5G networks offer higher download speeds and lower latency than older 4G networks while most actual 5G connections are supported by 5G non-standalone (NSA), wireless operators are increasingly deploying 5G standalone (SA) technology as 5G SA will drive new use cases and unlock the advanced capabilities of 5G.
In this article which is the first chapter of our 5G journey, we will discuss the 5G Standalone initial access and Cell search and downlink Synchronization :
Once the UE is powered on it will need to follow the steps according to Figure 1 :
Locate SSB position and decode SSB/PBCH
Using MIB parameter decode SIB
If remaining SIB are indicated by Broadcast expect and decode remaining SIs.
It is mandatory for UE to camp on the cell to decode MIB + SIB1 for 5G SA where other remaining SIs can be broadcasted or set on demand
In 5G SA SIB1 alone represent RMSI ( remaining minimum system information )
Figure 1
Cell search and Downlink synchronization
Compared with LTE where PSS and SSS & PBCH are always defined on center frequency on 5G SA UE will have to search for SSB on the synchronization raster which indicates the frequency positions of the synchronization block that can be used by the UE for system acquisition to do so a global synchronization raster is defined for all frequencies.
The frequency position of the SS block is defined as SSref with the corresponding number GSCN ( Global Synchronisation Channel Number ) for the corresponding ARFCN
The SSREF and GSCN calculation methodology for all the frequency ranges is defined on 3GPP 138 104 ( see table 5.4.3.1-1 ) .
So taking the example of ARFCN 643296 under band N78 and considering the UE is supporting the Band N78 means UE knows the ARFCN range and UE can derive the GSCN range for the same Band so SSref according to the bellow table is :
SSref = 3000+(451*1.44) = 3649.44 mhz 🧮
GSCN = 7499+ 451= 7950 🧮
So to find the SSB location the UE will need to proceed to the SSref scan as described in Figure 2 .
Figure 2
Once the SSRef scan is completed and UE finds SSB at GSCN and SSref location 3649.44 and decodes it will know the following information on the decoded MIB :
subCarrierSpacingCommon: Indicates the Subcarrier spacing for SIB1, Msg.2/4 for initial access and SI messages and the interpretation of this value varies with the frequency range ( FR1, FR2 ) in this example it corresponds to 30 kHz
If the UE acquires this MIB on a carrier frequency < 6 GHz like for our case, the value scs15or60 corresponds to 15 kHz and the value scs30or120 corresponds to 30 kHz. If the UE acquires this MIB on a carrier frequency >6 GHz, the value scs15or60 corresponds to 60 kHz and the value scs30or120 corresponds to 120 kHz.( see 3GPP 138.331 for further details )
ssb-SubcarrierOffset: corresponding to L1 parameter KSSB, the value representing the frequency domain offset in the number of subcarriers between SSB and the overall resource block grid.
Since the MIB is used on NSA and SA so how the UE should know that the cell is SA or NSA? 🤔
The answer is on 38.213 :
In the case of Non-Stand Alone(SA), the UE determines from MIB that a CORESET for Type0 PDCCH CSS set is present if kSSB ≤ 23 for FR1 or if kSSB ≤ 11 for FR2 so the UE can determine that the cell is SA if the CORESET is present
Also If a UE detects a SS/PBCH block and determines that a CORESET for Type0-PDCCH CSS set is not present, and for Kssb =31 for FR1 or Kssb=15 for FR2 the UE determines that there is no SS/PBCH block having an associated Type0-PDCCH CSS set within a GSCN range see upper part for GSCN calculation
The field pdcch-ConfigSIB1 may indicate the frequency positions where the UE may find a SS/PBCH and search space for SIB1.
SIB 1 Decoding
Once MIB is decoded the UE will store previously discussed parameters and will be able to locate the SIB1 some of the important parameters will be described in this section
SIB 1 alone is known as RMSi or remaining minimum system information and provides the most important parameter for attaching 5G cell and also more information about the scheduling of the other SIs ( periodicity, windows size ) and if the remaining SIs are transmitted in periodic or on demand.
In NR SIB1 is transmitted with 160ms periodicity
SIB 1 Cell selection Information
Similarly to LTE SIB1 will carry cell selection information in additionally to some introduced new features bellow is a description of some of the important parameters :
Q-Rxlevmin: Minimum required Rxlev to access the field value = SIb1 value x 2.
Q-Rxlevminoffset: is applied when a cell is evaluated for cell selection as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN.
more information about cell selection and reselection criteria is available on 3GPP 38.304.
Si Scheduling info
Si_BroadcastStatus : specify that for the corresponding SIB if it is being broadcasted or not ( on demand or broadcast )
Si_WindowLength: specify that the related SI should be transmitted somewhere within this window length starting at the SFN specified by si-Periodicity
servingCellConfigCommon
DownlinkConfigCommon: provides common downlink parameters of a cell including the Basic parameters of a downlink carrier and transmission
for further details see 3GPP 138.331
UplinkConfigCommon: provides common uplink parameters of a cell including the Basic parameters of an uplink carrier and transmission thereon for further details see 3PP 138.331.
ssb_PositionsInBurst: will indicate the SSB's position ( time domain position ) in the SSB_ Burst the first bit will represent SSB 0 and the second bit SSB 1 and so on where the bit 0 will means no SSB transmission on the given SSB position and 1 means active SSB and so on
The specification mentions that SSB burst is contained in a half frame ( see 3GPP 138.213)
Within an SSB burst set the beams are mapped in consecutive SS/PBCH blocks in increasing order of beam index and The beam indexes are numbered from 0 to L-1 where L represents the maximum number of beams where SS/PBCH blocks are broadcasted within a SS/PBCH burst set
Figure 3
Coming to the end of our first part the RACH,RRC and registration as a part of attach and registration will be covered in the upcoming article please subscribe or follow us on Linkedin to stay in Touch 🚧 .
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