Introduction to the Communication Process Digital Transmission MEEC José Manuel Bioucas Dias Instituto Superior Técnico, 2014
Outline 1. The communication process 2. Elements of a communication system 3. s of information 4. Communication channels 5. Modulation processes 6. Analog and digital communications 7. Theoretical limits for the communication process
The communication process The communication between a source and a user involves implicitly the transmission of information through the following processes, in chronological order: 1. Generation of the message signal (voice, music, picture, computer data) 2. Conversion of the message signal into a sequence of symbols suitable for communication (electrical, phonetic, visual, etc.) 3. The encoding of the symbols in an appropriate format for transmission over a given physical channel 4. Transmission of coded symbols to the user 5. Decoding the received symbols and re-creation of the original message signal with a given degradation caused by imperfections in the system
Elements of a communication system User Message signal Received signal Transmitter Receiver Transmitted signal Received signal Noise, interference, distortion Communication modes: 1. broadcasting: involves a single complex transmitter (expensive) and very simple receptors (inexpensive); the communication is unidirectional 2. Point-to-point: involves only one transmitter and one receptor connected with a link; communication is bidirectional
Fontes 0.03 0.02 Voice signal 0.01 0-0.01-0.02... aaaaaaa... -0.03 0 50 100 150 200 250 Video signal
s Atmosphere Ocean Twisted cable Coaxial cable Optical fiber Multi-path Digital media
Modulation process Message Desmodulated signal User Modulador Desmod. Modulated signal Received signal A modulation process involves two signals: 1) A modulating signal which encodes the message 2) A carrier signal The modulated signal carries the information about the modulating signal In general, the modulation is a reversible process
x T (V) P (V) Amplitude modulation (AM) 1 0.5 0-0.5-1 0 0.5 1 1.5 2 2.5 3 t (ms) 2 1.5 1.5 1 0.5 1 0 0.5-0.5-1 0 0 0.5 1 1.5 2 2.5 3 t (ms) -1.5 0 0.5 1 1.5 2 2.5 3 t (ms)
Frequency modulation (FM) 2 1 1.5 1 0.5 0.5 0 0-0.5-1 -1.5-0.5-2 0 0.5 1 1.5 2 2.5 3 t(ms) -1 0 0.5 1 1.5 2 2.5 3 t(ms)
Aim of modulation 1. Efficient transmission Selecting bands, then wavelengths suitable to the propagation media and antennas for transmission and reception 2. Sidestep hardware limitations Selecting bands, then wavelengths so that the bandwidth With respect to the frequency of the carrier remains in the range 1-10% 3. Avoiding noise and interferences Selecting bands so as to minimize noise and interference 5. Frequency allocation Use of frequency bands allocated by regulating entities 6. Signal multiplexing Simultaneous use of a single channel for multiple signals through multiplexing (FDM, TDM, CDM, WDM)
Digital communication Transmitted symbols Received symbols User encoder decoder encoder decoder Modulator Desmod.
Digital communication Transmitted symbols Received symbols User encoder decoder encoder decoder Modulator Desmod. 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 even when is severely degraded
Theoretical limits of the communication process + (band B) noise Information Capacity Theorem (Shannon) bits per second - transmitted mean power - noise mean power in the bandwidth B
Relevant events for the communications Year Event 1800-1837 Battery (Volta); The mathematical treatises by Fourier; Experiments with electric and magnetic fields (Oersted, Ampere, Faraday, Henry); Ohm s law 1838-1866 Telegraphy (Morse); Transatlantic cables 1845 Kirchoff circuits laws 1864 Maxwell equations predicts electromagnetic radiation 1876-1899 Telephone (Alexander Bell); Cable circuits; Theory of cable loading (Heaviside, Pupin, and Campbell) 1887-1907 Wireless telegraphy; Maxwell theory verified (Heinrich Hertz, Marconi e Papov); Commercial service begins 1904-1920 Triode (Lee De Forest); Basic filter types (Campbell); Experiments with AM modulation and broadcasting; Transcontinental telephone (Bell); multiplexed carrier telephony; Superheterodyne radio 1920-1928 Transmission theory (Carson, Nyquist, Jonhson, Hartley) 1936 FM theory (Armstrong) 1938-1945 Radar; Extensive use offm 1948-1950 Information and codification theory (Shannon) 1948-1951 Transistor (Bardeen, Brattain, Shockley) 1953 Color TV in the United States 1961 Integrated circuits 1961 Satellite communications 1966-1975 Wideband communication systems; Cable TV systems; Commercial satellite relay systems; Optical links using lasers and fiber optics; ARPANET 1978 Global Positioning System... 1991 Global System for Mobile Communications