RG vs RB vs RE: Understanding Resource Elements in 5G NR

This page compares Resource Grid (RG) vs Resource Block (RB) vs Resource Element (RE) and mentions the difference between Resource Grid (RG), Resource Block (RB), and Resource Element (RE) in the 5G NR system.

Introduction

As shown in Figure 1, a frame in 5G NR has a duration of 10 ms.

A frame consists of 10 subframes, each having a duration of 1ms, similar to LTE. Each subframe consists of 2^μ slots. Each slot can have either 14 (normal CP) or 12 (extended CP) OFDM symbols. All the subcarrier spacing options have 14 OFDM symbols. 60 KHz spacing supports both normal and extended CP types.

Figure 1: 5G NR Frame Structure - Slot, Mini-Slot

For μ = 0, there is 1 slot per subframe; for μ = 1, there are 2 slots per subframe; for μ = 2, there are 4 slots per subframe, and so on. The number of slots per frame is ten times the number of slots per subframe. Hence, for μ = 2, there are 40 slots/frame.

5G NR uses 4096 FFT points which consist of 3300 data subcarriers.

Figure 2: 5G NR Numerology

Like OFDMA used in mobile WiMAX and LTE, the resources are mapped across the time and frequency grid in 5G NR too. This is shown in the figure below.

Figure 3: RG, RB, RE in 5G NR Resource Grid

Resource Grid (RG)

• The resource grid, which consists of a number of subcarriers in the frequency axis and a number of OFDM symbols in the time axis, is known as the resource grid.

• There is one set of resource grids assigned per transmission direction (“x”) (uplink and downlink).

• There is one set of resource grids assigned for each of the antenna ports (“p”) as per the subcarrier spacing configuration (“μ”).

• 5G NR supports $(N_{grid,x})^{\mu}$ size, $(N_{SC})^{RB}$ subcarriers, and $(N_{symb,subframe})^{\mu}$ OFDM symbols.

  • Carrier bandwidth $(N_{grid})^{\mu}$ for “μ” is given by higher layer parameters, namely “carrierBandwidth” in the “SCS-SpecificCarrier IE”.
  • Starting position $(N_{grid,start})^{\mu}$ for “μ” is given by higher layer parameters, namely “offsetToCarrier” in the “SCS-SpecificCarrier IE”.

Resource Block (RB)

• RB (Resource Block) consists of a total of 12 consecutive subcarriers in the frequency domain.
• $(N_{SC})^{RB} = 12$
• The following table mentions $(N_{RB})^{\mu}$ for each of the subcarrier spacing configurations (“μ”).
• There are more than one RBs in a resource grid.
• Resource elements are grouped into PRBs (Physical Resource Blocks). Each PRB consists of 12 subcarriers, as mentioned.
• For each numerology and carrier, a resource grid of $(N_{RB,max})^{\mu}$ subcarriers and $(N_{symb,subframe})^{\mu}$ OFDM symbols are defined. Resource grids for all subcarrier spacing are overlapped.

Figure 4: 5G NR Resource Blocks

Resource Element (RE)

• Each element in the resource grid for antenna port (“p”) and subcarrier spacing configuration (“μ”) is known as a Resource Element.

• It is uniquely identified by $(k, l)_{p, \mu}$, where:

  • k = index in the frequency domain
  • l = symbol position in the time domain

• The resource element $(k, l)_{p, \mu}$ corresponds to a physical resource and the complex value $(a_{k,l})_{p, \mu}$.