5G/NR RACH PRACH
The first question question poping up in your mind when you first hear about the word RACH or RACH Process would be 'Why RACH ?', 'What is the functionality/purpose of RACH process ?', "Why we need this kind ki nd of complicated (looks overcomplicated) ?'. For sure, it is not for confusing you :), RACH has very important i mportant functionality especially in LTE (and in WCDMA as well). The main purpose of RACH can be described as follows. i) Achieve UP link l ink synchronization between UE and eNB ii) Obtain the resource resource for Message 3 (e.g, RRC Connection Connection Request) In most of the communication (especially digital comunication regardless of whether it is wired or wireless), the most important precondition is to establish the timing synchronization synchronization between the reciever and transmitter. So whatever communication technology technology you would study, you would see some kind of synchronization mechanism that is specially designed for the specific communication.
In NR (in LTE and WCDMA WCDMA as well), the synchronization synchronization in downlink downlink (Transmitter (Transmitter = gNB, Reciever = UE), this synchronization is achieved by the special synchronization channel (special physical signal pattern). This downlink sync signal gets broadcasted to everybody and it is get transmitted all the time with a certain interval. However in Uplink(Transmitt Uplink(Transmitter er = UE, Reciever = gNB), it is not efficient (actually (actually waste of energy and causing a lot of interference to other UEs) if UE is using this kind of broadcasting/always-on synchronization mechanism. You may easily understand this kind of problem. In case of uplink, this synchronization synchronization process should meet following criteria i) The synchronization process should happen only when there is immediate necessity ii) The synchronization should be dedicated to only a specific UE All the complicated/confusing stories in this page is mostly about the process specially designed mechanism to meet these criteria. Another purpose of RACH process is to obtain the resource for Msg3 (Message 3). RRC Connection Request is one example of Msg3 and there are several different types of Msg3 depending on situation. You would figure out this part in reading through this page and this is not very complicated to understand.
G-NR PRACH: 1.PRACH is a channel to carry preamble from UE for UL synchronization. 2.In 5G there are 13 types of preamble format supported. 3.Format 0, Format 1, Format 2,Format 2,Format 3 ,Format A1,Format A2,Format A3,Format A3,Format B1,Format B1,Format B2,Format B2,Format B3,Format B3,Format B4,Format B4,Format C0,Format C0,Format C1. C1. 4.Two 4.Two types types of preamb preamble le sequen sequences ces are there there in 5G, 5G, 839 and and 139 depending depending on PRACH PRACH preamble preamble format. format. 5.There are 64 preambles defined in each time -frequency PRACH occasion. 6.Following tables are used for different preamble sequence and PRACH format. sequence 839 are used for format 0,1,2 and 3 and 139 is used for others.
When we need RACH ? There are many situation that triggers RACH process. The list of cases are summarized in 38.300-9.2 38.300-9.2.6 .6 as follows. follows. The first half half of the list(i~iv) list(i~iv) is same as in LTE. The second second half of the list would be NR specific. We don't have RRC_INACTIVE state (item v), On-Demand SIB transmition(item vii) in LTE, we have a primitive types of BeamFormaing / BeamManagement in LTE but not as sophisticated sophisticated as in NR(item viii). We do have have CA(SCell addition) in LTE but we don't trigger RACH in any of CA activity in LTE(item vi). i) Initial access from RRC_IDLE; ii) RRC Connection Re-establishment procedure; procedure; iii) Handover; iv) DL or UL data arrival during RRC_CONNECTED RRC_CONNECTED when UL synchronisation status is "nonsynchronised"; v) Transition from RRC_INACTIVE; vi) To establish establish time alignment alignment at SCell additio addition; n; vii) Request for Other SI viii) Beam failure recovery.
Two types of RACH : Contention Based and NonContention Based
Typical 'Contention Based' RACH Procedure is as follows : i) UE --> NW : RACH Preamble (RA-RNTI, indication for L2/L3 message size) ii) UE <-- NW : Random Access Response (Timing Advance, T_C-RNTI, T_C-RNTI, UL grant for for L2/L3 message) iii) UE --> NW : L2/L3 message iv) Message for early contention resolution Now let's assume that a contention happened at step i). For example, two UEs sent PRACH .4 In this case, both of the UE will recieve the same T_C-RNTI T_C-RNTI and resource resource allocation at step ii). And A nd as a result, both UE would send L2/L3 message through the same resource allocation(meaning with the same time/frequency location) to NW at step iii). What would happen when both UE transmit the exact same information on the exact same time/frequency location ? One possibility is that these two signal act as interference to each other and NW decode neither of them.
In this case, none of the UE would have any response (HARQ ACK) from NW and they all think that RACH process has failed and go back to step i). The other possibility would be that NW could successfully decode the message from only one UE and failed to decode it from the other UE. In this thi s case, the UE with the successful L2/L3 decoding on NW side will get the HARQ ACK from Network. This HARQ ACK process for step iii) message is called cal led "contention resolution" process. Typical 'Contention Free' RACH Procedure is as follows : i) UE <--NW : RACH Preamble (PRACH (PRACH)) Ass Assign ignmen mentt ii) UE --> NW : RACH Preamble (RA-RNTI, (RA-RNTI, indication for L2/L3 message size) iii) UE <--NW : Random Access Response (Timing Advance, C-RNTI, UL grant for L2/L3 message)
PRACH: Overview
After a cell search, the UE establishes a connection with the cell through a random access procedure and obtains uplink synchronization.
PRACH: Used to transmit preamble sequences. The gNodeB measures the preamble to obtain the transmission transmission delay between between the gNodeB and the UE, and informs informs the UE of the uplink timing information through the timing advance command.
PRACH PRACH resour resources ces:: – Time domain: Time domain position (system frame, subframe, slot, and symbol), duration, and period. – Frequency domain: Start RB and the number of occupied RBs. – Code domain: Preamble sequence (root sequence
u
and cyclic shift parameter v )).. Each cell has 64 preamble
sequences, which are generated based on the values of
u
and v .
The UE is informed of the PRACH resources in the RMSI (SIB1).
5G NR PRACH function, 5G NR PRACH contents, mapping and physical layer processing of 5G NR PRACH (Physical Random Access Channel).
PRACH Preamble: Basic Format
The preamble sequence is classified into the long sequence and short sequence according to the preamble sequence lengths. •
•
The long sequence uses the sequence design in LTE. There are four formats for the long sequence. The maximum cell radius and typical scenarios in different formats are as follows: Form Fo rmat at
Sequ Se quen ence ce Le Leng ngth th
Subc Su bcar arri rier er Sp Spac acin ing g
Time Domain Duration
Occupied Bandwi dt dth
Maximum Cell Radius
Typi ca cal Sc Scenari os os
0
839
1.25 kHz
1.0 ms
1.08 MHz
14.5 km
Low speed and high speed, normal radius
1
839
1.25 kHz
3.0 ms
1.08 MHz
100.1 km
Ultra-wide coverage
2
839
1.25 kHz
3.5 ms
1.08 MHz
21.9 km
Weak coverage
3
839
5 kHz
1.0 ms
4.32 MHz
14.5 km
Ultra-high-speed
The short sequence is a new format in NR. In 3GPP Release 15, the subcarrier spacing can be {15,30} kHz on the sub-6 GHz band, and can be {16,120} kHz on the above 6 GHz band. Form Fo rmat at
Sequ Se quen ence ce Le Leng ngth th
Subc Su bcar arri rier er Sp Spac acin ing g
Time Domain Duration
Occupied Bandwi dt dth
Maximum Cell Radius
A1
139
15·2μ (μ=0/1/2/3)
0.14/2μ ms
2.16·2μ MHz
0.937/2μ km
small cell
A2
139
15·2μ
0.29/2μ ms
2.16·2μ MHz
2.109/2μ km
Normal cell
A3
139
15·2μ
0.43/2μ ms
2.16·2μ MHz
3.515/2μ km
Normal cell
B1
139
15·2μ
0.14/2μ
2.16·2μ
0.585/2μ
B2
139
15·2μ
0.29/2μ ms
2.16·2μ MHz
1.054/2μ km
Normal cell
B3
139
15·2μ
0.43/2μ ms
2.16·2μ MHz
1.757/2μ km
Normal cell
B4
139
15·2μ
0.86/2μ ms
2.16·2μ MHz
3.867/2μ km
Normal cell
C0
139
15·2μ
0.14/2μ
2.16·2μ
5.351/2μ
km
Normal Cell
C2
139
15·2
0.43/2 ms
9 297/2 km
Normal Cell
μ
μ
ms
ms
MHz
MHz
2 16·2 MHz μ
μ
km
Typi ca cal Sc Scenari os os
small cell
PRACH Preamble: Sequence Generation
ZC sequence generation: xu i e
j
ui ( i 1)
L RA
,
i
0,1, ..., LRA
1
– u: Indicates the root sequence index. The index of the first root sequence of each cell is configured by the gNodeB for the UE. If the first root sequence is insufficient to generate 64 preambles, the next root sequence is automatically used until 64 preambles are generated. – LRA indicates the length of the root sequence. The long sequence length is 839, and the short sequence length is 139.
Cyclic shift:
xu , v ( n)
xu ((n
Cv
) mod LRA )
– The cyclic shift Cv is as follows:
vN CS C v 0 f v, N CS
v 0,1,..., 0 N ZC N CS 1, N CS
for unrestricted sets
N CS 0
for unrestricted sets
v 0,1,...,
for restricted sets
– Ncs: restrictedSetConfig indicates the basic length of the cyclic shift. – The length of v is the number of preambles that can be generated by one root sequence. – unrestricted sets is used in low-speed scenarios. restricted sets is used in high-speed scenarios. LRA 1
Extension to the frequency domain:
yu ,v ( n)
xu,v (m) e
m 0
j
2 mn LRA
PRACH Preamble: Time Domain Structure
The preamble consists of two
A1, SCS 15 kHz
parts:
A2, SCS 15 kHz
– Cyclic prefix (CP)
A3, SCS 15 kHz
– Preamble sequence
B1,, SC B1 SCS S 15 kHz
Differences in the time domain
B2,, SC B2 SCS S 15kHz
of different preamble formats
B3,, SC B3 SCS S 15kHz
are as follows:
B4, SC SCS S 15kHz
– CP length
C0, SCS 15 kHz
– Sequence Sequence length length C2,, SC C2 SCS S 15 kH kHz z
– GP length – Number of sequence repetitions
0, SCS 1.25 1.25 kHz
CP
3, SC SCS S 5 kHz
CP
PUSCH, PUSC H, SCS15 kHz PUSCH, PUS CH, SCS30 kHz
PUSCH
Sequence
Sequence
GP
GP
PRACH: Time Domain Position
When a UE initiates the random access, the UE sends a preamble on the P RACH. RACH.
The time domain position of the PRACH is determined by the the frame number, subframe number, slot number, and occasion number, as shown in the following figures.
Radio frame where the
PRACH
PRACH is located
period …
…
…
…
Subframe Subframe where the PRACH is located
0
1
2
3
PRACH PRACH occasion
PRACH slot
5
6
7
8
PRACH: Frequency Domain Position
Frequency position (left) and number of occupied PRBs in frequency domain (right)
System
bandwidth Initial BWP
PRACH
Sequence Length
PRACH SCS
PUSCH SCS
PRACH PRBs (From the Perspective of PUSCH)
839
1.25
15
6
839
1.25
30
3
839
1.25
60
2
839
5
15
24
839
5
30
12
839
5
60
6
139
15
15
12
139
15
30
6
139
15
60
3
139
30
15
24
139
30
30
12
139
30
60
6
139
60
60
12
139
60
120
6
139
120
60
24
139
120
120
12
Beam Management in 5G NR
Introduction:: Introduction The 5G NR (New Radio) is the t he latest in the series of 3GPP standards which supports very high data rate with lower latency compare to its predecessor LTE (4G) technology. 5G NR supports FR1 and FR2 frequency bands. FR1 is sub -6 GHz, from 450 to 6000 MHz where as FR2 is mmwave mmwave band (from 24.25 24.25 GHz to 52.6 GHz). As the mmwave mmwave band uses very high frequency, frequency, it leads to propagation loss and other losses. To compensate for the losses, directional communication is essential at such frequencies. Antenna arrays with large number number of antenna elements make it possible due to smaller wavelengths. This concept provide beamforming gain to the RF link budget which helps in compensation of propagation loss. Moreover, large antenna array helps to achieve higher data rate due to spatial multiplexing technique. These directional links require accurate alignment of transmitted and received beams. In order to achieve alignment of beam pair and to have required end to end performance with desired delay, beam management operations are introduced in the 5G NR. Beam management operations are essential during Initial Access (i.e. IDLE mode) when UE is not in connection with gNB and during tracking (i.e. CONNECTED CONNECTED mode) mode) when UE is exchanging excha nging data data with the gNB (i.e. network). network).
One of the main new features in NR is the support for analog beam-forming, which is foreseen to be prevailingatmillimeter-wavefrequencies.Forthi prevailingatmillimeter-wavefrequencies.Forthispurposeanewfram spurposeanewframeworkcalledbeam eworkcalledbeammanagemen managementt has been developed in order to support analog beam-forming at both the BS and the UE side. Beam managementhasbeendefinedin3G managementhasbeendefinedin3GPPasasetofLay PPasasetofLayer1/2proceduresto er1/2procedurestoacquireandmain acquireandmaintainasetof tainasetof BSand/orUEbeams26 thatcanbeusedfordownlinkand thatcanbeusedfordownlinkanduplinktransmission/rec uplinktransmission/reception[1].Itincludes eption[1].Itincludes a number of features, such as: 1. 2. 3. 4. 5. 6. 7.
• Sweeping. Covering an angular sector by sweeping analog beams over the sector . sector . • Measurement. Measuring the quality of different beams. beams . • Reporting. Reporting beam information such as which beams are best and their measured qualities. • Determination. Selecting one or a few beams out of a number of candidate beams. • Indication. Indicating which beam or beams has been or have been selected for data transmission. • Switching. Switching to another beam if another beam gets higher quality than the current beam. • Recovery. Finding a new beam if the current beam cannot maintain a communication link due to, e.g., blockage.
Beamforming Digital vs Analog ●
Beamforming - scheduling in time/frequency/spatial time/frequency/spatial (beam(beam- or directional) directional) domain,
○
has to be considered when the scheduling prioritizations and li nk adaptation decisions are taken
Digital BF
●
BF weights attached in frequency domain (frequency subframe scheduling possible):
○ ○
Time
Different allocation in same symbol can be beam formed in different directions In one subframe, Control, UL and DL may have their separate BF
Analog BF
●
BF weights attached in time domain:
○ ○
All allocation in same symbol is beam formed in same direction In one subframe, Control, UL and DL may have their separate BF
Beam Management The beam management is nothing but a procedure with set of phases like, (a) Beam sweeping (b) Beam measurements (c) Beam determination (d) Beam reporting (e) Beam failure recovery
(a) Beam sweeping:
Beam Sweeping is a technique to transmit the beams in all predefined directions in a burst in a regular interval. For example, the first step in mobile terminal attach procedure is Initial Access, which is to synchronize with system and receive the minimum system system information broadcast. So a “SS Block” carries the PSS, the SSS and the PBCH, and it will be repeated in predefined directions (beams) in time domain in 5ms window, this is called a SS burst, and this SS burst will be repeated in 20ms periodicity typically. Below diagram illustrates the concept.
It’s understandable that above illustration of 20 beams based cell sector coverage diagram (in the previous section) will not have fixed beams (always on) with reference signals and synchronization signals, it’s just for visualization. So it’s clear now a 32 beams Nokia Nokia gNB will transmit transmit 32 SS SS blocks in different predefined directions (beams) in regular interval, the set of directions covered by the SS blocks may or may not cover the entire set of predefined directions available. The maximum number of predefined directions (beams / SS blocks) in the SS burst set is frequency dependent, like up to 3 GHz its “4”, from 3 GHz to 6 GHz its “8”, and from 6 GHz to 52.6 GHz its “64”
(b) Beam measurements / (c) Beam determination:
In IDLE mode the measurement is based on SS (Synchronization Signal), and in the connected mode it’s based on CSI -RS in DL and SRS in UL. The CSI-RS measurement window configuration like periodicity and time/frequency offsets are relative to the associated SS burst. The best beam needs to be searched periodically, by using the SS and CSI-RS measurement results. Like SS blocks, CSI-RS will also be covered using beam sweeping technique, considering the overhead in covering all the predefined directions, CSI-RS will be transmitted only in the subsets of those predefined directions (beams), based on the locations of the active mobile terminals. The SRS in UL is similar to LTE spec, the mobile terminal will transmit the SRS based based on gNB directions directions and and gNB will measure measure SRS SRS to determine determine the best best UL beam. The DL beam is determined by the mobile terminal, termi nal, the criterion is the beam should be received with maximum signal strength above a predefined threshold.
(d) Beam reporting: In IDLE mode, after t he mobile terminal selected a SS block (beam), for that SS block there is a predefined predefined one or more RACH RACH opportunities opportunities with with certain time time and frequency frequency offset and direction (special to this SS block only), so that the mobile terminal knows in which transmit (UL) beam to transmit the RACH preamble. This is a way for mobile terminal to notify the gNB which one is the best beam. The gNB (transmit/ receive point, point, TRP) will will be indicated to the mobile terminal in the system information, there is a one to one mapping between between beam sweeping (SS block) blocks. The UE will send PRACH preamble in the UL SS Block corresponding to the DL SS Block in which the best Signal strength is detected. detec ted. Below diagram illustrates the Rx beam to Tx beam mapping during initial access in 5G NR.
In connected mode, the mobile terminal will provide feedback using using control channel, in case of link failure and no directions can be recovered using CSI-RS, the mobile terminal will try to recover the link using the SS bursts.
Beam failure recovery:
When the mobile terminal is suffering from poor channel condition, it will get it as a beam failure f ailure indication from lower layers. The mobile terminal will request for a recovery by indicating a new SS block or CSI-RS, this will be done by starting a RACH procedure. The gNB will transmit a DL assignment or UL grant on the PDCCH to t o end the beam failure recovery.
BeamManagementProcedure
Although not explicitly stated in the specifications, specifications, downlink downlink beam management management has been divided divided into three procedures [1]: • P-1. P-1. The purpose of P-1 P-1 is to find initial BS Tx beam(s) and possibly possibly also UE UE Rx beam(s) by performing a beam sweep over a relatively wide angular sector. • P-2. P-2. This is used for beam refinement of the BS Tx beam(s) by performing performing a beam sweep in a more narrow angular sector than in P1. • P-3. P-3. This is used for performing an Rx beam sweep at the UE. In P-3, the BS Tx beam is fixed during the UE Rx beam sweep. There are similarities between the procedures and not all procedures are needed. Furthermore, P-2 can be a special case of P-1. An example of how the P-1, P-2, and P-3 procedures can be performed is schematically illustrated in Fig. 7.14. In P-1, the BS performs a beam sweep over an angular sector that covers the entire ent ire cell by transmitting a unique reference signal in each beam. To limit the number of beams in such a wide beam sweep the beams could be relatively wide to give an initial, coarse estimate of the best beam direction. The reference signal could be, e.g., the SSBs during initial access
or a periodic CSI-RS CSI-RS transmission that has been configured for beam management. The UE measures the power of the received reference signals from all BS Tx beams using a wide wide Rx beam and reports to the BS which beam has the highest received power. In P-2, the BS performs beam refinement by an aperiodic CSI-RS CSI-RS transmission using narrower beams in an angular sector around the best beam reported by the UE in P-1. The UE measures the power of the received CSI-RSs from these BS TX beams, st ill using a wide Rx beam, and it reports to the BS which of the narrow narr ow beams has the highest received power. In P-3, the BS transmits CSI-RS repeatedly in the best narrow beam reported by the UE in P-2 so that the UE can perform an Rx beam sweep to find its best bes t Rx beam by measuring the power of the received CSI-RS CSI-RS in each Rx beam. In the data transmission, the BS uses the best BS Tx beam found during P-2 and the UE uses the best UE Rx beam found during P-3. Notethatthisisjustoneexampleofhowtoperformbeammanagem Notethatthisisjustoneexampleofhowtoperform beammanagementandotherwaysarepossible. entandotherwaysarepossible. For example, P-1 could be a joint BS Tx/UE Rx beam sweep in which the UE sweeps its Rx beams for each BS Tx beam. The BS then has to repeat the reference signal transmissions transmissions in each BS BS Tx beam so that the UE UE can evaluate evaluate different different Rx beams for every every BS Tx beam. Therefor Therefore, e, this approach is more costly in terms of reference signaling overhead and beam ac quisition time. To provide robustness against blocking, a UE can be configured to monitor PDCCH on multiple beam pair links. For example, while data transmission is being performed on an active act ive beam pair link, the UE can monitor PDCCH on another beam pair as a backup link for swift fallback if t here should be a sudden blockage of the active link.
Beam management procedures. Schematic illustration of the beam management procedures P-1, P-2, and P-3.
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