Vodafone Chair Mobile Communications Systems, Prof. Dr.-Ing. G. Fettweis
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Introduction to Communications:
Digital Modulation
Eye Diagram and BPSK Transmission
Prof. Dr.-Ing. Gerhard Fettweis
Chair Mobile Communications Systems, TU Dresden
Digital Data Transmission (1)
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Transmit Filter
ƒ Block diagram of the transmitter for transmitting
digital data over a baseband channel
ƒ Pulses of different symbols don’t overlap
ƒ Ideal channel with infinite bandwidth (rectangular pulses!) can
accommodate baseband transmission without signal distortion
ƒ Error-free signal reconstruction possible
TU Dresden,
Gerhard Fettweis
Slide 2
Digital Data Transmission (2)
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Transmit Filter
ƒ Transmission over band-limited channel:
Use a pulse shape with better localization in the frequency domain!
ƒ Consequence:
Each received pulse is affected by adjacent pulses.
ƒ Inter-symbol interference (ISI)!
ƒ ISI is a major source of bit errors at the receiver!
ƒ Pulse shapes at transmitter and receiver have to be chosen carefully
to minimize ISI (e. g. root-raised cosine pulses).
TU Dresden,
Gerhard Fettweis
Slide 3
Visualization of Pulse Shape Properties
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Eye Diagram:
ƒ Signal form viewed within one symbol interval
ƒ Contributions from many overlapping pulses
ƒ Horizontal and vertical aperture affects system performance
Raised Cosine filter, r = 0.5
Raised Cosine filter, r = 1
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Filtered signal
Filtered signal
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TU Dresden,
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Gerhard Fettweis
Slide 4
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BPSK Transmission over Noisy Channel
BPSK Transmission over Noisy Channel
TU Dresden,
Gerhard Fettweis
Slide 5
Baseband Transmission: System Model
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Transmit Filter
Receive Filter
ƒ Baseband equivalent transmission
ƒ Transmitted signal has lowpass characteristics
ƒ Channel is modelled as an AWGN channel
TU Dresden,
Gerhard Fettweis
Slide 6
Eye Diagram: Noise Influence
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Eye pattern: α=1, noise free
Eye pattern: α=1, SNR=12dB
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ƒ Presence of the noise causes closing of the eye.
ƒ For low SNR, the eye is closed.
ƒ Decision errors are made at the receiver.
TU Dresden,
Gerhard Fettweis
Slide 7
BPSK Transmission: Noiseless Channel
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Eye pattern: α=1, noise free
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ƒ Grayscale picture of Barkhausenbau, TU Dresden
ƒ Intensity of a pixel is encoded with n=8 bits
ƒ Channel is noiseless and nondispersive: Error-free transmission
TU Dresden,
Gerhard Fettweis
Slide 8
BPSK Transmission over AWGN Channel
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Eye pattern: α=1, SNR=4dB
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ƒ
ƒ
TU Dresden,
Gerhard Fettweis
Slide 9
BPSK Transmission over AWGN Channel
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Eye pattern: α=1, SNR=0dB
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ƒ
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ƒ Bad quality of the received picture
TU Dresden,
Gerhard Fettweis
Slide 10
BPSK Transmission over AWGN Channel
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Eye pattern: α=1, SNR=-6dB
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ƒ
ƒ
ƒ Details are unrecognizable
TU Dresden,
Gerhard Fettweis
Slide 11