NR pdcch-ConfigSIB1 – ControlResourceSetZero & SearchSpaceZero

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In 5G, during initial network access, a UE does a completes a band scan for  SSB decodes the PBCH. The PBCH provides sufficient information lo allow the UE to proceed to decode System Information Block#1. SIB#1 information is transmitting on the PDSCH but these resources are indicated by PDCCH.

The PDCCH information for SIB#1 is transmited using DCI Format 1_0 within CommonSearchSpaceType0. The Control Resource Set (CORESET) and Common Search Space Type#0 configurations is provide provides by MIB as part of pdcch-ConfigSIB parameter. We call these information as CORESET#0 and Searchspace#0 because are are critical to decode Remaining System Information which SIB#1. Without them we could not decode the SIB#1

Key Pointer

  • NR Master Information Block (MIB) provides  IE pdcch-ConfigSIB1 which includes CORESET#0 and searchSpaceZero Info
  • pdcch-ConfigSIB1 IE is an 8 bits value
  • UE determines that a CORESET#0 for Type#0-PDCCH common search space is present if Kssb<=23 for FR1 and if Kssb is <= 11 for FR2
  • UE determines that a CORESET#0 for Type#0-PDCCH common search space is not present if Kssb > 23 (FR1) and Kssb >11 for FR2.
  • First 4 bits (MSB) determines the controlResourceSetZero index
  • Last 4 Bits (LSB) determines the searchSpaceZero index
  • controlResourceSetZero  indicates the no. of RB/symbols used to determine the CORESET seize of the Type#0 PDCCH Common Seasrch Space
    • CORESET bandwidth can be  24, 48 and 96 RBs and CORESET durations can be 1, 2 and 3 symbols
  • searchSpaceZero  indicates the PDCCH Monitoring Occasions i.e.  SFN & Slot information for UE need to monitor for the Type#0 PDCCH common search space

Decoding Information from Logs:

Below is the 5G NR MIB sample content, we will be taking this as an example for our further calculations of controlResoureSetZero and searchspacesetzero .

controlResourceSetZero: In above logs, we can see first four bit MSB are 1011 which equvivalent 11, from specificatio TS 38.213 we can get following table and map the index 11.

The result for index mapping will be CORESET#0 resource information as follow

  • Multiplexing pattern : 1
  • No. of RBs CORESET= 48
  • No. of Symbols for PDCCH= 1
  • Offset of RBs (Starting point)= 14

Using above infomration we can plot the CORESET#0 w.r.t. to SSB.

5g SIB 1 PDCCH Location

SearchSpaceSet : Search Space index is used to determine the Type 0 PDCCH Search Space Monitoring occasion, i.e. SFN & Slot Index where Type0-PDCCH carrying the SI-RNTI can be monitored.

Based on the CORSET Multiplexing pattern & Frequency Range following 38.213 13-11 to 13-14 tables are used to get the SFN and Slot Index. The table provides ”O” , no. of search space sets per slot, ”M” and First symbol index.

  • O: It specifies a slot offset relative to the start of the frame
  • M: it variable is related to the Number of Search Space Sets per Slot
  • First Symbol Index: specifics the starting symbol for the Search Space Set within the slot

From logs we can see four bits LSB of pdcch-ConfigSIB1  is 0010 which is equvivalent decimal 2, when mapping index 2 with following table.

  • O: 2
  • no. of search space sets per slot: 1
  • M:1
  • First symbol index: 0

Following formula can be used to calculate SFN and slots number.

It takes input as Numerology “u”=1  for 30khz SCS, ”N” frame per slot which 20 for 30KHz, “i” is SSB index we can be consider as 0. Put into above formula.

  • n0= [(2×2^1)+(0x1)]mod20=
  • no= [4] mode 20= 4
  • no+1=5

For the SS/PBCH block and control resource set multiplexing pattern 1, a UE monitors PDCCH in the Type0-PDCCH common search space over two consecutive slots starting from slot n0. So slot no will no=4 and n0+1 =5 with one symbol Index 0 in each slot allocated for Type#0 PDDCH.

Reference:

  • 3GPP TS 38.211, 5G NR Physical channels and modulation
  • 3GPP TS 38.213, 5G NR Physical layer procedures for control
  • LinkedPost by Naveen Chelikani

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