Experimental Evaluation of TDD T-Sync Accuracy in A-RoF based DAS using Different Delay Measurement Methods

In order to realize a large capacity mobile fronthaul, analog radio over fiber (A-RoF) is highly attractive from the perspective of simplification, especially for the high frequency band distributed antenna system (DAS). In recent years, mobile communication systems have adopted time division multiplexing, especially in high frequency bands, and each distributed antenna section (or remote radio unit (RRU)) must also synchronize its time. Generally, digital-RoF-based functions are required such as time synchronization (T-Sync) using precision time protocol, buffering, and digital-to-analog conversion in order to output radio waves. As a result, the RRU is becoming complicated. In this paper, we propose a T-Sync method for an A-RoF-based DAS that enables simplified RRUs by implementing transmission timing adjustment at the central station (CS) to compensate for A-RoF link delay. This approach decouples T-Sync implementation from RRU hardware, making it possible to achieve synchronization accuracy solely through delay measurement and compensation at the CS. Additionally, we experimentally compare and validate three different delay measurement methods, which form the core of this technology. Achieving high-precision T-Sync typically requires compensation for the internal processing delay of the delay measurement equipment, which varies depending on the device. Therefore, we need a delay measurement method that considers compatibility when modifying an RRU with equipment from different vendors or flexibility when operating multi-vendor RRUs in parallel within the same network. We find that only the optical direct return (ODR) method, which measures the round-trip time of the optical signal while excluding internal processing delays on the RRU side, thus avoiding delay fluctuations, offers acceptable reliability. By utilizing this ODR method, it is possible to achieve stable and highly accurate T-Sync with an error of less than 10 nanoseconds. According to third-generation partnership project requirements, this level of accuracy is sufficient for application in MIMO systems.