Mohammad J. Abdel-Rahman

Game-theoretic quorum-based frequency hopping for anti-jamming rendezvous in DSA networks

Establishing communications in a dynamic spectrum access (DSA) network requires the communicating parties to “rendezvous” before transmitting their data packets. Frequency hopping (FH) is an effective rendezvous method that does not rely on a predetermined control channel. Recently, “quorum-based” FH approaches have been proposed for asynchronous rendezvous in DSA networks. These approaches are highly vulnerable to jamming, especially when the attacker is an insider node (i.e., a compromised node). In this paper, we investigate the problem of two secondary users (SUs), a transmitter and a receiver, try to rendezvous in the presence of a third SU acting as a jammer. The jammer is aware of the underlying (quorum-based) rendezvous design. First, we consider the case when all SUs are time-synchronized and are aware of the “rendezvous channel.” We formulate the problem as a three-player game between the transmitter, receiver, and jammer. The transmitter and receiver try to maximize the number of successful rendezvous slots, while minimizing the number of jammed rendezvous slots. The jammer has the opposite objective. We show that this game does not have a pure Nash equilibrium (NE). Accordingly, we formulate a simplified two-player game between the receiver and jammer (assuming a uniform strategy by the transmitter), and derive multiple pure NE strategies. Next, we study the case when the rendezvous channel is unknown and obtain the Bayesian NE. Finally, the asynchronous case is addressed by exploiting the “rotation closure property” of quorum systems. Our numerical experiments show that uncertainty about the transmitters strategy improves the anti-jamming rendezvous performance. They also show that the rendezvous performance improves if the receiver and jammer are time-synchronized, and also improves if the receiver and jammer have a common guess about the transmitters strategy.

Stochastic Guard-Band-Aware Channel Assignment With Bonding and Aggregation for DSA Networks

Fading and shadowing along with the primary user dynamics make channel quality in dynamic spectrum access networks uncertain. Furthermore, the imperfect design of filters and amplifiers in wireless devices motivates the need for guard-bands (GBs) to prevent adjacent-channel interference. In this paper, we develop novel stochastic GB-aware sequential and batch channel assignment schemes that aim at maximizing the spectrum efficiency. In line with recent IEEE 802.11 and LTE standards, our schemes support bonding and aggregation. We propose two assignment models for each of the sequential and batch schemes: a static single-stage and an adaptive two-stage. In the static model, channel assignment is performed once such that the rate demands are probabilistically met. The adaptive model is a two-stage model, where the initial assignment may be corrected once uncertainties are partially revealed. We refer to our formulations of the sequential and batch static assignments as chance-constrained stochastic subset-s um problem (CSSP) and chance-constrained stochastic multiple s ubset-sum problem (CMSSP), respectively. Moreover, we develop stochastic formulations for the sequential and batch adaptive assignments, which we refer to as two-stage CSSP with recourse (CSSPR) and two-stage CMSSP with recourse (CMSSPR), respectively. Finally, we present computationally efficient simplified versions of CSSP and CSSPR with near-optimal performance.

Joint Adaptation of Frequency Hopping and Transmission Rate for Anti-Jamming Wireless Systems

Wireless transmissions are inherently vulnerable to jamming attacks. Frequency hopping (FH) and transmission rate adaptation (RA) have been separately used to mitigate jamming. When RA is used alone, it has been shown that a jammer who randomizes its power levels can force the transmitter to always operate at the lowest rate, by maintaining the average jamming power above a certain threshold. On the other hand, when only FH is used, a high throughput overhead is incurred due to frequent channel switching. In this paper, we propose to mitigate jamming by jointly optimizing the FH and RA techniques. This way, the transmitter can escape the jammer by changing its channel, adjusting its rate, or both. We consider a power-constrained “reactive-sweep” jammer who aims at degrading the throughput of the wireless link. The jammer sweeps through the set of channels, jamming a subset of them at a time, using the optimal jamming power. We model the interactions between the legitimate transmitter and jammer as a constrained zero-sum Markov game. The transmitters optimal defense strategy is derived by obtaining the equilibria of the constrained Markov game. This policy informs the transmitter when to hop to another channel and when to stay on the current channel. Furthermore, it gives the best transmission rate to use in both cases (hop or stay). The structure of the transmitters optimal policy is shown to be threshold type, whereby the transmitter stays on the same channel up to a certain number of time slots after which it hops. We analyze the “constrained Nash equilibrium” of the Markov game and show that the equilibrium defense strategy of the transmitter is deterministic. Numerical investigations show that the new scheme improves the average throughput and provides better jamming resiliency.

Game theoretic anti-jamming dynamic frequency hopping and rate adaptation in wireless systems

Wireless transmissions are inherently broadcast and are vulnerable to jamming attacks. Frequency hopping (FH) and transmission rate adaptation (RA) have been used to mitigate jamming. However, recent works have shown that using either FH or RA (but not both) is inefficient against smart jamming. In this paper, we propose mitigating jamming by jointly optimizing the FH and RA techniques. We consider a power constrained “reactive-sweep” jammer who aims at degrading the goodput of a wireless link. We model the interaction between the legitimate transmitter and jammer as a zero-sum Markov game, and derive the optimal defense strategy. Numerical investigations show that the new scheme improves the average goodput and provides better jamming resiliency.

Optimal channel assignment with aggregation in multi-channel systems: A resilient approach to adjacent-channel interference☆

Abstract Channel assignment mechanisms in multi-channel wireless networks are often designed without accounting for adjacent-channel interference (ACI). To prevent such interference between different users in a network, guard-bands (GBs) are needed. Introducing GBs has significant impact on spectrum efficiency. In this paper, we present channel assignment mechanisms that aim at maximizing the spectrum efficiency. More specifically, these mechanisms attempt to minimize the amount of additional GB-related spectrum that is needed to accommodate a new link. Similar to the IEEE 802.11n and the upcoming IEEE 802.11ac standards, our assignment mechanisms support channel bonding, and more generally, channel aggregation. We first consider sequential assignment (i.e., one link at a time), and we formulate the optimal ACI-aware channel assignment that maximizes the spectrum efficiency as a subset-sum problem. An exact exponential-time dynamic programming (DP) algorithm, a polynomial-time greedy heuristic, and an ∊ -approximation are presented and compared. Second, considering a set of links (batch assignment), we derive the optimal ACI-aware exponential-time assignment that maximizes the network’s spectrum efficiency. The optimal batch assignment is compared with the sequential assignment. Results reveal that our proposed algorithms achieve considerable improvement in spectrum efficiency compared to previously proposed schemes.

Fast and secure rendezvous protocols for mitigating control channel DoS attacks

The operation of a wireless network relies extensively on exchanging messages over a universally known channel, referred to as the control channel. The network performance can be severely degraded if a jammer launches a denial-of-service (DoS) attack on such a channel. In this paper, we design quorum-based frequency hopping (FH) algorithms that mitigate DoS attacks on the control channel of an asynchronous ad hoc network. Our algorithms can establish unicast as well as multicast communications under DoS attacks. They are fully distributed, do not incur any additional message exchange overhead, and can work in the absence of node synchronization. Furthermore, the multicast algorithms maintain the multicast group consistency. The efficiency of our algorithms is shown by analysis and simulations.

Multicast Rendezvous in Fast-Varying DSA Networks

Establishing communications between devices in a dynamic spectrum access (DSA) system requires the communicating parties to “rendezvous” before transmitting data packets. Frequency hopping (FH) is an effective rendezvous method that does not rely on a predetermined control channel. Previous FH-based rendezvous designs mainly target unicast rendezvous, and do not intrinsically support multicast rendezvous, where a group of nodes need to rendezvous simultaneously. Furthermore, these designs do not account for fast-primary user (PU) dynamics, leading to long time-to-rendezvous (TTR). In this paper, we exploit the uniform k-arbiter and Chinese Remainder Theorem quorum systems to develop three FH-based multicast rendezvous algorithms, which provide different tradeoffs between rendezvous efficiency (e.g., low TTR) and security (e.g., robustness to node compromise). Our rendezvous algorithms are tailored for asynchronous and spectrum-heterogeneous DSA systems. To account for fast PU dynamics, we develop an algorithm for adapting the proposed FH designs on the fly. This adaptation is done through efficient mechanisms for channel ordering and quorum selection. Our simulations validate the effectiveness of the proposed rendezvous algorithms, their PU detection accuracy, and their robustness to insider attacks.

Spectrum-efficient stochastic channel assignment for opportunistic networks

The uncertainty in channel quality due to fading and shadowing along with the unpredictability of primary user (PU) activity make channel assignment in opportunistic spectrum access (OSA) networks quite challenging. In this paper, we propose two per-link channel assignment models under channel uncertainty: a static single-stage and an adaptive two-stage. In the static model, channel assignment is performed once, such that the rate demands are met with a probability greater than a certain threshold. This model is appropriate for a distributed network with no centralized spectrum manager. The adaptive model is a two-stage assignment model, where the initial assignment may be corrected once the uncertainties are partially revealed, such that the excess spectrum is returned back to the spectrum manager. This adaptive model is more appropriate when a centralized spectrum manager is available. Our channel assignment algorithms account for adjacent channel interference (ACI) by introducing guard-bands between adjacent channels that are assigned to different links. These algorithms aim at maximizing the spectral efficiency, considering the impact of guard-bands. The static ACI-aware channel assignment problem is formulated as a chance-constrained stochastic subset-sum problem (CSSP), and the adaptive assignment problem is formulated as a two-stage chance-constrained stochastic subset-sum problem with recourse (CSSPR). We develop heuristic algorithms for both models and test their performance. Preliminary results demonstrate that the proposed heuristic algorithms are highly efficient.

Exploiting cognitive radios for reliable satellite communications

Summary Satellite transmissions are prone to both unintentional and intentional RF interference. Such interference has significant impact on the reliability of packet transmissions. In this paper, we make preliminary steps at exploiting the sensing capabilities of cognitive radios for reliable satellite communications. We propose the use of dynamically adjusted frequency hopping (FH) sequences for satellite transmissions. Such sequences are more robust against targeted interference than fixed FH sequences. In our design, the FH sequence is adjusted according to the outcome of out-of-band proactive sensing, carried out by a cognitive radio module that resides in the receiver of the satellite link. Our design, called out-of-band sensing-based dynamic FH, is first analyzed using a discrete-time Markov chain (DTMC) framework. The transition probabilities of the DTMC are then used to measure the ‘channel stability’, a metric that reflects the freshness of sensed channel interference. Next, out-of-band sensing-based dynamic FH is analyzed following a continuous-time Markov chain model, and a numerical procedure for determining the ‘optimal’ total sensing time that minimizes the probability of ‘black holes’ is provided. DTMC is appropriate for systems with continuously adjustable power levels; otherwise, continuous-time Markov chain is the suitable model. We use simulations to study the effects of different system parameters on the performance of our proposed design. Copyright

Dimensioning virtualized wireless access networks from a common pool of resources

Resource sharing in mobile wireless networks has been employed to reduce costs, extend coverage, and ease the entry of new players in the market. The introduction of programmability and virtualization is expected to amplify these benefits of resource sharing. In this paper, we study a new virtualization-based paradigm for resource sharing in mobile wireless networks. Specifically, we consider the problem of resource allocation, particularly when user demands are uncertain. We formulate several two-stage sequential stochastic allocation schemes that provide tradeoffs between cost and user satisfaction. These allocation schemes are studied under different resource provider pricing models. Our simulations demonstrate that: First, while reducing cost significantly, virtualization considerably improves user satisfaction, and virtualization gains increase with the number of operators that share resources. Second, the improvements in cost, user satisfaction, and resource usage increase substantially with the level of user clustering.