问题 问答题 简答题

阐述移动通信中时分多址的特点。

答案

参考答案:

(1)TDMA系统中几个用户共享单一的载频,其中每个用户使用彼此互不重叠的时隙,每帧中的时隙数取决于几个因素,例如调制方式、可用带宽等等;

(2)TDMA系统中的数据发射不是连续的,各移动台发送的是周期性突发信号,而基站发送的是时分复用信号。由于用户发射机可以在不用的时间(绝大部分时间)关掉,因而耗电较少;

(3)由于TDMA系统发射是不连续的,移动台可以在空闲的时隙里监听其它基站,从而使其越区切换过程大为简化。通过移动台在TDMA帧中的空闲时隙监听,可以给移动台增加链路控制功能,如使之提供移动台辅助越区切换(MAHOmoBILe assisted handoff)等等;

(4)同FDMA信道相比,TDMA系统的传输速率一般较高,故需要采用自适应均衡,用以补偿传输失真;

(5)TDMA必须留有一定的保护时间(或相应的保护比特),但是如果为了缩短保护时间而使时隙边缘的发送信号压缩过快,则发射频谱将展宽,并将对相邻信道构成干扰。

(6)由于采用突发式发射,TDMA系统需要更大的同步报头。TDMA的发射是分时隙的,这就要求接收机对每个数据突发脉冲串保持同步,此外TDMA需要有保护时隙来分隔用户,这使其与FDMA系统相比有更大的报头;

(7)TDMA系统的一个优点是在每帧中可以分配不同的时隙数给不同的用户,这样通过基于优先级对时隙进行链接或重新分配,可以满足不同用户的带宽需求。

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Viennese-born composer Frederick Loewe,小题1:        we remember from his classical musicals including “My Fair Lady” and “Camelot”, was not always famous. He studied piano with the great masters of Europe and achieved huge success as a 小题2:           (music) and composer in his early years. But 小题3:         he immigrated to the United States, he failed as a pianist. For a while he tried other types of work including gold mining and boxing. 小题4:_______            he never gave up his dream and continued to play the piano and write music.

During those difficult years, he could not always afford 小题5:        (make) payments on his piano. One day, bending over the keyboard, he could hear nothing but the music that he played with such 小题6:  inspiration. When he finished and looked up, he was astonished to find that he had an audience of three moving men who were seated on the floor.

They said nothing and made no movement toward the piano. Instead, they小题7:       (dig) into their pockets, pooled together enough money for the payment, placed 小题8:      on the piano and walked out, empty handed. 小题9:_________ (move)) by the beauty of his music, these men recognized excellence and responded 小题10:         it..

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