Adaptive Subcarrier Nulling: Enabling Partial Spectrum

Sharing in Wireless LANs

The University of Michigan

Kang G. Shinkgshin@eecs.umich.edu

Xinyu Zhangxyzhang@eecs.umich.edu

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 20MHz

20MHz

20MHz

20MHz

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

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

20MHz

20MHz

20MHz 20MHz

20MHz

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 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

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!