What is RB and PRB in 5G?
In 5G New Radio (NR), RB stands for Resource Block and PRB stands for Physical Resource Block. These are essential units of frequency-time resources that form the foundation of 5G radio communication. Both are used in the scheduling and allocation of radio resources between the gNB (5G base station) and the User Equipment (UE), such as smartphones or IoT devices.
Resource Block (RB)
A Resource Block is the basic unit of resource allocation in the frequency domain in 5G. It consists of 12 consecutive subcarriers in the frequency domain, where each subcarrier is typically spaced at 15 kHz, 30 kHz, 60 kHz, or even higher depending on the selected numerology (Subcarrier Spacing or SCS). The number of RBs available depends on the channel bandwidth and the numerology used.
Each RB spans:
- 12 subcarriers in the frequency domain
- One slot (which consists of 14 OFDM symbols) in the time domain
The RB is a logical structure used in resource allocation algorithms. It does not include physical-layer details like cyclic prefix or guard bands.
Physical Resource Block (PRB)
PRB is the actual physical implementation of the Resource Block in the 5G NR system. It represents a time-frequency resource grid structure on which the physical channels (PDSCH, PUSCH, etc.) are mapped. A PRB is essentially a 12-subcarrier by 1-slot physical structure that can be visualized on the time-frequency grid.
While RB refers to the logical structure, PRB includes additional physical aspects and is used in the actual transmission and reception of data. PRBs are allocated to UEs through the downlink and uplink scheduler in the gNB, and they carry user data, control information, and signaling messages.
Numerology and PRB Configuration
5G supports flexible numerology defined by different Subcarrier Spacing (SCS): 15 kHz, 30 kHz, 60 kHz, 120 kHz, and 240 kHz. The PRB size in the frequency domain (12 subcarriers) remains fixed, but the bandwidth of each PRB increases with higher SCS. This allows 5G to adapt to different use cases, such as low-latency communication (URLLC), enhanced mobile broadband (eMBB), and massive machine-type communication (mMTC).
Bandwidth and Number of PRBs
The total number of PRBs that can be configured in a carrier depends on the channel bandwidth and the selected numerology. For example:
- With 20 MHz bandwidth and 15 kHz SCS: around 106 PRBs
- With 100 MHz bandwidth and 30 kHz SCS: around 273 PRBs
The actual number can vary slightly due to guard bands and implementation-specific parameters.
What is the role of PRBs in data throughput?
PRBs directly influence the throughput of a 5G link. The more PRBs a user is allocated, the higher the potential data rate. Scheduling algorithms in the gNB dynamically assign PRBs based on channel quality, Quality of Service (QoS), and user demand.
What is the difference between RB and PRB?
The main difference lies in the level of abstraction:
- RB (Resource Block) is a logical resource unit used by higher-layer protocols and scheduling functions.
- PRB (Physical Resource Block) is the physical implementation of an RB on the actual time-frequency grid of the radio interface.
How are PRBs scheduled in 5G?
PRB scheduling in 5G is handled by the MAC layer of the gNB. The scheduler assigns PRBs to users based on various factors like channel conditions (CQI reports), buffer status, latency requirements, and priority levels. PRBs can be allocated dynamically every slot, providing efficient and fine-grained control of radio resources.
What is a Slot in 5G and how does it relate to PRBs?
A slot is a unit of time in the 5G frame structure, and each slot contains a certain number of OFDM symbols (typically 14). PRBs are mapped over these slots to transmit data. One PRB spans one slot in the time domain and 12 subcarriers in the frequency domain, forming the basic block of 5G communication.