# 2-Step RA v.s. 4-Step RA (Contention-based)
###### tags: `Master's Degree`
---
[TOC]

---
## **Must Know in RACH Procedure**
### **2-Step RA or 4-Step RA**
- If RSRP of the DL Pathloss Reference > msgA-RSRP-Threshold, set to 2-step RA; else, set to 4-step RA (3GPP TS 38.321 Release 16 5.1.1)
- 目前 RA 的決定是讓UE根據收訊品質來判斷 : 信號強的 UE 用 2-Step RA, 弱的選 4-Step RA
- In MsgA transmission, if Random Access procedure is not completed, set to 2-step RA (3GPP TS 38.321 Release 16 5.1.3a)

### **How RACH Procedure Start**
:::success
Triggered upon request of PRACH transmission by higher layers / by PDCCH order

**ROs (PRACH Occasions)** : time slots in which UE can transmit Preamble on PRACH
- Associating SSB indices with PRACH Occasions & preamble indices
- Multiple PRACH occasions in Time-Frequency domain in each RACH slot
- One-to-One, One-to-Many, Many-to-One association between SSB & ROs

**POs (PUSCH Occasions)** : time slots in which UE can transmit Data on PUSCH
- Associating PRACH Occasion & Preamble index selected for corresponding Preamble transmission
- Multiple PUSCH occasions in Time-Frequency domain in each slot
- One-to-One, One-to-Many, Many-to-One association between Preambles & PUSCH Resources

**PRU (PUSCH Resource Unit)**
- Associating specific PO & specific DMRS (Demodulation Reference Signal) port / sequence
- Defined as PO & DMRS port / sequence used for MsgA payload transmission
- Aiming to increase overall Capacity of 2SR procedure (e.g. small payload size for low coverage situation)
:::

### **RNTI (Radio Network Temporary Identifier)**
:::info
Used to differentiate and identify
- connected UE in cell / specific radio channel
- group of UEs in case of paging / for which power control is issued by eNB
- system information transmitted for all UEs by gNB
:::
:::spoiler Purpose of RNTI & mapping relationship with channel
:::

### **DCI (Downlink Control Information)**
:::warning
- Information of UL resource allocation & Descriptions about DL data transmitted to UE
- Once DCI data is constructed, it goes to channel coding process (over PDCCH)
:::
:::spoiler RNTI Types & DCI Format
:::

### **CRC (Cyclic Redundancy Check)**
:::danger
- Used for Error-Detection and Correction
- In RACH procedure, *DCI Format with CRC* scrambled by *RNTIs*
- Make UEs get their own msg. (T-F Resources, MCS, ...) by de-scrambling DCI
:::
:::spoiler CRC Generator and Checker
:::

---
## **PRACH Resources Selections / Allocations**
:::spoiler Simple Concept
:::

### **Time-Domain Resources Configuration & Determination**
> Parameter : ***prach-ConfigurationIndex*** in SIB1 (System Information Block 1)
> [3GPP TS 38.211 version 16.7.0 Release 16 - P.55](https://www.etsi.org/deliver/etsi_ts/138200_138299/138211/16.07.00_60/ts_138211v160700p.pdf)

| PRACH Config Index | Preamble Format | SF Number | Starting Symbol | Number of PRACH Slots within SF | Number of Time-Domain ROs within PRACH Slot | PRACH Duration |
|:------------------:|:---------------:|:----:|:---------------:|:--------------------------:|:--------------------------------------:|:--------------:|
- *SF Number indicates on which Subframes are configured with RO*
- *PRACH Starting Position* : Calculation is based on Starting Symbol (symbol index)
- symbol 0 for Index = 2 / symbol 16 for Index = 202
- *PRACH Length* : Calculation is based on Preamble Format
- 25.3 symbols for Index = 2 / 9.3 symbols for Index = 202

:::success
:::

### **Frequency-Domain Resources Configuration & Determination**
> Parameter :
> - ***msg1-FrequencyStart*** indicate Offset of lowest PRACH Occasion in Frequency-Domain with respective to PRB 0
> [3GPP TS 38.211 version 16.7.0 Release 16 - P.52](https://www.etsi.org/deliver/etsi_ts/138200_138299/138211/16.07.00_60/ts_138211v160700p.pdf)
> - ***msg1-FDM*** in SIB1 (System Information Block 1) indicate Number of ROs
> [3GPP TS 38.211 version 16.7.0 Release 16 - P.52](https://www.etsi.org/deliver/etsi_ts/138200_138299/138211/16.07.00_60/ts_138211v160700p.pdf) & 38.331

- *PRACH Starting Position* : Calculation is based on the related position with BWP (UL Bandwidth Part)
- *Number of PRBs (Physical Resource Blocks) in PRACH* : Calculation is based on table below
:::spoiler Table
:::
- *Multiple Frequency-Multiplexed ROs on same Time-Domain RO (ROs associated to one SSB are configured at the same time instance but different frequency locations)*

:::info
:::

### **PRACH Occasion Configuration & Determination (both 2SR / 4SR)**
- A 5G cell will transmit multiple SSBs
:::spoiler Beam & SSB (Synchronization Signal Blocks)
:::
- Each UE will choose one Best SSB among multiple SSBs based on the signal power / quality
- It is possible that multiple UEs select the same SSB
- After Preambles Generation & Knowing Time-Frequency-Domain Ressources
- Need to transmit Preamble(s) via PRACH
- ~~One PRACH Occasion takes place One Physical Resource Block~~

> Parameter :
> - ***ssb-perRACH-OccasionAndCB-PreamblesPerSSB***
> [3GPP TS 38.213 version 16.2.0 Release 16 - P.40](https://www.etsi.org/deliver/etsi_ts/138200_138299/138213/16.02.00_60/ts_138213v160200p.pdf)
> - ***ssb-perRACH-Occasion*** indicate that a RO corresponds to several SSBs
> - 1/8, 1/4, 1/2, 1, 2, 4, 8, 16
> - ***CB-PreamblesPerSSB*** indicate that how many Contention-Based Preambles are mapped on each SSB
> - 0 ~ 64
> - ***totalNumberOfRA-Preambles*** indicate Total Number of Preambles available per RO for both Contention-Based & Contention-Free RACH
> 3GPP TS 38.331

:::warning
- **Example 1 (4 *SSBs* in a Cell, on *Subframes* 9, 1 *Frequency-Domain Resource*)**
- ssb-perRACH-OccasionAndCB-PreamblesPerSSB = 4 : 13
- 1 RO -> 4 SSBs, 13 Contention-Based Preambles -> 1 SSB, 12 Contention-Free Preambles

:::
:::danger
- **Example 2 (4 *SSBs* in a cell, on *Subframes* 9, 2 *Frequency-Domain Resources*)**
- ssb-perRACH-OccasionAndCB-PreamblesPerSSB = 1 : 52
- 1 RO -> 1 SSB, 52 Contention-Based Preambles -> 1 SSB, 0 Contention-Free Preambles

:::

---
## **PUSCH Resources Selections / Allocations**
:::spoiler Configurations for the PO (of MsgA)
- POs can be separately configured from ROs
- Relative Time & Frequency location of PO can be configured with respect to associated RO
:::

### **Time-Domain Resources Configuration & Determination**
> Parameter :
> - ***startSymbolAndLength-MsgAPO*** indicate First PUSCH Occasion in each slot
> [3GPP TS 38.213 version 16.2.0 Release 16 - P.43](https://www.etsi.org/deliver/etsi_ts/138200_138299/138213/16.02.00_60/ts_138213v160200p.pdf)
> - ***msgA-PUSCH-TimeDomainOffset*** indicate Offset in number of slots from the start in a PRACH slot (actual time offset between PUSCH and start of each PRACH slot)
> [3GPP TS 38.213 version 16.2.0 Release 16 - P.44](https://www.etsi.org/deliver/etsi_ts/138200_138299/138213/16.02.00_60/ts_138213v160200p.pdf)
> - ***nrofMsgA-PO-perSlot*** indicate Number of Time-Domain PUSCH Occasions in each slot
> [3GPP TS 38.213 version 16.2.0 Release 16 - P.44](https://www.etsi.org/deliver/etsi_ts/138200_138299/138213/16.02.00_60/ts_138213v160200p.pdf)

:::success
:::

### **Frequency-Domain Resources Configuration & Determination**
> Parameter :
> - ***frequencyStartMsgA-PUSCH*** indicate Starting PRB of PUSCH Occasion is given by an offset value with respect to PRB 0
> [3GPP TS 38.213 version 16.2.0 Release 16 - P.43](https://www.etsi.org/deliver/etsi_ts/138200_138299/138213/16.02.00_60/ts_138213v160200p.pdf)
> - ***nrMsgA-PO-FDM*** indicate Number of PRBs (number of PUSCH Occasions available)
> [3GPP TS 38.213 version 16.2.0 Release 16 - P.43](https://www.etsi.org/deliver/etsi_ts/138200_138299/138213/16.02.00_60/ts_138213v160200p.pdf)
> - ***nrofPRBs-PerMsgA-PO*** indicate Number of PRB’s per PUSCH Occasion
> [3GPP TS 38.213 version 16.2.0 Release 16 - P.43](https://www.etsi.org/deliver/etsi_ts/138200_138299/138213/16.02.00_60/ts_138213v160200p.pdf)

:::info
:::

### **PUSCH Occasion Configuration & Determination (focusing on 2SR)**
- SSB Configuration is almost same as 4SR
- ~~One PUSCH Occasion takes place One Physical Resource Block~~

> Parameter :
> - ***msgA-DMRS-Configuration*** indicate DMRS configuration for PUSCH transmission in PO (Max Number of front-loaded DMRS Symbols)
> [3GPP TS 38.211 version 16.7.0 Release 16 - P.75](https://www.etsi.org/deliver/etsi_ts/138200_138299/138211/16.07.00_60/ts_138211v160700p.pdf)
> - ***msgA-MCS*** indicate MCS (Modulation and Coding Scheme) for data information in PUSCH transmission for PO
> [3GPP TS 38.213 version 16.2.0 Release 16 - P.44](https://www.etsi.org/deliver/etsi_ts/138200_138299/138213/16.02.00_60/ts_138213v160200p.pdf)
> - ***preamble-perPUSCHresourceunit*** indicate a numberof Preambles is associated with a PRU [WO2020227094A1]
> - *preamble-perPUSCHresourceunit* = 1, Preamble & PRU one-to-one mapping
> - *preamble-perPUSCHresourceunit* > 1, Preambles & PRU many-to-one mapping
> - *preamble-perPUSCHresourceunit* < 1, Preamble & PRUs one-to-many mapping

- :::spoiler Time & Frequency Resource Size of MsgA PUSCH
- Total Number of REs (Resource Elements) are determined based on Payload Size and MCS
- Payload Size is based on TB (Transport Block) Size threshold for Contention-Based Preamble
- MCS is derived from Table : MCS index table for PUSCH
- Number of Symbols is derived from Table : Default PUSCH Time-Domain Resource
:::
- :::spoiler MCS index table for PUSCH
:::
- :::spoiler Default PUSCH Time-Domain Resource
:::

- :::spoiler Trade-Off : Mapping between PRACH Resource Set and PUSCH Resource Set
- Multiple-to-One (All Preambles in RO are mapped to same Physical Resources in PO)
- More successful detected Preamble, associated PUSCH parts overlap, unsuccessful decoding
- Overhead of RACH procedure decreases
- One-to-One (Each Preamble in RO is mapped to separate Physical Resources in PO)
- Probability of failure in decoding of Data decreases
- Overhead of RACH procedure increases

- *Each PRACH Resource Subset can consist of one or more ROs and a set of PRACH Preamble Sequences*
- *A mapping between one / multiple PRACH Preambles and a PUSCH Occasion (associated with a DMRS Resource) is per PUSCH Configuration [3GPP TS 38.213 V16.1.0 (2020-03)]*
:::

- :::spoiler Trade-Off : Mapping between RO and PO in embodiments [WO2020227094A1]
- One-to-One mapping
- One-to-Many mapping
- Many-to-One mapping
- *If a PRU is not mapped / associated with a Preamble, PUSCH Resource is not used for 2SR*
- *There is One-to-One mapping between PRACH Preamble and PRU (PUSCH Resource Unit) in these three figures, but actually having one/more ROs mapping to one/more PRUs is possible because POs can consist of multiple PRUs*
:::

- :::spoiler Associating DMRS Resource for PUSCH transmission
- Front-loaded DMRS : DMRS Symbol in front of PUSCH Symbol in a slot (occupying 1-2 symbols)
- Additional DMRS : DMRS Symbol in middle of PUSCH Symbol (occupying 1-2 symbols)
- Config. Type 1 : Min RE Group in Frequency Domain is one RE
- Config. Type 2 : Min RE Group in Frequency Domain is two consecutive REs
- Mapping Type A : DMRS starts transmission from third / fourth OFDM Symbol
- Mapping Type B : DMRS starts transmission from scheduled start Symbol
:::

:::warning
- **DMRS (Antenna) Ports** : Refer to specific set of REs on which DMRS are transmitted (determines Location of DMRS within Resource Grid)
- DMRS configuration index determines Number of DMRS Ports & RE pattern of DMRS
- DMRS frequency offset determines Frequency Position of DMRS relative to Data Symbols
- **DMRS Sequences** : Predefined sequence of Symbols that is used to (de)modulate DMRS
- Selection based on CP (Cyclic Prefix) Length & Transmission Bandwidth of channel
- BS & UE need to use same DMRS Sequence in order to correctly (de)modulate Data

- *Slot configuration example of a MsgA payload transmission from 2 UEs within same PO*
- *PRU : {DMRS_conf1, DMRS_conf2}, associated with 2 different DMRS ports OR 2 different DMRS sequences*
:::

:::danger
- PRACH Slot (4 SSBs & 2 ROs) : 1 RO -> 2 SSBs
- PUSCH Slot (16 PRUs & 8 POs) : 1 SSB -> 4 POs, 1 PO -> 2 PRUs

- *Multiple PRUs can be contained in each PO*
- *Each PRU associates with specific DMRS configuration*
- *Multiple PUSCH Resource Groups with different configurations (different PO may be targeted for different applications and use cases)*
:::

---
## **4-Step RACH Procedure (in detail)**
- *We need to understand this procedure from UE's vision*
- :::spoiler Frame Structure : Message Configuration
:::

### **Message 1 (Random Access Preamble Transmission)**
> **On Shared PRACH**
:::success
:::spoiler **Structure / Function**
- Within each PRACH slot, network can configure one or more ROs (PRACH occasions)
- RO is Time-Frequency resources for PRACH preamble transmission
- UE will randomly select one PRACH preamble (RA ID) among available preambles
- PRACH preamble is associated with RA-RNTI (Random Access Radio Network Temporary Identifier)

:::spoiler **Notice**
- One UE can initiate multiple ROs, multiple UEs can initiate ROs at the same time also
- Multiple UEs can perform RA concurrently by using different PRACH preambles
- Number of ROs that a UE initiates can depend on various factors, such as amount of Data that UE needs to transmit, Traffic Load on network, and Resources that are available for allocation to UE
:::

### **Message 2 (Random Access Response Reception)**
> **On PDCCH (carry DCI) / PDSCH (carry coded data : RAR)**
:::info
:::spoiler **Structure / Function**
- RAPID (Random Access Preamble Identifier) field identifies specific PRACH preamble that UE transmitted
- Detect PRACH preamble (valid response)
- Calculate TA information (timing correction based on PRACH preamble receive timing)
- gNB initialize UL grant resources to one or more UEs trying to perform initial access
- Scheduling grant indicates what resource device should use for transmission
- Msg2 including TC-RNTI (Temporary Cell Radio Network Temporary Identifier) & Back-off indicator MAC header

:::spoiler **Notice**
- gNB scrambles CRC of DCI using RA-RNTI to transmit Msg2
- Multiple RAR to different UEs can be provided within same PDSCH transmission
- While multiple UEs decode PDCCH scrambled by same RA-RNTI (RA-RNTI can be addressed to multiple UEs), UE detects its own corresponding RA-RNTI
- If UE doesn't receive the RAR within RAR window, UE will start back-off to retransmit Msg1
- If RAPID from gNB and RAPID from UE isn't matched, UE will re-select preamble & will re-send Msg1
- If multiple UEs select & transmit same preamble, all of them will receive same Msg2
:::

### **Message 3 (Connection Request Transmission)**
> **On PUSCH**
:::warning
:::spoiler **Structure / Function**
- UE saves TC-RNTI & applies time correction
- Transmit RRC connection request using assigned UL grant resources

:::spoiler **Notice**
- Using HARQ in this part
- UE scrambles CRC using TC-RNTI to transmit Msg3
- So that gNB can use TC-RNTI to de-scramble CRC value & verify integrity of Msg3 message
- If multiple UEs select & transmit same preamble, all of them will send same Msg3 (*scrabling with same TC-RNTI*) on same UL grant resource (from the same Msg2)
:::

### **Message 4 (Contention Resolution Reception)**
> **On PDCCH (carry DCI) / PDSCH (carry coded data : CR)**
:::danger
:::spoiler **Structure / Function**
- *If multiple UEs send Msg3 to gNB, gNB will select one of these UE as successful access (no specific explanation about this), then gNB uses this UE's UE identity to MAC-CE packet, only one UE can identify thie packet by its corresponding UE identity*
- If gNB can decode data transmitted from UE successfully, CR performed at gNB side
- TC-RNTI is promoted to C-RNTI (Cell Radio Network Temporary Identifier) which is as permanent C-RNTI to UE
- If UE can detect CR identification in Msg4, contention is also resolved at UE side
- UE change from RRC_IDLE mode to RRC_CONNECTED mode
- gNB can have already identified UE

:::spoiler **Notice**
- Using HARQ in this part
- gNB scrambles CRC of DCI using TC-RNTI to transmit Msg4
- When handover, UE can still use same C-RNTI in other cell without reassignment
- UE is already in network (handover), UE has been assigned C-RNTI, UE will send C-RNTI within Msg3 to gNB
- gNB knows from Msg3 whether UE has valid C-RNTI or not & gNB will scrambles CRC of DCI using this C-RNTI
- Two possible cases of RA procedure failed -> UE retries new RA procedure (after back-off or without back-off, both are possible)
- Collision of decoding happens because multiple UEs want to transmit Msg3 data on the same resource
- Only one UE's Msg3 could be decoded by gNB because it has the correct information from gNB (e.g. correct TA)
:::

---
## **2-Step RACH Procedure (in detail)**
> UE ID needs to be sent in the payload of MsgA

- :::spoiler Use case of 2-Step RA Procedure
- UE is in RRC_CONNECTED active mode
- UE is in handover
- UE is in transitioning from RRC_CONNECTED inactive -> active mode
:::
- :::spoiler Flow Chart : 2-Step RA Procedure
> *In these flow charts, we need to check contention resolution after receiving MSGB, because successRAR and fallbackRAR are all included in MSGB at the same time (see MSGB MAC PDU), we need to check both to know whether RA is successful or failed*

- Flow Chart 1
- Flow Chart 2
:::
- :::spoiler Pros & Cons
- Shorter RA procedure enabling faster access
- Quickly diminishing if MsgA has to be repeated several times
- Reduce number of LBT operations with a corresponding reduction in overhead and delay
- Less efficient compared due to lack of tight time alignment for MsgA PUSCH transmission
- Extra guard times & guard bands may therefore be needed to handle intracell interference to / from MsgA PUSCH transmissions
- With fallback mechanism, it can be guaranteed that latency performance is no-worse than 4-step RA
:::

### **Message A (Msg1 + Msg3)**
> **On Shared PRACH (preamble) & PUSCH (payload : data)**
- *Both PRACH & PUSCH Resources are shared among a number of UEs for Contention-Based RA*
:::success
:::spoiler **Preamble Transmission**
- MsgA preamble : to identify UE & to estimate timing offset of different UEs (Same type of preambles is used as for 4-Step RA)
- Preamble selection and transmission is same as 4-Step RA
- If same RA configuration is used for 4-Step RA & 2-Step RA
- Same set of PRACH occasions
- Different set of preambles are used for 4-Step RA & 2-Step RA
- If different RA configuration is used for 4-Step RA & 2-Step RA
- Same set of preambles are used for 4-Step RA & 2-Step RA
:::
:::info
:::spoiler **PUSCH Transmission**
- MsgA payload : Time & Frequency resource is pre-configured, different UEs may share same resource
- MsgA PUSCH contents are same as Msg3 contents of 4-step RA
- Network should provide MCS(s) and Time-Frequency Resource size(s) of PUSCH in RRC configuration beforehand
- Mapping from PRACH slots to PUSCH resources : Time-Frequency resource & DMRS resource (estimate channel) will be allocated to UE
- Once UE has selected PRACH occasion & PRACH preamble, UE knows what PRU to use for corresponding MsgA PUSCH transmission
- Once network has detected PRACH preamble within PRACH occasion, network knows in what PRU corresponding MsgA PUSCH transmission is to be received
- Minimum time between PRACH transmission and PUSCH transmission is at least N symbols where value of N depends upon SCS of active UL BWP
- These two transmissions cannot be transmitted in a same slot
- Using HARQ in this part
:::

### **Message B (Msg2 + Msg4)**
> **On PDCCH / PDSCH**
:::warning
:::spoiler **UE receives MsgB successfully**
- Random Access Response is received on C-RNTI
- If UE has included C-RNTI MAC CE in MsgA, after receiving MsgB, UE considers that RA procedure is successfully completed
- Random Access Response is received on MsgB-RNTI
- If UE has not included C-RNTI MAC CE in MsgA, response from network (PDCCH) is addressed MsgB-RNTI
- MsgB contents could be fallbackRAR / successRAR / BI (Backoff Indicator)
:::
:::warning
:::spoiler **UE do not receive MsgB Transmission**
> Parameter :
> - ***msgA-TransMax*** : Maximum number of MsgA preamble transmissions performed before switching to 4-step RA
> - ***preambleTransMax*** : Maximum total number of re-transmit Msg1 preamble transmissions (MsgA + Msg1 attempts) after switching to 4-step RA

- UE retransmits MsgA for *msgA-TransMax - 1* times and then falls back to 4-step RA type
- UE starts to transmit Msg1, if no response still, it would continue to re-transmit Msg1 until *preambleTransMax*
:::
:::danger
:::spoiler **Network detect & decode MsgA PUSCH Transmission (successRAR)**
- gNB provides SuccessRAR including
- TA (Timing Adjustment) command (12 bits)
- Contention resolution identity (48 bits)
- C-RNTI (16 bits)
- RAR possibility includes RRC signaling message for connection set up (only one)
- It's not possible to multiplex RRC signaling messages to multiple UEs within same MsgB PDSCH
:::
:::danger
:::spoiler **Network cannot detect & decode MsgA PUSCH Transmission (fallbackRAR)**
- gNB provide FallbackRAR
- Contains same information as RAR of 4-Step RA (RAPID, UL grant, TA Command, TC-RNTI)
- Indicates UE should continue RA procedure as 4-Step RA
- With UL transmission of Msg3 using scheduling grant included with Fallback RAR
- Transmitting a required grant for transmission Msg3 (retransmission of MsgA-PUSCH using Msg3 of 4-step RA)
:::

---
## **Comparison**
:::success
**Message Contents**
- Considering MsgA preamble part of 2-step RA is kept the same with Msg1 of 4-step RA, size of MsgA payload part and Msg3 is also assumed the same

**PRACH & PUSCH Configurations Mapping**
- In 2-Step & 4-Step RA procedure : one Preamble corresponds to one Payload / PRU
- In 2-Step & 4-Step RA procedure : one Preamble corresponds to one Payload / PRB
- *Collision occurs : for both procedure are the same, when multiple UEs choose same preamble*
- For 2-Step RA procedure : If choose same preamble, collision occurs on PO; If choose different preamble, even they are mapping to same PO, DMRS could solve this problem

**PUSCH Transmission**
- In 2-Step RA procedure, multiple UEs transmit MsgA PUSCH in same Resources by using different DMRS Sequences & same DMRS Port or by using different DMRS Ports & same DMRS Sequence
- Improve resource utilization, but multi-UE interference will cause performance degradation
- In 4-Step RA procedure, different UEs will not be allocated to same Resource (no UE multiplexing) because Msg3 transmission resource is scheduled by gNB
- Better performance if total UL transmission resource is not limited, but decoder's code rate is higher
:::
:::info
**Resources Allocation (Msg3 PUSCH Payload)**
- In 2-Step RA procedure, BS directly reserves fixed resources
- Because Payload are always associated with Preamble, so BS will reserve a specific set of resources for the following UEs' allocation
- *i.e. in MsgA Payload Retransmission, UE will still transmit its data to the same resource (same as the previous allocation)*
- In 4-Step RA procedure, resource allocation is dynamic configuration
- Even Payload is related to Preamble also; however, BS will allocate resources to UEs from those possible & available resources (maybe the previous allocated resource is used to other purpose now)
- *i.e. in Msg3 Retransmission, UE will may transmit its data to the different resource (not same as the previous allocation)*
:::
:::warning
**Downlink signals**
- 4-Step RA
- Msg2 : 64 bits (sub-header 8 bits + timing advance command 12 bits + UL grant 27 bits + TC-RNTI 16 bits)
- Msg4 : 48 bits (contention resolution identity 48 bits)
- 2-Step RA
- MsgB (SuccessRAR) : 96 bits (sub-header 8 bits + timing advance command 12 bits + contention resolution identity 48 bits + C-RNTI 16 bits + others 12 bits)
- MsgB (FallbackRAR) : 64 bits (sub-header 8 bits + timing advance command 12 bits + UL grant 27 bits + TC-RNTI 16 bits)
:::
:::danger
**RA Procedure Comparison in Detail**

:::

---
## **Reference**
:::spoiler Thesis
- [Two-Step Random Access in 5G New Radio: Channel Structure Design and Performance](https://ieeexplore.ieee.org/document/9449057)
- [On Contention-based 2-step Random Access Procedure](https://ieeexplore.ieee.org/document/9299681)
- [On the Reliability of LTE Random Access: Performance Bounds for Machine-to-Machine Burst Resolution Time](https://www.researchgate.net/publication/321604390_On_the_Reliability_of_LTE_Random_Access_Performance_Bounds_for_Machine-to-Machine_Burst_Resolution_Time)
- [Modeling and Analysis of Random Access Channels With Bursty Arrivals in OFDMA Wireless Networks](https://ieeexplore.ieee.org/abstract/document/6975196)
- [Modeling and Estimation of One-Shot Random Access for Finite-User Multichannel Slotted ALOHA Systems](https://ieeexplore.ieee.org/abstract/document/6211364)
- [Power Saving Techniques for 5G and Beyond](https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9112193&tag=1)
:::
:::spoiler Patent
- [Method and appartus of performing backoff during 2-step random access procedure in mobile communication system](https://patents.google.com/patent/KR20210000227A/en)
- [Mapping between prach preambles and pusch resource units for 2-step rach](https://patentimages.storage.googleapis.com/ee/01/29/6ff9a01063cc9a/WO2020227094A1.pdf)
:::
:::spoiler Specifications
- [5G;NR; Medium Access Control (MAC) protocol specification (3GPP TS 38.321 version 15.3.0 Release 15)](https://www.etsi.org/deliver/etsi_ts/138300_138399/138321/15.03.00_60/ts_138321v150300p.pdf)
- [5G;NR; Physical layer procedures for control (3GPP TS 38.213 version 15.6.0 Release 15)](https://www.etsi.org/deliver/etsi_ts/138200_138299/138213/15.06.00_60/ts_138213v150600p.pdf)
- [5G;NR; Physical layer procedures for data (3GPP TS 38.214 version 15.3.0 Release 15)](https://www.etsi.org/deliver/etsi_ts/138200_138299/138214/15.03.00_60/ts_138214v150300p.pdf)
- [5G;NR; Multiplexing and channel coding (3GPP TS 38.212 version 16.3.0 Release 16)](https://www.etsi.org/deliver/etsi_ts/138200_138299/138212/16.03.00_60/ts_138212v160300p.pdf)
- [5G;NR; Physical layer procedures for control (3GPP TS 38.213 version 16.2.0 Release 16)](https://www.etsi.org/deliver/etsi_ts/138200_138299/138213/16.02.00_60/ts_138213v160200p.pdf)
- [5G;NR; Physical layer procedures for data (3GPP TS 38.214 version 16.2.0 Release 16)](https://www.etsi.org/deliver/etsi_ts/138200_138299/138214/16.02.00_60/ts_138214v160200p.pdf)
- [5G;NR; Physical channels and modulation (3GPP TS 38.211 version 16.7.0 Release 16)](https://www.etsi.org/deliver/etsi_ts/138200_138299/138211/16.07.00_60/ts_138211v160700p.pdf)
- [5G;NR; Medium Access Control (MAC) protocol specification (3GPP TS 38.321 version 16.10.0 Release 16)](https://www.etsi.org/deliver/etsi_ts/138300_138399/138321/16.10.00_60/ts_138321v161000p.pdf)
- [Pusch resource selection in 2-step random access](https://patentimages.storage.googleapis.com/d2/20/cd/f0933009d5d22d/WO2021029803A1.pdf)
- [R1-1902466 Intel Channel Structure](https://www.3gpp.org/ftp/Meetings_3GPP_SYNC/RAN1/Docs/)
- [3GPP TS 38.214 – NR; Physical layer procedures for data (h30, g10)](https://itecspec.com/archive/3gpp-specification-ts-38-214/)
:::
:::spoiler Documents
- 4-Step Random Access
- [4G/LTE - RACH](http://www.sharetechnote.com/html/RACH_LTE.html)
- [LTE Random Access Procedure](https://eventhelix.com/lte/random-access-procedure/lte-random-access-procedure.pdf)
- [5G-NR Standalone Access Registration](https://www.eventhelix.com/5g/standalone-access-registration/)
- [5G NR Random Access](https://cafetele.com/5g-nr-random-access/)
- [5G/NR - Initial Aattach](https://www.sharetechnote.com/html/5G/5G_CallProcess_InitialAttach.html)
- [5G/NR - Initial Access/RACH](http://www.sharetechnote.com/html/5G/5G_RACH.html)
- [NR RACH Process](https://www.linkedin.com/pulse/nr-rach-process-syed-mohiuddin/?articleId=6652741642669436928)
- [5G NR 随机接入RACH流程（3）-- Msg1之选择正确的PRACH时频资源](https://blog.csdn.net/GiveMe5G/article/details/102633678)
- [Random access procedure](https://www.nrexplained.com/ra)
- 2-Step Random Access
- [5G NR Rel16 两步接入/2-step RACH](https://blog.csdn.net/m0_45416816/article/details/121616415#:~:text=%E5%9B%9B%202-step%20RACH%20procedure%20%E5%9C%A8%E5%8F%91%E9%80%81%E4%BA%86MsgA%E4%B9%8B%E5%90%8E%EF%BC%8C2-step,RACH%E5%90%8C%E6%A0%B7%E8%A6%81%E5%9C%A8MsgB%20window%E5%86%85%E7%AD%89%E5%BE%85%E6%8E%A5%E6%94%B6MsgB%E3%80%82%20%E5%92%8C4-step%20RACH%E7%B1%BB%E4%BC%BC%EF%BC%8C%E4%BD%86%E8%BF%99%E9%87%8C%E5%B0%9D%E8%AF%95%E8%A7%A3%E7%A0%81%E7%9A%84%E6%98%AFMsgB-RNTI%E5%8A%A0%E6%89%B0%E7%9A%84DCI%EF%BC%8C%E5%92%8C4-step%20RACH%E4%B8%AD%E8%A7%A3%E7%A0%81RA-RNTI%E5%8A%A0%E6%89%B0%E7%9A%84DCI%E4%B8%8D%E5%90%8C%E3%80%82)
- [R16 5G NR Two-Step RACH](https://blog.csdn.net/s_zhchluo/article/details/125328004?spm=1001.2101.3001.6650.4&utm_medium=distribute.pc_relevant.none-task-blog-2%7Edefault%7EESLANDING%7Edefault-4-125328004-blog-121616415.pc_relevant_landingrelevant&depth_1-utm_source=distribute.pc_relevant.none-task-blog-2%7Edefault%7EESLANDING%7Edefault-4-125328004-blog-121616415.pc_relevant_landingrelevant&utm_relevant_index=9)
- [5G NR: 2-Step Random Access Procedure (Release-16)](https://howltestuffworks.blogspot.com/2020/04/5g-nr-2-step-random-access-procedure.html)
- [5G NR 2-Step RACH](https://www.techplayon.com/5gnr-2-step-rach/)
- [5G/NR - 2Step RACH](https://sharetechnote.com/html/5G/5G_RACH_2step.html)
- [2-Step RACH in R16 of 5G NR](https://wirelessbrew.com/5g-nr/2-step-rach-in-release-16-of-5g-nr/)
- [Two-Step Random Access for 5G System: Latest Trends and Challenges](https://www.researchgate.net/publication/347004291_Two-Step_Random_Access_for_5G_System_Latest_Trends_and_Challenges)
- [Chapter 17 - Random Access](https://www.sciencedirect.com/science/article/pii/B9780128223208000179)
- [RACH Occasion - NR LTE related tech oriented blog](https://info-nrlte.com/tag/rach-occasion/)
- [PRACH信道详解(二)：信号生成及时频资源分布](https://zhuanlan.zhihu.com/p/352261610)
- Other Knowledge
- [LTE：RA-RNTI、T-CRNTI、C-RNTI](https://blog.csdn.net/dxpqxb/article/details/104237992)
- [5G NR Radio Network Temporary Identifier (RNTI)](https://www.techplayon.com/5g-nr-radio-network-temporary-identifier-rnti/)
- [5G/NR - DCI 详解](https://blog.csdn.net/lizz2276/article/details/115282863)
- [Explain the Cyclic Redundancy Checks (CRCs)](https://www.tutorialspoint.com/explain-the-cyclic-redundancy-checks-crcs)
- [5G/NR - PUSCH DMRS](https://blog.csdn.net/kakamilan/article/details/104759214)
- [5G/NR - PUSCH DMRS](http://www.sharetechnote.com/html/5G/5G_PUSCH_DMRS.html)
- [DMRS在5G NR各种物理信道上的配置](https://blog.csdn.net/GiveMe5G/article/details/104480204)
- [DMRS在5G NR各种物理信道上的配置](https://www.tspweb.com/key/dmrs%C2%B7%205g.html)
- [5G Explained: Demodulation Reference Signals in 5G NR](https://www.mathworks.com/videos/5g-explained-demodulation-reference-signals-in-5g-nr-1566973057735.html)
- [上行参考信号DMRS for PUSCH](https://blog.csdn.net/zhangbin_leo/article/details/124805128)
- [NR PUSCH(三) 频域资源分配方式](https://blog.csdn.net/asd199086/article/details/127291940?spm=1001.2101.3001.6650.1&utm_medium=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7ERate-1-127291940-blog-104759214.pc_relevant_default&depth_1-utm_source=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7ERate-1-127291940-blog-104759214.pc_relevant_default&utm_relevant_index=2)
- [5G NR Transport Block Size (TBS) Calculation](https://www.techplayon.com/5g-nr-transport-block-size-tbs-calculation/)
:::

---
## **My other notes**
- [Pure ALOHA / Slotted ALOHA](https://hackmd.io/@YTL0307/Bk6j4INGO)
- [Multichannel Slotted ALOHA](https://hackmd.io/@YTL0307/HJijgDtXO)
- [RACH and 3GPP TN/NTN](https://hackmd.io/@YTL0307/rkEGVu2Eu)
- [3GPP-Based Satellite Integration into 5G](https://hackmd.io/@YTL0307/rk-u7gLHd)
- [MAC Scheduler : RACH Problems in NTN](https://hackmd.io/@YTL0307/BJnsSSHQj)