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Adaptive Subcarrier Nulling : Enabling Partial Spectrum Sharing in Wireless LANs

Adaptive Subcarrier Nulling : Enabling Partial Spectrum Sharing in Wireless LANs. Xinyu Zhang xyzhang@eecs.umich.edu. Kang G. Shin kgshin@eecs.umich.edu. The University of Michigan. Current WiFi channelization. Channel 20. Channel 1. Channel 2. Channel 1. Channel 11. Channel 6.

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Adaptive Subcarrier Nulling : Enabling Partial Spectrum Sharing in Wireless LANs

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  1. Adaptive Subcarrier Nulling: Enabling Partial Spectrum Sharing in Wireless LANs XinyuZhang xyzhang@eecs.umich.edu Kang G. Shin kgshin@eecs.umich.edu The University of Michigan

  2. Current WiFi channelization Channel 20 Channel 1 Channel 2 Channel 1 Channel 11 Channel 6 2.4GHz band 5 GHz band Common deployment: use 1, 6, 11 only 3 non-overlapping 20MHz channels 20 non-overlapping 20MHz channels Neighboring WLANs’s channels are either non-overlap or full-overlap

  3. Trends towards partial spectrum sharing (1/2) MHz MHz Standard MHz MHz MHz MHz ??? MHz Evolution of WiFi channel width Consequence: partial spectrum sharing between wideband and narrowband channels 20MHz 40MHz channel

  4. Trends towards partial spectrum sharing (2/2) Channel 1 Channel 11 Channel 6 Unmanaged, densely deployed WLANs Consequence: partially-overlapped channels between adjacent WLANs

  5. Is partial spectrum sharing beneficial? A. Mishra, V. Shrivastava, S. Banerjee, and W. Arbaugh, “Partially Overlapped Channels Not Considered Harmful,” in SIGMETRICS, 2006. (a) DSSS PHY (802.11b) (b) OFDM PHY (802.11a/g/n/ac…) Experiments: interference due to partial spectrum sharing Partially-overlapped channels cause severe interference for OFDM based 802.11 networks! Partially-overlapped channels cannot transmit concurrently

  6. Problems caused by partial spectrum sharing 20MHz 20MHz 40MHz channel 20MHz 20MHz 20MHz 20MHz 20MHz 20MHz 40MHz channel Partial channel blocking Middle channel starvation The middle-channel can transmit only when all other channels are idle When one channel is active, half of the other channel is wasted

  7. Problems caused by partial spectrum sharing WLAN B WLAN C WLAN A WLAN A is starved! Experimental observation WLAN A/B is blocked!

  8. Adaptive subcarrier nulling (ASN) 20MHz busy channel Null busy subcarriers Reuse other subcarriers 40MHz channel OFDM channel consists of small spectrum units (subcarriers) ASN nulls subcarriers occupied by neighboring WLANs, and reuse those idle subcarriers. Overall improvement in spectrum utilization: 26.7MHz 40MHz

  9. ASN enables partial spectrum sharing Spectrum utilization 20MHz 20MHz 20MHz 30MHz Middle- channel starved Fair access to shared spectrum 20MHz 20MHz 40MHz channel Middle-channel starved Fair access to shared spectrum 20MHz 20MHz 20MHz

  10. Challenges PHY layer MAC layer Performing subcarrier nulling on a per-packet basis Random access to part of the channel Sensing partially-occupied channel Detecting, synchronizing, and decoding a packet, without priori knowledge of its spectrum Fair access to shared subcarriers

  11. Sensing subband: temporal/frequency sensing Power-spectrum-density (PSD) Receiving time-domain samples Matching with known pattern Rugularize PSD

  12. Packet detection and TX/RX synchronization Redesigning the 802.11 preamble Ensure each subband contains a unique random sequence Cross-correlation for identifying random sequence

  13. Decoding bits from subbands Add preamble constellation mapping OFDM modulation Add pilot tones Outgoing packet OFDM signals OFDM signals modulated samples {0,1} OFDM demodulation Demapping Detect & Sync Channel estimation demodulated samples {0,1} Continuous channel update Workflow (Pilot-based update)

  14. ASN-aware medium access control (1/2) WLAN2 WLAN3 WLAN1 frequency 40MHz channel ASN with direct access (ASN-DA) Wideband (WLAN 1) manages backoff/sensing/transmission separately for each subband Wideband uses the entire bandwidth only when all other narrowbands are idle (which is rare)

  15. ASN-aware medium access control (2/2) ASN with water-filling access (ASN-WF) Wideband (WLAN1) adapts packet size to create access opportunity to an entire band

  16. Implementation SDR implementation of ASN PHY ns-2 simulation of ASN MAC Based on the GNURadio/USRP2 platform SINR based model with cumulative interference Components: subband sensing; packet detection/synchronization; packet decoding ASN PHY layer with subband sensing and SINR-based packet decoding model

  17. Accuracy of subband sensing Probability of sensing a false bandwidth is low in practical SNR range

  18. Packet decoding probability Br : fraction of bandwidth used for data transmission Decoding probability suffers negligible degradation when only a subband is used for transmission

  19. Solving partial channel blocking 20MHz 40MHz channel Access rate Throughput (Mbps) (# of transmissions per second)

  20. Fairness: ASN-DA vs. ASN-WF ASN-WF provides more fair access to shared subband than ASN-DA

  21. Solving middle-channel starvation 20MHz 20MHz 40MHz channel Access rate Throughput

  22. Conclusion Anomalies in partial spectrum sharing Adaptive subcarrier nulling (ASN) Partial channel blocking Middle channel starvation Null busy subcarriers and access idle subcarriers PHY layer: sensing and decoding partially used spectrum MAC layer: subband-level channel access Performance: highly efficient and fair access to partially-shared spectrum

  23. Thank you!

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