Wireless Coexistence Assurance Model for Interference Impact Prediction

1. IEEE 802.19Wireless Coexistence TAG An Analytic Coexistence Assurance Model Steve Shellhammer shellhammer@ieee.org Steve Shellhammer, Intel Corporation

2. An Analytic CA Model • Make reasonable approximations of PHY and MAC layers. • Provide a method of predicting the impact of interference in a timely manner. • Not a detailed model intended to predict absolute performance of either system. • Is intended to predict relative impact of interference. • Only considering non-hoppers at this point • Intended as a first-order approximation. Steve Shellhammer, Intel Corporation

3. Model of Interferer • Interferer sends pulses • When transmitting a pulse the interferer is models in the frequency domain as band-limited white noise of power PT PT B fc fc + B/2 fc - B/2 Steve Shellhammer, Intel Corporation

4. Model of Interferer • Based on our knowledge of the interferer traffic the temporal model of the interferer is a stochastic process of pulses. Need to consider various models. • Distribution of pulse durations • Distribution of spacing between pulses Steve Shellhammer, Intel Corporation

5. Model of Interferer • Pulse TP duration is a random variable • Space TS between pulses is a random variable. TP TS TP TS TP Steve Shellhammer, Intel Corporation

6. Example of Pulse Model • The interferer is sending TCP IP packets. • There is an AP far away sending ACK packets. So we don’t consider this an interferer. • Throughput is about half the data rate. • TP = 1.0 ms • TS is a uniform RV • TS = U(30, 1300) us Steve Shellhammer, Intel Corporation

7. Path Loss Model • Some standard path loss model will be recommended, like the one used in 802.15.2. • Other path loss models could be used. • Give a topology of devices you can determine the interference power at the receiver based on path loss model. pl(d) = path loss in dB, with d in meters. Steve Shellhammer, Intel Corporation

8. Topology of Wireless Devices • One possible topology Transmitter Is not interfered with due to distance from interferers Does not interfere due to distance from NUT System A Network Under Test Receiver d System B Interferer Primary Interferer Steve Shellhammer, Intel Corporation

9. Receiver Model • Model receiver filter as an ideal brick wall filter, as far as interference goes. • The portion of the interfering signal that is within the passband of the receiver filter is pass though undisturbed • Any portion of the interfering signal outside the filter passband is eliminated entirely. Steve Shellhammer, Intel Corporation

10. Receiver Model NI Interferer PSD at Receiver • Noise after the receiver filter is the same height as before the filter, but possibly a smaller bandwidth 1 Receiver Filter NI Steve Shellhammer, Intel Corporation

11. Bit Error Rate • It is assumed that there is formula for BER for the receiver in AWGN. ber() = BER versus SNR for AWGN. • There are two periods of stationarity when we want to calculate the BER (which will help us get PER) • During a portion of the received packet when there is no interference during the packet • During a portion of the received packet when there is no interference during the packet Steve Shellhammer, Intel Corporation

12. Bit Error Rate • BER when there is no interference is based on thermal noise. • Since this is not very high we can • Assume it is very low • Or set up realistic topology and calculate BER • Since absolute performance is not a primary concern method one is recommended. Steve Shellhammer, Intel Corporation

13. Bit Error Rate • BER when interference is present is based on equivalent AWGN. • Pick AWGN level that would give equivalent power after the receiver filter. Steve Shellhammer, Intel Corporation

14. Bit Error Rate 1 Receiver Filter BF NI BAF BAF ) ( NI BF BAF Steve Shellhammer, Intel Corporation

15. Effective AWGN • Power after receiver is NI BAF • To get the same power after filter we have to have, Neff BF = NI BAF • The issue is that the interfere may not be as wide as filter. So we are dropping the PSD and widening the bandwidth • This is another approximation Steve Shellhammer, Intel Corporation

16. Bit Error Rate Summary • We now have a method to calculate the BER when there is no interference and when there is interference. • Calculate Eb from path-loss • With no interference use N0 • With interference use Neff • Can also add N0 to Neff Steve Shellhammer, Intel Corporation

17. Packet Error Rate • A packet in the network under test (NUT) is sent from transmitter to the receiver. There is a (possible) overlap between that packet and an interfering pulse. TD T Steve Shellhammer, Intel Corporation

18. Probability Analysis • Calculate probability density function for the random variable T. (Work still to be done). • T is a mixed random variable. There will be a finite probability that T is zero, and some density function over the interval (0,TD) Steve Shellhammer, Intel Corporation

19. Probability Density of T • An example of a PDF for T ½ fT(t) 1/(2T) 0 TD Steve Shellhammer, Intel Corporation

20. Packet Error Rate • Step 1 • Calculate PER for a fixed value of T • Step 2 • Average over all values of T using the previously calculated PDF for T • Step 3 • If necessary, average over packet duration, TD, assuming it is variable Steve Shellhammer, Intel Corporation

21. Other Metrics • Calculate other metrics based on PER and necessary approximations (e.g. independence) • Throughput • Latency • Packet Loss Rate (assuming a fixed time to complete transmission) • Other Steve Shellhammer, Intel Corporation

22. Conclusions • Outlined an approach to analytic solution. • Next steps • Work out technique for determining PDF of collision time. • Write up document giving details. • Apply to an example and use for comparison with other techniques. Steve Shellhammer, Intel Corporation