Introduction to Communication Systems. Reference: Chapter 1, A.B. Carlson,
Communication Systems. – conveys information from its source to destination ...
EIE 331 Communication Fundamentals Lecturer: Room no.: Phone no.: e-mail:
Dr. W.Y.Tam DE604 27666265
[email protected]
web: www.en.polyu.edu.hk/~em/mypage.htm Normal Office hour: 8:00am – 5:30pm
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Assessment Examination (open book)
60%
Practical Two Experiments 20 * 1/3% Mini-project 20 * 2/3% (Attendance, preparation, report writing and demonstration) Test quizzes Assignments
10% 5% 5%
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Textbook Textbook F.G. Stremler, Introduction to Communication Systems, Addison Wesley. A. B. Carlson, Communication Systems, McGraw Hill, 4th ed., 2002 Reference S.S. Haykin, Communication Systems, Wiley, 2001.
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Introduction to Communication Systems Reference: Chapter 1, A.B. Carlson, Communication Systems. – conveys information from its source to destination some distance away.
Information
What is information?
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Information Example: signals which contain information • voice • music • still picture • video • Data • Multimedia signals
How to determine the information of a message?
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Signals Characteristics of information sources Example The frequency of voice is always limited to 0 - 5kHz. In most cases, people can understand the content if we transmit only 0 3kHz. Therefore, in telephone system, 3kHz is allocated for each voice channel. -- Bandwidth A monitor which refreshes at 30 frame/s to produce a visibly continuous display. Therefore, in VCD, the frame rate is 30 frame/s. --- Data rate
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Analog and Digital Signals Analog signal – Smooth – Continuous Digital signal – Sequence of symbols selected from a finite set of discrete elements – Examples: voice music stored in a CD keys you press on a computer keyboard
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Elements of a communication system Source Input transducer Source
Input Transducer
Input signal
Input signal
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Elements of a communication system Transmitter – Process the input signal – Produce a signal suited to the characteristics of the transmission channel – What is a channel ?
Source
Input Transducer
Transmitter
Transmitted Signal
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Transmitter Transmitter – Modulation – coding Source
Input Transducer
Transmitter
Encoder
Modulator
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Channel Channel – Electrical medium – Example: • Pair of wires • Coaxial cable • Optical fiber • Air Transmitter
Channel
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Channel: attenuation Channel is not perfect !! • Impairments (undesirable effects) – limit the transmission of signals – reducing the data rate and distance – Attenuation • Due to the losses, the magnitude of a signal decreases.
Transmitter
Channel Intro.12
Channel: distortion Distortion – Change the shape of the signal – Can be corrected
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Channel: Noise Noise – Random and unpredictable electrical signal – Example: thermal noise • presence in all subjects with temperature higher than 0 K. – Example: • a digit “1” can become “0” in the presence of noise.
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Interference Interference – Distort the signal – from competing signals in overlapping frequency • Example: other transmitters power lines switching circuits
• Example the signal in a telephone line may couple to another telephone line if they are bundled together.
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Receiver Amplifier – Compensate for transmission loss Filter – Reduce the effects of noise, interference and distortion Demodulator
Channel
Received Signal
Receiver
Output Signal
Decoder
Amplifier
Filter
Demodulator
Decoder Intro.16
Modes Simplex – One way transmission Full Duplex
Half-duplex – Transmit in either direction but not at the same time – Example: Walkie Talkie
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Fundamental limitations Technological problems – Hardware availability – Regulations Fundamental physical limitations – Bandwidth • Example: bandwidth of telephone line = 3.3kHz – Noise
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Bandwidth Bandwidth – Signal • Measure of speed (data rate) • High data rate Æ larger bandwidth – System • Transmission bandwidth
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Noise
Example: – Thermal noise Signal-to-noise power ratio – S/N – Channel capacity • The rate of information transmission cannot exceed the channel capacity
C = B log 2 (1 + S / N ) Intro.20
Channel capacity Example: Telephone line – B ≅ 3.3 kHz – S/N ≅ 30dB • i.e. S/N = 1000 – C = 3300log2(1001) = 3289 bit/s
C = B log 2 (1 + S / N )
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Modulation Modulation – Modulating signal • Message – Carrier wave – Example: mobile telephone system radio – Modulator • “Combine” modulating signal and carrier wave • Modulated signal – Demodulator • Extract the modulating signal from the modulated signal Intro.22
Modulation benefits and applications Efficient transmission – Example: Antenna size • For efficient radiation, physical dimensions > 0.1 wavelength • Audio signal contains frequency components down to 100Hz Î wavelength = 3km • Modulated frequency at 100MHz Æ wavelength = 3m
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Modulation benefits and applications Modulation to overcome hardware limitations – Fractional bandwidth • Bandwidth/center frequency • 1-10% • Example: FM radio Signal Bandwidth = 200kHz Fractional bandwidth = 200kHz/ 100MHz = 0.2% Intermediate frequency = 10.7MHz Fractional bandwidth = 200kHz/ 10.7MHz =
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Modulation benefits and applications Modulation to reduce noise and interference – Wideband noise reduction Modulation for frequency assignment
Channel 1
Channel 2
frequency
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Modulation benefits and applications Modulation for multiplexing – Combining several signals for simultaneous transmission on one channel. – Frequency-division multiplexing (FDM) • Each signal uses different carrier frequencies
Channel 1
Channel 2
frequency
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Coding Modulation – Signal-processing operation for effective transmission. Coding – Symbol-processing operation for improved communication when the information is digital – Channel coding – Source coding
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Channel Coding Channel coding – Introduce controlled redundancy to improve the performance reliability in a noisy channel – Example: • 1 become 11111 • 0 become 00000 • If the received bit pattern is 10011, the corrected output is
1
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Source coding Source coding – Reduce redundancy to achieve the desired efficiency – Example: • Symbol a, b, c P(a) = 0.8, P(b)=0.15 P(c)=0.05 a is encoded as 1 b is encoded as 10 c is encoded as 01
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