The cell-search procedure exploits two specially designed signals: the primary synchronization signal (PSS) and the secondary synchronization signal (SSS). The particular design of these signals allows a UE to acquire the correct time-frequency synchronization (including subframe boundaries), as well as the physical identity of a cell, the cyclic prefix length, and whether the cell operates in time division duplex (TDD) or frequency division duplex (FDD). With this information, a UE can correctly decode the system information broadcasted by the cell in the physical broadcast channel (PBCH), comprising a set of random-access preamble sequences and the corresponding time-frequency resources, known as the physical random-access channel (PRACH), that can be used to access the network.

Random-access is used in LTE for a number of purposes: for initial access to establish a radio link; to re-establish a radio link after a radio link failure; for handover; and for scheduling request when a dedicated resource has not been granted for scheduling purposes. A common aspect to these purposes is the acquisition of the exact uplink timing and to assign a unique identity, the C-RNTI, to the UE. A UE can perform random-access only on its primary component carrier, in either a contention-based or contention-free manner (the latter only used to re-establish uplink synchronization, handover, and positioning). Contention-based random-access consists of four steps:

Transmission, from the UE to the eNodeB, of a random-access preamble which allows the eNodeB to estimate the uplink timing of the UE.

Transmission, from the eNodeB to the UE, of a timing advance response to adjust the terminal timing estimate obtained at the first step.