【正文】 the data. CDMA also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal. CDMA can also effectively reject narrow band interference. Since narrow band interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information. Convolution 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。 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，解擴并恢復出原發(fā)的信號。 擴頻通信是數(shù)字通信，特別適合數(shù)字語音和數(shù)據同時傳輸，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。 CDMA采用 擴頻技術和特殊編碼方案（其中每個發(fā)射機分配一個碼），以允許多個用戶被復用在同一物理信道。不能接收解調不用于編碼數(shù)據的偽隨機序列的信號傳輸。由于擴展頻譜信號占有相當大的帶寬，只一小部分信號將經受衰落由于多路徑。 頻率復用是在蜂窩系統(tǒng)內的其他小區(qū)站點重復使用相同的無線電頻率的能力。選擇最佳的信號品質，直到軟切換完成