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Absolute time synchronization (timing) is a constant demand in various fields. There are many timing techniques at present. They are mainly based on dedicated equipments such as GNSS or specific protocols such as NTP, PTP, SIB16, etc. Different timing methods are applied according to the scenario, cost and demands for timing accuracy. Recently, the timing demands for some wireless scenarios have become increasingly urgent, like power IoT and auto-driving. However, due to the instability and complexity of radio link, the current methods can hardly meet the demands of high-precision and low cost simultaneously in mobile network. Here we propose a timing method over air interface based on physical layer signals (TAP). Periodic physical layer signals in both downlink and uplink channel are considered to reduce the impact of radio link instability on timing. We implemented the proposed method on an open source LTE software defined radio platform named OpenAirInterface and conducted a series of tests. Our tests prove that TAP can provide microsecond-level timing over the air interface, and it is more stable and precise than PTP. Further simulation shows that using TAP in 5G NR can improve the timing accuracy. INDEX TERMS Air interface, high-precision timing, IoT, physical layer, time synchronization.

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Orbital angular momentum (OAM) has been a popular topic due to the natural orthogonality of OAM waves with different modes. It is expected to greatly improve the channel capacity of wireless communication systems. From the perspective of networking, this letter proposes an OAM-based different modes networking method (OAM DMN) that sets adjacent cells with OAM carriers of different modes, so as to improve the signal quality and spectrum efficiency of wireless network and better adapt to the high-density base station distribution scenario. This letter firstly analyzes the characteristics of atmospheric channel on OAM waves by regarding turbulent as random phase screen based on Kolmogorov turbulence theory. Combining with stochastic geometry, we get the outage probability function of wireless cells. Then two types of OAM DMN schemes are proposed to test the performance of the network under different channel conditions. Simulation shows that OAM DMN wireless network has lower outage probability compared with other OAMbased and conventional networks and better performance on high-density base station distribution.

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