Adaptive Subcarrier Nulling: Enabling Partial Spectrum Sharing in Wireless LANs The University of Michigan Kang G. Shin Xinyu Zhang

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

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

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

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

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

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

Adaptive subcarrier nulling (ASN) 20MHz busy channel 40MHz channel Null busy subcarriers Reuse other subcarriers 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: 26.7MHz40MHz

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

Challenges PHY layer Performing subcarrier nulling on a per-packet basis MAC layer 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

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

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

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

ASN-aware medium access control (1/2) 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) WLAN1 WLAN2 WLAN3 frequency 40MHz channel

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

Implementation SDR implementation of ASN PHY Based on the GNURadio/USRP2 platform ns-2 simulation of ASN MAC 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

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

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

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

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

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

Conclusion Anomalies in partial spectrum sharing Partial channel blocking Middle channel starvation Adaptive subcarrier nulling (ASN) 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

Thank you!