Unit10_Digital_Com_Part2 - Unit 10 Digital...

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Unit 10 Digital Communications (Part 2) Digital Communications (Part 2) 10-1
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Unit 10: Outline 10.1 Communication Channels 10.2 Baseband Transmission 10.3 Bandpass Modulation and Demodulation Digital Communications (Part 2) 10-2
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Unit 10.1 Communication Channels Digital Communications (Part 2) 10-3
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Ideal Channel An ideal channel is a system that its output signal is exactly the same as its input signal. No loss of information in the transmission. Input Signal Ideal Channel Output Signal s(t) r(t) r(t) = s(t) Digital Communications (Part 2) 10-4
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Transmission Impairment In reality, no channel is ideal. Signal received is not exactly the same as signal transmitted. Impairment is mainly caused by Distortion Attenuation Noise Digital Communications (Part 2) 10-5
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Band-Limited Channels In reality, a channel can only accommodate a limited band of frequencies because of the physical properties of a medium. In practice, all channels are band-limited . For a band-limited channel, there is a highest acceptable frequency f H and a lowest acceptable frequency f L of the channel. The frequency components of a signal within the range [f L , f H ] can pass through the channel. Digital Communications (Part 2) 10-6
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Lowpass and Bandpass Channels If f L is 0 Hz or close to 0 Hz, a channel is called baseband / lowpass . If f L is much larger than 0 Hz, a channel is called bandpass . Digital Communications (Part 2) 10-7
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Channel Bandwidth The bandwidth of a channel is defined as W = f H – f L Digital Communications (Part 2) 10-8
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Binary Signal and Bit Rate Suppose we use two distinct voltages to represent 0 and 1. The bit duration (in sec.) is denoted byT b . The bit rate, R b , is defined as the number of bits carried by a signal in one second. We have R b = 1/T b . Rectangular Pulses Digital Communications (Part 2) 10-9
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Signal Bandwidth The frequency spectrum of a random binary signal is a continuous function of frequency. The signal bandwidth is inversely proportional to T b . Digital Communications (Part 2) 10-10
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Distortion For bandlimited (baseband) channels, the output r(t) is always a distorted version of s(t). If a binary signal is seriously distorted, the data carried by the binary signal cannot be fully recovered . The larger the channel bandwidth, the higher the bit rate can be supported , as major frequency components of s(t) can be kept. Channel with bandwidth W s(t) r(t) Binary signal Digital Communications (Part 2) 10-11
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Example Larger channel bandwidth, the spreading of the pulse will be slight. But it is wasteful if you provide more bandwidth than what is needed! Digital Communications (Part 2) 10-12
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Inter-Symbol Interference (ISI) When W is close to signal bandwidth, the spreading of the pulse will exceed a symbol duration and cause signal pulses to overlap, resulting in ISI.
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