In Post 12 we said: bits get mapped to I/Q symbols (points on the complex plane).
Now the story continues:
Why does 5G use 256QAM?
Because it can pack more bits into each I/Q symbol.
The progression (bits per symbol) 
QPSK → 2 bits/symbol
16QAM → 4 bits/symbol
64QAM → 6 bits/symbol
256QAM → 8 bits/symbol
More bits/symbol ⇒ potentially higher throughput with the same symbol rate.
The catch 
Higher-order QAM packs more constellation points into the same I/Q plane.
So points get closer together → noise, interference, and distortion have less room to “hide”.
That means higher-order modulation needs:
Better SNR
Better EVM (Tx/Rx quality)
Better channel conditions (less fading/interference)
The “adaptive” part (the real beauty) 
The system doesn’t always transmit 256QAM.
It keeps asking:
“How many bits can I safely fit into this channel right now?”
Good channel 
→ higher-order QAM
Bad channel 
→ lower-order modulation (more robust)
Where does this live? 
This is digital PHY (like we discussed in Post 12):
DU / PHY accelerator → QAM mapper → complex I/Q symbols
By the time it hits RF, the hardware just sees a waveform.
And next, that waveform gets organized across thousands of subcarriers…
Next: OFDM — what is it actually doing? 
Technical reference: 3GPP TS 38.211 defines NR modulation and OFDM baseband signal generation
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