Protecting the network layer from malicious attacks is an important and challenging security issue in mobile ad hoc networks (MANETs). In this paper, a security mechanism is proposed to defend against a cooperative gray hole attack on the well known AODV routing protocol in MANETs. A gray hole is a node that selectively drops and forwards data packets after it advertises itself as having the shortest path to the destination node in response to a route request message from a source node. The proposed mechanism does not apply any cryptographic primitives on the routing messages. Instead, it protects the network by detecting and reacting to malicious activities of any node. Simulation results show that the scheme has a significantly high detection rate with moderate network traffic overhead.

In multi-hop mobile ad hoc networks (MANETs), mobile nodes cooperate with each other without using any infrastructure such as access points or base stations. Security remains a major challenge for these networks due to their features of open medium, dynamically changing topologies, reliance on cooperative algorithms, absence of centralized monitoring points, and lack of clear lines of defense. Among the various attacks to which MANETs are vulnerable, malicious packet dropping attack is very common where a malicious node can partially degrade or completely disrupt communication in the network by consistently dropping packets. In this paper, a mechanism for detection of packet dropping attack is presented based on cooperative participation of the nodes in a MANET. The redundancy of routing information in an ad hoc network is utilized to make the scheme robust so that it works effectively even in presence of transient network partitioning and Byzantine failure of nodes. The proposed scheme is fully cooperative and thus more secure as the vulnerabilities of any election algorithm used for choosing a subset of nodes for cooperation are absent. Simulation results show the effectiveness of the protocol.Index Termsdistributed algorithm, mobile ad hoc network, packet dropping attack, routing security.

Recently the use of multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems has been proposed for signaling over frequency-selective fading channels. Although various aspects of these systems have been considered in the literature, the problem of the inherent high peak-to-average power ratio (PAPR) is not examined. In this paper, we consider PAPR reduction for MIMO-OFDM systems and propose alternate low-complexity algorithms that can be used in conjunction with the trellis shaping method. We show that a PAPR reduction in the order of 4-5 dB can be achieved at the cost of a slight reduction in the spectral efficiency. Furthermore, we compare the trellis shaping technique with other PAPR reduction techniques such as tone reservation and partial transmit sequences.

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