Where Does AC1900 Come From?
Ever since the RT-AC68U was outed a few days ago, speculation has been rampant about how ASUS came up with a 1.9 Gbps total throughput spec. So far, they haven't responded to requests for an explanation.
What we do know about the product is that it is Wi-Fi Certified and has 3x3 radios on both bands.
ASUS RT-AC68U Wi-Fi Certification
It is also likely that it is sporting Broadcom chips inside. The "dual core" most likely refers to the new BCM4708 as the main SoC CPU. Since Broadcom hasn't announced new 3x3 2.4 or 5 GHz radios it most likely uses the old 3x3 standby of Broadcom's BCM4360 for 5 GHz.
Since the math doesn't work for 5 GHz 160 MHz channel link rates, the extra 150 Mbps of link rate (from AC1750) is most likely in the form of a proprietary throughput enhancement on the 2.4 GHz side.
And, in fact that seems to be the case. Recent Broadcom announcements of new 1x1 and 2x2 client devices refer to "TurboQAM" technology and its ability to increase 2.4 GHz link rates by 33%. (This version of "TurboQAM" should not be confused with Broadcom's earlier use of the moniker with its DOCSIS 2.0 cable modem chips.) This Telecom Lead story from April says "Broadcoms TurboQAM technology implements the highest data-rate 256-QAM mode [emphasis mine] in 2.4 GHz for faster throughputs".
Looking up the highest 256-QAM mode for a 40 MHz channel in the 802.11ac link rate table below comes up with 200 Mbps per stream. Multiply times three streams and you get... 600 Mbps. Note that "normal" 802.11n radios support up to only 64-QAM, which provides a maximum 450 Mbps link rate for 3x3 devices.
802.11ac Data rate table
Although TurboQAM has been recently mentioned in 2x2 and 1x1 device announcements, it's uncertain whether it can be enabled for Broadcom's existing 2.4 GHz 3x3 mainstay, the BCM4331.
So perhaps the RT-AC68U will sport its "enterprise" BCM43431 equivalent, which does support TurboQAM.
Of course, like all proprietary throughput-tweaking technologies before it, TurboQAM is specific to Broadcom. But since they have almost 100% market share in draft 802.11ac at this point, we could see other manufacturers besides ASUS hop onto the AC1900 Mbps train.
ASUS responded with the following:
This 600Mbps functionality only works with the PCE-AC66 at the moment and boosts each 802.11n stream's throughput from 150Mbps to 200Mbps.
Broadcom responded with the following:
TurboQAM is a Broadcom technology incorporated into our 11ac chips which allow you to transmit and receive at 256 QAM while operating in the 2.4 GHz band.
All of the Broadcom 11ac chips support the TurboQAM feature including:
-1x1ac combo chips (used in HTC One and Galaxy S4)
-Enterprise 11ac solutions.
So it looks like any Broadcom-based draft 11ac product could get this bit of link rate enhancement when used with a Broadcom-based client.
The "our 11ac chips" in the Broadcom response is significant. TurboQAM requires newer 11ac devices and is not implemented on N devices. This means that routers using N chips like the BCM4331 and BCM43217 can't become "AC1900" routers. This includes all AC1750 routers on the market to date.
So, my best guess at this point is that that RT-AC68U uses two BCM4360's—one for 5 GHz and the other for 2.4 GHz—to achieve its 1900 Mbps total link rate.