AbstractWe consider a wireless sensor network whereKsensors must deliver their messages within a frame ofNtime slots by spending a given finite energy. If the messages are not transmitted within the required delay they become useless and the residual energy is wasted. The channel is block‐fading, with independent fades for each sensor and each slot. Sensors know the fading levels up to the current slot, but do not know the future fading levels. The receiver collects the signal on all the slots of the frame and performs joint decoding of all the messages. We characterize the region of long‐term average achievable rates and we show that the optimal policy tends to waterfilling in time (optimal policy without delay constraints) asNincreases. In this setting, we also characterize the long‐term average capacity region per unit energy by showing that the optimal policy is ‘one‐shot’, totally decentralized and extremely simple, i.e., every user spends all its available energy on the first slot of the frame, the fading gain of which is larger than a pre‐computed time‐varying threshold. Furthermore, the ‘one‐shot’ policy not only makes the most efficient use of the energy, but also reduces to the minimum the interference to other users as it makes all the users transmit with the minimum energy per bit required for reliable communications. These characteristics make the ‘one‐shot’ policy appealing for systems with severe energy limitation as wireless sensor ne