Discrete-time control analysis of transport channel synchronization in 3G radio access networks

Transport channel synchronization is a function of 3G access networks that operates in the link between Radio Network Controllers (RNCs) and Base Stations (Nodes B) and is required to support macro-diversity in the downlink direction. Its objective is to assure that every frame sent by the RNC arrives at the Node B on time to be transmitted over the air interface avoiding excessive buffering at the Node B. This is achieved by means of a timing adjustment algorithm that tracks the delay of the link and corrects the sending time of the frames in the RNC. However, when the link experiences abrupt delay variations, e.g. because of a sudden traffic increment in an intermediate node, the classic algorithm may loss frames and, depending on its configuration and the transport delay, it can show an undesired oscillatory behaviour. In this paper we analyze the response of this mechanism under sudden delay increments, providing useful guidelines to configure it in order to prevent oscillations, avoiding excessive signalling and eventual frame losses. Moreover, we propose a simple algorithm that overcomes the limitations of the classic scheme. The configuration of our proposal is addressed by means of discrete-time control theory focusing on stability and performance considerations. We show the influence of the delay on these issues and present an adaptive strategy to make the system robust under delay variations.