Cooperative Cross Layer MAC Protocols for Directional Antenna Ad Hoc Networks Andrea Munari Francesco Rossetto Michele Zorzi munari@dei.unipd.it rossetfm@dei.unipd.it zorzi@dei.unipd.it DEI - University of Padova - Via G. Gradenigo, 6/B, I-35131 Padova (PD), Italy This paper deals with some of the main problems that affect directional antenna ad hoc networks, especially deafness and collisions. By means of a thorough study of the possibili- ties offered by an antenna array, we propose cooperation as a novel technique for deafness suppression, together with a low complexity multiuser detector and a collision avoidance algorithm tailored for this type of networks. Building on these ideas, a new family of pro- tocols (called Cooperative MAC) is designed. A lower and an upper bound on its perfor- mance are compared with the Circular RTS/CTS protocol (CRCM), a known benchmark in this field. We are able to achieve interesting improvements. I. Introduction In the past decade a great deal of research has been devoted to wireless ad hoc networks [1], [2] due to the potential advantages of infrastractureless over infras- tructured communication systems. For instance, the possibility to exchange data in a peer-to-peer fashion anytime, anywhere is extremely attractive for confer- ence meetings or car networks. Moreover, some situa- tions may require data transfer or multimedia stream- ing in places where no infrastructure may be available at all (such as battlefields or disaster relief scenarios). However, wireless ad hoc networks are beset by two key problems. The former is the lack of a coordinating element, which may cause a severe degradation of the performance with respect to networks that can rely on a central node (such as cellular networks or WLANs with an access point). A lower degree of coordina- tion among terminals, in fact, can cause at the physical layer an intolerable level of interference, due to mul- tiple unorganized medium accesses. The consequent unreliability of the wireless links may affect all the upper layers. For instance, the ARQ protocols would issue more retransmissions, routes that converge at the same node would contend for access to that bottle- neck, and, last but not least, transport protocols like TCP would have to cope with very high packet loss rates. The overall performance (for instance in terms of throughput or delay) can heavily deteriorate [3]. This work was supported in part by the U.S. Army Research Office under the Multi-University Research Initiative (MURI) grant no. W911NF-04-1-0224 In wireless environments, the channel is highly time variant, due to changing propagation conditions (mul- tipath fading) and levels of interference. This factor is well known to limit the performance of these net- works. One of the most promising tools to increase the physical layer reliability is the introduction of smart antennas [4]. Such systems, by means of beamform- ing, are able to focus the transmitted energy toward a desired direction, adapting the antenna signals so that they may constructively combine in the aimed bear- ing. In addition, the same technique enables a node to improve the reception of the intended signal by re- jecting waveforms coming from unwanted directions. Therefore, the reduction of the generated and received interference makes it is possible to increase the num- ber of simultaneous communications in the network. Other key advantages related to smart antennas will be discussed in detail in Section III. While directional antennas can provide a significant boost to PHY performance, the introduction of direc- tionality for transmissions and receptions brings new issues that must be taken into account while designing a MAC protocol. First of all, simple extensions of pro- tocols designed for omnidirectional antennas (most notably IEEE 802.11) may be too conservative and not effective. For instance, consider the situation in Fig. 1. 1 If 802.11 DCF is used, a node that receives a handshake packet (RTS/CTS) intended for another destination is not allowed to start a new transmission in order to avoid a collision. If directional commu- 1 This example is drawn from [6]