5G NR Resource Block Definition and RB calculations

The technical details of 5G NR Resource Blocks (RB) and how they are calculated.

5G NR Resource Block Definition

In 5G New Radio (NR), a Resource Block (RB) is the smallest unit of resource allocation. It is defined in both the frequency and time domains:

Frequency Domain

• Subcarriers: Each RB consists of 12 consecutive subcarriers.
• Subcarrier Spacing: The spacing between subcarriers can vary based on the numerology (μ). Common subcarrier spacings are 15 kHz, 30 kHz, 60 kHz, 120 kHz, and 240 kHz.
• Bandwidth: The bandwidth of an RB depends on the subcarrier spacing. For example, with a 15 kHz subcarrier spacing, an RB spans 180 kHz (12 subcarriers * 15 kHz).

Time Domain

• OFDM Symbols: An RB spans one slot in the time domain. Each slot typically consists of 14 OFDM symbols for normal cyclic prefix (CP) and 12 OFDM symbols for extended CP.
• Slot Duration: The duration of a slot varies with the subcarrier spacing. For instance, with a 15 kHz subcarrier spacing, a slot duration is 1 ms.

Resource Block Calculations

Frequency Domain Calculations

1. Carrier Bandwidth (B):B = N \times \Delta fWhere:
   • ( B ) is the carrier bandwidth.
   • ( N ) is the number of subcarriers in an RB (typically 12).
   • ( \Delta f ) is the subcarrier spacing.
   For example, with ( \Delta f = 15 ) kHz:B = 12 \times 15 \text{ kHz} = 180 \text{ kHz}
2. Frequency Range:\text{Frequency Range} = [f – \frac{B}{2}, f + \frac{B}{2}]Where:
   • ( f ) is the carrier frequency.
   • ( B ) is the carrier bandwidth.

Time Domain Calculations

1. Slot Duration (( T_{\text{slot}} )):T_{\text{slot}} = N_S \times T_{\text{symbol}}Where:
   • ( T_{\text{slot}} ) is the slot duration.
   • ( N_S ) is the number of symbols per slot (typically 14 for normal CP).
   • ( T_{\text{symbol}} ) is the symbol duration.
   For example, with a 15 kHz subcarrier spacing, ( T_{\text{symbol}} ) is approximately 66.67 µs:T_{\text{slot}} = 14 \times 66.67 \text{ µs} \approx 1 \text{ ms}

Practical Example

Let’s consider a practical example with a 30 kHz subcarrier spacing:

• Frequency Domain:
  • Subcarrier spacing (( \Delta f )) = 30 kHz
  • Bandwidth of an RB = 12 * 30 kHz = 360 kHz
• Time Domain:
  • Slot duration (( T_{\text{slot}} )) = 14 * 33.33 µs ≈ 0.5 ms