Communication Systems full course Lecture 2.pdf
- 1. EEE 3105 : Communication Systems Course Instructor: Dr. Mosabber Uddin Ahmed
- 2. Noise in Communication System ▶ In communication system, noise is an unwanted and undesired quantity in electrical signals. ▶ Noise is a summation of unwanted or disturbing energy arises from; ❖ Natural source: includes elements such as atmosphere, sun, and other galactic sources. ❖ Man-made sources: includes sources as spark-plug ignition noise, switching transients and other radiating electromagnetic signals. ▶ Noise is ever present part of all systems and resulting degradation in the quality of transmitted and received signal. ▶ In analogsystems,noisedeterioratesthequality ofthereceivedsignal,e.g.theappearanceof “snow”onthe TVscreen,or “static” soundsduringanaudiotransmission. ▶ In digitalcommunication systems,noisedegradesthe throughputbecauseit requires retransmission ofdata packets orextra codingto recoverthedata in thepresenceoferrors.
- 3. Cont... Different types of noise are generated by different devices and different processes. ❖ Thermal noise: it’s a natural form of noise also called Johnson–Nyquist noise. It is unavoidable, and generated by random thermal motion of charge carriers (usually electrons), inside an electrical conductor, which happens regardless of any applied voltage. A communication system affected by thermal noise is often modelled as an additive white Gaussian noise (AWGN) channel. ❖ Shot noise: Shot noise in electronic devices results from unavoidable random statistical fluctuations of the electric current when the charge carriers (such as electrons) traverse a gap. If electrons flow across a barrier (in diode), then they have discrete arrival times. Those discrete arrivals exhibit shot noise. ❖ Flicker noise: also known as 1/f noise, is a signal or process with a frequency spectrum that falls off steadily into the higher frequencies, with a pink spectrum. It occurs in almost all electronic devices and results from a variety of effects. ❖ Burst noise: also known as popcorn noise (for popping or crackling sounds), consists of sudden step-like transitions between two or more discrete voltage or current levels, as high as several hundred microvolts, at random and unpredictable times. Each shift in offset voltage or current lasts for several milliseconds to seconds. ❖ Transit-time noise: If the time taken by the electrons to travel from emitter to collector in a transistor becomes comparable to the period of the signal being amplified, that is, at frequencies above VHF and beyond, the transit-time effect takes place and noise input impedance of the transistor decreases. From the frequency at which this effect becomes significant, it increases with frequency and quickly dominates other sources of noise.
- 4. Cont... While noise may be generated in the electronic circuit itself, additional noise energy can be coupled into a circuit from the external environment, by inductive coupling or capacitive coupling, or through the antenna of a radio receiver. ❖ Intermodulation Noise: Caused when signals of different frequency share the same medium. ❖ Crosstalk: Phenomenon in which a signal transmitted in one circuit or channel of a transmission systems creates undesired interference onto a signal in another channel. ❖ Interference: Modification or disruption of a signal travelling along a medium. ❖ Atmospheric Noise: This noise is also called static noise and it is the natural source of disturbance caused by lightning discharge in thunderstorm and the natural (electrical) disturbances occurring in nature. ❖ Industrial Noise: Sources such as automobiles, aircraft, ignition electric motors and switching gear, High voltage wires and fluorescent lamps cause industrial noise. These noises are produced by the discharge present in all these operations. ❖ Solar Noise: that originates from the Sun is called solar noise. Under normal conditions there is constant radiation from the Sun due to its high temperature. Electrical disturbances such as corona discharges, as well as sunspots can produce additional noise. The intensity of solar noise varies over time in a solar cycle. ❖ Cosmic Noise: Distant stars generate noise called cosmic noise. While these stars are too far away to individually affect terrestrial communications systems, their large number leads to appreciable collective effects.
- 5. SNR(Signalto NoiseRatio) ▶ In communication systems, signal-to-noise ratio (SNR), is a measure of signal strength relative to background noise in signal. 𝑆𝑁𝑅 = 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑠𝑖𝑔𝑛𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑛𝑜𝑖𝑠𝑒 𝑝𝑜𝑤𝑒𝑟 • The ratio is usually measured in decibels (dB). ▶ If the incoming signal strength in microvolts is Vs, and the noise level, also in microvolts, is Vn, then the signal-to-noise ratio, SNR in decibels is given by the formula: S/N = 20 log10(Vs/Vn). ▶ Ideally, Vs is greater than Vn, so a high signal-to-noise ratio is positive. This results in the signal being clearly readable. ▶ If Vs is less than Vn, then SNR is negative, representing a low signal-to- noise ratio. In this type of situation, reliable communication is generally not possible unless steps are taken to increase the signal level and/or decrease the noise level at the destination (receiving) computer or terminal.
- 6. Communication Channels • Communication channels provide pathways to convey information. A communication channel can be either a physical transmission medium or a logical connection over a multiplexed medium. The physical transmission medium refers to the material substance that can propagate energy waves, such as wires in data communication. And the logical connection usually refers to the circuit switched connection or packet-mode virtual circuit connection, such as a radio channel. ▶ There are two main kinds of communication channels i.e. ▶ Simplex ▶ Duplex ▶ Half Duplex ▶ Full Duplex
- 7. Cont.. ❖ Simplex: A simplex communication channel only sends information in one direction. For example, a radio station usually sends signals to the audience but never receives signals from them. The good part of simplex mode is that its entire bandwidth can be used during the transmission. ❖ Half Duplex: In half duplex mode, data can be transmitted in both directions on a signal carrier except not at the same time. At a certain point, it is actually a simplex channel whose transmission direction can be switched. Walkie-talkie is a typical half duplex device. An advantage of half-duplex is that the single track is cheaper than the double tracks. ❖ Full Duplex: A full duplex communication channel is able to transmit data in both directions on a signal carrier at the same time. It is constructed as a pair of simplex links that allows bidirectional simultaneous transmission. U sing the full duplex mode can greatly increase the efficiency of communication. • Unicast~hasonetransmitter& onereceiver,e.g.walkietalkie,website. • Broadcast~ hasonetransmitter& multireceiver,Wi-Fi,radio,mobiletower. • Multicast~oneormoretransmittertoaparticulargroupofreceivers,videoconference.
- 8. Channel Bandwidth & Rate of Communication • The bandwidth B of a channel is the range of frequencies that it can transmit with reasonable accuracy and precision. • For example, if a channel can transmit a signal with proper accuracy having frequency ranging from 0 Hz to 5000 Hz, the channel bandwidth is 5 kHz. ▶ The rate of information transmission is directly proportional to bandwidth B. • If channel bandwidth B can transmit N pulses per second then to transmit KN pulses per second, channel bandwidth should be KB. ▶ For example, consider the possibility of increasing the speed of information transmission by time compression of signal. If the signal is compressed in time by factor of 2, it can be transmitted in half time, speed of transmission is doubled. Transmitting this signal without distortion, channel bandwidth must also be doubled. ▶ The quality of information transmission is related to signal power i.e. increasing the signal power can reduce the effect of channel noise resulting in accuracy of transmission. ▶ To maintain a given rate & accuracy of information transmission, signal power and channel bandwidth are exchangeable i.e. reducing B to increase signal power and vice versa.
- 9. Cont.. • The two primary communication resources are Signalpower& bandwidthB. • The design of communication scheme having B and signal power depends on the application. E.g. Telephone channel has limited bandwidth of 3kHz but transmitted power is high, whereas space vehicles have infinite bandwidth but power is limited. • SNR is proportional to signal power, SNR and bandwidth are exchangeable. • If a given rate of information transmission requires a channel bandwidth B1 and signal- noise-ratio SNR1, then it is possible to transmit the same information over a channel bandwidth B2 and signal-noise ratio SNR2,
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