Evolution of Cell Identification: From 2G to 5G

 Evolution of Cell Identification: From 2G to 5G - Unraveling the Significance of Physical Cell Identity (PCI)

In any cellular system, whether it's 2G, 3G, 4G, or 5G, each cell needs a distinct identity to differentiate it from other cells, especially when cells are adjacent and using the same frequency, as is the case in 3G and 4G.
In 2G systems, each cell is distinguished by CI (Cell Identity), BSIC (Base Station Identity Code), and BCCH (Broadcast Control Channel) frequency. In 3G systems, the unique identifier for each cell is the Scrambling Code.

In 4G systems, the distinctive identifier for a cell is the PCI (Physical Cell Identity). There are 504 PCIs available, and the PCI for each cell is determined by the formula: PCI Number = Orthogonal Code + 3 * PN Code group. The system employs planning similar to the frequency reuse concept in 2G.
Each site is allocated a specific PN code from the 168 available, and each sector within the site is assigned one of the three Orthogonal codes (0, 1, or 2). For example, a site with three sectors might use PN 10, and each sector would have a different Orthogonal code (0, 1, or 2). Applying the formula yields the PCI numbers for each sector.
The reason for the complexity of the PCI system, with its PN codes, Orthogonal codes, and the formula, is for the Cell Search phase. During Cell Search, a mobile device needs to identify and select the best cell in its vicinity quickly. Knowing the PN code and Orthogonal code reduces the search space from 504 to just 3 possibilities, streamlining the cell selection process.
The mobile device listens to the PSS (Primary Synchronization Signal) to determine the Orthogonal code and the SSS (Secondary Synchronization Signal) to determine the PN Code group. This process, while intricate, significantly reduces the time needed for cell selection.
This concept is reminiscent of the PSC (Primary Scrambling Code) and SSC (Secondary Scrambling Code) in 3G.
In 5G, the PCI system remains, but with some variations, such as 1008 PCIs organized into 336 code groups. The PSS and SSS channels are also present in 5G, continuing the legacy of cell identification.