SS-Blocks in 5G-NR

SS Blocks

SS Blocks is Synchronization signal.
So now SS Blocks= PSS+SSS+PBCH in general.
SS Block used for initial cell search in idle/inactive state mobility.
SS Block can also be used on connected state mobility.
Cell search ,it is done in both the cases either device is connected to a network or it is in idle/inactive mode .
SS Blocks consist of 4 Symbols .

The SS block spans four OFDM symbols in the time domain and 240 subcarriers in the frequency domain.
PSS transmission is in the first OFDM symbol of SS Block and occupied 127 subcarriers in frequency domain.
SSS transmitted in 3^rd OFDM symbol and occupied same as PSS.
The PBCH is transmitted within the second and fourth OFDM symbols of the SS block. In addition, PBCH transmission also uses 48 subcarriers on each side of the SSS.

SS Block in Frequency Domain

In LTE PSS and SSS is always located at Centre frequency.
There is big drawback of PSS and SSS locating at carrier frequency is that suppose a device which has no any prior knowledge of frequency domain carrier position will search PSS and SSS at all possible position of carrier (carrier Raster)
Carrier/Channel Raster –distance between neighboring channel in frequency band.
So based on carrier raster SS Block location within each frequency band, a more limited set of possible locations of SS block that is referred to as the “synchronization raster”.

In NR Instead of searching for an SS block at each position of the carrier raster, a device thus only needs to search for an SS block on the sparser synchronization raster.
SS Block periodicity may vary from 5 ms up to 160 ms.
However, devices doing initial cell search or in inactive/idle state doing cell search CAN consider that the SS block is repeated at least once every 20 ms i.e 4 times longer than LTE.

SS Burst Set

The set of SS blocks within a beam-sweep is referred to as an SS burst set.
By applying beam-forming for the SS block, the coverage of a single SS block transmission is increased.
The periodicity of the SS burst set is flexible with (5 ms and160 ms),each SS burst set is always confined to a 5 ms time interval.
SS Blocks in group is SS Bursts.
SS Bursts then grouped into a SS Burst set.

PSS

SSS

Once a device has detected a PSS it knows the transmission timing of the SSS.
By detecting the SSS, the device can determine the PCI of the cell.
SSS ranges from (0, 1….335) There are 1008 different PCIs in 5G-NR as in LTE it is 504.

PBCH

The PBCH carries the master information block (MIB).
Below are the information carried by PBCH. Depending upon FR1 and FR2 the information may differ.

The SS-block time index identifies the SS-block location within an SS burst set.

The SS-block time index

This is sufficient for operation below 6 GHz (FR1) where there can be at most eight SS blocks within an SS burst set.

For operation in the higher NR frequency range (FR2) there can be up to 64 SS blocks within an SS burst.
CellBarred flag consist of two bits
1st PDSCH DMRS position indicates the time-domain position of the first DMRS symbol.
SIB1 numerology provides information about the subcarrier spacing used for the transmission SIB1 .
SIB1 configuration provides information for scheduling of SIB.
The CRB grid offset provides information about the frequency offset between the SS block and the common resource block grid.
Half-frame bit indicates if the SS block is located in the first or second 5 ms part of a 10 ms frame.

The half-frame bit and the SS-block time index, determine the cell frame boundary.

PBCH DMRS(DeModulation Reference Signal)

PBCH DMRS is a special type of physical layer signal which is used as a reference signal for decoding PBCH.
In LTE we can use CRS(Cell Specific Reference Signal) for PBCH decoding.
In 5G/NR there is no CRS. That’s why we need the DMRS dedicated for PBCH decoding.

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