5G(NR): Xn Based Handover

5G(NR): Xn Based Handover

 

Introduction:

The basic handover procedures is same in any networks, i.e. UE reports measurement report with neighbor cell PCI and signal strength to source cell, source cell take the decision to start handover procedure to best target cell and Target Cell completes the Handover procedure.

 

Impotent Pointers for Xn Handover:

1. Signal strength of both source gNB and target gNB should be reachable to UE during the HO. 
   
2. Xn Handover is similar X2 Handover in 4G LTE
   
3. XnAP interface must be established between source and Target gNB.
4. This type of Handover is only applicable for intra-AMF mobility, i.e. Xn handover cannot be used if Source and Target gNB is connected to different AMF
5. Xn Handover can be Intra Frequency HO and Inter Frequency HO
6. Source and Target gNB can be connected with two different UPFs
7. Tracking Area code should be same. Re-Registration is required after Successful Handover if the Source gNB and Target gNB belong to different Tracking Area (TAC)
8. Xn Handover is Faster as Compare to N2/NGAP Handover due to short signalling path and 5G Core involved in only for switch the PDU session path

High level setup diagram:

      where both the gNB is served by same AMF and UPF. and for XnHO, XnAP interface is active between source gNB and target gNB.

Signaling Exchange b/w Source gNB and target gNB.

 

5G(NR): NG Based Handover

 5G(NR): NG Based Handover

Introduction:

The basic handover procedures is same in any networks, i.e. UE reports measurement report with neighbor cell PCI and signal strength to source cell, source cell take the decision to start handover procedure to best target cell and Target Cell completes the Handover procedure.

•  In 5G NG Handover is very similar to S1 Handover in LTE. NG handover is also called inter gNB and Intra AMF Handover. NG handover take place when X2 interface is not available between source gNB and Target gNB or if X2 interface is there but XnHO is not permitted restriction is there at gNB configuration. 
• NG(N2) Handover can be Intra Frequency HO and Inter Frequency HO both.

• Below is the NG handover architecture in 5G.

 

Below is the flow diagram of NG(N2) handover.

5G-SA Call Flow: Messages mapped with channels:

 5G-SA Call Flow:

In this section of blog, we have tried to map some of the massages of 5G-SA call flow with channels (physical/Transport/Logical), SRBs over which messages are being transfer, mapped coreset and search space with the messages and RLC mode used during UE attached.

What is ARQ and HARQ?

What is ARQ and HARQ?

ARQ stands for Automatic Repeat Request. This is the protocol used at data link layer. it is an error-control strategy that is used in a two-way communication system.  It is used to achieve reliable data transmission over an unreliable source or service.

 It uses CRC(cyclic redundancy check) to determine, whether the packet received is correct or not. If the packet is received correctly at receiver side, receiver send ACK to the transmitter, but in case if the packet is not received correctly at receiver side, receiver send NACK to the transmitter. And then after receiving NACK from receiver, transmitter re-transmits the same packet again.

 

HARQ does not retransmit packet/PDU as it is; as done by ARQ technique. HARQ modifies certain physical parameters before retransmission.

  

The HARQ is a technique when the receiver gets a new data with some errors then it tries to make correction if the error is minor, but if the error is not minor then it sends re-transmission request to the sender. After getting the data again, it combines the new received data with previous erroneous data.

If some packets passed from HARQ to upper layer with a little bit errors which might be acceptable for some applications, but in any case there is one more mechanism which is ARQ or Automatic Repeat Request. The ARQ mechanism takes care of residual errors which passed from HARQ. If there is an error then it discards the packets and a new re-transmission is requested from the sender. ARQ is an error control protocol.

HARQ:
1. It works at Physical layer but controlled by MAC layer.
2. If the received data has an error then the receiver buffers the data and requests a re-transmission from the transmitter

3. HARQ  works for both UM and AM mode.

4. HARQ provides very fast retransmission which is suitable for high speeds (eg voice call).

ARQ: 
1. It works at RLC layer for RLC AM MODE ONLY.
2. If the received data has an error which is passed through HARQ then it is discarded, and a new re-transmission is requested from the transmitter.

3. ARQ is responsible for reliability.

5G-NR: NRF

Role of NRF in 5G architecture:

Network Repository Function (NRF)

The NRF maintains a record of all the 5G elements that are available in the network and their supported services. It allows other NF instances to subscribe and be notified of registrations from NF instances of a given type


The NRF supports discovery mechanisms that allows 5G elements to discover each other and get updated status of the desired elements.

The NRF supports the following functions:

• Maintains the profiles of the available NF instances and their supported services in the 5G core network
• Allows consumer NF instances to discover other providers NF instances in the 5G core network
• Allows NF instances to track the status of other NF instances

 It supports the above functions through the following services:

• Management Services (Nnrf_NFManagement)
• Discovery Services (Nnrf_NFDiscovery)

Management Services (nrf_NFManagement.

This will handles the following service operations:

• Receives and handles NFRegister service requests from the Nfs.
• Receives and handles NFDeregister service requests from the NFs
• Stores the registered profiles in its own data store using the database service.
• Receives and handles NFDeregister service requests from the Nfs.
• Receives and handles the Heart-beat messages from the NFs
• Monitors the heart-beat expiry, mark the NF profiles as suspended and take appropriate action on the suspended NF profiles.
• NF profile Retrieval.
• Receives and handles NFStatusSubscribe service requests from the NFs.
• Receives and handles NFStatusUnsubscribe service requests from the NFs.
• Receives and handles NFNotify service requests from the NFs
• Stores the subscription data in its own data store using the database service

Discovery Services (Nnrf_NFDiscoveryService)

This will handles the following service operations:

• Receives and handles NFDiscover service requests from the Nfs.

 

NRF Functions/Procedure Support:

The NRF interacts with every other element in the 5G core network and it supports the above functions through the following services over HTTPS protocols:

• Management Services
• Discovery Services

Supports the following Functions/Procedure-

1) NRF Management Services-
The NRF Management service is identified by the service operation name Nnrf_NFManagement. NRF supports the following management services.
 

Register NF instance (NFRegister):- It allows an NF Instance to register its NF profile in the NRF; it includes the registration of the general parameters of the NF Instance, together with the list of services exposed by the NF Instance. This service operation is not allowed to be invoked from an NRF in a different PLMN. 

 

Update NF instance (NFUpdate): Enables an NF instance to partially update or replace the parameters of its NF profile in the NRF. It also allows to add or delete services provided by the NF instance.

 

De-register NF instance (NFDeregister): It allows an NF Instance to de-register its NF profile in the NRF, including the services offered by the NF Instance. This service operation is not allowed to be invoked from an NRF in a different PLMN.

 

Subscribe to Status (NFStatusSubscribe): Enables an NF instance to subscribe the status changes of other NF instances registered in the NRF.

 

Unsubscribe to Status (NFStatusUnsubscribe): Enables an NF instance to unsubscribe the status changes of other NF instances.

 

Receive Notifications of Status (NFStatusNotify): Enables the NRF to notify changes status of NF instances to any subscriber of NF status. Changes also include information regarding newly registered and de-registered NFs.

2) Discovery Service:

The NRF Discovery service is identified by the service operation name Nnrf_NFDiscoveryService.

Nnrf_NFDiscoveryService- It also allows an NF to subscribe to be notified of registration, de-registration and profile changes of NF Instance along with their NF Services.

5G-NR: Synchronisation Signal Block(SSB/SS-Block)

 Synchronisation Signal Block(SSB/SS-Block)

3gpp technical specification:

• 38.213
• 38.211
• 38.214
• 38.101
• 38.102

In this blog, we will discuss 5G-NR Synchronization signal block. here we have all the SSB related stubs on one place. 

During cell search procedure Synchronization Signal (SS)/ Physical Broadcast Channel (PBCH) Blocks are used. where UE search for the synchronization Signals for getting a cell information to get attach with that cell and access the radio network services. 

Before proceeding to decode the system information messages which are transmitted on the PDSCH channel in downlink, UE must decode the PBCH. SS Blocks (SSB) are also used for RSRP, RSRQ and SINR measurements.

 

 

 

1- In time domain, it consist of 4 OFDM symbols,  and in Freq domain it consists of 20 RBs. (20x12 = 240 sub carrier) , 1RB = 12 subcarrier

     OFDM symbol 0 = PSS (sub carrier  range from 56 to 182)

     OFDM symbol 1 = PBCH (sub carrier  range from 0 to 230)

     OFDM symbol 2 = SSS (sub carrier  range from 56 to 182)

                 symbol 2 = PBCH(0 to 47 and 192 to 239)              

     OFDM symbol 3 = PBCH (sub carrier  range from 0 to 230)

 

        In LTE, the position of Syncronization signal is  straight forward located around DC carrier (Center 6 PRBS/72 subcarriers).

 

But in 5G, position of Synchronization signals not fixed and can be located any where across the Carrier Bandwidth.
 

 

 

 

In time domain the first symbol Position is determined by combination of Carrier Bandwidth (CBW) + Sub-carrier spacing (SCS).

 

SSB time domain resourse allocation:

 

Start symbol of SS block in respect of sub-carrier spacing.

 

 

In the SA mode, the location of the SSB needs to be obtained through cell search. 

SSB Frequency Domain Resources

In NSA mode operation, The location of SSB is determined by information provided by higher layer in RRC Reconfiguration message from eNB to UE, fields are highlighted in below snap.

Under recongigure with sync, below IEs are present.

Following information can be extracted from above RRC Message.

• Band = n78 is a TDD band known as TD3500 its frequency range is 3300MHz to 3800 MHz belongs to FR1
• SCS = 30Khz with Carrier Bandwidth= 51 RB’s which is 20 MHz with reference to following figure.

F_REF = F_REF-Offs + ΔF_Global (N_REF – N_REF-Offs)                   (i)

• absoluteFrequencyPointA: It represents the common reference point A, this reference point is the 0th RB of 273 RBs, which is the center point of RB#0. From above logs absoluteFrequencyPointA= 642722 (N_REF)
  • absoluteFrequencyPointA= 3000 MHz + 15* (642722 -600000) KHz= 3,640.83 MHz
• absoluteFrequencySSB: It represent the center frequency of SSB Block. A SSB block is 20 RBs results 20 * 12 =240 Subcarriers and from above logs snippet absoluteFrequencySSB= 643008 (N_REF)
  • absoluteFrequencySSB= 3000 MHz + 15* (643008 -600000) KHz= 3,645.12 MHz
• Carrier Center Frequency: The total number of RB’s=51 and Resource Block corresponding to center frequency is 51/2=26
  • Center Frequency= absoluteFrequencyPointA + 26 RBs * 12 * Subcarrier Spacing
  • Center Frequency= 3,640.83 + 26 * 12 *30 KHz = 3,650.19 MHz

Position of SSB from Point A

• offsetToPointA= It defines the frequency offset between point A and the lowest subcarrier of the RB overlapping with SSB. The unit for RB is expressed as 15KHz for FR1 and 60 KHz for FR2
• Kssb= it defines the frequency of RB#0 of SSB and The unit for RB is expressed as 15KHz for FR1 and 60 KHz for FR2

Frequency offset to SSB from Point A = offsetToPointA + Kssb 

• Difference between the SSB center frequency (absoluteFrequencySSB) and point A (absoluteFrequencyPointA)
  • 3,645.12 – 3,640.83 = 4290 KHz
• Difference between the Point A and 0th subcarrier RB#0 of SSB
  • 4290 – 10 (bottom 10 RB of SSB)* 12 *30 = 690 KHz
• Calculating No. of RBs= 690/180 =3.8
• offsetToPointA = 3
• Kssb = (690 – 3 * 12 *15) / 15 KHz = 10 Subcarriers
• offset to SSB from Point A = offsetToPointA + Kssb = 3 RBs + 10 Subcarrier

 

 

when the network is not using beam forming, it may transmit only one SSB and hence there can only one SSB starting position.