On the complexity of radio communication

A radio network is a synchronous network of processors that communicate by transmitting messages to their neighbors. A processor receives a message in a given step if and only if it is silent then and precisely one of its neighbors transmits. This stringent rule poses serious difficulties in performing even the simplest tasks. This is true even under the overly optimistic assumptions of centralized coordination and complete knowledge of the network topology. This paper is concerned with lower and upper bounds for the complexity of realizing various communication primitives for radio networks. Our first result deals with the broadcast operation. We prove the existence of a family of radius-2 networks on n vertices for which any broadcast schedule requires at least &OHgr;(log2 n) rounds of transmissions. This matches an upper bound of O(log2 n) rounds for networks of radius 2 proved earlier by Bar-Yehuda, Goldreich and Itai [BGI]. It is worth mentioning that this lower bound holds even under optimal centralized coordination, while the (randomized) algorithm of [BGI] is distributed. We then look at the question of simulating two of the standard message-passing models on a radio network. Both models can easily simulate the radio model with no overhead. In the other direction, we propose and study a primitive called the single-round simulation (SRS), enabling the simulation of a single round of an algorithm designed for the standard message models. We give lower bounds for the length of SRS schedules for both models, and supply constructions or existence proofs for schedules of matching (or almost matching) lengths. Finally we give tight bounds for the length of schedules for computing census functions on a radio network.