Wireless LAN & 802.11ac Wi-Fi Fundamentals

802.11ac Wi-Fi Fundamentals
Eric Johnson
March 2014

CONFIDENTIAL
© Copyright 2014. Aruba Networks, Inc.
All rights reserved
2 #AirheadsConf
Agenda
11ac Standards Physical Layer Overview
11ac Data Rates
Radio Realities
Transmitters
Receivers
Antennas
11ac Beamforming
11ac Products

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802.11ac Technology
Overview
Think of 11ac as an
extension of 11n
•  11n specification
introduced/leveraged:
•  2.4 and 5 GHz supported
•  Wider channels (40 MHz)
•  Better modulation (64-
QAM)
•  Additional streams (up to 4
streams)
•  Beam forming (explicit and
implicit)
•  Backwards compatibility
with 11a/b/g
11ac
introduces

•  5
GHz
supported

•  Even
wider
channels
(80
MHz

and
160
MHz)

•  Be?er
modulaAon
(256-‐QAM)

•  AddiAonal
streams
(up
to
8)

•  Beam
forming
(explicit)

•  Backwards
compaAbility
with

11a/b/g/n

•  Refer
to
h?p://www.
802-‐11.ac.net
for
in-‐depth

informaAon

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Wider Channels
•  80 MHz channel widths supported in first
generation
– 80 MHz is 4.5x faster than 20 MHz
– 80 MHz is contiguous
– Per packet dynamic channel width decisions
•  Future releases will allow for 160 MHz
channel widths
– 160 MHz can be either contiguous or in two non-
contiguous 80 MHz slices

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802.11ac Channels (FCC)
Channel
Freq (MHz)
UNII I and UNII II
2x 80 MHz
4x 40 MHz
8x 20 MHz
Band
Edge
Channel
Freq (MHz) 5850
US UNII III
1x 80 MHz
2x 40 MHz
5x 20 MHz
Channel
Freq (MHz)
UNII II extended
3x 80 MHz
6x 40 MHz
12x 20 MHz
36 4844 5240 56 6460 Band
Edge
5180 5200 5220 5240 5260 5280 5300 5320 5350
Band
Edge
5150
149 161157153
5745 5765 5785 5805
Band
Edge
5725
165
5825
100 112108 116104 120 128124
5500 5520 5540 5560 5580 5600 5620 5640
Band
Edge
5470
136 140 Band
Edge
5680 5700 5725
132
5660
144
5720
Weather
Radar

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802.11ac Channels (ETSI)
Channel
Freq (MHz)
UNII I and UNII II
2x 80 MHz
4x 40 MHz
8x 20 MHz
Channel
Freq (MHz)
UNII II extended
2x 80 MHz
5x 40 MHz
11x 20 MHz
36 4844 5240 56 6460 Band
Edge
5180 5200 5220 5240 5260 5280 5300 5320 5350
Band
Edge
5150
100 112108 116104 120 128124
5500 5520 5540 5560 5580 5600 5620 5640
Band
Edge
5470
136 140 Band
Edge
5680 5700 5725
132
5660

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Understanding 11ac Data Rates

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Terminology
•  Symbol: basic element containing 1 to 8 bits of
information
•  Tone/Sub-Carriers: OFDM is made up of many tones. Each
symbol is mapped to a tone.
•  Cyclic Extension: technique used in OFDM to protect
against multipath interference
–  You need cyclic extension but it is dead air and consumes transmit time
•  Guard Band: Space between channels. In these regions
tones have a constant value of zero amplitude
•  Pilot Tones: Used to train the receiver and estimate the
channel
•  Radio Channel: For Wi-Fi 20, 40, 80, or 160 MHz of
spectrum
•  Propagation Channel: everything that happens between
the transmitter and receiver
•  FEC: Forward Error Correction. Redundant information
that is sent to assist the receiver in decoding the bits.

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Sub-carriers
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
114 subcarriers (108 usable) for a 40 MHz HT mode (802.11n) channel
fc +10MHz-20MHz
+20MHz-10MHz
242 subcarriers (234 usable) for a 80 MHz VHT mode (802.11ac) channel
An 80+80MHz or 16MHz channel is exactly two 80MHz channels, for 484 subcarriers (468 usable)
fc +10MHz-20MHz +20MHz-10MHz-40MHz -30MHz +30MHz +40MHz
OFDM subcarriers used in 802.11a, 802.11n and 802.11ac
+10MHz-10MHz
Guard Tones

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QAM constellations
Amplitude +1
Amplitude -1
Quadrature-1
Quadrature+1
Amplitude +1
Amplitude -1Quadrature-1
Quadrature+1
Amplitude +1
Amplitude -1
Quadrature-1
Quadrature+1
16-QAM constellation 64-QAM constellation 256-QAM constellation
Constellation diagrams for 16-, 64-, 256-QAM

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How do I get to the data rate for
a given MCS?
•  Basic Symbol Rate
–  312.5 KHz
–  3.2 µs
•  Cyclic Extension
–  t/4 0.8 µs
–  t/8 0.4 µs
•  Bits Per Tone
–  BPSK 1
–  QPSK 2
–  16 QAM 4
–  64 QAM 6
–  256 QAM 8
11

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Raw Data Rates
•  #Tones * Bits per Tone * Symbol Rate
–  16 QAM, 20 MHz
–  52 * 4 * 0.3125 = 65 Mbps
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Correct for Cyclic Extension
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Apply FEC Coding
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Transmitters

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Transmitter Line Up
16
DAC
Symbol
Generation
Up
Convert PA

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Transmitter Terms
•  Conducted Power
–  This is the power that leaves the connectors
•  EIRP: Effective Isotropic Radiated Power
–  This is the conducted power (dBm) + antenna gain (dBi) in
the direction of interest – cable losses (dB)
•  Peak EIRP
–  This is what is regulated
–  It is the conducted power + peak gain – cable losses
•  dBm: log power ratio to milliwatt
•  dBi: antenna gain relative to isotropic
•  dBr: relative power eg:used with describing
transmit mask
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802.11 Symbol Stream
18
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64
15−
11.25−
7.5−
3.75−
0
3.75
7.5
11.25
15
Time (symbols)
LinearAmplitude

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802.11n Signal Frequency
Domain
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0 5 10 15 20 25 30 35 40
60−
50−
40−
30−
20−
10−
0
Frequency (MHz)
Amplitude(dB)
Digital Domain
After DAC
PA Non Linearity
0 5 10 15 20 25 30 35 40
60−
50−
40−
30−
20−
10−
0
a

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Transmitter Non-Idealities
•  DAC Quantization: this is due to the limited number of bits
in a practical Digital to Analog Converter
–  This noise source is not affected when the power is reduced
•  PA Non Linearity: OFDM has a high Peak to Average Ratio.
The peaks in the OFDM signal cause distortions which
manifest as noise like shoulders
–  Known as spectral regrowth
–  For every one 1 dB drop in tx power the regrowth drops by 3 dB
•  2 dB net
•  The in channel noise is referred to as EVM
–  Error Vector Magnitude
•  The out of channel noise interferes with other Wi-Fi
channels and determines how close we can space
antennas
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EVM
•  As the depth of modulation increase the
number of bits per symbol increases
•  The in-band noise introduces uncertainty wrt
to the actual symbol position
•  Higher order modulations decrease the
space between code points
•  To make higher order modulations work the
tx power needs to be reduced
•  The EVM noise will add with interference and
background noise
21
16 QAM

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BPSK 1/2 -‐5 -‐5
QPSK 1/2 -‐10 -‐10
QPSK 3/4 -‐13 -‐13
16QAM 1/2 -‐16 -‐16
16QAM 3/4 -‐19 -‐19
64QAM 2/3 -‐22 -‐22
64QAM 3/4 -‐25 -‐25
64QAM 5/6 -‐28 -‐27
256QAM 3/4 N/A -‐30
256QAM 5/6 N/A -‐32
802.11n

EVM
(dB)
802.11ac

EVM
(dB)
Modulation Coding
Rate
EVM Specfication and 22x tx
table
22

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Receivers

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Receiver Line Up
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ADC
Symbol
Decode
Down
Convert
LNA

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Receiver Impairments
•  Analog Compression
–  Modern LNAs have very effective input power tolerance
•  Digital Compression
–  This is where a high power signal hits the Automatic Gain
Control (AGC) Circuit. Gain drops and receiver sensitivity
degrades
–  The radio can be totally blocked if the power hits the Analog
to Digital Converter (ADC) and consumes all the bits
•  Intermodulation
–  Again, the effective linearity of modern LNAs reduces the
impact of this
25

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DAS Interference: Example
•  Without filtering any signal that hits the receiver
above -45 dBm will cause a reduction of
sensitivity
•  The degradation continues until about -15 dBm
at which point the signal is totally blocked
•  With a 100 mW (20 dBm) DAS system at 2100
MHz
–  Tx 20 dBm
–  Effective rx antenna gain 3 dBi
–  1st meter at 2100 MHz -39 dB
•  Power at 1m -19 dBm
–  No impact distance 40 meters
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Advanced Cellular Coexistence
•  Proliferation of DAS and new LTE bands at 2.6
GHz are creating issue for Wi-Fi solution
•  All new APs introduced by Aruba in the last 12
months and going forward have implemented
significant filtering into the 2.4 GHz radio portion
to combat this
•  Design solution
–  Use high-linear LNA followed with a high-rejection filter to achieve
rejection target and little sensitivity degradation;
–  Design target: Minimal Sensitivity degradation with -10dBm interference
from 3G/4G networks (theoretical analysis).

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Coverage Example
1.  Sample coverage for 3x3 11n AP (or 3x3 11ac AP with
11n clients) in HT40 mode
• Coverage area sustains MCS5 and up
360

405

450

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Coverage Example
2.  Upgrade to 3x3 11ac AP with 11ac clients, still using
40Mhz channels (VHT40)
• Radius for 600Mbps (MCS9) area is 1/4 of that for 450Mbps (MCS7)
360

405

450

540

600

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Coverage Example
3.  Equivalent range for clients using 80MHz channels
(VHT80)
– Rates roughly double, relative range for each of the MCS rates does not change, but
80MHz range is ~70% of equivalent (same MCS) 40MHz range
780

878

975

1170

1300

585

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Relative Range 802.11ac Rates
Datarate

40MHz
80MHz

MCS0
45
97.5

MCS1
90
195

MCS2
135
292.5

MCS3
180
390

MCS4
270
585

MCS5
360
780

MCS6
405
877.5

MCS7
450
975

MCS8
540
1,170

MCS9
600
1,300

Signal
level
and
rela@ve
range

-‐dB
r

MCS0
87
63

MCS1
85
50

MCS2
83
40

MCS3
79
25

MCS4
76
18

MCS5
71
10

MCS6
66
5.6

MCS7
63
4.0

MCS8
58
2.2

MCS9
51
1.0

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Pros and Cons of 802.11ac
•  Pros
1.  APs can accommodate more users/devices
•  Increased capacity
2.  Standards based Explicit Beam-forming increases SNR
•  Higher data rates over longer distances
3.  256-QAM
•  Increased throughput at high SNRs
•  Improved modulation and coding techniques
4.  Multi-User MIMO (future generations)
•  Improved utilization of RF capacity
5.  Use of 5 GHz spectrum
•  More non-overlapping channels
•  Quieter RF environment

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Pros and Cons of 802.11ac
•  Caveats
1.  Hardware update required to support 802.11ac
•  Some features will not be available on legacy devices
2.  Increased product cost
•  Small premium for 3x performance
•  Prices will come down
3.  Supporting 802.11ac will result in increased load on the
infrastructure
4.  AP-225 requires 802.3at (PoE+) for full functionality &
performance
•  However, no restrictions on 11ac radio with 802.3af POE
•  USB disabled, second Ethernet port disabled, 2.4GHz radio in
1x3:1SS mode

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Wave 2 of 11ac
•  What will wave 2 802.11ac deliver?
•  MU-MIMO
•  Use AP MIMO resources more effectively
•  Transmit data to multiple devices simultaneously: for example 4SS AP streaming
data to four 1SS clients simultaneously
•  4x4:4SS
•  Benefit of additional stream mostly for MU-MIMO
•  Not anticipating any 4x4:4SS client devices
•  Adds 33% to max datarate
•  VHT160
•  Doubles max datarate
•  Practical problem: only 2 VHT160 channels available in entire 5GHz band
•  Max 5GHz radio throughput triples again!
•  450 (11n 3x3 HT40), 1,300 (11ac 3x3 VHT80), 3,467 (11ac 4x4 VHT160)
•  When will it be available?
•  Radio chipsets available late 2014
•  Products in 2015

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Reasons not to wait for Wave 2
•  Unlikely to see any 4x4:4SS client devices
•  Use of VHT160 not practical for typical enterprise
deployment
•  MU-MIMO is a nice-to-have optimization.
•  How well it will work and what the real benefits are is still not entirely
clear
•  Requires new client devices (Wave 1 clients also not FW
upgradeable)
•  Wave 1 is here now (technology, products, market
momentum), offering huge advantages over 11n. Wave 2 is
the expected next step in the evolution of the technology.
•  In general: the next wave in technology is always around
the corner, something better is always coming Once Wave
2 is available, we’ll for sure be talking about Wave 3.
•  No different from when 11n 2x2 products were introduced and it was
clear that 3x3 products would be available within 18 months.

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11ad and what it means
•  60GHz band, three channels in most countries (each
2.16GHz wide), each providing up to 6.8Gbps PHY datarate
•  No MIMO
•  Challenges: Non-Line of Sight (NLOS) connections, range,
penetrating obstacles (and people)
•  Targeted to clean up a cluttered desk or TV cabinet
•  Likely not appropriate for traditional AP use. But can be
interesting for related applications like wireless docking,
high-capacity WLAN hotspots, AP backhaul/aggregation,
etc.
•  It is being investigated (but no product plans as of yet)
•  Standard is available, certification program in place
•  Wi-Fi Alliance WiGig Alliance

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Antennas

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Antenna Basic Physics
•  When the charges oscillate the
waves go up and down with the
charges and radiate away
•  With a single element the energy
leaves uniformly.
•  Also known as omni-directionally
38

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Building Arrays: 2 Elements
•  By introducing additional antenna elements we
can control the way that the energy radiates
•  2 elements excited in phase
39
λ/2
0
30
60
90
120
150
180
210
240
270
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330
Linear Plot
0
15
30
45
60
75
90
105
120
135
150
165
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270
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300
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345
dB Plot

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0
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•  By introducing additional antenna elements we
can control the way that the energy radiates
–  Equal amplitude
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Linear Plot
dB Plot
0
30
60
90
120
150
180
210
240
270
300
330

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0
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•  By shaping the amplitude we can control
sidelobes
–  Amplitude 1, 3, 3, 1
41
Linear Plot
dB Plot

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0
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Phase
•  By altering phase we can alter the direction that the energy
travels
•  4 elements excited with phase slope
–  Even amplitude
42
Linear Plot
dB Plot

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Beamforming: Notes
•  AP 22x series has 11ac beamforming support in 2.4 and 5 GHz
bands
•  Works with clients that support 11ac beamforming function
–  This is at a minimum all 11ac client devices using Broadcom chipsets
–  Support will have to come to all devices to compete with Broadcom offering
•  11ac beamforming is standards based
–  first standard that is doing this the “right” way
–  11ac beamforming represents the consensus view of the 1000’s of contributors
to the standards process
•  11ac beamforming is implemented in baseband.
–  It works with all antenna subsystems
–  The total number of beamforming combinations is effectively infinite
•  11ac actively tracks users so has a recent channel estimate
between the AP and client that is updated frequently
46

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Channel state information, implicit
and explicit beamforming estimation
47
Explicit feedback for beamforming (802.11n and 802.11ac)
1 (Beamformer) Here’s a sounding frame
2 (Beamformee) Here’s how I heard the sounding frame
3 Now I will pre-code to match how you heard me
sounding frames
Beamformed frames
feedback from sounding
Explicit feedback for beamforming
Beamformer Beamformee
Actual
CSI

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Line of Sight
•  3 stream AP
•  Smartphone
–  1 Antenna/1 Stream
Client
AP
0
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8090100
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260 270 280
290
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Simple Reflection
•  Let’s introduce two
reflection surfaces
and look at the
impact of one bounce
on each side
Client
AP
0
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20
30
40
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8090100
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Virtual
Antenna Pattern

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Multi Stream Client
•  The reflections allow
beamforming to send
different streams
with different
antenna pattern
through the system
Client
AP
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8090100
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Stream1
Stream2Stream3

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0
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11ac Beamforming across an
80 MHz channel
•  The standards based algorithm actually works out patterns
for each sub carrier
•  Below is the pattern for stream 1 at 5460, 5500, 5540 MHz

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AP-224/225 802.11ac 3x3 AP
•  Enterprise class 3x3 802.11ac
•  Aggregate TCP platform throughput performance >1Gbps
•  Two platform models:
–  AP-224: external antennas (3x, dual band)
–  AP-225: integrated antennas
–  “Advanced Cellular Coexistence” support
•  Dual radio:
–  802.11n 3x3:3 HT40 2.4GHz(450Mbps), support for “TurboQAM”
–  802.11ac 3x3:3 HT80 5GHz (1.3Gbps)
–  11ac beamforming supported in both bands
•  Wired interfaces
–  Network: 2x 10/100/1000Base-T Ethernet, with MACSec support
–  USB 2.0 host interface, console port, DC power
•  Will require 802.3at PoE (or DC power) for full functional operation
–  Functional, but capabilities reduced when powered from 802.3af POE
•  Enterprise temperature range, plenum rated, TPM
$1,295
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