1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: Channel Coherence due to mobile vehicles
Date Submitted: November
Source: John Geiger
Contact: John Geiger, General Electric Global Research
Voice: ,
Re:
Abstract: Model of channel coherence due to mobile vehicles
Purpose: Quantify the coherence bandwidth and coherence time of the channel as a result mobile vehicle NLOS paths for stationary AMI meters
Notice: This document has been prepared to assist the IEEE P 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 P
Slide 1

2. Introduction
Provide a two path model that agrees with the empirical measurements presented by Steve Shearer in the IEE g submission
Provides model data showing that the channel is not stationary in time or frequency over NLOS paths due to moving vehicles

3. 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.

4. 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)

5. Fading as a Function of Frequency & Time
f = 902 – 928 MHz
C= 3 x108 m/s
l= c/f
Dl= D / l
Rl= R / l
RPl= R1/l +R2/l
Transmitter
Receiver
FadingdB = 20Log( sin( abs(RPl(t, f) – Dl(f) )*p/2 ) ))

6. Fading as a function of time due to reflections of a vehicle moving at 13.4 m/s
Fades last approximately 150ms
Good agreement with Steve Shearer’s measurements
Fading is frequency and geometry dependent

7. 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

8. Fading as a function of frequency Five sweeps 100 msec apart
Fading due to reflections of a vehicle moving at 13.4 m/s

9. A 3 Dimensional View of the Channel
Fading due to reflections of a vehicle moving at 13.4 m/s

10. Approximations for Channel Coherence
Tc=9/16pfm=9l/16pv = 4.4ms
Bc=1/5Trms= 4MHz
Ref: T. Rappaport, “Wireless Communication: Principles and Practice”, 2nd Edition, Dec 2001

11. 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

12. Conclusion continued
OFDM will 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.