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802.11ac: Overcoming Test Challenges

National Instruments

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Why 802.11ac?

- Integration of WLAN into more products - Smartphones, Digital cameras, media players, gaming consoles

- Download data to mobile phones faster

- HD Video on multiple devices (>20 Mbps)

- Data sharing between multiple phones / tablets and PCs (>100 Mbps)

- Manufacturing floor automation (>1Gbps)

- Uncompressed video (3Gbps)

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1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

IEEE 802.11 Data Rate: 2

Mbps Use Case: Internet

IEEE 802.11b Data Rate: 11

Mbps Use Case: E-mail

IEEE 802.11a – Wi-Fi starts to become

ubiquitous. Data Rate: 54 Mbps Use Case: Rich-data

Web experience

IEEE 802.11n Data Rate : Up to 600 Mbps. Most

common is 150 Mbps. Enhanced range due to use of

MIMO Use Case: Medium-resolution

video streaming

IEEE 802.11ac Up to 6.93 Gbps.

First solution is < 1.8 Gbps. Use Case: Whole-home

coverage for video-consumption age

802.11 Evolution

IEEE 802.11g Data Rate: 54 Mbps

Use Case: Web experience

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802.11 a/b/g/n and ac comparison

802.11a/g

802.11n 802.11ac

Antenna Configuration 1x1 SISO 4x4 MIMO 8x8 MIMO

Highest Order Modulation

BPSK to 64-QAM BPSK to 64-QAM BPSK to 256-QAM

Channel Bandwidth 20 MHz 20 MHz and 40 MHz

mode 20, 40, 80, 80 + 80, and

160 MHz

FFT Size 64 64 (20 MHz), 128 (40

MHz)

64, 128, 256 512 (optional)

Year Introduced 1999 (802.11a) 2003 (802.11g)

2009 (draft) 2011 (draft)

Maximum Throughput 54 Mbit/s 600 Mbit/s 6.93 Gbit/s

Note: 1.56 Gbps (80 MHz, 4 Tx, MCS9) is average case 6.93 Gbps (160 MHz, 8x8, MCS9, short GI) best case

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802.11ac Key Requirements

- Backward compatible with 802.11a and 802.11n in the 5 GHz band

- Coexistence with 11a and 11n

- Higher data through put for wireless display, in home distribution of HDTV, manufacturing floor automation etc.

- Wider channel bw - 80 + 80 MHz and 160 MHz

- Optional 256 QAM

- Up to 8 spatial streams

- Multi-user MIMO (MU-MIMO)

- 400 ns short interval guard

- Space time block coding (STBC)

- Low density parity check (LDPC)

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802.11ac Details

• Operates only in 5-6 GHz band

• Mandatory 20,40 and 80 MHz channels

• Optional support for contiguous 160 MHz an non contiguous 80+80 MHz tx and rx

5170 – 5330 MHz

20 MHz

40 MHz

80 MHz

160 MHz

5490 – 5730 MHz 5735 – 5835 MHz

Non Contiguous

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802.11ac VHT PPDU

802.11n PPDU (Mixed Mode)

Frame Format (Compatibility with 802.11n)

- L-STF, L-LTF and L-SIG - Same fields as 11a/b/g/n. Transmitted for backwards compatibility

- VHT-SIG-A - Info required to interpret VHT packets (BW, number of streams, MCS, beamforming etc)

- VHT Short Training Fields (VHT-STF) - Improve automatic gain control estimation in MIMO transmission

- VHT Long Training Fields (VHT-LTF) - Mapping matrix for up to 4 VHT-LTFs (for 11n) and up to 8 VHT-LTFs(for 11ac)

- VHT-SIG-B - Length of data and MCS for multi user mode

L-STF L-LTF L-SIG HT-SIG HT-STF HT-LTFs HT Data

L-STF L-LTF L-SIG VHT-SIG-A VHT-STF VHT-LTFs VHT-SIG-B VHT-Data

2 Symbols 2 Symbols 1 Symbol BPSK

2 Symbol QBPSK

1 Symbol 1 Symbol / LTF 4 LTFs max

2 Symbols 2 Symbols 1 Symbol BPSK

1 sym BPSK 1 sym QBPSK

1 Symbol 1 Symbol / LTF 8 LTFs max

1 Symbol

Note: 1 Symbol = 4 us

Copyright

From Figure 22-19, IEEE P802.11ac/D2.0

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Multiple-user (MU) MIMO

• Multi-user (MU)-MIMO as extended SDMA concept

TX0

TX1

TX2

TX3

RX0

Rx1

RX2

RX3

RX0 TX0

TX1

TX0 RX0

Rx1

Spatial Expansion (TX Diversity)

Spatial Expansion (RX Diversity)

RX0 TX0

TX1

Space – Time block Coding (STBC)

TX0

TX1

TX0

TX1

RX0

Rx1

RX0

Rx1

MIMO 2X2

MIMO 4X2

MU-MIMO New to 802.11ac

- Downlink only - Total 8 streams max - Up to 4 users - Increase system efficiency

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Data Rate Calculation for 802.11ac E.g. 80 MHz bandwidth, 64 QAM, 800ns GI, Spatial Streams = 1

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Wider Channel BW for 802.11ac and Data Rates Calculation

Channel Bandwidth (MHz)

Subcarrier Spacing (kHz)

Total Subcarriers (IFFT Size)

Data Subcarriers Pilot Subcarriers

20 312.5 64 52 4

40 312.5 128 108 6

80 312.5 256 234 8

80 + 80 312.5 256 / ch 234 / ch 8 /ch

160 312.5 512 468 16

Channel BW Spatial Streams

Modulation Scheme

Code Rate Total Subcarriers

Data Subcarriers

NDPSC Max. Data Rate

20 MHz 1 256 QAM 3/4 64 52 312 86.7 Mbps

40 MHz 2 256 QAM 5/6 128 108 1440 400 Mbps

80 MHz 4 256 QAM 5/6 256 234 6240 1.733 Gbps

160 MHz 8 256 QAM 5/6 512 468 24960 6.933 Gbps

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Transmitter Block Diagram (Single User) P

HY

P

add

ing

Scr

amb

ler

En

cod

er

Par

ser

FC

C e

nco

de

r F

CC

en

co

de

r

Str

eam

Par

se

r

BCC Interleaver

BCC Interleaver

BCC Interleaver

Constellation Mapper

Constellation Mapper

Constellation Mapper

LDPC Tone Mapper

LDPC Tone Mapper

LDPC Tone Mapper

Sp

ace

tim

e b

lock

co

din

g (S

TB

C)

CSD

CSD

Sp

atia

l Map

pin

g

IDFT

IDFT

IDFT

Insert GI and

Window

Insert GI and

Window

Insert GI and

Window

Analog and RF

Analog and RF

Analog and RF

……

.

……

.

……

.

……

.

……

.

……

.

……

.

1 to 8 inputs BCC or LDPC used

1 to 8 inputs

From Figure 22-6, IEEE P802.11ac/D1.4

Making EVM and Spectral Mask Measurements on 802.11ac

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Test Challenges for 802.11ac

• 256 QAM requires better EVM

• Wider bandwidths are required • 80 MHz mandatory (calibration and

equalization algorithms are important)

• MIMO capabilities • Up to 4X4 • 8X8 possible

Modulation Coding Rate RMS EVM

BPSK 1/2 -5 dB

QPSK 1/2 -10 dB

QPSK 3/4 -13 dB

16 QAM 1/2 -16 dB

16 QAM 3/4 -19 dB

64 QAM 2/3 -22 dB

64 QAM 3/4 -25 dB

64 QAM 5/6 -27 dB

256 QAM 3/4 -30 dB

256 QAM 5/6 -32 dB

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Challenges with Higher Modulation Schemes

- Better EVM performance is required (>32 dB)

- Linearity and phase noise of analyzer / generator

- IQ modulator error - Phase noise - Error in LO - Non linearity

- NI WLAN Generation toolkit

- IQ Gain Imbalance - Quadrature Skew - I / Q DC Offset - Timing Skew - Carrier to Noise Ratio

-

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Error Vector Magnitude (EVM) for 802.11ac

- Test instrument specification needs to be at least -42 dB (10 dB better than spec)

- Achieved through

- Group delay compensation

- Flatness equalization across entire bandwidth

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Other EVM Measurements

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Transmit Spectrum Mask

- 160/80 MHz mask is extension of 40 MHz mask

- Measured with 100 kHz rbw

- For 80 + 80 mask is linear sum of the separate 80 MHz masks for values from -20 dBr to -40 dBr

Spectrum Mask for 20,40,80 and 160 MHz

Channel Size

A (MHz)

B (MHz)

C (MHz)

D (MHz)

20 MHz 9 11 20 30

40 MHz 19 21 40 60

80 MHz 39 41 80 120

160 MHz

79 81 160 240

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80 + 80 MHz Non-contiguous Spectrum Mask Values

-25 dBr

39 41 80 119 121 160 200 -39 -41 -80 -119 -121 -160 -200 In MHz

80+80 MHz • Non Contiguous • Cf separated by 160 MHz

Frequency overlap – Both masks have values between -20 and -40 dBr

• Neither between 0 and -20 dBr

From Figure 22-24, IEEE P802.11ac/D2.0

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Spectrum Mask Using WLAN Toolkit

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NI WLAN Toolkit

- Soft Front Panels for Acquisition and Generation including MIMO

- API and Examples for LabVIEW, C, .NET and TestStand

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Methods for MIMO

- PXI platform allows for tightly synchronized analysers and generators within 0.1˚ phase offset

- WLAN toolkits / API supports MIMO capability

- Up to 8x8 - Sharing of single LO for multiple VSAs + VSG

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NI PXI WLAN Test Solution

• NI PXI Test Solution • 80 MHz BW

• EVM <-45 dB

• Up to 8x8 MIMO capability with PXI synchronization

• Toolkits

o Soft front panels

o API for LabVIEW, C and .NET

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Quick look at 802.11ad (WiGig)

- Throughput up to 6.7 Gbps

- Frequency 57 – 66 GHz

- >2 GHz BW

- > 10 x 10 MIMO

- < 5m coverage (in room)

- Uncompressed HD Video key application

- Applications - Wireless sync

- Wireless display

- HD streams

- Cordless Computing

- Internet Access

Questions?

Resources ni.com/80211ac

ni.com/rf