david s. l. wei joint work with alex chia-chun hsu and c.-c. jay kuo

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


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


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


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


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


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

Adaptive Frequency Hopping

Fre

quen

cy

2.4 GHz2.4015

2.4835

2.4805

BT

2.4370

Wi-Fi

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


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


Determine Threshold

DIFS BW DATA

hdr

ACK

DIFS BW DATA1 ACK1 DATA2 ACK2


Retransmission

DIFS BW

Double the backoff window

ACKDATAh TSIFSTTBWDIFS

n

TTBWSIFSDIFS DATAoh

ACKhoh TSIFSTT 2


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


)'

)(''())(1'('1 n


If true, then state transition is beneficial


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


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


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


BW DATA1 ACK1 DATA2 ACK2

DATA2 ACK2DIFS BW

With optimization

DIFS DATA2 ACK2DIFS


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

PER equation validation

Result

Simulation

Threshold equation validation Throughput improvement

Result

Simulation: ACL link

Throughput of the Wi-Fi and BT in the presence of BT ACL link

Result

Simulation: 2 SCO links

Throughput of the Wi-Fi and BT in the presence of 2 SCO links between BT master/slave

Result

Simulation: delay

Average Wi-Fi delay vs. BT traffic load

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


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, …


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


Cognitive Radio: spectrum hole

F1

F2

F3

F4

F5

Exclude Spectrum

Gray space White space

Spectrum hole

Time

Frequency

Thanks ^_^

david s. l. wei joint work with alex chia-chun hsu and c.-c. jay kuo

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