Channel Coherence of Wireless Channels with Mobile Vehicles

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Channel Coherence due to mobile vehicles Date Submitted: November 18. 2009 Source: John Geiger Contact: John Geiger, General Electric Global Research Voice: +1 585-242-8474 , E-Mail: john.geiger@ge.com 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 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

2. 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

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 frequencyFive 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 • 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.