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Semiconductors for Wireless Communications
RF Pre-Distortion for Linearizing Power Amplifiers
Rajeev Krishnamoorthy
rajeev@scintera.com May, 2013
Semiconductors for Wireless Communications Semiconductors for Wireless Communications
Company Background
Semiconductors for Wireless Communications
What: Adaptive pre-distortion to linearize RF power amplifiers
• RF Power Amplifier Linearizer (RFPAL)
• Reduce distortion to increase PA efficiency and useable output power
• Lower system cost & reduce design time
• Target cellular wireless and microwave
How
• Single-chip programmable, high-performance, analog signal processing platform; lower power consumption and much lower cost (vs. DPD)
• Compute & Adapt in the digital domain, Apply in the analog/RF domain
• Programmability of digital; the simplicity, cost, size and power of analog
Why
• Higher PAR Waveforms: the move to more complex modulation schemes, multi-carrier systems, time-slotted waveforms, etc. increase signal variability (peak-to-average ratio)
• New Devices & Systems: A variety of PA architectures and transistor technologies put increasing demands on linearity & efficiency
• Cost: Push to lower CAPEX and OPEX • Electric bills is a non-trivial component of expenses
• Current digital solutions are expensive, large, and power-hungry, and require integration from BB to PA.
What do we do? And why?
3
Semiconductors for Wireless Communications
Company Timeline
4
2001 2007 2009 2011 2012
Company Founded
Focus for 1st 6 years: Optical & Wireline Markets
Shifted focus to Wireless Infrastructure
SC1887 in production Cellular (W)CDMA market
Add Microwave P2P (up to 60 MHz signal BW)
& Broadcast segments
SC1889 in production (Low-Cost SC1869)
Cellular 4G Time-slotted waveforms Improved performance
2010
SC1894 Expanded frequency range
(168 MHz – 4.2 GHz) Performance improvements Monitoring/control funcs.
2013
SC2200 for MIMO Systems
Semiconductors for Wireless Communications Semiconductors for Wireless Communications
Technology & Product
Semiconductors for Wireless Communications
Power Amplifiers & the Need to Linearize
Efficiency (%) and Inter Modulation Product (dBc) vs. output power (dBm)
PA output power (dBm) vs. input power (dBm)
6
Semiconductors for Wireless Communications
Predistortion Principle
7
No predistorion
With predistorion
5-10 x BW expansion
Semiconductors for Wireless Communications
Digital Pre-Distortion (DPD)
Benefits
Programmable, customizable for particular application, control
Challenges
High power consumption, high frequency reconstruction filter, wide band up-mixer, challenging clock generation to
achieve converters SNR, lack of modularity, cost…
8
Semiconductors for Wireless Communications
RFPAL (RF PA Linearization)
Benefits:
Lower power digital processor, easy reconstruction filter, relaxed up-mixer requirements, low power, low cost, easy to use; feedback path fully integrated; DAC only need transmit at 1x signal bandwidth
Challenges
Potentially … degree of control for end user
9
Semiconductors for Wireless Communications
RFPAL Application
10
Semiconductors for Wireless Communications
RFPAL in Microwave Backhaul Designs
RFPALBLN BLN
BLN
ATTENIF
Xcvr PADvr
Up-convertMixer
ALC
LO
DELAY
FeedbackDown-convert
Mixer
20 or 30dB Coupler
ATTEN
RMS EVM=4.6% RMS EVM=2.8%
128QAM, 28 MHz BW Before correction After correction
11
Easy to linearize split-mount or integrated backhaul systems
Semiconductors for Wireless Communications
RFPAL System Architecture
12
CLK
EEPROM
RFFB SPI
DIGITAL LOGIC
Generate Spectrum Power Analysis
RFOUT RFIN
ANALOG Correction Processor
Envelope
μP Initialize Calibrate
Adapt & Track Generate Corr. Func.
QPS
0
90
RF SIGNAL PROCESSOR
Down-Convert &
Sample
Semiconductors for Wireless Communications
RFPAL Signal Path
13
I
RF VGA
Q
QPSRFIN RFOUT
VOLTERRASERIES
VGA
VGA
VGA
CORRECTION ENGINE
· RFIN: 100MHz-3GHz· I,Q > 250MHz VBW· 50 DACs
Moves application of signal processing from digital to analog domain
Semiconductors for Wireless Communications
Analog Volterra Series
14
5
1m
p
2.m
2.m
4
1p
OUT )-(trc(t)V ~(t)r(t)V 2
IN
C2
C4
C6
C8
C10
X2
X4
X6
X8
X10
IN
Delay 1= 1
OUT
Coefficient DAC’s
Delay 2= 2
Delay 3= 3
Delay 4= 4
Semiconductors for Wireless Communications
15
Cost Function
Optimize Cost function
),(,log)( 2221112 ULUL PPfPPfC w
Use a Stochastic gradient search algorithm to optimize
)(min www
Copt
Semiconductors for Wireless Communications
Cost Surface & Adaptation
16
Starting Point
Concave Area
Global Optimum
0W
4p
0p
5p
1p
2p
3p
1W
2W
3W
4W
5W
0W
4p
0p
5p
1p
2p
3p
1W
2W
3W
4W
5W
Cost Surface (artist’s rendition)
Semiconductors for Wireless Communications
Extensible Approach
Basic idea: linearize by extracting parts of the error spectrum No need to down-convert the entire signal
Wideband Signal
Multi-band Signals
Semiconductors for Wireless Communications
SC1887 SC1869 SC1889 SC1894
Frequency of
operation
698 – 2800MHz 698 – 2800MHz 698 – 2800MHz 168 – 4200MHz
Max. signal BW 1.2 – 40/60MHz 1.2 – 20MHz 1.2 – 60MHz 25 kHz – 60MHz
Waveforms CDMA & WCDMA Same as SC1889 CDMA, WCDMA,
LTE, TD-LTE,
WiMAX, HSDPA
Same as SC1889
PAs Class A/AB &
Doherty
Class A/AB Class A/AB &
Doherty
Class A/AB &
Doherty
PA output power Up to 50 W Up to10W 5 W – 60W Up to 60W
Added features
(paid for add-ons)
RFFB RMS power
detector
RFFB RMS power
detector
RFIN & RFFB RMS
power detector,
gate bias control,
temp. & spectral
monitoring
Product Line Summary
18
SC2200 (MIMO) … 2H 2013
Semiconductors for Wireless Communications Semiconductors for Wireless Communications
Representative Results
Semiconductors for Wireless Communications
Macro: 30W LTE 20 MHz Using Freescale MRF8S2100 Doherty
-50 -40 -30 -20 -10 0 10 20 30 40 50-50
-40
-30
-20
-10
0
10
20
Frequency Offset from Carrier Center, MHz
PA
Out
put
Pow
er,
dBm
/ 3
0 kH
z
BO=0.5 SN460 hw3.1 PAM121 25 LTE20M1%100#8.21 2140.0 MHz 11/01/24 11:01:27 U
POUT-1
= 44.6 dBm
POUT-0
= 45.0 dBm
ACLR2L-1
-56.5
ACLR1L-1
-51.3
ACLR1U-1
-49.7
ACLR2U-1
-56.6
ACLR2L-0
-49.7
ACLR1L-0
-31.6
ACLR1U-0
-31.0
ACLR2U-0
-50.6
PSD-1
PSD-0
32 34 36 38 40 42 44 46 48-65
-60
-55
-50
-45
-40
-35
-30
-25
Pout, dBm
AC
LR,
dBc
SN460 hw3.1 PAM121 25 LTE20M1%100#8.21 2140.0 MHz 11/01/24 11:01:27 U
WP
ACLR1-0
ACLR1-1
ACLR2-0
ACLR2-1
AC
LR d
Bc
-50 -40 -30 -20 -10 0 10 20 30 40 50-50
-40
-30
-20
-10
0
10
20
Frequency Offset from Carrier Center, MHzP
A O
utpu
t P
ower
, dB
m /
30
kHz
BO=0.5 SN460 hw3.1 PAM121 25 LTE20M1%100#8.21 2140.0 MHz 11/01/24 11:01:27 U
POUT-1
= 44.6 dBm
POUT-0
= 45.0 dBm
ACLR2L-1
-56.5
ACLR1L-1
-51.3
ACLR1U-1
-49.7
ACLR2U-1
-56.6
ACLR2L-0
-49.7
ACLR1L-0
-31.6
ACLR1U-0
-31.0
ACLR2U-0
-50.6
PSD-1
PSD-0
32 34 36 38 40 42 44 46 48-65
-60
-55
-50
-45
-40
-35
-30
-25
Pout, dBm
AC
LR,
dBc
SN460 hw3.1 PAM121 25 LTE20M1%100#8.21 2140.0 MHz 11/01/24 11:01:27 U
WP
ACLR1-0
ACLR1-1
ACLR2-0
ACLR2-1
PAR = 8.2 dB, Amplifier P3dB = 53.9 dBm, hdrain=39%
Semiconductors for Wireless Communications
LTE with low traffic loading : E-TM2.0
0 1 2 3 4 5 6 7 8 9 10-20
-15
-10
-5
0
Time, ms
Sig
nal am
plitu
de,
dB
FS
LTE20M1-1#14.34.mat, dwell = 20 s, duty factor = 0.00 dB
peak across 20 us
12 kHz LPF
Am
plit
idue
Slot (Time)
Reso
urc
e B
locks (
Fre
qu
en
cy)
Blue – Uncorrected
Yellow – 100% Loaded
Magenta – Partial Loaded Signal Shown at Right
Semiconductors for Wireless Communications
31 32 33 34 35 36 37 38 39 40 410
5
10
15
20
25
30
35
40
45
50
Pout, dBm
Powe
r-add
ed e
ffici
ency
, %
SN21003 hw4.1 PAM347 25.0 WCDMA4-1111#7.75 2690.0 MHz 13/03/12 19:39:06
WP
PAE
31 32 33 34 35 36 37 38 39 40 41-65
-60
-55
-50
-45
-40
-35
-30
-25
Pout, dBm
AC
LR
, dB
c
SN21003 hw4.1 PAM347 25.0 WCDMA4-1111#7.75 2690.0 MHz 13/03/12 19:39:06 U
WP
ACLR1-0
ACLR1-1
ACLR2-0
ACLR2-1
5W (@ Antenna) Asymmetric Doherty Line-up
22
PA
28V
RF
Xcvr 37dBm
39.5dBm
SC1894 DVR
22
20MHz 4xWCDMA 7.8dB PAR
30 32 34 36 38 40 420
5
10
15
20
25
30
35
40
45
50
Pout, dBm
Pow
er-
added e
ffic
iency,
%
SN21003 hw4.1 PAM347 25.0 WCDMA4-1111#7.75 2650.0 MHz 13/03/07 20:44:14
WP
PAE
45%PAE
>20dB Improved Linearity
31 32 33 34 35 36 37 38 39 40 410
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Pout, dBm
EVM
, %
SN21003 hw4.1 PAM347 25.0 WCDMA4-1111#7.75 2690.0 MHz 13/03/12 19:39:06
WP
EVM-0
EVM-1
-20 -15 -10 -5 0 5 10 15 20-50
-40
-30
-20
-10
0
10
20
Frequency Offset from Carrier Center, MHz
PA
Outp
ut
Pow
er,
dB
m /
30 k
Hz
SN21003 hw4.1 PAM347 25.0 WCDMA4-1111#7.75 2690.0 MHz 13/03/12 19:39:06 U
POUT-1
= 39.1 dBm
POUT-0
= 39.2 dBm
ACLR2L-1
-55.0
ACLR1L-1
-50.6
ACLR1U-1
-50.6
ACLR2U-1
-52.5
ACLR2L-0
-34.1
ACLR1L-0
-32.2
ACLR1U-0
-33.9
ACLR2U-0
-36.5
PSD-1
PSD-0
Semiconductors for Wireless Communications
2.5W (@ Antenna) Asymmetric Doherty Line-up
23
PA
28V
28V
RF
Xcvr 34dBm
38dBm
SC1894 DVR
10MHz LTE 7.5 PAR
23
45%PAE
Semiconductors for Wireless Communications
2x1W (@ Antenna) Reference Product Line-up
24
DVR PARFPAL
12V
DVR PARFPAL
12VDual Xcvr
30dBm
30dBm
33dBm
33dBm
*
*
* Triquint PA requires special match for 33dBm.
24 26 28 30 32 34 36-65
-60
-55
-50
-45
-40
-35
-30
-25
Pout, dBm
AC
LR,
dBc
SN8072 hw 3.1 PAM231 25.0 1.81V 3.32V WCDMA4-1111#7.75 2140.0 MHz 12/02/07 18:39:23 U
WP
ACLR1-0
ACLR1-1
ACLR2-0
ACLR2-1
24 25 26 27 28 29 30 31 32 33 340
5
10
15
20
25
30
35
40
45
50
Pout, dBm
Pow
er-
added e
ffic
iency,
%
SN8072 hw3.1 PAM231 25.0 1.81V 3.32V WCDMA4-1111#7.75 2140.0 MHz 12/02/07 18:39:23
WP
PAE
20MHz 4xWCDMA 7.8dB PAR
-20 -15 -10 -5 0 5 10 15 20-50
-40
-30
-20
-10
0
10
Frequency Offset from Carrier Center, MHz
PA
Outp
ut
Pow
er,
dB
m /
30 k
Hz
SN8072 hw 3.1 PAM231 25.0 1.81V 3.32V WCDMA4-1111#7.75 2140.0 MHz 12/02/07 18:39:23 U
POUT-1
= 32.8 dBm
POUT-0
= 32.9 dBm
ACLR2L-1
-52.8
ACLR1L-1
-50.3
ACLR1U-1
-51.2
ACLR2U-1
-51.9
ACLR2L-0
-34.7
ACLR1L-0
-32.3
ACLR1U-0
-33.6
ACLR2U-0
-36.1
PSD-1
PSD-0
Semiconductors for Wireless Communications
Summary
25
• Technology: Programmable analog signal processing platform is ideally suited to address a wide range of markets & applications
• Initial focus on RF PA linearization
• Markets: Target markets expanding • Broadcast, Microwave backhaul, Public Safety, White space
• Advantages: Differentiated solution addressing customer issues • Low power consumption, low cost, small form factor and high performance
• Rfin/Rfout, simple design and integration of RFPAL enables fast time to market
• Supports a wide variety of
• Waveforms & modulations (2G/3G/4G, broadcast, microwave, …)
• Range of transmit powers (0.5W-60W)
• Infrastructure (macro base stations, repeaters, small cells, …)
• Technologies (LDMOS, GaN, GaAs, etc.)
• Architectures (class A, A/B, Symmetric & Asymmetric Doherty,…)
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