PRACH Basic-2

Home               LTE              NB-IoT             5G(NR-NSA)

In this section we will learn about PRACH and its basics as well as interview questions.

To handle the wide range of deployments for which NR is designed, NR RA preamble supports two different sequence lengths with different format configurations. 

For the long sequence of length 839, four preamble formats that is generated from LTE preambles and these supports mainly large cell deployment scenarios. These formats have subcarrier spacing of 1.25 or 5 KHz and can be used only in FR1. 

For the short sequence, nine different preamble formats (of length 139) are introduced in NR. This mainly targets the small or normal cell and indoor deployment scenarios.tis short preamble formats can be used in both FR1 with sub carrier spacing of 15 or 30 KHz and FR2 with sub carrier spacing of 60 or 120 KHz. 

Opposite to LTE, for the design of the short preamble formats the last part of each OFDM symbol acts as a CP for the next OFDM symbol and the length of preamble OFDM symbol equals the length of data OFDM symbols. 

There are many benefits of this new design. They can be listed as: 

• It allows the gNB receiver to use the same FFT (fast Fourier transform) for data and RA preamble detection. 
• Due to the composition of multiple shorter OFDM symbols per PRACH preamble, the new short preamble formats are more robust against frequency errors and time varying channels. 
• It supports the possibility of analog beam sweeping during PRACH reception in such a way that the same preamble can be received with different beams at the gNB. 

When does UE need to perform RACH in the case of LTE? 

RACH can be performed in below mentioned scenarios:- 

• Initial Access - UE tries to access the network in RRC idle state. 
• At the time of RRC connection re-establishment procedure. 
• Handover 
• When UL synchronization is lost :- It means from a certain duration network has not received anything from UE in UL. 
• When UE do not have any PUCCH resources available for SR (Scheduling Request). 
• When timing advance is required for positioning purpose in RRC connected state for UE. 

When does UE need to perform RACH in the case of NR? 

[TS 38.300] The RA procedure is triggered by a number of events: 

• Initial access from RRC_IDLE to RRC_Connectted state;
• Transition from RRC-INACTIVE to RRC_Connectted state; 
• RRC Connection Re-establishment procedure; ( in order for the UE to re-establish the wireless connection after the radio link fails) 
• Downlink or Uplink data arrival during RRC_CONNECTED when UL synchronization status is “non-synchronized; 
• Uplink data arrival during RRC_CONNECTED when there are no PUCCH resources for SR available; 
• Request by RRC upon synchronous reconfiguration (e.g. handover); 
• To establish time alignment at SCell addition; 
• SR failure;( re-acquire PUCCH resource through random access procedure) 
• Request for Other SI (when the UE is in RRC_IDLE state and RRC_CONNETTED state, request other SI through random access procedure) ; 
• Beam failure recovery. 

There are three ways to trigger the random access process: 

1) PDCCH order trigger: gNB tells the UE through special DCI format 1_0 that it needs to re-initiate the random access procedure, and tells the UE the ra-PreambleIndex, SSB Index, PRACH Mask Index, and UL / SUL Indicator indicating whether it is UL or SUL. 

2) MAC layer trigger: UE selects preamble to initiate random access procedure. 

3) RRC layer trigger: such as initial access, reconstruction, handover, RRC_INACTIVE transition to RRC_CONNECTED state, request for other SI, RRC request during synchronous reconfiguration, etc. 

To successfully send the preamble, the UE needs to: 

(1) select the SSB or CSI-RS; 

(2) select the preamble index; 

(3) select the PRACH resource used to send the preamble; 

(4) determine the corresponding RA-RNTI; 

(5) determine the target received power PREAMBLE_RECEIVED_TARGET_POWER.

In the next section we will learn more about RACH. 
So keep reading……. And Stay tune……………

Pinal Dobariya………..

Reference:

3GPP TS 38.321 
3GPP TS 38.300

3GPP TS 38.213