accurate physical layer modeling for realistic wireless network simulation · 2009-03-18 ·...

Accurate Physical Layer Modeling for Realistic

Wireless Network SimulationThe Need for Validation

Ruben Merz1 Cigdem Sengul1 Mustafa Al-Bado2

1Deutsche Telekom Laboratories, TU Berlin

2FG INET, TU Berlin

Workshop on ns-3 (WNS-3) / March 2nd, 2009

Ruben Merz — [email protected] — http://www.net.t-labs.tu-berlin.de/~ruben/ 1 / 15

http://www.net.t-labs.tu-berlin.de/~ruben/

The Tension Between Accuracy and Complexity

Simulator objectives

Short simulation time

Accurate simulation results

Wireless network simulator

Short simulation time = simplified models

Lack of validation with real environments



We do Need Simulators. Hence Validation

Why do we need simulators

Testbed: not necessary flexible, easily configurable

Example: protocol validation before deployement, traces replaying fordebugging

The Need for Validation

Trustworthy wireless simulations: usable and dependable simulator

Understand what the (valid) assumptions are

Accuracy/networking point view: what must be modeled precisely, whatcan be dropped



A Typical 802.11a/b/g TransceiverWistron CM9 mini-PCI http://www.ipmedia.cz/img.asp?attid=655


http://www.ipmedia.cz/img.asp?attid=655


A Typical 802.11a/b/g TransceiverWistron CM9 mini-PCI http://www.ipmedia.cz/img.asp?attid=655

Atheros5211/5111

McFarland et al. “A 2.4 & 5 Ghz dual band 802.11 wlan supporting data rates to 108 mb/s”, GaAs IC Symposium, 2002


http://www.ipmedia.cz/img.asp?attid=655


A Typical RF Frontend

RF transceiver Digital baseband

BPF

RX

TX

Transmitter LPF

Base

ban

d a

nd

MA

C

Receiver LPF

Offset control

8 MHz

I

Gain control

Frequencysynthesizer

DAC

QDAC

IADC

QADC

Meng at al. “Design and implementation of an all-CMOS 802.11a wireless LAN chipset”, IEEE Communications Magazine, 2003



A Typical RF Frontend




A Typical Baseband Engine

Pipelinecontrol

I and Q fromanalog

front-end

Rx gainto analogfront-end

Rx datato MAC

FIR

FIR

RemoveDC

offsetRotate

Frequencylock

Symboltiming

Channelestimate

andtracking

FFT ViterbiChannelcorrect

Deinterleave

Signaldetect

and AGC

ADC

Auto-correlate




A Typical Baseband Engine




What do We Have in ns-3.3

PRX(k) > γED SNIR(k, t) =Sk(t)

P

m 6=k m,t+NthPERR(k) = f(SNIR(k, t))

rand > PERR(k)

What’s missing?

Hardware specifics: RF frontend, non-standard transmission modeMultiple transmission channelsAntenna modeling / Multiple AntennasPropagationPacket detection and timing acquisition (synchronization) / Capture

Is the existing model sufficient/accurate? How to model/validate?What are the features that affects accuracy?



Accuracy Issue 1: Hardware Specifics

Power control granularity Output power spectrum not constant

5160 5180 5200 5220 5240 5260 5280 5300 5320 5340 5360−100

−90

−80

−70

−60

−50

−40

−30

−20

−10

Frequency (MHz)

Pow

er

Spectr

al D

ensity (

dB

m/1

.8M

Hz)

txpower=1

txpower=6

txpower=12

txpower=18

txpower=24

txpower=30

txpower=36

txpower=99

noise

MADWIFI, CM9 mini-PCI

Parameters dependent on transmission mode:output power, sensitivityProprietary features: antenna diversity

Kowalik et al., “Practical Issues of Power Control in IEEE 802.11 Wireless Devices”, 2008

Cheng et al., “Adjacent Channel Interference in Dual-radio 802.11a Nodes and Its Impact on Multi-hop Networking”, Globecom 06

Zargari et al., “A Single-Chip Dual-Band Tri-Mode CMOS Transceiver for IEEE 802.11a/b/g Wireless LAN”, IEEE Journal of Solid-Circuits, 2004



Accuracy Issue 2: Multiple Transmission Channels

5160 5180 5200 5220 5240 5260 5280 5300 5320 5340 5360−100

−90

−80

−70

−60

−50

−40

−30

−20

−10

Frequency (MHz)

Pow

er

Spectr

al D

ensity (

dB

m/1

.8M

Hz)

txpower=1

txpower=6

txpower=12

txpower=18

txpower=24

txpower=30

txpower=36

txpower=99

noise

0 5 10 15 20 25 30 35 400

2

4

6

8

10

12

Signal to Interference Ratio (dB)

UD

P T

hro

ug

hp

ut

(Mb

ps)

5220MHz 18dBm

5240MHz 18dBm

5240MHz 0dBm

Field Experiment Results

Channels are not orthogonal: ACI

Cheng et al., “Adjacent Channel Interference in Dual-radio 802.11a Nodes and Its Impact on Multi-hop Networking”, Globecom 06



Accuracy Issue 3: Antenna Pattern, RF Frontend

30 40 50 60 70 80 90100 200 3000

200

400

600

800

1000

1200

1400

Distance in Logarithmic Scale (meter)

Thro

ughput

(kbps)

Horizontal to Horizontal Elevated

Horizontal to Horizontal

Vertical to Vertical

Horizontal to Horizontal Cross!Polarized

Antennas: need orientation

Cheng et al., “Performance Measurement of 802.11a Wireless Links from UAV to Ground Nodes with Various Antenna Orientations”, ICCCN 06



Accuracy Issue 4: Channel Propagation

Propagation is a random process

Environment specific: indoor/outdoorFrequency specific

Channels are not symmetricSpace and time correlation

!"#

$" !# %#&" &' ((

)' ""

&##

!

"!!

#!!

$!!

%!!

&!!

'!!

!(!)"

!)"(!)#

!)#(!)$

!)$(!)%

!)%(!)&

!)&(!)'

!)'(!)*

!)*(!)+

!)+(!),

!),("

&

Reddy, Riley, “Measurement–Based Physical Layer Modeling for Wireless Network Simulations”, MASCOTS 07

Vyas et al., “Characterization of an IEEE 802.11a Receiver using Measurements in an Indoor Environment”, Globecom 06

Kurth et al., “Multi-Channel Link-level Measurements in 802.11 Mesh Networks”, 2006



Accuracy Issue 5: Synchronization and Capture,Automatic Gain Control (AGC)

Sync depends on SNR

Performance of sync and decodingnot equivalent

With several transmitters: captureeffects

Effect of AGC unknown0 5 10 15 20 25 30 35

0

10

20

30

40

50

60

70

80

90

100

SNR (dB)

Pro

b. o

f S

yn

c E

rro

r (%

)

Vyas et al., “Characterization of an IEEE 802.11a Receiver using Measurements in an Indoor Environment”, Globecom 06



Accuracy Issue 6: PER Computation

PERR(k) = f(SNIR(k, t))

= 1 − Πl (1 − Pe(k, l))

SNIR(k, t) calculationPER Computation

Existing Viterbi models: AWGN or Rayleigh channels, upper-bounds,asymptotic performanceOFDM modulation not modeledValidity of computation by block

Lacage, Henderson, “Yet Another Network Simulator”, 2006



What About ns-3.3

./!012 /:!012 35!012 8/!012 7:!012 78!0;2 9!012 5!012

6

.

76

7.

86

8.

36

3.

/6

!"#$%&'(")+E!012F

!&J(C$C+>?,"$@?A$'+E!012F

Indoor LOS, 2 nodes, 15 m

UDP, saturated 0

5

10

15

20

25

30

35

40

0 50 100 150 200 250

Exp54mb48mb36mb24mb18mb12mb

9mb6mb

According to http://www.atheros.com/pt/whitepapers/Methodology_Testing_WLAN_Chariot.pdf, should be 30.5 Mbps


http://www.atheros.com/pt/whitepapers/Methodology_Testing_WLAN_Chariot.pdf


Outlook and Summary

Ongoing and Future work

Channel modeling and validation

Ray-tracing?Testbed in simulator

Packet detection and timing acquisition validation, AGC

PER calculation validation

Detailed, bit-level PHY for 802.11

Large scale validation with network traces

PHY: need modeling and validation

Exhibit clearly what the underlying assumptions are

What are the elements of importance for the overall simulation accuracy?



accurate physical layer modeling for realistic wireless network simulation · 2009-03-18 ·...

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