CHALLENGES IN DESIGNING 5 GHZ 802.11AC WIFI POWER AMPLIFIERS

Yazhou Wang, Randy NaylorRadio & Wireless Week January 2014

Acknowledgement

RFMD Wireless Connectivity Business Unit Team (Boston,MA & Greensboro, NC)

Wireless Connectivity Business Unit 3

OUTLINE

RFMD 5 GHz 802.11ac PA Performance

WiFi Background

5 GHz 802.11ac PA Design Challenges

Summary

Evolution of IEEE 802.11 Standards WHAT IS 802.11AC?

Standard Year Released

Technology Details Frequency Bandwidth Highest data

rate

802.11 legacy 1997 DSSS 2.4 GHz 20 MHz 2 Mb/s

802.11b 1999 CCK 2.4 GHz 20 MHz 11 Mb/s

802.11a 1999 OFDM 5 GHz 20 MHz 54 Mb/s

802.11g 2003 OFDM 2.4 GHz 20 MHz 54 Mb/s

802.11n 2009 OFDM (64-QAM), MIMO

2.4 and 5 GHz 20 and 40 MHz 1×1: 150 Mb/s

4×4: 600 Mb/s

802.11ac 2012 OFDM (256-QAM), MIMO, MU_MIMO

5 GHz only 20, 40, and 80 MHz, 160 MHz optional

1×1: 866.7 Mb/s 8×8: 6.77 Gb/s (160 MHz BW)

IEEE 802.11: Wireless LAN MAC and PHY Specification, IEEE Standards Association, March 2012

Drivers for Higher WiFi Data Rates

Distribution of high-definition (HD) video through the office or home

Wireless display of HD images and video

Rapid file upload/download (sync devices)

WHY DO WE NEED 802.11AC?

802.11ac: The fifth generation of Wi-Fi, Technical White Paper, Cisco, 2012 802.11ac In-Depth, Aruba White Paper, 2013

Video Type Description Date Rate (Mbps)

Compressed Blu-rayTM 40

Lightly Comp. Motion JPEG2000 150

Uncompressed

HD 720p 1280×720p, 24 bits/pixel, 60 frame/sec 1300

HD 1080p (RGB) 1920×1080p, 24 bits/pixel, 60 frame/sec 3000

Example Data Rate:

Linearity Requirement on 802.11ac Transmitter/PA

256-QAM (802.11ac) requires lower error vector magnitude (EVM)than 64-QAM (802.11n) due to higher constellation density

Transmitter EVM spec: 2.5% (802.11ac) vs. 4% (802.11n)

Usually PA EVM spec: 1.8% (802.11ac) vs. 3% (802.11n)

802.11AC VS. 802.11N

IEEE 802.11 WLAN PHY Layer Operation and Measurement, Agilent Application Note EVM and related quantities

EVM Sources: 1. Additive Noise2. Nonlinear Distortion3. Linear Distortion, e.g., freq. response4. Phase Noise5. Spurious Signals6. Other Modulation Errors, e.g.,

quantization errors, offsets

Wireless Connectivity Business Unit 7

5 GHz 802.11ac PA Design Challenges

OUTLINE

WiFi Background

Summary

RFMD 5 GHz 802.11ac PA Performance

Design Challenge 1: Very Low EVM Requirement

Usually 802.11ac PAs need to achieve better than -35dB or 1.8% EVM Very stringent requirements for PA AM/AM and AM/PM distortion:

0.3dB Gain Imbalance or 2o Phase Imbalance can cause 1.8% EVM

Georgiadis, IEEE Trans on Vehicle Technology, March 2004

EVM 5.6%

1.8%

3.0%

4.0%

PA Gain Imbalance (dB)

PA P

hase

Imba

lanc

e (d

eg)

Design Challenge 1: Very Low EVM Requirement

11n PA meets 17.5 dBm output power @ 3% EVM, but can only meet10dBm output power @ 1.8% EVM (11ac requirement)

11n PA needs re-optimization to meet 11ac EVM at same output power

TYPICAL 802.11AC VS. 802.11N PA PERFORMANCE

802.11n PA: 20 MHz 11n waveform 802.11ac PA: 80 MHz 11ac waveform 802.11ac PA: 20 MHz 11n waveform

3 curves represent 3 frequencies (low, mid, high channel)

Design Challenge 2: Dynamic Operation and Transient Behavior

WiFi networks utilize Time Division Duplexing (TDD) – PA is pulsed onand off during usage (dynamic operation)

Good RF design provides for static mode performance, dynamicoperation also needs careful design of PA transient/thermal behavior

STATIC VS. DYNAMIC MODE

Static Mode: 100% Duty Cycle Dynamic Mode: 50% Duty Cycle

Static Mode: 100% Duty Cycle Dynamic Mode: 50% Duty Cycle

Baseline design Improved for dynamic mode

Design Challenge 2: Dynamic Operation and Transient Behavior Data payload of OFDM signal is compared against preamble of the

burst. Once PA is on amplitude must be flat during entire transmission

Any rise or droop contributes to AM/AM distortion and degrades EVM

1 symbol = 4 μs

Introduction to 802.11ac WLAN Technology and Testing, Agilent, 2012

overshoot slow turn on Ideal case

802.11ac Data Format

Usually PA turn-on time spec is <1us

Design Challenge 3: Achieve PAE and Linearity Simultaneously

Simple way to improve linearity (EVM) is to increase Icc; however, notacceptable to customers because of lower PAE

Need to achieve PAE & linearity simultaneously: optimize load,interstage match, bias circuits

Improve linearity (EVM) by increasing Icc Improve EVM by optimizing load, interstage match, and bias circuits

Baseline Improved EVM Design

Baseline Improved EVM Design

Design Challenge 4: Wide Operation Bandwidth RF bandwidth from 5170 to 5835 MHz (~15% fractional BW):

input/output match, EVM, Icc, Gain, Harmonics, Spectrum Mask, …

Balance between on-die matching network and die size, cost, ohmicloss

Wider channel bandwidth of 802.11ac (80/160 MHz): bias circuit musthave sufficient bandwidth to avoid limiting signal

Very flat gain and very little phase distortion along 80/160 MHzchannel to avoid EVM degradation

Source: IEEE Available 802.11ac channels in US

Design Challenge 5: Mass Production

Robust design for millions ofparts, not only one unit in thelab

Consider process variations:wafer parameter, dievariation, packaging, bondingwire, …

Design performance overtemperature (-40C to 85C)

Design performance overpower supply (3.0V to 4.8V)

EVM distribution in Production Test

Wireless Connectivity Business Unit 15

OUTLINE

RFMD 5 GHz 802.11ac PA Performance

WiFi Background

Summary

5 GHz 802.11ac PA Design Challenges

RFFM85xx & RFFM45xx WiFi Product Families 4.9-5.925 GHz 802.11a/n/ac WiFi FEM

2.5x2.5x0.4mm QFN

Integrated 5GHz PA, LNA with bypass, SP2T, Harmonic Filtering & Power Detector

Supports low power mode for improved efficiency

Minimal external SMD count RFFM8505 802.11ac FEM

RFFM85xx Dynamic EVM

Front End Module (FEM) achieves 19dBm power @ 1.8% EVM specin TX Mode, with very low backed-off EVM like “hockey stick”

If de-embedding TX switch loss (~0.8dB), PA gives 19.8dBm linearpower

VCC = 3.6V, 50% DUTY CYCLE, 80 MHZ 802.11AC D

ynam

ic E

VM (%

) Dynam

ic EVM (dB

)

+ 5210 MHz × 5530 MHz o 5775 MHz

-26

-28

-30.5

-34

-40

RFFM85xx Current Consumption

FEM consumes 200 mA Icc at rated power 19dBm: 11% PAE (Best inclass PAE @19dBm in the 5 GHz 802.11ac WiFi FEM market)

+ 5210 MHz × 5530 MHz o 5775 MHz

VCC = 3.6V, 50% DUTY CYCLE, 80 MHZ 802.11AC

PAE Analysis

FEM achieves 11% PAE at 19 dBm output power

If de-embedding Tx switch loss (0.8 dB), PAE goes up to 13.3%

If de-embedding loss of on-die match network and harmonic filter (0.7dB), PAE goes up to 15.6%

If de-embedding current consumption of supporting analog circuits,PAE goes up to 16.8%

If de-embedding two driver stages and only considering the PA stage,PAE goes up to 31.2%

On-die Match Network &

Harmonic Filter Driver1 Driver2 PA

FEM Die

Regulator, Biasing Circuit

VCC = 3.6V, 50% DUTY CYCLE, 80 MHZ 802.11AC

RFFM85xx Thermal Performance

Tmax is 125 C at 20 dBmoutput power (internal RFMDGaAs HBT process)

Even heat dissipation alongpower cells

Symmetrical layout design toeliminate phase imbalance

VCC = 3.6V, 50% DUTY CYCLE, 80 MHZ 802.11AC

+ 5210 MHz × 5530 MHz o 5775 MHz

RFFM85xx Gain

FEM achieves ~29 dB Gain at various 802.11ac channels, by using a3-stage amplifier configuration

Gain very flat up to 19 dBm output power, to avoid EVM degradation

VCC = 3.6V, 50% DUTY CYCLE, 80 MHZ 802.11AC

Wireless Connectivity Business Unit 22

OUTLINE

WiFi Background

Summary

5 GHz 802.11ac PA Design Challenges

RFMD 5 GHz 802.11ac PA Performance

Summary

802.11ac technology enables Giga-bit per second WiFi data transfer rates

EVM (over T,V) and PAE remain key challenges for 5 GHz 802.11ac PA

RFMD RFFM85xx and RFFM45xx WiFi module families achieve best in class overall performance

PA stage achieves 31.2% PAE @ 20.5 dBm output power, while meeting 1.8% 802.11ac EVM spec