5G/NR - Numerology
source: www.sharetechnote.com


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
  • Numerology and Supported Channels
  • Numerology in RRC message
  • Why different Numerologies ?




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
Description
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 pack 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 . 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]).




UE Capability

FeatureSetDownlinkPerCC ::= SEQUENCE {
supportedSubcarrierSpacingDL SubcarrierSpacing,
supportedBandwidthDL SupportedBandwidth,
channelBW-90mhz ENUMERATED {supported} OPTIONAL,
maxNumberMIMO-LayersPDSCH MIMO-LayersDL OPTIONAL,
supportedModulationOrderDL ModulationOrder OPTIONAL
}

supportedSubcarrierSpacingDL :Defines the supported sub-carrier spacing for DL by the UE indicating the UE supports simultaeous reception with same or different numerologies in CA. Note the UE shall support all mandated sub-carrier spacing for FR1/FR2. Same numerology for intraband NR CA including both continuous and non-continuous is mandatory with capability in both FR1 and FR2. Two mixed numerologies between FR1 band(s) and FR2 band(s) in DL are mandatory with capability if UE supports inter-band NR CA including both FR1 band(s) and FR2 band(s). Optional for other cases.