coexistence mechanism using dynamic fragmentation for interference mitigation between wi-fi and...
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Coexistence Mechanism Using Dynamic Fragmentation for Interference Mitigation between Wi-Fi and
Bluetooth
David S. L. Wei
Joint Work with
Alex Chia-Chun Hsu and C.-C. Jay Kuo
Outline
Overview of Wi-Fi and Bluetooth Previous Work Dynamic Fragmentation Algorithm Results Conclusion and Future Work
Content
Overview of Wi-Fi and Bluetooth 802.11/Wi-Fi 802.15.1/Bluetooth Coexistence in the UL band
Previous Work Dynamic Fragmentation Algorithm Results Conclusion and Future Work
Overview of Wi-Fi and Bluetooth
802.11 Wi-Fi
A dominating WLAN standard Medium to high date rate, medium range < 100m Use the ISM band Large selection of commodities
802.11a
802.11b
802.11g
802.11n
Standard
OFDM
DSSS (CCK)
CCK/OFDM
OFDM
Modulation
5GHz
2.4GHz
2.4GHz
2.4GHz
Frequency
6 ~ 54 Mbps
2~11 Mbps
20 ~ 54 Mbps
>100 Mbps
Data Rate
60 ft.
300 ft.
300 ft.
300 ft.
Max. Distance
Industrial, Scientific, medical
Overview of Wi-Fi and Bluetooth
802.11 Medium Access Control
CSMA/CA Carrier Sense Multiple Access / Collision
Avoidance Virtual Carrier Sensing
Request-to-send/Clear-to-send RTS/CTS Network Allocation Vector NAV
Distributed Inter Frame Space
Overview of Wi-Fi and Bluetooth
Example: DCF mode
Distributed Coordination Function
1
2
3
Time
DIFS
DIFS
Backoff Window
BW
BW
RTS
set NAV
Short Inter Frame Space
SIFS
CTS
DATA
ACK
BW*DIFS RTS
Overview of Wi-Fi and Bluetooth
802.15 Bluetooth
Popular WPAN standard Low data rate, low cost, short range < 10m use frequency hopping to avoid collision
1600 hops/s ~ 625 μs in every frequency channel SCO and ACL link
Synchronous Connection-Oriented link Real-time application: voice stream HV-3 link: a packet is generated every 6 time slots
Asynchronous ConnectionLess link Non-time-critical application: data traffic DH-1/3/5 link: one packet occupied 1/3/5 time slot
Overview of Wi-Fi and Bluetooth
Coexistence in the UL band
Time
Fre
quen
cy
2.4 GHz2.4015
2.4835
2.4805
79 MHZ
1 MHZ625 μs
BT
2.4370Channel 6
22 MHZ
Wi-Fi
Overview of Wi-Fi and Bluetooth
Coexistence in the UL band
Packet loss caused by interference Overlap both in time and in frequency Over the SNR threshold
Content
Overview of Wi-Fi and Bluetooth Previous works
Adaptive Frequency Hopping D-OLA and V-OLA Fragmentation
Dynamic Fragmentation Algorithm Results Conclusion and Future work
Previous Work
802.15.2 Coexistence Working Group
Many suggestions on improving coexistence Collaborative solutions
Devices could exchange information Collocated and under a central controller
Non-collaborative solutions No information exchange Most common scenario
Previous Work
Adaptive Frequency Hopping
Enhancement on BT, many variations Non-collaborative solution Distinguish good channels from bad ones Keep the hopping sequence on good
channels more frequently
Previous Work
D-OLA and V-OLA
Proposed by Chiasserini and Rao, Infocom 2004
Data OverLap Avoidance Use different BT packet length to avoid bad chann
els Voice OverLap Avoidance
Wi-Fi estimate the interference pattern of real-time packet
Shorten Transmission or Postpone Transmission Increase delay Not a pure non-collaborative solution
Previous Work
Fragmentation
DIFS BW DATA
hdr
ACK
DIFS BW DATA1 ACK1
No fragmentation
2 fragments
DATA2 ACK2
Previous Work
Fragmentation
Adaptive Fragmentation from 802.15.2 2001 Adjust fragmentation according to Packet Error R
ate PER Many rounds before reach optimal length
Optimal Fragmentation by Howitt 2005 Complexity is too high to determine the optimal fra
gment length at run time No resolution on collision and interference Need simple run time solution
Content
Overview of Wi-Fi and Bluetooth Previous works Dynamic Fragmentation Algorithm
Interference model State diagram of DFA Determine threshold Optimization
Results Conclusion and Future Work
Dynamic Fragmentation Algorithm
Interference model
NBTfFiWi PPER )1(1
DATAhdr ACK
625 μs
366 μs
Pf : Probability of BT hops on Wi-Fi frequency
N : # of BT time slot overlapped by Wi-Fi packet
τBT : Traffic load of BT
σ : utilization of BT time slot
BTfNP
N is crucial
Dynamic Fragmentation Algorithm
State Diagram of DFA
2 states State 1, no fragmentation State 2, DATA → n fragments PER greater than P2, one state
up if possible, further fragmentation
PER lower than P1 → one state down if possible
How to choose P1, P2?
1 2
PER≤P2PER>P2
PER≥P1PER<P1
ACKDATA
hACKDATA
h TSIFSn
TTSIFSTSIFS
n
TTBWDIFS )(
n
TTnBWSIFSDIFS DATAoh
Dynamic Fragmentation Algorithm
Determine Threshold
DIFS BW DATA
hdr
ACK
DIFS BW DATA1 ACK1 DATA2 ACK2
DIFS BW DATA1 ACK1 DATA1 ACK1
Retransmission
DIFS BW
Double the backoff window
ACKDATAh TSIFSTTBWDIFS
n
TTBWSIFSDIFS DATAoh
ACKhoh TSIFSTT 2
Dynamic Fragmentation Algorithm
Determine Threshold
}{)}({n
TTBWSIFSDIFSR
n
TTnBWSIFSDIFS DATA
ohDATA
oh
Time to transfer a packet with n fragments and suffer R retransmissions
))(())(1(1 n
TTRnBWSIFSDIFSR DATAohR i
Compare the transmission time before and after state transition
))(())(1(1 n
TTRnBWSIFSDIFSR DATAohR i
)'
)(''())(1'('1 n
TTRnBWSIFSDIFSR DATAohR i
If true, then state transition is beneficial
Dynamic Fragmentation Algorithm
Determine Threshold
p
pnREnRE i
1][][
R iBWE ][
Case 1: less or equal to BW upper-bound
Case 2: greater than BW upper-bound
slotiREa nTREi ])[)12(2( ][1
21
slotibbaba nTREab ])[)12()(2)12(2( 1
21
Before state transition
12
12
max
min
b
a
CW
CW
P is the current PER
Assume geometric distribution
Dynamic Fragmentation Algorithm
Determine Threshold
p
pn
p
pnRE
NN
'
'
'1
'']'[
BTfFiWi NPPER
pN
Np
N
N
p
p
''
''
How to find PER after state transition?
'][
R iBWE Same as previous slide
Now we have all the parameters to calculate a theoretically correct threshold
Dynamic Fragmentation Algorithm
Optimization
Timing
Cause
Solution
Wi-Fi
From the beginning of a transmission
Traffic Jam
CSMA/CA
Collision
Most likely not
Coexistence (BT)
Coexistence Mechanism
Interference
Transmission failure on following fragments is due to Interference
DIFS BW DATA1 ACK1 DATA2 ACK2
BW DATA1 ACK1 DATA2 ACK2
DATA2 ACK2DIFS BW
With optimization
DIFS DATA2 ACK2DIFS
Dynamic Fragmentation Algorithm
Optimization
))(())(1(1 n
TTRnBWSIFSDIFSR DATAohr i
)'
)(''())(1'('1 n
TTRnBWSIFSDIFSR DATAohr i
If true, then state transition is beneficial
Only first fragment needs backoff window when retransmission
DFAm : with node mobility
DFAs : static network (throughput performance can be optimized)
Content
Overview of Wi-Fi and Bluetooth Previous work Dynamic Fragmentation Algorithm Results Conclusion and Future work
Result
Simulation: 2 SCO links
Throughput of the Wi-Fi and BT in the presence of 2 SCO links between BT master/slave
Content
Overview of Wi-Fi and Bluetooth Previous works Dynamic Fragmentation Algorithm Results Conclusion and Future work
Simple non-collaborative mechanism Increase collision/interference resolution Improve throughput and delay Built a reliable, powerful model
Conclusion and Future work
Conclusion
If PER > 0.6, DFAs 56%, DFAm 30%
Conclusion and Future Work
Cognitive Radio
Existing policy fragmented the
spectrum Bandwidth is scarce and expensive
Good frequencies are taken
Recent measurements by FCC shows 70% of the allocated spectrum is not utilized (US)
Improve spectrum efficiency Unlicensed bands
Need new solution for upcoming wireless service
Conclusion and Future work
Cognitive Radio
Paradigm shift – Cognitive Radio by Mitola 1999
“radio or system that senses its operational electromagnetic environment and can dynamically and autonomously adjust its radio operating parameters to modify system operation, such as maximize throughput, mitigate interference, facilitate interoperability,…”
IEEE 802.22, FCC, DARPA XG, OverDRiVE, SWRF, WWRF, …
Conclusion and Future work
Cognitive Radio
Cognitive radio requirements coexist with legacy wireless systems use their spectrum resources do not interfere with them
Cognitive radio properties RF technology that "listens" to huge swaths of spectrum Knowledge of primary users’ spectrum usage Rules of sharing the available resources Embedded intelligence to determine optimal transmission
based on primary users’ behavior
Conclusion and Future work
Cognitive Radio: spectrum hole
F1
F2
F3
F4
F5
Exclude Spectrum
Gray space White space
Spectrum hole
Time
Frequency