Cogito Communications Communications Limited
Line of Site Link Planning
Microwave Link Planning
http://www.cogitocommunications.com
K. Bretherick
Cogito Communications Limited
Line of Site Link Planning
Contents 1.0 Microwave Link Planning .....................................................3 1.1 Link Budget.................................................................... 3 1. !resnel "one.................................................................. # 1.3 K $ factor....................................................................... % 1.& 'ain (ttenuation............................................................. ) 1.# Practical *+am,les of 'adio Link Planning ........................10
K. Bretherick
of 11
Cogito Communications Limited
1.0
Microwave Link Planning
1.1
Link Budget
Line of Site Link Planning
-ne of the most im,ortant design features of a Microwave Link is the Link Budget /n recent ears several software suites have een ,roduced that greatl sim,lif this ,rocess. 2owever in order to understand the ,rocess we will discuss in more detail the various stages in the Link Budget ,rocess. 4he starting ,oint of an Link Budget are the e5ui,ment ,arameters of the intended microwave e5ui,ment to e used and these are6 '! out,ut ,ower usuall e+,ressed in dBm or 7atts. 'eceiver sensitivit usuall e+,ressed as a Bit *rror 'ate 8B*'9 against a given '! signal level for e+am,le B*' 10$3 $:; dBm. (ntenna to e used this will usuall have a stated gain for e+am,le &# dBm. /t should e noted that this gain is /sotro,ic and not indicating an '! am,lification. 4he other ma
!ree s,ace loss can e e+,ressed with the sim,le calculation elow LdB = !."" # !0 log $ # !0 log % 7here
LdB is the loss in dB > is the distance in Km. f is the transmit fre5uenc.
4hus calculating the link udget is a sim,le e+ercise in arithmetic. -n the ,lus side? 4ransmitter -ut,ut Power. (ntenna gain near end @ far end.
-n the minus side? !ree s,ace loss wave guide losses near end @ far end. circulator losses near end @ far end.
K. Bretherick
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Cogito Communications Limited
Line of Site Link Planning
!rom our e5ui,ment ,arameters we can select the B*' figure 10$; 8errored seconds9 or 10$3 8 severel errored seconds9 this will then give us a figure for our fade margin this is the difference from the calculated figure and the B*' signal level.
Antenna Gain
Antenna Gain
Free Space Loss
Wave guide losses Transmitter
Transmit Output Power
Wave guide losses Receiver
Receiver Threshold
!igure 1? Link Budget
K. Bretherick
& of 11
Cogito Communications Limited
Line of Site Link Planning
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Path Length 7,.8* 9m: SPF %!&!"A Latitude Longitude A-imuth !levation Antenna #L
Fre2uenc3 4 )'''.' $15 4 ./ '.,' 6F 4 +'.''
'( )' (* % '* (+ *, ! *+.'* deg (+ m ASL '.'/ '.' m AGL
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PPF 0!#1!T Latitude Longitude A-imuth !levation Antenna #L
'( () '' % '* ), ) ! '+.( deg *) m ASL '.'/ '.' m AGL
snedpbec.pl
!igure ? Path Profile 1.! &resnel 'one 7hat is the !resnel "oneA 4he !renel "one is the Microwave energ that arrives at the receiving antenna 1:0 degrees or a half wavelength out of ,hase with the direct wave determines the oundar of what is called the first fresnel zone as illustrated. !or a s,ecific fre5uenc all ,oints within a microwave link from which a wave could e reflected with a total additional ,ath length of one half$wavelength 1:0 degrees form an elli,se that defines the first !resnel one radius at ever ,oint along the ,ath. Calculated ?$
nd1 nd !n 1%.3
f>
where d1 >istance from one end of the ,ath to the reflection ,oint Km. d >istance from the other end of the ,ath to the reflection ,oint Km. > d1 @ d f fre5uenc Dh o n numer of fresnel one 1st nd etc. o =ormal e+ce,ted standard is that no ostacle should im,inge within 0.% of the first !resnel one radius. (n variation of this will result in signal degradation.
K. Bretherick
# of 11
Cogito Communications Limited
Line of Site Link Planning
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Path Length 7,.8* 9m: SPF %!&!"A Latitude Longitude A-imuth !levation Antenna #L
'( )' (* % '* (+ *, ! *+.'* deg (+ m ASL '.'/ '.' m AGL
Fre2uenc3 4 )'''.' $15 4 ./ '.,' 6F 4 +'.''
!"R#O$
PPF 0!#1!T Latitude Longitude A-imuth !levation Antenna #L
'( () '' % '* ), ) ! '+.( deg *) m ASL '.'/ '.' m AGL
snedpbec.pl
!igure 3? !renel "one
K. Bretherick
; of 11
Cogito Communications Limited
Line of Site Link Planning
1. ( ) %actor 'adio waves travelling through the atmos,here do not follow true straight lines. 4he are refracted or ent. 4he ma also e diffracted. 4he velocit of an electromagnetic wave is a function of the densit of the media through which it travels. 7hen this coincides with a transition from one air mass densit to an air mass with another densit the difference in refractive inde+ results in the ,henomena of refraction and diffraction. 4he asolute ulge. 4he amount of earth ulge in metres at an ,oint ma e determined the formula?$ refractive inde+ of a sustance is its inde+ with res,ect to a vacuum and is ,racticall the same value as its inde+ with res,ect to air. /t is the change in the refractive inde+ that determines the ,ath of an electromagnetic wave through the atmos,here or how much the wave is ent from a straight line. 4o determine tower height we must estalish the ,osition and height of ostacles in the ,ath etween stations with which we want to communicate radio link sstems. 4o each ostacle height we will add earth ulge. 4his is the numer of feet or meters an ostacle is raised higher in elevation 8into the radio ,ath9 owing to curvature or earth h = 0.0*+ d1d! 4his will give the earth ulge ased on a unent radio signal as we have stated aove the radio signal is su
h = 0.0*+ d1d!
( 7here K effective earth radius 4rue earth radius
K. Bretherick
% of 11
Cogito Communications Limited
Line of Site Link Planning
/f the K factor is greater than 1 the ra eam is ent toward the earth which essentiall allows us to shorten radio link towers. /f K is less than 1 the earth ulge effectivel is increased and the ,ath is shortened or tower height must e increased. Most designers within the industr refer to normal refraction and e5uate this to a K factor of &E3 or 1.33. 4his follows a rule of thum that a,,lies to refraction in that a ,ro,agated wave front ends toward the region of higher densit that is toward the region having the higher inde+ of refraction. -ne should e careful when using a K factor of 1.33 at all times es,eciall on radio links over water. /n order to cater for variations from the 1.33 norm most ,ath ,rofiles will allow the use of more than one K factor in these cases the norm is to sti,ulate K factors of 0.% and 1.33. /t can ee seen therefore that there are man factors that influence the 2eight of radio ,ath ostacles the are summarised elow.
Radio Lin9 Obstructed
5 Factor
Fresnel
!arth 0ulge
Flat !arth !igure &? !actors associated with height
K. Bretherick
: of 11
Cogito Communications Limited
1."
Line of Site Link Planning
,ain -ttenuation
4his is the final factor that has to e taken into consideration in radio link design. /n general fre5uencies elow 10 Dh rain attenuation is considered insignificant. 2owever aove this figure and rain starts to ecome a ma
K. Bretherick
) of 11
Cogito Communications Limited
1.
Line of Site Link Planning
Practical amples o% ,adio Link Planning
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Path Length 7*'.' 9m: PPF GALAT" R=T"!R Latitude Longitude A-imuth !levation A n te nn a #L
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Fre2uenc3 4 )'''.' $15 4 ./ '.,' 6F 4 + '.''
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SPF S">"TA Latitude Longitude A-imuth !levation A nt en n a #L
'() + * % '* ; '* )* ! (.; deg ) m ASL + ) .' m A GL
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!igure #? Path Profile
K. Bretherick
10 of 11
Cogito Communications Limited
pgarssiv.pl
Line of Site Link Planning
PPF GALAT" R=T"!R !levation 7m: Latitude Longitude A-imuth Antenna T3pe Antenna 1eight 7m: Antenna Gain 7d0i: Radome Loss 7d0:
SPF S">"TA
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Fre2uenc3 7$1-: Polari-ation Path Length 79m: Free Space Loss 7d0: Field $argin 7d0: Atmospheric Absorption Loss 7d0: %et Path Loss 7d0:
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R@ Signal 7d0m: Thermal Fade $argin 7d0:
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Geoclimatic Factor Gra-ing Angle 7mr: Path "nclination 7mr: Average Annual Temperature 7deg #: &iversit3 T3pe Worst $onth $ultipath wa3 7sec: Worst $onth $ultipath wa3 76: Annual $ultipath wa3 7sec: Annual $ultipath wa3 76: Annual $ultipath * wa3 76?sec: Rain Region Rain Rate 7mmChr: Rain Attenuation 7d0: Annual Rain * wa3 76?sec: Annual $ultipath D Rain 76?sec:
K. Bretherick
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