When you first turn on your phone or your phone loses signal, it doesn’t know which frequency to use to connect to a cell tower. This is because there are many different LTE bands, and each band uses a different frequency range.

For example, LTE Band 5 operates in the 850 MHz frequency range, LTE Band 3 operates in the 1800 MHz frequency range, and LTE Band 25 operates in the 1900 MHz frequency range.

your phone has to search for cells on all of the frequencies that it supports. This is called a “blind search” because the phone doesn’t know which frequency is the best one to use.

For example, let’s assume that your device support LTE Band 5,3,25.”

This means that your phone can connect to cells on LTE Bands 5, 3, and 25.

Initial Acquisition

When the UE is ON, UE starts to search for cells on all of the frequencies that it supports.

Let’s say that you are in a city with three cell towers. One cell tower is on LTE Band 5, another cell tower is on LTE Band 3, and the third cell tower is on LTE Band 25. When you turn on your phone, it will start to search for cells on all three of these bands. If it finds a cell on LTE Band 5, it will connect to that cell. If it finds a cell on LTE Band 3, it will connect to that cell. And if it finds a cell on LTE Band 25, it will connect to that cell.

Let’s elaborate the above statement, the above process will go through multiple steps to attach the UE.

Let’s elaborate the above steps involve in Initial acquisition procedure:

• RSSI Measurement or Scan: The UE tunes to each and every channel that it supports and measures the Received Signal Strength Indicator (RSSI). RSSI is a measure of the power of the signal that the UE is receiving.

The UE tunes to each and every channel within LTE Bands 5, 3, and 25.

For each channel, it measures the RSSI (Received Signal Strength Indicator), which represents the power of the signal received from that channel.

It creates a list of channel numbers and their corresponding RSSI values.

• Filter by RSSI Thresholding: The UE then goes through the list of RSSI measurements from above step and identifies all the channels that have an RSSI value greater than a certain threshold. This threshold is determined by the UE implementation and is not specified by 3GPP.

The UE applies an RSSI threshold to the list generated in above step.

It selects only the channels with RSSI values greater than this threshold.

This step eliminates channels with weak signals, focusing on those with stronger indications of potential cells

• Synchronization and Physical Cell ID (PCID) Detection (PSS and SSS decoding): The UE then performs synchronization with each of the channels selected in above and decodes the reference signal to detect the Physical Cell ID (PCID). PCID is a unique identifier for each cell within a given LTE band.

To decode the PCI ID the UE first decode the PSS and then SSS.

The first signal to be detected is PSS then SSS in Time domain and frequency domain

• Cell Selection: The UE then selects the cell with the strongest RSSI and valid PCID detection. This ensures that the UE connects to the strongest and most reliable cell within the available bands.

• MIB Decoding: The UE then decodes the Master Information Block (MIB) to obtain basic information about the system, such as the system frame number and system bandwidth.

• SIB Decoding: The UE then decodes the System Information Block (SIB) to obtain additional information about the cell, such as the available frequency bands, supported services, and channel bandwidth, cell access related, RACH related decoding.

The diagram also shows two additional steps:

• Storage List Search (SLS): This step is optional and is only performed if the UE has a list of previously known cells stored in its memory. The UE will first search for these cells before performing a blind scan of all available bands.
• Deviated Band Search (DBS): This step is also optional and is only performed if the UE does not find any cells during the SLS or blind scan. In this case, the UE will search for cells in all available bands, even if they are outside of its supported frequency range.

Now let’s focus on to understand many points which are involved in above discussion:

• The first signal to be detected is PSS then SSS in Time domain and frequency domain.
• Minimum 6PRBs required to acquire PSS and SSS.
• For UE camp on, basically as default its use 1.4MHZ where 6RB will utilize means (6*12)72 subcarrier where 10 subcarrier will be reserved and will use for DTX/Guard band purpose and remaining 62 subcarrier will be utilized for camp on process.
• In TDD achieve radio frame and slot synchronization but in FDD UE identify the central of channel bandwidth.
• From PSS and SSS the PCI ID decoding will be done.

PSS (Primary synchronization Signal (First signal decoded by UE):

PSS stands for Primary synchronization Signal and the purpose of PSS is for slot synchronization.

The PSS serves as a reference signal that is embedded in the LTE frame structure. When the UE detects the PSS in the time domain, it extracts the timing information embedded within the signal. This timing information includes the frame number, subframe number, and slot number. The UE device then uses this information to align its internal clock with the base station’s clock, ensuring that it is ready to receive the next downlink transmission at the expected time.

Slot synchronization is particularly important in TDD (Time Division Duplexing) mode, where the uplink and downlink transmissions share the same frequency band. By synchronizing with the base station’s slot timing, the UE can avoid interference between its uplink and downlink transmissions.

Periodicity – 5ms (shown below in Grid)

Position of PSS in FDD and TDD frame structure:

• In Frequency Division Duplex (FDD), the Primary Synchronisation Signal (PSS) undergoes dual broadcasts within each radio frame, specifically during time slots 0 and 10. This broadcast utilizes the central 62 subcarriers located in the final symbol of both time slots 0 and 10.
• Contrastingly, in Time Division Duplex (TDD), the PSS is transmitted using the central 62 subcarriers associated with the third symbol of time slot 2 (subframe 1) and also the third symbol of time slot 12 (subframe 6).

Information Elements of PSS (Primary Synchronisation Signal):

• Number of PSS recorded: Total PSS results while scanning
• PSS Indices (0,1,2): 0,1,2 value will be used in formula to calculate the proper PCI.
• PSS peak Value: Based on good RSSI it will give the list with reference to energy in dB

In below grid PSS position is been marked

LTE Resource Grid for FDD

LTE Resource Grid for FDD

Mapping of PSS

The Primary Synchronization Signal (PSS) is a sequence of 62 consecutive symbols that is used to synchronize the receiver with the transmitter in an LTE (Long Term Evolution) system. The PSS is mapped into the first 31 subcarriers either side of the DC subcarrier, which means that it occupies six resource blocks with five reserved subcarriers each side. This is shown in the figure below.

The PSS is a known sequence of symbols that is easy for the receiver to detect. By correlating the received signal with the PSS, the receiver can determine the frequency offset and timing of the transmitter. This allows the receiver to demodulate the data symbols and decode the received data.

The use of the PSS is an important part of the LTE synchronization process. Without the PSS, the receiver would not be able to synchronize with the transmitter and demodulate the received data.

Generation of PSS:

• The PSS is based on a frequency-domain Zadoff-Chu sequence, which is a type of spreading sequence that has good autocorrelation properties.
• This means that the PSS can be easily detected by the receiver, even in the presence of noise.
• Zadoff-Chu sequences are a type of Frank-Zadoff sequence that was defined by D.C. Chu in 1972.
• These sequences have the useful property of having zero cyclic autocorrelation at all nonzero lags.
• This means that when the sequence is correlated with itself, the correlation is zero at all lags except for zero.
• This property makes Zadoff-Chu sequences ideal for use as synchronization signals, as they can be easily detected even in the presence of noise.
• The PSS is a sequence of 62 consecutive symbols that is mapped into the first 31 subcarriers either side of the DC subcarrier.

SSS (Secondary synchronization Signal (Second signal decoded by UE):

Secondary Synchronization Signal, is a crucial component of LTE’s synchronization process, along with the Primary Synchronization Signal (PSS).

SSS provides more precise timing information and aids in cell identification, ensuring accurate frame synchronization between the User Equipment (UE) and the eNodeB.

Periodicity – 5ms (shown below in Grid)

Position of PSS in FDD and TDD frame structure:

In FDD: SSS is broadcast using the central 62 subcarriers belonging to the second to last symbol of time slots 0 and 10.

In TDD: SSS is broadcast using the central 62 subcarriers belonging to the last symbol of time slots 1 and 12.

Information Elements of SSS (Secondary Synchronisation Signal):

• Frame Boundary Info: Frame boundary information refers to the starting and ending points of an LTE frame. This information is crucial for synchronizing the UE with eNodeB.
• EARFCN: It is a unique identifier for a carrier frequency in LTE before calculating PCI.

CP (Cyclic Prefix): Cyclic Prefix, is a portion of the LTE frame that is inserted at the beginning of each symbol. It is used to mitigate the effects of inter-symbol

Generation of SSS:

• SSS Generation from m-Sequences
• The Secondary Synchronization Signal (SSS) is generated using three maximum length sequences (m-sequences), each of length 31.
• An m-sequence is a pseudorandom binary sequence that can be generated by cycling through every possible state of a shift register of length m.
• This results in a sequence of length 2m–1.
• To generate the SSS, the three m-sequences are combined using a scrambling code. The scrambling code is different for each cell, which allows the UE to identify the cell it is attempting to connect to.

In below grid SSS position is been marked

LTE Resource Grid for FDD

LTE Resource Grid for TDD

Finally, the PCI will be decoded based on PSS and SSS decoding.

Formula for PCI in LTE

The PSS takes on three values (0, 1, or 2), and the SSS takes on 168 values (0 to 167). This results in a total of 504 possible PCI values (0 to 503).