adaptive subcarrier nulling : enabling partial spectrum sharing in wireless lans
DESCRIPTION
Adaptive Subcarrier Nulling : Enabling Partial Spectrum Sharing in Wireless LANs. Xinyu Zhang [email protected]. Kang G. Shin [email protected]. The University of Michigan. Current WiFi channelization. Channel 20. Channel 1. Channel 2. Channel 1. Channel 11. Channel 6. - PowerPoint PPT PresentationTRANSCRIPT
Adaptive Subcarrier Nulling: Enabling Partial Spectrum
Sharing in Wireless LANs
The University of Michigan
Kang G. [email protected]
Xinyu [email protected]
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 1 Channel 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
802.11??
802.11ac
802.11ac
802.11n
802.11a/b/g
802.11-2007
802.11-2007
160
80
40
20
10
5
Standard
???
MHz
MHz
MHz
MHz
MHz
MHz
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, “PartiallyOverlapped Channels Not Considered Harmful,” in SIGMETRICS, 2006.
(a) DSSS PHY (802.11b) (b) OFDM PHY (802.11a/g/n/ac…)
Partially-overlapped channels cause severe interference for OFDM based 802.11 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 20MHz 20MHz 20MHz20MHz
20MHz20MHz
Problems caused by partial spectrum sharing
Experimental observation
WLAN B WLAN CWLAN 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:
26.7MHz 40MHz
ASN enables partial spectrum sharing
20MHz20MHz
20MHz 20MHz20MHz
20MHz 30MHz
40MHz channel
20MHz 20MHzMiddle- 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 PSDMatching with known pattern
Packet detection and TX/RX synchronization
Redesigning the 802.11 preambleEnsure 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 PHYBased on the GNURadio/USRP2 platform
ns-2 simulation of ASN MACSINR 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
Br : 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 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!