5G/NR - Frame Structure                                                          Home : www.sharetechnote.com

 

 

 

 

 

Frame Structure

 

There has been long long discussions on frame structure both in academia and in 3GPP and now we have pretty clear agreements on what a NR(5G) radio frame would look like. In this page, I will describe on NR Frame Structure that is specified in 3GPP specification (38.211). If you are intrested in those long discussions and histories about show these specification came out for your personal interests and study, refer to 5G Frame Structure Candidate page.

 

 

 

 

 

Numerology - Subcarrier Spacing

 

Comparing to LTE numberology (subcarrier spacing and symbol length), the most outstanding diffrence you can notice is thet NR support multiple different types of subcarrier spacing (in LTE there is only one type of subcarrier spacing, 15 Khz). The types NR numerology is summarized in 38.211 and I converted the table into illustration to give you intuitive understanding of these numerology.

As you see here, each numerology is labled as a parameter(u, mu in Greek). The numerology (u = 0) represents 15 kHz which is same as LTE. And as you see in the second column the subcarrier spacing of other u is derived from (u=0) by scaling up in the power of 2.

 

 

 

 

Numerology and Slot Length

 

As illustrated below, Slot length gets different depending on numerology. The general tendancy is that slot length gets shorter as subcarrier spacing gets wider. Actually this tendancy comes from the nature of OFDM. You would see further details on how the slot length is derived in Radio Frame Structure section.

 

 

 

 

 

 

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.>

Numerology

Subcarrier Spacing

(kHz)

CP type

Supported for Data

(PDSCH, PUSCH etc)

Supported for Sync

(PSS,SSS,PBCH)

0

15

Normal

Yes

Yes

1

30

Normal

Yes

Yes

2

60

Normal,Extended

Yes

No

3

120

Normal

Yes

Yes

4

240

Normal

No

Yes

 

 

 

OFDM Symbol Duration

 

Parameter / Numerlogy (u)

0

1

2

3

4

Subcarrier Spacing (Khz)

15

30

60

120

240

OFDM Symbol Duration (us)

66.67

33.33

16.67

8.33

4.17

Cyclic Prefix Duration (us)

4.69

2.34

1.17

0.57

0.29

OFDM Symbol including CP (us)

71.35

35.68

17.84

8.92

4.46

 

 

 

Numerology - Sampling Time

 

Sampling time can be defined differently depending on Numerogy (i.e, Subcarrier Spacing) and in most case two types of Timing Unit Tc and Ts are used.

  • Tc = 0.509 ns
  • Ts = 32.552 ns

 

See Physial Layer Timing Unit page to see how these numbers are derived and to see some other timing units.

 

 

 

Radio Frame Structure

 

As described above, in 5G/NR multiple numerologies(waveform configuration like subframe spacing) are supported and the radio frame structure gets a little bit different depending on the type of the numerology. However, regardless of numerology the length of one radio frame and the length of one subfame is same.  The length of a Radio Frame is always 10 ms and the length of a subframe is always 1 ms. 

Then what should get different to accomondate the physical property of the different numerology ? The anwer is to put different number of slots within one subfame. There is another varying parameter with numerology. It is the number of symbols within a slot. However, the number of symbols within a slot does not change with the numberology, it only changes with slot configuration type. For slot configuration 0, the number of symbols for a slot is always 14 and for slot configuration 1, the number of symbols for a slot is always 7.

 

Now let's look into the details of radio frame structure for each numerology and slot configuration.

 

 

< Normal CP, Numerology = 0 >

 

In this configuration, a subframe has only one slot in it, it means a radio frame contains 10 slots in it. The number of OFDM symbols within a slot is 14.

 

 

 

< Normal CP, Numerology = 1 >

 

In this configuration, a subframe has 2 slots in it, it means a radio frame contains 20 slots in it. The number of OFDM symbols within a slot is 14.

 

 

 

< Normal CP, Numerology = 2 >

 

In this configuration, a subframe has 4 slots in it, it means a radio frame contains 40 slots in it. The number of OFDM symbols within a slot is 14.

 

 

 

< Normal CP, Numerology = 3 >

 

In this configuration, a subframe has 8 slots in it, it means a radio frame contains 80 slots in it. The number of OFDM symbols within a slot is 14.

 

 

 

< Normal CP, Numerology = 4 >

 

In this configuration, a subframe has 16 slots in it, it means a radio frame contains 160 slots in it. The number of OFDM symbols within a slot is 14.

 

 

 

< Normal CP, Numerology = 5 >

 

In this configuration, a subframe has 32 slots in it, it means a radio frame contains 320 slots in it. The number of OFDM symbols within a slot is 14.

 

 

 

< Extended CP, Numerology = 2 >

 

In this configuration, a subframe has 8 slots in it, it means a radio frame contains 80 slots in it. The number of OFDM symbols within a slot is 12.

 

 

 

 

Slot Format

 

There are a lot of different slot format defined in 38.211 (since v 2.0.0).  The concept would be similar to legacy LTE TDD Subframe configuration, but main differences from LTE TDD subframe configuration are

  • in NR slot format, DL and UL assignment changes at a symbol level (in LTE TDD the UL/DL assignment is done in a subframe level)
  • in NR slot format, there are much diverse patterns comparing to LTE TDD subframe configuration (Not a good news for FPGA or DSP engineers :)
  • 38.211 - Table 4.3.2-3 applies only for DCI with SFI_RNTI (i.e, DCI 2_0).

Even though all the slot format looks like a TDD structure, these can be deployed in FDD mode as well (See Frequency Band page).

 

 

<38.211 - Table 4.3.2-3: Slot formats>

D : Downlink, U : Uplink, X : Flexible

 

Symbol Number in a slot

Format

0

1

2

3

4

5

6

7

8

9

10

11

12

13

0

D

D

D

D

D

D

D

D

D

D

D

D

D

D

1

U

U

U

U

U

U

U

U

U

U

U

U

U

U

2

X

X

X

X

X

X

X

X

X

X

X

X

X

X

3

D

D

D

D

D

D

D

D

D

D

D

D

D

X

4

D

D

D

D

D

D

D

D

D

D

D

D

X

X

5

D

D

D

D

D

D

D

D

D

D

D

X

X

X

6

D

D

D

D

D

D

D

D

D

D

X

X

X

X

7

D

D

D

D

D

D

D

D

D

X

X

X

X

X

8

X

X

X

X

X

X

X

X

X

X

X

X

X

U

9

X

X

X

X

X

X

X

X

X

X

X

X

U

U

10

X

U

U

U

U

U

U

U

U

U

U

U

U

U

11

X

X

U

U

U

U

U

U

U

U

U

U

U

U

12

X

X

X

U

U

U

U

U

U

U

U

U

U

U

13

X

X

X

X

U

U

U

U

U

U

U

U

U

U

14

X

X

X

X

X

U

U

U

U

U

U

U

U

U

15

X

X

X

X

X

X

U

U

U

U

U

U

U

U

16

D

X

X

X

X

X

X

X

X

X

X

X

X

X

17

D

D

X

X

X

X

X

X

X

X

X

X

X

X

18

D

D

D

X

X

X

X

X

X

X

X

X

X

X

19

D

X

X

X

X

X

X

X

X

X

X

X

X

U

20

D

D

X

X

X

X

X

X

X

X

X

X

X

U

21

D

D

D

X

X

X

X

X

X

X

X

X

X

U

22

D

X

X

X

X

X

X

X

X

X

X

X

U

U

23

D

D

X

X

X

X

X

X

X

X

X

X

U

U

24

D

D

D

X

X

X

X

X

X

X

X

X

U

U

25

D

X

X

X

X

X

X

X

X

X

X

U

U

U

26

D

D

X

X

X

X

X

X

X

X

X

U

U

U

27

D

D

D

X

X

X

X

X

X

X

X

U

U

U

28

D

D

D

D

D

D

D

D

D

D

D

D

X

U

29

D

D

D

D

D

D

D

D

D

D

D

X

X

U

30

D

D

D

D

D

D

D

D

D

D

X

X

X

U

31

D

D

D

D

D

D

D

D

D

D

D

X

U

U

32

D

D

D

D

D

D

D

D

D

D

X

X

U

U

33

D

D

D

D

D

D

D

D

D

X

X

X

U

U

34

D

X

U

U

U

U

U

U

U

U

U

U

U

U

35

D

D

X

U

U

U

U

U

U

U

U

U

U

U

36

D

D

D

X

U

U

U

U

U

U

U

U

U

U

37

D

X

X

U

U

U

U

U

U

U

U

U

U

U

38

D

D

X

X

U

U

U

U

U

U

U

U

U

U

39

D

D

D

X

X

U

U

U

U

U

U

U

U

U

40

D

X

X

X

U

U

U

U

U

U

U

U

U

U

41

D

D

X

X

X

U

U

U

U

U

U

U

U

U

42

D

D

D

X

X

X

U

U

U

U

U

U

U

U

43

D

D

D

D

D

D

D

D

D

X

X

X

X

U

44

D

D

D

D

D

D

X

X

X

X

X

X

U

U

45

D

D

D

D

D

D

X

X

U

U

U

U

U

U

46

D

D

D

D

D

D

X

D

D

D

D

D

D

X

47

D

D

D

D

D

X

X

D

D

D

D

D

X

X

48

D

D

X

X

X

X

X

D

D

X

X

X

X

X

49

D

X

X

X

X

X

X

D

X

X

X

X

X

X

50

X

U

U

U

U

U

U

 

U

U

U

U

U

U

51

X

X

U

U

U

U

U

X

X

U

U

U

U

U

52

X

X

X

U

U

U

U

X

X

X

U

U

U

U

53

X

X

X

X

U

U

U

X

X

X

X

U

U

U

54

D

D

D

D

D

X

U

D

D

D

D

D

X

U

55

D

D

X

U

U

U

U

U

U

XU

U

U

U

U

56

D

X

U

U

U

U

U

D

X

U

U

U

U

U

57

D

D

D

D

X

X

U

D

D

D

D

X

X

U

58

D

D

X

X

U

U

U

D

D

X

X

U

U

U

59

D

X

U

U

U

U

U

D

X

U

U

U

U

U

60

D

X

X

X

X

X

U

D

X

X

X

X

X

U

61

D

D

X

X

X

X

U

D

D

X

X

X

X

U

62-255

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Why we need so many different types of slot formats ? Obviously it is not just to make your job difficult :). It is to make NR scheduling flexible especially for TDD operation. By applying a slot format or combining different slot formats in sequence, we can implement various different types of scheduling as in the following example (these examples are based on 5G NEW RADIO : Designing For The Future (Ericsson Technology Review))

 

DL-heavy transmission with UL part

Slot (e.g, slot format 28)

Slot(e.g, slot format 28)

D

D

D

D

D

D

D

D

D

D

D

D

X

U

D

D

D

D

D

D

D

D

D

D

D

D

X

U

 

 

UL-heavy transmission with DL Control

Slot(e.g, slot format 34)

Slot(e.g, slot format 34)

D

X

U

U

U

U

U

U

U

U

U

U

U

U

D

X

U

U

U

U

U

U

U

U

U

U

U

U

 

 

Slot aggregation for DL-heavy transmission (e.g, for eMBB)

Slot(e.g, slot format 0)

Slot (e.g, slot format 28)

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

X

U

 

 

Slot aggregation for UL-heavy transmission (e.g, for eMBB)

Slot

Slot

D

X

U

U

U

U

U

U

U

U

U

U

U

U

D

U

U

U

U

U

U

U

U

U

U

U

U

U

 

 

 

TDD DL/UL Common Configuration

 

See  TDD DL/UL Common Configuration page.

 

 

 

Resource Grid

 

The resource grid for NR is defined as follows. If you just take a look at the picture, you would think it is almost identical to LTE resource grid. But the physical dimmension (i.e, subcarrier spacing, number of OFDM symbols within a radio frame) varies in NR depending on numerology.

 

 

The maximum and minimum number of Resource blocks for downlink and uplink is defined as below (this is different from LTE)

 

< 38.211 v1.0.0-Table 4.4.2-1: Minimum and maximum number of resource blocks.>

 

Following is the table that I converted the downlink portions of Table 4.4.2-1 into frequency Bandwidth just to give you the idea on what is the maximum RF bandwidth that a UE / gNB need to support for single carrier.

 

u

min RB

Max RB

sub carrier spacing

(kHz)

Freq BW min

(MHz)

Freq BW max

(MHz)

0

24

275

15

4.32

49.5

1

24

275

30

8.64

99

2

24

275

60

17.28

198

3

24

275

120

34.56

396

4

24

138

240

69.12

397.44

5

24

69

480

138.24

397.44

 

 

 

SS/PBCH

 

SS(PSS and SSS) and PBCH in NR is transmitted in the same 4 symbol block as specified in the following table.

 

 

< Frequency Domain Resource Allocation >

 

Overall description on the resource allocation for SS/PBCH block is described in 38.211 - 7.4.3.1 Time-frequency structure of an SS/PBCH block and followings are the summary of the specification.

  • SS/PBCH block consists of 240 contiguous subcarriers (20 RBs)
  • The subcarriers are numbered in increasing order from 0 to 239 within the SS/PBCH block
  • The UE may assume resource elements denoted as 'Set to 0' in Table 7.4.3.1-1 are set to zero.
  • Subcarrier 0 in an SS/PBCH block corresponds to subcarrier k_ssb(k0 in older spec)  in Common Resource Block
    • is obtained from the higher-layer parameter offset-ref-low-scs-ref-PRB
    • offset-ref-low-scs-ref-PRB corresponds to the FrequencyInfoDL.absoluteFrequencyPointA. Data type is ARFCN-ValueNR and the range of the value is INTEGER (0..3279165) in integer.
  • There are two types of SS/PBCH Block
    • Type A
      • k_ssb(k0 in older spec) = {0,1,2,...,23}
        • 4 LSB bits of k_ssb is informed to UE via ssb-subcarrierOffset in MIB
        • The MSB bit is informed to UE via a bit within the PBCH Data ()  
        • is expressed in terms of 15 Khz subcarrier spacing
      • u (numerology) = {0,1}
      • is expressed in terms of 15 Khz subcarrier spacing
    • Type B
      • k_ssb(k0 in older spec) = {0,1,2,...,11}
        • 4 LSB bits of k_ssb is informed to UE via ssb-subcarrierOffset in MIB
        • is expressed in terms of the subcarrier spacing provided by the higher-layer parameter subCarrierSpacingCommon in MIB  .
      • u (numerology) = {3,4}
      • is expressed in terms of 60 Khz subcarrier spacing
  • Following table shows the time domain (OFDM symbol number) and frequency domain (Subcarrier Number) within SS/PBCH bloc.

< 38.211- Table 7.4.3.1-1: Resources within an SS/PBCH block for PSS, SSS, PBCH, and DM-RS for PBCH >

 

This table can be represented in Resource Grid as shown below. Note that the position of PBCH DM-RS varies with v and the value v changes depending on Physical Cell ID.

 

 

 

< Time Domain Resource Allocation >

 

Following table indicates the first OFDM symbol number (s) where SS/PBCH is transmitted. This is based on 38.213 - 4.1 Cell Search.

 

The document states as follows :

    For a half frame with SS/PBCH blocks, the number and first symbol indexes for candidate SS/PBCH blocks are determined according to the subcarrier spacing of SS/PBCH blocks as follows.

This mean that [38.213 - 4.1 Cell Search] specifies SS/PBCH location in time domain as illustrated below.

 

Subcarrier Spacing

OFDM Symbol (s)

f <= 3 Ghz

3 Ghz < f <= 6 Ghz

6 Ghz < f

Case A :

15 KHz

{2,8} + 14 n n = 0,1 n = 0,1,2,3  
s = 2,8,16,22 s = 2,8,16,22,30,36,44,50  

Case B :

30 Khz

{4,8,16,20}+28n n = 0 n = 0,1  

s = 4,8,16,20

s = 4,8,16,20,32,36,

     44,48

 

Case C :

30 Khz

{2,8} + 14 n n = 0,1 n = 0,1,2,3  
s = 2,8,16,22

s = 2,8,16,22,30,36,44,50

 

Case D :

120 Khz

{4,8,16,20} + 28n

 

 

n=0, 1, 2, 3, 5, 6, 7, 8, 10, 11, 12, 13, 15, 16, 17, 18

 

 

s = 4,8,16,20,

     32,36,44,48,

     60,64,72,76,

     88,92,100,104,

     144,148,156,160,

     172,176,184,188,

     200,204,212,216,

     228,232,240,244,

     284,288,296,300,

     312,316,324,328,

     340,344,352,356,

     368,372,380,384,

     424,428,436,440,

     452,456,464,468,

     480,484,492,496,

     508,512,520,524

Case E :

240 Khz

{8, 12, 16, 20, 32, 36, 40, 44} + 56n

   

n=0, 1, 2, 3, 5, 6, 7, 8

 

 

s = 8,12,16,20,

     32,36,40,44,

     64,68,72,76,

     88,92,96,100,

     120,124,128,132,

     144,148,152,156,

     176,180,184,188,

     200,204,208,212,

     288,292,296,300,

     312,316,320,324,

     344,348,352,356,

     368,372,376,380,

     400,404,408,412,

     424,428,432,436,

     456,460,464,468,

     480,484,488,492

 

 

Followings are examples of SSB Transmission for each cases. For the simplicity, I set the frequency domain location of SSB block to be located at the bottom of the system bandwidth, but in reality the frequency domain location can change to other location (e.g, center frequency of the system bandwidth). The main purpose of these examples is o show the time domain location (transmission pattern) of each cases. In real deployment, it is highly likely (but not necessarily) that the frequency domain location of the SSB located around the center frequency.

 

The example below shows how you can correlate the above table to the SSB transmission plot shown in the following examples.

 

 

 

< Case A : f <= 3 Ghz >

 

This plot is created by Matlab 5G library. See this page for the Matlab code and more examples.

 

 

 

< Case A : 3 Ghz < f <= 6 Ghz >

 

This plot is created by Matlab 5G library. See this page for the Matlab code and more examples.

 

 

 

< Case B : f <= 3 Ghz >

 

This plot is created by Matlab 5G library. See this page for the Matlab code and more examples.

 

 

 

< Case B : 3 Ghz < f <= 6 Ghz >

 

This plot is created by Matlab 5G library. See this page for the Matlab code and more examples.

 

 

 

< Case C : f <= 3 Ghz >

 

This plot is created by Matlab 5G library. See this page for the Matlab code and more examples.

 

 

 

< Case C : 3 Ghz < f <= 6 Ghz >

 

This plot is created by Matlab 5G library. See this page for the Matlab code and more examples.

 

 

 

< Case D : 6 Ghz < f >

 

This plot is created by Matlab 5G library. See this page for the Matlab code and more examples.

 

 

 

< Case E : 6 Ghz < f >

 

This plot is created by Matlab 5G library. See this page for the Matlab code and more examples.

 

 

 

 

Reference

 

[1] 3GPP TSG RAN WG1 Meeting NR#3 : R1- 1716650 Comparison of PBCH DMRS mapping schemes

[2] 3GPP TSG RAN WG1 Meeting NR#3 : R1-1715841 Remaining Details on PBCH design and contents

[3] 3GPP TSG RAN WG1 Meeting AH_NR#3 : R1-1716609 - On remaining details of NR DL DMRS

[4] 3GPP TSG RAN WG1 NR Ad-Hoc#3 : R1-1716574 - Discussion on time domain resource allocation

[5] 5G NEW RADIO : Designing For The Future (Ericsson Technology Review)

[6] Making 5G NR a reality (Qualcomm)