This letter introduces a novel method to address communication delay and communication frequency in distributed state estimation using a unified structure. Starting from a general type of delay, which can be time-varying, unknown, and different for each channel, two estimation strategies are proposed which use either a unified upper-bound of the delay in all channels, or a sender specific upper-bound in order to improve the performance. The strategies guarantee convergence of the estimation error, and they can be applied to study how communication frequency (or asynchronous communication) affects the convergence rate of the estimation. Thus, the task of determining a necessary communication frequency for obtaining specified performance of distributed estimation is considered, and this task is approached as a special case of the one tailored to delay. Effectiveness of the techniques is confirmed by numerical examples for both types of tasks.
