Department of Information Engineering University of Padova, ITALY

ICC 2009 June 14-18 in Dresden, Germany

Department of Information EngineeringUniversity of Padova, ITALY

Carrier–Sense ARQ: Squeezing Out Bluetooth

Performance while Preserving Standard Compliancy

[email protected]

Andrea Zanella

IEEE ICC 2009 June 14-18 in Dresden, Germany

Special Interest Group on NEtworking & Telecommunications


Motivations Bluetooth was designed to be integrated in portable

battery driven electronic devices

Energy Saving is a key issue!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!High Throughput is very welcome! Bluetooth v2.0 + EDR (Enhanced Data Rate) promise bit rates up to 3

Mbps and faster node connections


Retransmissions

Automatic 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

detection for bluetooth,” VTC 2002

MASTER

SLAVE

A B B BB

G F H

NAK

ACK

H

B

A X B X DPCK DPCK


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 Carrier sensing is generally provided by last generation chipset

Carrier-Sense ARQ CS-ARQ

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 is needed

Busy channel: slave sent back ACK or NAK that got lost RTX may generate DUPCK!!!


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


CS-ARQ: state diagram


Fetch a new packet

Fetch a new packet

Tx packet & wait for feedback

Tx packet & wait for feedback

Slave’s feedback

Slave’s feedbackNAK ACK

No valid pck rcvd

Carrier SenseCarrier SenseIDLE

Tx POLLTx POLL Slave’s feedback

Slave’s feedback

ACK

NAK

No valid pck rcvd

BUSY


Mathematical model (I) Assumptions:

Single slave piconet Saturated links

Master and slave have always packets waiting for transmission

Unlimited retransmission attempts Packets are transmitted over and over again until positive acknowledgement

Static Segmentation & Reassembly policy Unique packet type per connection

Independent error events in successive transmissions Justified by FH

Model CS-ARQ behavior described by by means of a 2-State Markov Chain

Define appropriate reward functions Data, Energy, Time

Apply renewal reward theorem to get system performance Throughput, energy efficiency, energy balancing, …


Mathematical Model (II) 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

AA SS

PAS

PSA

1-PAS 1-PSA

Reception Event Index

Slaves rx

Master rx

State transition probabilities depend on the reception events… Ds = Data successful

AC ok, HEAD ok, CRC ok Df = Data failure

AC ok, HEAD ok, CRC error Hf = HEAD failure

AC ok, HEAD error Af = AC failure

AC error


Mathematical Model (III) 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

€

π A =PAS

PAS + PSA

The steady-state probabilities are, then,

€

πD =PSA

PAS + PSA

AA SS

PAS

PSA

1-PAS 1-PSA


Reward Functions

∑∈

=EjE

xjj

xDD )()(

π

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

∑∈

=EjE

xjj

xWW )()(

π∑∈

=EjE

jjTT π

Performance indexes Energy Efficiency: Energy Efficiency:

Goodput: G( )( ) T

DD

T

DG

MS )()(

lim+

==∞→ τ

ττ

( )( ) )()(

)()(

limMS

MS

WW

DD

W

D

+

+==

∞→ τ

τξ

τ


Performance Analysis

Results

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


Goodput


Std ARQ 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

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


Conclusions

CS-ARQ yields some performance gain almost for free Only requires CS capability and very limited software

changes Furthermore, CS-ARQ is

Fully Standard compliant Complementary to other performance enhancement

schemes Stand-Alone (suffices to be implemented by the

master)

Department of Information Engineering University of Padova, ITALY

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