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• General concepts• Energy-saving mechanisms• Integrated circuits (IC)
– Processors
• Wireless interfaces
2
Source of energy consumption
• IC– Computing/Switching– Radiation (wireless)
• Discrete components
• Display
4
Energy characterization
• Equipment setup
5
Power Supply
- +
Target
0.1 ohmSense Resistor
Voltage Sampling Device
6
Differential measurement
Extra energy/power consumption of an event obtained through differential measurements
Extra energy consumption for writing “x”
Write “x” with stylus/touchscreen
0
0.4
0.8
1.2
1.6
0 0.5 1 1.5
Time (s)
Pow
er (W
)
0
0.4
0.8
1.2
1.6
0 0.5 1 1.5
Time (s)
Pow
er (W
)
Rule No. 1• Focus on the big one!
– Amdahl’s Law• Reduction of the power of α % of the system by p% leads to
α∙p % reduction of the total power
7
α
Rule No. 2
• Minimize static energy consumption– IC consumes static power when it is merely
powered
8
Rule No. 3
• Minimize activity– When not doing things useful
• Turn it off• Stop the clock• Check the manual for power-saving modes• Be aware of state transition overhead
– Interrupt-driven instead of polling
9
Processors
• Dynamic voltage scaling• Power-saving states
– Clock gating– Power-down different subsets of components
• As-fast-as-possible or As-slow-as-possible?
11
Busy power vs. delay vs. energy
fVCaP dddyn ⋅⋅⋅= 2
)( Tdd
dd
VVVt−
∝
Analysis and Design of Digital ICs, Hodges et al
13
Core 2 Duo for example• Intel® Core™2 Duo processor
– T7800 at 2.6GHz– T7700 at 2.4GHz available on Thinkpad T61p– 0.75-1.35V, 35Watts
• Intel® Core™2 Duo Low Voltage– L7500 at 1.6GHz available on Thinkpad X61– 0.75-1.3V, 17Watts
• Intel® Core™2 Duo Ultra Low Voltage– U7500 at 1.06GHz available on Dell D430– 0.75-0.975V, 10Watts
14
Given workload L and deadline T
• L measured by # of CPU cycles• Clock speed f ≥ L/T
• Time to finish: t = L/f
• Energy to finish: P ∙ t= a∙C ∙V2 ∙f ∙t= a∙C ∙V2 ∙L
16
Effect of lower clock speed (f)
Power consumption
P= a∙C ∙V2 ∙f
Energy consumption
E=P ∙ t= a∙C ∙V2 ∙f ∙t= a∙C ∙V2 ∙L
17
Effect of lower supply voltage (V)
Power consumption
P= a∙C ∙V2 ∙f=k∙V3=x∙f3
Energy consumption
E=P ∙ t= a∙C ∙V2 ∙f ∙t= a∙C ∙V2 ∙L
Maximum clock speed
f= b∙V
18
Given workload L and deadline Tsingle processor
• The processor can run at any frequency (voltage)– f= b∙V
• The processor can be complete off when work is done (zero power when idle)
• To minimize energy consumption, at which frequency should the processor run?– f ≥ L/T (in order to meet the deadline)– E=P ∙ t= a∙C ∙V2 ∙f ∙t= a∙C ∙V2 ∙L– f=????
19
Given workload L and deadline TM processors
• The workload can be divided without overhead: L = L1+L2+…+LM (L ≥ Li≥0)
• To minimize energy consumption, at which frequency should processor i run?– f i= Li/T and V = u ∙ Li
– Ei= a∙C ∙V2 ∙Li=w∙Li3
22
Given workload L and deadline TM processors
• The workload can be divided without overhead: L = L1+L2+…+LM (L ≥ Li≥0)
• To minimize the TOTAL energy consumption, how should the workload be allocated?– E= E1+E2+…+EM= w∙L1
3+w∙L23+…+w∙LM
3
– = w(L13+L2
3+…+LM3)
23
From high school
• [(a+b)/2]2≤ (a2+b2)/2
≥ ≥ ≥
Quadratic mean Arithmetic mean Geometric mean harmonic mean
24
From high school (Contd.)
• [(a+b)/2]3≤ (a3+b3)/2 ( for a, b ≥0)
– E= w(L13+L2
3+…+LM3) ??? (L1+L2+…+LM)3
25
Jensen’s Inequality (finite form)
• ϕ (x) is convex– ϕ (t∙x1+(1-t)∙x2)≤ t∙ ϕ (x1)+(1-t) ∙ϕ (x2)
http://en.wikipedia.org/wiki/Jensen%27s_inequality#Proof_1_.28finite_form.29
27
• ai=1/n• ϕ (x) =x2 (Convex)
• ϕ (x) =x3(Convex for x≥0)– E= w(L1
3+L23+…+LM
3)=w∙M ∙ (L13+L2
3+…+LM3)/M
– ≥ w∙M ∙[(L1+L2+…+LM)/M] 3=w∙L3/M2
≥
28
Idle power (Static power)
)( Tdd
dd
VVVt−
∝ Tstatic eTP
α−
∝ 2 ddVddstatic eVP ⋅∝ γ
When IC is idle but not powered off, e.g. SRAM31
Multiple power/clock domains
TI OMAP 2 architecture, ISSCC 2005
Multimedia phone: NTT DoCoMo 3G FOMA 902ito be released with OMAP2420
32
Given workload L and deadline Tsingle processor
• One processor can run at any frequency (voltage)– f= b∙V
• The processor can be complete off when work is done (zero power when idle) Given Pstatic– Given energy overhead of shutting down the
processor (Eoverhead)• To minimize energy consumption, at which
frequency should the processor run?
37
Check the assumptions (Contd.)
• The workload can be divided without overhead: L = L1+L2+…+LM (L ≥ Li≥0)
• Communication cost between processors!!!
38
Wireless interfaces
• Stay connected
• Establish connection
• Transfer data
• Transmit vs. receive
39
41
Power consumption (SMT5600)
Lighting: Keyboard, 73, 3% Lighting: Display I,
148, 5% Lighting: Display II, 61, 2%
LCD, 13, 0%
Speaker, 45, 2%
Bluetooth, 440, 16%
GPRS, 1600, 58%
Compute, 370, 13%
Cellular network, 17, 1%
Flight mode: Sleep, 3, 0%
42
Power consumption (T-Mobile)
1
10
100
1000
10000
IDLE-Flight m
ode
Com
puting
LCD
LCD
lighting
Keyboard lighting
Speaker
Discoverable
Paging
Connected
Transmission
Connected
Transmission
Connected
Transmission
Pow
er (m
W)
Bluetooth Wi-Fi Cellular
43
Power consumption (Contd.)
• Theoretical limits– Receiving energy per bit > N * 10-0.159
• N: Noise spectral power level• Wideband communication
Distance: d
Propagation constant: a (1.81-5.22)
PRXPTX∝ PRX*da
44
Power consumption (Contd.)
• What increases power consumption– National regulation (FCC)
• Available spectrum band (Higher band, higher power)• Limited bandwidth• Limited transmission power
– Noise and reliability– Higher capacity
• Multiple access (CDMA, TDMA etc.)– Security– Addressability (TCP/IP)– More……
Wasteful wireless communication
45
TimeMicro power management
SpaceDirectional communication
SpectrumEfficiency-driven cognitive radio
Time waste
• Network Bandwidth Under-Utilization– Modest data rate required by applications
• IE ~ 1Mbps, MSN video call ~ 3Mbps– Bandwidth limit of wired link
• 6Mbps DSL at home
47470 0.2 0.4 0.6 0.8 1
0
200
400
600
800
1000
1200
1400
Time (s)
Dat
a S
ize
(Byt
e)
0
20
40
60
80
100
Time Energy
Idle
inte
rval
s in
busy
tim
e (%
)
User1 User2 User3 User4
49
Wireless system architecture
Application
Transport
Network
Data link
Host computer
RF front ends
BasebandNetwork interface
Network protocol stack Hardware implementation
Physical
50
Power consumption (Contd.)
Baseband processor
Antenna interface
LNA
Low-noise amplifier
PA
Power amplifier
Intermediate Frequency (IF) signal processing
Local Oscillator (LO)
Physical Layer
IF/B
aseb
and
Conv
ersio
n
MAC Layer & above
>60% non-display power consumed in RF
RF technologies improve much slower than IC
51
Power consumption (Contd.)
67%
18%
8%
6%
1%
PA
FS
Mixer
Source: Li et al, 2004
Components Power (mW)
Power amplifier (PA) 246
Frequency synthesizer (VCO/FS)
67.5
Mixer 30.3
LNA 20
Baseband processing 5
52
Circuit power optimization
• Major power consumers
Baseband processor
Antenna interface
LNA
Low-noise amplifier
High duty cycle
PA
Power amplifier
High power consumption
Intermediate Frequency (IF) signal processing
Local Oscillator (LO)
Almost always on
Physical Layer
IF/B
aseb
and
Conv
ersio
n
MAC Layer & above
Huge dynamic range 105
53
Circuit power optimization (Contd.)
• Reduce supply voltage– Negatively impact amplifier linearity
• Higher integration– CMOS RF– SoC and SiP integration
• Power-saving modes
54
Circuit power optimization (Contd.)
• Power-saving modes– Complete power off
• (Circuit wake-up latency + network association latency) on the order of seconds
– Different power-saving modes• Less power saving but short wake-up latency
55
Power-saving modes
Baseband processor
Antenna interface
LNA
Low-noise amplifier
PA
Power amplifier
Intermediate Frequency (IF) signal processing
Local Oscillator (LO)
Physical Layer
IF/B
aseb
and
Conv
ersio
n
MAC Layer & above
Radio Deep Sleep Wake-up latency on the order of micro seconds
56
Power-saving modes (Contd.)
Baseband processor
Antenna interface
LNA
Low-noise amplifier
PA
Power amplifier
Intermediate Frequency (IF) signal processing
Local Oscillator (LO)
Physical Layer
IF/B
aseb
and
Conv
ersio
n
MAC Layer & above
Sleep Mode Wake-up latency on the order of milliseconds
Low-rate clock with saved network association information
57
Network power optimization
• Use power-saving modes– Example: 802.11 wireless LAN (WiFi)
• Infrastructure mode: Access points and mobile nodes
58
802.11 infrastructure mode• Mobile node sniffs based on a “Listen Interval”
– Listen Interval is multiple of the “beacon period”• Beacon period: typically 100ms
• During a Listen Interval– Access point
• buffers data for mobile node• sends out a traffic indication map (TIM), announcing buffered
data, every beacon period– Mobile node stays in power-saving mode
• After a Listen Interval– Mobile node checks TIM to see whether it gets buffered
data– If so, send “PS-Poll” asking for data
59
Buffering/sniffing in 802.11
Gast, 802.11 Wireless Network: The Definitive Guide
802.15.1/Bluetooth uses similar power-saving protocols: Hold and Sniff modes
60
Power profile of 802.11b
0
200
400
600
800
1000
1200
1400
1600
1 401 801 1201
Pow
er (m
W)
Time (1/4000 s)
61
Network variations
10080604020
20406080
100
1 21 41 61 81 101 121 141 161 181 201 221 241 261 281 301
Sign
al s
tren
gth
(%)
Distance (4-5 meters)
10080604020
20406080
100
1 21 41 61 81 101 121 141 161 181 201 221
Sign
al s
tren
gth
(%)
Distance (2-3 meters)
Cellular
Wi-Fi
Cellular
Wi-Fi
10080604020
20406080
100
1 121 241 361 481 601 721 841 961 1081 1201 1321 1441 1561 1681 1801 1921 2041 2161 2281 2401
Sign
al s
tren
gth
(%)
Time (minutes)
Cellular
Wi-Fi