Digital Modulation - Vodafone Chair Mobile Communications Systems

Vodafone Chair Mobile Communications Systems, Prof. Dr.-Ing. G. Fettweis chair
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)
Transmit Filter
Block diagram of the transmitter for transmitting
digital data over a baseband channel
Pulses of different symbols don’t overlap
Ideal channel with infinite bandwidth (rectangular pulses!) can
accommodate baseband transmission without signal distortion
Error-free signal reconstruction possible
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TU Dresden, Gerhard Fettweis Slide 2

Digital Data Transmission (2)
Transmit Filter
Transmission over band-limited channel:
Use a pulse shape with better localization in the frequency domain!
Consequence:
Each received pulse is affected by adjacent pulses.
Inter-symbol interference (ISI)!
ISI is a major source of bit errors at the receiver!
Pulse shapes at transmitter and receiver have to be chosen carefully
to minimize ISI (e. g. root-raised cosine pulses).
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TU Dresden, Gerhard Fettweis Slide 3

Visualization of Pulse Shape Properties
Filtered signal
Eye Diagram:
Signal form viewed within one symbol interval
Contributions from many overlapping pulses
Horizontal and vertical aperture affects system performance
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Raised Cosine filter, r = 0.5 Raised Cosine filter, r = 1
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1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5
TU Dresden, Gerhard Fettweis Slide 4
Filtered signal
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BPSK Transmission over Noisy Channel
BPSK Transmission over Noisy Channel
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TU Dresden, Gerhard Fettweis Slide 5

Baseband Transmission: System Model
Transmit Filter Receive Filter
Baseband equivalent transmission
Transmitted signal has lowpass characteristics
Channel is modelled as an AWGN channel
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TU Dresden, Gerhard Fettweis Slide 6

Eye Diagram: Noise Influence
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Eye pattern: α=1, noise free
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Presence of the noise causes closing of the eye.
For low SNR, the eye is closed.
Decision errors are made at the receiver.
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TU Dresden, Gerhard Fettweis Slide 7
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Eye pattern: α=1, SNR=12dB
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BPSK Transmission: Noiseless Channel
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Eye pattern: α=1, noise free
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Grayscale picture of Barkhausenbau, TU Dresden
Intensity of a pixel is encoded with n=8 bits
Channel is noiseless and nondispersive: Error-free transmission
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TU Dresden, Gerhard Fettweis Slide 8

BPSK Transmission over AWGN Channel
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Eye pattern: α=1, SNR=4dB
time[s]
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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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Bad quality of the received picture
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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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time[s]
Details are unrecognizable
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TU Dresden, Gerhard Fettweis Slide 11