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We know, digital signal is on time domain show disperse sex and have two kinds of state only 1 with 0, when stone's throw is deferent (100 meters the following) can use base band to transmit, should want to undertake remote when transmitting, be about to take carrier transmission kind. Carrier transmission system is in making digital signature to carrier wave send transmission channel again, it conveys with base band aching it is in the output of digital signal end increases a modulator only, a demodulator increases before the number inputs a mouth and other part is identical.
One, base band transmits a system
In digital communication system, the code that channel coder outputs still needs to alternate through pattern of sling-load, turn into comfortable at transmission pattern of sling-load. Commonly used base band transmits a code to basically have the following kinds: 1, bipolar do not clear code; 2, odd polarity does not clear code; 3, bipolar clear code; 4, odd polarity clears code; 5, graceful thorough is ripped pile up especially. Here alleged bipolar it is to point to with pulse and negative pulse represent digital signal respectively 1 with 0; Alleged odd polarity is to point to with pulse and zero do not represent digital signal 1 with 0; Alleged clearing is to represent the first yard yuan pulse is tightened afterwards is to represent the 2nd yard then yuan pulse, both between without time-interval, namely alleged clear. Graceful thorough is ripped be being piled up especially is with half symbol wide first hind negative (1, 0) signal of pulse delegate number 1, and with half symbol after be being lost first pulse (0, 1) delegate number signal 0, if pursue,D-1 place is shown. Bipolar do not clear in the code, if 0 with 1 occurrence probability identical, losing voltage as it happens to offset without dc heft, consequently advantageous to transmitting and have stronger interference rejection capacity.
Convey in base band in the system, use multichannel answer to use a technology normally, multichannel answer is used is to will come from each information of different message source to press some kind of means to incorporate for all the way, convey through same channel sink, sink gives each signal to give different user by corresponding means depart again. The means that multichannel answer uses has: 1, frequency is divided answer with; 2, time answer is used; 3, pile up cent answer to use; 4, cent answer is used; 5, time is compressed answer with etc. Use time answer morely in digital communication to use a technology, alleged time answer using is the different time slot that uses each signal same channel will have communication, because time answer uses each signal when transmitting to not be on same time,convey, not easy generation hands in harmonic intermodulation lack fidelity, what so time answer uses a system is nonlinear demand of lack fidelity index is not high.
In time answer two main parts of an apparatus should use in using a system: It is answer receive implement, its function is a principle that a few signal press time answer to use synthesis closes signal of word of a movement in martial arts for. Another is cent is received implement, it and answer receive implement the function is contrary, it is a signal closing a road reductive the digital signal that is a few spur track. Answer receive implement receive with cent implement outfit call digital answer to receive facility together. Digital answer receives must solve two problems: One is synchronous, one is answer receive. Synchronism is mixed by time system code fast adjust unit composition, the in-house clock of time system gives answer receive implement provide time base mark signal, code fast adjusting unit is a code fast different each spur track is adjusted with answer receive implement the digital signal that times signal finishs synchronism, answer receiving is to finish answer replace Wu, collect signal of each spur track receive all the way signal.
Signal of digital base band is rectangular wave, because bag of rectangular wave pulse contains rich harmonic heft, it is so in finite channel bandwidth, when transmitting, will surely arise lack fidelity, cause for this meeting bigger pile up by accident rate. The harmonic that piles up yuan of place generation as a result of every again is to be handed in each other on time domain fold, generated the interference between the code so, piling up yuan of weaveform fortunately is given out by certain interval, should be in only the undee amplitude of specific hour is done not have lack fidelity, pairs of although other part is lack fidelity very big codes yuan second birth court decision also does not have an influence.
2, the carrier wave of digital signal is deferent
Should undertake when digital signal longer of the distance when conveying, be about to use the means with deferent carrier wave. The carrier wave of digital signal conveyed to basically be distinguished even if add the segment of modulation and demodulation what convey with base band, it is to be in answer receive implement hind increased a modulator, receive in cent implement before increase a demodulator just.
Digital signal has value of a few disperse only, this is just like like control switch to choose to have the oscillation of different parameter with digital signal, the modulation means that is this number signal calls key to accuse. Modulation means has extent key to accuse (ASK) ; Frequency moves key accuses (FSK) ; Phase shift key accuses (PSK) .
1, extent key accuses (ASK)
Extent key accuses to be able to pass multiplier and switch circuit to come true. Carrier wave is in digital signal 1 or be connected below the control of 0 or break, put through for the state carrier of 1 in signal, right now transmission letter has carrier wave to appear on the road; Fall for the condition of 0 in signal, carrier wave is closed, right now transmission letter does not have carrier wave to convey on the road. So in sink we are OK according to carrier wave have without reductive give digital signal 1 with 0. Double what bolt to binary extent the frequency band width that accuses signal is width of signal of binary base band, weaveform is like graph D-2.
2, frequency moves key accuses (FSK)
Frequency moves the oscillation source that bolts accusing is to use F1 of two different frequency and F2 will represent signal 1 with 0, its weaveform is shown like graph D-3 place, use digital signal 1 output alternately with the 0 oscillation sources that go controlling two independence. Move to binary frequency key charges modulation means, its are effective bandwidth is B=2xF+2Fb, the biggest frequency that the bandwidth that XF is signal of binary base band also is FSK signal slants, the bandwidth as a result of digital signal namely Fb value is great, so binary frequency moves the signal bandwidth B that key accuses is bigger, frequency band utilization rate is minor.
3, phase shift key accuses (PSK)
In phase shift key accuses in, be in wave is phasic the control that gets signal of digital base band, if be in signal of binary base band,be 0 when, carrier wave is phasic for 0, for 1 when carrier wave is phasic for π , carrier wave is phasic have one to one correspondence concern with base band signal, its weaveform is like graph D-4.
3, make into the tones of Chinese characters that make number more
What what discuss above is the circumstance in signal of binary digit base band, in applying actually, we often are called with a kind enter more make (be like 4 into make, 8 into make, 16 into make etc) base band signal. Make carrier wave parameter M is planted into the tones of Chinese characters that make number more take a cost differently, compare binary digit modulation to have two outstanding advantages into modulation of the word that make number more: It is to have make digital signal contain more information to make frequency band utilization rate higher at be being entered more; 2 it is below same news rate duration grows, can raise a code yuan energy, reduce the interference between the code that causes as a result of channel character thereby. Because all over relation, here talks way of uses at most a kind of modulation only: Enter more make phase shift key accuse (MPSK) .
Enter more make phase shift key accuse to call heterogeneous to make again, the condition that because base band signal has M to plant,differs, its carrier wave is so phasic M is planted different extraction is worth, these take a value to be commonly wait for interval. Make in heterogeneous move key accuses to have move to be mixed absolutely move relatively two kinds, actual in use 4 photographs to move to bolt absolutely mostly accuse (4PSK, have weigh QPSK) , what 4 photographs make is phasic have 0, π / 2, π , 3 π / 4 kinds 2, part corresponding 4 kinds of condition 11, 01, 00, 10. QPSK signal can express to be Fct of π of I(t)COS2 π Ft-Q(t)SIN2, among them the first is inphase heft, the 2nd calls orthogonal heft, so QPSK calls key of orthogonal phase shift to accuse modulation again.
From go up knowable, the frequency band utilization rate of QPSK is 2 times of binary digit modulation more corresponding, but this is cost with sacrificial power utilization rate. Because as pile up yuan of distance between to reduce each into the addition that make, go against the refreshment of signal, frank meeting is piled up to increase subsequently by accident when noise and interference especially. To solve this problem, we must enhance signal power (the accretion that the letter a confusion of voices that raises signal namely leads than avoid to be piled up by accident) , this makes power utilization rate was reduced. For this whether a kind of method makes frequency band utilization rate increases each to pile up yuan of distance between not quite small? This introduced a kind to call QAM (orthogonal extent modulation) . The characteristic of QAM is to be piled up each yuan between not only range is distinct, phasic different also, attribute extent and the modulation kind that phasic photograph ties, in QPSK each are piled up yuan range is same it is phasic and different only, its are so average power is higher, because QAM is piled up each yuan range is distinct, average power is so lesser. Because this is below the situation with average same power, QAM is piled up each yuan n takes a cost but prep above QPSK is piled up each yuan take a cost, make believe a confusion of voices to be compared and rise thereby.
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