LTE Cell Search Procedure Explained

This tutorial explains the LTE cell search procedure used by the User Equipment (UE). A cell search procedure allows the UE to achieve time and frequency synchronization with an LTE cell and to detect the Physical Layer Cell ID (PCI) of that cell.

There are two main cell search procedures in LTE:

1. Initial Synchronization: This is performed when the UE is initially powered on.
2. Detecting Neighbor Cells: This is done in preparation for handover to maintain connectivity as the UE moves.

LTE employs a hierarchical cell search scheme, similar to WCDMA. Let’s break down the steps involved:

Step 1: Initial Power-Up and Frequency Scanning

After being switched on, the UE tunes its Radio Frequency (RF) receiver and starts measuring the wideband Received Signal Strength Indicator (RSSI). It does this for specific frequencies (channels dictated by higher layers) across all the frequency bands it supports. The UE then ranks the cells it detects based on their signal strength. Essentially, it’s trying to find the strongest signal available.

Step 2: Primary Synchronization Signal (P-SS) Detection

The UE then uses locally stored Primary Synchronization Signals (P-SS) and Secondary Synchronization Signals (S-SS) to correlate with the received signals. The UE first looks for the Primary Synchronization Signal (PSS).

The PSS is located in the last OFDM symbol of the first time slot of the first and fifth subframes.

Finding the PSS allows the UE to synchronize at the subframe level. It also helps with:

• Slot Timing Detection: Knowing when the slots start and end.
• Physical Layer ID Detection: Identifying the Physical Layer ID (which will be 0, 1, or 2).

Step 3: Secondary Synchronization Signal (S-SS) Detection

The Secondary Synchronization Signals (S-SS) are located in the same subframe as the P-SS, but in the OFDM symbol immediately before the P-SS. By decoding the S-SS, the UE can determine the Physical Layer Cell Identity Group number (from 0 to 167).

The S-SS provides information for:

• Radio Frame Timing Detection: Knowing when the radio frames start and end.
• Physical Layer Cell ID: Fully determining the PCI (combined with information from the P-SS).
• Cyclic Prefix Length Detection: Understanding the length of the cyclic prefix used in the cell.
• FDD or TDD Detection: Identifying whether the cell is using Frequency Division Duplexing or Time Division Duplexing.

The figure below illustrates the LTE cell search procedure.

Step 4: Cell Reference Signal and Channel Estimation

Once the UE has determined the PCI for a cell, it knows the location of the cell’s Reference Signals (RS). These Reference Signals are crucial for:

• Channel Estimation: Determining the characteristics of the radio channel.
• Cell Selection / Reselection: Deciding which cell to connect to or switch to.
• Handover Procedures: Seamlessly transferring the connection to another cell.

After performing channel estimation using the RS, Minimum Mean Square Error (MMSE) equalization is used. MMSE equalization attempts to remove the effects of channel impairments (like fading and interference) from the received symbols, improving the quality of the signal.