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.
During
cell search operations which are carried out when a UE is powered ON, mobility
in connected mode, idle mode mobility (e.g. reselections), inter-RAT mobility
to NR system etc., the UE uses NR synchronization signals and PBCH to derive
the necessary information required to access the cell.
Similar
to LTE, two types of synchronization signals are defined for NR; Primary
Synchronization Signal (PSS) and the Secondary Synchronization
Signal (SSS). The Synchronization Signal/PBCH
block (SSB) consists of PSS, SSS and Physical Broadcast Channel (PBCH).
Synchronization
signals can also be used by the UE for RSRP and RSRQ measurements.
For
details about PSS, visit: 5GNR: Primary Synchronization Signal (PSS), for SSS, visit 5GNR: Secondary Synchronization Signal (SSS) and for PBCH/MIB, visit 5GNR: PBCH and Master Information Block (MIB).
Physical-layer Cell Identity (PCI)
- There are 1008 unique PCIs defined in 5G NR, double of
that in LTE (504).
- 1008 NR PCIs are divided into 336 unique PCI groups
and each group consisting of three different identities.
- PCI of a cell can be calculated using;
NIDCell = 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.
Time and Frequency Structure of an SSB
Time-frequency
structure of an SSB is shown figure below.
- PSS, SSS and PBCH are always together in consecutive
OFDM symbols.
- Each SSB occupies 4 OFDM symbols in the time domain
and spread over 240 subcarriers (20 RBs) in the frequency domain.
- PSS occupies first OFDM symbol and span over 127
subcarriers.
- SSS is located 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).
Summary of frequency resources occupied by SSB:
-
The following table (from 38.211) summarizes resources within an SSB for
PSS, SSS, PBCH and DM-RS for PBCH.
-
The complex-valued symbols corresponding to resource elements denoted as
'Set to 0' in table below are set to zero.
-
As can be seen from the below table, the location of PBCH DM-RS depends
upon PCI (v = NIDcell
mod 4) of the cell (PCI already determined by the UE using PSS/SSS)
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
|
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.
- A UE can assume a default periodicity of 20ms
during initial cell search or idle mode mobility.
SS Burst Set:
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.
- The network sets the SSB periodicity via RRC parameter
ssb-PeriodicityServingCell which can take values in the range {5ms,
10ms, 20ms, 40ms, 80ms, 160ms}.
- The maximum number of candidate SSBs (Lmax)
within an SS burst set depends upon the carrier frequency/band as shown in the table
below.
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 unpaired spectrum, 2.4 GHz is used | |
- 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.
- Note that when the network is not using beam forming, it may transmit only one SSB and hence there can only be one SSB starting position.
SCS
|
OFDM starting symbols of the candidate SSBs
|
fc ≤ 3 GHz*
Lmax = 4
|
3 GHz* < fc ≤ 6 GHz
Lmax = 8
|
fc > 6 GHz
Lmax = 4
|
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}
|
*SCS = 30 kHz case: for paired spectrum, 3 GHz, for unpaired spectrum, 2.4 GHz is used
Entries within curly brackets denote OFDM starting symbols for the candidate SSBs |
||||
- Note that when the network is not using beam forming, it may transmit only one SSB and hence there can only be one SSB starting position.
- As an example, the timing of candidate SSBs within the
SS burst set is illustrated in the figure below for the case of SCS = 15 kHz
and carrier frequency between 3 GHz and 6 GHz.
Information about Active SSBs in SS Burst Set via SIB1
- ServingCellConfigCommonSIB (given below) within SIB1 indicates which SSBs within
SS burst set are active.
- The network informs the UEs about which SSBs are being transmitted using ssb-PositionsInBurst within ServingCellConfigCommonSIB.
- The field inOneGroup within ssb-PositionsInBurst
informs the UE which SSBs (and thereby the time domain positions of the SSBs) are
being transmitted. Value 0 in
the bitmap indicates that the corresponding SSB is not transmitted while value
1 indicates that the corresponding SSB is transmitted. The interpretation of this field varies depending upon
operating band.
·
fc
≤ 3 GHz: As already
discussed, maximum number of SSBs within SS burst set equals to four, so
4 bits are good enough. Only
the 4 leftmost bits of inOneGroup are
valid; the UE ignores the 4 rightmost bits.
·
3
GHz < fc ≤ 6 GHz: As already discussed, maximum number of SSBs within SS burst
set equals to eight, so all 8 bits are needed/valid.
·
fc
> 6 GHz: As already
discussed, maximum number of SSBs within SS burst set is 64.
- For this case, an additional field called groupPresence
is defined to indicate which groups are active. There can potentially
be 64 SSBs which are divided into 8 groups. The first/leftmost bit corresponds
to the SSB index 0-7, the second bit corresponds to SSB index 8-15, and so on.
Value 0 in the bitmap indicates that the SSBs according to inOneGroup are
absent. Value 1 indicates that the SSBs are transmitted in accordance with inOneGroup.
- inOneGroup shall be interpreted as follows. The first/leftmost
bit corresponds to the first SSB index in the group (i.e., to SSB index 0, 8,
and so on); the second bit corresponds to the second SSB index in the group
(i.e., to SSB index 1, 9, and so on), and so on.
- This procedure is illustrated with an example in the
figure below.
Information about Active SSBs in SS Burst Set via dedicated signalling
- ServingCellConfigCommon (given below) provided via dedicated RRC signalling can
also be used to indicate which SSBs within SS burst set are active.
- The network informs the UEs about which SSBs are being transmitted using ssb-PositionsInBurst within ServingCellConfigCommon.
-
The
network makes sure to configure the same pattern for ssb-PositionsInBurst field in both ServingCellConfigCommonSIB and ServingCellConfigCommon.
- The field ssb-PositionsInBurst informs the UE
which SSBs (and thereby the time domain positions of the SSBs) are being
transmitted. Value 0 in the
bitmap indicates that the corresponding SSB is not transmitted while value 1
indicates that the corresponding SSB is transmitted. The interpretation of this field varies depending upon
operating band.
· fc ≤ 3 GHz: As already discussed, maximum number of SSBs within
SS burst set equals to four, so 4 bits are good enough. For this purpose, a shortBitmap of
length 4 is defined.
· 3 GHz < fc ≤ 6 GHz: As already discussed, maximum number of SSBs
within SS burst set equals to eight, so 8 bits are good enough. For this purpose, a mediumBitmap of
length 8 is defined.
· fc > 6 GHz: As already discussed, maximum number of SSBs within
SS burst set is 64. For
this purpose, a longBitmap of
length 64 is defined.
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.
SSB details in Frequency Domain
- As discussed before, the SS/PBCH block span over
240 subcarriers (20 RBs) in the frequency domain.
Synchronization
Raster:
-
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.
-
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.
-
The synchronization raster indicates the frequency positions of the SSB
that can be used by the UE for system acquisition when explicit signalling of
the SSB position is not present.
-
The synchronization raster and the subcarrier spacing of the SSB is
defined separately for each band. For more details refer to 38.104, section
5.4.3.
-
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.
-
As the SSB is not RB aligned with resource block grid, the network
informs the UE about the exact frequency-domain (relative) position on the
carrier via MIB and SIB1;
Determination of kSSB:
-
The quantity kSSB is the subcarrier offset from subcarrier#0 in
Common Resource Block#0 (referred to as Point A) to the subcarrier#0 of the lowest Resource Block (NSSBCRB) that overlaps with the SSB.
-
NSSBCRB is obtained from SIB1 parameter offsetToPointA.
-
For SSB type A (sub-6 GHz), 4 LSBs of kSSB are obtained from MIB parameter ssb-SubcarrierOffset and an
additional bit (MSB) encoded within PBCH to represent 24 values (0, 1, 2,
…,23).
-
For SSB type B (mmWave), 4 LSBs of kSSB are obtained from MIB parameter ssb-SubcarrierOffset to
represent 12 values (0, 1, 2, …,11).
-
If ssb-SubcarrierOffset is not provided, kSSB is derived from the frequency difference between the SSB and Point A.
-
For SSB type A, Numerology (μ) ∈ {0, 1} and kSSB ∈ {0, 1, 2, ..., 23} with the quantities kSSB and NSSBCRB expressed in terms of 15 kHz
subcarrier spacing.
-
For SSB type B, Numerology (μ) ∈ {3, 4} and kSSB ∈ {0, 1, 2, ..., 11} with the quantities kSSB expressed in terms of the subcarrier spacing provided by the MIB parameter.
subCarrierSpacingCommon and NSSBCRB expressed in
terms of 60 kHz subcarrier spacing.
SSB numerologies and the corresponding operating bands
-
The network can use different subcarrier spacings for user data and SSB.
-
SSB numerologies and the corresponding operating bands are summarized
below (Table 5.4.3.3 from 38.104).
NR Operating Band(s)
|
SS Block
SCS
|
SS Block Bandwidth
(20 RBs = 240 SCs)
|
| n1, n2, n3, n5, n7, n8, n12, n20, n25, n28, n34, n38, n39, n40, n41, n50, n51, n66, n70, n71, n74, n75, and n76 |
15 kHz
|
3.6 MHz (= 240 * 15 kHz)
|
| n5, n41, n66, n77, n78, and n79 |
30 kHz
|
7.2 MHz (= 240 * 30 kHz)
|
| n257, n258, n260 and n261 |
120 kHz
|
28.8 MHz (= 240 * 120 kHz)
|
240 kHz
|
57.6 MHz (= 240 * 240 kHz)
|
- The SCS(s) associated with each band is clearly defined by 3GPP which reduces UE’s processing power/time for cell search.
-
As the SSB always occupies 20 RBs and there are 12 subcarriers in each
RB, there are a total of 240 subcarriers. So, the bandwidth occupied by SSB = 240 * subcarrier spacing.
For instance, 15 kHz SCS leads to 240*15 kHz = 3.6 MHz and 30 kHz SCS leads
240*30 kHz = 7.2 MHz and so on.
· Note for example that a frequency band supporting
30 kHz or higher SCS, can’t support 5 MHz channel bandwidth, because, for 30
kHz, the minimum required bandwidth is 7.2 MHz.
· From the above table, it is clear that subcarrier
spacings 15 and 30 kHz are applicable to sub-6 GHz operating bands, while 120 and 240 kHz are applicable to FR2 frequency bands.
Other details on SSB:
-
Antenna port p = 4000 is used for transmission of PSS, SSS, PBCH
and DM-RS for PBCH.
-
The same CP length and SCS for the PSS, SSS, PBCH and DM-RS for PBCH.
