碼分多址通信系統(tǒng)、擴頻通信外文翻譯-其他專業(yè)-預(yù)覽頁
【正文】 volution encoding and interleaving can be used to assist in recovering this lost data. CDMA signal are also resistant to multipath fading. Since the spread spectrum signal occupies a large bandwidth only a small portion of this will undergo fading due to multipath at any give time. Like the narrow band interference this will result in only a small loss of data and can be overe. Another reason CDMA is resistant to multipath interference is because the delayed versions of the transmitted pseudorandom code, and will thus appear as another user, which is ignored at the receiver. In other words, as long as the multipath channel induces at least one chip of delay, 天 the multipath channel induces at least one chip of delay,the multipath signals will arrive at the other words, as long as the multipath channel induces at least one chip of delay, the multipath signals will arrive at the receiver such that they are shifted in time by at least one chip from the intended signal. The correlation properties of the pseudorandom codes are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, and it is thus ignored. Some CDMA devices use a rake receiver, which exploits multipath delay ponents to improve the performance of the system. A rake receiver bines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with a signal correlation tuned to the path delay of the strongest signal. Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system. In the FDMA and TDMA systems frequency planning is and important consideration. The frequencies used in different cells must be planned carefully to ensure signals from different cells do not interfere with each other. In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudorandom codes. Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system。幾種方式組合的混合系統(tǒng)也經(jīng)常得到應(yīng)用。 由于擴頻后的信號，它的發(fā)射功率很低，傳輸時大多數(shù)淹沒在空間的噪聲中，因此很難被其他接收機截獲，只有具有相同的擴頻碼（或稱 PN 碼）的接收機，才能獲得處理增益，解擴并恢復(fù)出原發(fā)的信號。由于擴頻系統(tǒng)中采用的 PN 碼具有很好的自相關(guān)性，互相關(guān)性很弱，不同路徑傳來的信號很容易被分開，并可在時間相位上重新對齊，形成幾路信號功率的疊加，從而改善了接收系統(tǒng)的性能增加了系統(tǒng)的可靠性，易于同頻使用，提高了無線頻譜利用率。 擴頻通信是數(shù)字通信，特別適合數(shù)字語音和數(shù)據(jù)同時傳輸，擴頻通信自身具有加密功能，只能在 PN 碼相同的用戶之間進行通信，隱蔽性好，保密性強，便于開展各種通信業(yè)務(wù)。它不應(yīng)該與被用在移動電話標準的要求的 cdmaOne， CDMA2021（ cdmaOne 的的 3G 演進）和 WCDMA 相混淆（用于 GSM 的載波 3G標準），它往往被簡稱為 CDMA，并且作為一個潛在的信道接入方法。 CDMA采用 擴頻技術(shù)和特殊編碼方案（其中每個發(fā)射機分配一個碼），以允許多個用戶被復(fù)用在同一物理信道。然而，擴頻技術(shù)用比所需的最小信號帶寬大幾個數(shù)量級的傳輸帶寬。不能接收解調(diào)不用于編碼數(shù)據(jù)的偽隨機序列的信號傳輸。 CDMA 還可以有效抑制窄帶干擾。由于擴展頻譜信號占有相當(dāng)大的帶寬，只一小部分信號將經(jīng)受衰落由于多路徑。換句話說，只要在多徑信道引起至少一個芯片的延遲，多徑信號將到達接收機，這樣，它們比預(yù)定信號中在時間上移位一個芯片延遲。 頻率復(fù)用是在蜂窩系統(tǒng)內(nèi)的其他小區(qū)站點重復(fù)使用相同的無線電頻率的能力。在每一個小區(qū)內(nèi)可以重復(fù)使用相同的頻率，所以在 CDMA 系統(tǒng)中不需要頻率規(guī)劃 。選擇最佳的信號品質(zhì)，直到軟切換完成。 結(jié)束語 擴頻通信在發(fā)展的初始階段，就已經(jīng)實現(xiàn)了理論與技術(shù)上的重大突破，在此之后的發(fā)展過程中主要是 硬件的改善和性能的提高
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