Basics

5G supports two deployment options

1) SA Mode (Stand Alone Mode) :
  • gNode B is connected to 5G core via Ng interface.
2) NSA mode (Non Stand Alone Mode) :

  • LTE Core is used . 
  • eNodeB acts as master node and gNodeB acts as secondary node.
  • Also known as ENDC (eUTRA-ngRAN Dual Connectivity). Various options are possible under this
    • Option 3 : EPC has connectivity only via eNB. Control/User plane traffic goes via eNB only.
    • Option 3a : Control plane connection to Core network is via eNB. User plane is connected to both eNB and gNB.
    • Option 3x: Combination of both the above. Control Plane is connected via eNB and User plane is connected via gNB.


Numerology





  1. As numerology increases, resource usage in frequency domain increases while in time domain it decreases.
  2. Cyclic prefix duration also reduces with higher numerology as symbol duration reduces


Bandwidth Part (BWP)
  • Indicates the group of continuous PRBs out of whole bandwidth.
  • Helps UE to use narrower bandwidth for monitoring control channels.
  • Maximum 4 UL and 4 DL BWP can be assigned to any UE.
  • At any point, only 1 UL and 1 DL BWP will be active.
  • Each BWP can have its own sub carrier spacing and cyclic prefix.
  • In TDD case, a BWP pair (UL & DL BWP with same BWP-ID) must have same center frequency.
  • It can be Common(cell specific) or UE specific BWP.
  • Following are few types of BWPs
    • InitialDownlinkBWP : Sent in SIB1.  CORESET#0 will be part of this BWP only.
    • InitialUplinkBWP : 
    • Default BWP : Once BWP in-activity timer expires at UE, it moves to this BWP for monitoring control channels.

BWPs were configured by gNodeB in SIB1 message.
SIB1->ServingCellConfigCommonSIB -> DownlinkConfigCommonSIB/UplinkConfigCommonSIB 



Parameters related to BWP configuration :

locationAndBandwidth  : Frequency domain resources. Interpreted in RIV format. First PRB will be found as per the SCS of this BWP.
 subcarrierSpacing :  SubcarrierSpacing used in this BWP.
CyclicPrefix : Indicates whether Normal prefix or extended prefix used in this BWP.

CORESET (Control Resource Set) :
  • Defined as set of continuous PRBs allocated for 1-3 symbols for reception/transmission of Control information.
  • Can be Common or UE specific.
  • Max 3 CORESET/BWP can be assigned. Hence 1 UE can have max = 12 CORESET assigned which includes both common & UE specific.
  • CORESET IDs are unique among BWPs.
  • CORESET#0 is assigned by MIB. SIB1, initial RACH etc takes place in this CORESET only.
  • If CORESET#0 is not indicated, it means cell will be considered as not suitable for camping.
  • Control information is transmitted in terms of CCE.
1 CCE = 6 REGs.
1 REG = 12 REs/symbol.

Out of these 12 REs, 3 REs are used for DMRS (Demodulation reference signal) signals. hence 9 REs are used for passing the control information.

Parameters related to CORESET configuration:

  • FrequencyDomainMapping : Bitmap indicating the No of Continour PRBs assigned for CORESET.  Each bit indicates 6 PRBs. Leftmost bit indictaes the first RB in BWP. eg. Bitmap 100000 means 6 continuous RBs starting from RB0 in that BWP
  • Duration : Length of CORESET in time domain. it can be 1-3 symbol long.
  • CCEToREGMapping: Maps CCE with REG in case of interleaving. 

SEARCH SPACE:
  • Each search space is attached to a CORESET.
  • It is dedicated area where UE will do blind decoding for those kind messages with respective RNTI.
  • Few rtypes of Search spaces
    • PagingSearchSpace (Type2-PDCCH) - If configured, blind decoding with P-RNTI is done in this location.
    • raSearchSpace (Type1-PDCCH)- If configured, UE will do blind decoding with RA-RNTI in this location.
    • searchSpaceSIB1 (Type0-PDCCH)- if configured, blind decoding with SI-RNTI will be preformed here.
    • SearchSpaceSystemInformation.
    • SearchSpaceZero: Always associated with CORESET#0.



Parameters related to Search Space:

SearchSpaceID 
CORESET ID
MonitoringSlotPeriodicityAndOffset : Indicates the slots & periodicity  for PDCCH Monitoring. e.g. sl1 means, monitor PDCCH in every slot. sl4 means, monitor PDCCH in every 4th slot.

MonitoringSymbolsWithinSlot : Bitmap Indicates the symbols to be monitored within the slot. 
SearchSpaceType: UE specific or common search space.
DCI_FOrmat_x_x - DCI formats, that can be monitored in this search space.


LTE Vs 5G

Feature/Property
LTE
5G
Resource Block
12 subcarrier * 7 symbols makes 1 RB. Hence RB defines resources in frequency and time domain both.
1RB = 12 subcarrier. It just defines resources in frequency domain and not in time domain.
SubCarrier Spacing (SCS)
Fixed sub carrier spacing of 15KHz
Variable SCS , 15KHz, 30 KHz, 60 KHz, 120 KHz, 240 KHz. This is called numerology. As it increases, frequency domain resources increases in size for one RE.
Slot Duration
Fixed as 0.5ms
Slot duration changes with Numerology as more number of slots are present per sub-frame (1ms).
TTI
TTI is fixed as time taken by 1 slot i.e. 0.5 ms
TTI varies as per numerology because number of timeslot in a sub-frame increases with numerology hence timeslot duration reduces. e.g. for 30Khz SC, TTI = 0.25ms ()
Prefix
Prefix duration is fixed
 As numerology increases, slot duration decreases, and hence Prefix size reduces. Small prefix is suitable for small cells though large prefix is required for macro cells. Hence for mmwave, prefix length is very small.
Network components
MME
SGW
PGW
eNB - Node B handling radio interface with UE
AMF -  similar to MME. Handles registration procedure
UPF - User plane function for transmit/reception of data
SMF - For PDU session establishment.
gNB - 5G Node B handling radio interface with UE
Initial Synchronization
Location of PSS and SSS signals are fixed on slot 0 & 5 on symbol 5 & 6.
Position for MIB is also fixed.
Takes 6 RBs around centre frequency.
PSS,SSS and MIB together is called as SSB block. SSB block location is not fixed and has to be calculated in frequency and time domain based on the band and numerology. Once start symbol  4 continuous symbols in time domain are used for SSB block which will provide PSS, SSS and MIB information
Takes 24 RBs.
Not located around centre frequency.  GSCN is defined which is the centre of SSB block.
DMRS
Cell Reference signal was defined for demodulation of the signa;
For each of the PDSCH/PUSCH/PBCH data, DMRS signals are defined. Its position is variable and gNB configures the same to UE. It is present in the same location which is defined for PDSCH/PUSCH/PBCH data.
Bandwidth Part
Whole channel bandwidth is assigned to UE.
Due to large channel bandwidth, whole bandwidth is divided into multiple chumks. Each UE can be assigned max 4 UL and 4 DL BW.  UE will be monitoring only 1 BWP at any instance. Each BWP can have different numerology
CORESET
UE will monitor the whole bandwidth for control channel
UE will be configured with CORESET which is where control channel will be present at any time. 1 BWP can have max 3
RACH
Takes 6 RBs
Number of RBs can vary depending on SCS of PUSCH and PRACH.
Interfaces
X2 -  eNB <->eNB
S1-AP - eNB<->MME
S1-U - eNB<->SGW
Xn - gNB<->gNB
N2 -gNB <->AMF
N3 - gNB <->UPF
N4 - SMF <->UPF
Protocol Stack
PDCP, RLC, MAC, RRC
SDAP, PDCP, RLC, MAC, RRC
Modulation Scheme
QPSK, 16QAM, 64 QAM
QPSK, 16 QAM, 64 QAM, 256 QAM



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