5G mMTC Random Access Procedure (RAP) Abstraction5gopenplatform.org/se2/RandomAccess_Technical_Report.pdf · 2019-02-28 · NPSS and NSSS are located in different subframes. NPBCH

y-m-d 문서번호 페이지

2018-11-30

과제명: 개방형 5G 표준 모델 개발

5G mMTC Random Access Procedure

(RAP) Abstraction


2018-11-30

2

구 분 소속 성명 날짜 서명

작성자

한양대학교 전자공학과 Mamta

Agiwal 2018-11-30

한양대학교 전자공학과 Hu Jin 2018-11-30

검토자

한양대학교 전자공학과 Hu Jin 2018-11-30

사업책임자

버 전 v02

발행일 2018.11.30

상 태


2018-11-30

3

문서 정보

버전 v02

작성자 Mamta Agiwal/Hu Jin

검토자 Hu Jin

발행일 2018.11.30

상태 초안

개정 이력

개정일자 버전 개정내역 작성자 확인자

2018-07-20 v01 1차 작성 Hu Jin

2018-07-20 V02 2차 작성 Hu Jin


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Contents

1. Introduction

2. Narrow Band Internet of Things

3. Random Access Procedure for NB-IoT

4. New Radio (NR) Access Technology

5. Random Access Procedure for mmWave Communications

6. Reference


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1. Introduction

By 2020, every person on earth is expected to have at least 4 internet-connected devices, resulting

in the Internet of Things (IoT) with almost 30 billion devices [1]. Moreover, not only have the

number of machine type connections risen by 300% over the last few years, the economic influence

of IoT (Internet of Things) is expected to be in the range of $2.7 trillion to $6.2 trillion by 2025

[2]. The legacy LTE access-layer technologies, optimized over voice and high data traffic for

limited number broadband connections, are inadequate for such massive number of sporadic IoT

applications. Third Generation Partnership Project (3GPP) has substantiated a new radio access

technology called Narrowband Internet of Things (NB-IoT) to provide improved coverage with

respect to LTE, massive device connectivity along with ultra-low device costs, complexity and

power consumption [3,4]. Standardization for NB-IoT was introduced in 3GPP Release 13. In

LTE Release 14 and 15, NB-IoT was further enhanced for improved user experience in selected

areas through the integration of features such as increased peak data rates, increased positioning

accuracy, introduction of the lower device power class, multicast, improved non-anchor carrier

operation, and authorization of coverage enhancements [5].

Figure 1: 5G work directions

At the same time, next-generation of wireless communications, i.e., fifth-generation (5G), has been

progressively gaining momentum for its capability to address the exponentially growing device

volumes and data demands. 5G wireless supports three generic work directions, (i) massive mobile

broadband (mMBB), (ii) massive machine-type communications (mMTC), and (iii) ultra-reliable


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low latency communication (uRLLC) as summarized in Figure1 [6]. While broadband

communication is optimized over high data rates, several of mMTC and uRLLC applications are

expected to be focused on low data transmissions. Towards 5G wireless systems, 3GPP is

continuously developing new specifications with focus on New Radio (NR) access technologies.

For the timeline, in January 2018, the initial version of Release 15 was introduced and it is almost

frozen while accepting amendments on the contents already included. Furthermore, Release 16,

"5G phase 2", would be completed in December 2019 and the release is expected to meet the ITU

IMT-2020 submission requirements [7].

In this report, we focus on the random access procedure (RAP) introduced in 3GPP Release 15. It

is notable that Release 15 is almost about to be freeze and it includes the contents for both the NB-

IoT as well as 5G-NR. While for NB-IoT systems, RAP becomes more important due to novel

NPRACH design and massive connectivity, it is distinctive in 5G wireless systems due to

introduction of mmWave directional air interface. Therefore, in this report, we review the RAP in

terms of two contents: NB-IoT and 5G-NR.

In section 2, we review and analyze the novel features of NB-IoT designed to operate at180 kHz

system bandwidth. In section 3, we analyze the performance of RAP in NB-IoT with focus on the

design of novel RAP procedures. In section 4, we review and analyze (NR) access technologies

and subsequently, in section 5 we highlight the RAP in 5G communications.

2. Narrowband Internet of Things (NB-IoT)

The radio interface, NB-IoT, is specifically optimized for ultra-low machine type traffic [5]. It is

designed with the basic fundamental of low complexity for reduced device costs and minimized

battery consumption. Towards these regards it is adapted to work at 180 kHz system bandwidth,

that corresponds to only one physical resource block (PRB) in LTE [8]. Although it is an

independent radio interface, it is based on existing LTE functionalities as clear from its integration

in the existing LTE specifications [5]. NB- IoT can be characterized based on its peculiar

requirements

Low Complexity

Low data rate

Long battery life

Low device cost

Low deployment cost

Extended coverage

Support for a massive number of devices.


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2.1 Fundamental Features of NB-IoT

To meet the aforesaid objectives, various techniques were studied and the followings are some of

the key features of NB-IoT:

NB-IoT devices operate at 180 kHz bandwidth, which corresponds to only one physical

resource block in legacy LTE system.

NB-IoT operates in half duplex mode and half duplex FDD operation is allowed. This

makes it possible to operate LTE FDD time multiplexed, avoiding the duplex filter [9].

• NB-IoT uses the limited max transmission power (maximum transmit power of 23 dBm).

• NB-IoT provides 20dB additional link budget, enabling about ten times better area

coverage [10].

NB-IoT supports a reduced peak rate for user data (170kbps in DL and 250kbps in UL).

Max. coupling loss 164dB can be achieved [3].

Multi-carrier operation of NB-IoT is supported.

Up to 3 coverage enhancement (CE) levels are supported.

In NB-IoT, repetition of transmissions is an important means of achieving coverage

enhancement.

NB-IoT performs repetitive transmission for almost every channel to make them decodable

even when the signal quality is very poor.

Downlink of NB-IoT is based on OFDMA with the same 15 kHz subcarrier spacing as

LTE. However, uplink of NB-IoT supports both multi-tone and single-tone transmissions.

Single-tone transmission further supports 15 kHz and 3.75 kHz numerologies [3].

For NB-IoT, with 3.75kHz subcarrier spacing numerology, the slot duration extends to 2ms

and remains compatible with the LTE numerology.

NPSS, NSSS, NRS, NPDCCH, NPDSCH physical signals and channels are supported in

the downlink. However, they can be primarily multiplexed in time [3].

NPSS and NSSS resource element mapping and transmission pattern are different from

LTE. NPSS and NSSS are located in different subframes.

NPBCH carries the Narrowband Master Information Block (MIB-NB) that contains 34 bits.

It is spread over a time period of 640ms.

NB-IoT Reference Signal (NRS) is located in symbol 5,6 and 12,13 of every subframe

except the subframe for NPSS, NSSS subframe.

NPRACH is a newly designed channel since the legacy LTE Physical Random Access

Channel (PRACH) uses a bandwidth of 1.08 MHz, more than NB-IoT uplink bandwidth.

NPUSCH has two formats.


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NPDCCH uses aggregation levels (AL) 1 or 2 for transmitting a DCI. With AL-1, two

DCIs can be multiplexed in one subframe, otherwise each subframe only carries one DCI

(i.e. AL-2), giving rise to a lower coding rate and improved coverage [3].

In NB-IoT, three DCI types are defined: N0: Used to schedule Uplink transmissions, N1:

Used to schedule Downlink transmissions and N2: Used to schedule Paging or Direct

Indication [4].

NB-IoT supports eDRX for power saving. With eDRX, the DRX cycle is extended beyond

10.24 s with a maximum value of 10485.76 s for NB-IoT [11].

NB-IoT is expected to support the battery life of ten years with battery capacity of 5 Wh.

2.2 NB-IoT Design Overview

At the physical layer, NB-IoT occupies a small bandwidth of 180 kHz. NB-IoT reuses the LTE

design extensively with several simplifications and optimizations.

NB-IoT has three deployment options: (i) in-band, (ii) guard-band and (iii) stand-alone, as

shown in Figure 2. While in-band mode utilizes 1 PRB of LTE frequency band resource for

deployment, guard-band deployment employs edge frequency band of LTE. Stand-alone mode

utilizes independent frequency band that does not overlap with the LTE frequency band.

Figure 2: NB-IoT deployment options

Enhanced Coverage: Up to 3 coverage enhancement (CE) levels (CE level 0, CE level 1, CE

level 2) are supported in NB-IoT. One example of different CE levels is shown in Figure 3. While

CE level 0 corresponds to normal coverage, CE level 2 mapped to the worst case, where the

coverage may be assumed to be very poor. Covering power of NB-IoT can be as high as 164 dB

for independent deployment mode. In order to realize coverage enhancement, repeated

transmission is adopted.


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Figure 3: Different Coverage Enhancement Levels

Transmission Mode

For downlink, NB-IoT adopts QPSK modem and OFDMA technology with sub-carrier

spacing of 15 KHz [12]. Uplink includes single sub-carrier and multiple subcarrier, BPSK or

QPSK modem and SC-FDMA technology. Single sub-carrier technology can adopt sub-carrier

spacing of 3.75 kHz or15 kHz with 48 or12 continuous sub-carriers respectively as shown in Figure

4. Multiple subcarrier adopts sub-carrier spacing of 15 kHz with 12 continuous sub-carriers that

can be combined into 3, 6, or 12 continuous sub-carriers. In summary, there is no changes at all in

Downlink and small changes in Uplink as given in the following table 1.

Figure 4: NB-IoT Subcarrier


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Table 1: NB-IoT Key Parameters

2.3 Physical Layer of NB-IoT

In this section, the NB-IoT physical layer design is introduced in detail in both uplink (UL) and

downlink (DL). These design features highlight the novel features of NB-IoT and bring forth

methods in which these channels help fulfil the objectives of NB-IoT. The difference between NB-

IoT and legacy LTE are also expressed so as to understand the working of NB-IoT more clearly.

Most of the physical layer changes in NB-IoT compared to LTE are motivated by the requirements

on low device cost, deep coverage, and long battery lifetime. The low device cost is enabled by

reduced transmit and receive bandwidths and other simplifications. The deep coverage is mainly

achieved through repetition techniques. The long battery lifetime is made possible by the

introduction of long sleeping cycles and efforts to keep the overhead from both higher and lower

layer control signaling as small as possible.

2.3.1 Frame Structure

The downlink transmission in NB-IoT is based on 15 KHz subcarrier spacing. Thus, for 15 KHz,

the slot, subframe, and frame durations are identical to those in LTE as shown in the Figure 5.

However, the uplink transmission can have sub-carrier spacing of either 3.75 kHz or 15 kHz. The

3.75 kHz sub-carrier spacing defines new frame structure as shown in Figure 6 [12]. Similar to

LTE the slot format has 7 number of OFDM symbols per slots. Moreover, the of cyclic prefix (CP)

duration is also identical to those in LTE. All illustrations in this article assume the normal CP

length.

Figure 5: Frame Structure for 15 KHz Figure 6: Frame Structure for 3.75 KHz


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2.3.2 Resource Grid

One physical resource block (PRB) spans 12 subcarriers with the 15 kHz subcarrier spacing and

48 subcarriers with the 3.75 kHz subcarrier spacing correspond to 180 kHz. The resource grid for

the frame based on 15 KHz subcarrier spacing is shown in Figure 7. It comprises of 20 slots within

a radio frame and is same as in legacy LTE resource grid. The resource grid for the frame based

on 3.75 KHz subcarrier spacing is shown in Figure 8 and comprises of 48 subcarriers. Since the

subcarrier spacing reduces by the factor of 4, the slot duration increases by the same factor. Thus,

each slot is of 2ms and there are 5 slots within a radio frame. It is to be noted that the number of

OFDM symbols within each slot is same (7) for both 3.75 KHz and 15 KHz resource grid.

Figure 7: resource grid for the frame based on 15 KHz subcarrier spacing


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Figure 8: resource grid for the frame based on 3.75 KHz subcarrier spacing

2.3.3 RU (Resource Unit)

This is a new concept of resource assignment in NB-IoT compared to legacy LTE. An RU is a

basic unit for NPUSCH allocation and it can take different configurations as highlighted in the

table 2. Figure 9 shows various NB-IoT Uplink Resource Units (RUs). The Figure 9 also

highlights the multi-tone and single-tone transmissions supported for uplink in NB-IoT.

Table 2: NPUSCH Formats


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Figure 9: NB-IoT Resource Units

2.3.4 Signal Generation In NB-IoT downlink is based on OFDM waveform. OFDM Signal can be generated as

2.4 Physical Channels

NB-IoT physical channels are based on legacy LTE design to a large extent. In this section, we

present overview of NB-IoT downlink and uplink channels while focusing at the aspects that

differentiate them from legacy LTE.


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2.4.1 Downlink channels

NB-IoT supports the following physical channels in the downlink:

Narrowband Primary Synchronization Signal (NPSS)

Narrowband Secondary Synchronization Signal (NSSS)

Narrowband Physical Broadcast Channel (NPBCH)

Narrowband Reference Signal (NRS)

Narrowband Physical Downlink Control Channel (NPDCCH)

Narrowband Physical Downlink Shared Channel (NPDSCH)

NPSS and NSSS: Figure10 shows NPSS and Figure11 shows NSSS [4]. Unlike LTE, NPSS and

NSSS are located in different subframes. In legacy LTE system PSS and SSS are located in same

subframes. While NPSS is transmitted in every radio frame, NSSS is transmitted in every two

radio frames (i.e. in even frame). Both PSS and SSS are transmitted in every radio frame in legacy

LTE networks. Due to narrow bandwidth NB-IoT can carry small number of data in single OFDM

symbol. Thus, multiple OFDM symbols (11 OFDM Symbols) are used to accommodate the

synchronization signal which is long enough to achieve good correlation. Therefore, the entire RB

is used to carry the NPSS. Likewise, NSSS fills out 11 OFDM symbols from the end of a subframe

9 as it is transmitted at the ninth subframe of every two radio frames.

Figure 10: Narrow Band Primary Synchronization Signal Figure 11: Narrow Band Secondary Synchronization Signal

NPBCH: NPBCH carries the Narrowband Master Information Block (MIB-NB). The MIB-NB

contains 34 bits.

4 bits indicating the most significant bits (MSBs) of the System Frame Number (SFN)

2 bits indicating the two LSBs of the hyper frame number

4 bits for the SIB1-NB scheduling and size

5 bits indicating the system information value tag

1 bit indicating whether access class barring is applied


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7 bits indicating the operation mode with the mode specific values

11 spare bits for future extensions

These bits are transmitted over a time period of 640ms, i.e. 64 radio frames [4]. As shown in

Figure12, MIB-NB is split into 8 blocks. First block is transmitted on the first subframe (SF0) as

shown in Figure 12. It is repeated in SF0 of the next 7 consecutive radio frames, respectively. The

transmissions are arranged in 8 independently decodable blocks of 80 ms duration. Majorly,

NPBCH is different from PBCH (LTE) resource element mapping and the transmission cycle.

Figure 12: Narrow Band Master Information Block

NRS: Similar to LTE, NB-IoT also supports cell specific reference(CRS) signal called NRS. NRS

is transmitted in all SFs. To avoid NRS overwriting over legacy CRS, the RE mapping (the

position in resource map) is modified. The signal generation expression for NRS is similar as

RS(LTE) with the exception in the Cell ID parts as indicated below.


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NPDCCH and NPDSCH: . NB-IoT physical channels are primarily multiplexed in time as shown

in Figure 13. It can also be seen from the figure there are a several subframes that can be allocated

to carry NPDCCH or NPDSCH. Similar to LTE NPDCCH consists of control channel elements

(NCCEs). For a single PRB, two NCCEs are defined, one is mapped to the upper six subcarriers

while the other to lower six as shown in Figure 14. Thus, there are two different types of NPDCCH

format (Format 0 and Format 1). NPDCCH Format 0 takes up only one NCCE and NPDCCH

format 1 takes up two NCCEs [8]. NRS also provides phase reference for the demodulation of the

downlink channels. Thus, it is time-and-frequency multiplexed with information bearing symbols

in subframes that carry NPDCCH and NPDSCH. 8 resource elements per subframe per antenna

port is used for NRS. NPDSCH carries user data and broadcast information not transmitted on

NPBCH The maximum transport block size of NPDSCH is 680 bits. Compared to legacy LTE

system, it supports maximum TBS greater than 70,000 bits without spatial multiplexing [8].

Figure 13: Time multiplexing between NB-IoT downlink physical channels and signals.

Figure 14: CCE allocation in the NPDCCH:


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It is to be noted that NB-IoT within the system bandwidth do not block the legacy LTE reference

signal. Thus, for In-Band deployment the first three symbols of each subframe is always reserved

for legacy LTE and are not used for NB-IoT. NB-IoT within the system bandwidth does not block

the legacy LTE Control Signal region. For, In Band operation NPBCH occupies a whole

subframe except the first three symbols at subframe 0. NPSS occupies a whole subframe except

the first three symbols and the last subcarrier at subframe 5 of every radio frame. NSSS occupies

a whole subframe except the first three symbols and the last subcarrier at subframe 9 of every even

radio frame. NB-IoT Reference Signal is located in symbol 5,6 and 12,13 of every subframe except

the subframe for NPSS and NSSS. Figure 15 shows the NB-IoT frame structure for In-band

deployment. The frame structure for guard band and Standalone Deployment is different than the

aforesaid as shown in Figure 16. In this case, NPBCH occupies a whole subframe except the first

three symbols at subframe 0. NPSS occupies a whole subframe except the first three symbols and

the last subcarrier at subframe 4 of every radio frame. NSSS occupies a whole subframe except

the first three symbols and the last subcarrier at subframe 9 of every even radio frame.

Figure 15: Frame Structure for In-Band Deployment (Even Radio Frame)


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Figure 16: Frame Structure for Guardband/Standalone Deployment (Even Radio Frame)

2.4.2 Uplink Channels

NB-IoT supports the following physical channels in the uplink:

Narrowband Physical Uplink Shared Channel (NPUSCH)

Narrowband Physical Random Access Channel (NPRACH)

NPUSCH: has two formats. While Format 1 is used for carrying uplink data, Format 2 is used

for signaling HARQ acknowledgement. Though, Format 1 uses the same LTE turbo code for

error correction, it only supports maximum transport block size of 1000 bits that is less than

LTE. NPUSCH Format 1 supports multi-tone transmission and UE can be allocated with 12,

6, or 3 tones. 6-tone and 3-tone formats are not used in LTE. They are exclusively introduced

for NB-IoT devices which, due to coverage limitation, are not able to benefit from higher UE

bandwidth allocation. NPUSCH also supports single-tone transmission for both 15 kHz and

3.75 kHz.

Details of NPRACH along with the NB-IoT, random access Procedure is explained in next chapter.


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3. Random Access Procedure for NB-IoT

In this section, we present the details of NPRACH followed by Random Access Procedure (RAP).

3.1 NPRACH Novelty

Figure 17: NPRACH Design

The time frequency resource on which random access preambles are transmitted is referred the

narrowband physical RA channel (NPRACH). Similar to LTE, random access procedure is used

by devices to acquire uplink resource for data transmission. However, NPRACH is a differently

designed channel compared to legacy LTE Physical Random Access Channel (PRACH). While

PRACH uses a bandwidth of 1.08 MHz, NB-IoT uplink bandwidth is limited to only 180KHz,

which is far less. Thus, NPRACH preamble is transmitted within 180 KHz range. NPRACH is

based on single-tone transmission.

It is set of 48 subcarriers (with the subcarrier spacing of 3.75 KHz) and is periodically allocated

specifically for preamble transmission. Every NPRACH preamble is made up of 4 symbol groups.

In turn, one symbol group comprises of one Cyclic Prefix (CP) and 5 symbols. To reduce the CP

overhead in NB-IoT, each N-sample OFDM symbol is repeated 5 times and then is added with a

single CP.

Up to three NPRACH resource configurations are supported in a cell. Each configuration

corresponds to a different coverage level. An NPRACH band may comprise of 12, 24, 36, or 48


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subcarriers for each coverage class. Thus, there are 12, 24, 36, or 48 orthogonal preambles such

that each can be uniquely identified by its hopping signature. A device can randomly select one

preamble (initial subcarrier) to transmit for accomplishing the random access procedure. If more

than one device selects the same initial subcarrier, it results in a collision that needs to be resolved

using the random access procedure.

Each NPRACH resource configuration is given by –periodicity, number of repetitions, starting

time, frequency location, and number of subcarriers as listed in table 3 [13].

Table3: NPRACH RRC Parameters and their Description

PHY

Parameter

RRC Parameter Description

N𝑝𝑒𝑟𝑖𝑜𝑑𝑃𝑅𝐴𝐶𝐻 nprach-Periodicity NPRACH resource periodicity. ms40, ms80, ms160,

ms240, ms320, ms640, ms1280, ms2560

N𝑠𝑐𝑜𝑓𝑓𝑠𝑒𝑡𝑃𝑅𝐴𝐶𝐻 nprach-SubcarrierOffset Frequency location of the first subcarrier allocated to

NPRACH. n0, n12, n24, n36, n2, n18, n34, spare1

N𝑠𝑐𝑃𝑅𝐴𝐶𝐻 nprach-NumSubcarriers Number of subcarriers allocated to NPRACH. n12, n24,

n36, n48

N𝑠𝑐_𝑐𝑜𝑛𝑡𝑃𝑅𝐴𝐶𝐻 nprach-NumCBRA-StartSubcarriers Number of starting sub-carriers allocated to contention

based NPRACH random access.

N𝑟𝑒𝑝𝑃𝑅𝐴𝐶𝐻 numRepetitionsPerPreambleAttempt Number of NPRACH repetitions per attempt. n1, n2, n4,

n8, n16, n32, n64, n128

N𝑠𝑡𝑎𝑟𝑡𝑃𝑅𝐴𝐶𝐻 nprach-StartTime NPRACH starting time. ms8, ms16, ms32, ms64, ms128,

ms256, ms512, ms1024,

N𝑀𝑠𝑔3𝑃𝑅𝐴𝐶𝐻 nprach-SubcarrierMSG3-RangeStart Fraction for calculating starting subcarrier index for the

range of NPRACH subcarriers reserved for indication of

UE support for multi-tone msg3 transmission. zero,

oneThird, twoThird, one

3.2 Random Access Procedure

Similar to LTE, the contention-based RAP in NB-IoT comprises of four steps. RAP can be

performed on the anchor carrier or one of the non-anchor carriers for which NPRACH resource

has been configured in system information [13].


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Figure18: Random Access Procedure for NB-IoT

Step1: In NB-IoT, RAP serves multiple purposes, for instance, initial access when establishing a

radio link, scheduling request, uplink synchronization, etc. An NB-IoT device measures RSRP on

the anchor carrier. Based on RSRP measurements, device decides its enhanced coverage level.

NPRACH resources are mapped to the enhanced coverage levels and each set is further

partitioned into random access preamble groups for single tone and multi-tone Msg3

transmission. The device randomly selects a preamble based on the group(s) of subcarriers

available for the identified enhanced coverage level. For NB-IoT devices, the Random Access

Response (RAR) Window Size and Contention Resolution (CR) Timer also corresponds to the

selected enhanced coverage level and NPRACH. The device instructs the physical layer to transmit

a preamble with the number of repetitions required for preamble transmission corresponding to the

selected preamble group using the selected NPRACH that corresponds to the selected enhanced

coverage level along with corresponding RA-RNTI, subcarrier index and preamble received target

power [13].

Step2 : Once the randomly selected preamble is transmitted, the NB-IoT device monitors the

PDCCH for RAR identified by the RA-RNTI. For NB-IoT devices RA-RNTI is evaluated as

where SFN_id gives the index of the first radio frame of the specified NPRACH. Carrier_id

provides the index for uplink carrier associated with the specified NPRACH and it is 0 for the

anchor carrier [13]. When PDCCH scheduling RAR is not received within the given RAR window,

or when the received RAR does not contain RA-RNTI corresponding to the transmitted preamble,

the RAR reception is considered as not successful. The RAP is reattempted after the Back-OFF


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time with the increased power either in the same enhanced coverage level or in the next coverage

level.

Step3: If NB-IoT device receives RAR successfully, it transmits message 3 to the resource block

that is specified in the RAR. Subsequently, it starts the contention resolution (CR) timer. If more

than one devices selected the same preamble in step1, they would transmit message 3 in the same

resource block causing collision. In case of collision, either one or none of the devices would be

able to successfully transmit the message 3. The eNB sends CR message to only those devices

from which it received message 3 in step 4.

Step 4: If CR message is received before the contention resolution timer expires, the RAP is

considered to be successfully completed. However, the devices for which CR message is not

received, perform random backoff and retransmit a new attempt in a newly chosen initial subcarrier

in the next NPRACH.

3.3 Coverage Enhancement and Random Access Procedure

NB-IoT provides massive connectivity for devices with ultra-low costs, complexity and power

consumption while at the same it offers coverage extension beyond the existing cellular

technologies [3]. In Fig.19 (a) we highlight an example with three CE levels such that the coverage

can be extended for devices deployed as CE level changes from CE0 to CE2. CE2 represents the

environments with high path losses. Another example of extended CE can be considered as in

indoor support where the penetration losses are high. In order to achieve the desired coverage

enhancement, the repetition of data and the associated control signaling has been proposed.

Figure19: (a) CE Levels (b) Sequential transitions to different CE levels

CE2

CE1

CE0


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Repetition would cause spectral efficiency loss and some of the recent works in NB-IoT are

focused on evaluation of optimal number of repetitions and assessment of collision/success

probability. We believe, not only spectral efficiency, but the power consumption of the NB-IoT

devices would also be immensely affected by the increase in number of repetitions.

Though the four step RAP for NB-IoT is similar to LTE’s, it is to be noted that the collided devices

can retransmit either in the same CE level or in the next higher CE level. The decision is made

based on the maximum number of attempts that a device can perform in each CE level and in all

CE levels. Figure 19 (b) shows an example of retransmission in different enhanced coverage levels.

(NPTmax,G be the total maximum attempts and NPTmax,b be the maximum number of attempts

in enhanced coverage)

It is to be noted that a very large number of NB-IoT devices are expected to access the network.

Hence, there is a high probability of collision preambles. In LTE power ramping is performed after

every collision which makes sense since number of devices is limited and collision probability is

low. However, in NB-IoT the collided devices first try to reattempt the random access in their

initially chosen CE level. After the maximum number of allowed attempts in the current CE level

are exhausted, the process restarts in the next higher CE level. Not only there is power ramping at

every reattempt, the number of repetitions also increases with increase in the CE level. In dense

NB-IoT deployment, the collisions would be many. To increase power levels as well as repetition

value would degrade the power performance of the random access procedure. This is the major

challenge since the NB-IoT devices inherently are power constrained.

Figure 20 Random Transition to different CE levels


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CE level based Power Ramping (CEPA) mechanism can help in improving the energy efficiency

of random access procedure in high congestion NB-IoT environment. In CEPA mechanism the

device that fail RAP do not indiscriminately ramp their power ramping first in the identified CE

level and they do not sequentially move to the higher CE level as in figure 19 (b). Rather, in CEPA

the devices transits randomly between different CE levels based on RSRP measurement as shown

in Figure 20. If the measured RSRP falls below the attempt at which the collision happened, the

device chooses the lower CE level with less power level. This makes sense for the NB-IoT

landscape where the collisions are more often due to massive connectivity rather than due to poor

channel conditions. To analyses CEPA, we consider a three state semi-Markov model.

3.4 Analysis of Random Access Procedure for different CE levels

In CEPR mechanism we propose to curtail the power consumption of NB-IoT devices in a highly

congested environment. For analysis, we consider a three state semi-Markov model as shown in

Figure 21. We consider three CE levels as three states and all the state transitions are random.

Semi-Markov modelling is considered since it provides flexibility such that the time in any given

state (CE level) is variable. It is to be noted that a device in CE0 would spend less time as the

required signal repetitions are small. Similarly, the repetitions and therefore the time in CE2 is

more as there are more number of signal repetitions.

Figure 21 Markov Model to CE level Transitions


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It is considered that the NB-IoT devices are uniformly distributed over the three CE levels. The

arrivals of new and backlogged devices in each CE level follow Poisson process with a rate of λi.

If the NB-IoT device initially in CE level CE0 fails random access, it measures RSRP. If the

measured RSRP is still above the threshold for CE1 (TH1), the device retries random access in

CE0 with probability P11. However, if the RSRP is below the threshold, the device can either

transit to CE level 1 (state S2) with probability P12 or CE level 2 (state S3) with probability P13

based on the measurement. The transition probabilities P11, P12 and P13 are estimated as:

Where, Ω denotes probability of channel conditions and N1 as number of repetitions in level CE0.

Similarly, for CE1 with N2 number of repetitions the probabilities P21, P22 and P23 are obtained

as:

Similarly, for the CE level CE2 (state S3), with N3 allowable re-attempts, the state transition

probabilities are

The state probability matrix can be obtained as


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Next, we estimate holding time for different states.

Finally, we can calculate the average power of states as PW1, PW2 and PW3 as


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4. New Radio (NR) Access Technology for 5G Wireless

5G wireless systems, quickly coming into the limelight offers to fulfill the new requirements for

future communication that include enhanced Mobile Broadband (eMBB), massive Machine Type

Communication (mMTC) and Ultra Reliable Low Latency Communications (URLLC) as shown

in the figure 1.

Major technical 5G goal include [15]:

10-100 times higher typical user data rate. In a dense urban environment user data rate

would range from 1Gbps to 10Gbps.

1,000 times more mobile data per area (per user). The volume per area (per user) would be

over 100 Gbps/km2.

Support for 10-100 times more connected devices.

10 times longer battery life for low-power massive machine communications where

machines such as sensors or pagers will have a battery life of a decade.

Support of ultra-fast application response times, where the end-to-end latency will be less

than 5ms with high reliability.

Ability to fulfil these requirements under a similar cost and energy dissipation per area as

in today’s cellular systems.

Towards 5G communications. a number of related 3GPP work items have been carried out starting

from 3GPP - 5G Workshop in early 2015 and subsequently followed by several 3GPP meetings

and discussions.

• Release 15: The industry agreed for accelerate 5G standardization schedule. Release 15

(i.e. 5G phase 1) basically comprises of non-standalone (NSA) as well as full standalone

(SA). The non-standalone mode revolves around enabling the enhanced Mobile Broadband

(eMBB) use-case and anchors the connection in LTE while 5G NR carriers are used to

increase data rates and take advantage of the reduced latency. The full Release 15

specification almost completed around the June 2018 and enabled standalone 5G NR with

user and control plane using the 5G next-generation core network (5G NGC). The NSA

and SA releases share the same physical layer specifications and thus NSA hardware

equipment are expected to be forward compatible with the SA standard [16]. 3GPP

officially launched New Radio (NR) specification work with New specification series

38.xxx.

(i) 38.1xx: RF test specifications (UE and BS)

(ii) 38.2xx Layer 1 (physical layer) specifications

(iii) 38.3xx Layer 2 / Layer 3 specifications


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(iv) 38.4xx Core network specifications

(v) 38.5xx UE conformance testing specifications for RF, RRM and protocol testing

• Some of the key 3GPP NR specifications addressed in this report can be found in:

o 38.201 Physical layer; General description [17]

o 38.300 Overall description; Stage-2 [18]

o 38.321 MAC protocol specification [19]

o 38.401 Architecture description [20]

o 38.410 NG general aspects and principles [21]

• 3GPP is already looking towards upcoming Release 16 where major action items include

all deployment scenarios with special focus on massive IoT use cases.

Figure 22 clearly highlights the 3GPP standardization time line [22].

Figure 22: 5G- NR time line

4.1 Fundamental Features of 5G To meet the objective of 5G, various techniques were studied and followings are some of the

major characteristics of 5G-NR.

• Roughly, two large frequency ranges are specified in 3GPP. One is sub 6 GHz and the other is

in millimeter range (mmWave). Depending on the ranges, the maximum bandwidth and

subcarrier spacing varies.

• For sub 6 GHz, the maximum bandwidth is 100 MHz and for mmWave range the maximum

bandwidth is 400 MHz.

• Compared to LTE numerology (subcarrier spacing and symbol length), the most outstanding

difference is that NR supports multiple different types of subcarrier spacing. Sub carrier

spacing(s) of 15KHz, 30KHz,60KHz, 120KHz,240KHz are supported [23].


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• Slot length becomes different depending on numerology. Slot length gets shorter as subcarrier

spacing gets wider.

• OFDM symbol duration (µs) also varies with subcarrier spacing. While OFDM symbol

duration is 66.67 µs for 15 KHz subcarrier spacing, it is reduced to 4.17 µs for 240 KHz

subcarrier spacing [23].

• The length of a Radio Frame is always 10 ms and the length of a subframe is always 1 ms.

However, there are different number of slots within one subframe.

• Numerology selection is not a static. Different numerology (Subcarrier Spacing) can be

used in various different situation and purposes. Moreover, some subcarrier spacing (15,

30 KHz) are used only in Sub 6 GHz and some subcarrier spacing (120 KHz) can be used

in mmWave range only.

• NR does not use support CRS (Cell Specific Reference Signal). Thus, NR PDCCH and NR

PSCCH requires Demodulation Reference Signal (DMRS) whereas LTE PDCCH and

PDSCH does not use DMRS.

• In NR, PSS/SSS/PBCH are bundled into a specific area in the downlink resource grid called

Signal Synchronization Block (SS Block).

• In NR there are two different types of SIBs. One type is the one being transmitted

periodically like SIBs in LTE and the other type is the one being transmitted only when

there is the request from UE.

• Beam forming and beam management are crucial in high frequency mmWave deployment.

Special reference signals are used to let UE knows of the identification of each Beam that

the network transmits. Beam sweeping is important.

• In NR slot format, DL and UL assignment changes at a symbol level while in LTE TDD

the UL/DL assignment is done in a subframe level.

• In NR slot format, there are diverse patterns comparing to LTE TDD subframe

configuration. It is to make NR scheduling flexible especially for TDD operation.

4.2 Novel Design Considerations of NR In this section we first delineate different deployment scenarios for NR which are designed

for heterogeneous, flexible and forward compatible coverage. Next we highlight novel NR

numerologies. Beamforming is crucial for mmWave based 5G communication and is also

presented in this section.

4.2.1 Deployment scenarios of NR.

LTE/LTE-A and NR coexistence offers different deployment scenarios. While sometimes

LTE/LTE-A and NR can have same coverage for some cases their coverages can be different [24].

Thus, the following deployment scenarios are feasible:

(i) LTE/LTE-A eNB as the Master Node: In this the LTE/ LTE-A eNB provides an anchor

carrier (in both control and user planes). The NR gNB is used as a booster carrier and

the data aggregates is supported via the evolved packet core (EPC).


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(ii) NR gNB as the Master Node: In standalone system NR gNB provides both control and

user planes services via the next generation core. A collocated enhanced LTE (eLTE)

eNB is supports additional booster carriers for dual connectivity.

(iii) eLTE eNB as the Master Node: In standalone system the eLTE eNB offers both control

and user plane functionalities via the next generation core. It can also be collocated

with NR gNB to provide booster carriers.

(iv) Inter-Radio Access Technology (RAT) Handover between (e)LTE/LTE-A eNB and

NR gNB: To support handover between eNB and gNB, LTE/ LTE-A eNB is connected

to EPC. Both EPC and NR gNB is connected to the next generation core. Alternately,

eLTE eNB connects to the next generation core and handover between eNB and gNB

is fully managed through the next generation core.

Due to several possible scenarios, heterogeneous deployment of NR is possible with different

coverage. For achieving mobility up to 500 km/h, multiple cyclic prefix (CP) lengths needs to be

adopted in NR. In practice, CP length is also affected by the carrier frequency and subcarrier

bandwidth. Therefore, there can be multiple combinations of physical transmission parameters

(subcarrier, symbol durations, CP length) in NR resulting in different numerologies [24].

4.2.2 Operating Bandwidth

In NR, there are roughly two large frequency range specified in 3GPP: (i) sub 6 GHz and (ii)

millimeter wave (mmWave). The exact frequency ranges (FRs), FR1(sub 6 Ghz) and FR2

(millimeter wave) [25,26] are defined as in table4.

Table 4: NR Frequency Range

While for sub 6 GHz, the maximum bandwidth is 100 MHz, it is 400 MHz for mmWave bandwidth.

Some subcarrier spacing (15, 30 KHz) are exclusively used only in Sub 6 GHz, some (120KHz)

exclusively by mmWave and some (60 KHz) are used by both. The gNB can support different UE

channel bandwidths within the same spectrum for transmitting to and receiving from different UEs

connected to it. Transmission of multiple carriers to the same UE (Carrier Aggression) or multiple

carriers to different UEs within the supported gNB channel bandwidth is feasible.

From a UE perspective, the UE channel bandwidth supports a single NR RF carrier in the uplink

or downlink at the UE. Channel bandwidth for each UE carrier is flexible, but can only be

completely within the gNB channel bandwidth. The relation between channel bandwidth,

transmission bandwidth configuration and guardband [27] is shown in Figure 23.


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Figure23: Channel Bandwidth and Transmission Bandwidth

Tables 5 and 6 provides maximum transmission bandwidth configuration resource blocks (NRB)

for each UE channel bandwidth with respect to the subcarrier spacing for FR1 and FR2. In lower

frequency (like sub 3Ghz, sub 6 Ghz), there is not much of wide band spectrum left for new

technology. Thus, in order to pack as many subcarriers as possible in these limited spectrum, we

need to get subcarrier spacing as small as possible. That's why small subcarrier spacing like 15

KHz, 30 KHz, 60 KHz is used for sub 6 GHz. Wide subcarrier spacing like 120 KHz or 240 KHz

is for the operation in very high frequency like mmWave. As carrier frequency gets higher, the

degree of frequency drift by moving transmitter or receiver gets higher (Doppler spread gets wider

as carrier frequency gets higher). To tolerate this kind of wide range of frequency drift (or shift),

we need to use wider subcarrier spacing.

Table5: Maximum transmission bandwidth configuration (NRB) for FR1

Table6: Maximum transmission bandwidth configuration (NRB) for FR2

FR1 BW

SCS

(KHz)

5 MHz 10 MHz 15 MHz 20 MHz 25 MHz 40 MHz 50 MHz 60 MHz 80 MHz 100

MHz

NRB NRB NRB NRB NRB NRB NRB NRB NRB NRB

15 25 52 79 106 133 216 270 - - -

30 11 24 38 51 65 106 133 162 217 273

60 - 11 18 24 31 51 65 79 107 135

FR2 BW

SCS

(KHz)

50

MHz

100

MHz

200

MHz

400

MHz

NRB NRB NRB NRB

60 66 132 264 -

120 32 66 132 264


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The Tables 7 and 8 provides minimum guardband for each UE channel bandwidth with respect to

the subcarrier spacing for FR1 and FR2.

Table7: Minimum Guardband for each UE Channel Bandwidth for FR1

Table8: Minimum Guardband for each UE Channel Bandwidth for FR2

Since multiple numerologies are supported, when they are being multiplexed in the same symbol,

the minimum guardband on each side of the carrier is the guardband applied at the configured

channel bandwidth for the numerology that is transmitted immediately adjacent to the guard band

[28] as clear from figure. Furthermore, for UE channel bandwidth of value greater than 200MHz,

the guardband applied adjacent to 60kHz subcarrier spacing would be the same as the guardband

defined for 120kHz subcarrier spacing for the same UE channel bandwidth.

Figure 24: Guard time and Numerology

FR1 BW

SCS

(KHz)

5 MHz 10 MHz 15 MHz 20 MHz 25 MHz 40 MHz 50 MHz 60 MHz 80 MHz 100

MHz

15 242.5 312.5 382.5 452.5 522.5 552.5 692.5 - - -

30 505 665 645 805 785 905 1045 825 925 845

60 - 1010 990 1330 1310 1610 1570 1530 1450 1370

FR2 BW

SCS

(KHz)

50

MHz

100

MHz

200

MHz

400

MHz

60 1210 2450 4930 -

120 1990 2420 4900 9860


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4.2.3 Numerologies of NR

Figure25: Flexible Numerologies in NR

NR should cover very wide range of operating frequency (e.g., sub 3 GHz, sub 6 GHz and

mmWave (over 25 GHz). Thus, to obtain high efficiency and performance, more than a single

numerology (subcarrier space) is supported to cover the whole range.

In this section, we describe on NR Frame Structure that is specified in 3GPP specification

(38.211).

• While LTE supports only one type of subcarrier spacing (15KHz), NR supports multiple

types of subcarrier spacing as given in table 9 and Figure 26. This is one of the major

difference between LTE and NR numerology.

• In OFDM, number of subcarrier that can be packed into a specific frequency range is

directly related to spectrum efficiency. More is the number of subcarriers that can be

packed into a frequency range (the narrow subcarrier spacing), the more data can be

transmitted/ received.

Table 9: Cyclic Prefix in NR


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Figure26 : Subcarrier Spacing

Due to various possibilities for subcarrier spacing, the slot length also becomes different [30]

depending on numerology such that the slot length gets shorter as subcarrier spacing gets wider.

However, slots are align at symbol boundaries as shown in Figure27.

Figure27: Slots and Subframes

Though majority of the numerologies can be used for any type of physical channels, there are a

few exceptions [18], as clear from the table 10.


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Table 10: Supported Transmission Numerologies

Table11 provides details about OFDM Symbol Duration. There is inverse-proportional

relationship between subcarrier spacing and OFDM symbol length. Narrow subcarrier spacing

results into longer OFDM symbol length. In turn, longer OFDM symbol can be assigned with more

room for CP which makes the signal more tolerable to fading channel.

Table 11: OFDM Symbol and Cyclic Prefix Duration in NR

Parameter /

Numerology (u)

Subcarrier

Spacing

(KHz)

OFDM

Symbol

Duration (us)

Cyclic Prefix

Duration (us)

0 15 66.67 4.69

1 30 33.33 2.34

2 60 16.67 1.17

3 120 8.33 0.57

4 240 4.17 0.29

4.2.4 Frame Structure of NR

Regardless of numerology, the length of one radio frame and one sub fame remains the same in

the time domain. The length of a radio frame is always 10 ms and the subframe length of NR is 1

ms. Each subframe is composed of an integer number of slots. Each slot comprises of 14 OFDM

symbols [30]. The details of radio frame structure for 15KHz and 30KHz numerology and slot

configuration can be expressed as in Figure 28 (a) and 28 (b). While one subframe is equal to one

slot for 15KHz sub carrier, one subframe is two slots for 30KHz sub carrier. However, for both

the cases, each slot contains 14 symbols.


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Figure 28 (a): Slots for 15 KHz Subcarrier Spacing Figure 28(b): Slots for 30 KHz Subcarrier

Similarly, the configurations for 60 KHz and 120 KHz subcarrier spacing (Figure 29) can be

expressed as

Figure29 (a): Slots for 60 KHz Subcarrier Spacing Figure 29 (b): Slots for 120 KHz Subcarrier

Time-division multiplexing (TDM) scheme in NR is more flexible than that in LTE, such that,

OFDM symbols in a slot are either all downlink or all uplink or at least one downlink part along

with at least one uplink part. Moreover, to support small size packet transmissions, mini-slots are

additionally adopted in NR, where each mini-slot is composed of lesser number of OFDM symbols.

Thus, mini-slot can be considered as the minimum unit for resource allocation/scheduling. As

shown in Figure30, each slot/mini slot can carry control signals at the beginning and/or ending

OFDM symbol-(s). This enables the gNB to immediately allocate resources especially useful for

uRLLC when urgent data arrives. Moreover, in NR, different subcarrier spacing(s) can be

multiplexed within a subframe [24] as clear from the figure.


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Figure 30: Mini slots and Slots

4.2.5 5G/NR - Beam Management

Even though use of Sub 6 GHz would not be precluded from the deployment of 5G(NR), high

frequency deployment would be one of the most important characteristics of 5G (NR). The

vast availability of under exploited mmWave spectrum (6 GHz to 300 GHz) promises to

address increasing bandwidth-appetite of the wireless industry. Though, mmWave spectrum

offers exciting new opportunities for broadband applications, it suffers from excessive path

and propagation losses compared to current frequency bands [15]. The compact form factor

of embedded antenna arrays, enables directive beamforming to overcome excessive mmWave

propagation and path losses. According to researchers, with the use of beamforming transmit

and receive antennas, the propagation properties of the mmWave communications are found

to be comparable to legacy frequency bands [15]. Thus, with high propagation challenges of

mmWaves, directional wireless access becomes important in 5G communications. In a

directional communication, several antenna elements focus their electromagnetic energy to

provide a high beamforming (BF) gain that overcomes large isotropic path loss of mmWaves.


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Figure 31: Beamforming in 5G

However, in a directional beam formed communication, one beam can provide only a

limited spatial coverage and multiple beams are required to cover the entire cell.

Simultaneous transmission over all the beams is would not be possible due to the

complexity of antenna module. As defined by 3GPP, an antenna array model comprises

uniform rectangular arrays of panels ( Mg×Ng), where Mg is the number of panels in a

column and Ng is the number of panels in a row [31]. In each panel, M and N antenna

elements are placed in vertical and horizontal directions, respectively, as shown in Figure

32. These antenna panels are either cross-polarized (P = 2) or co-polarized (P = 1) [31].

Thus, as shown in Figure 32, the tuple (M, N, P, Mg, Ng) describes the rectangular panel

array antenna [31]. Since simultaneous transmission over all the beams would not be

possible, beam sweeping needs to be performed to cover the entire cell. In beam sweeping

operation, different beams are transmitted in different time intervals in a predetermined

way. This directional beam communication complicates several legacy mechanisms and

release 15 address several modifications to accommodate beam based communication.

BRS is a special reference signal to let UE knows of the identification of each Beam that

the network transmits [32]. It occupies 8 subcarriers (5th~12th subcarrier) in every RB

(resource bloc) except the 18 RBs at the center of the frequency. It is to be noted that 18

RBs at the center is reserved for PSS, SSS, ESS. It is transmitted at every symbols (i.e,

symbol 0 ~ 13) in subframe 0 and 25 as shown below. The data is based on Pseudo Random

Data (Gold Sequence).

4.2.6 Synchronization Signal Block

SS Block(SSB) stands for Synchronization Signal Block. Unlike in LTE, SSB refers to

Synchronization/PBCH block because synchronization signal and PBCH channel are

packed as a single block that always moves together. The components of this block are as

follows:


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Synchronization Signal: PSS (Primary Synchronization Signal), SSS (Secondary

Synchronization Signal)

PBCH : PBCH DMRS and PBCH (Data)

Even though it is a small package that resides in a radio frame, it has many different components

in it and the way it works is pretty complicated. Moreover, time Domain transmission pattern of

SS Block in NR [33] is very complicated as shown in figure33. Every SSB resides within a SSB

burst Set (i.e., all of the SSBs within the 5 ms period of the SSB transmission) as shown in figure33.

SS block is assigned with a unique numbers starting from 0 and increasing by 1. This number reset

to 0 in the next SS Burst Set (i.e, next 5 ms span after SSB transmission cycle (e.g., 20 ms). This

unique number (i.e., SSBlock Index) is informed to UE via two different parts within SSB.

One part is carried by PBCH DMRS (i_SSB parameter)

Another part is carried by PBCH Payload.

Figure 32: Antenna Panel in 5G

SSBs in SSB burst confined to 5ms window. SS burst set transmission is periodic. An idle UE

assumes default periodicity of 20ms. Several SSB frequency locations can be supported within

wideband carrier. Number of possible candidates SSB locations (L) in the SS burst set can be (i)

L=4 for up to 3 GHz (ii) L=8 from 3 to 6 GHz, (iii) L=64 from 6 to 52.6 GHz. UE has to identify

the SSB with within the SSB burst set. The mapping of SSBs in SS Burst set is configured by

parameters in SSB [33]. UE identify the best SSB.

http://www.sharetechnote.com/html/5G/5G_PBCH_DMRS.html#Sequence_Generation

http://www.sharetechnote.com/html/5G/5G_PBCH.html#PBCH_Payload_Generation


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Figure 33: Signal Synchronization Block

5 Random Access Procedure for 5G

The overall logic of NR RACH is similar to LTE RACH process (Based on TR 38.804). However,

there are several modifications in view of beam based communication and flexible numerologies.

The major differences are explained as:

Similar to LTE, the main purpose of random access is (i) to achieve uplink synchronization

between UE and eNB and (ii) to obtain the resource for Message 3 (e.g, RRC Connection

Request). Most of the reasons for random access procedure (PDCCH order, RRC trigger,

Time synchronization, etc.) However, the random access procedure can also be initiated

when there is beam failure indication by lower layer (i.e. when serving Beam fails) [34].

Moreover, in NR the supplementary uplink (SUL) can be configured and the SUL carrier

can also be selected for performing Random Access Procedure. One of the reason of SUL

configuration could be that the UE near cell edge may use low frequency supplementary

carrier while the one near the gNB can use the primary carrier as explained in the figure34.

Figure 34: Supplementary Carrier in NR


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The major difference between LTE RACH and NR RACH is due to Beamforming. Beam

forming is supported by default (especially in mmWave) in NR. So for Beamforming mode,

UE need to detect and select a best beam for random access procedure. This beam selection

process is the fundamental difference between LTE RACH and NR RACH process.

SSBs are be mapped to preambles (and/or) PRACH resources. Thus, based on the identified

SSB , the UE selects the PRACH resource AND/OR Preamble that are associated with the

selected SSB. SSBs are identified based on SS-RSRP.

Figure 35: PRACH and SSBs

Beam sweeping in crucial process in NR, and the time index of SSB gives the idea about

the beam. The mapping of SSBs to preambles/PRACH resources assist in the identification

of the beam that can be used for communication between UE and gNB as explained in the

later part of this section.

NR also use various types of Preamble Format as shown in Figure 36.

Figure 36: Preamble Formats in NR


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Power ramping counter increments if UE TX beam is not changed or SSB is not changed,

assuming that there is no power ramping counter suspension notification. Kindly note that

New- Power ramping counter suspension notification is defined for beam switching.

However, if SSB is changed power ramping is not done.

Figure 37: Power Ramping in NR

The RA-RNTI associated with the PRACH in which the RA Preamble is transmitted, is

computed for 5G-NR as:

For contention free RACH, while multiple msg1 transmissions can be performed (for

different beams), single RAR window is used by UE to monitor multiple RA-RNTIs. When

RAR is received, UE stops multiple preamble transmission.

Figure38: UE Monitors Multiple RA-RNTIs

s_id : index of first OFDM symbol of the specified PRACH (0 ≤ s_id < 14),

t_id: index of first slot of the specified PRACH in a system frame (0 ≤ t_id < 80),

f_id: index of specified PRACH in the frequency domain (0 ≤ f_id < 8),

ul_carrier_id UL carrier for Msg1 (0 for normal carrier, and 1 for SUL carrier)


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Table 12: RACH Parameters in NR

Parameter Description

prach-ConfigIndex The available set of PRACH occasions for the transmission of the random access preamble

preambleReceivedTargetPower Initial RA preamble power

preamblePowerRampingStep The power-ramping factor.

ra-PreambleIndex: Random access preamble

preambleTxMa The maximum number of preamble transmission

ra-Msg3SizeGroupA (per cell) the threshold to determine the groups of Random Access Preambles

deltaPreambleMsg3 ∆PREAMBLE_Msg3

ra-ResponseWindow Time window to monitor RA response(s);

ra-ContentionResolutionTimer Contention Resolution Timer

messagePowerOffsetGroupB the power offset for preamble selection

rsrp-ThresholdSSB RSRP threshold for selection of SSB and corresponding RA Preamble and/or PRACH

occasion

csirs-Threshold RSRP threshold for selection of CSI-RS and corresponding RA Preamble and/or PRACH

occasion

sul-RSRP-Threshold RSRP threshold for the selection between the NUL carrier and the SUL carrier

ra-ssb-OccasionMaskIndex PRACH occasion associated with an SSB the MAC entity may transmit a RA Preamble

ssb-perRACH-OccasionAnd

CB-PreamblesPerSSB

number of SSBs mapped to each PRACH occasion and the number of Random Access

Preambles mapped to each SSB

numberOfRA-PreamblesGroupA number of RA Preambles in Preamble group A for each SSB, if Preambles group B is

configured

PCMAX,f,c of the SUL carrier if SUL carrier is selected for performing Random Access Procedure

PCMAX,f,c of the NUL carrier if NUL carrier is selected for performing Random Access Procedure

the set of Random Access Preambles and/or PRACH occasions for beam failure recovery request, if any

Set of Random Access Preambles for SI request and corresponding PRACH resource(s), if any;

UE transmits PRACH based on the set of resources mapped to the selected SSB time index.

On identifying the SSB, UE may randomly select from the resources associated with the

SSB. Since UE transmits PRACH/Preamble mapped to SSBs in message 1, it is able to

notify the gNB about its best beam.

Now we delineate four steps of the contention based NR random access procedure. However,

before the start of random access procedure certain parameters are to be known. Table12 gives the

parameters configured by RRC for random access procedure [34].


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Step1: In NR, UE select an SSB by measurement of SS-RSRP which is above SSB threshold.

Since preambles (and /or) PRACH resources are associated with SS blocks, UE set the Preamble

Index randomly from the RA preambles associated with the selected SS block. Subsequently, it

determines the next available PRACH occasion from the PRACH occasions corresponding to the

selected SS block. It selects power for transmission. If it is a re attempt and if SS block selected is

not changed and there is no Power Ramping Counter Suspension Notification, the power ramping

counter is incremented. RA-RNTI associated with the PRACH in which the RA Preamble is

transmitted is computed. The UE instructs the physical layer to transmit a preamble along with

corresponding RA-RNTI and preamble received target power [36.321]. It is to be noted that UE

should initially perform beam sweeping and SSB measurements as shown in figure to identify best

SSB. Subsequently, same TX beam direction as in the DL TX beam can be used for message 1

transmission.

Figure39: Beam Sweeping and Message 1 Transmission

Step2: Once the randomly selected preamble is transmitted, UE monitors the PDCCH for RA

Response (RAR). The gNB transmits the RAR addressed to RA-RNTI. RA-RNTI is evaluated as

RAR conveys RA preamble identifier, Timing alignment information, Temporary C-RNTI and

UL grant for message 3. Since gNB is not aware of the fact that which UE has transmitted RAR,

nor does it know the UE’s location or beam, it transmits RAR using all TX beams, where each TX

beam transmission is repeated for each RX beam of UE as shown in figure 40.

Step3: If UE receives RAR successfully, it transmits message 3 to the resource block that is

specified in the RAR. For transmitting message 3, UE uses the best uplink TX beam which is the

TX beam that was used to transmit preamble in step 1. Thus, in step 3, UE can avoid beam

sweeping. Subsequently, it starts the contention resolution (CR) timer. If more than one devices

selected the same preamble in step1, they would transmit message 3 in the same resource block

causing collision. In case of collision, either one or none of the devices would be able to


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successfully transmit the message 3. The gNB sends CR message to the device from which it

received message 3 in step 4.

Figure 40: Beam sweeping and RAR Transmission

Step 4: If CR message is received before the contention resolution timer expires, the RAP is

considered to be successfully completed. For CR resolution message the beam pair that was

identified in message 2 can be used. However, the devices for which CR message is not received,

perform random backoff and retransmit a new attempt. For the new attempt the power ramping is

performed if the same SSB is identified for retransmission. If, however, new SSB is identified

power ramping is avoided as shown earlier in Figure37.

5.1 Random Access Challenge of Preamble Distribution over several SSBs

3GPP proposed the association between the periodically broadcasted SSBs and random access

resources (preambles/PRACH occasions) [29,30], that offers to resolve the aforesaid random

access complexity as shown in the figure 41.


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Figure41 Random Access Procedure with RA Resource mapping to beams/SSBs

The association facilitates the gNB to identify the beam visible to UE and the subsequent MSG2

is transmitted on this beam only rather than performing beam sweeping and then identifying the

beam to transmit MSG2. For MSG3 transmission, the UE can use information added to RAR for

determining its best uplink beam. The beams identified in such a manner are further used for MSG4

transmission and reception instead of beam sweeping. While this association simplifies the random

access procedure it brings forth the challenge of association of the preambles to the beams since

the number of preambles is limited.

Figure 42 shows a random distribution of backlogged users over different beams in the cell. Since

the user distribution is random in space and time while some beams may have more backlogged

users at some point of time while others may have less. The dynamics in variable in both space

and time. In such a case, if preambles are uniformly distributed over different beams, while some

beams may suffer sever collisions, others may have preambles which are ideal. Additionally, it is

to be noted that while the number of preambles are fixed to max of 64 (for both contention based

and contention free operation), the number of beams increases with the increase in the frequency

of operation.

It is more logical to distribute RA preambles amongst different beams based on the number of UEs

in each beam rather than the uniform distribution as shown in figure 42. In the user based


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distribution, the beams with higher number of active users are allocated more preambles and the

beams with less number backlogged users are mapped to less number of preambles. This presents

two work items (i) to identify the number of users in each beam and then (ii) to distribute preambles

over different beams such that there is fair chance of random access to all users in different of the

beam that they attempt the random access. We would also like to ensure that the preambles left

ideal in beams with less users is very low.

Figure 42: Users and Preable distribution over different beams.

Since the number of number of users in each beam is dynamic and we consider that the preambles

are also dynamically distributed, to take into account both the factors together we consider the

optimal transmission probability of different beams as the supporting factor that enables preamble

distribution. At time t, let na backlogged users in beam Ba attempt to access the network using Ua

number of random access preambles that are mapped to the beam Ba. Pa(Sk| na) represents

the probability of transmission success over random access preamble ua such that ua belongs to

set [Ua] and it can be evaluated as


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Where, Ea is the transmission probability. With the aim to obtain the maxima, we nullify the first

derivative of equation. Thus, the optimal transmission probability is obtained as

We use this optimal transmission probability to distribute preambles among beams as it considers

both the important parameters: (i) RA preambles and (ii) the number of UEs. The preambles can

be moved dynamically from beam with a higher value of optimal transmission probability to the

beam with a lower value in a manner that the optimal transmission probability in both the beams

become same.

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resource management scheme in NB-IoT systems. IEEE Access, 6, pp.15051-15064.

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[3] Y. P. E. Wang et al., ``A primer on 3GPP narrowband Internet of Things,'' IEEE Commun. Mag., vol.

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[4] ETSI TS 136211 v13.2.0, 2016 , “EUTRAN : Physical channels and modulation”

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[6] Parkvall, Stefan, Erik Dahlman, Anders Furuskar, and Mattias Frenne. "NR: The new 5G radio access

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[10] Lin, Xingqin, Ansuman Adhikary, and Y-P. Eric Wang. "Random Access Preamble Design and

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[17]3GPP TS 38.201, V15.0.0 (2017-12), “ NR; Physical layer; General description.”

[18] 3GPP TS 38.300, “NR: Overall description; Stage-2.”

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[26] R4-1706982, “WF on band specific UE channel bandwidth,” NTT DOCOMO, 3GPP TSG RAN

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[27] 3GPP TS 38.101-2, “NR - UE radio transmission and reception - Part 2: Range 2 Standalone”, 2018.

[28] 3GPP TS38.104, NR; Base Station (BS) radio transmission and reception, 2018.

https://www.gsma.com/

http://www.3gpp.org/news-events/3gpp-news/1929-nsa_nr_5g


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[29] 3GPP TS 38.300, “NR: Overall description; Stage-2.”

[30] 3GPP TS 38.211, “NR - Physical channels and modulation.”

[31] 3GPP TR 38.802 v2.0, “ Study on New Radio (NR) Access Technology: Physical Layer Aspects.”

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[32] White paper, Rohde & Schwarz, “The 5G Technology Ecosystem.” 2017.

Documents

5G mMTC Random Access Procedure (RAP) Abstraction5gopenplatform.org/se2/RandomAccess_Technical_Report.pdf · 2019-02-28 · NPSS and NSSS are located in different subframes. NPBCH