Guest editorial for special issue on cross-layer design in ad hoc and sensor networks

For well over a decade, the international research community has invested significant effort on wireless ad hoc and sensor networks. Initially, the classical layered network architecturewas followed in thedesignof such systems, but to obtain higher gains in performance, the crosslayer approach emerged. For example, this may include the introduction of new interfaces between adjacent and non-adjacent layers, the coupling of layer design, vertical calibration of parameters across layers, and the formation of a super-layer by merging the services of adjacent layers. It is our great pleasure to bring you this special issue presenting state-of-the-art research contributions that include aspects of cross-layer design in ad hoc and sensor networks. A total of thirty-nine manuscripts were received in response to our solicitation. Each paper underwent a round of rigorous review. After two additional rounds of review we decided to accept eight papers for publication (an acceptance rate of about 20.5%). We organize this issue in a bottom-up fashion, presenting cross-layer design involving the physical layer (PHY) up to the application layer. We begin with an article by Koutsopoulos and Tassiulas that exploits interactions between the PHY and medium access control (MAC) protocols. It studies the problem of fast neighbor positioning in wireless networks with directional antennas. Fast neighbor positioning is a prerequisite for speeding up other network functionalities, such as medium access. Using a polling methodology combined with use of directional beams, a tradeoff between user positioning and medium access delay is explored. Methodologies for minimizing the delay by appropriately selecting the beam width and the persistence probability of the collision resolution protocol are investigated. Simulations assess the effectiveness of the proposed framework. Leonardi, Palazzo, Rametta, and Knightly propose CSMA/ CARD, a receiver-initiated MAC protocol to mitigate starvation in multi-hop wireless networks arising from asymmetry in channel state. Receivers sense collisions at the physical layer and construct a historical profile to predict if a sender attempted to initiate a transmission. The receiver