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Characterization of an IEEE 802.11a Receiver using Measurements in an Indoor Environment

Characterization of an IEEE 802.11a Receiver using Measurements in an Indoor Environment. Amit K. Vyas (amitvyas@cs.stanford.edu) Fouad A. Tobagi (tobagi@stanford.edu) Presented in-part at GLOBECOM 2006. Motivation. Wireless Link Performance depends on various factors, such as

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Characterization of an IEEE 802.11a Receiver using Measurements in an Indoor Environment

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  1. Characterization of an IEEE 802.11a Receiver using Measurements in an Indoor Environment Amit K. Vyas (amitvyas@cs.stanford.edu) Fouad A. Tobagi (tobagi@stanford.edu) Presented in-part at GLOBECOM 2006 Wireless Mesh Networks (AIIT)

  2. Motivation • Wireless Link Performance depends on various factors, such as • Channel Realization (varies with time, distance and specific locations) • Data Rate • Packet Size • Transmission Power • Mobility • Interference and Blocking in the Presence of multiple nodes • How does the link performance vary with these factors ? • How do we accurately represent wireless links in simulators ? • These are essential for • Realistic assessment of network performance • Performance improvement by proper tuning of operational parameters and design of better protocols Wireless Mesh Networks (AIIT)

  3. Wireless Link Performance • Two components: • Wireless Channel Characteristics • Depends on environment – varies with time, distance, specific locations, mobility • Path loss studied by a number of people using measurements at 5 GHz in different settings – Medbo et al [1], Bergljung et al [2], Nobles et al [3], Cheung et al [4] • Receiver Characteristics (PER Performance) • Depends on particular device – varies with received signal strength, noise, interference, data rate and packet size • No published measurements for OFDM/802.11ag (to the best of our knowledge) Wireless Mesh Networks (AIIT)

  4. Outline of Talk • Measurement Setup & Environment • Wireless Channel Characteristics • Receiver Model • Synchronization • Data Reception • Conclusions Wireless Mesh Networks (AIIT)

  5. Measurement Setup • Measurement Tool • Developed at Stanford university in collaboration with France Telecom R&D – based on open source Multi-band Atheros Driver for WiFi (also known as MADWIFI) • Captures packet-level traces – includes micro-second timestamps, SNR, different PHY errors (timing error; signal parity error; illegal rate, length or service field; error due to power drop; CRC error etc.), and complete MAC header • Consists of kernel module and user module, which interact through IOCTL. Uses a double buffering system to capture packets in real time, while data is transferred to user space • Systems • HP/Compaq Laptops (nc6110) running Redhat Fedora Core 4 • IEEE 802.11 a/b/g wireless cards (Orinoco Gold 8480-WD Combo Card, from Proxim Inc) Wireless Mesh Networks (AIIT)

  6. Measurement Environment Stanford University’s Computer Science Building – third floor Wireless Mesh Networks (AIIT)

  7. Variability with Time and Locations • Three locations at the same distance in the same rooms (11 m, across 4 walls) • Locations = ±1 m laterally • SNR ~ [12, 26] dB, i.e. range of 14 dB • Shows the margin of safety that should be allowed in placement of devices or relays to guard against variability with time and small location changes Wireless Mesh Networks (AIIT)

  8. Path Loss in Different Scenarios Prior Work Results (γ = Path Loss Exponent σ = Standard Deviation) • γ = 1.3 – 2.15 • σ = 2.2 – 4.2 • γ = 2.9 – 5.0 • σ = 2.1 – 3.5 • γ = 4.1 – 7.0 • σ = 2.7 – 5.01 • Lower γ in R-R and R-C indicates lower attenuation in our walls • Higher σ in R-R is because cases include different number of walls at each distance, e.g. cases with both 3 & 4 walls at 10 m Wireless Mesh Networks (AIIT)

  9. Path Loss Exponents Wireless Mesh Networks (AIIT)

  10. Carrier Synchronization Physical Layer (PLCP) Header Data Reception (CRC check) Receiver Characteristics • Packet Reception using 802.11a: • Synchronization • Preamble is 16 μs long – 10 short symbols and two long symbols • PLCP Header (Signal field) • Information about the type of modulation, coding rate used and length of transmission for the data part of the packet • It is always encoded using BPSK, and sent at 6 Mbps Wireless Mesh Networks (AIIT)

  11. Carrier Synchronization • Synchronization – Easily achieved for SNR values > 25 dB Never achieved for SNR values < 10 dB • Two cases of intermediate SNRs illustrated above, resulting in intermediate percentage of synchronization errors Wireless Mesh Networks (AIIT)

  12. Synchronization for Different Wireless Cards • Each point represents % sync error in one second interval (400 packets) vs. SNR averaged over that interval • Fitted model works well for wireless cards ‘c1’ and ‘c2’ • Anomalous behavior observed for wireless card ‘c3’ Wireless Mesh Networks (AIIT)

  13. Synchronization Model • Difference between 5th and 95th percentiles is not much • For representation in simulator, one may use the average, 50th percentile, or provide a table with complete distribution Wireless Mesh Networks (AIIT)

  14. Impact of Interference Without Interference – Sync Error vs. SNR With Interference – Sync Error vs. SINR • Current simulators consider 802.11 packet interference to be equivalent to noise – we would expect some difference because interference is a more controlled signal • Our experiments show around 5 dB shift in the Sync Error curve • Enough to make a significant difference in simulation results, especially in multihop networks, where the effect is compounded Wireless Mesh Networks (AIIT)

  15. Data Reception at 6-24 Mbps • No CRC Errors observed – if synchronization is achieved, no errors observed in data reception Wireless Mesh Networks (AIIT)

  16. Data Reception at 36-54 Mbps • 36 Mbps - Small spread between 5th and 95th percentile • 54 Mbps - Heavy tail and much wider spread (not illustrated on figure due to loss of clarity) Wireless Mesh Networks (AIIT)

  17. Higher Sensitivity at 54 Mbps • In one test scenario (left figure), even at a high average SNR (~45 dB), fluctuation of 2-3 dB results in PER fluctuating between 0 and 80% • In another test scenario (right figure), PER is around 10% even though average is only around 30 dB Wireless Mesh Networks (AIIT)

  18. Impact of Interference With Interference – CRC Error vs. SINR Without Interference – CRC Error vs. SNR • Around 5 dB shift in the CRC Error curve, similar to impact of interference on synchronization • So for both synchronization and data reception, interference is better behaved than noise Wireless Mesh Networks (AIIT)

  19. Conclusions • Measured Path Loss in an Indoor Environment • Empirical Model for 802.11a Receiver • Models for Synchronization and Data Reception • Gains from Study • Simulator Models – Path loss parameters and receiver model for an indoor environment and receiver • Impact of Interference - Around 5 dB shift in error curves with and without interference • Margin of Safety – Best to provision for 10-15 dB fluctuation with time and small location changes • Stable Operation – Receivers considered are very sensitive to fluctuations at 54 Mbps – better to operate at 24/36 Mbps, which yields good performance with minimum sensitivity to SNR fluctuations Wireless Mesh Networks (AIIT)

  20. References • J. Medbo and J.-E. Berg, “Simple and accurate path loss modeling at 5 GHz in indoor environments with corridors,” in Proceedings of the Vehicular Technology Conference. IEEE 52nd VTS-Fall VTC, vol. 1, Sept. 24-28, 2000, pp. 30-36. • C. Bergljung and P. Karlsson, “Propagation characteristics for indoor broadband radio access networks in the 5 GHz band,” in Proceedings of the Ninth IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, vol. 2, Sept. 1998, pp. 612-616. • P. Nobles, D. Ashworth, and F. Halsall, “Indoor radiowave propagation measurements at frequencies up to 20 GHz,” in Proceedings of the Vehicular Technology Conference. IEEE 44th, vol. 2, June 1994, pp. 873-877. • D. Cheung and C. Prettie, “A path loss comparison between the 5 GHz UNII band (802.11a) and the 2.4 GHz ISM band (802.11b),” White Paper, Intel Labs, Intel Corporation, Jan. 2002. Part of this talk published as: A. Vyas, F. Tobagi and R. Narayanan, “Characterization of an IEEE 802.11a Receiver using Measurements in an Indoor Environment,” in Proc. IEEE GLOBECOM 2006. The 49th Annual IEEE Global Telecommunications Conference, San Francisco, California (USA), Nov 27 – Dec 1, 2006. Wireless Mesh Networks (AIIT)

  21. Thank You !! Wireless Mesh Networks (AIIT)

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