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).
The
UE needs to first decode PBCH/MIB in order for it to receive other system
information transmitted on PDSCH.
For more details about SSB,
visit 5G NR: Synchronization Signal/PBCH block (SSB), for details about
PSS, visit: 5GNR: Primary Synchronization Signal (PSS) and for SSS, visit 5GNR: Secondary Synchronization Signal (SSS).
Physical Broadcast Channel (PBCH) and
MIB
- MIB is mapped on the BCCH logical channel and carried
on BCH transport channel. BCH is then mapped on PBCH.
- MIB is transmitted with a periodicity of 80ms and is
repeated within the 80ms.
- MIB provides
the UE with parameters (e.g. CORESET#0 configuration) required to acquire SIB1,
more specifically, information required for monitoring of PDCCH for scheduling
PDSCH that carries SIB1.
Information carried within/by PBCH and
MIB
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.
-
The
information carried by MIB is shown below:
-
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.
|
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)
|
|
|
* 4 additional (LSB) bits are implicitly carried by PBCH scrambling sequence. |
||||
·
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 to 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 (0, 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.
- For more information about kSSB and its usage, visit 5G NR: Synchronization Signal/PBCH block (SSB).
·
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.
- SSB Index for sub-6 GHz (Lmax = 8):
Each one of the 8 PBCH scrambling sequences (section 7.3.3.1 from 38.211) used
for PBCH scrambling implicitly indicates 1-out-of-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.
- The half-frame bit information together with SSB index
are used to achieve frame synchronization.
PBCH time-frequency structure
- The UE knows the timing of PBCH after detecting PSS.
- PBCH/MIB is transmitted periodically at SSB
periodicity.
- It can be seen from the above figure that 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 (including PBCH DM-RS) occupying 576 subcarriers across three OFDM symbols (240+48+48+240 = 576).
- The total number of resource elements (REs) occupied
by PBCH per SSB is equal to 576. This includes REs for PBCH as well as REs for
DM-RS which is required for coherent demodulation of PBCH.
- PBCH DM-RS occupies 144 REs which is one-fourth of
total REs (576 REs) and the remaining bits are occupied by PBCH payload (432
REs)
-
The following table (from 38.211) summarizes resources within an SSB. The
location of PBCH DM-RS depends upon PCI (v = NIDcell mod 4) of
the cell (PCI is 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
|
PBCH Payload Generation and other processing information
- Layer-1, after receiving the BCH data (24-bits),
appends Eight additional timing related bits to generate PBCH payload
bits (32-bits).
- A 24-bit CRC is appended to the PBCH payload which
yields 56-bits in total.
- These 56-bits would result in 512-bits after channel
coding (Polar coding) which then becomes 864-bits after rate
matching. Since QPSK is used, just 432 REs are needed to transmit
432 QPSK symbols (864-bits).
- As discussed already, there are 576 REs dedicated for
PBCH, remaining 144 REs (576-432) are used for PBCH DM-RS (see Table 7.4.3.1-1
from 38.211).
- The following figure illustrates PBCH payload
generation.
PBCH Misc.
- For PBCH;
·
Polar
coding is used.
·
QPSK
modulation is used.
- PBCH uses a single antenna transmission scheme.
- PBCH, SSS and PSS within the same SSB uses the same
antenna port for transmission.
Reference: 3GPP TS 38.211, 38.212,
38.213, 38.331, and 38.300
