CELL SEARCH IN NEW RADIO — 5G
In this document, we will focus on cell search procedure in New Radio (NR)-5G
Cell search and PBCH :
In NR-5G, Cell search concept is close to what we had in LTE. The UE has to decode Primary Sync Signal and Secondary Sync Signal to decode time slot information and physical cell id. But that is the only similarity. In NR, PSS, SSS and PBCH are transmitted in one block called the Sync Signal Block(s). SSB is transmitted in 4 OFDM symbols across 240 subcarriers and in pre-defined bursts across the time domain on the configured PRBs. The bursts periodicity in terms of time slots depends on which subcarrier spacing numerology is configured. Major OEMs have opted to start 5G support with 30 KHz for LTE-NR coexistence, 30 kHz SCS is beneficial
• To avoid collision with the LTE CRS
• SS block is sandwiched between LTE CRS
It is to be noted that location of SSB in frequency domain can vary for each cell or bandwidth part. Bandwidth parts is not in scope of this document will be discussed in a separate document.
Different Subcarrier spacing, as mentioned in Figure: 2, has been introduced in NR-5G to enable multiple services like –
1. eMBB (Enhanced Mobile Broadband)
2. mMTC (machine to machine type communication)
3. URLLC (Ultra reliable low latency communication)
These subcarrier spacings are segregated in terms of frequency band in use by the cell as shown below. <6Ghz frequency bands, also known as midband/ lowband, fall under FR1 category and mmWave frequency band, which are above 6Ghz are under FR2 category. Different numerologies can be used in a single cell by using different bandwidth parts.
The subcarrier spacing can vary between SSB and transmission channels like PDSCH/ PDCCH.
There are 2 methods by which a UE can access NR-5G cell.
1. EUTRA- NR- Dual Connectivity (ENDC)
2. Standalone NR cells.
In ENDC, cell search information, like frequency, cell id, RACH parameters, etc, are provided by network in RRC Reconfiguration message via LTE eNB. The UE can attempt RACH to access the NR cell. In standalone NR cell search, UE scans the frequency band on sync raster based on which frequency band the UE is performing cell search on and is defined by 3gpp 38.104, 5.4.3.3.
The synchronization raster indicates the frequency positions of the synchronization block that can be used by the UE for system acquisition when explicit signalling of the synchronization block position is not present.
The synchronization raster and the subcarrier spacing of the synchronization block is defined separately for each band.
The size of broadcast control channel (BCCH) message is of 24 bits including 1 choice bit as shown below.
The UE decodes MIB to read other common parameters such as show in figure above.