一、直放站的優點

直放站與基(ji)站相比較，其優點主要體現(xian)在如下(xia)幾個方面：

1、同等覆(fu)蓋面(mian)積(ji)時，使用直(zhi)(zhi)放(fang)站(zhan)(zhan)投資較低。在平(ping)原地(di)區室(shi)外一個(ge)(ge)全(quan)向(xiang)(xiang)基(ji)站(zhan)(zhan)可(ke)以(yi)有(you)10km覆(fu)蓋半(ban)徑；一個(ge)(ge)全(quan)向(xiang)(xiang)直(zhi)(zhi)放(fang)站(zhan)(zhan)可(ke)以(yi)有(you)4km覆(fu)蓋半(ban)徑；就覆(fu)蓋面(mian)積(ji)而言，六個(ge)(ge)直(zhi)(zhi)放(fang)站(zhan)(zhan)約相當于一個(ge)(ge)基(ji)站(zhan)(zhan)。六個(ge)(ge)直(zhi)(zhi)放(fang)站(zhan)(zhan)的(de)設備價(jia)約為一個(ge)(ge)基(ji)站(zhan)(zhan)的(de)80%。但考(kao)慮到機房租用和裝修(xiu)、交直(zhi)(zhi)流電源(yuan)、空(kong)調、傳輸系統和電路租金等費用，六個(ge)(ge)直(zhi)(zhi)放(fang)站(zhan)(zhan)的(de)費用只相當于于一個(ge)(ge)基(ji)站(zhan)(zhan)的(de)50%，甚(shen)至更低。

2、覆蓋(gai)更(geng)為靈活。一個基站基本上(shang)是圓形覆蓋(gai)，多個直放站可(ke)以(yi)組(zu)織成多種(zhong)覆蓋(gai)形式(shi)。如“一”字型(xing)排(pai)開，可(ke)以(yi)覆蓋(gai)十(shi)幾至幾十(shi)公里的路段。也可(ke)以(yi)組(zu)織成“L”型(xing)、“N”型(xing)和(he)“M”型(xing)覆蓋(gai)，特別(bie)適合(he)于山(shan)區組(zu)網。

3、在(zai)(zai)組網初期(qi)，由(you)于用戶(hu)較少，投資效(xiao)益較差，可以用一部分直放站代替(ti)基(ji)站。用戶(hu)發(fa)展起來后(hou)現(xian)更(geng)換(huan)為(wei)基(ji)站，替(ti)換(huan)下(xia)來的直放站再進(jin)一步放置在(zai)(zai)更(geng)邊緣的地區，這樣(yang)一步步地滾動發(fa)展。

4、由于不(bu)需要土建和傳輸電路(lu)的(de)施工，建網迅速(su)。

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二、直放站的缺點

但(dan)直放站(zhan)與基(ji)站(zhan)相比也有明顯(xian)的不足，主要表現在：

1、不能增加(jia)系(xi)統容(rong)量。

2、引入直(zhi)放站后(hou)，會給基站增加約(yue)3dB以上(shang)的噪音，使原基站工(gong)作(zuo)環境惡化，覆蓋半徑減少(shao)。所以一個(ge)基站的一個(ge)扇區只(zhi)能帶(dai)兩個(ge)以下的直(zhi)放站工(gong)作(zuo)。

3、直放(fang)站(zhan)只(zhi)能頻分不能碼分，一(yi)個直放(fang)站(zhan)往往將(jiang)多個基(ji)站(zhan)或多個扇區(qu)的(de)信號(hao)加(jia)以放(fang)大。引入過多的(de)直放(fang)站(zhan)后，導致(zhi)基(ji)站(zhan)短碼相位混亂導頻污染嚴重(zhong)，優(you)化工作困難，同時加(jia)大了不必要的(de)軟切換。

4、直(zhi)放站(zhan)的網管功(gong)能和設備檢測功(gong)能遠不(bu)(bu)如(ru)基站(zhan)，當直(zhi)放站(zhan)出現問題(ti)后不(bu)(bu)易察覺。

5、由于受隔離度的(de)要求(qiu)限(xian)制，直(zhi)放站(zhan)的(de)某些安裝(zhuang)條件要比基站(zhan)苛刻的(de)多(duo)，使直(zhi)放站(zhan)的(de)性(xing)能(neng)往往不能(neng)得到(dao)充分發揮(hui)。

6、如果直(zhi)放(fang)站(zhan)(zhan)自激或直(zhi)放(fang)站(zhan)(zhan)附近有干(gan)擾源，將對(dui)原網造(zao)成嚴重影響。由于(yu)直(zhi)放(fang)站(zhan)(zhan)的工作(zuo)天線較高，會將干(gan)擾的破壞(huai)作(zuo)用(yong)大(da)面(mian)積擴(kuo)大(da)。CDMA是一(yi)個同頻系統，周邊(bian)的基站(zhan)(zhan)均有可(ke)能受(shou)到堵(du)塞而癱瘓。

根據相關資料，有引起國(guo)(guo)家(jia)直(zhi)(zhi)放(fang)(fang)(fang)站和基站的安裝比例高達2比1以上；由于中國(guo)(guo)的人口密度很大(da)，直(zhi)(zhi)放(fang)(fang)(fang)站和基站的安裝比例不應(ying)(ying)過大(da)，如果(guo)沒有光纖直(zhi)(zhi)放(fang)(fang)(fang)站，只對射頻耦(ou)合(he)型室外直(zhi)(zhi)放(fang)(fang)(fang)站而(er)言，這(zhe)一比值應(ying)(ying)不大(da)于1。在規(gui)劃時，直(zhi)(zhi)放(fang)(fang)(fang)站作為滾動發展(zhan)的過渡(du)設備，一次性安裝直(zhi)(zhi)放(fang)(fang)(fang)站的比率應(ying)(ying)進(jin)一步減(jian)少。在大(da)中城市的市區(qu)和通話密度較高的地區(qu)應(ying)(ying)不使(shi)用射頻耦(ou)合(he)型室外直(zhi)(zhi)放(fang)(fang)(fang)站。

直(zhi)放站不能增加系(xi)統(tong)容量，卻可以彌補CDMA系(xi)統(tong)基站的覆蓋不足，由于(yu)價(jia)格低、安裝方便、在GSM、CDMA系(xi)統(tong)中采(cai)用直(zhi)放站不失(shi)為(wei)網絡(luo)優化的一種較好的解決方案(an)。

另外由于CDMA系統(tong)的(de)頻率(lv)復用(yong)率(lv)為1，直(zhi)放站在(zai)(zai)CDMA系統(tong)和GSM系統(tong)中(zhong)的(de)使用(yong)存在(zai)(zai)著差異。直(zhi)放站的(de)使用(yong)將與(yu)整個系統(tong)的(de)性(xing)(xing)能(neng)相關，而在(zai)(zai)GSM系統(tong)中(zhong)直(zhi)放站的(de)使用(yong)僅與(yu)幾個相關的(de)通道性(xing)(xing)能(neng)有關。因而，合理(li)的(de)規劃直(zhi)放站網絡，嚴格的(de)工(gong)程勘測及(ji)施工(gong)對提高CDMA網絡的(de)性(xing)(xing)能(neng)是(shi)十分必要(yao)的(de)。

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施主基站的信(xin)號以外(wai)，還要注(zhu)意(yi)(yi)保證直放站的施主天(tian)線(xian)(xian)與業(ye)務天(tian)線(xian)(xian)之間的隔離要求，避免直放站的性能惡化。以下介(jie)紹一些在直放站天(tian)線(xian)(xian)選(xuan)擇與安裝過程中的注(zhu)意(yi)(yi)點：

1、根據具(ju)體的(de)(de)信(xin)(xin)號(hao)情況，以及(ji)覆(fu)蓋的(de)(de)需(xu)要(yao)，選擇(ze)合適(shi)的(de)(de)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)增益(yi)。由于直(zhi)放站(zhan)(zhan)屬(shu)于同頻(pin)中繼系統，所以一(yi)般(ban)不(bu)(bu)(bu)能采用(yong)全向(xiang)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)，否則可能引起系統自激。施(shi)主(zhu)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)與施(shi)主(zhu)基(ji)站(zhan)(zhan)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)之間是(shi)點對點的(de)(de)通信(xin)(xin)，所以應(ying)(ying)選擇(ze)具(ju)有高增益(yi)和(he)窄(zhai)水(shui)平波束的(de)(de)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)應(ying)(ying)當最適(shi)合。一(yi)般(ban)采用(yong)角(jiao)反(fan)射(she)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)、對數(shu)周(zhou)期天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)和(he)拋(pao)物面天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)，以避(bi)免引入不(bu)(bu)(bu)必(bi)要(yao)的(de)(de)導頻(pin)信(xin)(xin)號(hao)；業(ye)務天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)根據需(xu)要(yao)覆(fu)蓋區(qu)域(yu)(yu)的(de)(de)不(bu)(bu)(bu)同特點來選擇(ze)。如(ru)要(yao)覆(fu)蓋一(yi)個很(hen)大區(qu)域(yu)(yu)，這種天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)可以是(shi)普(pu)通基(ji)站(zhan)(zhan)使(shi)(shi)用(yong)的(de)(de)定(ding)向(xiang)型天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)，但須(xu)具(ju)有較高的(de)(de)增益(yi)；要(yao)進行隧道覆(fu)蓋時，可選用(yong)八木(mu)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)或螺旋狀天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)；在(zai)(zai)室(shi)內(nei)環(huan)境(jing)(jing)下(xia)，經常(chang)要(yao)求使(shi)(shi)用(yong)特殊(shu)設(she)計的(de)(de)室(shi)內(nei)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)，室(shi)內(nei)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)通常(chang)需(xu)設(she)計得不(bu)(bu)(bu)易引人(ren)注目，但不(bu)(bu)(bu)需(xu)要(yao)象普(pu)通的(de)(de)基(ji)站(zhan)(zhan)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)那(nei)樣(yang)應(ying)(ying)具(ju)備在(zai)(zai)惡劣環(huan)境(jing)(jing)的(de)(de)要(yao)求，室(shi)內(nei)業(ye)務天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)網(wang)絡(luo)要(yao)引入較多的(de)(de)電纜和(he)導致(zhi)分配器損耗，因(yin)而(er)通常(chang)僅使(shi)(shi)用(yong)于覆(fu)蓋較小的(de)(de)區(qu)域(yu)(yu)。無論在(zai)(zai)哪種場合下(xia)，業(ye)務天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)的(de)(de)發(fa)射(she)方向(xiang)應(ying)(ying)該嚴格(ge)控制(zhi)，以保證業(ye)務信(xin)(xin)號(hao)不(bu)(bu)(bu)會饋(kui)入施(shi)主(zhu)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)；如(ru)果施(shi)主(zhu)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)與施(shi)主(zhu)基(ji)站(zhan)(zhan)滿足(zu)視距傳輸(shu)，拾取的(de)(de)信(xin)(xin)號(hao)相對純凈。施(shi)主(zhu)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)采用(yong)方向(xiang)性好的(de)(de)窄(zhai)波束定(ding)向(xiang)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)(xian)(xian)，也(ye)有利于提高拾取信(xin)(xin)號(hao)的(de)(de)純凈度。

2、天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)方向圖：安(an)裝天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)時(shi)，應使一副天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)的(de)(de)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)方向圖的(de)(de)零點對應另一副天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)的(de)(de)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)方向圖的(de)(de)零點。天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)通常背靠背安(an)裝，這種情(qing)況下選擇(ze)高前后比的(de)(de)天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)很(hen)重要。一般要求天(tian)(tian)(tian)(tian)(tian)線(xian)(xian)(xian)的(de)(de)前后比最好在30db以上。

3、垂直(zhi)(zhi)分離(li)：直(zhi)(zhi)放(fang)站天(tian)線在垂直(zhi)(zhi)方向(xiang)通常波(bo)瓣(ban)較(jiao)窄(zhai)，當業(ye)務天(tian)線和施主(zhu)天(tian)線垂直(zhi)(zhi)安裝時，它們的垂直(zhi)(zhi)面方向(xiang)圖會有零點相對，可(ke)以(yi)獲得較(jiao)高的天(tian)線隔離(li)度(du)。

4、微(wei)波中繼(ji)處，天(tian)線(xian)要用(yong)(yong)高(gao)增益拋(pao)物面天(tian)線(xian)，安裝時要注意：如果采(cai)用(yong)(yong)柵格拋(pao)物面天(tian)線(xian)，那么需要極(ji)(ji)化(hua)正(zheng)交(jiao)安裝，兩個(ge)天(tian)線(xian)的(de)極(ji)(ji)化(hua)要和(he)基(ji)站側、用(yong)(yong)戶(hu)側的(de)所用(yong)(yong)天(tian)線(xian)的(de)極(ji)(ji)化(hua)一致；天(tian)線(xian)要安裝在平臺(tai)的(de)兩端，最好能上下錯開，這樣(yang)可以增大兩個(ge)天(tian)線(xian)之間的(de)隔離度(du)。

5、環(huan)(huan)境因素：天(tian)線周圍環(huan)(huan)境可能影響天(tian)線隔離度，環(huan)(huan)境因素包括：天(tian)線正面近處是(shi)否(fou)存(cun)在反射(she)物(wu)、天(tian)線安裝塔(ta)的材質、施主天(tian)線和(he)業務天(tian)線間是(shi)否(fou)存(cun)在屏蔽物(wu)或設置屏蔽網。

可以考慮將施(shi)主(zhu)基(ji)站(zhan)(zhan)的(de)(de)施(shi)主(zhu)扇(shan)區的(de)(de)天線采用(yong)與(yu)其他扇(shan)區和(he)基(ji)站(zhan)(zhan)不同(tong)的(de)(de)交(jiao)叉極化方式，以便直放站(zhan)(zhan)有效地選擇(ze)來自(zi)施(shi)主(zhu)扇(shan)區的(de)(de)導頻；在直放站(zhan)(zhan)應用(yong)中，對于光(guang)纖(xian)直放站(zhan)(zhan)而(er)言，天線隔(ge)離要(yao)求同(tong)普通(tong)直放站(zhan)(zhan)的(de)(de)要(yao)求基(ji)本一致。