计算机网络第四版习题答案(中文版)(12)

计算机网络第四版习题答案(中文版)

2-44 In a typical mobile phone system with hexagonal cells, it is forbidden to reuse a frequency band in an adjacent cell. If 840 frequencies are available, how many can be used in a given cell? 在一个典型的移动电话系统中，蜂窝单元为六角形，在相邻的单元内禁止重新使用频段。如果总共有840个频率可以使用的话，则任何一个给定的单元内可以使用多少个频率？

每个单元有6个邻居。如果中间的单元使用频段组合A，它的六个邻居可以分别使用的频段组合B, C, B, C, B, C。换句话说，只需要3个单一的单元。因此，每个单元可以使用280个频率。

2-50 Suppose that A, B, and C are simultaneously transmitting 0 bits, using a CDMA system with the chip sequences

of Fig. 2-45(b). What is the resulting chip sequence?

FIG 2-45(b)

结果是通过对A、B、C求反再将这三个码片序列相加得到的。

结果是(+3 +1 +1 1 3 1 1 +1).

2-53 A CDMA receiver gets the following chips: (-1 +1 -3 +1 -1 -3 +1 +1). Assuming the chip sequences defined in Fig. 2-45(b), which stations transmitted, and which bits did each one send?

一个CDMA接收器得到了下面的时间片（-1+1-3+1-1-3+1+1）。假设时间片序列如图2.45b中所定义，请问那些移动站传输了数据？每个站发送了什么位？

Just compute the four normalized inner products:此处答案中的~疑为-号之误？

( 1 +1 3 +1 1 3 +1 +1) d ( 1 1 1 +1 +1 1 +1 +1)/8 = 1

( 1 +1 3 +1 1 3 +1 +1) d ( 1 1 +1 1 +1 +1 +1 1)/8 = 1

( 1 +1 3 +1 1 3 +1 +1) d ( 1 +1 1 +1 +1 +1 1 1)/8 = 0

( 1 +1 3 +1 1 3 +1 +1) d ( 1 +1 1 1 1 1 +1 1)/8 = 1

结果是 A 和 D 发送了1位, B发送了0位, C 没有发送。