Holdover Algorithm Of LTE ENodeB Timing Module Based On GPS Disciplined OCXO

Timing system is one of the critical components in LTE base station. A technology tomaintain the frequency accuracy of timing system in LTE Base Station during GPS holdoveris described in this thesis. When GPS receiver loses signals from satellites, this system is ableto calibrate the OCXO and generate clock which meets the requirement from the wirelessnetwork operators.Usually base station will use aging model to compensate the frequency drift. The agingfactor of the modern OCXO is very small and produces little impact on the frequency stabilityregarding to the accuracy required by LTE base station. The temperature contributesimportant impact instead since the operation conditions of Base station vary widely. A linearfrequency-temperature model is introduced to address this issue and Recursive Least Squaremethod is applied. The frequency of OCXO can be calibrated by the data from the linearmodel when GPS receiver enters holdover state, to keep the Base Station performing.The design of prototype is presented inside. The functionality of frequency calibration isimplemented on one single FPGA. The key components of timing system are GPS receiver,TIE(Time Interval Error) detector, control module, DAC and OCXO. The testing result isanalyzed and the technology is proved to be feasible.