Wireless sensor networks (WSNs) are a collection of nodes organized into a cooperative network with sensing, processing and transmitting capabilities. WSNs are becoming an increasingly prominent technology that can be used in diverse application areas. In WSNs, cooperative relay stations are projected as one of the most cost effective solutions to meet the demanding requirement of capacity enhancement. In this paper, major concerns of the wireless sensor networks addressed are optimizing the number of relay stations required for covering the desired percentage of sensor nodes by optimal placement of relay stations and optimal assignment of the sensors to the relay stations. The joint problem of relay station placement and coverage is formulated into a mixed integer program which is solvable by commercial GAMS software with Xpress-MP Solver. Sensitivity analysis is carried out, along with a case study to demonstrate the performance gain of the model.