After the power on, the UE knows:
- the UE category and capability.
- the preferred PLMN.
the carriers.
The UE needs to know:
- The frame synchronization to be able to decode the DL radio frame.
- The cell parameters to be able request a connection.
The UE can use:
- The PSS: Primary Synchronization Signal.
- The SSS: Secondary Synchronization Signal.
- The BCH, the broadcast channel.
The synchronization signals provide the cell id to the UE.
LTE supports 510 different cell identities.
- They are divided into 170 cell id groups and there are 3 cell ids per group.
- Cell id = 3* Cell_Group_id(0 to 169 Provided by the SSS) + Cell_id_in_group(0 to 2 Provided by the PSS )
The Primary Synchronization Signal (PSS):
- is used for the slot synchronization.
- is on the last OFDM symbol of slots 0 & 10 in the 1st & 6th sub-frames of each frame.
- carries one of the 3 cell id in group sequence.
The Secondary Synchronization Signal (SSS):
- is used for the frame synchronization.
- is on the same slot as PSS.
- is on the next to last OFDM symbol of slots 0 & 10 in the 1st & 6th sub-frames of each frame.
- carries one of the 170 unique cell group identifiers.
The PSS, the SSS and the BCH are carried over 6 Resource Blocks (RB) whatever the bandwidth
6 RBs = 6 * 12 Sub-ca = 72 Sub-carriers
By this way, these signals are independent from the bandwidth and can decode this signal without knowing it.
How Synchronization Channel Works in LTE?
The Synchronization Channel (SCH) in LTE is crucial for aligning devices (UE) with the network’s timing and frequency, ensuring smooth communication. It helps the device synchronize its clock with the eNB (evolved NodeB) to avoid interference between transmissions.
Functions of Synchronization Channel
- Primary Synchronization Signal (PSS): This signal helps the device determine the frame timing and the frequency synchronization. It ensures that the device can detect the frame structure of the LTE network.
- Secondary Synchronization Signal (SSS): After the PSS, the SSS helps the device identify the cell identity and further refine its time and frequency synchronization with the network.
How Synchronization Channel Works
When a device (UE) is powered on, it searches for the PSS and SSS to synchronize with the eNB. Once the device detects these signals, it can align its internal clock and frequency with the network, ensuring that both uplink and downlink transmissions are properly timed. This synchronization allows the device to communicate without interference.
Steps of Synchronization
- Step 1: PSS Detection – The device detects the PSS, which provides the timing alignment of the LTE frame.
- Step 2: SSS Detection – After detecting the PSS, the device detects the SSS, which helps in identifying the cell and fine-tuning the synchronization.
- Step 3: Synchronization Confirmation – With both signals detected, the device has achieved synchronization with the network and can proceed with the Random Access Procedure to establish communication.
Why Synchronization Channel Matters
Synchronization is crucial in LTE to avoid data transmission conflicts, especially in TDD systems where the same frequency is used for both uplink and downlink. Without proper synchronization, devices would struggle to send and receive data at the correct times, leading to poor performance and interference.