作者Leon (Achilles)
站內comm_and_RF
標題Re: [請益] QPSK & 2-PAM*2
時間Thu Oct 25 11:14:34 2007
※ 引述《IronHand (蹂躪之爪牙)》之銘言：
: 1.為什麼QPSK(4-PSK) & 兩個independent 2-PAM系統combined
: 的performance會相同呢
用 signal space 的 dimension 來看.
: 2.Consider a PSK system.
: It is suggested to modify the system to enhance its BER performance.
: There are two choices.
: The first is to double the average transmitting power level.
: The second is to duplicate the system on another carrier,
: i.e. to use two independent radios duplicating the same information
: data stream. Which one will you choose? Why?
因為我覺得推文裡面的回答有些問題, 我就直接回文寫我的想法.
at first, you need to define what channel are you using ?
if it's AWGN .... then, transmit same information twice is just
like a repetition code. And repetition does NOT have any coding gain.
我寫個例子:
S_0 : transmit -1,
S_1 : transmit +1,
if you double the transmitting power, it will be sqrt(2) and -sqrt(2).
repetition,
S_0 : (-1,-1)
S_1 : (+1, +1)
Both cases have the same minimum distance.
They should have the same performance.
_
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