Synchronization and Cell Search
A User Equipment (UE) wishing to access an LTE cell must first undertake a cell search procedure. This chapter focuses on the aspects of the physical layer that are designed to facilitate cell search. The way this relates to the overall mobility functionality and protocol aspects for cell reselection and handover is explained in Chapters 3 and 22. The performance requirements related to cell search and synchronization are explained in Section 22.2.
At the physical layer, the cell search procedure consists of a series of synchronization stages by which the UE determines time and frequency parameters that are necessary to demodulate the downlink and to transmit uplink signals with the correct timing. The UE also acquires some critical system parameters.
Three major synchronization requirements can be identified in the LTE system:
1. Symbol and frame timing acquisition, by which the correct symbol start position is determined, for example to set the Discrete Fourier Transform (DFT) window position;
2. Carrier frequency synchronization, which is required to reduce or eliminate the effect of frequency errors1 arising from a mismatch of the local oscillators between the transmitter and the receiver, as well as the Doppler shift caused by any UE motion;
3. Sampling clock synchronization.
7.2 Synchronization Sequences and Cell Search in LTE
The cell search procedure in LTE begins with a synchronization ...