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11. A. what be studied B. what should study C. what to study D. what studied
12. A. In three years B. For three years C. After three years D. Three years
13. A. never would be B. would be never C. would not be ever D. would never be
14. A. I didn't either B. Nor I did C. So didn't I D. Also didn't I
15. A. Not al1 B. Neither C. Nor D. Both
16. A. since B. while C. before D. whereas
17. A. understand B. recognize C. suggest D. inform
18. A. speaking B. to say C. to speak D. saying
19. A. a lot B. much C. a bit D. more
20. A. as B. being C. to be D. but
13. A. never would be B. would be never C. would not be ever D. would never be
14. A. I didn't either B. Nor I did C. So didn't I D. Also didn't I
15. A. Not al1 B. Neither C. Nor D. Both
16. A. since B. while C. before D. whereas
17. A. understand B. recognize C. suggest D. inform
18. A. speaking B. to say C. to speak D. saying
19. A. a lot B. much C. a bit D. more
20. A. as B. being C. to be D. but
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問(wèn)答題
在M＞4時(shí)，MQAM與MPSK哪一種抗噪性能更好？
答案：
MQAM（M-ary Quadrature Amplitude Modulation，M進(jìn)制正交振幅調(diào)制）和MPSK（M-ary Phase Shift Keying，M進(jìn)制相移鍵控）都是數(shù)字調(diào)制技術(shù)，它們?cè)诓煌膽?yīng)用場(chǎng)景和條件下有不同的性能表現(xiàn)。
在M>4時(shí)，MQAM和MPSK的抗噪性能取決于多種因素，包括信噪比（SNR）、調(diào)制階數(shù)M、以及具體的實(shí)現(xiàn)方式等。通常，MQAM在高階調(diào)制（即M值較大時(shí)）時(shí)，其星座點(diǎn)之間的距離會(huì)比MPSK小，因此在相同的信噪比條件下，MQAM的誤碼率（BER）會(huì)比MPSK高，即抗噪性能相對(duì)較差。
MPSK由于其相位的離散性，每個(gè)符號(hào)之間的相位差是固定的，這使得它在高信噪比環(huán)境下具有更好的性能。然而，隨著M的增加，MPSK星座點(diǎn)之間的最小相位差會(huì)減小，這會(huì)降低其抗噪性能。
在實(shí)際應(yīng)用中，MQAM和MPSK的選擇通常取決于系統(tǒng)對(duì)數(shù)據(jù)速率和誤碼率的要求，以及可用的帶寬資源。在帶寬受限的情況下，MQAM可能更受歡迎，因?yàn)樗軌蛟诮o定的帶寬內(nèi)傳輸更多的數(shù)據(jù)。而在對(duì)誤碼率要求較高的應(yīng)用中，MPSK可能更受青睞。
總結(jié)來(lái)說(shuō)，在M>4時(shí)，如果只考慮抗噪性能，MPSK通常比MQAM有更好的性能，尤其是在高信噪比環(huán)境下。然而，實(shí)際選擇哪種調(diào)制方式還需要綜合考慮其他因素，如頻譜效率、實(shí)現(xiàn)復(fù)雜度、系統(tǒng)設(shè)計(jì)要求等。
