6月 21, 2015

802.11ac: The Fifth Generation of Wi-Fi Technical White Paper

1. Executive Summary

802.11ac is a faster and more scalable version of 802.11n. It couples the freedom of wireless with the capabilities of Gigabit Ethernet.

802.11ac achieves its raw speed increase by pushing on three different dimensions:

More channel bonding
Denser modulation
More multiple input, multiple output (MIMO)

The design constraints and economics that kept 802.11n products at one, two, or three spatial streams haven’t changed much for 802.11ac, so we can expect the same kind of product availability, with first-wave 802.11ac products built around 80 MHz and delivering up to 433 Mbps (low end), 867 Mbps (mid-tier), or 1300 Mbps (high end) at the physical layer.
Second-wave products may promise still more channel bonding and spatial streams, with plausible product configurations operating at up to 3.47 Gbps.
802.11ac is a 5-GHz-only technology, so dual-band APs and clients will continue to use 802.11n at 2.4 GHz. However, 802.11ac clients operate in the less crowded 5-GHz band.

Second-wave products could also come with a new technology, multiuser MIMO (MU-MIMO). Whereas 802.11n is like an Ethernet hub that can transfer only a single frame at a time to all its ports, MU-MIMO allows an AP to send multiple frames to multiple clients at the same time over the same frequency spectrum. That’s right: with multiple antennas and smarts, an AP can behave like a wireless switch.

One thing not to worry about is compatibility. 802.11ac is designed in a deep way to coexist efficiently with existing 802.11a/n devices, with strong carrier sense, a single new preamble that appears to be a valid 802.11a preamble to 802.11a/n devices, and extensions to request-to-send/clear-to-send (RTS/CTS) to help avoid collisions with users operating on slightly different channels.

2. What Is 802.11ac?

First, 802.11ac is an evolution of 802.11n. If you are already familiar with the channel bonding, MIMO, and aggregation introduced by 802.11n, and you don’t need a refresher, read on.

2.1 Drivers for 802.11ac

Figure 1. How 802.11ac Accelerates 802.11n

2.2 How Does 802.11ac Go So Fast?

Wireless speed is the product of three factors:

channel bandwidth
constellation density(content modulation)
number of spatial streams.

802.11ac pushes hard on the boundaries on each of these, as shown in Figure 1.
For the mathematically inclined, the physical layer speed of 802.11ac is calculated according to Table 1.
Table 1. Calculating the Speed of 802.11n and 802.11ac:

PHY	Bandwidth (as Number of Data Subcarriers)	×	Number of Spatial Streams	×	Data Bits per Subcarrier	÷	Time per OFDM Symbol	=	PHY Data Rate (bps)
802.11n or 802.11ac	56 (20 MHz)		1 to 4		Up to 5/6 × log₂(64) = 5		3.6 microseconds (short guard interval)
	108 (40 MHz)						4 microseconds (long guard interval)
802.11ac only	234 (80 MHz)		5 to 8		Up to 5/6 × log₂(256) ≈ 6.67
	2 × 234 (160 MHz)

For instance, an 80-MHz transmission sent at 256QAM with three spatial streams and a short guard interval delivers


 234 × 3 × 5/6 × 8 bits/3.6 microseconds = 1300 Mbps.

Immediately we see that increasing the channel bandwidth to 80 MHz yields 2.16 times faster speeds, and 160 MHz offers a further doubling. Nothing is for free: it does consume more spectrum, and each time we’re splitting the same transmit power over twice as many subcarriers, so the speed doubles, but the range for that doubled speed is slightly reduced (for an overall win).

The speed is directly proportional to the number of spatial streams. More spatial streams require more antennas, RF connectors, and RF chains at transmitter and receiver. The antennas should be spaced one-third of a wavelength (3/4 inch) or more apart, and the additional RF chains consume additional power. This drives many mobile devices to limit the number of antennas to one, two, or three.

Going from 64QAM to 256QAM also helps, by another 8/6 = 1.33 times faster. Being closer together, the constellation points are more sensitive to noise, so 256QAM helps most at shorter range where 64QAM is already reliable. Still, 256QAM doesn’t require more spectrum or more antennas than 64QAM.

2.3 How Do We Make 802.11ac Robust?

Table 2. Important Data Rates of 802.11a, 802.11n, and 802.11ac:

Nominal Configuration	Bandwidth (MHz)	Number of Spatial Streams	Constellation Size and Rate	Guard Interval	PHY Data Rate (Mbps)	Throughput (Mbps)^*
802.11a
All	20	1	64QAMr3/4	Long	54	24
802.11n
Amendment min	20	1	64QAMr5/6	Long	65	46
Low-end product (2.4 GHz only+)	20	1	64QAMr5/6	Short	72	51
Mid-tier product	40	2	64QAMr5/6	Short	300	210
Max product	40	3	64QAMr5/6	Short	450	320
Amendment max	40	4	64QAMr5/6	Short	600	420
802.11ac 80 MHz
Amendment min	80	1	64QAMr5/6	Long	293	210
Low-end product	80	1	256QAMr5/6	Short	433	300
Mid-tier product	80	2	256QAMr5/6	Short	867	610
High-end product	80	3	256QAMr5/6	Short	1300	910
Amendment max	80	8	256QAMr5/6	Short	3470	2400
802.11ac 160 MHz
Low-end product	160	1	256QAMr5/6	Short	867	610
Mid-tier product	160	2	256QAMr5/6	Short	1730	1200
High-end product	160	3	256QAMr5/6	Short	2600	1800
Ultra-high-end product	160	4	256QAMr5/6	Short	3470	2400
Amendment max	160	8	256QAMr5/6	Short	6930	4900
^*Assuming a 70 percent efficient MAC, except for 802.11a, which lacks aggregation. ⁺Assuming that 40 MHz is not available due to the presence of other APs.

2.3.1 Technology Overview

By design, 802.11ac is intended to operate only in the 5-GHz band, as shown in Table 3. This avoids much of the interference at 2.4 GHz, including Bluetooth headsets and microwave ovens, and provides a strong incentive for users to upgrade their mobile devices (and hotspot APs) to dual-band capability so that the 5-GHz band is more universally usable.

As we’ve already seen, 802.11 introduces higher-order modulation, up to 256QAM; additional channel bonding, up to 80 or 160 MHz; and more spatial streams, up to eight. There is an alternative way to send a 160-MHz signal, known as “80+80” MHz, discussed later (see Section 2.3.6).

Because of the wider channel bandwidths of 802.11ac, it is much more likely that an 80-MHz AP will overlap with another 20- or 40-MHz AP - and similarly an 80- or 160-MHz AP - or even several of them, all potentially on different channels. To enable reliable operation amid this complexity, 802.11ac mandates extensions to the RTS/CTS mechanism, stronger clear-channel assessment (CCA) requirements, and new primary channel selection rules. See Section 2.3.4.

802.11ac continues some of the more valuable features of 802.11n, including the option of a short guard interval (for a 10 percent bump in speed) and an incrementally better rate at range using the advanced low-density parity check (LDPC) forward error-correcting codes. These LDPC codes are designed to be an evolutionary extension of the 802.11n LDPC codes, so implementers can readily extend their current hardware designs.

Various space time block codes (STBCs) are allowed as options, but (1) this list is trimmed from the overrich set defined by 802.11n, and (2) STBC is largely made redundant by beamforming. 802.11n defined the core STBC modes of 2×1 and 4×2 and also 3×2 and 4×3 as extension modes, but the extension modes offered little gain for their additional complexity and have not made it to products. Indeed, only the most basic mode, 2×1, has been certified by the Wi-Fi Alliance. With this experience, 802.11ac defines only the core 2×1, 4×2, 6×3, and 8×4 STBC modes, but again only 2×1 is expected to make it to products: if you had an AP with four antennas, why would you be satisfied with 4×2 STBC when you could - and should - be using beamforming?

What 802.11ac also gets right is to define a single way of performing channel sounding for beamforming: so-called explicit beamforming. Although optional, if an implementer wants to offer the benefits of standards-based beamforming, there is no choice but to select that single mechanism, which can then be tested for interoperability.

2.3.2 Differences Between 802.11ac and 802.11n

Given the power of A-MPDU and the 802.11n channel access mechanism, 802.11ac actually didn’t need to innovate much in the MAC. Indeed, extensions to the RTS/CTS mechanism are the only new mandatory MAC feature.

2.3.3 Standards-Based Beamforming

An 802.11ac AP operating on 80 MHz (or 160 MHz and so on) should still be capable of allowing 802.11a or 802.11n clients to associate. Thus, beacons are sent on one 20-MHz channel, known as the primary channel, within that 80 MHz. The AP and all clients associated with the AP receive and process every transmission that overlaps this primary channel and extract virtual carrier sense from the frames they can decode.

However, the AP could be near other uncoordinated APs. Those APs could be preexisting 802.11a or 802.11n APs, and their primary channels could be any 20 MHz within the 80 MHz of the 802.11ac AP. The different APs and their associated clients then have a different virtual carrier sense and so can transmit at different times on the different subchannels, including overlapping times.
For this reason, 802.11ac defines an enhanced RTS/CTS protocol. RTS/CTS can be used to determine when channel bandwidth is clear and how much, around both the initiator and the responder, as shown in Figure 3.
Figure 3. RTS /CTS Enhanced with Bandwidth Signaling:

When an 802.11ac device sends an RTS

this initiating device has to verify that the 80-MHz channel is clear in its vicinity
the RTS is normally sent in an 802.11a Physical Protocol Data Unit (PPDU) format
the basic 802.11a transmission, which is 20 MHz wide, is replicated another three times to fill the 80 MHz (or another seven times to fill 160 MHz).

every device that hears the RTS has its virtual carrier sense set to busy
the device addressed by the replicated RTS responds with CTS
the CTS is sent, every nearby device receiving a CTS can understand on its primary channel.
the initiator can switch to a narrower bandwidth on the fly

2.3.5 All A-MPDUs

2.3.6 Channelization and 80+80 MHz

802.11ac adopts a keep-it-simple approach to channelization. Adjacent 20-MHz subchannels are grouped into pairs to make 40-MHz channels, adjacent 40-MHz subchannels are grouped into pairs to make 80-MHz channels, and adjacent 80-MHz subchannels are grouped into pairs to make the optional 160-MHz channels
Figure 4. 802. 11ac Channelization (United States):

Figure 5. Example of Parallel Transmissions with Two BSSs on the Same 80 MHz but with Different Primary 20-MHz Subchannels:

It is entirely allowed for two 80-MHz 802.11ac APs to select the same 80-MHz channel bandwidth:

one AP to put its primary 20-MHz channel within the lower 40 MHz
the other AP to put its primary 20-MHz channel within the upper 40 MHz
802.11n clients associated with the 1st AP can transmit 20 or 40 MHz as usual, at the same time that 802.11n clients associated with the 2nd AP transmit 20 or 40 MHz in parallel.
any 802.11ac client that sees the whole 80 MHz as available to invoke a very high-speed mode and to transmit across the whole 80 MHz.

The ability to have overlapped APs but different primary channels is made possible by:

The enhanced secondary CCA thresholds mandated by 802.11ac, which are up to 13 dB more stringent than the secondary CCA thresholds defined by 802.11n
The addition of a bandwidth indication to the RTS/CTS exchange

802.11ac also introduces a noncontiguous 80+80 MHz mode.
It is the 160-MHz bandwidth but is transmitted in two separate 80-MHz segments, each of which can lie on any allowed 80-MHz channel. APs and clients only ever transmit on 80+80 or receive on 80+80; they are never expected to transmit on one 80-MHz segment and receive on the second 80-MHz segment.
As shown in Figure 6. 80+80 MHz has 13 options versus the 2 options for 160 MHz (ignoring regulatory issues).

Unfortunately, an 80+80 MHz device is much more complicated than a 160-MHz device, since the 80+80 MHz device needs twice as many RF chains.

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