IEEE 802.11ac entered the market almost 6 years after the transition from 802.11a/b/g to 802.11n. During that time all large-scale, corporate, commercial 802.11 WiFi networks had upgraded their infrastructure away from 802.11b/g to 802.11n and all mobile devices, laptops, and computers were connecting in the 802.11 WiFi environment. It wasn't effective to do a "1-for-1" swap, replacing 802.11n access points with 802.11ac access points due to the higher signal quality requirements for full 802.11ac capability. This meant that WiFi networks needed new 802.11ac WiFi network design services and the installation of new cabling and access point drops. Planning for 802.11ac also required a careful assessment of the wired Ethernet and fiber backhaul to be sure that switches, interconnecting Ethernet and fiber, routers, and firewalls all had port-level support for higher aggregate data throughput. The timeline for 802.11ac evolution is as follows:

Each spatial stream carries data. Two spatial streams provide twice the throughput as does one. Multiple spatial streams comprise a technology referred to as "MIMO" (Multiple-Input, Multiple-Output"). A device requires a separate antenna for each supported MIMO spatial stream. If a device has only 1 antenna it can only support 1 spatial stream. For example, the Broadcom BCM4335 Wi-Fi chipset, used in some Samsung smartphones, supports only 1 spatial stream. Some mobile tablets and laptops have two antennas and support two spatial streams (hence, twice the potential throughput). The number of achievable spatial streams will be arbitrarily limited by the hardware capabilities of the device. 

Even if an access point and a device had hardware support for 3 spatial streams there is a statistical reality that, in most cases, only results in, at most, 2 spatial streams being the norm. It's statistically unlikely that 4 spatial streams could be consistently maintained in any real-world environment. Also, signal reflections are needed to allow multiple reflective paths on which different spatial streams will travel. This is fine indoors where walls provided reasonable reflection. Outdoors there are often few usable reflection points thus allowing for only a single spatial stream. The number of spatial streams used, along with the set of other aspects of an 802.11ac (or 802.11n and 802.11ax) connection are identified by a numeric value called the "Modulation Coding Scheme Index" or "MCS Index". You can view the MCS Index Table here.

Only real-world, on-site testing can establish the probability of achieving more than 1 spatial stream (albeit, achieving 2 streams is typical indoors.)

If you double the channel width you double the throughput capacity of the channel. All 802.11ac devices support both 20 MHz and 40 MHz channel widths. A limited number of devices support 80 MHz channels (not many mobile devices provide this support) and even fewer devices support 160 MHz channels.

When noise or interference is present on a channel it can corrupt the bits being transmitted resulting in reduced overall throughput and capacity. The wider the channel, the greater the probability that some noise or interference will enter the channel. 80 MHz and 160 MHz channel widths are mostly unusable for this reason and, in many cases, even 40 MHz channels can suffer from unacceptable performance degradation.

On-site, real-world 802.11ac testing can provide measurements as to whether or not use of a 40 MHz channel will improve performance. Use of 80 MHz and 160 MHz channels is typically unrealistic.

MIMO (multiple streams transmitted and/or received) was discussed earlier, Multi-User MIMO implies that some streams are destined for one particular device while, at the same time, some streams are destined for other devices. A MU-MIMO transmission sends data to two (or more) destinations at the same time resulting in increased throughput to the devices (since everyone doesn't need to wait their turn to receive data; multiple devices can receive data simultaneously.

For MU-MIMO to work there are three requirements: 1) the transmitter must have an antenna array (preferable at least 3 antennas) in order to beamform (aim) the spatial streams in the correct directions and, 2) the MU-MIMO recipient devices (probably mobile, hand-held devices) need to transmit "sounding packets" so that the access point can calculate the client device's location in the reflective environment and, 3) there has to be at least one dedicated spatial stream for each recipient client in the MU-MIMO transmission. Referring back to the previous discussion of MIMO Spatial Streams it should be clear that the proverbial "planets aren't typically going to align" to make MU-MIMO a game changer for network network capacity and throughput.

Only on-site RF testing can truly determine whether MU-MIMO is going to be usable in any particular environment. Connect802's experience has been that it's a functionality that sounds great but doesn't always live up to the expectations.

"Modulation" refers to the way an electromagnetic signal is "jiggled around" and manipulated to represent data bits. There are various patterns that modify the amplitude and waveform phase to create unique bit patterns. The simplest modulation scheme is called "BPSK" (Binary Phase-Shift Keying) which specifies that if the phase of the transmitted signal remains the same into the next bit time period then that next bit is a binary 0, and if the phase shifts by 180 degrees then the next bit is a binary 1. It's simple, either a 1 or a 0; and that's what's used in every 802.11 WiFi standard going back to 1 Mbps 802.11b. As the modulation scheme gets more intricate it becomes possible to have not just two patterns (1 and 0) but up to 256 unique patterns of phase and signal amplitude change. This would be referred to as "256 Quadrature Amplitude Modulation" (or, "256-QAM"). Every possible value of a single byte (8 bits) could be transmitted with a single burst of modulated waveform! 256-QAM has twice the throughput of 64-QAM which has 4 times the throughput of 16-QAM (the three QAM modulation schemes used).

The "gotcha" with high QAM modulation rates is that the more intricate and complex the waveform is, the more likely it will be corrupted by environmental noise or interference. In practice, it's usually not possible to maintain a 265-QAM connection and even 64-QAM suffers from the realities of many real-world environments. Modulation, coupled with channel width and other aspects of the 802.11ac connection, are quantified in the Modulation Coding Scheme (MCS) Index. You can view the MCS Index Table here.

What levels of 802.11ac modulation can be obtained in your environment? Only on-site, real-world testing can answer that question. Don't, however, set your expectations on the highest specified modulation and bit-rate levels.

An omnidirectional antenna (a dipole "stick") propagates electromagnetic RF signals in all directions perpendicular to the antenna element (to visualize the RF pattern, imagine putting a doughnut down over the antenna). With a "beamforming antenna", greater RF signal energy is propagated in some particular direction as opposed to other directions. This means that the signal density from a beamforming antenna is greater in the direction of an intended recipient because the overall transmitted power is not spread evenly around the antenna. A beamforming antenna uses sophisticated circuitry, and requires multiple antennas, to shape and direct the RF energy. With more transmit antennas comes greater beam focus and more possible directions to which the beam can be pointed.

Prior to 802.11ac, beamforming was implemented by some vendors (notably, Ruckus Wireless with its patented BeamFlex technology implementing up to 19 separate antenna elements in the access point). The 802.11ac standard provided specifications that required a standardized approach to beamforming. The most notable aspect included the implementation of "sounding packets" sent by client devices to let the access point know where they were located. An access point detects a sounding packet arrive on its multiple antennas. The phase relationship between the multiple received versions of the sounding packet are different because the received signal took different paths through the air. When the access point transmits back to that client it matches the phase relationship of the sounding packet and the result is that the transmission is directed back to the client. (That's an extermely simplified, broad description of beamforming but it should provide a basic idea of the process).

As discussed earlier regarding MIMO Spatial Streams, not all devices have more than one, single antenna. Just like a device can't utilize MIMO without multiple antennas, so too a device can't transmit a beamformed RF signal without multiple antennas. This means that mobile devices generally don't use beamforming.