89
1 INTRODUCTION
TheAutomaticIdentificationSystem,orinshortAIS,
is a rising safety equipment in maritime navigation
and positioning, and originally designed for the
militaryuse,workingonthefrequency161.975MHz
and 162.025 MHz, transmitting power 12.5 W and 2
W(K.Nausetal.2007).TheAISmodula
tethedataat
the speed of 9600 bps, and the mode is GMSK
(GuassianMinimumShiftKeying),whichmeanshigh
efficiency(YanXinpingetal.2010).TheAISterminals
in 20 nautical miles share the 2 frequency in the
TDMA (Time Division Multiple Access), which
dividesoneminuteinto2250piecesasthechannels,
supports them exchanging the name, call sign, ID
number, size, speed, vector, ROT and so on. As a
highlyint
egratedwirelessequipment,theAIScovers
the dead zones of RADAR, making the navigation
muchmorereliable. In2006,theAISissuggestedto
beafundamentalequipmentinnewlybuiltshipsby
IMO(TaoLinminetal.2004).
WhentheAISisadop
tedininland‐rivers,theAIS
becomesamaritimemanagementdevice,morethana
navigation tool, but also face some new problems
(Wang Feizhou et al. 2001). First, the inland river
wouldcauseobservableattenuationontheAISsignal,
whichwouldma
ketheAISdatalinklessreliablethan
that on the open sea. As well as that, the AIS data
package contains the CRC checksum, but no error
tolerance,whichmeansanybiterror,wouldmakethe
wholepackageinvalid.
Someresearchesarerepresentativeasbelow.With
the help of hardware‐in‐the‐loop simulation, it is
possibletofindtherelat
ionshipbetweenthePERand
AISsignalreceivingfieldstrength,andprovethatin
specificdistanceorabove,theAISdatalinkturnsout
to be unreliable (Ma, 2009). Considering the
PER Estimation of AIS in Inland Rivers based on Three
Dimensional Ray Tracking
F.Ma,X.M.Chu&C.G.Liu
IntelligentTransportationSystemResearchCenter,WuhanUniversityofTechnology
EngineeringResearchCenterofTransportationSafety(MinistryofEducation)
ABSTRACT: The Automatic Identification System (AIS) is an important maritime safety device, which is
populousininlandrivers.Comparedwiththatinopensea,thePackageErrorRate(PER)ofAISininlandriver
has increased sharply due to it
s complex environment. With the help of hardware in loop simulation, it is
possible to make statistical calculation on the PER under a given field strength and describe the data by
quadratic rational fraction. Meanwhile, in the three dimensional software environments, the signal field
strengthisab
letobecalculatedbytheraytrackingmethod,whichexhaustsallthepossiblepropagationpaths,
includingdirectway, reflection,diffractions,and theothermediumattenuation matters.Beyondthat, inthe
model,thepropagationgeographyinformationininlandriversisrequiredtobesimplifiedinsomeway,orthe
computationoftheraytra
ckingistoohardtoget.ThepapersettheChangjiangWuhanchannelasthefield
testingregion,andallthedeviationsarelessthan5%insunnyweather,whichprovesthemethodaccurateand
effective.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 8
Number 1
March 2014
DOI:10.12716/1001.08.01.10
90
uncertainty of AIS data link, there is a limitation of
theAISmessagesinsomearea,suchastheport,the
anchoring berths, and it is also possible that all the
AIS network would be overloaded, that no AIS
messages could be transmitted (Zhang, 2010). To
solve the problem, the
AIS terminals could adopt
intelligent transmit power to minimize the coverage
toprotecttheAISdatalink,basedonthepresentAIS
targets number and the distance to the basestations
(Ma,2009).However,majorityoftheresearchesabout
AIS did not take the reliability into account; they
totally believe the
AIS track shows how the targets
moved.SomeresearchersadopttheAISsignalasan
instance to prove the radar target, and find how to
combine them into one object (J.F.Vesecky, 2009). In
the maritime management area, some organization
takestheAISsignalasasourcetodothestatisticson
thevesselstrafficflow,lookingforfeaturesaboutthe
channels (K.Hasegawa, 2008). The other researchers
analyzetheAISdatatofindoutthecollisionrisk(Lin
Changchuan, 2008). According to the all the details
above, it is obviously that the AIS is fundamental
information for all the maritime area, some
organizationshave beendoingthe researcheson the
reliability. Their essays show that, field strength
estimation would be the chief problem of AIS
reliabilityestimation.Itisapitythat,thereisnopaper
to discuss the AIS field strength propagations
estimationmodel.
To solve the problem, the paper proposes a
methodtoestimatethefieldstrengthoftheAISsignal
basedontheraytracking.Thesecondpartdiscussthe
simulation platform, and the third part shows the
three dimension ray tracking method, the last part
proposesafiledtesting.
2 THERELATIONSHIPBETWEENPERANDTHE
SIGNALFIELDSTRENGTH
In the research of RF system, the simulationis very
common,whichincludesthesoftwaresimulationand
hardware‐in‐loop simulation. The software
simulation is more popular in two areas, one is the
performancestudy inspecifichardwarecircuit,such
as the ADS suite from Aglient, the Pspice from
ORCAD,which
isdesignedforimprovetheproducts
andlowerthedevelopmentcost(DzvonkovskayaAet
al.2010).
The software mentioned are very limited in the
AIS study, for the AIS usually covers a large area
which is more 80 km, contains varieties of
dimensions, even two completely same terminals
would show
different performance in very small
changein environment. The simulation focus onthe
micro view of AIS terminal is meaningless.
Meanwhile, the inland‐river environment stands for
the complex landforms, the GSM and CDMA
simulation tools are unable to describe the un‐
continuous transmission medium. So the AIS
performanceis unable
to bestudiedby thesoftware
simulation, the hardware‐in‐loop simulation may be
thekey(GrassoR2010).
The hardware‐in‐loop simulation introduces the
real target into the process, which use the real
transmitter as the simulating model, combines the
hardwareandsoftware.IntheRFstudies,thiskind of
simulationisverycommoninthemilitaryareas,such
as the radars, the guided weapons. The most
outstandingcharacteristicofthissimulation,isitcan
simulateverycomplextransmissionprocess.
The AIS is very suitable to use the hardware‐in‐
loop simulation, for it is just the system which
containsverycomplextransmissionenvironment.
The structure of the simulation is designed as
Figure1,whichincludesthesoftwareandhardware.
Thehardwarecontainsthesignalsource,attenuation
simulator, interference simulator, and receiver. And
the software would control all the hardware to
simulate the attenuation and interference in specific
environment.
Figure1.Hardware‐in‐the‐loopsimulationsystemstructure
ofradiofrequency
The real hardware‐in‐loop AIS simulation
platformisdesignedasFigure2.Thesignalsourceis
developed by the STM32 MCU and CMX7042 DSP,
which produces AIS baseband signal; the Aglient
8920AwillmodulatethebasebandintoAISworking
frequencyinsettingfieldstrength;theother38920A
will produce
the interference signal; all the signal
would be combined and sent into the AIS receiver,
AISbasestationSAABR40;andthesoftware willdo
thestaticsonhowmanypackageshavebeenmissed.
SAAB R40
STM32
CMX7042
aglient 8920A
aglient 8920A
aglient 8920A
aglient 8920A
software simulation controller（IBM PC）
combiner
E
xt Mod
Figure2. The real hardware‐in‐loop AIS simulation
platform
Withthehelpoftheplatform,wefoundthat,the
relationshipbetweenfieldstrengthandPERwouldbe
decribedasequation1,
32 2 2
32 1
2.764 10 ' 1.863 10 ' ' 2.821 10
7.532 10 ' 1.548 10 ' 1
xx
y
xx
(1)
originalsignal
signal
attenuation
interfering
signal2
interfering
signa1
receiver
software
control
control
interface
simulation
interface
91
where,
'99xx
,
The x stands for the field strength, and the y
standsforthePER.
3 FIELDSTRENGTHESTIMATION
3.1 Raytrackingmodel
After the discussion about the relationship between
field strength and PER, thefiled strength prediction
becomes the problem. When in the complex
environment, the ray tracking model is a very high
efficienttool.
Therearethreecondit
ionsinwirelesssignalfield
strength prorogation, which are direct waves,
diffractions,andreflections.Themultiplywillbethe
finalfieldstrength.According tothis,theraytracking
methodaimstodotheexhaustivecalculationsabout
direct wave, diffractions, and reflections, and set up
a
ttenuation algorithmic model for each condition, to
gettherealfieldstrengthoftestspot.
Infact,allthewirelesssignalspropagateunderthe
rule of Maxwellʹs equations. But the solution to the
Maxwell’sequationisverydifficulttogetinpractical
applications, as the wavelength, paths will cause
heavy computations. To simplify the equations, it is
requiredtha
tthewavelengthshouldbefarlessthan
thesizeofbarriersintheraytrackingmethod,andwe
canassumethatthewirelesssignals’wavelengthare
close to 0 and the mediums are homogeneous, and
ignore most of the electromagnetic wave
charact
eristics. Therefore, the field strength will be
shownasbelow.
0
jk
EEe
(2)
0
jk
H
He
(3)
where:
2/k
,Vacuumpropagationcoefficient
00
(, ,)
E
Exyz
,Spatialpositionrealfunction
00
(, ,)HHxyz
,Spatialpositionrealfunction
The
0
E
,
0
H
and
k
change with the shaft
direction gradually, and the wavelength is assumed
tobe0,sothechangeinthecross‐sectionisnegligible.
Undertheassumption,itisreasonabletoanalyze
thewirelesswavereflectionsandinfractionswiththe
geometrical optics. When all the propagation paths
turn to be known, the field strength will be the
mult
iplyofthe
E
.Thereflectionsanddiffractionswill
bediscussedbelow.
3.2 Reflectionmodel
Onthesurfaceofperfectmedium,thereisnolossin
the wireless signal reflections. When the signal is
propagatedtotheboundaryofdifferentmediums,the
reflectionandrefractionwillappear.InFigure3.,the
full line stands for a bounda
ry, the signal is
transmitted from the point O, and the incident of
angleisθ,thereflectionangleisθ
R,refractionangleis
θ
r, and the dotted line stands for the spreading of
fieldstrength.
θθ
R
θ
r
O
Figure3.Thereflectionandrefraction
InthereflectionangleθR,thevariableRwillbethe
keytosolvethefieldstrengthattenuation,where:
2
2
cos sin
cos sin
r
r
R
(4)
2
2
cos sin
cos sin
rr
rr
R
(5)
r
,Relativedielectricconstant
η,Reflectionefficiency.
In the reflection, there will be some energy
consumedbythemediumssurface.ForAIS,itwillbe
theconcrete;weuseηtostandfortheenergyratethat
canbereflected.
3.3 Diffractionmodel
The typical diffraction will be shown as below in
Figure 4. The wireless signal will p
ropagate to an
intersectionoftwoedges,0andn.Theincidentangle
isα
1,thereflectionangelisα2,andthediffractionwill
spreadfromtheintersection.
92
α
1
α
2
RB
SB
0
n
Figure4.Diffraction
Comparing with the reflections, the diffraction is
much more complex, and the analytic geometry is
unable to describe. By now, the UTD theory is the
only way to make the analysis, the simplification is
listed as below. It is necessary to introduce the
transitionfunctionf(x)todescribethe
gradualchange
ofthediffractionwirelesssignal,anduseRvariableto
describe two different edges. According to the
equations(4)and(5),therearetwovalueofRwhen
parallelorvertical.
Thediffractionparameter D can be calculated by
theheuristicequationbelow:
(1) (2) (3) (4)
00
[]
nn
DD RRD RD RD
(6)
ηisthesamemeaningasthereflection.Transition
functionisshownasfollows.
/4
() () 2 ()
(2 ln sin 2 )
22
j
iii
e
DctgFk
nk
where:
(1)
21
[( )]/2n
(2)
21
[( )]/2n
(3)
21
[( )]/2n
(4)
21
[( )]/2n
2
( ) 2 exp( ) exp( )
X
FX j X jX j d
,
3.4 Fieldstrengthestimationmodel
Basedontheequations(2)and(3),itiseasytosetup
theestimationmodeltotheAISsignalfieldstrength
when we know all the propagation paths, reflection
parameter R and diffraction parameter D (Hao
Ruijingetal.2007).
Directpropagation,
0
j
kd
LOS
e
EE
d
(7)
Reflection,
()
12
0
12
jk s s
R
e
EER
ss
(8)
Diffraction,
12
()
0
1
1212
()
jk s s
D
E
s
ED e
ssss
(9)
where:
2/k
Vacuumpropagationcoefficient
E0theorigintransmitpowerlevel,
ddirectpropagationpathdistance,
s
1 start point to reflections or diffractions point
distance
s
2reflectionsordiffractionspointtoreceivepoint
distance,
Rreflectionsparameter,accordingto(4)or(5)
Ddiffractionsparameter,accordingto(6)
whenthepropagationincludes morethanonepath,it
needs to use the equations above to do the
summation,justasequation(10).
total i
i
EE
(10)
Thepathlosswillbesolvedasequation(11)
0
||
20 lg( )
4||
total
tr
E
L
GG E
(11)
where:
G
rReceiveantennagain
GtTransmitantennagain
λWavelength
Infact,thewirelesssignalwillattenuatemarkedly
in each reflection and diffraction. Therefore, it is
logical to ignore the signalswhich havereflected or
diffracted more than specific ti mes, to make the
calculationmucheasier.
3.5 RayTrackingModel
Due to the massive calculated amount,
the ray
trackingisconsideredtobeonlysuitableforthefield
strength estimation in small area, such as the GSM,
CDMA,andWIFI. In theinland rivers, the situation
seemstobemuchmorecomplex,allthefactorsonthe
land would take influence on the propagation of
wireless
signal, the high buildings, the woods, the
mountains.Itisimpossibletoexhaustallthefactorsto
93
do the analysis, and the simplification is necessary.
Luckily,mostofthefactorsabovetakeverytinyeffect
iononthesignal,thesimplificationsarefeasible.
Figure5.InlandRivers
TheAISisrunningonthefrequency161.975MHZ
and 162.025MHZ, the wavelength is about 1.852
meters.Thechiefprincipleofraytrackingisthat,the
signal wavelength must be far less than the size of
barriers,so,allthematterswhicharenotsmallthan
thelimitedsizecouldbeignored.
Besidesthat,dueto
the bulwarks and shelter forests, the AIS signal is
very hard to escape from the channel and most the
spreadsignalwouldattenuatetoverylowlevel.Just
like the figure 5, in most cases, the top of the AIS
targets are below the top of
the bulwarks. Even a
small part of the signal propagate out of the
bulwarks, they would be consumed by the shelter
woods.
So,itisreasonabletodothesimplification,which
we assume all the propagations are running in the
rivers,justbetween thebulwarks.The main barriers
in the channel
would be the bridges, bulwarks,
sandbars,levees,docks,anchorpiles,notincludethe
buoyswhose size arejust neartothe wavelength of
AISsignal.
AsthesituationinFigure6,thesignalpropagation
betweenAandBinsomeinlandriverchannelhasa
lot of paths. The banks
are just like the mirrors, the
edgeKwouldformA’imageofA,andtheK1andK2
edgeswouldformB’andB’’imagesofB,theA’and
B’, B’’ is reachable, so there are 2 different paths by
reflectionwiththehelpofK,K1,K2.The
diffractionis
similar to this. With all the equations above and
accumulate all the paths, the prediction of field
strengthbecamepossible.
diffraction path
reflection path
direct path
A
B
A’
B’
B’’
M
N
reflectin
g
surface K2
reflecting surface K1
reflecting surface K
Figure6.The2dimensionalraytracking
4 PERESTIMATIONANDRESULT
Based on the 2 dimensional ray tracking, the 3
dimensional ray tracking is also available, as the
Figure 5, between A and B, the propagation paths
wouldbeAtoC,thenB,alsoAtoE,thenB.Allwe
should do is exhausting all
the paths in define
channel.
TheAwassettingtotransmittingat1W,andthe
distancebetweenAandBis4.7km,thetestingtime
was6:00to18:00,didthestaticsonevery1hour,and
theweatherissunny,foggyinthemorning.
Figure6.adimensionsstructureofthefieldtestingarea
TheFigure6 shows adimensionsstructure of the
field testing area. And the A point field testing is
shown in Figure 6. In 12 hours testing, the PER
predictionandtestresultareshowninTable1.
Table1.ThePERpredictionandtestresult
_______________________________________________
Time(h)Test(%)
_______________________________________________
1st45.8
2nd44.2
3rd37.2
4th27.2
5th27.2
6th26.5
7th28.5
8th26.0
9th25.1
10th29.0
11th21.9
12th21.0
_______________________________________________
Estimation26.1
In the testing, we could infer that, in the good
weather,afterthefoggy,theestimation isveryclose
tothetestvalue.
5 CONCLUSION
TheAISisspreadingintheinlandrivers,andbecome
an essential part of modern maritime management,
theperformanceofAISisaninevitableproblem.
The
hardware‐in‐loop simulation shows the relationship
betweenPERandfieldstrength,meanwhilethethree
dimension ray tracking mode will calculate the
attenuation, so the PER estimation become possible,
and also been proved. In the further study, the ray
tracking model needs to be improve to adapt the
weatherchanging.
BulwarksChannel
A
B
C
D
E
94
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