## 5G NR Frame Structure

Exploring the 5G NR Frame Structure used in 5G New Radio networks and 5G Radio equipment: including gNodeB and 5G CPE devices

Frame Structure

The 5G NR frame structure is defined by the 3GPP and here we present details of the NR Frame Structure that is specified in 3GPP specification (38.211).

Numerology – Subcarrier Spacing

Compared to LTE numerology (subcarrier spacing and symbol length), the most outstanding difference you can notice is that 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 labeled 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 tendency is that slot length gets shorter as subcarrier spacing gets wider. Actually this tendency 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 and additional info.>

OFDM Symbol Duration

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

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.

What changes to accommodate the physical property of the different numerology ? Answer 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 numerology, 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 look at 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.

< 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

Slot Format indicates how each of symbols within a single slot is used. It defines which symbols are used for uplink and which symbols are used for downlink within a specific slot. In LTE TDD, if a subframe (equivalent to a Slot in NR) is configured for DL or UL, all of the symbols within the subframe should be used as DL or UL. But in NR, the symbols within a slot can be configured in various ways as follows.

• We don’t need to use every symbols within a slot (this can be a similar concept in LAA subframe where only a part of subframes can be used for data transmission).
• Single slot can be devided into multiple segments of consecutive symbols that can be used for DL , UL or Flexible.

Theoretically we can think of almost infinite number of possible combinations of DL symbol, UL symbol, Flexible Symbol within a slot, but 3GPP allows only 61 predefined symbol combination within a slot as in following table. These predefined symbol allocation of a slot called Slot Format. (For the details on how these Slot Format is being used in real operation, refer to Slot Format Combination page).

<38.213 v15.7 -Table 11.1.1-1: Slot formats for normal cyclic prefix>

Why we need so many different types of slot formats ? Key goal 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:

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 dimension (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.

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 that the contents(value) of the resource elements denoted as ‘Set to 0’ in Table 7.4.3.1-1 are set to zero. (This mean that the contents of the gray colored resource element in the SSB diagram shown below is filled with zeros).
• k_ssb corresponds to the gap between Subcarrier 0 of SS/PBCH block and Common Resource Block • is obtained from the higher-layer parameter OffsetToPointA
• 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 (Sub 6)
• k_ssb(k0 in older spec) = {0,1,2,…,23}
• 4 LSB bits of k_ssb value can 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}, FR1 (sub 6 Ghz)
• is expressed in terms of 15 Khz subcarrier spacing
• Type B (mmWave)
• k_ssb(k0 in older spec) = {0,1,2,…,11}
• the whole k_ssb value can be 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}, FR2 (mmWave)
• is expressed in terms of 60 Khz subcarrier spacing

NOTE : Actually understanding k_ssb and in the resource grid often get confusing and hard to visualize. The following is an example where the SubcarrierSpacingCommon is equal to 30KHz, and k_ssb=2, where in such a case the center of the first subcarrier of the SS/PBCH Block (which has 15KHz SCS) coincides with the center frequency of the subcarrier 1 of 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 means that [38.213 – 4.1 Cell Search] specifies SS/PBCH location in time domain as illustrated below.

< Start Symbols for each subcarrier spacing and frequency >

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.