一、直放站的優點

直放站與基(ji)站相(xiang)比(bi)較，其優點主要體(ti)現在如(ru)下幾個方面：

1、同等(deng)覆(fu)蓋(gai)面積(ji)時，使用(yong)直(zhi)(zhi)(zhi)放站(zhan)投(tou)資(zi)較低(di)。在平原地(di)區室外一(yi)個(ge)(ge)(ge)(ge)全向基(ji)(ji)站(zhan)可(ke)以(yi)有10km覆(fu)蓋(gai)半(ban)徑；一(yi)個(ge)(ge)(ge)(ge)全向直(zhi)(zhi)(zhi)放站(zhan)可(ke)以(yi)有4km覆(fu)蓋(gai)半(ban)徑；就(jiu)覆(fu)蓋(gai)面積(ji)而言，六(liu)(liu)個(ge)(ge)(ge)(ge)直(zhi)(zhi)(zhi)放站(zhan)約(yue)相當(dang)(dang)于(yu)一(yi)個(ge)(ge)(ge)(ge)基(ji)(ji)站(zhan)。六(liu)(liu)個(ge)(ge)(ge)(ge)直(zhi)(zhi)(zhi)放站(zhan)的設備價約(yue)為一(yi)個(ge)(ge)(ge)(ge)基(ji)(ji)站(zhan)的80%。但(dan)考慮到機房租用(yong)和(he)裝修、交直(zhi)(zhi)(zhi)流電源、空調、傳(chuan)輸系統和(he)電路租金(jin)等(deng)費用(yong)，六(liu)(liu)個(ge)(ge)(ge)(ge)直(zhi)(zhi)(zhi)放站(zhan)的費用(yong)只相當(dang)(dang)于(yu)于(yu)一(yi)個(ge)(ge)(ge)(ge)基(ji)(ji)站(zhan)的50%，甚至更低(di)。

2、覆(fu)(fu)蓋(gai)更為靈活。一個基站(zhan)基本上是圓(yuan)形覆(fu)(fu)蓋(gai)，多個直(zhi)放站(zhan)可以(yi)組(zu)織成多種覆(fu)(fu)蓋(gai)形式。如“一”字型排開，可以(yi)覆(fu)(fu)蓋(gai)十(shi)幾至幾十(shi)公(gong)里的(de)路(lu)段。也可以(yi)組(zu)織成“L”型、“N”型和(he)“M”型覆(fu)(fu)蓋(gai)，特(te)別(bie)適(shi)合(he)于山區組(zu)網(wang)。

3、在組網(wang)初期，由于用(yong)戶較少，投資效益(yi)較差，可(ke)以用(yong)一(yi)(yi)部分直放(fang)站代替(ti)基(ji)站。用(yong)戶發展起來后(hou)現(xian)更(geng)換(huan)為基(ji)站，替(ti)換(huan)下來的(de)直放(fang)站再進(jin)一(yi)(yi)步放(fang)置在更(geng)邊(bian)緣的(de)地區(qu)，這樣(yang)一(yi)(yi)步步地滾動發展。

4、由于不(bu)需要土建和傳輸電(dian)路的(de)施工，建網迅速。

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二、直放站的缺點

但(dan)直放(fang)站與基站相(xiang)比也(ye)有明顯(xian)的不足，主(zhu)要表現在：

1、不能增加系統容(rong)量。

2、引入直放站后，會給基站增加約3dB以上的(de)噪音(yin)，使原基站工作環(huan)境惡化(hua)，覆蓋半徑減少。所(suo)以一(yi)個(ge)基站的(de)一(yi)個(ge)扇區(qu)只能帶兩個(ge)以下的(de)直放站工作。

3、直放站只能(neng)頻分不能(neng)碼分，一(yi)個(ge)直放站往往將多(duo)個(ge)基站或多(duo)個(ge)扇區的信號加以放大。引入過多(duo)的直放站后，導致基站短(duan)碼相位混亂(luan)導頻污染嚴重，優化工(gong)作困難，同時加大了(le)不必要的軟切換。

4、直放站(zhan)的網管功能和設備檢(jian)測功能遠不如基(ji)站(zhan)，當直放站(zhan)出(chu)現問題(ti)后不易察覺(jue)。

5、由于(yu)受隔離度的(de)(de)要求限(xian)制，直放(fang)站(zhan)的(de)(de)某些安裝條(tiao)件要比基站(zhan)苛(ke)刻(ke)的(de)(de)多，使直放(fang)站(zhan)的(de)(de)性能往(wang)往(wang)不(bu)能得到充(chong)分發揮。

6、如果直(zhi)放站(zhan)(zhan)自激或直(zhi)放站(zhan)(zhan)附近有(you)干(gan)(gan)擾源，將對原網造成嚴重(zhong)影響。由(you)于直(zhi)放站(zhan)(zhan)的(de)工(gong)作天線(xian)較高(gao)，會將干(gan)(gan)擾的(de)破壞作用(yong)大(da)面積擴大(da)。CDMA是一個同(tong)頻系統，周邊的(de)基(ji)站(zhan)(zhan)均(jun)有(you)可(ke)能受到堵塞而癱瘓。

根據相關資料，有(you)引(yin)起國家直(zhi)(zhi)放(fang)(fang)(fang)站(zhan)(zhan)和(he)(he)基站(zhan)(zhan)的(de)(de)(de)安(an)裝(zhuang)(zhuang)比(bi)(bi)例高(gao)達2比(bi)(bi)1以上；由于(yu)中(zhong)國的(de)(de)(de)人口密度很大(da)，直(zhi)(zhi)放(fang)(fang)(fang)站(zhan)(zhan)和(he)(he)基站(zhan)(zhan)的(de)(de)(de)安(an)裝(zhuang)(zhuang)比(bi)(bi)例不(bu)應(ying)(ying)過(guo)大(da)，如果(guo)沒(mei)有(you)光纖直(zhi)(zhi)放(fang)(fang)(fang)站(zhan)(zhan)，只對射(she)頻耦(ou)合型室外直(zhi)(zhi)放(fang)(fang)(fang)站(zhan)(zhan)而言，這一(yi)比(bi)(bi)值應(ying)(ying)不(bu)大(da)于(yu)1。在規劃時，直(zhi)(zhi)放(fang)(fang)(fang)站(zhan)(zhan)作為(wei)滾動發展的(de)(de)(de)過(guo)渡設備，一(yi)次(ci)性安(an)裝(zhuang)(zhuang)直(zhi)(zhi)放(fang)(fang)(fang)站(zhan)(zhan)的(de)(de)(de)比(bi)(bi)率應(ying)(ying)進一(yi)步減少。在大(da)中(zhong)城市的(de)(de)(de)市區(qu)和(he)(he)通話密度較(jiao)高(gao)的(de)(de)(de)地區(qu)應(ying)(ying)不(bu)使用射(she)頻耦(ou)合型室外直(zhi)(zhi)放(fang)(fang)(fang)站(zhan)(zhan)。

直(zhi)放(fang)站(zhan)(zhan)不能增加系統(tong)容量，卻(que)可以(yi)彌補(bu)CDMA系統(tong)基站(zhan)(zhan)的(de)覆蓋不足，由于(yu)價格(ge)低(di)、安裝方(fang)便、在GSM、CDMA系統(tong)中采用直(zhi)放(fang)站(zhan)(zhan)不失為網絡優化(hua)的(de)一種較(jiao)好(hao)的(de)解決方(fang)案。

另外(wai)由于CDMA系(xi)(xi)(xi)統(tong)(tong)(tong)的頻率復用率為(wei)1，直(zhi)放(fang)站(zhan)在(zai)CDMA系(xi)(xi)(xi)統(tong)(tong)(tong)和GSM系(xi)(xi)(xi)統(tong)(tong)(tong)中的使用存在(zai)著差異。直(zhi)放(fang)站(zhan)的使用將(jiang)與(yu)整個(ge)(ge)系(xi)(xi)(xi)統(tong)(tong)(tong)的性(xing)(xing)能(neng)相關(guan)，而(er)在(zai)GSM系(xi)(xi)(xi)統(tong)(tong)(tong)中直(zhi)放(fang)站(zhan)的使用僅與(yu)幾個(ge)(ge)相關(guan)的通道性(xing)(xing)能(neng)有關(guan)。因而(er)，合理的規劃直(zhi)放(fang)站(zhan)網絡，嚴格的工程勘測及施工對提(ti)高CDMA網絡的性(xing)(xing)能(neng)是十(shi)分必要的。

三、直放站如何選擇安裝

在進行直放站天線選擇與安裝的過程中，除了要保證直放站良好接收(shou)施主(zhu)基站(zhan)(zhan)(zhan)的(de)(de)信號以外，還要(yao)注意保證直放(fang)(fang)站(zhan)(zhan)(zhan)的(de)(de)施主(zhu)天線(xian)(xian)與業務天線(xian)(xian)之間的(de)(de)隔離要(yao)求，避免直放(fang)(fang)站(zhan)(zhan)(zhan)的(de)(de)性能(neng)惡化。以下介紹(shao)一些在直放(fang)(fang)站(zhan)(zhan)(zhan)天線(xian)(xian)選擇與安裝過程(cheng)中的(de)(de)注意點：

1、根據具體的(de)(de)信(xin)(xin)號(hao)(hao)情況，以(yi)(yi)(yi)及覆(fu)蓋的(de)(de)需要(yao)，選擇(ze)合適的(de)(de)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)增益(yi)。由(you)于(yu)直放站(zhan)(zhan)(zhan)屬于(yu)同(tong)頻中(zhong)繼系統，所以(yi)(yi)(yi)一(yi)般(ban)(ban)不(bu)能(neng)采用全向(xiang)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)，否(fou)則可(ke)能(neng)引起系統自(zi)激。施(shi)主(zhu)(zhu)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)與施(shi)主(zhu)(zhu)基(ji)站(zhan)(zhan)(zhan)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)之(zhi)間是點對(dui)點的(de)(de)通(tong)(tong)信(xin)(xin)，所以(yi)(yi)(yi)應選擇(ze)具有高增益(yi)和(he)窄水平(ping)波(bo)束的(de)(de)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)應當最適合。一(yi)般(ban)(ban)采用角反射天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)、對(dui)數周期天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)和(he)拋物面天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)，以(yi)(yi)(yi)避免引入(ru)不(bu)必要(yao)的(de)(de)導頻信(xin)(xin)號(hao)(hao)；業(ye)務(wu)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)根據需要(yao)覆(fu)蓋區域的(de)(de)不(bu)同(tong)特點來選擇(ze)。如要(yao)覆(fu)蓋一(yi)個很(hen)大區域，這種(zhong)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)可(ke)以(yi)(yi)(yi)是普(pu)通(tong)(tong)基(ji)站(zhan)(zhan)(zhan)使(shi)用的(de)(de)定向(xiang)型(xing)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)，但(dan)須(xu)具有較高的(de)(de)增益(yi)；要(yao)進(jin)行隧道覆(fu)蓋時，可(ke)選用八(ba)木(mu)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)或螺旋(xuan)狀天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)；在室(shi)內(nei)環境下(xia)，經常(chang)(chang)要(yao)求(qiu)使(shi)用特殊設計(ji)的(de)(de)室(shi)內(nei)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)，室(shi)內(nei)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)通(tong)(tong)常(chang)(chang)需設計(ji)得不(bu)易引人注目(mu)，但(dan)不(bu)需要(yao)象普(pu)通(tong)(tong)的(de)(de)基(ji)站(zhan)(zhan)(zhan)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)那樣(yang)應具備在惡劣(lie)環境的(de)(de)要(yao)求(qiu)，室(shi)內(nei)業(ye)務(wu)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)網(wang)絡要(yao)引入(ru)較多的(de)(de)電纜和(he)導致分配器損耗，因而通(tong)(tong)常(chang)(chang)僅使(shi)用于(yu)覆(fu)蓋較小的(de)(de)區域。無論在哪種(zhong)場合下(xia)，業(ye)務(wu)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)的(de)(de)發射方向(xiang)應該嚴格控制(zhi)，以(yi)(yi)(yi)保證業(ye)務(wu)信(xin)(xin)號(hao)(hao)不(bu)會饋入(ru)施(shi)主(zhu)(zhu)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)；如果(guo)施(shi)主(zhu)(zhu)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)與施(shi)主(zhu)(zhu)基(ji)站(zhan)(zhan)(zhan)滿足(zu)視距(ju)傳輸，拾取的(de)(de)信(xin)(xin)號(hao)(hao)相對(dui)純凈(jing)。施(shi)主(zhu)(zhu)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)采用方向(xiang)性好的(de)(de)窄波(bo)束定向(xiang)天(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)(xian)，也有利(li)于(yu)提高拾取信(xin)(xin)號(hao)(hao)的(de)(de)純凈(jing)度。

2、天(tian)(tian)(tian)線(xian)(xian)方向(xiang)圖：安裝天(tian)(tian)(tian)線(xian)(xian)時，應使一(yi)副天(tian)(tian)(tian)線(xian)(xian)的(de)(de)(de)天(tian)(tian)(tian)線(xian)(xian)方向(xiang)圖的(de)(de)(de)零點(dian)對(dui)應另一(yi)副天(tian)(tian)(tian)線(xian)(xian)的(de)(de)(de)天(tian)(tian)(tian)線(xian)(xian)方向(xiang)圖的(de)(de)(de)零點(dian)。天(tian)(tian)(tian)線(xian)(xian)通常背靠(kao)背安裝，這(zhe)種(zhong)情況下選擇(ze)高前后比的(de)(de)(de)天(tian)(tian)(tian)線(xian)(xian)很(hen)重要。一(yi)般要求天(tian)(tian)(tian)線(xian)(xian)的(de)(de)(de)前后比最好在30db以上。

3、垂(chui)直(zhi)分離：直(zhi)放站天線(xian)在垂(chui)直(zhi)方向通常波瓣較(jiao)窄(zhai)，當業務天線(xian)和(he)施主天線(xian)垂(chui)直(zhi)安裝時，它(ta)們(men)的垂(chui)直(zhi)面方向圖會有(you)零(ling)點相對，可(ke)以(yi)獲得較(jiao)高(gao)的天線(xian)隔離度。

4、微波中繼(ji)處，天(tian)線(xian)(xian)(xian)要用(yong)(yong)高增益拋物面天(tian)線(xian)(xian)(xian)，安裝(zhuang)(zhuang)時要注(zhu)意：如(ru)果采(cai)用(yong)(yong)柵格拋物面天(tian)線(xian)(xian)(xian)，那么需要極化(hua)正交(jiao)安裝(zhuang)(zhuang)，兩個天(tian)線(xian)(xian)(xian)的極化(hua)要和基站側(ce)、用(yong)(yong)戶(hu)側(ce)的所(suo)用(yong)(yong)天(tian)線(xian)(xian)(xian)的極化(hua)一致；天(tian)線(xian)(xian)(xian)要安裝(zhuang)(zhuang)在平臺的兩端(duan)，最(zui)好(hao)能(neng)上下錯(cuo)開，這樣可(ke)以增大兩個天(tian)線(xian)(xian)(xian)之間的隔離度。

5、環(huan)(huan)境因素：天(tian)(tian)(tian)線(xian)周圍(wei)環(huan)(huan)境可能影(ying)響天(tian)(tian)(tian)線(xian)隔離度，環(huan)(huan)境因素包(bao)括：天(tian)(tian)(tian)線(xian)正面近處是否存在(zai)反射(she)物、天(tian)(tian)(tian)線(xian)安裝塔的材質、施主天(tian)(tian)(tian)線(xian)和業務天(tian)(tian)(tian)線(xian)間是否存在(zai)屏蔽物或設(she)置屏蔽網。

可以考慮將(jiang)施(shi)主(zhu)基站(zhan)(zhan)(zhan)的(de)施(shi)主(zhu)扇(shan)區的(de)天線采用與其他扇(shan)區和基站(zhan)(zhan)(zhan)不同的(de)交(jiao)叉極化方式，以便直(zhi)放站(zhan)(zhan)(zhan)有效地(di)選擇來自(zi)施(shi)主(zhu)扇(shan)區的(de)導頻；在(zai)直(zhi)放站(zhan)(zhan)(zhan)應用中(zhong)，對于光纖直(zhi)放站(zhan)(zhan)(zhan)而言(yan)，天線隔離要求同普通(tong)直(zhi)放站(zhan)(zhan)(zhan)的(de)要求基本一致。