The issue with the X12 and 3xCA is the amount of spatial streams it can handle.
The X12 modem can process up to six spatial streams when leveraging 3-way carrier aggregation and 2×2 MIMO, but only four spatial streams (one component carrier) when attached to a 4×4 MIMO capable basestation.
256QAM itself is not a spatial stream, but a form of modulation and how the data is handled. 2^8 bits per Hz as opposed to 2^6... 8 is ~33% more than 6, which is where we get the 33% faster peak speeds. Same goes for UL.
Galaxy S7 - Carrier Aggregation Band 30 10 MHz PCC + Band 4 10 MHz SCC + Band 12 10 MHz TCC
Quadrature Amplitude Modulation (QAM) is best described like this... most if not all of you will get it.
As I said in my earlier post, the difference is 2^6, or 64 (QAM) vs 2^8, or 256 (QAM)
All data is sent in binary... either a 0 or a 1. With higher order modulation, you get to send two more pairs (8-6 is 2, 8 is 33% larger than 6) of binary. All data measurement is done on a bits/Hz ratio. With 64QAM, you're sending 6 bits per Hz, with 256QAM, you're sending 8 bits per Hz.
Also, fun fact, 21.1 Mbps HSPA+ is the result of AT&T going from 16 to 64QAM on UMTS.
Hope that clears it up for you!
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