<aside> đź’ˇ The entire resource grid, Visualized
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Just like how a point in Cartesian coordinates is expressed as an (x, y) pair, we describe resource elements (REs) in this grid by their slot # and carrier frequency. Slot # is determined by scheduling/SFI, and is confined to 1 of several predefined patterns. Carrier frequency is a bit more complex, described by an integer called “ARFCN”.
For frequencies below 3GHz, ARFCN = 200*frequency (MHz), giving a granularity of 1 MHz/200 = 5kHz. For frequencies above 3GHz, the granularity increases to 15kHz but the numbers keep increasing from 600000 upwards (i.e. 600001 is 15kHz above 600000, which is 5kHz above 599999).
Notice that with numerology 0 (15kHz) spacing, the ARFCN can only be changed in increments of 3, since 3*5kHz = 15kHz. Similarly, numerologies 1, 2, etc admit ARFCN spacings of 6, 12, etc.
To switch to a different band (in 5G, devices always work in terms of bandwidth parts, or BWPs), we need to tell the UE where in the frequency grid the new BWP is, then redo the entire attach process starting from RACH. Although we could communicate everything using ARFCN, variable subcarrier spacing means it’s sometimes easier to describe relative offsets, e.g. 5 PRBs up or 3 subcarriers away. Compared to LTE, 5G tends to let the UE do more calculations (to save time/spectrum at the tower?), so the UE does the work of translating these relative offsets into carrier frequencies.
The first step of the attach process is to locate the new SSB (which contains PBCH, which carries MIB/SIB1, which contains CORESET0, which tells the UE where PDSCH is), so we give the UE 2 numbers: offsetToPointA (units of PRBs), and k_SSB (units of subcarriers). You can view offsetToPointA as coarse location, while k_SSB gives the exact $f_c$ of the “lowest” part of the SS/PBCH block.
5G’s dynamic slot-based scheduling and use of BWPs means that all these UEs need a common “absolute” locator, which they can use as a reference to find their own BWP(s). This point is called point A, and it’s the lowest subcarrier (#0) of the lowest PRB (CRB0). The location of point A is described by absoluteFrequencyPointA (as an ARFCN) in SIB1, and is decided by the NR frequency band, subcarrier spacing, and channel bandwidth. If these change, then absoluteFrequencyPointA is recalculated.
<aside> đź’ˇ Calculator for point A
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Another way to describe the location of a BWP is the RRC parameter locationAndBandwidth. Within the RRC message, a related parameter is carrierBandwidth, measured in PRBs.
<aside> đź’ˇ There are formulas to calculate this parameter, but usually these tables suffice.
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http://howltestuffworks.blogspot.com/2019/11/5g-nr-resource-blocks.html