The terms RB (Resource Block) and RBG (Resource Block Group) represent a shift from the Control Channel (which uses REG/CCE) to the Data Channel (which uses RB/RBG).
- For Control (PDCCH): We use RE REG CCE Aggregation Level. Why? Control messages are small, vital, and need to be scattered to survive interference.
- For Data (PDSCH): We use RE RB RBG. Why? User data (like a video stream) is large. Allocating it tiny bit by tiny bit (REG by REG) would be a nightmare for the scheduler. The system needs larger “containers” to move bulk data efficiently.
1. Resource Block (RB)
The Resource Block is the standard “cargo container” for user data.
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Definition: An RB is a block of time and frequency.
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Dimensions: It measures 12 Subcarriers (Frequency) 7 Symbols (Time).
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Total Capacity: (Note: This assumes “Normal Cyclic Prefix.” If the network uses “Extended Cyclic Prefix” for long-distance coverage, the time duration drops to 6 symbols, making the RB slightly smaller at 72 REs ).
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Usage: When the network sends you data, it doesn’t give you individual subcarriers; it gives you these blocks of 84 REs at a time.
2. Resource Block Group
The RBG is simply a bundle of multiple RBs.
- Definition: A unit comprised of multiple consecutive RBs.
- Why it exists: If you have a 20 MHz bandwidth, you have 100 RBs available. If the scheduler had to say “User 1 gets RB 1, RB 3, RB 5, RB 8…”, the instruction message itself (the DCI) would be huge.
- The Solution: The scheduler groups RBs into “Groups” (RBGs). Instead of assigning single RBs, it assigns a whole Group at once.
- Variable Size: The document notes that the number of RBs inside one RBG is not fixed; it differs depending on the System Bandwidth.
- Small Bandwidth: 1 RBG might = 1 RB.
- Large Bandwidth (20MHz): 1 RBG might = 4 RBs.