Multiple Access in Connectionless Networks Using Cooperative Transmission∗

The paper calculates the throughput of Carrier Sense Multiple Access (CSMA) assuming that the set of propagation delays among the nodes (the so called “vulnerable times”) are very different and shows that considering the worst case propagation time can be largely pessimistic in assessing the performance of these networks. The application of our analysis is a large ad-hoc wireless network of locally interconnected nodes where, to transmit a packet, each node floods the entire network to avoid routing. Assuming that the nodes access the channel using CSMA, collisions occur if the network starts to be flooded by multiple sources which, due to the various delays (“vulnerable time”) necessary for the first signal to reach them, can fail to sense that the channel is already in use and that the network is being flooded by other nodes. The paper considers, as flooding strategy, a scheme where the packets are broadcasted using multiple levels of relays at the physical layer, where the nodes progressively echo, on a symbol by symbol basis, what they receive from the near neighbors. In this scheme, called Opportunistic Large Array (OLA), the intermediate nodes that act as relays cannot collide unless they carry information from different sources. OLA also achieves a cooperative wireless advantage compared to other broadcasting techniques that comes from the fact that multi-user interference in OLA is equivalent to active multi-path propagation and thus is a benign source of diversity and signal enhancement.