LTE Initial Setup

Reference:

3GPP TS 36.213: EUTRAN Physical Layer Procedures

3GPP TS 36.331: EUTRAN RRC Protocol Specification

LTE Cell Search:

When the UE is powered up it needs a network to attach itself. The first towards it is Cell search. Cell Search is a procedure by which a terminal can find a potential cell to attach too.

As a part of cell search procedure the terminal obtains the identity of cell and estimates the frame timing of the identified cell. LTE supports 510 different cell identifiers divided into 170-cell identity group of 3 identities each.

LTE provides two signals in downlink;

-       Primary Synchronization Signal

-       Secondary Synchronization signal.

In first step of cell search, UE uses primary sync signal to find the timing on 5 ms basis. This signal is transmitted twice in each frame(as LTE frame is of 10 ms).

Terminal can use this signal to identify the frame timing with a 5 ms ambiguity. Here terminal locks it local oscillator frequency to the base station carrier frequency. The terminal also finds an identity within the cell. It also obtains partial knowledge about reference signal structure.

In the next step terminal detects the cell identity group and determines the frame timing using secondary synchronization signal.

Random Access Procedure

To transmit data terminal needs a connection setup with the network. So a terminal has to ask for one. Random access procedure is used to establish uplink and unique terminal ID.

-       First step consists of UE transmitting a Random Access Preamble allowing the eNB to estimate the transmission timing of the terminal.

-       In the next step network transmits a Random Access Response. This consists of timing advance command to adjust the terminal transmit timing, based on timing measurement received in the first step. In addition to establish uplink synchronization this step also assigns uplink resources to be used in next steps to the terminal. Temporary identity is also assigned to UE for further communication with the network. This response is sent on PDCCH.

-       Third step consists of transmission of mobile terminal identity to the network using UL-SCH. The exact content of this signal depends on the state so of terminal whether the network previously knows it or not. (RRC_IDLE)

-       4^th step consists of contention resolution message from network to terminal on DL-SCH.

RRC Procedures

There are two RRC states in LTE. RRC_Idle & RRC_Connected.

In RRC_Idle there is no signaling radio bearer established, that is there is no RRC connection.

In RRC_Connected there is a signaling radio bearer established

Signaling Radio Bearers(SRB) are defined as Radio bearers that are used only to transmit RRC and NAS messages. SRB’s are classified into

Signaling Radio Bearer 0: SRB0: RRC message using CCCH logical channel.

Signaling Radio Bearer 1: SRB1: is for transmitting NAS messages over DCCH logical channel.

Signaling Radio Bearer 2: SRB2: is for high priority RRC messages. Transmitted over DCCH logical channel.            

RRC Procedures:

-       Paging

o   To transmit paging info/system info to UE in RRC_IDLE state.

-       RRC Connection Establishment

o   The purpose is establishing SRB1.

o   This procedure is initiated by UE when upper layers requests of a signaling connection when UE is in RRC_IDLE mode.

-       RRC Connection Reconfiguration

o   The purpose is to establish/modify/release radio bearers.

o   Also to perform handovers

o   Network initiated procedure(?)

-       RRC Connection Re-Establishment

o   To re-establish RRC connection which involves SRB1 resumption and reactivation.

-       Initial Security Activation

o   Activate security upon RRC establishment.

o   eNB initiated procedure.

RRC release procedure.