The document discusses key aspects of synchronization signal blocks (SSBs) in 5G NR, including: 1) An SSB consists of PSS, SSS and PBCH which enable cell search and detection of physical layer cell ID. 2) SSBs are transmitted periodically with configurable periodicities and occupy 4 OFDM symbols in the time domain. 3) In the frequency domain, SSBs are transmitted on synchronization rasters called GSCNs which have wider steps than LTE to facilitate faster cell search.

1. NR- SSB
Presentation
1

2.  Cell search is the procedure for a UE to acquire time and frequency
synchronization with a cell and to detect Physical layer Cell ID (PCI) of the
cell, done by decoding SSB.
 The Synchronization Signal/PBCH block (SSB) consists of PSS, SSS
and Physical Broadcast Channel (PBCH).
 There are 1008 unique PCIs defined in 5G NR, double of that in LTE (504).
 PCI of a cell can be calculated using;
NID
Cell = 3 * NID
(1) + NID
(2) where NID
(1) ∈ {0,1, … ,335} and NID
(2) ∈ {0,1,2}
The UE derives PCI group number NID
(1) from SSS and physical-layer
identity NID
(2) from PSS.
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3. Time and
Frequency
Structure of
an SSB
SSS is in located the third OFDM symbol and span over 127 subcarriers. There are 8 un-used
subcarriers below SSS and 9 un-used subcarriers above SSS.
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4.  SSS is in the third OFDM symbol and span over 127 subcarriers. There are
8 un-used subcarriers below SSS and 9 un-used subcarriers above SSS.
 PBCH occupies two full OFDM symbols (second and fourth) spanning 240
subcarriers and in the third OFDM symbol spanning 48 subcarriers below
and above SSS. This results in PBCH occupying 576 subcarriers across three
OFDM symbols (240+48+48+240 = 576).
 PBCH DM-RS occupies 144 REs which is one-fourth of total REs and
remaining for PBCH payload (576-144 = 432 REs).
 Location of PBCH DM-RS depends upon PCI (v = NID
cell mod 4) of the cell
(PCI already determined by the UE using PSS/SSS)
4

5. Channel or Signal OFDM symbol number ‘l’ relative to the start of an SSB
Subcarrier number ‘k’
relative to the start of an SSB
PSS 0 56, 57, ..., 182
SSS 2 56, 57, ..., 182
Set to ‘0’ 0 0, 1, ..., 55, 183, 184, ..., 239
2 48, 49, ..., 55, 183, 184, ..., 191
PBCH 1, 3 0, 1, ..., 239
2
0, 1, ..., 47,
192, 193, ..., 239
DM-RS for PBCH 1, 3 0+v, 4+v, 8+v,...,236+v
2
0+v, 4+v, 8+v,…,236+v
192+v, 196+v,...,236+v
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6. SSB details in Time Domain
 Each SSB spans across 4 OFDM symbols in the time domain.
 An SSB is periodically transmitted with a periodicity of 5ms, 10ms, 20ms, 40ms, 80ms or
160ms.
 While longer SSB periodicities enhances network energy performance, the shorter
periodicities facilitate faster cell search for UEs. Longer SSB for FR2 and Shorter SSB for FR1
 A UE can assume a default periodicity of 20ms during initial cell search or idle mode mobility (
what if it not 20 ms?)
 To enable beam-sweeping for PSS/SSS and PBCH, SS burst sets are defined. An SS burst
set comprised of a set of SSBs, each SSB potentially be transmitted on a different beam.
 SS burst set consists of one or more SSBs.
 SSBs in the SS burst set are transmitted in time-division multiplexing fashion.
 An SS burst set is always confined to a 5ms window and is either located in first-half or in the
second-half of a 10ms radio frame.
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7.  The maximum number of
candidate SSBs (Lmax) within an
SS burst set depends upon the
carrier frequency/band as
shown in the table below.
 Q: For 30khz SCS, how many
symbols present in 5ms?
Carrier Frequency
Max. No. of Candidate SSBs
within SS Burst Set (Lmax)
fc ≤ 3 GHz* 4
3 GHz* < fc ≤ 6 GHz 8
fc > 6 GHz 64
*SCS = 30 kHz case: for paired spectrum, 3 GHz, for
spectrum, 2.4 GHz is used
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8.  Within a 5ms half frame, the starting OFDM symbol index for a candidate SSB within SS burst set depends upon subcarrier spacing
(SCS) and carrier frequency/band (summarized in the below table). See section 4.1 from 38.213 for full details
 How many slots in a 120Khz SCS?
SCS
OFDM starting symbols of the
candidate SSBs
fc ≤ 3 GHz*
Lmax = 4
3 GHz* < fc ≤ 6 GHz
Lmax = 8
fc > 6 GHz
Lmax = 64
CaseA:
15 kHz
{2,8} + 14n n = 0,1 {2,8,16,22}
n = 0, 1, 2,
3 {2,8,16,22,30,36,
44,50}
NA
CaseB:
30 kHz
{4,8,16,20} + 28n n = 0 {4,8,16,20}
n = 0, 1 {4,8,16,20,32,36,
44,48}
NA
CaseC:
30 kHz
{2,8} + 14n
n = 0,
1 {2,8,16,22}
n = 0, 1, 2,
3 {2,8,16,22,30,36,
44,50}
NA
CaseD:
120 kHz
{4,8,16,20} + 28n NA NA
n=0,1,2,3,5,6,7,8,10,11,12,13,15,
16,17,18
{4,8,16,20 … 508,512,520,524}
CaseE:
240 kHz
{8,12,16,20,32,36,40,44} + 56n NA NA
n=0,1,2,3,5,6,7,8
{8,12,16,20 … 480,484,488,492}
8

9. Frame Synchronization:
 Due to beamforming of SSBs, a UE wouldn’t be able to decode all SSB at
the same time. The received SSB might be anywhere within the SS burst
set, which means that the UE can’t determine the relative location of the
SSB in time, so no frame synchronization yet.
 In order for the frame synchronization to be achieved, the MIB includes a
time index so that the UE knows the relative position of the SSB in time.
 SSB index together with half-frame bit value embedded in PBCH helps
the UE to calculate frame boundary.
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10. 10

11.  UE shall assume the reference-signal sequence r(m) for an SS/PBCH block is
defined by
 Where c(n)is given by clause 5.2. The scrambling sequence generator shall
be initialized at the start of each SS/PBCH block occasion with
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12.  MIB ::= SEQUENCE {
systemFrameNumber BIT STRING (SIZE (6)), => 6 bits
subCarrierSpacingCommon ENUMERATED {scs15or60, scs30or120}, => 1 bit
ssb-SubcarrierOffset INTEGER (0..15), => 4 bits
dmrs-TypeA-Position ENUMERATED {pos2, pos3}, => 1 bit
pdcch-ConfigSIB1 INTEGER (0..255), => 8 bits
cellBarred ENUMERATED {barred, notBarred}, => 1 bit
intraFreqReselection ENUMERATED {allowed, notAllowed}, => 1 bit
spare BIT STRING (SIZE (1)) => 1 bit
}
BCCH-BCH-MessageType indication 1 Bit
Ref: 38.331
Total of :23rBits
Q: SFN Range?
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13.  PBCH carries critical information required for further system access (e.g. to
acquire SIB1). In this section, all the information/fields included in MIB and the
information that is carried by PBCH (excluding MIB contents) are discussed in
detail.
 MIB contents are same over 80ms period and same MIB is transmitted over all
SSBs within the SS burst set. The information such as SSB index is unique and
dedicated to an SSB, so MIB can’t carry such information and hence the
approach of carrying some of the information over PBCH outside of MIB is
adapted.
 PBCH payload size including 24-bit CRC is 56-bits. The following table
summarizes the number of bits occupied by the information/field within
PBCH/MIB.
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14. Information/field
Number of Bits
Total Carried by MIB
Carried by PBCH (excluding
MIB contents)
System Frame Number (SFN) 10 6 4
Sub Carrier Spacing (for SIB1, Initial access Msg-2/4, paging,
SI-messages)
1 1 0
SSB Subcarrier Offset
FR1 ____ 5
4
1
FR2 ____ 4 0
dmrs-TypeA-Position 1 1 0
PDCCH Config for SIB1 8 8 0
Cell Barring Information flag 1 1 0
Intra-Frequency Reselection allowed/not allowed flag 1 1 0
SSB Index
FR1 ____ 0
0
0*
FR2 ____ . 3 3*
half-frame bit 1 0 1
Spare bits 1 1 0
Reserved bits
FR1 ____ 2
0
2
FR2 ____ 0 0
BCCH-BCH-MessageType indication 1 1 0
CRC bits 24 0 24
Total Number of bits 56 (FR1 or FR2) 24 32 (FR1 or FR2)
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15.  SFN (6-bits): Similar to LTE, SFN in 5G NR takes 10 bits and ranges from 0 to 1023. The 6 MSB bits of
the 10-bit SFN are part of MIB. The 4 LSB bits of the SFN are conveyed in the PBCH transport block as
part of channel coding.
 subCarrierSpacingCommon (1-bit): Carried within MIB. Subcarrier spacing used for SIB1, Msg-2/4 for
initial access, paging and broadcast of SI-messages. This bit indicates either 15 kHz or 30 kHz for FR1
and either 60 kHz or 120 kHz for FR2
 SSB Subcarrier Offset (4 or 5-bits): Corresponds to kSSB, which is the frequency domain offset between
SSB and the overall resource block grid in number of subcarriers. For reception of SIB1, the UE needs
know where the overall resource block grid starts. This field takes 5 bits for FR1 and 4 bits for FR2
Only 4 bits are carried by MIB parameter ssb-SubcarrierOffset.
For FR1, 4 LSBs of kSSB are obtained from MIB parameter ssb-SubcarrierOffset and an additional bit
(MSB) is
encoded within PBCH to represent 24 values (0, 1, 2, …,23).
For FR2, 4 LSBs of kSSB are obtained from MIB parameter ssb-SubcarrierOffset to represent 12 values
1, 2, …,11).
This may also indicate that this cell does not provide SIB1 and that there is hence no CORESET#0
configured in MIB. In this case, the field pdcch-ConfigSIB1 indicates the frequency positions where the UE
may find SSB with SIB1 or the frequency range where the network does not provide SSB with SIB1. The UE
determines from MIB that a CORESET#0 is not present if kSSB > 23 for FR1 or if kSSB > 11 for FR2
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16.  dmrs-TypeA-Position (1-bit): Carried within MIB. This field defines the position of first DM-RS symbol for
downlink (PDSCH) and uplink (PUSCH);
For downlink, this bit is only relevant for PDSCH mapping Type A. The position of first DM-RS symbol is set to 3
if dmrs-TypeA-Position is set to pos3 and is set to 2 if dmrs-TypeA-Position is set to pos2.
For uplink, this bit is only relevant for PUSCH mapping Type A. The position of first DM-RS symbol is set to 3 if
dmrs-TypeA-Position is set to pos3 and is set to 2 if dmrs-TypeA-Position is set to pos2.
 pdcch-ConfigSIB1 (8-bits): Carried within MIB. This field is used to configure CORESET#0 and search space#0 (of
the initial BWP) which is the most important information the UE should know in order for it to monitor for
scheduling (PDCCH) of SIB1.
This CORESET configuration also provides and activates the initial bandwidth part in the downlink.
If the field ssb-SubcarrierOffset indicates that SIB1 is absent (explained above), the field pdcch-ConfigSIB1 indicates
the frequency positions where the UE may find SSB with SIB1 or the frequency range where the network does not
provide SSB with SIB1.
• cellBarred (1-bit): Carried within MIB. This field indicates whether or not UEs in the cell are allowed to access the
cell; ‘barred’ indicates, the UEs are not allowed to access the cell.
• intraFreqReselection (1-bit): Carried within MIB. This field controls cell selection/reselection to intra-frequency
cells when the highest ranked cell is barred (as indicated by cellBarred) or treated as barred by the UE.
• SSB Index (0 or 3-bits): This information is not conveyed by MIB, instead, PBCH payload carries the required 3 bits.
Index of the SSB within SSB burst set which is very important piece of information for achieving frame
synchronization. The maximum number of candidate SSBs (Lmax) within an SS burst set depends upon the carrier
frequency
16

17. SSB Index for less than sub-6 GHz : Each one of the 4/8 PBCH scrambling sequences
(section 7.3.3.1 from 38.211) used for PBCH scrambling implicitly indicates 1-out-of-4/8
SSB indices. In this case, there is no need of explicit bits to indicate SSB index.
SSB Index for above 6 GHz(Lmax = 64): Each one of the 8 PBCH scrambling sequence
(section 7.3.3.1 from 38.211) used for PBCH scrambling implicitly indicates 3 LSB bits of
SSB index. In order to represent 64 SSB indices, another 3 bits (MSB) are required which
are explicitly carried by PBCH payload.
half-frame bit (1-bit): This bit is set to ‘0’ if SSB is transmitted in the first half-frame of the
10ms frame or set to ‘1’ if SSB is transmitted in the second half-frame of the 10ms frame.
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18. SSB details in Frequency Domain
 In LTE, the frequency domain position of PSS/SSS is always fixed around carrier center
frequency. In NR, based on the frequency band, a set of possible frequency locations
for SSB are defined, this is called synchronization raster. The UE only need to search
for SSB on this raster, called as GSCN(Global Synchronization Channel Number)
 Unlike in the case of LTE, the UE doesn’t need to search for SSB on all carrier raster
positions, instead the UE just need to search for SSB in a sparser synchronization
raster.
 UE needs to search SSB based on ARFCN raster, it would take too long time since
ARFCN raster is very narrow. So it would be good to define a SSB searching frequency
in wider steps. This is the usage/purpose of GSCN.( In general 100Khz Frequency raster
for FR1 & 60/120KHz Frequency raster in FR2)
 In case of GSCN, as you see in the following table, the granularity is 50 or 150 or 250
Khz under 3Ghz and it has the granularity of 1.44 Mhz in above 3 Ghz frequency
range and below 24.25Ghz, and the granuilirity jumps to 17.28 Mhz if the frequency
goes over 24.25 Ghz.
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19. The SSB is not RB aligned with the resource block grid. Instead, there is an arbitrary offset
between the edge of the SSB RBs and the edge of the resource block grid.
Frequency range SS block frequency position GSCN Range of GSCN
0 – 3000 MHz
N * 1200kHz + M * 50 kHz
N=1:2499, M={1,3,5} (Default
M=3)
3N + (M-
3)/2
2 – 7498
3000 – 24250 MHz
3000 MHz + N * 1.44 MHz
N= 0:14756
7499 + N 7499 – 22255
24250 – 100000 Mhz
24250.08 MHz + N * 17.28 MHz,
N = 0:4383 22256 + N 22256 – 26639
< 38.104 v15.7.0 - Table 5.4.3.1-1: GSCN parameters for the global frequency raster >
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20. 20

21.  Point A serves as a common reference point for resource block grids. For all subcarrier
spacings, the lowest subcarrier (subcarrier #0) of Common RB #0 (discussed later) is
referred to as Point A.
 After decoding SSB, the UE doesn’t automatically know starting PRB of the bandwidth
part. The UE needs to first determine the position of Point A using one of the following
parameters;
 1. offsetToPointA represents frequency offset between Point A and the lowest subcarrier
of the common resource block which overlaps with the start of SSB. This field is provided
by the network via FrequencyInfoDL-SIB as part of SIB1.
 2. absoluteFrequencyPointA represents the frequency-location of point A expressed as
in ARFCN. It provides absolute frequency position of the reference resource block
(Common RB 0) whose lowest subcarrier is Point A.
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22. Thank You
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