The received signal strength indicator (RSSI) is used to estimate locations of sensor nodes in the wireless sensor networks such as ZigBee based on IEEE802.15.4. The location estimation method requires a prior knowledge of the network environment and the investigation of the RSSI maps. Since it is difficult to characterize propagation environments experimentally, stochastic methods related to the wave propagation were proposed until now. The paper proposes a new approach to investigate wave propagation parameters of the sensor field using the computer simulations based on the computational electromagnetics. The analysis method employs the finite-difference time-domain technique and gives us pretty good results of indoor propagation characteristic. Its reliability and effectiveness are shown for the ZigBee sensor network by comparing the calculated RSSIs with the measured ones.