RACH in 5G-NR

#RACH process or RA process is performed by a UE to get uplink synchronization with the gNB and also to request the resources from gNB to send data in Uplink (Scheduling Request). RACH process is a PHY and MAC layer process. UE uses PRACH Channel to send RACH Request in UL.

For a UE to initiate RACH it must get some information from gNB before it sends a RACH request to the gNB.?UE gets this information from gNB in below message.

<< RRC Reconfiguration message >> via LTE Anchor eNB in the case of NSA (Non-Standalone).?

<< System Information Broadcast Message (SIB1) >> in the case of SA (Standalone).

The above messages provide RACH related information to the UE as RACH-ConfigCommon which contains the parameters as shown below :

UE needs following information to start the RACH process: -

• Preamble Format
• Frequency domain Resources
• Time domain resources (Slots and Symbols)
• RACH Occasions available per SSB
• Number of Contention based preambles
• Initial Power for PRACH transmission

#PRACHFormats

PRACH Formats in NR : PRACH Formats are categorized into two types: -

• Long Format (0, 1, 2, 3)
• Short Format (A1, A2, A3, B1, B2, B3, B4, C0, C2)

These PRACH formats differs in CP length, Sequence Length, GP length and number of repetitions. Below Table from 3gpp Spec. 38.211 gives the detailed information of PRACH formats.

#prach-ConfigurationIndex :?The value of this parameter points to a table (specified below) for prach-ConfigurationIndex in 3gpp Spec.38.211.

The table defines which PRACH format is used and when to send PRACH in time domain, the number of RACH occasions available and its periodicity.

For Example: - Below image explains the PRACH position in time domain, number of RACH occasions available and its periodicity for TDD pattern 3-10-1-2 and PRACH config Index 41.

Now as we know that one SSB spans over?4 symbols in time domain?and 240 sub carriers in frequency. In FR1 we can use up-to?8 SSBs and up-to?64 SSBs in FR2. And an SSB burst containing multiple SSB beams would?span over 5ms (Half Frame Wise) and these SSB beams radiate in different directions. A UE would probably see more than one beams but it will report the strongest beam and thereby successfully establish a connection or maintain an existing connection.

UE will get predefined RACH occasions associated with multiple SSBs. By the virtue of the RACH Occasion, the eNB/gNB can figure out the SSB to which a specific UE is associated or linked to.

Now, as per?3gpp Spec. 38.213

It means that while associating a RACH Occasion to an SSB, the?resources are used in the increasing order of frequency before increasing in time.

So, Now as per?3gpp Spec. 38.331

#msg1-FDM:?

This parameter defines the number of PRACH transmission occasions FDMed in one time instance. Up-to eight RACH occasions can be used in one time -instance, meaning we can have up-to 8 frequency resource instances over one instance of time. It actually specifies how many RACH Occasions are allocated in frequency domain.

#msg1-FrequencyStart:?This value refers to the Offset of lowest PRACH transmission occasion in frequency domain with respective to PRB 0. The value is configured so that the corresponding RACH resource is entirely within the bandwidth of the UL BWP.

#ssb-perRACH-OccasionAndCB-PreamblesPerSSB :

--value?1/8 corresponds to one SSB associated with 8 RACH occasions and value?n4 in ENUMERATED part corresponds to 4 Contention Based preambles per SSB

--value?1/4 corresponds to one SSB associated with 4 RACH occasions and value?n8 in ENUMERATED part corresponds to 8 Contention Based preambles per SSB

?and so on...

The total number of CB preambles in a RACH occasion is given by :-

CB-preambles-per-SSB * max(1, SSB-per-rach-occasion)

Examples: -

?#zeroCorrelationZoneConfig: This value helps to get the Ncs value for the mentioned type of restricted sets as per below mentioned tables in 3gpp Spec.38.211.

The Ncs value is used to calculate the cyclic shift Cv as per below formula.

#restrictedSetConfig: This parameter lets the UE know what types of restricted set of Ncs is used by the gNB.

For FR1 there are three types of restricted sets: Unrestricted set, restricted set Type A and restricted set Type B.

The concept of restricted set is used to mitigate the impact of Doppler frequency offset in the case of high mobility scenario. Unrestricted sets are intended to use with frequency offset which do not exceed half of subcarrier spacing. Restricted sets Type A are intended to use with frequency offset which do not exceed subcarrier spacing. Restricted sets Type B are intended to use with frequency offset which do not exceeds twice of subcarrier spacing.

For FR2 there are no restricted sets because FR2 uses short preamble formats and restricted sets are applicable for long preamble format only.

#prach-RootSequenceIndex: It indicates the starting logical root sequence. One PRACH Occasion corresponds to 64 preambles. The preambles are generated in the increasing order of first increasing cyclic shift of the logical root sequence and then the increasing order of the logical root sequence index until there are 64 preambles. Refer below mentioned table in 3gpp Spec.38.211 for logical root sequence.

#preambleTransMax :?If RACH procedure fails at?Msg2?or?Msg4?fails, UE will trigger Msg1 retransmissionbefore declaring a failure and the maximum number of Msg1 retransmissions is defined by the parameter?preambleTransMax. i.e., How many times a UE can transmit MSG1 before declaring RACH failure is given by this parameter.

#preambleReceivedTargetPower: It is the initial Random Access Preamble power. This is used by UE to calculate the amount of power needed by the UE to send Msg1 so that it reaches gNB .

The calculation of Power needed by UE for PRACH is defined below as per 3gpp Spec. 38.213 [Clause 7.4]

#powerRampingStep :?This value indicates the amount of power UE needs to increase for RACH transmissions if previous RACH attempt failed.

If previous RACH attempt failed then, as per 3gpp Spec. 38.321 set PREAMBLE_RECEIVED_TARGET_POWER to

?preambleReceivedTargetPower + DELTA_PREAMBLE + (PREAMBLE_POWER_RAMPING_COUNTER – 1) × PREAMBLE_POWER_RAMPING_STEP

#ra-ResponseWindow:?Msg2 (RAR) window length in number of slots. UE scans for PDCCH looking for msg2 response from gNB within the ra-ResponseWindow, if UE did not receive msg2 within the window it will consider RACH failure and start msg1 transmission again.

#totalNumberOfRA-Preambles:?totalNumberOfRA-Preambles?indicates the total number of preambles available per PRACH occasion for both contention based and contention free RACH. This IE has a range of 0-63, If this IE is missing then All 64 preambles are available and if this IE is present then less than 64 preambles are available to UE’s.

#ra-ContentionResolutionTimer :?The initial value for the contention resolution timer. value sf16 corresponds to 16 subframes, and so on.

?#msg1-SubcarrierSpacing:?Defines Subcarrier spacing of PRACH.

You can also visit below mentioned page to get further details on RACH.

https://www.dhirubhai.net/pulse/4-step-ra-2-step-5g-nr-ajay-kumar-singh/