Initial cell selection: In this procedure, UE scans the frequency bands without the use of prior stored data. Once the suitable cell is found, the UE camps on the selected cell.
• Stored information cell selection: In this procedure, the UE utilizes the carrier frequency information, cell parameters etc. of the previous measurements. Once the suitable cell is selected, UE camps on it.
Cell selection criteria S has to be met to select the suitable cell which states that:
Where, 𝑄𝑟𝑥𝑠𝑟𝑣𝑚𝑟𝑎𝑦=𝑀𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑅𝑥 𝑙𝑒𝑣𝑒𝑙 𝑣𝑎𝑙𝑢𝑒 (𝑅𝑆𝑅𝑃) 𝑆𝑟𝑥𝑠𝑟𝑣=𝐶𝑒𝑙𝑙 𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝑅𝑥 𝑙𝑒𝑣𝑒𝑙 𝑣𝑎𝑙𝑢𝑒 (𝑑𝐵) 𝑄𝑟𝑥𝑠𝑟𝑣𝑚𝑠𝑠=𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑚𝑖𝑛𝑖𝑚𝑢𝑚 𝑅𝑥 𝑙𝑒𝑣𝑒𝑙 𝑣𝑎𝑙𝑢𝑒 (𝑅𝑆𝑅𝑃) 𝑄𝑟𝑥𝑠𝑟𝑣𝑚𝑠𝑠𝑠𝑜𝑜𝑦𝑟𝑠=𝑂𝑓𝑓𝑠𝑒𝑡 𝑡𝑜 𝑠𝑖𝑔𝑛𝑎𝑙𝑙𝑒𝑑 𝑄𝑟𝑥𝑠𝑟𝑣𝑚𝑠𝑠(𝑑𝐵) 𝑃𝑠𝑠𝑚𝑝𝑟𝑠𝑦𝑎𝑠𝑠𝑠𝑠=𝑃𝑜𝑤𝑒𝑟 𝐶𝑜𝑚𝑝𝑒𝑛𝑠𝑎𝑡𝑖𝑜𝑛 (𝑑𝐵)
Once the suitable cell has been selected and the camped upon, the UE starts to measure neighbouring cells for the reselection process. UE measures the neighbouring cells in the neighbour cell list of the serving cell. To decrease the frequency of neighbouring cell measurements, a threshold signal level is defined for the serving cell so that UE does not need to perform the measurement if the serving cell measured value exceeds the threshold. The threshold has been defined for both inter-frequency and intra-frequency cell selections.
Intra-frequency measurement criteria is that if 𝑆𝑦𝑟𝑟𝑣𝑠𝑠𝑔𝑠𝑟𝑠𝑠≤Sintrasearch, intra-frequency neighbour search should be initiated and inter-frequency measurement criteria is that if 𝑆𝑦𝑟𝑟𝑣𝑠𝑠𝑔𝑠𝑟𝑠𝑠≤Snon-intrasearch , inter-frequencies or inter-RAT frequency neighbour search should be initiated. 𝑆𝑦𝑟𝑟𝑣𝑠𝑠𝑔𝑠𝑟𝑠𝑠 is the 𝑆𝑟𝑥𝑠𝑟𝑣 value of the serving cell.
Cell re-selection is performed when the above mentioned measurement criteria is ful-filled. In the case of intra-frequency cell re-selection, ranking criteria is used. The serv-ing cell and the neighbouring cells are ranked based on measured data. The best ranked cell is re-selected.
The serving cell is ranked as
While the neighbouring cell is ranked as
𝑄𝑚𝑟𝑎𝑦,𝑦 is the measured RSRP value of the serving cell while 𝑄𝑚𝑟𝑎𝑦,𝑠is the measured RSRP value of the neighbouring cell. The hysteresis value 𝑄ℎ𝑦𝑦 is added so that the frequent cell-reselection is prevented. Once the neighbouring cell is better ranked, the cell transition occurs after 𝑇𝑟𝑟−𝑦𝑟𝑠𝑟𝑠𝑠𝑠𝑠𝑠 time. This also helps to reduce the frequency of cell re-selection.
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