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Symbol Timing Synchronization for Uplink Multi-User Transmission in IEEE 802.11ax WLAN

Along with the proliferation of mobile devices and applications, IEEE 802.11 wireless local area network (WLAN) has become one of the most popular Internet access technologies. This leads to dense deployment of WLAN devices, and the calls for better user experience in such high-density WLAN environments are mounting. To keep pace with this increasing demand for high efficiency WLAN, IEEE 802.11ax, whose key advance is adopting uplink multi-user transmission (UL MU), is currently entering the final phase of standardization. The UL MU in 802.11ax WLAN, however, has a difficulty that stems from the intrinsically distributed nature of WLAN. Specifically, concurrent frames from multiple stations are loosely synchronous in time, thus resulting in symbol timing misalignment which leads to poor decoding performance. In this paper, we dissect the performance degradation caused by symbol timing misalignment and examine the required symbol timing conditions in receiving UL MU. Based on the analysis, we present a new two-step synchronization method to accommodate asynchronously arriving UL MU. We develop an 802.11ax link-level simulator and comparatively evaluate the proposed synchronization method via extensive simulations. Simulation results demonstrate that the proposed method successfully stabilizes UL MU data delivery even with large UL MU arrival timing deviations and the use of short cyclic prefix.