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 SSS, visit: 5G NR: Secondary Synchronization Signal (SSS) and for PBCH/MIB, visit 5G NR: PBCH and MasterInformation 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.
Primary Synchronization Signal
(PSS)
- PSS helps the UE determine physical-layer identity NID(2) and synchronization up to the periodicity of the PSS.
- PSS is generated by using a BPSK modulated m-sequence
of length 127. In LTE, Zadoff-Chu sequence is used for the generation of PSS.
- As the PSS identifies one out of three identities for NID(2), three different m-sequences are required.
- Based on the PCI used in the cell, the basic
length-127 m-sequence is cyclic shifted
by 0, 43 or 86 steps to generate the required (1-out-of-3) m-sequence. The
details are as shown in the figure below.
-
The
time-frequency structure of SSB is illustrated in the figure below. It can be
seen that PSS occupies first OFDM symbol within the SSB and span over 127
subcarriers.
- The m-sequence of length 127 is mapped on to 127 subcarriers. The mapping is in the increasing order from subcarrier number 56 through 182.
- PSS is transmitted periodically at SSB periodicity.
- Once UE decodes PSS, it knows one out of three
identities for NID(2). In order to derive the PCI, the UE still need to
decode SSS from which PCI group number NID(1) is derived.
