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Department of Information Engineering
University of Padova, Italy
On the Impact of Fading and Inter-piconet Interference on
Bluetooth Performance
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Department of Information Engineering
University of Padova, Italy
WPMC02 WPMC02 Honolulu, Hawaii, October 27-30, 2002
On the Impact of Fading and Inter-piconet Interference on
Bluetooth Performance
Andrea Zanella, Andrea M. Tonello, Silvano Pupolin
{zanella,tonello,pupolin}@dei.unipd.it
Department of Information Engineering
University of Padova, Italy
WPMC02 WPMC02 Honolulu, Hawaii, October 27-30, 2002
A note on the use of these ppt slides:We’re making these slides freely available to all, hoping they might be of use for researchers
and/or students. They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. In return for use, we only ask the
following:If you use these slides (e.g., in a class, presentations, talks and so on) in substantially
unaltered form, that you mention their source.If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and put a link to the authors webpage:
www.dei.unipd.it/~zanella
Thanks and enjoy!
October 27-30, 2002 WPMC02 Honolulu Hawaii 4
Outline of the contentsOutline of the contents
Bluetooth basic Motivations &
Purposes System model Results Conclusions
October 27-30, 2002 WPMC02 Honolulu Hawaii 6
Main CharacteristicsMain Characteristics
Radio Band: ISM (2.45 GHz)
Available (almost) worldwide Royalties-free: cost saving
Interference Immunity Interference in ISM band has different strength and nature:
baby monitors, microwave ovens, lighting devices, WLAN,… Frequency Hopping Spread Spectrum (FHSS) modulation
Since most radio systems are band-limited, it is easy to finding a part of the 80-MHz-wide ISM band with low interference
FH_CDMA provides a good multiple access scheme Modulation Scheme
Signal Bandwidth of 1MHz (Frequency Hopping) GFSK (K=0.3): robustness, demodulation is easy, low-cost radio
units
October 27-30, 2002 WPMC02 Honolulu Hawaii 7
Bluetooth piconetBluetooth piconet
Two up to eight Bluetooth units sharing the same channel form a piconet
In each piconet, a unit acts as master, the others act as slaves
Channel access is based on a centralized polling scheme
active slavemaster
parked slavestandby
slave1
slave2
slave3
master
October 27-30, 2002 WPMC02 Honolulu Hawaii 8
FH & TDDFH & TDD
Each piconet is associated to frequency hopping (FH) channel The pseudo-random FH sequence is imposed by the master Time is divided into consecutive time-slots of 625 s Each slot corresponds to a different hop frequency
Full-duplex is supported by Time-division-duplex (TDD) Master-to-slave (downlink) transmissions start on odd slots Slave-to-Master (uplink) transmissions start on even slots
625 s
t
t
master
slave
f(2k) f(2k+1) f(2k+2)
October 27-30, 2002 WPMC02 Honolulu Hawaii 9
AC HECaccess code packet header payload
72 54 0-2745
CRC
General Packet FormatGeneral Packet Format
Access Code (AC) All packet exchanged in a piconet have the same AC Packets that don’t satisfy AC test are immediately
discarded Packet Header
Contains, among other information, slave active member receiver address, ARQ flags, payload format, header checksum field (HEC)
If the HEC test fails, the packet is immediately discarded Payload
If the CRC test fails, the packet is negative acknowledged
PAYL
October 27-30, 2002 WPMC02 Honolulu Hawaii 10
Multi-slot packets Multi-slot packets
f(k)
625 s
f(k+1) f(k+2) f(k+3) f(k+4)
f(k+3) f(k+4)f(k)
f(k)
f(k+5)
f(k+5)
f(k+5)
Packets can extend over one, three or five consecutive slots
Carrier frequency remains unchanged
Multi-slot packets reduce bandwidth losses due to header & guard time (220)
Max PAYL size
216 bit
1464 bit
2712 bit
October 27-30, 2002 WPMC02 Honolulu Hawaii 11
ACL data packet formatsACL data packet formats
ACL: Asynchronous data packet formats
1, 3, or 5 slot long Unprotected or protected by 2/3
FEC
Protected packet formats (DM): medium data capacity higher protection against errors
Unprotected packet formats (DH) higher data capacity more subject to errors
Erroneous packets are automatically retransmitted (Stop&Wait ARQ)
0
50
100
150
200
250
300
350
By
tes
1 slot 3 slots 5 slots
Paylod Capacity
Medium rate High rate
October 27-30, 2002 WPMC02 Honolulu Hawaii 12
Aims of the workAims of the work
Motivations &Purposes
October 27-30, 2002 WPMC02 Honolulu Hawaii 13
MotivationsMotivations
Scenario Massive Bluetooth presence: many independent
piconets overlap in the same area
Questions How does inter-piconet interference affect single
user performance? What’s the impact on multi-slot packets? Is there an easy way to derive performance
bounds?
October 27-30, 2002 WPMC02 Honolulu Hawaii 14
Aims of the workAims of the work
Previous works Either based on simulations or making restrictive
assumptions as Fixed length packets Destructive interference Absence of fading
Our contributions Simple method to evaluate impact of fading and inter-
piconet interference on Bluetooth performance taking into account
Fading & capture effects Packet format (FEC and packet length) Arbitrary number of potential interferers
October 27-30, 2002 WPMC02 Honolulu Hawaii 15
Hypothesis and Hypothesis and assumptionsassumptions
System Model
October 27-30, 2002 WPMC02 Honolulu Hawaii 16
Interferers modelInterferers model
We focus on a Target Receiver (TR) TR is r0 meters apart from its
transmitter TR can receive any pck type
Np “potential” interferers Uniformly distributed around TR, s.t.
fr(r)=2r/D2
Use only 1-slot long pcks Nodes are slot-synchronous
Ne “effective” interferers per slot
epe
e
nNn
e
peN n
NnP
79
78
79
1
r0
DTarget receiver
Interf. Piconet 2Interf. Piconet 3
Interf. Piconet 1
TimeTime
Frequency
carr
iers
Frequency
carr
iers
October 27-30, 2002 WPMC02 Honolulu Hawaii 17
Bit Error ProbabilityBit Error Probability
Prx: received power PI: interference
power N0: noise power RI: SIR for BER=1e-
3 R0: SNR for
BER=1e-3
*
00
RNRP
PrxBER
II
* Zürbes et al. “Radio Network Performance of Bluetooth,” ICC 2000
October 27-30, 2002 WPMC02 Honolulu Hawaii 18
Propagation model
Slow Flat Fading (envelope=) flat on 1 MHz channel constant along the entire packet Rice or Rayleigh distributions
WSSUS Signals from different transmitters incur independent fading Because of FH, successive pcks experiment independent
fading
2 rPrx
For Rayleigh fading, we get
D
Df i ,
22 22
e
e
N
iiNIP
1
Power-addictive interference
Ne interferers powers add up to PI
October 27-30, 2002 WPMC02 Honolulu Hawaii 19
jjCT
jok j
AC
72
000
0 172
,
1830
2000 113, okHEC
Conditioned PERConditioned PER
AC HEC PAYLOAD72 bits 54 bits h=2202745 bits
CRC
Correlator Threshold (CT)
2-time bit rep. (1/3 FEC)
DHn: Unprotected
DMn: (15,10) Hamming FEC
1515
014
000
00
1115,,:DMn
1,,:DHnh
ok
hok
hPL
hPL
00: BER: BER
λλ00: normal. : normal. useful useful powerpower
Λ: Λ: normal. normal. interfer. interfer. powerpower
October 27-30, 2002 WPMC02 Honolulu Hawaii 20
Average PERAverage PER
AC HEAD PAYL220 µs
Tslot=625 µs
,0BP
,,405 0okPL
0,,220 0okPL··
==
',0 AP
',0 okAC
',0 okHEC ',,280 0 okPL
0
0001
000 21
rfrPPdPER nBAxn
0
0)(0
0)( ,Peenee
p
e
e nnBAn
N
neNBA fPdnP
··
==
··
Target PacketTarget Packet
Effective Effective InterferersInterferers
October 27-30, 2002 WPMC02 Honolulu Hawaii 22
Performance MetricsPerformance Metrics
We focus on forward-channel performance: Packet type Dxk x{M,H}, k{1,3,5} Disregard errors in the backward channel
Performance metrics Average Packet Error Probability: PEPxk
Probability of Dxk pck dropping due to unrecoverable errors
Average Forward Throughput: xk
Average number of useful user data bit transmitted in the forward direction per unit of time
Throughput Crossing Point: Nx
Number of potential interferers for which x5 x3
October 27-30, 2002 WPMC02 Honolulu Hawaii 23
Results (1)Results (1) Parameters
Rayleigh fading (KT=- dB) r0=8 m, D = 10 m Different pck formats
Results PEP curves for DMk and
DHk get close each other as the number Np of potential interferers increases
FEC does not give benefit As expected, Dx3 formats
outperform Dx5 in terms of throughput for Np10
October 27-30, 2002 WPMC02 Honolulu Hawaii 24
Results (2)Results (2) Parameters
Interferers: Rayleigh fading Target Receiver:
Rayleigh: KT=- dB Ricean: KT=6 dB
Results The crossing point Nx is
greater than 10 when transmitter an receiver are within the nominal coverage range (10 m)
The presence of LOS has a marginal impact on Nx, but the throughput at the crossing point is higher
Shorter formats are better
October 27-30, 2002 WPMC02 Honolulu Hawaii 25
Analysis AccuracyAnalysis Accuracy
Analysis vs Simulations Simulator does
consider time offsets, fading, …
Results =(theor-sim)/sim
Bound is tight for Np<10, r0 <3 or r0 >7
Bound is loose for Np>10, r0 5
October 27-30, 2002 WPMC02 Honolulu Hawaii 26
ConclusionsConclusions
In case of inter-piconet interference DMn formats achieve very poor performance Dx5 and Dx3 formats show a performance tradeoff for
increasing number of interferers The performance crossing point Nx depends on the
distance r0 between transmitter and receiver and marginally on the presence of LOS
In case of LOS, short formats may be more suitable than long ones
Model Accuracy Analytic bound becomes loose for high number of
potential interferers and intermediate values of r0
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