Energy is often the most constrained resource in networks of battery-powered devices, and as devices become smaller, they spend a larger fraction of their energy on communication (transceiver usage) not computation. As an imperfect proxy for true energy usage, we define energy complexity to be the number of time slots a device transmits/listens; idle time and computation are free. In this paper we investigate the energy complexity of fundamental communication primitives such as broadcast in multi-hop radio networks. We consider models with collision detection (CD) and without (No-CD), as well as both randomized and deterministic algorithms. Some take-away messages from this work include: 1. The energy complexity of broadcast in a multi-hop network is intimately connected to the time complexity of leader election in a single-hop (clique) network. Many existing lower bounds on time complexity immediately transfer to energy complexity. For example, in the CD and No-CD models, we need $Omega(log n)$ and $Omega(log^2 n)$ energy, respectively. 2. The energy lower bounds above can almost be achieved, given sufficient ($Omega(n)$) time. In the CD and No-CD models we can solve broadcast using $O(frac{log nloglog n}{logloglog n})$ energy and $O(log^3 n)$ energy, respectively. 3. The complexity measures of Energy and Time are in conflict, and it is an open problem whether both can be minimized simultaneously. We give a tradeoff showing it is possible to be nearly optimal in both measures simultaneously. For any constant $epsilon>0$, broadcast can be solved in $O(D^{1+epsilon}log^{O(1/epsilon)} n)$ time with $O(log^{O(1/epsilon)} n)$ energy, where $D$ is the diameter of the network.
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