Computer Networks GroupUniversität Paderborn

ChSim – A wireless channel simulator for OMNeT++

Simulation workshop

TKN, TU BerlinSeptember 08, 2006

(c) 2006, Stefan Valentin, stefanv@upb.de

2

Outline

• Introduction

• Example scenario, results & modeling

• PHY interface & program structure

• Integration to joint simulator framework

• Conclusion & wishlist for joint simulator

(c) 2006, Stefan Valentin, stefanv@upb.de

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Introduction

• Wireless channels may vary…

• ChSim generates channel state statistics for multiple links• Leads to transmission rate and error statistics

… vs. time in mobile scenarios

… vs. frequency in multipath environments

(c) 2006, Stefan Valentin, stefanv@upb.de

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Scenario example

• A mobile wireless network:

• At runtime: Definition of relative speed and distance by mobility model

• Included: Manhattan grid, (Random waypoint)• Fixed setup is possible

• Mobility parameters are passed to channel models• Included: Clarke’s fading model (Rayleigh), path loss, shadowing,

and correlation

relative distance

relative speed

(c) 2006, Stefan Valentin, stefanv@upb.de

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Modeling fading

• Time selective fading:• Frequency of each fading path ist

shifted according to mobility speed• Doppler shift: fd ~ vrel

• Paths overlap at receiver• Clarke’s model

1. Simplification: There are k discretefading paths

2. Simplification: Isotropic antenna gain pattern• I.e., scatterers are located in a ring

around the mobile receiver⇒ No ray-trace 3D models needed ☺

• Frequency selective fading• Parameterized by delay spread

• Delay variance of paths

(c) 2006, Stefan Valentin, stefanv@upb.de

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Fading result example

• Channel states with various speeds

(c) 2006, Stefan Valentin, stefanv@upb.de

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Correlation example

• Useful for multiple channel systems, e.g. MIMO, OFDM• Chsim supports link and frequency correlation

0 100 200 300 400 500 600 700 800 900 1000

-30

-20

-10

0

10Channel gain of two uncorrelated links

Samples

H [d

B]

0 100 200 300 400 500 600 700 800 900 1000-40

-30

-20

-10

0

10Channel gain of two correlated links ρ=0.9

H [d

B]

Samples

(c) 2006, Stefan Valentin, stefanv@upb.de

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Interface to physical layer

• Output of Chsim: Attenuation factor h(t)

• One h per time-sample, frequency-band, and link

• With transmission power & noise the PHY:

• Calculates: SNR = h(t) × Es/N0• Selects: Transmission rate using

rate thresholds• Calculates: Error statistics using

coding gain and thresholds• Drawback compared to G.-E.:

• All PHY calculations have to be done per channel sample

• However, with interleaving block fading can be assumed

• Limits sample frequency to one sample per MAC frame

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-10

-5

0

5

10

15

20

25

E s/N

0at

rece

iver

[dB

]

SNR per symbol, Ptx

=-9dBm, 50m distance and single 2ms block fading channel with 1m/s motion speed

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

1

2

3

4

5

6x 107Ideal rate allocation for the 8 IEEE 802.11a rates, 2ms adaptation cycle, and 2 different bound sets

chos

en tx

-rat

e [B

its/s

]Time [s]

Bounds: L=13500Bytes Bounds: L=1500BytesBounds: L=1500Bytes (min. Gcode

) Bounds: Proxim MP.11a default

PHY not included in ChSim, 802.11a/g PHY implementations

exist

(c) 2006, Stefan Valentin, stefanv@upb.de

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Program structure

• Basic structure: • MobileStation[n]:

• Channel[m]:

• channelStateCalculator:• Channel modeling methods

• errorCalculator:• Empty: no error model in

ChSim

(c) 2006, Stefan Valentin, stefanv@upb.de

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Integration to joint simulator framework

• TriggerGen and Filewriter: • Natural connection to higher layers

• Integrate these (and other channel models) as a:• Single compound module btw. terminals or• OMNeT++ NED connection

• Preferred but requires OMNeT++ changes

• Wish list for joint simulator:• Separation and clean interfaces for/btw.:

• Mobility model (Mobility framework implementations should be used)• Channel • PHY

• Arbitrarily amounts of channels per node and MAC link• E.g. for MIMO

• Different time resolutions for Channel and PHY/MAC modules

(c) 2006, Stefan Valentin, stefanv@upb.de

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Conclusion & future work

• Introduced: ChSim, our contribution to the joint simulator framework

• Provides channel statistics• Analog channel models close to physical scenario

• Advantage vs. G.-E.: Physical parameters can be used directly or considered as factors

• E.g. Analyze performance of MIMO system with channel correlation• Disadvantage vs. G.-E.: Computational overhead ≈ 4 × G.-E.

• Future work:• Revise correlation model, and add further methods• Release 802.11a/g PHY• Documentation

• For joint simulator:• Include more, e.g. digital, channel models• PHY, channel, mobility models should be clearly separated and

connected using clean interfaces

(c) 2006, Stefan Valentin, stefanv@upb.de

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Thank you – Questions?

• Download Chsim at http://wwwcs.upb.de/cs/cn/projects.html

(c) 2006, Stefan Valentin, stefanv@upb.de

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Frequency selective fading

0 5 10 15 20 25 30 35 40 45 50-30

-25

-20

-15

-10

-5

0

5

10

Frequency band

H [d

B]

Delay spread = 10ns = 150ns = 5µs

(c) 2006, Stefan Valentin, stefanv@upb.de

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Rayleigh fading

-70 -60 -50 -40 -30 -20 -10 0 100

1000

2000

3000

4000

5000

6000

7000

8000Histogram of a slow fading channel generated with ChSim

(c) 2006, Stefan Valentin, stefanv@upb.de