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QAM

24.1

QAM
Introduction
QAM stands for Quadrature Amplitude Modulation. The idea is to use both(prenominal) phase and premium modulation to try and get more information done a channel of a given bandwidth. As weve storied before, trying to use AM on a radio set channel is quite enigmaatic due to the amplitude fluctuations introduced by fading. However, for point-to-point, or fixed links, this is often not a problem and QAM is feasible. QAM is also used extensively for modems. If

s (t ) = a mI (t ) cos ?c t ? mQ (t ) sin ?c t
2 2

[

]

(24.1)

the amplitude is A(t ) = a mI (t ) + mQ (t ) and the phase is tan ?(t ) = mQ (t ) / mI (t ) . Allowing mI , mQ to take on values other than ± 1 will in general result in both phase and amplitude

variations. For spokesperson, let mI , mQ take on some(prenominal) of the four values 3, 1, 1,3. Then in that respect atomic number 18 16 possible combinations of mI , mQ . The result is 16-QAM. The configuration diagram is shown below. An example 16-QAM signal is shown in Fig. 24.2

0111

0011

1011

1111

0110

0010 a

1010

1110

0100

0000

1000

1100

0101

0001

1001

1101

Figure 24.1: 16-QAM constellation diagram.



The dapple labels are a Gray code designed so that a symbol fracture causes only one bit computer error. Since there are sixteen constellation points, each point or symbol, can represent four bits. Demodulation is now a bit trickier. We would calculate amplitudes along both the cos ?c t and ? sin ?ct axes of the constellation diagram. We then find which constellation point we are closest
EE432: RF Engineering for Telecommunications Scott Hudson, Washington deposit University 09/27/03

QAM

24.2

to and assume that was the symbol transmitted. For example, the gray box in Fig. 24.1 would represent the decision region for the symbol 1000. We make an error if the noise is large enough to take us kayoed of this box. If a is the distance illustrated, then for the 1000 symbol, the probability of error would be Pe ,sym =...If you want to get a full essay, coiffure it on our website: Orderessay



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