copyright 2002

On the Throughputof Bluetooth

Data Transmissions

Matthew C. Valenti

Assistant Professor

Lane Dept. of Comp. Sci. & Elect. Eng.

West Virginia University

Morgantown, WV

mvalenti@wvu.edu

Max Robert and Jeffrey H. Reed

Mobile & Portable Radio Research Group

Virginia Tech

Blacksburgh, VA

This work was supported by the Office of Naval Researchunder grant N00014-00-0655

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Motivation & Goals

Motivation Why Bluetooth? Custom error control = better QoS Suitable benchmarks?

• QoS analysis of bottom two layers: Radio/Baseband

Goal of this study To obtain analytical expressions for the maximum

average throughput of Bluetooth data transmissions

Can be used as a “benchmark” to compare custom error control protocols and practical receiver designs

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Overview of Talk

Overview of the analysis Baseband layer analysis

• Exact maximum average throughput over a binary symmetric channel (BSC)

• Benchmark for custom error control Radio layer analysis

• Upper bound on throughput of noncoherent reception over AWGN and quasi-static Rayleigh fading channels

• Benchmark for receiver implementation

Extensions of this work Custom error control in Bluetooth Analysis of other QoS parameters: delay & delay

jitter

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Bluetooth Protocol Stack

Application

Presentation

Session

Transport

Network

Data Link

Physical

Applications

RFCOMM/

Service Discovery Protocol (SDP)

Logical Link Control & Adaptation Protocol (L2CAP)

Host Controller Interface (HCI)

Link Manager

Link Controller

Baseband

RadioOSI Reference Model

Bluetooth

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Bluetooth Baseband Layer

Physical links Asynchronous Connection-Less (ACL) Synchronous Connection Oriented (SCO)

Automatic-Repeat Request (ARQ) ACL packets use a CRC code and ARQ.

• Stop-and-wait protocol (single-bit-flag). SCO packets do not use a CRC or ARQ.

Forward Error Correction (FEC) Packets may be coded with a FEC Both ACL & SCO can use (15,10) Hamming code. SCO packets can use a triple repetition code.

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Frequency Hopping

Transmissions are broken into 625 sec slots Each piconet is synchronized to the master’s clock Time-Division Duplexing (TDD)

• Master may only begin transmitting on even indexed slots• Slaves may only begin transmitting on odd indexed slots

A transmission may last for 1, 3, or 5 slots

Frequency Hopping Radio hops after each transmission

• Does not hop during a multislot transmission Hops through 79 channels

• Each channel is 1 MHz wide• Some countries only allow 23 channels

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ACL Packet Structure

Access Code PayloadPacketHeader

72 bits 54 bits

PayloadHeader

Payload Data CRC

0-2744 bits

8 or 16 bits

16 bits

0-2712 bits

ACL Packet Types

There are 6 ACL packets that use ARQ:

Packet Type

Duration (slots)

Payload data

length (Bytes)

Hamming FEC code?

Peak

throughput

DM1 1 17 Yes 108.8 kbps

DH1 1 27 No 172.8 kbps

DM3 3 121 Yes 387.2 kbps

DH3 3 183 No 585.6 kbps

DM5 5 224 Yes 477.9 kbps

DH5 5 339 No 723.2 kbps

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Probability of Retransmission

A retransmission will occur unless all of the following events occur: Sf: Destination radio synchronizes with the access code of

forward packet Hf: Destination radio decodes forward packet header Lf: Destination radio decodes payload of fwd packet Sr: Source radio synchronizes with the access code of the return

packet Hr: Source radio decodes the return packet header

Thus the probability of retransmission is:Pr() = 1 – P[Sf]P[Hf]P[Lf]P[Sr]P[Hr]

where is the error probability of the demodulator (BER)

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Synchronization

In practice, synchronization is achieved by: Comparing the hard outputs of the demodulator with a stored

copy of the 72 bit access code Synchronize if the received and stored copies of the access code

agree in T bit positions T is a threshold set to an acceptable false alarm rate.

• Minimum Hamming distance between distinct access codes is 13, and therefore up to 6 errors can be tolerated.

• Thus for ML decoding, set T = 66.

Therefore synchronization occurs if there are no more than 72-T bit errors:

T

k

kk

kSP

72

0

72)1(72

][

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Decoding the Header

Header is protected by (3,1) repetition code. 18 information bits (including an 8 bit CRC) 18 three bit code words (triplets) = 54 code bits Code can correct a single error per triplet

Thus probability of correctly decoded header:

1823

181

0

3

131

)1(3

][

k

kk

kHP

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Decoding the Payload

High rate DH packets are uncoded and therefore require that all m bits are correct:

where m=240, 1496, or 2744

Medium rate DM packets have (15,10) single-error correcting Hamming code, thus:

where M=16,100, or 183 is the number of Hamming code words in the payload

MM

k

kk

kLP 1415

1

0

15 1151)1(15

mLP )1(

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Number of Transmissions

Let N be the total number of transmissions before success In order to transmit exactly N times

• First N-1 transmissions must fail• Last transmission must succeed

N is a geometric random variable w/ pmf

Where it is assumed that: is the same throughout a frame (quasi-static)• Channel is uncorrelated from Tx to Tx.

p n P PN r ri

n

[ ] ( ) ( )

11

1

b g

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Average Throughput

Data rate is function of N:

D is number of occupied slots (including return)• D = 2 for Dx1, 4 for Dx3, or 6 for Dx5• We assume only 1 return slot (asymmetric traffic)

K is number of data bits • K= 136, 216, 968, 1464, 1792, or 2712

Average throughput is Ravg = EN[R]

bps )10625)(( 6

DN

KR

Throughput in BSC Channel

10-5 10-4 10-3 10-2 10-10

100

200

300

400

500

600

700

800

Da

ta R

ate

in k

bps

DH5

DH3

DH1

DM5

DM3

DM1

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Bluetooth Radio Layer

Bluetooth uses GFSK modulation Gaussian pulse shaping BT = 0.5 Nonorthogonal frequency shift keying

• 0.28 h 0.35• We assume h = 0.32

1 Megabaud channel symbol rate 1 MHz occupied bandwidth

Reception is normally noncoherent

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Demodulator Error Rate

Because of the ISI induced by Gaussian pulse shaping, an exact expression for noncoherent detection is complicated. Highly implementation dependent

Instead, we use an expression for the exact error performance of noncoherently detected FSK over full-response channels:

This is a lower bound for all receiver architectures

2

2

112

112

)2(sinc

b

a

h

1

2/

2/1

)()(2

1

)(2

1),()(

22

k

k

oba

abkIb

aab

oIe

abIebaQ

5 10 15 200

100

200

300

400

500

600

700

800

=Es/No in dB

Dat

a R

ate

in k

bps

DH5

DH3

DH1

DM5

DM3

DM1

Throughput in AWGN

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Quasi-Static Fading

Quasi-static fading SNR remains same for entire transmission. SNR changes from transmission to transmission.

• Envelope may be Rayleigh, Rician, or Nakagami Good model for Bluetooth

• Short transmissions, uncorrelated channels

Retransmission probability

pdf of the SNR for Rayleigh fading

)()( rr PEP

)(exp1

)( uf

800

0 5 10 15 20 25 300

100

200

300

400

500

600

700

=Es/No in dB

Dat

a R

ate

in k

bps

DH5

DH3

DH1

DM5

DM3

DM1

Throughput in Quasi-StaticRayleigh Fading

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Custom Error Control

There is a seventh ACL packet type AUX1 Occupies one slot CRC & ARQ are turned off 29 bytes of payload data

Can use AUX1 to transport a custom code Because ARQ shut off, data is delivered from

Bluetooth device to host regardless of errors Perform FEC encoding & decoding on host

computer Implement ARQ on the host computer No modification of Bluetooth standard is needed

Dat

a R

ate

in k

bps

5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 100

50

100

150

Es/No in dB

BCH coding bound

BCH t=10

DM1DM 3

Example: BCH Coding in AWGN

Adaptive Coding forQuasi-Static Fading

0 5 10 15 20 250

100

200

300

400

500

600

700

800

Es/No in dB

Dat

a R

ate

in k

bps

DM1

DM3

DM5DH5

BCH10

Adaptive BCH

Fully Adaptive

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Application of Turbo Codesto Bluetooth

Turbo codes are capable of achieving near capacity performance.

Technical challenges: Long block lengths needed Soft-decision decoding is desirable

Solutions: Rate Compatible Turbo Codes (RCPT)

• Start with a rate ⅓ turbo code• 1 frame of data = 4 Bluetooth AUX1 packets• There are 8 packets of parity information• Only transmit parity as needed

“Pseudo” soft-decision decoding• Use the Received Signal Strength Indicator (RSSI)

thro

ughp

ut (

kbps

)

4 5 6 7 8 9 100

20

40

60

80

100

120

140

160

180

200

Es/No in dB

TC4 w/ RSSI

TC4 no RSSI

DM1 DM3

DM5

Performance of Turbo Coded Bluetooth in AWGN

Other QoS Parameters

Delay If packet is transmitted N times, then the delay is =(DN)(625 x 10-6)

Average delay is mean of Delay jitter is standard deviation of

4 5 6 7 8 9 100

2

4

6

8

10

12

14

16

18

20

Es/No in dB

Avg

. de

lay

in m

sec

TC4 w/ RSSITC4 no RSSI

DM1

DM3

DM5

4 5 6 7 8 9 100

2

4

6

8

10

12

14

16

18

20

Es/No in dB

Del

ay ji

tter

in m

sec

TC4 w/ RSSITC4 no RSSI

DM1

DM3

DM5

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Conclusion

Throughput of Bluetooth can be obtained analytically Exact at the baseband layer Upper bound at the radio layer

Analysis can be extended to other QoS parameters Average latency & latency jitter Residual error rate (after timeout)

Provides a benchmark Compare custom error control codes Compare receiver implementations

Further work More exact expression for BER of demodulator Implement custom coding using AUX1 packet