Optimal Computation of Symmetric Boolean Functions in Collocated Networks

Abstract: We consider collocated wireless sensor networks, where each node has a Boolean measurement and the goal is to compute a given Boolean function of these measurements. We first consider the worst case setting and study optimal block computation strategies for computing symmetric Boolean functions. We study three classes of functions: threshold functions, delta functions and interval functions. We provide exactly optimal strategies for the first two classes, and a scaling law order-optimal strategy with optimal preconstant for interval functions. We also extend the results to the case of integer measurements and certain integer-valued functions. We use lower bounds from communication complexity theory, and provide an achievable scheme using information theoretic tools. Next, we consider the case where nodes measurements are random and drawn from independent Bernoulli distributions. We address the problem of optimal function computation so as to minimize the expected total number of bits that are transmitted. In the case of computing a single instance of a Boolean threshold function, we show the surprising result that the optimal order of transmissions depends in an extremely simple way on the values of previously transmitted bits, and the ordering of the marginal probabilities of the Boolean variables. The approach presented can be generalized to the case where each node has a block of measurements, though the resulting problem is somewhat harder, and we conjecture the optimal strategy. We further show how to generalize to a pulse model of communication. One can also consider the related problem of approximate computation given a fixed number of bits. In this case, the optimal strategy is significantly different, and lacks an elegant characterization. However, for the special case of the parity function, we show that the greedy strategy is optimal.