并且为各基站提供信号强度比较以确定何时进行切换Word文档格式.docx
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相邻的小区之间载频数量不平衡的现象越来越多,不同的频率之间切换的问题开始显露。
Asthecapacityofasinglecarriergradually(insufficient),soinsomeurbanareasoperatorsstarttoaddnewcarriers.
Thesignsofcarriersimbalancearegettingmoreandmore,theswitchingproblemsamongdifferentfrequenciescometolight.
CDMA的软切换技术在降低掉话率方面作用明显,但它仅在相同载频之间的切换才能发挥作用。
CDMA’ssoft-handofftechnologyplaysanevident/obviousroleinreducingcalldroprate,however,thistechnologycanswitchonlyownssamecarriers.
对于不同载频之间的切换,只能采用硬切换实现。
Hard-handoffistheonlywayforthosedifferentcarrierstoachievehandoff.
硬切换的成功率相对较低,尤其是不同基站不同载频之间的硬切换,很容易造成掉话。
Thepossibilityofsuccessiscomparativelylowforhard-handoff,whichwasrealizedamongdifferentBTS&
carries.Inparticular,itisveryeasytocausecalldropsintheprocessofhard-handoff.
伪导频技术可以有效提高不同载频之间的切换成功率,是网络优化的重要手段之一。
Pseudo-pilottechnologycaneffectivelyimprove/enhancethepossibilitiesofsuccessanditisalsooneoftheimportantmeans/wayofnetworkingoptimization.
伪导频切换的原理
handoffprinciplesonpseudopilot-frequency
导频信号是基站连续发射未经调制的直接序列扩频信号
Pilot-frequencyisadirectsequence,spreading-frequenceandunmodulatedsignalwhichistransmittedcontinuously/continualfromBTS.
在没有伪导频设备的情况时,手机漫游在A基站下,使用载频FA2通信。
Intheabsenceofpseudo-pilotfacilities/equipment,cellphonewillcommunicatebywayofcarrierFA2whenroaminginthesiteA.
当手机逐渐远离A基站,靠近B基站,B基站则只有载频FA1提供服务。
WhenacellphonegraduallygetsfarawayfrombasestationAandgetsclosertobasestationB.ThenitisonlybasestationB’scarrierFA1thatservesthecommunications.
手机收到的A基站FA2的信号越来越弱,而B基站FA1信号逐渐增强,只能采用硬切换的方式进行切换,而且会产生30毫秒的中断。
WhenacellphonereceivessignalofFA2fromstationAandthesignalgetsweakerandweaker,however,thesignalofFA1fromstationBgetsstrongerandstronger.Underthiscircumstances,hardhandoffistheonlysolution/wayandtheinterruptionwilllastfor30milli-seconds.
不同基站的异频硬切换的成功率很低,非常容易形成掉话的现象。
Iftheprocessofhard-handoffofpilot-frequency’sdeploysamongdifferentbase-stations,thepossibilityofsuccesswillbeverylowandthephenomenonofcalldropwillbeeasilycreated/generated/formed.
如果我们在B基站安装了伪导频设备,并且手机处于载频FA2服务之下,从A基站移动到B基站时,手机会不断检测附近基站的导频信号强度。
Ifthepseudopilot-frequencyequipmentwasinstalledinStationB,andCarrierFA2servescellphone.Thecellphonewillalwaysdetecttheintensityofpilot-frequencysignalfromBase-stationaround.
当T_ADD参数超过门限值时,手机会主动向A基站发送PSMM(功率强度测量)消息。
WhenparametersofT_ADDexceedthresholds,thecellphonewillsendthemessageofPSMMactively.
A基站收到消息后,查询相邻基站的配置信息,发现B基站的FA2的导频信号实际上是伪导频信号,不具备提供业务信道的可能,但B基站的FA1可以提供服务信道。
HavingreceivedthemessageofPSMM,stationAdesks/checksmessageregardingconfigurationintheneighbouringbase-stations.Itsmessagefindsoutthatpilot-frequencyofFA2frombase-stationB,inpractice,ispseudo-pilotsignal.Soithasnopossibilitiesofprovidingtrafficchannel,however,FA1ofbase-stationBcanprovideservicechannel.
A基站向手机发送EHDM(增强型切换定向)消息,通知手机切换到载频FA1,同时将切换参数发送给手机。
Base-stationAtransmitsthemessageofEHDMtocellphone,informscellphoneofswitchingintocarrierFA1andsendsswitchingparameterstocellphone.
手机首先立刻切换到A基站的载频FA1下,然后按照软切换的方式从A基站的载频FA1切换到B基站的载频FA1,从而保证切换的顺利进行。
Atfirst,thecellphonewillbehandoffedtocarrierFA1inbase-stationA,andthenitwillbehandoffedcarrierFA1inbase-stationAtocarrierFA1inbase-stationBbywayofsofthandoffsoastomakesure/guaranteethesmoothhandoff.
几种常用的伪导频实现方案
(Severalcommonsolutionstorealizepseudo-pilot):
伪导频技术由CDMA技术标准拥有者高通公司提出之后,由于对有效降低掉话率,作用非常明显,因而得到了广泛的应用。
根据使用方式的不同,大致可以分为以下三类:
Pseudo-pilottechnologieswasraisedbycorporationofQualcomwhoistheownerofstandardCDMA.Itwaswidelyusedduetoplayinganveryobviousroleinreducingdroprates.Accordingtodifferentapplicationways,generallyitwasdividedintothreecategories.
一、基站自提供方式
wayofbase-station’sself-offering
基站在设计的时候就考虑到伪导频切换功能。
Theswitchingfunctionofpilot-frequencywastakenintoconsiderationwhendesigningbase-stations.
在数字基带处理时,从正常载频信道中提取出导频信号,用于伪导频的发射。
Whenprocessingdigitalbase-band,thesignalofpilot-frequencywastakenoutfromnormalcarrierchannelanditwouldbeusedfortransmittingpseudopilot-frequency.
这样可以保证伪导频信号只包括导频信号,而且和正常载频导频信号保持高度一致。
Bydoingsoitcanguaranteepilot-frequencyalonewhichwas/isincludedinpseudo-pilotsignal.Anditcanalsomaintainsahighdegreeofconsistencywithnormalcarrier&
pilotsignal.
这种方式显然是最佳的实现方式。
Obviously,thisisthebestwaytorealizepseudo-pilot.
但遗憾的是,不少厂家的基站并不支持。
However,itisapitythatmanybase-stations(providers/suppliers)factoriesdon’tsupportthisway,especially/inparticular,micro-cellularasarepresentative.
尤其是微蜂窝基站为代表,为了减少成本,厂家往往省去伪导频的功能。
Inordertoreducecosts,eliminating/takingasidethefunctionofpseudo-pilotiswhatfactories/manufacturersalwaysdo.
也为后面两种方式留下了市场空间。
Bydoingso,marketspacehasbeenleftfortwowaysfollowing.
二、纯导频方式
wayofpurepilot-frequency
纯导频方式是采用专门的信道发生器模拟出纯粹的导频信号。
Thewayofpurepilot-frequencyisonethatadoptsdedicated-channelgeneratortosimulatepurepilotsignal.
由于只发射纯导频,对伪导频所在的载频的干扰减小。
Duetothesingletransmissionofpurepilot-frequency,bydoingsoitcanreducecarrierinterferencewherepseudo-pilotliesin.
但由于导频信号需要自己产生,要使用一些昂贵的modem芯片,而且内部结构比较复杂。
However,becausethepilot-signalisforitselftogenerate,itwillusesomeexpensivemodem-chips.Andtheirstructuresarecomparativelycomplicated.
三、移频方式
wayoffrequency-shifting
移频方式实现起来相对简单,具体地说从基站射频信号处将所有信号(包括同步、寻呼和业务信道信号)都耦合到新载频上进行发射。
Frequency-shiftingisarelativelysimplewaytorealizepilot-frequency,specificallyspeaking,includingsignalin/on/throughsynchronous,pagingandtrafficchannel.Allofthesesignalswillbetransmittedbywayofcouplingtonewcarriers.
伪导频设备不仅发射导频信号,而且还要发射同步信号、寻呼信号和业务信道信号,这样为保证伪导频的覆盖范围与基站的覆盖范围相似,所需要发射的功率将与基站的发射功率保持同步。
Pseudo-pilotequipmentcantransmitnotonlypilotsignal,butalsosynchronous,pagingandtrafficsignal.Bydoingsoitwillguaranteethesimilarcoverageareabetweenpseudo-pilotandbase-stations.Power(pseudo-pilot)needstotransmitwillbesynchronizedwithbase-stationpower.
分析比较(analysisandcompare)
基站自提供方式和纯导频方式从技术本质上看属于同一种技术,我们重点分析一下纯导频方式和移频方式的优缺点。
Base-station’sself-offeringwayandpurepilotwayaltogetherbelongtothesametechnologyintermsoftechnicalessence.Here,wewillattachgreatimportancetopurepilotandfrequency-shiftingtoanalyzetheiradvantagesanddisadvantages.
纯导频方式结构复杂,导频信号发生器设备成本也较高,但其所需发射的信号纯粹,对发射功率的要求也减少到最小,一般不超过4w。
Pure-pilotmode/wayownsacomplicatedstructureandpilot-signalgenerator’sprice
iscomparativelyhigh.However,whatthesignalofthismodeneedstotransmitispure.Sominimizethepowertransmission,generallyspeaking,tolessthan4w.
而对于移频方式,伪导频设备转发了正常载频的全部信号,因此所需要发射的功率将与基站正常载频的发射功率相同,在国内基站的发射功率通常为20w,这样,伪导频的发射需要20w的高功放,成本较高,因此移频方式和纯导频方式综合成本相差不多。
Asformodeoffrequency-shifting,pseudo-pilotequipment/devicetransmitsallsignalsofnormalcarrierfrequencies.Sothepowerneedstotransmitwillbethesameasbase-station’snormalcarrierfrequencies.Powertransmitsfrombase-station,inourcountry,usually20w.20w’shighpoweramplifierisneededfortransmissionofpseudo-pilotanditscostiscoparativelyhigh.Sothecomprehensivecostwillbeclosertoeachother/almostthesame.
纯导频方式的覆盖范围相对固定,而CDMA基站的信号是有呼吸效应的,实际的覆盖范围会随着用户数量不断变化,纯导频方式的信号覆盖范围不能保持和原基站载频的同步,对切换的成功率产生负面影响。
Modeofpurepilotownsarelativelystablecoveragearea,thesignalofCDMA’sbase-stationownscellbreathingeffect.Sotherealareaofcoveragewillchangeassubscribersnumbers.Coverageofsignalisnotcapableofsynchronizingwiththeoriginalbasestationcarrierfrequency.Themodewillhaveasideeffectonhandoff’spossibilityofsuccess
移频方式却恰恰很好地解决了这个问题。
However,theway/modeoffrequency-shiftingwellhandlesthisproblem.
移频方式在发射伪导频的同时,也发射了业务信道等信号。
Modeoffrequency-shiftingtransmitspseudo-pilotsignalandtrafficsignalaswell.
这些不需要的信号会对周围的基站产生不必要的干扰,降低了周围基站的信号质量和用户容量。
Thoseunwantedsignalswillcauseunnecsssaryinterferencetosurroundingbase-stations,asaresultofit,thesignalqualityandsubscribercapacityof(?
)willbereduced.
而纯导频方式则对周围基站的干扰降到了最小。
Themodeofpure-pilot,however,willminimizeinterferencetosurroundingbase-stations.
综合起来,我们认为纯导频方式对网络影响小,适合在基站密集的地区使用。
Comprehensivelyspeaking,webelievetheapproachofpure-pilothasasmallimpactonthenetwork,soitissuitableforusinginareaswhichdeploydensebase-stations.
移频方式对网络一定的影响,但切换成功率较好,适合在城市边缘地区使用。
Approachoffrequency-shiftingwillhaveaneffectonnetwork.However,itspossibilityofsuccessisbetter.Soitissuitableforareaslocatingtheedgeofthecitytouse.
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