doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

Slide 1

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)(WPANs)

Submission Title: Channel Coherence due to mobile vehicles Date Submitted: November 18. 2009Source: John GeigerContact: John Geiger, General Electric Global ResearchVoice: +1 585-242-8474 , E-Mail: john.geiger@ge.comRe: Abstract: Model of channel coherence due to mobile vehiclesPurpose: Quantify the coherence bandwidth and coherence time of the channel as a result mobile vehicle NLOS paths for stationary AMI metersNotice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

Slide 1

doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

Introduction

• Provide a two path model that agrees with the empirical measurements presented by Steve Shearer in the IEE802.15-09-0742-00-004g submission

• Provides model data showing that the channel is not stationary in time or frequency over NLOS paths due to moving vehicles

doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

Objective

• Quantify the coherence bandwidth and coherence time of the channel as a result mobile vehicle NLOS paths for stationary AMI meters– Provide a mathematical model– Predict how fast the channel is changing and its

impact on packet dwell time– Predict the effect of mobile vehicles on frequency

selective fading and its effect on propagation. How much of the spectrum is faded for how long and how fast is it changing.

doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

Assumptions

• The radio transmitter has a NLOS refracted fixed path and a reflected path fixed or mobile path to the receiver

• The radio receiver’s direct and reflected paths are at the same average signal level

• The vehicle is moving at 13.4 m/sec (30 mph)

doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

Fading as a Function of Frequency & Time

f = 902 – 928 MHzxm/scfD= D / R= R / RP= R1/ +R2/

Transmitter Receiver

FadingdB = 20Log( sin( abs(RP(t, f) – D(f) )*

915 MHz906 MHz 924 MHz

20 dB

1.5 MHz@ 20 dB

D = 40 meters

R =

20

me

ters

V = 13.4 m/s, 30mph

R1 R2

doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

1350 1400 1450 1500 1550 1600 1650 1700-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

time in milliseconds

addi

tiona

l los

s du

e to

veh

icle

ref

lect

ion

914

914.5

915915.5

916

• Fades last approximately 150ms• Good agreement with Steve Shearer’s measurements• Fading is frequency and geometry dependent

Fading as a function of time due to reflections of a vehicle moving at 13.4 m/s

doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

Fading as a Function of Frequency

• Non-Stationary frequency fades typically between 1 to 3 MHz wide will appear in the spectrum due to the reflected signals from moving vehicles

• Fades will last typically 150 ms for vehicle speeds of 30 mph• Stationary objects will create stationary frequency nulls in the spectrum • Small changes in the environment will cause the fades to change drastically

900 905 910 915 920 925 930-20

-15

-10

-5

0

5

10Amplitude vs Frequency at mid-time

Frequency (MHz)

Pat

h lo

ss (

dB)

doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

Fading as a function of frequencyFive sweeps 100 msec apart

900 905 910 915 920 925 930-25

-20

-15

-10

-5

0

5

10Amplitude vs Frequency - 5 sweeps 100 msec apart

Frequency (MHz)

Path

loss

(dB

)

Fading due to reflections of a vehicle moving at 13.4 m/s

doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

900

910

920

930

1200

1400

1600

1800

-50

-40

-30

-20

-10

0

Frequency (MHz)Time (msec)

Am

plitu

de (

dB)

A 3 Dimensional View of the Channel

Fading due to reflections of a vehicle moving at 13.4 m/s

doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

Approximations for Channel Coherence

c=9/16fm=9v = 4.4ms

Bc=1/5rms= 4MHz

Ref: T. Rappaport, “Wireless Communication: Principles and Practice”, 2nd Edition, Dec 2001

doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

Conclusions

• For non-line-of-sight paths, multipath reflections from fixed objects will cause deep stationary nulls.

• Nulls will move in frequency with time due to reflections from vehicles as they pass by.

• Typical null bandwidth is ~2 MHz at 20db point

• Typical null duration is ~100 mS• Coherence time of the channel will be on the

order of 5-10ms

doc.: IEEE 802. 15-09-0798-00-004g

Submission

November 18, 2009

John GeigerGE Digital Energy

Conclusion continued

• Given the results presented: OFDM may be ineffective unless very wide, over 10 MHz.– not practical within the 900 ISM band

• Frequency hopping is required to dodge moving nulls caused by multipath.

• Packet size must be kept under 100 ms to have a high probability of success.