Transcript Ingegneria dell'Informazione

Department of Information Engineering
University of Padova, ITALY
Special Interest Group on
NEtworking & Telecommunications
Carrier–Sense ARQ: Squeezing Out Bluetooth
Performance while Preserving Standard
Compliancy
Andrea Zanella
[email protected]
IEEE ICC 2009 June 14-18 in Dresden, Germany
ICC 2009
June 14-18 in Dresden, Germany
Motivations

Bluetooth was designed to be integrated in portable
battery driven electronic devices 
Energy Saving is a key issue!




Units periodically scan radio channel for valid packets
Scanning takes just the time for a valid packet to be recognized
Units that are not addressed by any valid packet are active for less
than 10% of the time
WPAN market is expanding and it aims at becoming the
standard the facto for short range communications 
High Throughput is very welcome!

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Bluetooth v2.0 + EDR (Enhanced Data Rate) promise bit rates up to 3
Mbps and faster node connections
June 14-18 in Dresden, Germany
Retransmissions
A
B
B
B
B
B
NAK
MASTER
ACK
G
SLAVE
A
 Automatic
F
X
H
H
B
X
DPCK
DPCK
Retransmission Query (ARQ):
 Each
data packet is transmitted and retransmitted until positive
acknowledge is returned by the destination
 Negative
acknowledgement is implicitly assumed!

Errors on return packet determine transmission of duplicate packets (DUPCK)

Slave filters out DUPCKs by checking their sequence number
 DUPCKs
waste energy & throughput!!!
A. Zanella "A Mathematical Framework for the Performance Analysis of Bluetooth with
Enhanced Data Rate" , IEEE Transactions on Communications, August 2009
 M. Valenti and M. Robert, “Custom coding, adaptive rate control, and distributed
June 14-18 in Dresden, Germany
ICC 2009detection for bluetooth,” VTC 2002

Aims of the work

Goal:


Improve performance by reducing the number of DUPCKs!!!
Method

Enhance the native ARQ scheme with Carrier Sense to avoid
useless retransmissions


ICC 2009
Carrier sensing is generally provided by last generation chipset
Carrier-Sense ARQ  CS-ARQ
June 14-18 in Dresden, Germany
Key idea

Carrier-sense can be used to infer
transmission outcome in case slave’s reply
is not received!

Idle channel: slave did not received master’s
pck and, hence, did not reply
 RTX

Busy channel: slave sent back ACK or NAK
that got lost
 RTX
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is needed
may generate DUPCK!!!
June 14-18 in Dresden, Germany
Key idea (cont)
To avoid tx of DUPCKs, any time the
slave’s feedback id doubtful (busy channel
with no valid packet) enter a soliciting
phase
 In the soliciting phase

Suspend the ARQ scheme
 Keep sending POLL packets until a valid
frame is returned by the slave
 Resume the ARQ scheme using the ACK info
carried
by
the
slave’s
frame
June 14-18 in Dresden, Germany
ICC 2009

CS-ARQ: state diagram
Fetch a new
packet
Tx packet & wait
for feedback
NAK
Slave’s
feedback
ACK
ACK
No valid pck rcvd
BUSY
IDLE
Carrier Sense
Slave’s
feedback
Tx POLL
No valid pck rcvd
ICC 2009
June 14-18 in Dresden, Germany
NAK
Mathematical model (I)

Assumptions:

Single slave piconet

Saturated links


Unlimited retransmission attempts


Unique packet type per connection
Independent error events in successive transmissions


Packets are transmitted over and over again until positive acknowledgement
Static Segmentation & Reassembly policy


Master and slave have always packets waiting for transmission
Justified by FH
Model

CS-ARQ behavior described by by means of a 2-State Markov Chain

Define appropriate reward functions


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Data, Energy, Time
Apply renewal reward theorem to get system performance

Throughput, energy efficiency, energy balancing, …June 14-18 in Dresden, Germany
Mathematical Model (II)

PAS
1-PAS
1-PSA
A
S
PSA

State A  Standard ARQ phase


Master transmits packets that have never been
correctly received by the slave
State S  Soliciting Phase

Master transmits POLLs waiting for a valid
reply frame
State transition probabilities depend
on the reception events…

AC ok, HEAD ok, CRC error
AC ok, HEAD error
Af = AC failure

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AC ok, HEAD ok, CRC ok
Hf = HEAD failure


Master rx
Df = Data failure


Slaves rx
Ds = Data successful


Reception
Event Index
AC error
June 14-18 in Dresden, Germany
Mathematical Model (III)

PAS
1-PAS
1-PSA
A
S


PSA

State A  Standard ARQ phase
Master transmits packets that have never been
correctly received by the slave
State S  Soliciting Phase

Master transmits POLLs waiting for a valid
reply frame
The steady-state probabilities are, then,
PAS
A 
PAS  PSA
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PSA
D 
PAS  PSA
June 14-18 in Dresden, Germany
Reward Functions

For each state j we define the following reward functions

Tj= Average amount of time spent in state j

Dj(x)= Average amount of data delivered by unit x{M,S}

Wj(x)= Average amount of energy consumed by unit x{M,S}

The average amount of reward earned in state j is given by
T

 jT j
E j E

Performance indexes


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D
( x)


( x)
jDj
W
( x)

E j E
 W
j
( x)
j
E j E
Energy Efficiency: 
D  D  D
  lim
 (S )
(M )
  W  
W W
Goodput: G
D  D
G  lim

  T  
(S )
(M )
(S )
(M )
D
T
June 14-18 in Dresden, Germany
Performance Analysis
Results
ICC 2009
June 14-18 in Dresden, Germany
Case study scenario
The model is general and can be applied
to any other scenario
 We here focus on a simple scenario as an
example of the results that analysis
enabled by the model
 Case-study

Rayleigh fading
 Same SNR at master & slave
 Downlink traffic only

ICC 2009
June 14-18 in Dresden, Germany
Std ARQ
goodput
Goodput

CS-ARQ yields from 20 kb/s to 100 kb/s goodput gain, provided that the correct
packet format is selected


Single-slot packet formats do not bring any improvement
2EDR is preferbale for SNR<20 dB, 3EDR for SNR>20 dB
ICC 2009
June 14-18 in Dresden, Germany
Energy efficiency
Std ARQ
energy
efficiency


CS-ARQ yields from 5% up to 40% energy efficiency gain
2EDR is preferbale for SNR<20 dB, 3EDR for SNR>20 dB
ICC 2009
June 14-18 in Dresden, Germany
Conclusions

CS-ARQ yields some performance gain almost
for free


Only requires CS capability and very limited software
changes
Furthermore, CS-ARQ is



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Fully Standard compliant
Complementary to other performance enhancement
schemes
Stand-Alone (suffices to be implemented by the
master)
June 14-18 in Dresden, Germany