Understanding NR5G Synchronisation Signal Block (SSB)
In this document we will explore NR5G SSB in detail. We looked into a brief introduction on SSB in NR cell search in previous document CELL SEARCH IN NEW RADIO — 5G.
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.
A UE assumes that reception occasions of a physical broadcast channel (PBCH), Primary Sync Signal(PSS), and Secondary Sync Signal(SSS) are in consecutive symbols and form a SS/PBCH block. The UE assumes that SSS, PBCH DM-RS, and PBCH data have the same Energy per Resource Element(EPRE). The UE may assume that the ratio of PSS EPRE to SSS EPRE in a SSB in a corresponding cell is either 0 dB or 3 dB. These assumptions by UE are critical for channel estimation of SS blocks and cell search.
SSB mapping in time domain
In time-domain, the first symbol is the PSS, second symbol is PBCH, third symbol is SSS and fourth symbol is PBCH. The first symbol index of candidate SSB are determined according to subcarrier spacing of the SSB. Where index 0 corresponds to the first symbol of the first slot in a half-frame. To enable the beam sweeping of SS and PBCH, the transmission of SS blocks is organised in a periodical series of SS burst set such that the transmission of SS blocks within SS burst set is confined to a 5ms window (half radio frame). Within this 5ms window, number of possible candidate SS block location is L.
Based on the subcarrier spacing, the number of slot/symbols can vary in 5/10ms time period We have 5 different categories in process to identify the SSB start symbol index:
The set of SS Blocks transmitted as mentioned in the table, define a SSB burst. Start symbol index defines the slot where the first symbol of SSB will be transmitted, and 3 other symbols will follow. You can also refer to python based utility to identify the start symbols indexes for all the cases mentioned above using 3GPP_NR_5G_SSB_Indexes.py.
A UE determines the 2 LSB bits, for L = 4 (start symbol indexes in half frame), or the 3 LSB bits, for L > 4, of a SSB index per half frame from a one-to-one mapping with an index of the DM-RS sequence transmitted in the PBCH. For L = 64, the UE determines the 3 MSB bits of the SSB index per half frame by PBCH payload bits. The SSB periodicity can be configured by higher layer parameter ssb-periodicityServingCell. The UE may assume half frame periodicity i.e. 5ms if not configured by higher layer. For initial cell selection, the UE assumes SSB periodicity to be 20ms i.e. 2 frames.
SSB resource mapping in frequency domain
In the frequency domain, an SS/PBCH block consists of 240 contiguous subcarriers with the subcarriers numbered in increasing order from 0 to 239 within the SS/PBCH block. The location of PSS, SSS, PBCH and DMRS for PBCH are given by table below –
Where v is given by Cell-Id Mod 4.
As we discussed in the previous document, SSB location is not fixed in cell bandwidth like LTE, which had PBCH and PSS/SSS around centre frequency. The position of SSB in frequency domain is identified by higher-layer parameter offsetToPointA and by the higher-layer parameter ssb-SubcarrierOffset. The quantity Kssb is the subcarrier offset from subcarrier 0 in SSB common resource block to subcarrier 0 of the SS/PBCH block. Where SSB common resource block is obtained from the higher-layer parameter offsetToPointA and the 4 least significant bits of Kssb are given by the higher-layer parameter ssb-SubcarrierOffset. If ssb-SubcarrierOffset is not provided, Kssb is derived from the frequency difference between the SS/PBCH block and Point A. SS block signals would apply default numerology defined per frequency band/range so that UE in initial access can assume single numerology for these signals. However, in case of NSA carrier access, NR supports the indication of SCS used for SS block transmission so that the mixed numerology between the data/control can be avoided.
For SS/PBCH block in numerology 0 and 1, SSB common resource block is expressed in terms of 15Khz subcarrier spacing.
For SS/PBCH block in numerology 3 and 4, SSB common resource block is expressed in terms of 60Khz subcarrier spacing.
Below is a diagram showing the time and frequency allocation of SS/PBCH block.
PBCH
Data arrives to the coding unit every 80ms. The following steps can be identified –
1. Payload generation
2. Scrambling
3. CRC addition to transport block
4. Channel coding
5. Rate matching
PBCH payload is of 56 bits including 24 bit CRC. The content consists of :
- Timing information : 3 bit MSB of SSB index + 1 bit half frame index
2. 10 bit SFN
3. 8 bit RMSI configuration
4. 4 bit SSB-Subcarrier offset
5. 1 bit subcarrier spacing for SIB1, MSG2, MSG4
6. 1 bit for first DL-DMRS position
7. 1 bit cell barring
8. 1 bit intra frequency cell re-selection