User Manual

FiberDistributedAntennaSystem(FiberDAS)
29
Intheillustrationin Figure15onpa ge28 thereisaninputsignaltotheremoteof‐80dBm.Then eachstepofthe
chainhasga
inorat tenuationasshown,thefi berlossisintotal40dB,theFOIissetto20dBgain,thelossthrough
thePOIis35dB,theBIUissetto7dBgainandthelossonthejumperbetweentheMasterUnitandtheRa
dioBase
Stationis2dB.
Thisme
ansthatthesignallevelenteredintotheRemoteUnitisseenbythebasestation.Thisisconsideredan
optimalpointsettingfortheuplinkwhenitcomestosignallevel.Ifthenetgainintheuplinkispositive,wealsoput
noiseontothebasestationandwi
lldesensitizeitsreceiver.Thismaynotbeaproblem,iftheBaseStationis
dedicatedtoonlytheFiber‐DASsystemthenapositivenetgainisnotaproblembecauseanydesensitizationcaused
byincreasedgainiscompensatedbyanequalincreaseintheusefulsignalslevel.Thusma
intainingthesameC/I.
Howeverifth
eBaseStationisalsocoveringanoutdoorareaorhasotherantennasconnectedtothesamesector
thenapositivenetgainwillcauseadesensitizationofthereceiverfortheotherantennaandthisisgenerally
speakingabadthing.
Thesy
stemalsohasathermalnoiseloadthatitwillputonthebasestation,justlikeanantenna.Thenoisefigureof
thesystemcanbedeterminedbyusingtheFiber‐DAScalculatorexcelsheet.Thetotalnoiseisalsoanaccumulation
ofthenoiseposedbyeachchainandthene
tgainofthesystem.
Ifweha
veasystemwith4equallysetupchains,andeachchainhasanoiseloadof3dBandthenetgainis0dB
thenthenoiseloadonthesystemwillbearound9dB.
Ifwedec
reasethenetgainintheuplinkwecanlowerthisnoiseasthesystemNFcanbeconstruedastheNF+Net
gainuplinktimesthenumberofequalchains.Sincethechainsarenotinfactequal,theywillhavedifferentNFand
differentgainslightlyitma
ybeagoodideatocalculatethetotalnoiseload:
Table 41
Chain NF Gain N. Load Comment
Total Noise Load, Unadjusted
Byloweringthenetgainto‐5dBonallchainswegetthefollowing:
Table 42
Chain NF Gain N. Load Comment
Total Noise Load, Adjusted
ThisissimilartotheBTSownnoisefigurewhichgenerallyis2‐4dBdependingonthesystem.Thiskindof
desensitizationthenbecomesatradeoffbetweencoverageareaintheuplinkandthenoiseloadonthebase
station.
13.2-1.2 2.0
1 2.3 +2.3 4.6
2 2.8 +4.0 6.8 This is very high
34.0-2.8 1.2
Total 1 0.2
1 3.2 -5 -1.8
1
2.3 -5 -2.7
2 2.8 -5 -2.2
34.0 -5 -1
Total 4.1 Similar to a BTS own NF