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PDCCH is the heart of the data call, if the UE is unable to decode and get the DCI info carried in the PDCCH, then no matter what the radio conditions are UE can never know the DL/UL grants for PDSCH/PUSCH hence end user experience would be no data.
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So bear minimum requirement is at any cost UE should be able to decode the PDCCH and derive the DCI information carried by PDCCH.
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In LTE PDCCH is located across the channel bandwidth and number of PDCCH symbols are conveyed by CFI carried in DL PCFICH. (pretty straight forward for the UE to understand)
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How ever in 5G NR due to the large bandwidth that UE has to handle it is further broken down into Bandwidth Parts and with in each Band Width part 5G NR PDCCH CORESET region is localized to specific PRBs on Frequency domain.
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