Throughput and Loss Packet Performance of DCF with Variable Transmit Power

1. Throughput and Loss Packet Performance of DCF with Variable Transmit Power Steven D. Gray and Venkatesh Vadde [steven.gray, venkatesh.vadde]@nokia.com Nokia Research Center Irving, TX S. Gray, Nokia

2. Outline • Motivation • CCA performance • Probability of detection analytical model • Simulation • Test cases • DCF simulation • Throughput and delay S. Gray, Nokia

3. Motivation for Analysis • Proposals have been offered to allow STAs to adjust power on an individual basis • In discussion, many expressed the opinion that this will cause “hidden terminals” • Quantitative results are offered to illustrate the performance (delay and throughput) of a networking using two different power settings • (E)DCF and PCF/HCF • DCF STAs must transmit at a power level to achieve a target packet loss rate AND keep others off the channel • PCF/HCF must only transmit at a power level to achieve a target packet loss rate S. Gray, Nokia

4. CCA Probability of Detection • IEEE802.11a states: “A start of a valid OFDM transmission at receiver level equal or greater than 6 Mbits/s sensitivity (-82dBm) shall cause CCA to indicate Busy with probability >90% within 4 us. If the preamble portion was missed, the receiver shall hold the CS signal Busy for any signal 20 dB above the minimum 6 Mbits/s sensitivity (-62 dBm)” S. Gray, Nokia

5. CCA Test: Analytical Model • Many ways exist for doing CCA. One method is to use a delay and correlate filter based upon the periodicity of the PLCP Preamble • General structure of a delay and correlate filter are the sampled time-series from the PLCP Preamble Dthe periodicity in the short symbols S. Gray, Nokia

6. CCA Test: Analytical Model • General approach for CCA is to define where is a normalizing signal such that S. Gray, Nokia

7. CCA Test: Analytical Model Consider a specular channel Let the received signal be modeled as is the transmitted OFDM signal is background noise S. Gray, Nokia

8. The essence of a delay and correlate filter is where conditioned on a valid PLCP preamble being sent • If is complex Gaussian then is exponential CCA Test: Analytical Model S. Gray, Nokia

9. CCA Test: Analytical Model • Probability of detection can be defined by considering the distribution of a sum of non iid exponential random variables (CLT can not typically be invoked because number of multipaths are few in number i.e., five) • For ease of analysis, two multipaths are considered S. Gray, Nokia

10. CCA Test: Analytical Model If is distributed exponential with the pdf for z is then if S. Gray, Nokia

11. CCA Test: Analytical Model • To simplify analysis, background noise is modeled as a constant level • The probability of detection can be defined as S. Gray, Nokia

12. Test Case PLCP Preamble Detection SNRSTA1= 8 dB SNRSTA2= 8 dB AP 60 meters OBS 20 meters LOS STA 1 STA 2 63 meters OBS TX Power = 5 dBm TX Power = 23 dBm • The signal power of STA 1 seen at STA2 = -11 dB • The signal power of STA 2 seen at STA1 = 7 dB S. Gray, Nokia

13. Fresnel distance Reference path loss (1 meter) Test Case • Line of Sight (LOS) path loss • Obstucted Line of Sight (OBS) path loss Feuerstein, M.J., Blackard, K.L., Rappaport, T.S., Seidel, S.Y., and Xia, H.H., “Path Loss, Delay Spread and Outage Models as Functions of Antenna Height for Microcellular System Design,” IEEE Transactions on Vehicular Technology, Vol. 43, No 3, pp. 487- 498 August 1994. S. Gray, Nokia

14. Test Case • Two tap channel • tap 1 = 0.7 • tap 2 = 0.3 • Probability of false • alarm at SNR = 6 dB • is approx 9 % • Pd = 0.011 at • SNR = - 11dB S. Gray, Nokia

15. DCF Simulation Procedure • Traffic evenly distributed between different users • Poisson arrivals for traffic; • Uniformly distributed packet sizes [500, 1500] bits • 12Mb/s PHY assumed, average packet length = 1000 bits • Average PHY-related packet error rate = 1% • Simulations averaged over 106 slot-times S. Gray, Nokia

16. Hidden-node DCF Simulations • Assume 10-30 users, randomly allocated power levels • 2 power-levels defined: High and Low • Probability{detection of low-power users} = 1% • Probability{detection of high-power users} = 90% • Number of retransmissions allowed = 3 • Initial back-off value = 16 slot-times • (1 slot-time = 9 micro-sec) S. Gray, Nokia

17. Throughput Results - 1 10-30% of network throughput can be lost with hidden-nodes S. Gray, Nokia

18. 4 x 10 14 12 10 Arvls 8 Txmns,0L:100H Packets 6 Txmns,50L:50H 4 2 0 10 20 30 40 50 60 70 80 DCF Traffic Load (%) Throughput Results - 2 S. Gray, Nokia

19. 4 x 10 14 12 10 Total Arrived pkts 8 Packets 6 High power pkts Txed 4 2 Low power pkts Txed 0 0 20 40 60 80 DCF Traffic Load (%) Throughput of High-vs-Low Power Packets Network composition assumed = 50%low:50% High Conclusion: Low power STAs suffer poor throughput as load is increased S. Gray, Nokia

20. Dropped Packet Results Up to 70 % increase in dropped packets for network S. Gray, Nokia

21. Conclusions • Throughput is decreased when large variations of power are used by STAs in a BSS • Low power STAs increase the number of hidden terminals • Multiple retransmission of packets required to ensure success • Battery saving desired with power reduction may be lost S. Gray, Nokia