Today, we will be discussing how a User Equipment (UE) finds a 5G cell after being switched on. This topic is of utmost importance if you genuinely wish to comprehend the process of searching for a 5G cell.
When you first power on your device, it attempts to detect and connect to a new cell. The UE mobile has no prior knowledge of which frequency to use for camping. Hence, it performs a blind search. Let’s explore two types of frequency scans: SLS (Storage List Search) and FBS (Full Band Search).
SLS involves checking and scanning the frequencies stored in the mobile before the Cell Jammer ON (CJON). In other words, the mobile might have stored certain frequencies it previously used to camp on a cell. If no suitable physical cell ID is found during the SLS search, the UE initiates a FBS search. In this case, it scans all frequencies in the entire band.
For instance, let’s assume your device supports 5G bands n1, n25, and n66. During the scan, UE will utilize NR ARFCN (Absolute Radio Frequency Channel Number) ranges for each band:
- Band n1: ARFCN range is from 422000-434000. (2110 MHz-2170 MHz)
- Band n25: ARFCN range is from 386000 – 399000. (1930 Mhz-1995 MHz)
- Band n66: ARFCN range is from 422000 – 440000. (2110 MHz-2200 MHz)
The primary objective of the UE is to find a suitable physical cell ID to connect to the cell. In the initial step, the UE tunes to each supported channel and measures RSSI (Received Signal Strength Indicator), which represents the energy power it can detect. This measurement does not involve any channel coding process, and the UE does not attempt to decode the synchronization or reference signal at this stage. It merely measures the power of each channel and creates a list of channel numbers along with their corresponding RSSI values.
In the next step, the UE analyzes the list from the previous step and identifies all channels with RSSI values greater than the threshold value.
The threshold value depends on the UE chipset implementation and is not determined by 3GPP. Subsequently, the UE narrows down the list to these selected frequencies.
In summary, the UE’s main goal is to efficiently identify a suitable physical cell ID to establish a connection with the 5G cell. The process involves careful scanning, measurement, and analysis of available frequencies to ensure a successful connection.
In the first step, the physical layer compiles a list of nine frequencies during the scanning process. These frequencies will be considered for further analysis.
Moving to step two, the physical layer filters the list obtained in step 1, considering only those frequencies whose RSSI values exceed the threshold value.
In step 3, the process slightly differs from the previous LTE (LTE) process. The UE now performs a synchronization process and decodes SSB (Synchronization Signal Block) by scanning the synchronization raster. Here, it aims to decode the SSB block to obtain synchronization information.
- Why is the UE trying to decode the SSB?
- What is the synchronization raster, and how does the UE find 5G NR SSB?
- What are the frequency and time domain locations of the SSB?
- Decoding the SSB is essential for finding the synchronization information and identifying the physical cell ID.
- The synchronization raster and decoding SSB are crucial steps in obtaining synchronization information and identifying the 5G NR SSB.
- The frequency and time domain locations of the SSB are determined during the decoding process.
In non-standalone mode, NR (5G) is activated, and the UE doesn’t need to perform a blind search for SSB. The required configuration information, such as frequency and subcarrier spacing, is provided through RRC (Radio Resource Control) connection reconfiguration messages, eliminating the need for blind searches.
In a standalone mode, the UE performs a blind search to detect the Synchronization Signal Block (SSB) since it needs to find this before receiving any Radio Resource Control (RRC) messages. As a result, the UE has to perform a blind search to identify the SSB location. The reason for conducting a blind search in standalone mode is to identify the SSB, which is essential for decoding the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) to find the physical cell ID. The PSS and SSS are contained within the SSB block.
To search for the SSB, the UE relies on the Absolute Radio Frequency Channel Number (ARFCN) register, and the concept of a global synchronization number channel raster (GSCN) comes into play.
The GSCN is similar to the concept of the channel raster in LTE, where the channel raster is 100 kHz. However, in 5G NR, the frequency interval of the channel raster is not fixed at 100 kHz like LTE. Due to the narrower frequency range of 5G, scanning every 100 kHz interval would be time-consuming and impact UE performance.
- To address this issue, 5G NR adopts a different approach. The SSB’s position is not fixed due to different subcarrier spacings, but there is a limited set of possible locations in each band, known as synchronization raster or GSCN positions. To efficiently search for the SSB, the UE performs sparse and specific searches at these possible locations, similar to the concept of the channel raster in LTE.
- The GSCN represents the center frequency point of the SSB, and the UE scans the entire bandwidth using this synchronization raster. The UE performs a narrow-width scan at each step, aiming to reach the central frequency point of the SSB within this synchronization raster. This process helps to quickly locate the SSB without having to scan the full bandwidth at a narrow interval.
- The width of the synchronization raster, or the scanning step, is wider than the 100 kHz channel raster in LTE. This wider scanning step optimizes the search for the SSB, reducing the time it takes for the UE to identify the SSB location.
In conclusion, the concept of GSCN or synchronization raster is employed in 5G NR to efficiently search for the SSB, reducing search time and improving UE performance compared to LTE’s fixed 100 kHz channel raster.
For 5G NR, a wider frequency width is used to scan the entire frequency spectrum. In contrast to LTE, where the positions of the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) are fixed at the carrier’s center frequency, the position of the Synchronization Signal Block (SSB) in 5G NR is not fixed. The SSB can be placed at various locations within the carrier’s frequency range.
To locate the SSB, the UE employs a small scanning width to try and reach the central frequency point of the SSB, known as SSREF or GSCN. The GSCN represents the multiple of the synchronization cluster, and it helps in locating the SSB in both frequency and time domains.
To understand the relationship between SSB center frequency point and GSCN, there is a conversion formula from Ssref to GSCN and vice versa.
The formula to calculate SSREF is Ssref = N * 1.2 MHz + M * 15 kHz, where N and M are integer values.
Additionally, the GSCN can be calculated as GSCN = 3 * N + (M – 3)/2 for a specific frequency range (0-3000 MHz).
UE scans GSCN as indicated in the below table
Table 126.96.36.199-1: Applicable SS raster entries per operating band
NR operating bands in FR1
NR operating bands in FR2
- In conclusion, the positioning of the SSB is variable, and the UE uses GSCN or synchronization raster to efficiently search for the SSB location in 5G NR. By knowing the values of N and M, the UE can calculate the GSCN and SSREF, facilitating the search process for the SSB.
- SSB follows the GSCN raster in standalone mode, where the UE scans the entire bandwidth with special granularity to locate the SSB’s central frequency point in both time and frequency domains.
- To find the SSB location, the UE performs a frequency scan across the entire band, using a special granularity called the synchronization raster.
- The synchronization raster divides the carrier bandwidth into finite locations where SSBs can potentially be deployed. By scanning the band with the synchronization raster, the UE tries to reach the central frequency point of the SSB.
The relationship between SSB center frequency point (Ssref), Global Synchronization Channel Number (GSCN), and 5G ARFCN (Absolute Radio Frequency Channel Number) is essential for the SSB search process.
- By knowing the ARFCN range, the UE can calculate GSCN and Ssref.
- Similarly, knowing GSCN allows the UE to calculate Ssref.
In the case of Band N1, for example, the GSCN range is from 5279 to 5419, with a step size of one. This means the UE will perform 140 scans to cover the entire GSCN range.
- Through these steps, the UE can find the SSB location in the frequency and time domains, enabling successful cell searching and connection establishment in 5G NR.
- In the 1st scan, the GSCN value is 5279, and the corresponding M value is 1. Using the GSCN formula, we calculate the N value as 2640.
N for Band N1, for scan 1st ,M=1
So, with N=1760 and M=1, we can find the Ssref value using the Ssref formula as follows:
Ssref = N * 1200 kHz + M * 50 kHz
Ssref = 1760 * 1200 kHz + 1 * 50 kHz
Ssref = 2112.05 MHz
Thus, in the 1st scan, the Ssref position is at 2112.05 MHz
Next, we will calculate the ARFCN value for this frequency. Using the AFCN formula:
ARFCN (Nref) = (FREF – FREF_offset) / delta F global + NREF_offset
ARFCN = (2112.05 – 2110)/ 0.005 + 422000
ARFCN = 422410
So, in the 1st scan, the calculated ARFCN value is 422410.
- However, even in the 1st scan, the SSB is not located. The UE continues with the scan process, increasing the GSCN value by one in each step, until it successfully detects the SSB location.
- The search process continues until the UE finds the SSB location in the frequency and time domains, allowing it to successfully establish a connection in 5G NR.
- In the 140th scan, the GSCN value is 5419, and the corresponding M value is 5.
Using the GSCN formula, we calculate the N value as 1656. So, with N=5418 and M=3, we can find the Ssref value using the Ssref formula
Throughout the entire scanning process, the UE tried different GSCN values and calculated the corresponding Ssref and ARFCN values. It continued this process until it successfully found the SSB location in the frequency and time domains at 5th and 119th scan.
- In a standalone mode, the UE searches for the synchronization signal to establish synchronization.
- The SSB block is also used for measuring RSRP, RSRQ, and SINR.
- It occupies 20 resource blocks in the frequency domain and four symbols in the time domain.
- The SSB block includes PSS (Primary Synchronization Signal), SSS (Secondary Synchronization Signal), and PBCH (Physical Broadcast Channel).
- According to 3GPP specifications, the bandwidth of the SSB block is fixed at 20 resource blocks.
- All UEs must support this bandwidth capability to decode the SSB. The SSB consists of 240 continuous subcarriers (12 subcarriers per resource block).
SS-Block (PSS, SSS and PBCH)
- Within the SSB block, PSS occupies 127 carriers within the first symbol, while SSS occupies 127 carriers within the third symbol of every assembly.
- PBCH occupies 20 resource blocks within the second and fourth symbols. Additionally, PBCH-DMRS occupies eight resource blocks within the third symbol.
- The SSB block is transmitted using antenna port 4000. Resource blocks are allocated to the PBCH to accommodate both the PBCH payload and PBCH demodulation.
- DMRs (Demodulation Reference Signals) occupy 25% of the resource elements allocated to the PBCH. The PBCH payload utilizes the remaining 75%.
- The PCI mode four rule is used to determine the location of DMRs, which are dependent on the Physical Layer Cell Identity (PCI).
- The UE deduces the PCI from the PSS and SSS to locate the DMRS when decoding the PBCH payload.
- PBCH DMRs are a special type of physical layer signal used as a reference signal for PBCH decoding in 5G. In LTE, CRS (Cell-Specific Reference Signals) are used for PBCH decoding.
- The PBCH carries important system information, including MIB (Master Information Block) data. It uses QPSK modulation with two bits allocated per resource element. The location of DMRs is determined by the PCI mode four rule.
In conclusion, the SSB block plays a crucial role in synchronization and system information decoding for 5G cells. The PBCH, along with PSS and SSS, enables UE to synchronize with the base station and access essential system information for communication.
Time Domain Resource Allocation
- 5ms half frame interval considered.
- Focus on Orthogonal Frequency Division Multiplexing (OFDM) symbol index within a frame.
SS Burst and SSB:
- Reference to synchronization signals (SS) burst set.
- Candidate Synchronization Signal Block (SSB) within the burst set.
Factors Affecting SSB Index:
- Dependent on Subcarrier Spacing (SCS).
- Also influenced by Carrier Frequency/Band.
Below is the example of case A :
Subcarrier spacing: 15KHz.
Maximum number of SSBs in SS burst set =4
SSB starting symbols: 2,8,16,22.
Reference: 3gpp 38.213 4.1 (howltestuffwork)
Primary Synchronization Signal (PSS)
PSS Role and Synchronization:
- Primary Synchronization Signal (PSS) aids User Equipment (UE) in:
- Determining physical-layer identity NID(2).
- Achieving synchronization up to PSS periodicity.
- PSS created using BPSK-modulated m-sequence of length 127.
- LTE uses Zadoff-Chu sequence to generate PSS.
- NID(2) identification requires three distinct m-sequences due to PSS variations.
PSS Sequence Variation:
- PSS m-sequence of length 127 cyclically shifted by 0, 43, or 86 steps.
- Shifting depends on the Physical Cell Identity (PCI) used in the cell.
- Mapping on 127 subcarriers sequentially from subcarrier 56 to 182.
Transmission and Timing:
- PSS transmitted periodically at the same rate as Synchronization Signal Blocks (SSBs).
- Occurs at Specific Subcarrier Spacing (SSB periodicity).
Decoding and PCI Derivation:
- Decoding PSS reveals one of three identities for NID(2).
- To derive PCI, decoding Secondary Synchronization Signal (SSS) is necessary.
- PCI group number NID(1) obtained from decoded SSS.
Secondary Synchronization Signal (SSS)
SSS Placement and Timing:
- SSS location illustrated within SSB’s time-frequency structure.
- SSS occupies third OFDM symbol in SSB, spanning 127 subcarriers.
- UE identifies SSS timing after detecting PSS.
- SSS transmitted periodically in alignment with SSB timing.
- Occurs at the same SSB periodicity.
Frequency Domain Characteristics:
- SSS takes up the same frequency space as PSS.
- Shared frequency filter for PSS and SSS detectors.
Associations and Sequences:
- 336 SSS sequences linked to 1-of-3 PSS sequences.
- Reflecting the total of 1008 possible Physical Cell Identities (PCIs).
- SSS crafted using BPSK-modulated Gold sequence of length 127.
- Gold sequences offer numerous mutually low-correlated sequences, advantageous for the substantial count of SSS sequences (336).
- Gold sequence produced by multiplying two BPSK-modulated m-sequences.
- Cyclic shifts for m-sequences determined by PCI.
- SSS sequence, length 127, mapped onto 127 subcarriers.
- Subcarriers ordered sequentially from subcarrier 56 to 182.
Physical-layer Cell Identity (PCI)
- In 5G NR, a total of 1008 distinct Physical Cell Identities (PCIs) are established.
- This is twice the number found in LTE, which has 504 PCIs.
- Within the 1008 NR PCIs, they are organized into 336 distinct PCI groups.
- Each group comprises three distinct PCI identities, contributing to the 1008 total.
The UE derives PCI group number NID(1) from SSS and physical-layer identity NID(2) from PSS.
location of the SSB on a carrier bandwidth
To determine the actual location of the SSB on a carrier bandwidth and its position from an absolute frequency point, we can follow these steps:
- First, we need to know the carrier bandwidth and subcarrier spacing.
- Next, we look at the absolute frequency point A, and the SSB’s absolute frequency.
- Now, we can calculate the frequency difference between the SSB block and point A.
- The frequency difference is obtained by subtracting the absolute frequency of the SSB from the absolute frequency of point A.
Frequency Difference = Absolute Frequency Point A – Absolute Frequency SSB
- Next, we need to find the SSB subcarrier offset. So, we derive the SSB from the frequency difference between the SSB block and point A.
- Now, we know that the SSB block consists of 240 contiguous subcarriers in increasing order from 0 to 239. Each RB contains 12 subcarriers.
- The frequency difference we calculated represents the offset from the first subcarrier of the SSB block.
- We divide the frequency difference by the subcarrier spacing to find the SSB subcarrier offset:
SSB Subcarrier Offset = Frequency Difference / Subcarrier Spacing