5G NR Frame Structure | 5G Frame As Per NR Standard

 5G NR Frame structure. It describes 5G frame as per NR (New Radio) 3GPP standard. The 5G NR frame structure depicts subframes,slot and symbol configurations.

Introduction:
5G NR Supports two frequency ranges FR1 (Sub 6GHz) and FR2 (millimeter wave range, 24.25 to 52.6 GHz). NR uses flexible subcarrier spacing derived from basic 15 KHz subcarrier spacing used in LTE.

5G NR Frame Structure

 

A frame has duration of 10 ms which consists of 10 subframes having 1ms duration each similar to LTE technology. Each subfame can have 2μ slots. Each slot typically consists of 14 OFDM symbols. The radio frame of 10 ms are transmitted continuously as per TDD topology one after the other. Subframe is of fixed duration (i.e. 1ms) where as slot length varies based on subcarrier spacing and number of slots per subframe. As shown below, it is 1 ms for 15 KHz, 500 µs for 30 KHz and so on. Subcarrier spacing of 15 KHz occupy 1 slot per subframe, subcarrier spacing of 30 KHz occupy 2 slots per subframe and so on. Each slot occupies either 14 OFDM symbols or 12 OFDM symbols based on normal CP and extended CP respectively.

Each 5G NR frame is divided into two equal size half frames with 5 subframes in each. Half frame-0 consists of subframes 0 to 4 and half frame-1 consists of subframes 5 to 9. Uplink frame-i start TTA time duration before the start of downlink frame-i.
➤TTA = (NTA + NTA, offset)*Tc
Where,
Physical layer timing unit, Tc = 1/(Δfmax*Nf)
Δfmax = 480 KHz , Nf = 4096
Tc = 0.509 ns

NTA, offset is defined in TS 38.133 document as per table 7.1.2-2.

• OFDM symbols in a slot can be classified as 'downlink', 'flexible', or 'uplink'.
• Signaling of slot formats is mentioned in subclause 11.1 of TS 38.213 document. Table below mentions the same.

• In a slot in a downlink frame, the UE shall assume that downlink transmissions only occur in 'downlink' or 'flexible' symbols.
• In a slot in an uplink frame, the UE shall only transmit in 'uplink' or 'flexible' symbols.
• A UE not capable of full-duplex communication is not expected to transmit in the uplink earlier than NRx-Tx*Tc after the end of the last received downlink symbol in the same cell where NRx-Tx is given by TS 38.101 document.

Slot Formats In 5G NR Frame Structure (Normal CP)

As mentioned in the table, a slot can be all downlink, all uplink or mixed (i.e. combination of downlink and uplink). Here mixed refers to static, semi-static or dynamic. Slot aggregation is supported in 5G NR and hence data transmission can be scheduled to span one or multiple slots. Slot format indication informs UE whether an OFDM symbol is downlink, uplink or flexible. The figure below depicts the same.

Number of resource blocks per subframe is depicted in the following figure.

The figure depicts resource grid of 5G NR with symbols in time axis and subcarriers in frequency axis. 12 subcarriers form one PRB (Physical Resource Block). 5G NR supports 24 to 275 PRBs in a single slot. Occupied BW of 34.56 MHz (minimum) and 396 MHz (maximum) can be achieved for 120 KHz subcarrier spacing. One SS/PBCH Block occupies 4 OFDM Symbols in time domain and 24 PRBs in frequency domain. 5G NR SS consists of PSS and SSS as specified for LTE.