1747-M5
測(cè)量信號(hào)傳播時(shí)間特性的定位技術(shù)
最后一類(lèi)主要的定位技術(shù)是通過(guò)測(cè)量基站到移動(dòng)站之間射頻信號(hào)傳輸時(shí)間特性來(lái)實(shí)現(xiàn)的。這類(lèi)定位技術(shù)實(shí)現(xiàn)起來(lái)比較容易，與AOA相比較，在同等條件下（參與定位的基站數(shù)目相同）能夠提供更高的定位精度，因而在實(shí)際中應(yīng)用得最廣泛。它主要有兩種具體的實(shí)現(xiàn)手段。第一種是測(cè)量信號(hào)到達(dá)時(shí)間（TOA），即由基站向移動(dòng)站發(fā)出特定的測(cè)距命令或指令信號(hào)，并要求移動(dòng)站對(duì)該指令進(jìn)行響應(yīng)。基站會(huì)紀(jì)錄下由發(fā)出測(cè)距指令到收到移動(dòng)站確認(rèn)信號(hào)所花費(fèi)的時(shí)間T，該時(shí)間主要由射頻信號(hào)在環(huán)路上的傳播時(shí)延、移動(dòng)站的響應(yīng)時(shí)延和處理時(shí)延、基站的處理時(shí)延組成。如果能夠準(zhǔn)確地得到移動(dòng)站和基站的響應(yīng)和處理時(shí)延，就可以算出射頻信號(hào)的環(huán)路傳播時(shí)延Td。因?yàn)闊o(wú)線(xiàn)電波在空氣中以光速c傳播，所以基站與移動(dòng)站之間的距離估值dm=c*Td/2。當(dāng)有三個(gè)基站參與測(cè)量時(shí)，就可以根據(jù)三角定位法來(lái)確定移動(dòng)站所在的區(qū)域，如圖2所示。
由于這種定位方法是以時(shí)間為基準(zhǔn)的，多徑效應(yīng)和非視線(xiàn)傳輸（NLOS）所帶來(lái)的傳輸時(shí)延增加是產(chǎn)生測(cè)距和定位誤差的主要原因，所以在實(shí)際的系統(tǒng)中，測(cè)距結(jié)果dm一般都要大于基站與移動(dòng)站之間的實(shí)際距離d。為了克服NLOS以及多徑效應(yīng)帶來(lái)的不利影響，提高定位精度，參與同次定位的基站數(shù)目N一般都要大于3，這樣可以縮小圖2中陰影區(qū)域的面積。另外對(duì)于每次測(cè)量的結(jié)果都要應(yīng)用一些定位算法，使定位估計(jì)值在某種準(zhǔn)則下達(dá)到誤差最小。例如，T是每個(gè)基站測(cè)得的TOA，i為參與測(cè)量的基站編號(hào)，在某坐標(biāo)系下，移動(dòng)站的位置估計(jì)是（x，y），基站i的位置是（xi，yi）。以函數(shù)fi=c*Ti-作為基站BS測(cè)距的性能測(cè)度，也就是基站BS的測(cè)距誤差。在理想狀態(tài)下，即當(dāng)（x，y）是移動(dòng)站的實(shí)際位置，并且移動(dòng)站到每一個(gè)基站無(wú)線(xiàn)信號(hào)都是視線(xiàn)傳輸（LOS）的，那么對(duì)每一個(gè)參與測(cè)量的基站來(lái)講，fi應(yīng)該為零。但在實(shí)際中，由于受到NLOS傳輸和多徑效應(yīng)的影響，一般不可能求得（x，y）使fi=0（i=1，2，。..，N）都成立。所以整個(gè)定位系統(tǒng)來(lái)講，可以用參與定位的基站的測(cè)距誤差的加權(quán)平方和F作為系統(tǒng)性能測(cè)度函數(shù)，并以使F最小的（x，y）作為一次定位測(cè)量的結(jié)果。式中ai是基站BS在測(cè)量結(jié)果中的加權(quán)系數(shù)，其大小反映了BS到MS測(cè)距的精確性和可信程度。
另一種基于信號(hào)傳輸時(shí)間特性的定位方法是測(cè)量不同基站接收到同一移動(dòng)站的定位信號(hào)的時(shí)間差（TDOA），并由此計(jì)算出移動(dòng)站到不同基站的距離差。移動(dòng)站到任何兩個(gè)基站的距離差d可以在兩個(gè)基站之間給出一條雙曲線(xiàn)，移動(dòng)站一定處于該曲線(xiàn)之上。當(dāng)同時(shí)有N個(gè)基站參與測(cè)距時(shí)（N≥3），由多個(gè)雙曲線(xiàn)之間的交匯區(qū)域就是對(duì)用戶(hù)位置的估計(jì)，如圖3所示。這種方法要求所有參與測(cè)量的基站的時(shí)鐘是嚴(yán)格同步的。與TOA相比，它的主要好處是不需要精確地求得基站和移動(dòng)站的響應(yīng)和處理時(shí)延。與TOA一樣，TDOA的定位誤差也是主要來(lái)自射頻信號(hào)的非視線(xiàn)傳輸和多徑效應(yīng)。解決這一問(wèn)題的主要途徑也是通過(guò)增加參與定位的基站數(shù)目和采用高精度的估計(jì)算法。
1747-M5
1747-M5
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