subclass of sender-based algorithms. We propose an efficient sender-based flooding algorithm based on 1-hop neighbor information that reduces the time complexity of computing forwarding nodes to O(n). In Lius algorithm, n nodes are selected to forward the message in the worst case, whereas in our proposed algorithm, the number of forwarding nodes in the worst case is 11. In the second part of the paper we propose a simple and highly efficient receiver-based flooding algorithm. When nodes are uniformly distributed, we prove that the probability of two neighbor nodes broadcasting the same messageneighbor nodes broadcasting the same message exponentially decreases when the distance between them decreases or when the node density increases. The analytical results are confirmed using simulation.

Localized Broadcasting with Guaranteed Delivery and Bounded Transmission Redundancy

The common belief is that localized broadcast algorithms are not able to guarantee both full delivery and a good bound on the number of transmissions. In this paper, we propose the first localized broadcast algorithm that guarantees full delivery and a constant approximation ratio to the minimum number of required transmissions in the worst case. The proposed broadcast algorithm is a self-pruning algorithm based on one round of information exchange. Using the proposed algorithm, each node determines its forwarding status in 0(DeltaG logDeltaG), where DeltaG is the maximum node degree of the network. By extending the proposed algorithm, we show that localized broadcast algorithms can achieve both full delivery and a constant approximation ratio to the optimum solution with message complexity O(N), where JV is the total number of nodes in the network and each message contains a constant number of bits. We also show how to save bandwidth by reducing the size of piggybacked information. Finally, we relax several system-model assumptions, or replace them with practical ones, in order to improve the practicality of the proposed broadcast algorithm.

MAC design for analog network coding

Most medium access control (MAC) mechanisms discard collided packets and consider interference harmful. Recent work on Analog Network Coding (ANC) suggests a different approach, in which multiple interfering transmissions are strategically scheduled. Receiving nodes collect the results of collisions and then use a decoding process, such as ZigZag decoding, to extract the packets involved in the collisions. In this paper, we present an algebraic representation of collisions and describe a general approach to recovering collisions using ANC. To study the effects of using ANC on the performance of MAC layers, we develop an ANC-based MAC algorithm, CMAC, and analyze its performance in terms of probabilistic latency guarantees for local packet delivery. Specifically, we prove that CMAC implements an abstract MAC layer service, as defined in [14, 13]. This study shows that ANC can significantly improve the performance of the abstract MAC layer service compared to conventional probabilistic transmission approaches. We illustrate how this improvement in the MAC layer can translate into faster higher-level algorithms, by analyzing the time complexity of a multi-message network-wide broadcast algorithm that uses CMAC.

An Efficient Binary Locally Repairable Code for Hadoop Distributed File System

In the Hadoop distributed file systems (HDFSs), to lower costly communication traffic for data recovery, the concept of locally repairable codes (LRCs) has been recently proposed. With regard to the immense size of modern energy-hungry HDFS, computational complexity reduction can be attractive. In this letter, to avoid finite field multiplications, which are the major source of complexity, we put forward the idea of designing binary locally repairable codes (BLRCs). More specifically, we design a BLRC with a length of 15, rate of 2/3, and minimum distance of 4, which has the minimum possible locality among its type. We show that our code has lower complexity than most recent non-binary LRC in the literature while meeting other desirable requirements in HDFS such as storage overhead and reliability.

Time-efficient randomized multiple-message broadcast in radio networks

Multiple-message broadcast, or k-broadcast, is one of the fundamental problems in network communication. In short, there are k packets distributed across the network, each of them has to be delivered to all other nodes. We consider this task in the model of multi-hop radio network, in which n nodes interact by transmitting and receiving messages. A message transmitted at a round reaches all neighbors of the transmitter at the end of the same round, but may not be successfully received by some, or even all, of these neighbors. More specifically, a node receives a message at a round if this is the only message that has reached this node in this round. Due to this specific interference-prone nature of radio networks, many communication tasks become more challenging and more costly than in other types of networks, especially in ad-hoc setting in which each node knows only its own id and linear estimates on the basic network parameters, such as the number of nodes n, diameter D and maximum node degree Δ. We design a new randomized k-broadcast algorithm combining the bestof two worlds: efficient randomized transmission schedules and network coding. We show that our algorithm accomplishes multi-broadcast in O(log Δ) amortized number of communication rounds per packet, with high probability. This improves over the best previous solution of Bar-Yehuda, Israeli and Itai, which guarantees only O(log Δ log n) of amortized number of rounds per packet, with high probability.

Some research issues in cognitive radio networks

The cognitive radio (CR) technology will allow a group of potential users to identify and access available spectrum resources provided that the interference to the users for whom the band has been licensed is kept below a prescribed level. However, this research area is at a very immature stage because various research challenges have to be addressed and solved. In this paper our objective is to present an overview of some research issues for CR networks. Specifically, we present some research and development in CR networks with focus on i) information- theoretic aspects, ii) spectrum sensing, iii) link adaptation, iv) advanced transceiver design, and v) admission control. We also discuss some important research problems related to these specific topics that needs to be addressed before deployment of CR systems in practice.

Leveraging channel diversity to gain efficiency and robustness for wireless broadcast

This paper addresses two primary questions: (i) How much faster can we disseminate information in a large wireless network if we have multiple communication channels available (as compared to relying on only a single communication channel)? (ii) Can we still disseminate information reliably, even if some subset of the channels are disrupted? In answer to the first question, we reduce the cost of broadcast to O(log log n) rounds/hop, approximately, for sufficiently many channels. We answer the second question in the affirmative, presenting two different algorithms, while at the same time proving a lower bound showing that disrupted channels have unavoidable costs.

A Class of Binary Locally Repairable Codes

An (n, k) erasure code that can recover any coded symbol by at most r other coded symbols is called a locally repairable code (LRC) with locality r. LRCs have been recently implemented in distributed storage systems. Coding complexity reduction can be significantly decreased by using binary LRCs (BLRCs) as they eliminate costly multiplication calculation. In this paper, motivated by the recently erasure codes with d = 4 used in practice, we propose BLRCs when (r + 1) | n and d = 4. We prove that our proposed binary codes are optimal for r ∈ {1, 3}, meaning that neither their locality nor their minimum distance can be improved by non-binary codes. For r ≥ 4, our proposed binary codes offer near-optimal code rate, with a rate gap of O(log r/n) compared with optimal nonbinary codes. While keeping the bulk of code structure binary, we eliminate this rate gap by using fields with sizes as small as r + 2 for only two redundant symbols. These non-binary codes still eliminate the need for costly multiplications in many operations including a single failure repair (a dominant repair scenario). Using the construction of spanning BLRC with d = 4 as a backbone, we also construct LRCs with minimum distance d ≥ 6. Furthermore, we obtain a closed-form equation for the mean-time to data-loss of arbitrary erasure codes.