文档介绍：数据通信与网络chapter05
5-1 DIGITAL-TO-ANALOG CONVERSION
Digital-to-analog conversion is the process of changing one of 8000 bps and a baud rate of 1000 baud. How many data elements are carried by each signal element? How many signal elements do we need?
Solution
In this example, S = 1000, N = 8000, and r and L are unknown. We find first the value of r and then the value of L.
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Figure Binary amplitude shift keying
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Figure Implementation of binary ASK
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Figure Bandwidth of MFSK used in Example
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Example
We need to send data 3 bits at a time at a bit rate of 3 Mbps. The carrier frequency is 10 MHz. Calculate the number of levels (different frequencies), the baud rate, and the bandwidth.
Solution
We can have L = 23 = 8. The baud rate is S = 3 MHz/3 = 1000 Mbaud. This means that the carrier frequencies must be 1 MHz apart (2Δf = 1 MHz). The bandwidth is B = 8 × 1000 = 8000. Figure shows the allocation of frequencies and bandwidth.
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Figure Bandwidth of MFSK used in Example
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Figure Binary phase shift keying
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Figure Implementation of BASK
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Figure QPSK and its implementation
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Example
Find the bandwidth for a signal transmitting at 12 Mbps for QPSK. The value of d = 0.
Solution
For QPSK, 2 bits is carried by one signal element. This means that r = 2. So the signal rate (baud rate) is S = N × (1/r) = 6 Mbaud. With a value of d = 0, we have B = S = 6 MHz.
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Figure Concept of a constellation diagram
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Example
Show the constellation diagrams for an ASK (OOK), BPSK, and QPSK signals.
Solution
Figure shows the three constellation diagrams.
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Figure Three constellation diagrams
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Quadrature amplitude modulation is a combination of ASK and PSK.
Note
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Figure Constellation diagrams for some QAMs
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5-2 ANALOG AND DIGITAL
Analog-to-analog conversion is the representation of analog i