5G-Cell Selection Process
Bikas Singh
Hiring for Telecom Companies.EXP-19 Years .Teacher, Corporate Trainer, Career Coach.18 Years of Exp. AT&T(USA), Nokia. Expert-5G/4G/3G/2G Protocol Testing & Log Analysis & RF Optimization. NAS/RRC/PDCP/RLC/MAC/PHY.
Cell Search Process in 5G
When a UE first powers up it, conducts a cell search procedure to find an appropriate carrier. In the cell search procedure, the UE tunes to a frequency location, synchronizes in time with the DL signal and detects the physical cell ID of the cell. In 5G NR, the gNB transmits a SS/PBCH at certain frequency positions defined by the synchronization raster, which is also known by the UE. For the cell search procedure, the UE, which is aware of the synchronization raster, only these specific scans frequency locations across the entire spectrum. At each of the synchronization raster frequency locations, the UE searches for the SS/PBCH block or the primary synchronization signal, secondary synchronization signal and PBCH DM-RS signals.
?The following key parameters, defined by the specification (TS 38.101), are essential for cell search:?
Synchronization Raster
?This raster is a set of frequency locations where the SS/PBCH block is transmitted. For frequencies less than 2650 MHz, the frequency delta between frequency locations is approximately 900 kHz. Similarly, for frequencies between 2400 MHz and 24.25 GHz, the frequency delta between frequency locations is 1.44 MHz.
?SS/PBCH subcarrier spacing
?For each operating band, a known subcarrier spacing used for the SS/PBCH.
?For example,
?in the following NR operating bands the SS/PBCH subcarrier spacing are:
?Operating Band: n7 (FDD), UE transmit frequencies: 2500 MHz- 2570 MHz, gNB transmit frequencies: 2620 MHz- 2690 MHz, SCS: 15 KHz
?Operating Band: n79 (TDD), UE transmit frequencies: 4400 MHz-5000 MHz, gNB transmit frequencies: 4400 MHz-5000 MHz, SCS: 30 KHz?
After the UE tunes to different synchronization raster frequency locations, the UE detects the PSS, which is easier to detect as gNB transmits one of three sequences on the PSS. Next, the UE detects the SSS, which is harder to detect as the gNB transmits one of 336 sequences on the SSS. At this stage, the UE has determined the PCI from PSS and SSS. Next, the UE detects the PBCH DM-RS. As discussed in chapter 3, the time domain structure and repetitions of the SS/PBCH allows the UE to determine the start of the frame and slots.
System Information
The system information can be either Minimum System Information or Other System Information. Minimum System Information includes Master Information Block and System Information Block Type 1 that is broadcasted by the cell periodically and is essential for initial access and also points to other system information.
?Other System Information is System Information Block Type 2 and above. Other System Information can be either optionally broadcasted with a periodicity, or it can be transmitted to a UE in RRC connected mode or dedicated mode.
?Other information transmitted in dedicated mode can be either network or UE triggered. In UE triggered, the UE makes the request by the random access procedure.
?Currently 3GPP specifications for System Information Bock Type 1 and above have not been defined. (See TS38.331)
Master Information block (MIB)
The MasterlnformationBlock (MIB) is transmitted on the PBCH with a periodicity of 80 ms. Within the 80 ms, repetitions are made. The MIB includes parameters that are needed by the UE to receive SystemlnformationBlock Type1.?
The following are some of the key parameters:
?? halfFramelndex: This provides the half frame that could be first half (Slots 0, 1, 2, 3, 4) or the second half (Slots 5, 6, 7, 8, 9).
?? systemFrameNumber: The frame number is derived from this field.?
? subCarrierSpacingCommon: This provides the subcarrier spacing that is used to transmit SIB1.
?? pdcchConfigSIB1: This provides the bandwidth or search space for the PDCCH or CORESET (number of resource blocks and OFDM symbols).?
? cellBarred: This broadcast parameter indicates if the cell is barred or not barred.
Cell Selection Summary?
For successful cell selection, the UE must complete cell search successfully, read the MIB to determine if cell is not barred, find the preferred PLMN ID of the network and the and Rx-levmin thresholds in Minimum System Information.
?This flow chart describes the initial cell selection procedure.
?1. The initial cell selection process starts with Cell Selection. During cell search, the UE acquires time/frequency synchronization and PCI. If any of these procedures fail, the UE will restart the whole procedure with another 5G cell.
?2. Once the cell search is successful, the UE tries to decode the Minimum System Information (MIB).?
3. If MIB indicates the cell is barred, then the UE acquires another 5G cell, or else it continues.
?4. The UE reads Minimum System Information that is broadcasted to find PLMN ID and RxLevmin thresholds.?
5. If the PLMN ID matches, then the UE continues to check if the measured RSRP is greater than Rx-levmin.?
6. If RSRP is greater than Rx-Levmin, then the UE selects the cell and camps on the cell to acquire Other System Information.