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
more details about SSB, visit 5G NR: Synchronization Signal/PBCH block (SSB),
for details about PSS, visit: 5GNR: Primary Synchronization Signal (PSS) and for PBCH/MIB, visit 5G NR: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.
Secondary Synchronization Signal (SSS)
-
The
time-frequency structure of SSB is illustrated in the figure below. It can be
seen from that the SSS is located in the third OFDM symbol within the SSB and
span over 127 subcarriers.
- UE knows the timing of SSS after detecting PSS.
- SSS is transmitted periodically at SSB periodicity.
- In frequency domain, SSS occupies same length as PSS
so that, the same frequency filter can be shared between PSS and SSS detectors.
- As the total number of PCIs are 1008, there are 3 PSS
sequences and 336 SSS (=1008/3) sequences associated to 1-of-3 PSS sequence defined.
- SSS is generated by using BPSK modulated Gold
sequence of length 127. Gold sequences can generate much larger number of
mutually low-correlated sequences than its sequence length. This is very useful
since the number of SSS sequences (336) are too large as compared to the
sequence length (127).
- The Gold sequence is generated by multiplying two BPSK
modulated m-sequences with cyclic shifts determined by PCI. The details are as
shown in the figure below.
- The SSS sequence of length 127 is mapped on to 127
subcarriers. The mapping is in the increasing order from subcarrier number 56
through 182.
- Once UE decodes PSS, it knows one out of three
identities for NID(2) and after decoding SSS, PCI group number NID(1) is known to the UE. Using NID(1) and NID(2), the PCI can be calculated as NIDCell = 3 * NID(1)
+ NID(2)
