Point A in 5G-NR

“Point A” is a fundamental concept in 5G wireless networks that serves as a reference point within the frequency domain. It is a fixed reference point used to establish the structure of frequency resources, including subcarriers and resource blocks. The placement of “Point A” significantly influences how various communication components are positioned within the spectrum.

  • Positioning of Reference Point A:
  • “Point A” is positioned at the starting point of the frequency grid for resource allocation.
  • It is associated with subcarrier 0 of common resource block 0, regardless of the subcarrier spacing used in the system.
  • Importantly, “Point A” can be located outside the actual carrier frequency range, serving as an abstract reference.
  • Role as a Reference:
  • “Point A” acts as a critical reference from which the entire frequency structure is described.
  • It helps establish a consistent framework for allocating subcarriers and resource blocks across different communication scenarios.
  • Locating “Point A”:
  • Devices, such as user equipment (UE) or base stations, locate “Point A” based on system information.
  • The discovery of “Point A” is facilitated by decoding the broadcasted System Information Block 1 (SIB1).
  • Once SIB1 is decoded, the device knows the precise location of “Point A” within the frequency spectrum.

Let’s consider a practical example involving a 5G base station and a user equipment (UE) operating within a frequency band from 2.8 GHz to 3.5 GHz. Within this band, “Point A” is positioned at 3.0 GHz.

Subcarrier Allocation:

  • “Point A” at 3.0 GHz serves as the starting reference point for subcarrier allocation.
  • Subcarriers are distributed in both directions from “Point A” at regular intervals, such as every 15 kHz.

Resource Block Structure:

  • The base station divides the spectrum into resource blocks, each comprising a certain number of subcarriers.
  • For instance, a resource block might consist of 12 consecutive subcarriers.

Channel Allocation:

  • Different channels, including control channels and data channels, are allocated based on the positioning of “Point A.”
  • These channels are placed relative to the reference point for consistent and organized communication.

UE Discovery and Communication:

  • The UE detects synchronization signals (SSBs) broadcasted by the base station.
  • By decoding the SIB1 carried in these signals, the UE learns the location of “Point A” (3.0 GHz).

Resource Allocation:

  • All physical resource blocks used for transmitting actual signals are positioned relative to “Point A.”
  • The base station allocates resources based on the reference point to ensure efficient and controlled communication.

In this example, “Point A” at 3.0 GHz serves as the anchor for establishing the entire frequency structure. It guides how subcarriers, resource blocks, and channels are organized and allocated, leading to well-structured communication within the 5G network.

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