Bluetooth / IEEE 802.11 Coexistence Reliability of IEEE 802.11 WLANs in Presence of Bluetooth Radios...

Bluetooth / IEEE 802.11 Coexistence

Reliability of IEEE 802.11 WLANs in Presence of Bluetooth Radios

Jim Zyren

[email protected]


Conditions for Collision• Frequency Overlap

– Probability of collision is reduced by using a narrower occupied channel width

• Time Overlap– Probability of collision is reduced by minimizing

transmit time (ie transmit at higher data rate)

• Sufficient Interference Energy– Interference energy can be reduced by:

• spatial separation

• tighter filtering

• processing gain


FHSS and DSSS Power Spectral Densities

2.400 GHz

t0 t2t1

FHSS Networks are Frequency Agile2.4835 GHz

2.400 GHz

DSSS Networks typically use 3 fixed non-overlapping Channels2.4835 GHz


DSSS vs. Bluetooth• Early study of impact of Bluetooth on IEEE

802.11 DSSS system– IEEE 802.11 DSSS WLAN @ 11 Mbps

– Dense environment of BT piconets• one DSSS WLAN node per 25 m2 of office space

• BT piconet co-located with each DSSS node

– 0 dBm BT tx power

– +20 dBm DSSS tx power


Enterprise Environment

40 m

IEEE 802.11 AP

IEEE 802.11 STA

BT Piconet

Composite BT/DSSS Network Topology

L.O.S. , range < 8mr3.3 , range > 8m

Lpath = 20 log (4 r / ) r < 8m= 58.5 + 33 log( r/8 ) r > 8m

where: = wavelength @ 2.45 GHz (0.1224 m) r = range (m)

Simplified Propagation Model


Impact of BT Interference Depends on Range from AP

Range to AP (m) Rx DSSS Signal(dBm)

BT InterferenceThreshold (dBm)

Radius of BTInterference (m)

# BT Interferors inRange

4 -32.3 -42.3 1.3 1

10 -41.4 -51.4 3.7 2

20 -51.4 -61.4 10.0 13

20 m4 m

10 m

IEEE 802.11 AP

IEEE 802.11 STA

BT Piconet


BT Piconet User ModelFunction Traffic BT Packet Structure

Paging 1 time / connection IDE-mail 15 / day @ 10 kbytes DH1

Telephony 10 calls / day @ 1 min. each HV3

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

BT Idle 1 Call 2 Calls 3 Calls 4 Calls Paging E-mail Blocked

BT Interference Condition

Pro

bab

ilit

y

R = 20m

R = 10m

R = 4m

BT Single Piconet Utilization

BT Composite Effects


Interference Model

1.94 dwell periods

625 sec

259 sec

BT Transmission slots

1500 byte DSSS Hi Rate Packet(1210 sec)


Thruput of DSSS WLAN vs. Piconet Load

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

1500 750 500 375 300 250 150 100 75

Packet Payload (bytes)

Th

rou

gh

pu

t (M

bp

s)

BT Idle

33% BT Load (1 call)

Paging

100% BT Load (Email)

Effects shown are for a single BT Piconet operating in close proximityto IEEE 802.11 DSSS WLAN.


Availability Curve for DSSS WLAN (r = 4 m)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

1500 750 500 375 300 250 150 100 75


Th

rou

gh

pu

t (M

bp

s)

97.8%

99.9%

100.0%

For r = 4m, only one BT piconet is close enough to cause interference


Availability Curve for DSSS WLAN (r =10 m)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

1500 750 500 375 300 250 150 100 75


Th

rou

gh

pu

t (M

bp

s)

95.6%

99.7%

99.9%

100.0%

At r = 10 m, 2 piconets are close enough to interfere with DSSS receiver


Availability Curve for DSSS WLAN (r = 20 m)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

1500 750 500 375 300 250 150 100 75


Th

rou

gh

pu

t (M

bp

s)

74.9%

95.6%

98.4%

99.5%

At r = 20 m, 13 piconets are close enough to interfere with DSSS reception. IEEE802.11 Hi Rate WLAN can still provide peak THROUGHPUT (7.2 Mbps) with 75%

certainty, and 3.5 Mbps THROUGHPUT with over 99% certainty


Average Effect Over 8 Hour Day

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

1500 750 500 375 300 250 150 100 75


Th

rou

gh

pu

t (M

bp

s)

BT Idle

R = 4m

R = 10m

R = 20m

Average Effect over 8 hour working day is shown as a function of range from DSSS node to DSSS AP.


Summary of BT/DSSS Results• Degree of BT interference depends on:

• local propagation conditions

• density of BT piconets

• BT piconet loading

• DSSS susceptibility to BT interference increases as a function of range from DSSS node to DSSS AP

• DSSS Hi Rate systems retain high throughput and have graceful degradation in the presence of BT interference

• Based on these user models, DSSS Hi Rate WLANs are very reliable in the presence of significant BT interference

• Results are preliminary. Must be verified by lab tests.


IEEE 802.11 FHSS vs. Bluetooth• Preliminary evaluation

– interaction between single BT transmitter and an IEEE 802.11 FHSS link

– 0 dBm BT Tx power, +20 dBm IEEE 802.11 tx power

– Impact estimated under specific BT conditions: Bluetooth idle (establish baseline IEEE 802.11 throughput) BT telephony w/HV3 packet (33% BT piconet load) BT file transfer w/DH1 packet (100% BT piconet load) BT in PAGE mode (worst case)

– Influence of Range and receiver filtering• IF filtering determines bandwidth of susceptibility to BT interference

(about 3 MHz for IEEE 802.11 FHSS Rx)


Rx Desense Values (2FSK)Bluetooth IEEE 802.11 FH HomeRF

CCI -14 dB (2) --- ---

+/- 1 channel -4 dB (2) --- ---

+/- 2 channels 30 dB 30 dB 0 dB



>+/- 5 channels 40 dB 40 dB 35 dB

(1) Vales stated in terms of interference-to-signal ratio(2) Interim values. To be finalized within 18 mos. after release


Influence of Range (1 Mbps)

Signal-to-Interference Ratio depends on:- relative BT and IEEE 802.11 Tx power- Range from IEEE 802.11 node to AP- Range from IEEE 802.11 node to BT Tx

At 1 Mbps and 0 dBm BT Tx Power:- Bandwidth of interference susceptibility is 3 MHz- BT Tx must be approximately 50% closer to node than AP (0 dBm BT Tx power, +20 dBm IEEE 802.11FH Tx Power)

802.11 FH AP

802.11 FH Node

Link Distance (D)

BT Tx Interference Range (D/2)


Throughput of WLAN Operating @ 1Mbps (2FSK) vs BT Load

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1500 750 500 375 300 250 150 100 75


Th

rou

gh

pu

t (M

bp

s)

BT Idle

Telephony (33% BT load)

e-mail (100% BT load)

Paging

Note: Raw data rate for 2FSK = 1 Mbps


Influence of Range (2 Mbps)

At 2 Mbps and 0 dBm BT Tx Power:- Bandwidth of interference susceptibility is 3 MHz- BT Tx must be within roughly same distance to node as AP (0 dBm BT Tx power, +20 dBm IEEE 802.11FH Tx Power)

802.11 FH Access Point

802.11 FH Node

Link Distance (D)

BT Tx

Interference Range (D)


Throughput of WLAN Operating @ 2 Mbps (4FSK) vs BT Load

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

1500 750 500 375 300 250 150 100 75

Packet payload (bytes)

Th

rou

gh

pu

t (M

bp

s)

BT Idle

Telephony (33% BT load)

e-mail (100% BT load)

Paging

Note: Raw data rate for 4FSK = 2 Mbps


Summary of FHSS/BT Results• Narrow channel width of FHSS systems help avoid

interference in frequency domain– Occupied channel for FHSS is about 1 MHz under current FCC

rules (same width for 2FSK and 4FSK)

• Slower transmission rate requires longer transmit time– increases chances BT will hop into channel during transmission and

collide (advantage for 4FSK)

• Multi-level FSK more susceptible to ACI/CCI– 4FSK transmission can be jammed by a weaker BT signal

(advantage 2FSK)

Bluetooth / IEEE 802.11 Coexistence Reliability of IEEE 802.11 WLANs in Presence of Bluetooth Radios...

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