Improved Precision Time Protocol with Relative Clock Phase Information

Sammanfattning: A common time reference among nodes is one of the key requirements in telecommunication, distributed control systems and industrial automation systems. For instance, 3GPP LTE TDD standard requires at least ± 1.5 µsec time accuracy among base stations in order to resolve uplink and downlink transmission. Certain other emerging technologies such as wireless positioning, coordinated antenna beamforming have far more stringent timing requirements often in the order of sub-nanoseconds. For example, in wireless positioning method such as LTE OTDOA [1], every nano-second loss of precision translates into approximately 30 cm of positioning estimation error. In modern packet switched backhaul networks, time distribution protocols are used to distribute timing information from high quality clock source to network nodes. The accuracy and precision of the time distribution protocol improve if it runs as close to hardware as possible so that variable software queuing delays are reduced or eliminated. IEEE 1588-2008 Precision Time Protocol, PTP with hardware timestamping promises higher precision com- pared to purely software based protocols. However, network asymmetry, variable queuing delays, and timestamping errors in underlying transport, limit the highest time synchronization precision of most commercial PTP deployments to a few hundred nanoseconds. In this work, the operation and estimation capability of IEEE 1588-2008 Precision Time Protocol (PTP) is formally analysed and PTP improvement in the form of super-imposed clock timing aware signal exchange protocol, is proposed. The proposed protocol operating alongside PTP provides independent clock parameter estimates without impacting any existing PTP infrastructure. In addition, it provides relative clock phase offset estimate which is otherwise not detectable through standard PTP. Furthermore, this work through qualitative and quantitative analysis, demonstrates how the supplementary estimates from the proposed method can be used to improve overall clock synchronization accuracy.