pptx - Xinyu Zhang

Report
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
5 GHz band
20 non-overlapping 20MHz channels
Channel 1
Channel 2
Channel 20
2.4GHz band
3 non-overlapping 20MHz channels
Common deployment: use 1, 6, 11 only
Channel 1
Channel 6
Channel 11
Neighboring WLANs’s channels are either non-overlap
or full-overlap
Trends towards partial spectrum sharing (1/2)
Evolution of WiFi channel width
802.11-2007
802.11-2007
Standard 802.11a/b/g
802.11n
802.11ac
5 MHz
10 MHz
20 MHz
40MHz
80 MHz
802.11ac
160 MHz
??? MHz
802.11??
Consequence:
partial spectrum sharing between
wideband and narrowband channels
20MHz
40MHz channel
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?
A. Mishra, V. Shrivastava, S. Banerjee, and W. Arbaugh, “Partially
Overlapped Channels Not Considered Harmful,” in SIGMETRICS, 2006.
Experiments: interference due to partial spectrum sharing
(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
20MHz
20MHz
40MHz channel
20MHz
When one channel is active, half of the other channel is wasted
Middle channel starvation
20MHz
20MHz
40MHz channel
20MHz
20MHz
20MHz
The middle-channel can transmit only when all other channels
are idle
Problems caused by partial spectrum sharing
Experimental observation
WLAN B
WLAN C
WLAN A
WLAN A is
starved!
WLAN A/B is
blocked!
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
ASN enables partial spectrum sharing
20MHz
20MHz
20MHz
20MHz
40MHz channel
20MHz
20MHz
20MHz
Spectrum utilization
20MHz
30MHz
Middlechannel
starved
Fair access
to shared
spectrum
Middlechannel
starved
Fair access
to shared
spectrum
Challenges
PHY layer
Sensing partially-occupied channel
Performing subcarrier nulling on a per-packet basis
Detecting, synchronizing, and decoding a packet, without priori
knowledge of its spectrum
MAC layer
Random access to part of the channel
Fair access to shared subcarriers
Sensing subband: temporal/frequency sensing
Receiving time-domain samples
Power-spectrum-density (PSD)
Matching with known pattern
Rugularize PSD
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
Decoding bits from subbands
Workflow
Add
constellation
OFDM
Add pilot
mapping
modulation preamble
tones
Outgoing
OFDM
OFDM
modulated
{0,1}
packet
signals
signals
samples
OFDM
Demapping
demodulation
Channel
demodulated
{0,1}
estimation
samples
Detect &
Sync
Continuous
(Pilot-based
channel
update)
update
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)
WLAN2
WLAN3
WLAN1
40MHz channel
frequency
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
Components:
subband sensing; packet detection/synchronization;
packet decoding
ns-2 simulation of ASN MAC
SINR based model with cumulative interference
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
20MHz
40MHz channel
Throughput
(Mbps)
Access rate
(# of transmissions per second)
Fairness: ASN-DA vs. ASN-WF
ASN-WF provides more fair access to shared subband than
ASN-DA
Solving middle-channel starvation
20MHz
20MHz
40MHz channel
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!

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