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In this paper, we discuss problems in densely deployed wireless networks. Particularly, we focus on wireless local area networks (WLANs) because they have enabled us to provide seamless and high capacity wireless access easily and inexpensively. However, recently, channel interference between different services has become a serious problem because access points (APs) of WLANs are located too densely. In the carrier sense multiple access with collision avoidance used in WLANs, the hidden terminal (HT) and the exposed terminal (ET) problems occur depending on the distance between stations and the carrier sensing range. In the higher dense deployment mentioned above, the HT and ET problems occur complicatedly. Therefore, we propose an AP cooperation system that detects the HT and ET problems between stations (STAs). In our system, APs are operated with time synchronization and obtain the information of connected STAs from received frames. The HT and ET problems are identified from the integration of the information obtained at different APs. The effectiveness is verified by simulations.