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 .
  1.        PSS- 1 Symbol
  2.        SSS- 1 Symbol
  3.        PBCH- 2 Symbols
  • With all downlink transmission in NR,SS Block transmission is based on OFDM.
  • 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 3rd 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.
Time/frequency structure of a SS Block

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

  • PSS is the first signal to be searched by any device.
  • PSS Ranges from 0,1,2.

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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