Abstract-In this paper we study the performance bound of a wireless sensor network which is capable of estimating the true values of several active targets. We assume that sensors can observe each target separately, assuming either targets are orthogonal in frequency or physically distinguishable. The aforementioned assumption is of practical relevance for targets with different physical nature, e.g., heat, humidity, pressure etc. Another interesting example of separable observation is power grids where the voltage of each node can be exclusively estimated at any of its neighboring nodes by measuring the flowing current and applying the Ohm's law. Even though such a sensor network is not wireless, the current work provides a suitable framework for state estimation and planning of the smart meters in smart grids. We then propose a novel unbiased estimator. Moreover, as an additional design variable, we perform power allocation and optimal fusion of data to further improve the performance of the proposed estimator. The optimal fusion rules are provided in closed-form, while power allocation is optimally done by means of convex optimization.