2. 计算机
  3. CDMA for cellular systems can be described as follows．As with FDMA，each cell is allocated a frequency (71)，which is split into

CDMA for cellular systems can be described as follows．As with FDMA，each cell is allocated a frequency (71)，which is split into

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问题 CDMA for cellular systems can be described as follows．As with FDMA，each cell is allocated a frequency  (71)，which is split into two parts，half for reverse(mobile unit to base station)and half for (72)  (base station to mobile uni0．For full—duplex   (73) ，a mobile unit uses both reverse and forward channels．Transmission is in the form of direct—sequence spread (74)，which uses a chipping code to increase the data rate of the transmission，resulting in an increased signal bandwidth．Multiple access is provided by assigning (75) chipping codes to multiple users，SO that the receiver can recover the transmission of an individual unit from multiple transmissions．
(75)
选项 A、concurrent
B、orthogonal
C、higher
D、10wer
答案B
解析 用于蜂窝系统的CDMA技术可以描述如下。就像FDMA一样，每一个小区被分配了一个频带，该频带划分为两半，一半用于反向信道(从移动终端到基站)，一半用于正向信道(从基站到移动终端)。在全双工通信中，移动终端使用了反向和正向两个信道。传输以直接序列扩频方式进行，用一个码片来增加传输的数据速率，同时也产生了额外的信号带宽。通过把正交的各个码片指定给不同的用户就可以实现多路访问，这样接收者就能够从多个传输中提取并恢复需要的传输单元。
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