Position:? See by the graph, the extent n on inphase axis and orthogonal axis is no longer 2 however 4 (16QAM) mixes 6 (32QAM) , the digital rate that can transmit also will be original 4 times to 5 times (take no account of boil fall factor) . But can not increase transmission number to lead without limit through increasing n progression. Because of the addition that counts as n, the interval between n is reduced, noise tolerance is reduced, pile up by accident below same noise condition increase. Also meet on time axis such, each are phasic and removed reduce, the interference between the code increases, shake and time the problem can make receive the effect to become poor. Graph 5 - the 4 constellation that are 64QAM pursue, what 64QAM and 256QAM use at date of telecommunication of number of be issued to lower levels is deferent. The frequency band utilization rate of 64QAM can amount to 5bit ／ Hz. If the general block diagram of QAM modulator pursues 5 - 5 are shown. ? ?
String - and the input that transformer is rate Rb 2 divide into two rate to be into alignment
two n alignment, 2 - L n transformer is every rate, two n alignment becomes rate to be
? The L n signal of M, multiply with the carrier wave photograph of two orthogonality respectively next, addition hind generates MQAM signal namely. In the M=64 when 64QAM modulation.
? The demodulation of MQAM signal can use method of demodulation of be concerned with likewise, its block diagram also is drawn in the graph 5 - 5 in. The signal of L n base band of inphase road and orthogonal road is used have (L - 1) the door is restricted the court decision of n implement after the court decision, restore to give rate to be equal to respectively binary alignment, final classics and - the binary alignment that strings together transformer to compose sequence of two binary systems a rate to be Rb.
? Cent answer uses 4 orthogonal frequency
The number is transmitted in a very big question is much diameter problem, namely the double image problem in TV. If reflex,be near many periodic centers, can bring the interference between serious code to the court decision. Must use balanced reach tap defer line to adjust wait for a method to reduce this kind of interference. Use orthogonality frequency divides answer with (OFDM: Method of Orthogonal Frequency Division Multiplex Ing) overcomes another train of thought of the effect that reflection or double image cause effectively, its are basic the method is a carrier wave before becomes many carrier, turn signal of high number rate into small amount to lead signal, respectively modulation is on every carrier wave. Because digital rate is reduced greatly, bit cycle grows substantially greatly, because this reflexes the influence of wave,reduce greatly. Because OFDM is of orthogonality between carrier wave each, although have between each carrier wave,overlap accordingly part, orthogonal sex also can be used to part signal of each carrier wave when demodulation. Can make full use of so bandwidth, arrange as far as possible much carrier wave. Graph 5 - 6 show the spectrum that gave OFDM, the interval between each carrier wave is the reciprocal of symbolic cycle, either carrier wave is in other carrier wave locally the value all is (Sinx ／ X) . Hundreds even the method that thousands of carrier wave produces cannot be used normally Suo Xiangpin leads synthesizer, and should use turn over disperse Fu Lixie to alternate (the carrier wave that IDFT) will come to to produce place to need an amount at the same time. Another profit that uses this kind of method is, should make a certain position has or very simple without carrier wave, should make only relevant data is " 1 " or " 0 " . European DVB, HDTV and DAB system use OFDM modulation means.
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