wp designed speed 802.11n

7/30/2019 Wp Designed Speed 802.11n

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White Paper |

Peter Thornycroft

Enterprise

Designed for Speed:Network Infrastructure in an

802.11n World


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1 Designed for Speed Aruba Networks

Table of contentsSummary ..................................................................................................................................................................... 3Introduction................................................................................................................................................................. 4Benefits for the Enterprise .......................................................................................................................................... 4

Benefits for the user ............................................................................................................................. 5Realizing these benefits with a mixed client base requires careful planning ......................................... 5

802.11n migration strategies for enterprises ............................................................................................................... 6Network design with 802.11n ............................................................................................................... 6Wired backhaul from APs & wired LAN design ..................................................................................... 7Power consumption.............................................................................................................................. 7Implications for rogue APs and WIDS .................................................................................................. 8Good-neighbor (or bad-neighbor) strategies ........................................................................................ 8Changes between draft-2.0 and 802.11n .......................................................................................... 8

Technology in 802.11n ............................................................................................................................................. 12Techniques for high-throughput PHY ...................................................................................................................... 12

High Throughput PHY: Maximum Ratio Combining ........................................................................... 13High Throughput PHY: space-time block coding ............................................................................... 13High Throughput PHY: Spatial Division Multiplexing ......................................................................... 14

High throughput PHY: Transmit Beamforming (TxBF) ......................................................................................... 19MIMO, STBC, SDM & Beamforming ..................................................................................................................... 21

802.11n MIMO configurations and terminology .................................................................................. 22Hierarchy of MIMO techniques ........................................................................................................... 22High Throughput PHY: 40 MHz channels .......................................................................................... 24High Throughput PHY: Shorter guard interval ................................................................................... 25High Throughput PHY: More subcarriers ........................................................................................... 26High Throughput PHY: New Modulation Rates .................................................................................. 26

Techniques to enhance the MAC .............................................................................................................................. 29MAC layer enhancements: Frame aggregation ................................................................................. 29MAC layer enhancements: Multiple Traffic ID Block Acknowledgement (MTBA) ............................... 30MAC layer enhancements: Reduced inter-frame spacing (RIFS) ...................................................... 30MAC layer enhancements: Spatial multiplexing power save (SM power save) .................................. 30MAC layer enhancements: Power Save Multi-poll (PSMP) ................................................................ 31Compatibility Modes and Legacy Support in 802.11n ............................................................................................. 32Greenfield, High-throughput and non-HT modes ................................................................................ 34Phased coexistence Operation (PCO)................................................................................................ 34Other mechanisms for coexistence: RTS/CTS & CTS-to-self ............................................................ 36Other mechanisms for coexistence: 40 MHz-intolerant indication ...................................................... 36Using 802.11n in the 2.4 GHz band ................................................................................................... 3620/40 MHz channel numbering in the 2.4 GHz band .......................................................................... 37Using 802.11n in the 5 GHz band ...................................................................................................... 38

Use of 20/40 MHz channels, coexistence and protection mechanisms .................................................................... 3820/40 MHz operation and fallback to 20 MHz ..................................................................................... 38

New Wi-Fi Alliance 802.11n certifications .............................................................................................................. 41Migration strategies .................................................................................................................................................. 43Different paths to enterprise-wide 802.11n ......................................................................................... 43

Greenfield .......................................................................................................................................... 43Summary of Greenfield enterprise WLAN recommendations: ........................................................... 44

AP-overlay ......................................................................................................................................... 45Summary of Overlay recommendations: ........................................................................................... 45

AP substitution ................................................................................................................................... 46Summary of AP substitution recommendations: ............................................................................... 46

Other considerations when planning an upgrade ...................................................................................................... 47


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Aruba Networks Designed for Speed 2

Conclusion ................................................................................................................................................................ 48Appendix .................................................................................................................................................................. 49

Note on expected real-world cell capacity with 802.11n .................................................................... 49Forms of MIMO ....................................................................................................................................................... 50Channel estimation ................................................................................................................................................... 51Glossary of terms used in this note ........................................................................................................................... 52 References................................................................................................................................................................. 53About Aruba Networks, Inc. ..................................................................................................................................... 54


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Two steps into the 802.11n adoption wave

Summary

The IEEE 802.11n standard and Wi-Fi Alliance 802.11n certification herald a new world for enterprisewireless networks. 802.11n brings significantly higher data rates and more reliable coverage than previous

Wi-Fi technology: it represents a significant upgrade in performance.

The first Wi-Fi Alliance 802.11n draft-2.0 certification, dating from June 2007, was based on a snapshot of

the then-unfinished IEEE 802.11n specification. Over the next two years, many millions of draft-2.0

products were shipped, and in enterprise WLANs draft-2.0 already accounted for 30% of all access points

shipped in 2Q2009, according to the DellOro Group and Aruba Networks.

In September 2009, 802.11n passed the second milestone in its rollout, as the IEEE concluded its ratification

of the standard, and the Wi-Fi Alliance released its new 802.11n certification program. The new

certification accepts all previous draft-2.0 products as compliant, so all equipment that was certified as

draft-2.0 can immediately use the 802.11n Wi-Fi logo. The new version adds a number of features, but

they are all optional. Those of us who questioned whether the final certification would render draft-2.0

devices obsolete have been proved wrong. Those who understandably insisted on waiting for a final

specification can now move ahead.

The accumulated experience with draft-2.0 equipment has allowed us to update this booklet first

published in September 2007 to include experience gained from real-world deployments of draft-2.0

equipment, and to give a more concrete view of what 802.11n means for enterprise networking. We can

already see, for instance:

802.11n offers 5x to 7x the performance of 802.11a/g;

The indoor environment offers sufficient multipath that multi-spatial-stream transmission is the norm

rather than the exception; and

MAC aggregation contributes significantly to throughput for many applications.

It is still true that the performance benefits are only fully realized in a legacy-free environment, as even a few

older (802.11a/b/g) clients on an access point can drastically reduce overall performance compared to a

uniform 802.11n network. Fortunately, the PC vendors long ago standardized on draft-2.0 capabilities, and

the installed base of enterprise clients now comprises a significant and growing percentage of high-

performance PCs. Indeed, Aruba Networks university customers report that by the fall 2009 entry, the

penetration of 802.11n-capable clients already approached the 50% mark.

Also, the migration to 802.11n poses some challenges in network design. For best performance, LAN edge

switch ports and cabling to the access points require an upgrade to Gigabit Ethernet even more important

now the 802.11n certification extends to higher data rates, and 802.11n overlays may be necessary if high-

speed services are to be assured. But the concern that 802.11n access points were power-hungry many on

the market still exceed the 802.3af Power over Ethernet limits is transient. From early 2009, all newly-

designed dual-radio enterprise 802.11n access points are likely to comply with 802.3af. 802.11n migration

strategies still require careful planning, but the constraints are becoming less restrictive.


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In this paper, we will explain the advanced technology introduced in the final 802.11n certification,

allowing enterprise network managers to understand its benefits and to plan their own upgrade strategies.

Introduction

Wi-Fi technology has carved a path of ever-increasing performance from the earliest pre-802.11 standards

through 802.11b to 802.11a/g, with peak data rates rising from 2Mbps to 54Mbps. The latest set ofinnovations is a package known as 802.11n.

We refer in this paper to two key documents that shape the industry, and will endeavor to maintain

consistency in using these terms:

IEEE 802.11n is a technical standard developed by the IEEE, (formerly named the Institute of

Electrical and Electronic Engineers) 802.11 working group. The draft 2.0 milestone was attained

in March 2007, and IEEE 802.11n was finally completed and ratified in September 2009. The earlier

draft was the basis for the Wi-Fi Alliance draft-2.0 certification, but it was never formally

published, and is now superseded by the final 802.11n standard.

Meanwhile 802.11n is an interoperability certification developed by the Wi-Fi Alliance, a tradeassociation of companies interested in promoting 802.11 products (Wi-Fi is a Wi-Fi Alliance

brand). 802.11n is a certification awarded by the Wi-Fi Alliance to indicate a product has passed a

set of tests that ensure it will inter-operate with other 802.11n products. For this certification, the

Wi-Fi Alliance took parts of the IEEE 802.11n standard, and developed a series of tests involving a

testbed of early 802.11n-compliant equipment: this certification tests only a subset of the full IEEE

802.11n functionality.

The history of IEEE 802.11n draft 2.0 and the draft-2.0 certification are included in this

document because of the large amount of installed draft-2.0 equipment. Although this is fully

interoperable with new equipment, as has been the case throughout the history of Wi-Fi, newer

802.11n products include options enabling better performance than draft-2.0 devices.

Benefits for the Enterprise

802.11n includes a number of complex technological advances which are explained in detail later in this

paper. Many of these features have already demonstrated astonishing performance improvements in draft-

2.0 equipment, and 802.11n enables even greater performance.

Increased capacity. 802.11n enables increased data rates, improving the usable data capacity of an access

point from perhaps 15-20 Mbps with 802.11a/g to 150-300 Mbps (see appendix for more analysis). Draft-2.0

equipment already demonstrates a 5x 7x improvement in application throughput over 802.11a/g, and

802.11n allows for a 50+% increase over draft-2.0. Given that this capacity will be spread over a number

of simultaneous users, performance will match or exceed that of a wired 100Mbps Ethernet connection, thestandard for desktop connectivity.

Improved range. An 802.11g connection from AP to client can usefully extend up to 60 meters in open,

unobstructed areas but this range drops to only 20 meters in office environments. 802.11n increases this

through multiple-input, multiple-output (MIMO) techniques which involve driving multiple antennas on the

access point and the client. The use of MIMO improves the connection data rate for a given range, and

somewhat extends the range at the edge of a cell, useful if a network is designed for coverage rather than

capacity.


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More uniform reliable coverage. Coverage in Wi-Fi networks can be spotty. A user may have a good

signal in one location, but moving the client a short distance, stepping in front of it, or even opening a door

across the room can affect the received signal strength, moving the client into a coverage null and reducing

performance. One contributor to this issue is multipath propagation, and the best technology to counter this

to date has been antenna diversity nearly every Wi-Fi device sports two antennas, and switches between

them because when one is in a multipath null, the other should still have a workable signal. The MIMOtechnology in 802.11n is extremely effective in reducing the effect of multipath nulls by allowing antennas to

work together to recover the original signal: the effect is that the incidence and severity of signal nulls is

greatly reduced, especially as a mobile client moves across the network.

Lower network costs. In a homogeneous 802.11n network, improved range and more reliable coverage can

allow APs can be spaced further apart. This reduces costs in a number of ways: fewer APs, lower

installation costs, possibly fewer LAN edge switch ports, and fewer outdoor APs to cover campus areas

between buildings. But to date, it has been difficult to realize these gains because of the need to support

legacy clients, and because it is usually more important to increase data rates than to extend range (most

enterprise WLANs are designed for capacity rather than coverage).

Benefits for the userEnd users benefit from higher data rates, higher throughput and more uniform coverage. For a given group

of users on an access point, each will now have access to more bandwidth. The WLAN is no longer the

limiting factor for application performance, overcoming a significant obstacle to un-wiring the desktop and

leading eventually to the All-Wireless Workplace. One example of an application that benefits from 802.11n

performance is video, both interactive and broadcast, which many organizations are beginning to use as an

internal communications tool. Earlier 802.11 networks can support video, but its demands, at 1 to 4 Mbps per

channel, stress 802.11a/g installations. 802.11n provides 5x the data performance, as well as incorporating

features optimized for video.

Realizing these benefits with a mixed client base requires careful planning

While the promise of 802.11n is great, some factors make it difficult for enterprises to realize its full benefits

in the short-term.

The presence of legacy clients impairs 802.11ns performance. The benefits outlined above are easy to

achieve in a greenfield network, where all APs and all clients are 802.11n-capable. However, very few

deployments are completely new, particularly on the client side. As long as older clients exist, they will

affect the performance of the network by connecting at much lower rates than 802.11n clients, effectively

slowing APs to near-802.11a/g rates. Legacy devices are also unable to take advantage of the improvements

in range and uniformity of coverage offered by 802.11n. If 802.11n APs are spaced farther apart to lower

costs, legacy clients will likely run into more coverage problems than before.

Despite the limitations, 802.11n deployments are moving forward rapidly. A number of factors mitigate thepicture painted above. Firstly, a rising percentage of the client base is 802.11n-capable. Draft-2.0 devices

(equivalent to 802.11n in this context) approached 50% of the client population on some university campuses

at the fall entry of 2009, and this percentage will only increase, as nearly all new PCs are now 802.11n-

capable. Secondly, the legacy poisoning effect is only significant at high AP loading, and the average

offered traffic for most WLANs is very low: networks are dimensioned for infrequent peak loading

conditions. Most of the time, an 802.11n client of an 802.11n AP will be able to see its full bandwidth.


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Thirdly, network managers with specific performance needs, such as broadband video, add 802.11n APs as

an additional WLAN overlay, limiting the clients on those APs to ensure they connect only at 802.11n rates.

Naturally, there can be issues when users with legacy PCs find they cant access these high-speed services,

but the increased penetration of 802.11n over time is a positive trend.

Lastly, most WLAN vendors implement some kind of per-user bandwidth control, even though it is only a

partial solution to the problem. As long as even a few legacy clients exist, the expected capacity

improvements are limited.

When we wrote the first edition of this booklet, in mid-2007, the issue of mixed-client network performance

was a significant concern. Fortunately, it has turned out to be less significant than we first feared, and many

802.11n networks are working well in mixed mode even though with high traffic they do not deliver the

data capacity of a pure 802.11n network. Time is our greatest ally, as the percentage of older PCs and other

802.11b/a/g clients fades.

802.11n migration strategies for enterprises

802.11n includes a number of mechanisms for successful coexistence with legacy 802.11 clients andnetworks. As noted above, these operating modes impair the performance advantages of a greenfield

802.11n network, but they do offer complete support for older 802.11 clients. Thus, 802.11n access points

can be installed before any 802.11n clients are present, and operate in 802.11a/b/g mode. It is also possible

to mix 802.11n access points within an 802.11a/b/g network, replacing selected existing access points, or

deploying new 802.11n access points as an overlay in parallel with an existing 802.11 network: a later

section of this booklet provides more detail on migration strategies.

Network design with 802.11n

For RF planning purposes, 802.11n differs from 802.11a/g in utilizing MIMO and the option of a 40 MHz

channel, which provides approximately twice the data rate of a 20 MHz channel, but in doing so uses twice

the RF spectrum. Because the 2.4 GHz band has a limited number of channels, the 40 MHz option is notusually deployed there, although the many other enhancements in 802.11n still provide significant

performance improvements in a 20 MHz channel. However, the 40 MHz channel is popular in the 5 GHz

band where channels are plentiful.

Most RF planning tools rely on modeling the reduction of signal strength over distance and across or through

RF obstructions. For most purposes, this reduces to a simple calculation of dB loss at a given distance from

the AP; knowing the transmit power, the data rate achievable at a given distance from the AP can be easily

calculated. For instance, to plan an Aruba network, the required inputs are the dimensions of the floorplan,

the minimum data rate desired and the cell overlap factor. Since the propagation characteristics for 802.11n

signals are little different from 802.11a/g, existing RF planning tools such as this one need little

modification. The output of the RF planning tool will be, as before, a set of suggested AP locations to satisfythe input parameters.

The new factors that must be considered are the number of antennas on both AP and client, and whether the

design should account for legacy clients.

The type, placement and attitude of the antennas, particularly on the AP, can be important to 802.11n

performance; most enterprise office deployments use the articulated captive antennas supplied on the AP.


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Spacing the antennas by at least half a wavelength (6.25cm for 2.4GHz or 2.7cm for 5.5GHz) allows good

MIMO capability: closer spacing may restrict performance.

Wired backhaul from APs & wired LAN design

With 802.11a/g, the maximum data rate on the air is 54 Mbps, and actual, achievable data throughput tops

out at about 26 Mbps for UDP and 21 Mbps for TCP traffic. Such a rate is easily supported on a 10/100

Mbps Ethernet connection for backhaul from the AP.

802.11n rates, however, can move well beyond 100 Mbps, at least for peak traffic. A dual-radio, draft-2.0

access point with two spatial streams can generate up to 600Mbps peak traffic, albeit half-duplex, and even

this is too much for a single 100 Mbps Ethernet connection. The new 802.11n certification tests to three

spatial streams, (IEEE 802.11n allows for up to four) which could generate 900 Mbps if both radios operate

in 40 MHz channels. Thus, vendors have three options in designing APs:

Continue with the current design, with one 10/100 Ethernet connection, realizing that at peak loads

this will be the network bottleneck. In the short-term this may be acceptable: traffic peaks will be

widely spaced since clients do not often connect at maximum data rates.

Use multiple 10/100 Ethernet connections to the AP. Two (or more) cable drops and two LAN edgeswitch ports are needed for each AP location.

Provide Gigabit Ethernet connections (10/100/1000). This comprehensively accommodates the

traffic load, but may require that cabling is upgraded to Category 5e or Category 6, and that LAN

edge switches provide GE ports. Thus far most infrastructure vendors, including Aruba, are

providing this GE option.

Other areas of the LAN should be checked for traffic capacity but will probably not require upgrades. This is

because the traffic on the upstream connection from the edge switch still represents the same aggregate

number of users and applications as when clients were all wired.

Power consumption

The power consumption of the early generations of draft-2.0 APs is greater than for 802.11a/g APs,

exceeding 802.3af limits (12.95W maximum can be delivered to a Class 3 device under 802.11af, now

properly termed clause 33 of updated 802.3-2005). This means the edge switches or in-line power injectors

must support the unratified 802.3at standard. Alternatively, the AP must use a local power brick.

But the longer-term trend in 802.11n access points is to reduce power consumption, even as performance

increases, and the requirement for 802.3at will not last for long. Already, the first generation of 802.11n-

compliant silicon uses higher levels of integration (fewer chips in the AP), more sophisticated clock

management and smaller-geometry processes to reduce power needs below the 802.3af limit, even for APs

with three driven antennas.

Based on currently-available (late 2009) 802.11n technology, single-radio and dual-radio APs with up to

three driven (transmitting) antenna chains should be within 802.3af limits. APs with four transmitting

antennas per radio or more than two radio units will be borderline or into the 802.3at range, although over

time we expect this limit to be attainable.


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Implications for rogue APs and WIDS

Most 802.11n enterprise infrastructure is multi-purpose, also providing RF monitoring and wireless intrusion

detection (WIDS). 802.11n access points are important for identifying rogue APs as employees adopt

consumer products at home, find they work and bring them to the office. While most of these devices use

beacons and other frames that can be recognized by 802.11a/g access points, some can be configured for

greeenfield operation. In this mode, the only way to reliably identify these devices is to use another

802.11n device such as an 802.11n AP in monitor mode.

To date, WIDS vendors have not found the need for enhancements to identify characteristics of 802.11n-

specific attacks: as most attacks are at higher levels than the PHY, the current WIDS functionality has proven

sufficient for draft-2.0 and should be adequate for 802.11n environments.

Good-neighbor (or bad-neighbor) strategies

802.11n networks will often overlap with other 802.11 networks. In some cases the network manager will

wish to cooperate with these neighbors, while in others it may be better to use non-coexisting modes,

supporting higher throughput, and to ignore occasional interference. 802.11n allows a number of strategies.

Several forms of interference are possible:

If an 802.11n network is operating in greenfield mode, its transmissions will not be comprehensible

to 802.11a/b/g APs and clients. Therefore, even though neighboring APs may be operating in the

same RF channel, the transmissions from one cell (AP and clients) will appear as noise bursts to the

other. This interference will work in both directions: 802.11n transmissions are likely to be affected

as much as the legacy network.

An 802.11n network using a 40 MHz channel will have just this effect in the 5 GHz band. In 2.4

GHz, because the channel boundaries of 802.11n do not line up with the traditional channels used in

802.11b/g (channels 1, 6, 11), transmissions will inevitably overlap in frequency. This is one reason

such 40 MHz operation at 2.4 GHz is not recommended. However, it is quite reasonable to use

802.11n in 20 MHz channels in the 2.4 GHz band.An 802.11n network operating in mixed-mode is entirely compatible with overlapping 802.11a/b/g

cells. In this mode, all transmissions are prefixed by a preamble that uses legacy modulation,

allowing such networks to coexist.

Any 802.11n device, client or AP, can indicate that it is 40 MHz intolerant. Any device hearing

this indicator must inform its AP, and the AP must indicate and execute a switch to 20 MHz

operation.

Another optional mechanism, Phased Coexistence Operation (PCO) allows an AP to alternate between 20

MHz channel 40 MHz channels. The AP sets the Network Availability Vector (NAV) on the 20 MHz

channels to inhibit transmissions for a time, during which it and whatever 40 MHz-capable clients it supports

switch to 40 MHz operation.

Changes between draft-2.0 and 802.11n

An enormous amount of work went into the IEEE 802.11 standard as it progressed from draft-2.0 (March

2007) to the final version (from draft-11.0). But none of the changes was significant enough to affect

backwards-compatibility. Most of the changes were in just a few areas that were not part of the Wi-Fi

Alliances draft-2.0 certification, such as beamforming.


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This allowed the Wi-Fi Alliance to incorporate the changes from draft-2.0 to 802.11n as optional

extensions. In fact, equipment that was certified as draft-2.0 is automatically qualified for the 802.11n label,

with full backwards and forwards compatibility.

At the technical level, the new Wi-Fi Alliance 802.11n certification adds a number of features as options

that were not part of the original certification. These features are:

Short guard interval

Greenfield preamble

Transmitting with A-MPDU MAC aggregation

40 MHz operation in the 2.4 GHz band with coexistence mechanisms

40 MHz operation in the 5 GHz band

Transmitting in HT duplicate mode

Space-time block coding (2x1)

Transmitting up to 3 spatial streams

A number of these options were already part of draft-2.0, and some of them were already tested in the

receive direction (e.g. for A-MPDU, a device had to be able to receive correctly from a testbench device), but

there was no option to test for correct transmission before.

Also, even though a number of these features were not part of the draft-2.0 certification, they were still

available in commercial equipment. All enterprise WLAN vendors operate at 40 MHz channels in both 2.4

GHz and 5 GHz, for instance, and some products already support three spatial streams. The practical

performance difference between draft-2.0 devices and 802.11n devices may not be uniformly significant.

Three other questions exercised the Wi-Fi Alliance as it considered the 802.11n certification:

Whether to allow 40 MHz channels in the 2.4 GHz band. An AP using a 40 MHz channel in the 2.4

GHz band will interfere with any other 2.4 GHz equipment within range, not only Wi-Fi equipment

but Bluetooth, DECT and other wireless protocols. Even with a 20 MHz channel, interference is

common, but devices have the option to find unused parts of the band. The 40 MHz channel takes so

much of the band that it is difficult for neighboring devices to avoid it, hence a debate over whether

it should be allowed. The resolution is that a 40 MHz channel is tested and certified in the 2.4 GHz

band, but only if coexistence mechanisms are also supported. The main coexistence mechanisms are

the 40 MHz intolerant bit, that can be broadcast by any 802.11 or other device, a periodic AP scanfor other 802.11 activity, and the use of the CTS protection mechanism for frames such as

greenfield frames that would not be detected by older Wi-Fi equipment.

How to label products to inform the buyer of their capabilities and performance. Since the IEEE

802.11n standard allows a wide range of options, performance of 802.11n devices will vary. For

instance, a device supporting two spatial streams in a 20 MHz channel and an 800 nsec guard

interval would have a top rate of 130 Mbps, while three spatial streams in a 40 MHz channel and a


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400 nsec guard interval can reach 450 Mbps. Both are 802.11n (see below for the debate on single

SS APs, technically they are not recognized as 802.11n). The wide performance range caused

considerable apprehension. Some equipment vendors were concerned that the 802.11n logo should

not be devalued by lower-end devices: there should be a minimum threshold of performance, hence

the decision to deny the 11n logo to single-stream APs. At the other extreme, many felt it important

to allow higher-end vendors to market their equipment with a higher-performance label, and this

gave rise to the tagline concept (see below). Still others complained that while the number of spatial

streams is an indicator of performance, it is one of many factors and should not be singled out; while

cynics suggested that the buying public will likely not read or understand taglines in any case, and

the complexity of 802.11n features can never be successfully explained. The compromises resulting

from this debate are that single-stream APs are tested but not given the 802.11n logo, while a set of

three taglines defines different categories of equipment, based on functionality and, hopefully,

predicting relative performance.

Should single-stream APs be allowed? This is a derivative of the previous question, but fiercely

debated, as many vendors plan to build low-cost APs with only a single radio, but using 802.11n

silicon. The argument why should these not be 802.11n centers on performance. Customers

purchasing 802.11n labeled equipment will be expecting a level of performance perhaps 100

Mbps. A single spatial stream AP is intrinsically limited to a top rate of 150 Mbps, whereas the

second stream doubles this to 300 Mbps. Should the Wi-Fi Alliance mandate a minimum level of

performance, implied by a 2 SS baseline certification? Many clients only support 1 SS, but that will

not be important if they are dedicated devices such as Wi-Fi phones, where the higher performance

will never be seen, so the question was whether to certify a single-stream AP. The final answer is

that a single-stream AP will be certified, but it is supposed to carry an 802.11g logo and the tag line

with some n features. Time will tell whether equipment vendors hew to these labeling guidelines,

or find other ways to market their features and performance claims.

Here is the Wi-Fi Alliance lineup of taglines

Wi-Fi Alliance 802.11n certification taglines

Client device Access Point

Capabilities Tagline Capabilities Tagline

1x1 client devices None 1x1 access points (a

special certification)

With some n

features

Receive two-spatial

streams

Transmit & receive A-

MPDUIf operating at 5 GHz,

support 40 MHz channel

2x2, 1x2 or 2x3 MIMO

Dual-stream n Transmit & receive two-

spatial streams


MPDUIf operating at 5 GHz,


2x2 MIMO

Transmit STBC (2x1)

Dual-stream n

Transmit & receive three

spatial streams

Multi-stream n Transmit & receive three

or more spatial streams

Multi-stream n


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MPDU

If operating at 5 GHz,


3x3, 3x4 or 4x4 MIMO


MPDU

If operating at 5 GHz,


3x3 or 4x4 MIMO

Transmit STBC (2x1)

The tagline is optional, displayed at the vendors discretion. It is quite possible for a device to be

certified and display the 802.11n logo with a different combination of options, not lining up with

any tagline.

The concept of device classes developed for the draft-2.0 certification has effectively been

abandoned. Originally there was provision for PC devices, the baseline class, and also for HH

(handheld) and CE (consumer electronics) products. The rationale behind this was that not all

equipment required all 802.11n features, so the full list of required features could be shortened for

devices with defined purposes. Although there is some vestigial language in the 802.11n test plan

for HH and CE device classes, it is unlikely that a test suite will ever be defined for them.


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Technology in 802.11n

The IEEE 802.11n standard includes several key technologies and features. This section includes a brief

technical treatment of all significant new technologies. The Wi-Fi Alliance 802.11n certification is a sub-set of

the techniques in this section.

High throughput PHY (physical layer): MIMO. The new PHY uses Orthogonal Frequency DivisionMultiplexing (OFDM) modulation with additional coding methods, preambles, multiple streams and beam-

forming. These features can support higher data rates, longer ranges, and a much larger range of data rates

than earlier 802.11 standards. The MIMO technique that is synonymous with 802.11n belongs in this

section.

High throughput PHY: 40 MHz channels. Two adjacent 20 MHz channels are combined to create a single 40

MHz channel. This simple technique, already used in some point-to-point bridges and consumer equipment,

more than doubles the effective data rate under a given set of RF conditions.

Efficient MAC: MAC aggregation. Two MAC aggregation methods are supported to efficiently pack

smaller packets into a larger frame. This reduces the number of frames on the air, and reduces the time lost

to contention for the medium, improving overall throughput.

Efficient MAC: Block Acknowledgement. Particularly for streaming traffic such as video, a performance

optimization where one acknowledgement can cover many transmitted frames, so an ack is no longer

required for every frame. This technique was first introduced in 802.11e.

Power Saving: power save multi-poll. This is an extension of the APSD concept introduced in 802.11e.

Techniques for high-throughput PHY

IEEE 802.11n marks a significant increase in complexity over previous versions of IEEE 802.11, giving it

excellent performance, but making it increasingly difficult for the layman to understand. Several different

techniques are used, and at times combined, to give the improved PHY performance. We will examine some

of these techniques in this section, particularly those dealing with multiple antenna systems.

In legacy 802.11 equipment, the radio unit only drives one antenna at a time, and only receives on one

antenna at a time, usually the same antenna. Although such equipment often has two antennas, the radio

input and output is switched from one to the other, so only one at a time is carrying a signal. IEEE 802.11n

allows multiple antennas to be used simultaneously for either or both the transmit and receive functions.

Four distinct algorithms may be used, although not all at once:

Maximum ratio combining is a receiver function, where signals received on multiple antennas,

whether from one or a number of transmit antennas, can be combined to improve the signal-to-noise

ratio. MRC is an antenna diversity technique that can increase range for a given data rate.

Space-time block coding can be used where there are multiple transmit antennas, regardless of thenumber of receive antennas. STBC is an antenna-diversity technique that improves the signal-to-

noise ratio at the receiver, also increasing range for a given data rate.

Spatial division multiplexing is the technique most often associated with MIMO. Rather than

increasing range, SDM sends different spatial streams of data from each transmit antenna to each

receive antenna. Since these streams carry different data, the overall data rate of the system is

increased. Under good conditions, a MIMO system of two transmit and two receive antennas

doubles the achievable data rate over a single-antenna system.


14/56


Transmit beamforming is a technique where signals sent to multiple transmit antennas can be

phased such that the RF power at a targeted receive antenna is maximized. Although TxBF can be

used with non-802.11n clients, it is most effective when the 802.11n receiver cooperates by returning

a feedback message to allow the transmitter to optimize its beam.

High Throughput PHY: Maximum Ratio Combining

MRC is an established technique using multiple receive antennas to extract a better signal: it requires more

than one receive antenna chain, but can work with a single transmit antenna. Since 802.11n equipment

already features multiple independent receive chains, MRC is easily added to 802.11n products.

In the diagram above, each receive antenna would receive an identical signal under line-of-sight conditions.

However, due to noise and multipath effects, one or both of the antennas will often receive an impaired

signal. MRC allows the receiver to process both signals independently, and then combine them, weighted by

the strength of each signal, to extract a more accurate replica of the transmitted data-stream than it could

from a single antenna. In simple terms, when one antenna is in a null and has a bad signal, the other is

likely to have a good signal. The receiver will only suffer when all antennas have bad signals

simultaneously. As the number of antennas increases, the probability of bad conditions for all of them

simultaneously becomes progressively smaller.

In order for MRC (and MIMO techniques) to be effective, the receive antennas must receive different

versions (distorted by noise & interference) of the original transmitted signal. Accomplishing this goal

usually means separating the antennas by at least half a wavelength, in the order of 3cm for a 5GHz signal.

MRC is not explicitly covered in 802.11n because it can be implemented at the receiver only, with no

changes at the transmit end. However, most 802.11n chip vendors now implement a form of receive

diversity such as MRC.

Although receiver-feedback is not required, MRC works better if the RF channel can be characterized. In

simple terms, this involves sending a known sequence of symbols from the available transmit antennas to

calibrate the system. Because the receiver knows the symbols that were sent, it can determine the type of

distortion introduced by the RF channel. 802.11n intrinsically provides for these known sequences in the

long training fields (LTF). As MRC is only implemented in 802.11n receivers, this is part of the usualalgorithm.

High Throughput PHY: space-time block coding

STBC is another diversity technique for improving SNR, but is applied when the number of transmitting

antenna chains exceeds the number of receive antennas. STBC uses coding to transmit different (but known)

copies of the data-stream from different antennas; assuming the receiver knows the code; it will be able to

extract the original data with fewer errors than when a single transmit antenna is used.


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The form of space time coding used in 802.11n is the Alamouti code. This spreads one spatial stream over

two space time streams, taking a pair of data-stream bits, and performing operations on them in consecutive

time intervals.

When processing this sequence of two symbols from two space-time streams, the receiver is able to re-

constitute the original data-stream even in the presence of channel noise and distortion. STBC uses the time

dimension: consecutive symbol intervals contain the same pair of basic symbols, but modified according to

the code. An efficient code such as Alamouti allows minimum complexity for the transmitter and receiver,

but maximum improvement in SNR under normal channel impairments.

802.11n defines STBC codes to work with as many as 4 space-time streams. This technique can be used

when the number of transmit antennas exceeds the number of receive antennas: it can also be used in

conjunction with MRC. STBC requires both channel characterization at the receiver, and knowledge at the

transmitter and receiver of the STBC code in use.

High Throughput PHY: Spatial Division Multiplexing

802.11n achieves its most dramatic improvements in data rate through the use of MIMO (Multiple Input,

Multiple Output) spatial division multiplexing. SDM requires MIMO, specifically the transmitting and

receiving stations must each have multiple RF chains with multiple antennas it does not work where either

station has only a single antenna chain. The diagram shows the simplest MIMO system, with two

transmitting and two receiving antenna chains (2 spatial streams).


16/56


Each antenna is connected to its own RF chain for transmit and receive. The baseband processing on the

transmit side can synthesize different signals to send to each antenna, while at the receiver the signals from

different antennas can be decoded individually. (We will simplify this explanation by showing only one

direction of transmission, practical systems will transmit in both directions.)

Under normal, line of sight conditions, the receiving antennas all hear the same signal from the transmitter.

Even if the receiver uses sophisticated techniques to separate the signals heard at antennas 1 and 2, it is leftwith the same data. If the transmitter attempts to send different signals to antennas A and B, those signals

will arrive simultaneously at the receiver, and will effectively interfere with each other. There is no way

under these conditions to better the performance of a non-MIMO system: one might as well use only one

antenna at each station. If noise or interference affects the signals unevenly, MRC or STBC techniques can

restore it to a clear-channel line-of-sight condition, but in the absence of multipath, only one stream can be

supported, and the upper bound on performance is a clear-channel single-stream.

(Nearly all 802.11 stations built before 802.11n actually use two antennas. However, they do not use MIMO

the single radio unit switches from one antenna to the other, so only one is used at any time. Using two

antennas in this way helps to negate the effects of multipath, as when one antenna is in a multipath null, the

other is likely to have a better signal. It is generally reckoned that using antenna diversity in this way

improves overall reception by perhaps 3-6 dB, although the effect is of course statistical. MIMO is different

in that both antennas are driven by and receiving signals at all times, and those signals need not be identical.)


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However, if there is sufficient RF distortion and especially multipath in the path, receiving antennas will see

different signals from each transmit antenna. The transmit antenna radiates a signal over a broad arc,

scattering and reflecting off various objects in the surrounding area. Each reflection entails a loss of signal

power and a phase shift, and the longer the reflected path, the more delay is introduced relative to a line-of-

sight signal. In the past, multipath has been the enemy of radio systems, as the receiver sees a dominant

signal (usually line of sight), and all the multipath signals tend to interfere with this dominant signal,effectively acting as noise or interference and reducing the overall throughput of the system. Multipath

effects also change over time, as objects in the path move, and movement of reflecting objects results in a

Doppler shift of the frequency of the received signal, further complicating the mechanisms needed to counter

multipath.

To understand how MIMO works, first consider the signal each receive antenna sees in a multipath

environment.


18/56


In this example there are 3 multipath signals arriving at antenna 2. The strongest is signal a, and the

information carried in this signal will be decoded. Other signals arrive at lower power levels, and they are

time-shifted (or phase-shifted) compared to a, so it is likely they will degrade the overall signal-to-noise ratio

associated with a.

When multiple antennas are considered, however, MIMO offers considerable gains in throughput. The

example above shows that each receive antenna receives its dominant signal from a different transmit

antenna: receiver 1 tunes to transmitter A while receiver 2 uses transmitter B. When the system understands

this, it can take advantage by transmitting different signals from each antenna, knowing each will be received

with little interference from the other. Herein lies the genius of MIMO.

(In practice the technique is more sophisticated, using RF channel characterization as explained earlier in this

paper: it is not necessarily the case that individual signal paths can be drawn between pairs of transmit-

signal

processingMACetc

S1 in

S2 in

V11

V21

V12

V22

signal

processingMACetc

S1 out

S2 out

U11

U21

U12

U22

signal

processingMACetc

S1 in

S2 in

V11

V21

V12

V22

signal

processingMACetc

S1 out

S2 out

U11

U21

U12

U22


19/56


receive antennas, but given that the nature of the cross coupling is known at the receiver, and that

mathematical conditions for the channel are favorable, this is the overall effect.)

The diagram above shows a more detailed explanation of MIMO implementation. At the transmit side,

signal processing provides real and imaginary outputs S1in and S2in. These are then mixed with different

weights V11 etc, before the signals are combined and delivered to the transmit antennas. A similar mixing

function processes signals from the receive antennas using weights U 11 etc. Provided the RF characteristics

are known, the weights U11 can be calculated and set for optimum throughput, given the RF channel

conditions.

The most favorable case would be where each transmit-receive pair operates with a completely independent

RF path: a 2x2 (2 transmitting & two receiving antennas) system will have double the throughput of a

single-antenna 1x1 system, and a 3x3 configuration could extend to triple the throughput. IEEE 802.11n

defines MIMO configurations from 2x1 to 4x4 antennas and up to four spatial streams (The Wi-Fi Alliance

802.11n certification only tests to three spatial streams).

MIMO is the most difficult aspect of 802.11n to understand: multipath (reflected RF between transmitter

and receiver) is normally the enemy of performance, but with MIMO it can be used constructively. Line of

sight normally gives the best performance, but with MIMO it provides just baseline data rates. (Note,however, that reflected signals are usually much weaker than primary, line-of-sight signals. Even though

losing line-of-sight may allow use of more RF paths and hence the additive MIMO effect, the signal-to-noise

ratio of each path may be considerably worse than previously. It is difficult to predict the relative weight of

these two opposing effects.)

One key question in MIMO systems is how to tune the transmit signals at different antennas for optimum

reception at the receiver. 802.11n offers different methods for this. With implicit feedback the MIMO

transmitter characterizes signals from the receiver, and assumes that the channel is reciprocal reflections

and impairments operate equally in both directions. This is a reasonable approximation for most purposes,

but better performance is achieved when the receiver sends explicit feedback messages to the transmitter;

with these, the transmitter can accurately tune its signals for optimum reception and best signal tointerference and noise ratios at the receiver.


20/56


High throughput PHY: Transmit Beamforming (TxBF)

Transmit beamforming is a technique that has been used in radio for many years. Beamforming allows an

AP to focus its transmission to a particular client in the direction of that client (and vice versa for a client

with multiple antennas), allowing higher signal to noise ratios and hence higher data rates than would

otherwise be the case.

By carefully controlling the time (or phase) of the signal transmitted from multiple antennas, it is possible to

shape the overall pattern of the received signal, emulating a higher-gain, or directional antenna in the

direction of the target. The same implicit and explicit feedback mechanisms used to characterize the MIMO

channel allow beamforming.

In practice, beamforming may be used when MIMO with SDM is not effective. This is because

beamforming aims to produce a single, coherent RF signal at the receiver, while SDM relies on multiple,

independent signals. Also, contrasting with other adaptive antenna or beam steering technologies, the

802.11n, beam is not based on an indication of the direction of a client, but rather on the actual RF conditions

at its antennas; hence the requirement below for explicit feedback messages.

IEEE 802.11n defines three modes for beamforming. The first is implicit, so named because it relies on thebeamformee sending a sounding frame to the beamformer, which then assumes that the RF channel is

reciprocal, using the weights derived from the received signals to set the transmitting beam pattern. The

diagram below illustrates the implicit beamforming exchange.

Implicit feedback relies on the assumption that the RF channel is reciprocal that the multipath reflections

and attenuation from the beamformee to the beamformer are identical to the reverse direction. Experience

shows this is a good assumption, and implicit beamforming benefits from simplicity the beamformer does


21/56


all the calculation, and there is no need to transmit channel state information from the beamformee to the

beamformer.

But the implicit method has a significant disadvantage. Although the RF channel itself is reciprocal, the RF

amplifiers in the stations transmit and receive antenna chains have different characteristics, over amplitude,

phase and frequency. These effects are not accounted for in the implicit model; because the beamformers

transmit chains are not used in sounding. They can be corrected with a calibration table, but this is often not

practicable, as the effects change with time and temperature they must be measured at the time of

transmission, with cooperation between the beamformer and beamformee.

Thus, the implicit beamforming method is only useful if the channel state is measured before and during the

transmissions - which makes it just as complicated as other forms of beamforming. This calibration effect

A

B

1

1. Beamformer requests sounding frames

AB

2. Beamformee transmits sounding frames

A

B

1

13. Beamformer uses channel assessment fromsounding frames to form transmitted beam

Implicit transmit beamforming in 802.11n

A

B

1

2. Beamformee calculates correctbeamforming weights for the beamformer,

and returns the V matrix

A

B

1. Beamformer transmits sounding frames

A

B

1

13. Beamformer uses V matrix to form transmitted beam

Explicit transmit beamforming in 802.11n


22/56


explains why beamforming to legacy 802.11 clients is unsatisfactory in practice: legacy clients cannot

participate in channel state information (CSI) calibration, so beamforming in this regard is imprecise and

ineffective.

The discussion above established that beamforming works best with explicit feedback messages from the

client. IEEE 802.11n provides three methods, variations on an explicit beamforming procedure. In all cases,

the beamformer first transmits sounding frames to characterize the RF channel and transmit and receive

chains. On receipt of these frames, the beamformee sends information back to the beamformer, allowing it

to set up the optimal beam.

In the first method, the beamformee returns raw channel state information. This is a set of complex factors

indicating how each beamformee antenna hears each beamformer spatial stream. The beamformer then takes

the CSI matrix and calculates the weights it must use to optimize the beam.

In the second and third methods of explicit beamforming, the beamformee calculates the V matrix, the set

of weights the transmitter should use to maximize its signal, or SNR. In the explicit full matrix method, the

full V matrix is returned to the beamformer, which can then use it for subsequent transmissions.

These are satisfactory models, except that the amount of data in the CSI or V matrix can be very large. Itincludes coordinates for each OFDM sub-carrier, per-transmit stream, per-receive antenna and with a

reasonable degree of precision: this can be a large amount of data. In order to reduce the amount of data,

IEEE 802.11n specifies an option where a matrix compression technique is used: the compressed V matrix

method. Compressing the V matrix reduces overhead on the air.

All three IEEE 802.11n beamforming methods are complex, and as yet not well-characterized in practical

networks. Experience with indoor WLANs indicates that spatial multiplexing is generally more effective

than beamforming, but development organizations will continue to advance the technology.

MIMO, STBC, SDM & Beamforming

In this note, we use the term MIMO (Multiple Input, Multiple Output) for any system where the transmitter

has a number of antenna transmit chains (antennas that can be powered simultaneously with independent

signals) and/or receiver chains. Many commentators use terms such as SIMO, SISO: we see these as

degenerate forms of MIMO, but there is a brief explanation in the Appendix. A MIMO system can use a

variety of techniques to improve range and/or data rate.


23/56


The key technique associated with MIMO is SDM (spatial division multiplexing). For our purposes, these

are related terms: SDM provides a MIMO system with its superior data throughput. SDM allows

transmission of multiple streams of data, enabling higher data throughput due to the multiple antenna chains.

The diagrams explain the difference between MIMO/SDM, STBC, MRC, and transmit beamforming as used

in this document. While SDM is a multiplexing technique to increase overall data rate, STBC and MRC are

diversity techniques that improve the signal to noise and interference ratio, SNR or SINR. These techniques

can be combined under some conditions.

802.11n MIMO configurations and terminology

A full specification of an 802.11n system includes a number of parameters. MIMO is often defined as MxN:

e.g. 2x2, 3x3. In this case M refers to the number of transmit antennas configured, and N to the number of

antennas at the receiver.

Next the number of spatial streams must be specified. While for most access points this will be the same as

the number of antennas, many clients, particularly where power consumption, processing or size is a concern,

may have asymmetric capabilities. IEEE 802.11n offers MIMO specifications up to 4x4, with 4 spatial

streams, while the Wi-Fi Alliance 802.11n certifies up to 3 spatial streams. (The Wi-Fi Alliance draft-2.0certification tested up to 2 spatial streams.)

Hierarchy of MIMO techniques

MIMO as defined here includes diversity techniques (STBC for multiple transmit antennas and MRC for

multiple receivers) and SDM where there are multiple parallel paths that can be used to increase data rate.

The logical combination of these techniques in IEEE 802.11n is somewhat confusing: the diagram below

shows the general relationship.


24/56


The first decision is how many spatial streams to use. A spatial stream in this context carries data that is

independent of other spatial streams: spatial streams are combined by SDM techniques. The number of

spatial streams can be no greater than the smaller of the number of transmit or receive antenna chains in the

system, where the system covers a transmitting and a receiving unit. Most current 802.11n chipsets process

two or three spatial streams.

After the number of spatial streams is defined, there may be excess antennas at either the receiver or

transmitter. For instance, take the case where an 802.11n access point sports three driven antennas, while its

PC clients have two. Transmitting towards the client, the access point has an excess antenna. This

transmitter chain can be used to provide better performance by implementing STBC. The STBC encoder can

take one of the two spatial streams from the SDM block, and expand it to form three signals to drive the

transmit antennas.

At the receiver, antenna chains are programmed for the STBC used at the transmitter, and may use whatever

MRC capabilities the designer implements. In this case one would not expect any gains from MRC, as there

are two antennas receiving two spatial streams. The combined system gains bandwidth from SDM, and

some SNR gains from STBC: in practice, SNR may translate to a higher data-rate for a given range, or a

longer range for a given data-rate, its the implementers choice.

The diagram below shows the reverse link, where the transmitter has two antenna chains and the receiver

three. Now, it is not possible to use an STBC gain, as there are no excess transmit antennas, but the MRC

processing the receiver realizes from its excess antenna will provide approximately the same gain.

MAC

etc

Spatial

DivisionMultiplexor

Space-TimeBlock Coder MAC

etc

Spatial

DivisionMultiplexor

Space-Time

BlockDecoder

andMaximal

RatioCombiner

RF

channel

Data flow in this direction

Originaldata

stream

Split intoNS spatial

streams

Split againinto NSTS

space-timestreams

Receivedsignals from

availableantennas

Re-builtinto NS

spatialstreams

Originaldata

stream

MAC

etc

Spatial

DivisionMultiplexor

Space-TimeBlock CoderMAC

etc

Spatial

DivisionMultiplexor

Space-TimeBlock Coder MAC

etc

Spatial

DivisionMultiplexor

Space-Time

BlockDecoder

andMaximal

RatioCombiner

MAC

etc

Spatial

DivisionMultiplexor

Space-Time

BlockDecoder

andMaximal

RatioCombiner

RF

channel


Originaldata

stream

Split intoNS spatial

streams

Split againinto NSTS

space-timestreams

Receivedsignals from

availableantennas

Re-builtinto NS

spatialstreams

Originaldata

stream


25/56


High Throughput PHY: 40 MHz channels

All earlier versions of 802.11 have used 20 MHz channels, defined in the 2.4 GHz and 5 GHz bands. Somevendors have increased throughput, particularly in point-to-point bridging applications by using two adjacent

channels simultaneously, usually modulating each channel separately then combining the streams at the far

end.

802.11n specifies operation in the same 20 MHz channels used by 802.11b/g in the 2.4 GHz and 802.11a in

the 5 GHz bands, but adds a mode where a full 40 MHz wide channel can be used. As might be expected,

this offers approximately twice the throughput of a 20 MHz channel. However, while in the 5 GHz band the

channels are defined as pairs of existing 20 MHz channels, they do not line up with commonly-used 20 MHz

channels in the 2.4 GHz band, as these channels are not adjacent. This means that when a 40 MHz channel is

used in 2.4 GHz, it will interfere with at least one other 802.11b/g channel.

MAC

etc

Spatial

Division

Multiplexor

Space-Time

Block Coder MAC

etc

Spatial

Division

Multiplexor

Space-Time

Block

Decoder

and

MaximalRatio

Combiner

RF

channel


Original

data

stream

Split into

NS spatial

streams

Split again

into NSTSspace-time

streams

Received

signals from

available

antennas

Re-built

into NSspatial

streams

Original

data

stream

MAC

etc

Spatial

Division

Multiplexor

Space-Time

Block Coder MAC

etc

Spatial

Division

Multiplexor

Space-Time

Block

Decoder

and

MaximalRatio

Combiner

RF

channel


Original

data

stream

Split into

NS spatial

streams

Split again

into NSTSspace-time

streams

Received

signals from

available

antennas

Re-built

into NSspatial

streams

Original

data

stream


26/56


High Throughput PHY: Shorter guard interval

The diagram shows how the guard interval is used in OFDM. 802.11n uses complex modulation techniques

with OFDM, where blocks of input data are coded into a single OFDM symbol at RF.

For best (least-error) decoding, the symbol must arrive at the receiver without any interference or noise.

Previous sections of this document have shown how 802.11n uses MIMO to improve reception of multipath,but this only works symbol-by-symbol. Inter-symbol interference occurs when the delay between different

RF paths to the receiver exceeds the guard interval, causing a reflection of the previous symbol to interfere

with the strong signal from the current symbol: a form of self-interference.

The optional 400 nsec short guard interval in 802.11n can be used when the path difference between the

fastest and slowest RF paths is less than that limit. Experience with draft-2.0 equipment has been

encouraging: the shorter guard interval is usually attained in the indoor enterprise environment.


27/56


High Throughput PHY: More subcarriers

Through advances in implementation, it is now possible to squeeze more OFDM subcarriers in a 20 MHz or

40 MHz channel (each subcarrier allows more data to be transmitted over the RF channel).

The additional subcarriers effectively add bandwidth to the channel, allowing increased data rates for a given

modulation type (see the section below on new modulation rates). The number of subcarriers is increased

from 48 to 52 in a 20 MHz channel, and to 108 in a 40 MHz channel.

High Throughput PHY: New Modulation Rates

Radio systems have to adapt to the signal and noise characteristics of the RF path, and they accomplish this

by changing the modulation rate. For a given SNR (signal to noise ratio) the system will change the

modulation rate to provide the best compromise between raw data rate and error rate: at any point,modulating for a higher data rate will increase the error rate and at some point the increased error rate will

decrease the overall data throughput. This is a continuous decision-making process, the transmitter relying

on feedback from the receiver about its SNR to adjust the transmit modulation.

While 802.11a and g specify 8 rates (6, 9, 12, 18, 24, 36, 48 and 54 Mbps), 802.11n provides many more:

over 300. However, the basic set is of 8 rates:

Basic rates (Mbps) of 802.11n; 20 MHz channel; single stream; 400 nsec GI; equal modulation

MCS 0-7 7.2 14.4 21.7 28.9 43.3 57.8 65.0 72.2

(IEEE 802.11n indexes rates as Modulation and Coding Schemes (MCS), and the MCS references areincluded in these tables for reference.)

This is a set of rates for one spatial stream in a 20 MHz RF channel and with the 400 nsec guard interval and

equal modulation on all spatial streams. This basic set of rates is comparable to the 802.11b/g rates above:

each rate is improved by about 20% (e.g. 18 to 21.7 Mbps) by using slightly wider bandwidth, more

subcarriers and the shorter guard interval. The 72.2 Mbps rate has no equivalent in 802.11a/g: it uses 5/6

coding, a higher rate than the previous maximum of 3/4.

52 subcarriers (48 usable) for a 20 MHz

non-HT mode (legacy 802.11a/g) channel

fc +10MHz-10MHz

26 carriers 26 carriers

56 subcarriers (52 usable) for a 20

MHz HT mode (802.11n) channel

fc +10MHz-10MHz


114 subcarriers (108 usable) for a 40 MHz HT mode (802.11n) channel

fc +10MHz-20MHz


+20MHz-10MHz

52 subcarriers (48 usable) for a 20 MHz

non-HT mode (legacy 802.11a/g) channel

fc +10MHz-10MHz


fc +10MHz-10MHz


56 subcarriers (52 usable) for a 20

MHz HT mode (802.11n) channel

fc +10MHz-10MHz


fc +10MHz-10MHz


114 subcarriers (108 usable) for a 40 MHz HT mode (802.11n) channel

fc +10MHz-20MHz


+20MHz-10MHz


28/56


Other rates are generally derived as multiples of the basic rates above:

Rates (Mbps) of 802.11n 20 MHz channel; two streams; 400 nsec GI; equal modulation

MCS 8-15 14.4 28.9 43.3 57.8 86.7 115.6 130.0 144.4

Rates (Mbps) of 802.11n 40 MHz channel; single stream; 400 nsec GI; equal modulation

MCS 0-7 15.0 30.0 45.0 60.0 90.0 120.0 135.0 150.0

The 40 MHz channel allows slightly more than twice the data rate of a 20 MHz channel.

Rates (Mbps) of 802.11n 20 MHz channel; single stream; 800 nsec GI; equal modulation

MCS 0-7 6.5 13.0 19.5 26.0 39.0 52.0 58.5 65.0

The longer 800nsec guard interval restricts data rates below the 400 nsec option.

For a given situation, the range of choices will be smaller than the tables above or below would indicate,

because some of these factors are fixed for a given system:

20 MHz or 40 MHz channel. As discussed elsewhere in this paper, 40 MHz channels are widespread

in the 5 GHz band. However, a 40 MHz channel will not often be feasible for an enterprise

deployment in the 2.4 GHz band.

Spatial streams. As described above, the number of supported spatial streams cannot be larger than

the number of antenna chains. But it can be smaller, because of silicon processing capabilities or

because the client has fewer antenna chains than the access point. For instance, some 3x3 systems

only support 2 spatial streams. Also, where there is insufficient RF path isolation between streams,

even a 2x2:2SS system may not be able to support 2 diverse streams.

Guard interval. The guard interval is the time between OFDM symbols in the air. Normally it will

be 800 nsec: the option is for a 400 nsec guard interval. In practice, the shorter guard interval can beused most of the time when indoors.

Convolutional coding. When data arrives at the PHY layer for transmission, it is scrambled and

coded. This alters its spectral characteristics in order to achieve the best signal-to-noise ratio, and

also includes built-in error correction, known as convolutional coding. The 802.11n standard

includes BCC (block convolutional coding), as included in previous 802.11 standards, but also adds

an option for LDPC (low density parity check) coding, which can improve effective throughput

under certain RF conditions.

Modulation. All spatial streams may use the same (equal) modulation, or they may carry different

(unequal) modulation and coding. An example might be where there are three spatial streams with

good MIMO characteristics, but one stream has a high noise floor or a low signal level: under these

conditions the weak stream would support a lower data rate than the other streams. In practice,current equipment implements equal modulation only.


29/56


Data rate ranges (min & max) for various 802.11n system parameters

MCS Equal

Modulation

Channel

width (MHz)

Guard

Interval (nsec)

Number of

streams

Minimum PHY

rate (Mbps)

Maximum PHY

rate (Mbps)

Number of

rates

0-7 YES 20 800 1 6.5 65 8

0-7 YES 20 400 1 7.2 72.2 8

8-15 YES 20 800 2 13 130 8

8-15 YES 20 400 2 14.4 144.4 8

16-23 YES 20 800 3 19.5 195 8

16-23 YES 20 400 3 21.7 216.7 8

24-31 YES 20 800 4 26 260 8

24-31 YES 20 400 4 28.9 288.9 8

0-7 YES 40 800 1 13.5 135 8

0-7 YES 40 400 1 15 150 8

8-15 YES 40 800 2 27 270 8

8-15 YES 40 400 2 30 300 8

16-23 YES 40 800 3 40.5 405 8

16-23 YES 40 400 3 45 450 8

24-31 YES 40 800 4 54 540 8

24-31 YES 40 400 4 60 600 8


30/56


Techniques to enhance the MAC

MAC layer enhancements: Frame aggregation

A client (or AP) must contend for the medium (a transmit opportunity on the air) with every frame it wishes

to transmit. This results in contention, collisions on the medium and backoff delays that waste time that

could be used to send traffic. 802.11n incorporates mechanisms to aggregate frames at stations, and thus

reduce the number of contention events. Many tests have shown the effectiveness of reducing contentionevents in prior 802.11 standards. For instance, in 802.11g, a given configuration can send 26 Mbps of data

using 1500 Byte frames, but when the frame length is reduced to 256 Bytes, throughput drops to 12 Mbps.

With MAC-layer aggregation, a station with a number of frames to send can opt to combine them into an

aggregate frame (MAC MPDU). The resulting frame contains less header overhead than would be the case

without aggregating, and because fewer, larger frames are sent, the contention time on the wireless medium

is reduced.

Two different mechanisms are provided for aggregation, known as Aggregated MSDU (A-MSDU) and

Aggregated-MPDU (A-MPDU). The figure below shows the general architecture:

In the A-MSDU format, multiple frames from higher layers are combined and processed by the MAC layer

as a single entity. Each original frame becomes a subframe within the aggregated MAC frame. Thus this

method must be used for frames with the same source and destination, and only MSDUs of the same priority

(access class, as in 802.11e) can be aggregated.

An alternative method, A-MPDU format, allows concatenation of MPDUs into an aggregate MAC frame.Each individual MPDU is encrypted and decrypted separately. Since MPDUs are packed together, this

method cannot use the earlier 802.11 per-MPDU acknowledgement mechanism for unicast frames. A-

MPDU must be used with the new Block Acknowledgement function of 802.11n.

In order to accommodate aggregated MAC frames, the maximum frame length accepted by the PHY is

increased from 4095 in previous standards to 65535 in 802.11n.

MAC processing

P1 P2 P3

P1 P2 P3

MACheader

P1 P2 P3

P1 P2 P3

MACheader

P1 P2 P3

MAC processing

MSDU (MAC Service Data Unit)

MPDU (MAC Protocol Data Unit)MAC

headerMAC

header

Aggregated MSDU format (A-MSDU) Aggregated MPDU format (A-MPDU)

Applications

PHY layer

MAC processingMAC processing

P1 P2 P3

P1 P2 P3

MACheader

P1 P2 P3

P1 P2 P3

MACheader

P1 P2 P3

MAC processing

MSDU (MAC Service Data Unit)

MPDU (MAC Protocol Data Unit)MAC

headerMAC

header

Aggregated MSDU format (A-MSDU) Aggregated MPDU format (A-MPDU)

Applications

PHY layer

MAC processing


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MAC layer enhancements: Multiple Traffic ID Block Acknowledgement (MTBA)

Earlier 802.11 standards demanded an ack frame for every unicast data frame transmitted. The new block

ack feature allows a single ack frame to cover a range of received frames. This is particularly useful for

streaming video and other high-speed transmissions, but when a frame is corrupted or lost, there will be a

delay before a non-acknowledge is received and re-transmission can be accomplished: this is not often a

problem with broadcast video, where re-transmission is often not feasible, given the time constraints of the

media, but may be problematic for other real-time applications.

The format of the Block Ack is a bit-map to acknowledge each outstanding frame: it is based on a

mechanism originally defined in 802.11e. The bit-map identifies specific frames not received, allowing

selective retransmission of only those required.

MAC layer enhancements: Reduced inter-frame spacing (RIFS)

When a station (client or AP) has a number of frames to send sequentially, it must pause between frames,

seizing the medium before each transmission. This time on the air lost due to contention constitutes

overhead for the overall network. Prior to 802.11n, the pause between frames transmitted by the same

station was set at SIFS (single inter-frame spacing). 802.11n defines a smaller inter-frame spacing, RIFS

(reduced inter-frame spacing). RIFS cannot be used between frames transmitted by different stations, and it

can only be used when the station is transmitting in 802.11n HT (greenfield) mode, so all the other clients of

the access point are designed for RIFS operation. It accomplishes similar goals to the MAC aggregation

functions explained earlier, but arguably with less implementation complexity. 802.11n defines a RIFS

interval as 2 usec, whereas SIFS is 16 usec.

MAC layer enhancements: Spatial multiplexing power save (SM power save)

The basic 802.11n power save mode is based on the earlier 802.11 power save function. In this mode, the

client notifies the AP of its power-save status (intention to sleep), then shuts down, only waking for DTIMs

(Delivery Traffic Indication Maps) broadcast by the AP, while the AP buffers downlink traffic for sleeping

stations between DTIMs.


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Power save in 802.11n is enhanced for MIMO operation with SM power save mode. Since MIMO requires

maintaining several receiver chains powered-up, standby power draw for MIMO devices may be

considerably higher than for earlier 802.11equipment.

To mitigate this, a new provision in 802.11n allows a MIMO client to power-down all but one RF chain

when in power save mode. When a client is in the dynamic SM power-save state, the AP sends a wake-up

frame, usually an RTS/CTS exchange, to give it time to activate the other antennas and RF chains. In static

mode, the client decides when to activate its full RF chains, regardless of traffic status.

MAC layer enhancements: Power Save Multi-poll (PSMP)

PSMP is a new application of the existing APSD mechanism: in IEEE 802.11n it has the same extensions,

scheduled- and unscheduled-PSMP (viz. S-APSD, U-APSD), even though scheduled-PSMP may never be

implemented in a Wi-Fi Alliance certification.

Unscheduled PSMP is the simpler mode: it is very similar to U-APSD, supporting both trigger-enabled and

delivery-enabled options. Each sleep interval is considered and signaled independently, with the client

determining when to wake to receive or transmit data. In the diagram above, the sleep frame informs the

AP that the client will stop receiving frames until further notice. When the client wishes to communicate, it

sends a regular or trigger frame to the AP, and both parties then transmit whatever data is queued. At the end

of this exchange, the client can indicate its return to sleep mode.


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Scheduled PSMP is very similar to the S-APSD function introduced in 802.11e. The client requests a

reservation for a T-Spec (traffic specification) from the AP, giving details of data rate, frame size, frame

interval and access class (QoS priority) of the traffic streams it wishes to send and receive. The AP, once it

has admitted this T-Spec, defines and a polling schedule for the client. Since there may be several clients

using S-PSMP, the AP defines global PSMP SP (service period) for S-PSMP traffic, informing other stations

they cannot transmit during these intervals. Once a PSMP SP is declared, the AP first transmits data in the

downlink direction to all applicable S-PSMP clients during the DTT (downlink transmission time), then

accepts traffic from clients during the UTT (uplink transmission time).

S-PSMP is a very efficient way to transmit streaming or periodic traffic over 802.11n: there is no contention

for the medium, as everything depends on a published schedule. However, it is likely that S-PSMP will not

be incorporated in Wi-Fi Alliance certifications or in products for some time.

Compatibility Modes and Legacy Support in 802.11n

One of the most difficult aspects of 802.11n is operation in the presence of earlier 802.11 technologies.

Because it operates in the same bands as legacy 802.11, and is developed by the same standards bodies, and

because there are already more than 100 million Wi-Fi devices in use world-wide, 802.11n is designed tosupport earlier forms of Wi-Fi. This includes:

Support for legacy clients. 802.11a/b/g clients can connect to 802.11n APs. They will not be able to

use 802.11n features, and their performance will be only marginally improved when connecting to an

802.11n AP; and

Awareness of neighboring or overlapping 802.11a/b/g networks. This is particularly important when

using the new 40 MHz channel capability, which would impair the performance of such networks.


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As explained elsewhere in this note, working with legacy 802.11 clients and networks degrades the

performance of 802.11n considerably, 802.11a/b/g clients will see very comparable performance whether

they are using an 802.11a/b/g or 802.11n access point. In addition, working with legacy clients poisons the

802.11n cell: its capacity will be severely degraded as soon as even one legacy client is present. This does

not negate the need for legacy operation, but it does increase the urgency of upgrading the client population

to 802.11n. The diagram below shows how introducing an 802.11a client into an 802.11n cell reduces thethroughput (based on data rates alone: the overhead introduced by co-existence mechanisms will further

reduce throughput).


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Greenfield, High-throughput and non-HT modes

802.11n defines three modes of client compatibility; the mode chosen depends on the extent of legacy

802.11a/b/g client support:

High Throughput (HT). In HT or Greenfield mode, the AP does not expect to connect to any

legacy 802.11 clients, and indeed, assumes that there are none operating in the area. Apart from the

beacon and some control frames at 20 MHz, no indication is available that will allow older devices

to understand the remaining part of the transmission: it is all in HT-format.

Non-HT format. This is essentially legacy mode. The frames are all in 802.11a/g format (PHY and

MAC), so they can be understood and decoded by 802.11a/b/g clients. This mode gives essentially

no performance adva

wp designed speed 802.11n

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