Cross–Layer Design in Wireless Ad Hoc Networks with Multiple Antennas

An ad hoc wireless network is a collection of wireless nodes that self-configure to form a network without the aid of any established infrastructure. Some or possibly all of these nodes are mobile. These networks are extremely compelling for applications where a communications infrastructure is too expensive to deploy, cannot be deployed quickly, or is simply not feasible. There are numerous potential applications for adhoc wireless networks, ranging from multi-hop wireless broadband Internet access, to sensor networks, to building or highway automation, to voice and video communication for disaster areas. The lack of established infrastructure, the network and channel dynamics, and the nature of the wireless medium offer an unprecedented set of challenges in supporting demanding applications over ad hoc wireless networks. The wireless channel is inherently a broadcast medium, so transmissions from different nodes interfere with each other. The quality of wireless links vary over time and space due to interference, multipath fading, and shadowing refer to (setton et al., 2005). Ad hoc networks are harder to design than wired networks because of problems that arise from the every nature of wireless communication. One of these problems, namely the hidden terminal that makes collision. To avoid collisions, a collision avoidance method could be used, as in the well known IEEE 802.11 DCF, which recommends the use of a bidirectional signaling flow made of Request-To-Send (RTS) and Clear-To-Send (CTS) frames before packet transmission, closing the data exchange with an Acknowledgement packet. This scheme forces other nodes in the proximity of the sender and the receiver to defer their own transmissions while a data exchange is in progress, even if they sense a free channel. Fig. 1. shows a typical scenario of "hidden-terminal". Suppose that the B station is in the range of transmission of both A and C, but A and C do not feel another, and suppose that A is transmitting to B. According to the DCF protocol, if C has a packet to be sent to B, listens to the channel and it senses free, because it can not hear the transmission of A. Then begins to transmit, causing a collision at node B. Many authors are working to solve the hidden terminal problem. In (Choudhury et al., 2006), the authors focused on purely directional transmission and designed multi-hop MAC (MMAC), a routing-aware protocol that bridges longer distances by both coordinating farther nodes using RTS/CTS exchanges over multiple hops and exploiting the higher gain and lower overall interference achieved by directional communications. In (Gatsis et al., 2010) the authors dealt with optimal cross-layer design for wireless ad hoc networks.