Cooperative diversity avails MIMO-class benefits (e.g., improved BER, outage behavior) to single-antenna wireless devices through cooperative relaying. The MIMO-class benefit is enabled through cooperative relaying where a single-antenna device act as a relay to enhance communication between its partner and a destination. The performance enhancements are envisaged with the inherent assumption that relays adhere to the cooperation strategy at all times. However, in a practical setting where relays might exhibit malicious or selfish behavior the promised performance improvements might be curtailed severely. As malicious and selfish relays constrain the cooperation benefits differently, we treat them separately. A malicious behavior is characterized by a relay that violates the cooperation strategy with the objective of disrupting communication between source and destination at the expense of its own power. In the absence of a mechanism to detect malicious behavior, cooperative diversity exhibits severe performance degradation depending on the degree of maliciousness.
As cooperation incurs cost, the relay has to expend its own resources (power and bandwidth) to provide a reliable communication to cooperation partner. In return, the helping partner is rewarded with the same benefits when the partner reciprocates acting as a relay. However, in practice, there are no mechanisms to ensure reciprocation in cooperative communication. Due to lack of such a mechanism, especially power-limited partners may be tempted to refuse cooperation while reaping benefits of it at no cost. The presence of such free-riders can degrade the envisaged performance improvement.
Due to the dependency on relays' conformance to rules of cooperation, misbehaving relays can severely curtail the envisaged MIMO-class benefits in the absence of a mechanism to enforce cooperation. Since inherently trusted relays cooperate by performing physical-layer operation on source signal, cooperative diversity presents a new security challenge at the physical layer. Existing physical-layer security techniques are unable to mitigate such malicious behavior as such schemes are primarily designed to prevent signal detection and interception by unauthorized sources.
We have shown both analytically and by simulation the performance degradation in cooperative diversity in the presence of misbehaving relays. We have also developed a statistical detection scheme to mitigate effects of malicious behavior. The goal of cooperative communication security research at ISIS, is to develop a game-theoretic model where cooperation is conditioned based on past behavior of partners. That is, cooperation will be confined to a group of wireless devices whose disposition to cooperate is known a priori.