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Even though Numerology is one of the most widely discussed item at 3GPP RAN1 meeting before NR Technical Specification is finalized, it sounds very vague to me. I think I understand what it indicates, but I still don't understand the meaning of Numerology in ordinary dictionary can be associated with what it means in NR.

 

 

 

Definition of Numerology

 

The very simple defination of Numerology based on the usage of the term in 3GPP specification would be 'subcarrier spacing  type'. In LTE, we don't need any specific terminology to indicate the subcarrier spacing since there is only one subcarrier spacing, but in NR there are several different types of subcarrier spacing as summarized in the following table.

 

< 38.211 - Table 4.2-1: Supported transmission numerologies >

 

 

To help you understand the meaning of each numerology more intuitively I tried to visualize the table as follows.  (NOTE: Numberology 5 was defined in an early specification, but removed from the specification.)

 

 

 

 

Numerology and Supported Channels

 

Not every numerology can be used for every physical channel and signals. That is, there is a specific numerologies that are used only for a certain type of physical channels even though majority of the numerologies can be used any type of physical channels. Following table shows which numerologies can be used for which physical channels.

 

< 38.300-Table 5.1-1: Supported transmission numerologies and additional info.>

Numerology

Subcarrier Spacing

(kHz)

CP type

Supported for Data

(PDSCH, PUSCH etc)

Supported for Sync

(PSS,SSS,PBCH)

PRACH

N/A

1.25

 

No

No

Long Preamble

N/A

5

 

No

No

Long Preamble

0

15

Normal

Yes

Yes

Short Preamble

1

30

Normal

Yes

Yes

Short Preamble

2

60

Normal,Extended

Yes

No

Short Preamble

3

120

Normal

Yes

Yes

Short Preamble

4

240

Normal

No

Yes

 

 

 

 

Numerology in RRC message

 

Numerology selection is not a static. Different numerology (Subcarrier Spacing) can be used in various different situation and purpose. The subcarrier spacing for different situation and purpose is defined in various places in RRC messages as follows.

 

Message/ASN Sequence

IE

Descrition

MIB

subCarrierSpacingCommon

Subcarrier spacing for SIB1, Msg.2/4 for initial access and SI-messages

BandwidthPart-Config

subcarrierSpacing

Subcarrier spacing to be used in this BWP. It is applied to at least PDCCH, PDSCH and corresponding DMRS

LogicalChannelConfig

allowedSubCarrierSpacing

 

ReferenceSignalConfig

subcarrierSpacing

 

CSI-RS-ResourceConfig-Mobility

subcarrierSpacing

subcarrier spacing of CSI-RS. It can take the same values available also for the data channels and for SSB

RACH-ConfigCommon

msg2-SubcarrierSpacing

 

RACH-ConfigCommon

msg3-SubcarrierSpacing

 

RACH-ConfigDedicated

rar-SubcarrierSpacing

 

ServingCellConfigCommon

subcarrierSpacingCommon

Subcarrier spacing for SIB1, Msg.2/4 for initial access and SI-messages.

Values 15, and 30 kHz are applicable for carrier frequencies < 6GHz; Values 60 and 120 kHz are applicable for carrier frequencies > 6GHz

ServingCellConfigCommon

subcarrierSpacingSSB

Subcarrier spacing of SSB. Used only for non-initial access (e.g. SCells, PCell of SCG).

If the field is absent the UE shall assume the default value of the band.

BasebandParametersPerCC

subCarrierSpacing

 

 

 

 

Why different Numerologies ?

 

Now you have some important questions on NR Numerology design. You might have these questions even before you went through all the details described above. The question that I had when I first heard of this kind of multiple numerology (basically multiple subcarrier spacing) was 'WHY we need this kind of multiple numerologies ?'.

One thing for sure is that it is not for making your life difficult as an engineer. Then, what would be other (more technical) reason ?

 

More technical (practical reason) is

  • NR should cover very wide range of operating frequency (e.g, sub 3 Ghz, sub 6 Ghz and mmWave(over 25 Ghz).
  • Due to physics, it is hard (almost impossible) to come up with single numerology (subcarrier space) that can cover the whole of these range without sacrificing too much of efficiency or performance.

 

Can you be more specific ?

  • In OFDM, number of subcarrier that can be packed into a specific frequency range is directly related to spectrum efficiency (how many bits can be transmitter per Hz per second). The more subcarriers you can packed into a frequency range (i.e, the narrow subcarrier spacing you use), the more data you can transmit (or recieve).
  • Based on Physics(anti-proportional relationship between subcarrier spacing and OFDM symbo length), Narrow subcarrier spacing means longer OFDM symbol length. With longer OFDM symbol, we can assign more room for CP(Cyclick Prefic). With longer CP, we can make the signal more tolerable to fading channel(Ref [33]).
  • In lower frequency (like sub 3Ghz, sub 6 Ghz), we don't have much wide band spectrum left for this new technology. 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 we use small subcarrier spacing like 15 Khz, 30 Khz, 60 Khz in NR numerology.
  • Then, why we don't use even smaller subcarrier like 10 Khz, 5 Khz etc ? As you know, in OFDM maintaining orthogonolity between subcarriers is critical (See Overview section of OFDM page). The transmitted signal would go through various fading channel causing the drift of each subcarrier and the degree of the drift gets even more serious when the transmitter or reciever moves faster. So the narrower of subcarrier spacing you use, the tolerance to fading gets weaker.
  • Then we need very wide subcarrier spacing like 120 Khz or 240 Khz ? It is for the operation in very high frequency like mmWave. As carrier frequency gets higher, the degree of frequency drift by moving transmitter or reciever gets higher (i.e, 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.
  • There is another reason for wider subcarrier spacing in mmWave. For the reason as explained here, we would use beamforming (Massive MIMO based beamforming), for the implementaion of beamforming controlling the phase of the signal is critical and it is difficult to control the phase of the signal with narrow subcarrier spacing (Ref [33]).
  • As frequency goes higher, the degree of phase noise would increase. So we need to implement more sophisticated mechanism for phase noise estimation and correction. It is easier to implement this kind mechanism with wider subcarrier spacing(Ref [33]).  

 

 

 

Reference

 

[1] 3GPP R1-166225. 3GPP TSG RAN WG1 Meeting #86 - Considerations on numerology for support of flexible guard lengths

[2] 3GPP R1-166346. 3GPP TSG RAN WG1 Meeting #86 - Forward compatibility consideration for NR frame structure

[3] 3GPP R1-166360. 3GPP TSG RAN WG1 Meeting #86 -  URLLC numerology and frame structure design

[4] 3GPP R1-166363. 3GPP TSG RAN WG1 Meeting #86 - Scaled Numerology Control Design for NR

[5] 3GPP R1-166364 . 3GPP TSG RAN WG1 Meeting #86 - NR numerology scaling and alignment

[6] 3GPP R1-166471. 3GPP TSG RAN WG1 Meeting #86 - Discussion of NR Numerology

[7] 3GPP R1-166490. 3GPP TSG RAN WG1 Meeting #86 - Numerology impact on power efficiency for mMTC

[8] 3GPP R1-166637. 3GPP TSG RAN WG1 Meeting #86 - Discussion on numerology multiplexing

[9] 3GPP R1-166676. 3GPP TSG RAN WG1 Meeting #86 - TDD frame structure with mixed numerology

[10] 3GPP R1-166747. 3GPP TSG RAN WG1 Meeting #86 - Evaluation results of OFDM-based waveform in DL and UL single numerology case

[11] 3GPP R1-166748. 3GPP TSG RAN WG1 Meeting #86 - Evaluation results of OFDM-based waveform in DL and UL mixed numerology case    

[12] 3GPP R1-166749. 3GPP TSG RAN WG1 Meeting #86 - Evaluation results of OFDM-based waveform in UL single numerology and asynchronous case

[13] 3GPP R1-166753. 3GPP TSG RAN WG1 Meeting #86 - Discussion on symbol alignment across scaled numerology

[14] 3GPP R1-166754. 3GPP TSG RAN WG1 Meeting #86 - Discussion on numerology support

[15] 3GPP R1-166795. 3GPP TSG RAN WG1 Meeting #86 - Discussion on numerology aspects of NR synchronization signal

[16] 3GPP R1-166878. 3GPP TSG RAN WG1 Meeting #86 - Discussion on alignment for different numerology multiplexing

[17] 3GPP R1-166879. 3GPP TSG RAN WG1 Meeting #86 - Further evaluation results on different numerology

[18] 3GPP R1-166939. 3GPP TSG RAN WG1 Meeting #86 - Numerology evaluation results for high speed scenario

[19] 3GPP R1-166940. 3GPP TSG RAN WG1 Meeting #86 - Discussion on numerology multiplexing in NR

[20] 3GPP R1-166941. 3GPP TSG RAN WG1 Meeting #86 - On design of mixed numerology in a NR carrier

[21] 3GPP R1-167004. 3GPP TSG RAN WG1 Meeting #86 - Discussion on numerology

[22] 3GPP R1-167034. 3GPP TSG RAN WG1 Meeting #86 - On PA impact to in mixed numerology with narrow band allocation

[23] 3GPP R1-167035. 3GPP TSG RAN WG1 Meeting #86 - On PA impact to in mixed numerology with wide band allocation

[24] 3GPP R1-167040. 3GPP TSG RAN WG1 Meeting #86 - On scalable numerology  

[25] 3GPP R1-167106. 3GPP TSG RAN WG1 Meeting #86 - Phase Noise Measurement/Modeling and LLS for High Frequency Numerology

[26] 3GPP R1- 167107. 3GPP TSG RAN WG1 Meeting #86 - Discussion and Evaluation on Numerology Design for High Speed Train Scenario

[27] 3GPP R1-167218. 3GPP TSG RAN WG1 Meeting #86 - Numerology and Frame Structure for NR-Unlicensed

[28] 3GPP R1-167260. 3GPP TSG RAN WG1 Meeting #86 - Resource block and guard band arrangement supporting mixed numerology

[29] 3GPP R1-167261. 3GPP TSG RAN WG1 Meeting #86 - On resource block grouping and multi-cell coordination aspects for mixed numerology support    

[30] 3GPP R1-167394. 3GPP TSG RAN WG1 Meeting #86 - Views on NR numerology    

[31] 3GPP R1-167527. 3GPP TSG RAN WG1 Meeting #86 - Discussion on NR Numerology   

[32] 3GPP R1-167564. 3GPP TSG RAN WG1 Meeting #86 - Impact of numerology on the non-CP based waveforms

[33] NR Wide Bandwidth Operations by Jeongho Jeon, Intel Corporation