CHAPTER 1: INTRODUCTION TO COMMUNICATION SYSTEMS (BEB31803) DR. NOORSALIZA BINTI ABDULLAH DEPARTMENT OF COMMUNICATION ENGINEERING FACULTY OF ELECTRICAL AND ELECTRONICS ENGINEERING OVERVIEW ON COMMUNICATION SYSTEMS TRANSMISSION IMPAIRMENTS Figure 1.1 : (a) Basic block diagram for communication (b) example Transmitter Receiver Signal source Base band converter Modulation and power amplification Transmission (Electromagn etic Field) Subsystem synchronization Amplification and demodulation Base band inverter Synchronization system Base band processing Electromagnetic field Figure 1.2 : (a) Basic transmitter block diagram (b) basic receiver. Terminology Electronic communication : transmission, reception and processing of information between 2 or more locations using electronic circuit. Information : analog or digital signal that had been converted to electromagnetic energy Transmitter : collection of one or more electronic devices or circuits. That convert the original source into a signal that is more suitable for transmission over a given transmission medium Receiver : collection of electronic devices and circuits that accepts the transmitted signal from the transmission medium and converts them back to their original form Base band converter: to convert the signal source into base band waveform for the carrier signal before transmission. Can be either analog or digital system. Subsystem synchronization: synchronizing connection between the TX and RX for recovery processes. Transmission medium: provides a means of transporting signal from the TX to the RX. eg : copper wire (signal as electrical current flow), optical fiber cable (signal in e/magnetic light wave), free space (signal in e/magnetic radio wave) Transmission impairments : any undesired effect on the signals while traveling from the transmitter to the receiver, such as noise, attenuation, interference and other losses caused by the atmosphere or the medium itself. Noise: random, undesired electrical energy that enters the communication system via the communication media (i.e. inserted between TX and RX) and interferes with the transmitted message. Attenuation : drop in signal power due to distance travel by the signal. Interference : noise signal that has the same frequency as the information signal. Types of Signals analog signal : a continuously varying voltage or current e.g. sound, video digital signal : binary pulses or codes Figure 1.3 : Examples of signals (a) analog (b) digital. Analog Signals Components of Speech Frequency range (of hearing) 20 Hz-20 kHz Speech 100Hz-7kHz Easily converted into electromagnetic signal for transmission Sound frequencies with varying volume converted into electromagnetic frequencies with varying voltage Limit frequency range for voice channel ~ 300-3400Hz Digital Signal From computer terminals etc. Bandwidth depends on data rate ELECTROMAGNETIC SPECTRUM Electromagnetic wave is a signal where its electrical and magnetic field change at fixed rate. Frequency range for communication start roughly from 200kHz until few giga Hertz (GHz). Frequency (f) no. of times a periodic motion occurs in a given period of time Hertz (Hz) or cycles per second Period = time for one repetition (T) T = 1/ f cycle one complete alternation of a waveform wavelength (ì) distance traveled by an electromagnetic wave during one period ì = cT ìf = c c = 3 x 10 8 ms -1 (speed of light in free space) Wavelength •Distance between corresponding points of two consecutive waves f c = ì λ is wavelength c is velocity of light f is frequency Designation Freq. Range (Hz) ì range (m) ELF 30 – 300 10 7 – 10 6 VF 300 – 3 k 10 6 – 10 5 VLF 3 k – 30 k 10 5 – 10 4 LF 30 k – 300 k 10 4 – 10 3 MF 300 k – 3 M 10 3 – 10 2 HF 3 M – 30 M 10 2 – 10 1 VHF 30 M – 300 M 10 1 – 10 0 UHF 300 M – 3 G 10 0 – 10 -1 SHF 3 G – 30 G 10 -1 – 10 -2 EHF 30 G – 300 G 10 -2 – 10 -3 Table 1: Frequency range (a) designation (b) applications Extremely Low Frequency (ELF) ac power line distribution (50 and 60 Hz) low freq telemetry signal Voice Frequency (VF) human speech (most intelligent sound) Very Low Frequency (VLF) upper end of human hearing range musical instrument government and military (eg. submarine) Low Frequency (LF) marine and aeronautical navigation as subcarriers Medium Frequency (MF) AM radio broadcasting marine and aeronautical comm application High Frequency (HF) Also known as short wave (SW) 2-way radio communication SW radio broadcast amateur radio and citizen band (CB) Very High Frequency (VHF) mobile radio marine and aeronautical communication FM broadcast TV amateur radio Ultra High Frequency (UHF) * freq > 1GHz is known as microwave TV land mobile communication cellular phone military certain radar and navigation system microwave and satellite radio system amateur radio Super High Frequency (SHF) microwave and satellite radio system radar specialized form of 2-way radio Extremely High Frequency (EHF) seldom used in radio communication except in very sophisticated, expensive and specialized application satellite communication Radar * freq > 300 GHz are not referred as radio wave Infrared refers to electromagnetic radiation generally associated with heat anything that produced heat generate infrared signal eg : light bulb, human body astronomy (to detect stars) electronic photography heat-seeking guidance system (weapon) TV remote control visible light optical communication Bandwidth Portion of the electromagnetic spectrum occupied by the signal Frequency range over which a receiver or other electronic circuits operate. Difference between the upper and lower limit frequency, limits of the signal, or equipment operation range Channel bandwidth Range of frequencies required to transmit the desired information i.e. an audio signal (3kHz) being modulated by a 1000kHz carrier signal using AM modulation TRANSMISSION MEDIUM Guided – coaxial cable, twisted pair, fiber optic, waveguide. Unguided – wireless (terrestrial, spacewave, free space, earth wave). Characteristics and quality determined by medium and signal. For guided, the medium is more important. For unguided, the bandwidth produced by the antenna is more important. Key concerns are data rate and distance. Characteristics of Guided Media Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair 0 to 3.5 kHz 0.2 dB/km @ 1 kHz 50 µs/km 2 km Twisted pairs (multi-pair cables) 0 to 1 MHz 0.7 dB/km @ 1 kHz 5 µs/km 2 km Coaxial cable 0 to 500 MHz 7 dB/km @ 10 MHz 4 µs/km 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km Characteristics of Wireless Propagation Signal travels along three routes Ground wave Follows contour of earth Up to 2MHz AM radio Sky wave 2 MHz < f < 30 MHz Amateur radio, BBC world service, Voice of America Signal refracted from ionosphere layer of upper atmosphere Line of sight Above 30MHz cellular phone TRANSMISSION IMPAIRMENTS Signal received may differ from signal transmitted Analog - degradation of signal quality Digital - bit errors Caused by Attenuation and attenuation distortion Delay distortion Noise Attenuation Signal strength falls off with distance Depends on medium Received signal strength: must be enough to be detected must be sufficiently higher than noise to be received without error Attenuation is an increasing function of frequency Delay Distortion Propagation velocity varies with frequency Noise Will be discuss later TYPES OF ELECTRONIC COMMUNICATION Can be classified in three ways Transmission mode (one-way, two-way) Analog or digital system Baseband or broadband transmission Transmission Mode One-way (Simplex) info travels in 1 direction only receive-only, transmit-only eg. Radio and TV broadcasting, telemetry system Two-way (duplex) a) half duplex both direction, but only one way at a time 2-way-alternate, either-way, over-and-out e.g. police radio b) Full duplex Both directions at the same time 2-way-simultaneous, both-way e.g. telephone Analog Or Digital System Analog system energy is transmitted and received in analog form both info and carrier are analog signals Digital system Digital transmission a true digital system where digital pulses are transferred bet. 2 or more points no analog carrier original source info may be in digital or analog signal if analog signal convert to digital pulses prior to transmission and converted back to analog signal at the RX require a physical medium between TX-RX Digital radio transmission of digitally modulated analog carriers between 2 or more points modulating signal and demodulated signals are digital pulses the digital pulses could originate from a digital transmission system, from a digital source i.e. computer, or a binary encoded analog signal transmission medium may be physical facility or free space Advantages of Digital Transmission Digital technology Low cost LSI/VLSI technology Data integrity Longer distances over lower quality lines Capacity utilization High bandwidth links economical High degree of multiplexing easier with digital techniques Security & Privacy Encryption Integration Can treat analog and digital data similarly Baseband Or Broadband Transmission Baseband transmission putting the original signal (analog or digital) directly into the medium eg : in many telephone and intercom system, it is the voice itself that is placed on the wires & transmitted Broadband transmission original signal is used to modulate a carrier for transmission over the medium when baseband signal is incompatible with the medium Analog Signals Carrying Analog and Digital Data Digital Signals Carrying Analog and Digital Data Encoding Techniques Digital data, digital signal Analog data, digital signal Digital data, analog signal Analog data, analog signal Digital Data, Digital Signal Need to know Timing of bits - when they start and end Signal levels Factors affecting successful interpreting of signals Signal to noise ratio Data rate Bandwidth Example Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Bipolar -AMI Pseudoternary Manchester Differential Manchester B8ZS HDB3 Digital Data, Analog Signal Public telephone system 300Hz to 3400Hz Use modem (modulator -demodulator) Example Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PSK) Analog Data, Digital Signal Digitization Conversion of analog data into digital data Digital data can then be transmitted using digital encoding such as NRZ-L Digital data can then be converted to analog signal Analog to digital conversion done using a codec Example Pulse code modulation Delta modulation Analog Data, Digital Signal Digitizing Analog Data Analog Data, Analog Signals modulate analog signals to the higher frequency Types of analog modulation Amplitude Frequency Phase Modulation : process of changing one or more properties (amplitude, frequency, phase) of the carrier in proportion with the info signal MODULATION Why? It is extremely difficult to radiate low frequency signals from an antenna in the form of electromagnetic energy it is possible theoretically but impractical realistically c= ìf f +, ì| antenna length usually 1/2 or 1/4 of ì for voice signal (300 - 3000 Hz), require very large antenna expensive to construct and consume more pore (aperture). Info signal often occupy the same frequency band, and if signals from 2 or more sources are transmitted at the same time, they would interfere with each other i.e. all commercial FM station broadcast voice and music signals that occupy the AF from 300 Hz - 15 kHz to avoid interference, each station converts its into to a different frequency band more space at higher frequency many channels can be formed to carry many simultaneous communication without interference Modulation Techniques let v(t) = V c sin (2tft + u) general expression for a time varying sine wave of voltage as a high frequency carrier signal modulating signal modulation performed analog AM FM PM l l l v(t) = V c sin (2t . f . t + u) l l l digital ASK FSK PSK QAM MULTIPLEXING Transmission of info from more than one source over the same transmission medium increase the no. of communication channel more info transmitted reduce cost and higher utilization of the transmission line Frequency Division Multiplexing (FDM) Multiple signals share common BW of a single communication channel Useful BW of medium exceeds required bandwidth of channel each signal occupies a separate portion of the BW Each signal modulates a different sub-carrier freq Sub-carriers are linearly mixed to form a composite signal that is usually used to modulate a final carrier for transmission carrier frequencies separated so signals do not overlap (guard bands) Channel allocated even if no data • at the RX, the recovering of the individual signal is done with a DEMUX whose main component is BPF tuned to the individual sub-carrier freq. For analog signal, i.e. radio broadcast FDM System Time Division Multiplexing (TDM) Each channel is assigned a time slot and may transmit for a brief period using the entire BW of the medium Data rate of medium exceeds data rate of digital signal to be transmitted signal sources takes times to transmit Time slots do not have to be evenly distributed amongst sources for both analog and digital signal TDM System TDM of Analog and Digital Sources Wavelength Division Multiplexing (WDM) Similar of FDM coupling light at 2 or more discrete wavelengths, ì into and out of an optical fiber Multiple beams of light at different frequency Each colour of light (ì) carries separate data channel unlike FDM (same time, same transmission path), different ì travels at different speed and did not take the same path, but enter the fiber at the same time and same transmission medium each arrives at the RX at a slightly different time WDM System Fiber cable ì 1 , ì 2 ….. ì n ì 1 ì 2 ì n ì 1 ì 2 ì n Laser optic source To laser optical detector Gain Ratio output to the input Output has greater amplitude than the input Most amplifiers are power amplifier, the same procedure can be used to calculate power gain, A p . A p = P out /P in FIgure 1.4 Amplifier Gain in out V V V input output A = = Example 1.1 What is the gain of an amplifier that produces an output of 750 mV for 30 µV input? Example 1.2 The power output of an amplifier is 6 W. The power gain is 80. What is the input power? Example 1.3 Three cascade amplifier have power gains of 5, 2, and 17. The input power is 40 mW. What is the output power? Attenuation Refers to loss introduced by a circuit Output is less than input For cascade circuit, total attenuation is A T =A 1 x A 2 x A 3 ….. Voltage divider network may introduce attenuation in out V V A n Attenuatio = Figure 1.5 Voltage divider introduces attenuation Attenuation can be offset by introducing gain Figure 1.6 Total attenuation in cascaded network Figure 1.7 Gain offsets the attenuation Figure 1.8 Total gain is the product of the individual stage gains and attenuation Example 1.4 A voltage divider shown in Figure 1.7 has values of R 1 = 10kO and R 2 = 47kO. 1. What is the attenuation? 2. What amplifier gain would you need to offset the loss for an overall gain of 1? Example 1.5 An amplifier has gain of 45,000, which is too much for the amplification. With an input voltage of 20 µV, what attenuation factor is needed to keep the output voltage from exceeding 100mV? Let A 1 = amplifier gain = 45,000; A 2 = attenuation factor; A T = total gain. DECIBEL Gain and attenuation often expressed in decibels, rather than ratio value (decimal) Decibel unit of measurement originally created as a way of expressing the hearing response of human ear to various sound levels. A decibel is one-tenth of a bel. Using decibel, total gain or attenuation can be calculated by simply adding the gains and the attenuation expressed in decibel. 20log 20log 10 log out in out in out in V For votage dB V I For current dB I P For power dB P = = = Beside performing ratio operation, decibel is also used to expressed power in communication. A notation is added after the dB simbol dBW, dBm, dBµ etc. For dBm, reference level 1mW A larger unit, dBW has reference value of 1W. dBm and dBW are decibel units used for expressing power in communication. Example 1.6 A microphone has output value of -50dBm, calculate the actual output power? Bit Error Rate Another significant measure of system performance in term of noise is bit error rate (BER) Specify the number of bits that are corrupted or destroy as data are transmitted from TX to the RX BER of 10 -6 indicate that 1 bit out of 1 million bits is corrupted in the transmission Several factor contribute to BER is Bandwidth Transmission speed Transmission medium Environment Transmission distance Transmitter and receiver performance
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