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In IoT networks, mitigation of channel access contention is one of the crucial requirements due to a massive number of devices connecting to the shared communication channel. In order to satisfy this requirement, IEEE 802.11ah introduces the restricted access window (RAW) mechanism which divides wireless stations (STAs) into several STA groups and allocates a dedicated RAW slot to each STA group. Since STA grouping plays an important role in this mechanism, several grouping methods have been proposed. However, the IoT networks have various network non-uniformity such as in STA deployment. Although the existing method cannot handle this non-uniform characteristic enough to maintain fairness among STAs. We take up non-uniform STA deployment, heterogeneous traffic patterns and multiple data rates at STAs as the potential network non-uniformity. This paper proposes a sector-based grouping method with adjustable central angles, which can handle all three factors of the network non-uniformity. The performance for the proposed method is evaluated through computer simulation to show that the proposed method can improve the network performance even when STAs with various traffic patterns and data rates are deployed non-uniformly.