EIE339 Digital Transmission and Switching Systems

Transcription

1 EIE339 Digital Transmission and Switching Systems Lecturer: Dr. W.Y.Tam Office: DE604 Telephone no.: address: Continuous Assessment Tests 5% Assignments and quizzes 5% Practical work 50% A.1

2 Textbook & References Textbook Haykin, S.S., Communication Systems, Wiley, 001. References Stallings, W., Data and Computer Communication s, Prentice Hall, Shanmugam, K.S., Digital and Analog Communication Systems, Wiley, A.

3 Transmission impairments After this lecture, you will be able to describe different types of impairments during transmission Attenuation distortion Delay distortion noise Reference: Section.3 Transmission Impairments Data and computer communications, Prentice Hall. A.3

4 Introduction Introduction The purpose of a communication system is to transmit information-bearing signals or baseband signals through a communication channel separating the transmitter from the receiver. The received signal differs from the transmitted signal due to various transmission distortion. Example: Analog signal: snow flakes appear in TV Digital signal: a binary 1 is transformed into binary 0 The most significant distortion are Attenuation distortion Delay distortion Noise A.4

5 Attenuation Strength of a signal falls off with distance over any transmission medium. For guided medium, this strength is generally expressed as αx V ( x) = V (0) e α : a function of frequency and the attenuation is expressed as attenuation : 0log 10 V ( x) / V (0) = [ V ( x) / V (0)] = e αx 0log 10 e αx (no unit) (unit : db) 0log10[ V ( x) / V (0)] = 0ax[ log10 e] { 0log [ V ( x) / V (0)]}/ x = 0a[ log e] (unit : db/km) A.5

6 Attenuation For unguided medium, attenuation is more complex function of distance and the makeup of the atmosphere. Example: Signals transmit from a satellite to the ground station passing through different layers of atmosphere including the ionosphere. Impacts of attenuation received signal must have sufficient strength so that the electronic circuitry in the receiver can detect and interpret the signal received signal must maintain a level sufficiently higher than noise to be received solved by adding amplifiers or repeaters at regular intervals attenuation is a function of frequency --- attenuation distortion solved by equalizing attenuation across a band of freqeuncy A.6

7 Impacts of Attenuation Example: Thee attenuation as a function of frequency for typical leased line is shown below. (attenuation is measured relative to the attenuation at 1000Hz, which is the reference frequency specified in North American) The solid line shows the attenuation of a typical leased line. The dashed line shows the equalized attenuation of a typical leased line.. A.7

8 Delay distortion This distortion is caused by the fact that the velocity of propagation of a signal through a guided medium varies with frequency. Delay distortion is critical for digital data. Because of delay distortion, some of the signal components of one bit position will spill over into other bit position, causing intersymbol interference (ISI), which is a major limitation to maximum bit rate. A.8

9 Delay distortion Example Equalizing techniques can also be used for delay distortion. A.9

10 Noise Any unwanted signals that are inserted somewhere between transmission and reception. A major limiting factor in communications system performance. Four categories Thermal noise Intermodulation noise Crosstalk Impulse noise A.10

11 Thermal Noise Occurs in all transmission media and in all communication equipment arising from random electron motion. Every equipment element and the transmission medium contribute thermal noise to a communication system, provided the temperature of that element of medium is above absolute zero. It cannot be eliminated (unless lowering the temperature) and therefore places an upper bound on communication systems performance. Characterized by a uniform distribution of energy over the frequency spectrum and a normal (Gaussian) distribution of levels. Thermal noise in a bandwidth of B Hz is N o = ktb (unit : watts) k : Boltzman's constant = T : Temperture in Kelvin -3 J / K A.11

12 Thermal Noise Example At room temperature, T = 17 C or 90K 3 N o = = 4 10 W / Hz For a 10MHz bandwidth, N o = 4 10 = W 6 A.1

13 Intermodulation Noise Intermodulation noise is produced when there is some nonlinearity in the transmitter, receiver, or intervening transmission system Example, if a signal composes of two sinusoids, through an nonlinear device, e i 1 1 ( t) = cosπ f t + cos πf t, passing eo ( t) = ei + e ( t) i (t) e i e o (t) the second term is e i ( t) = [ cosπf t + cosπf t] = cos = 1 πf t + cos 1 πf t + cosπf t cosπf t [ 1+ cos4πf t] / + [ 1+ cos4πf t] / + cosπ ( f + f ) t + cosπ ( f f )t 1 output signal cotains sinsods at frequency f 1 1 1, f, f1, f, f1 + f, f 1 1 f The derived signals could interfere with an intended signals A.13

14 Crosstalk Crosstalk refers to unwanted coupling between signal paths. Example: hear another conversation when using the telephone Three causes of crosstalk electrical coupling between transmission media Example: occur by electrical coupling between nearby unshielded cable such as twisted pair Poor control of frequency response (i.e., defective filter or poor filter design) Nonlinearity performance in analog (FDM) multiplex systems Example: coaxial cable lines carrying multiple signals A.14

15 Impulse noise is non-continuous Impulse Noise irregular pulses or noise spikes of short duration of relatively high amplitude. Generated from a variety of causes, including external electromagnetic disturbances such as lightning minor annoyance for analog data voice transmission may be corrupted by short clicks with no loss of intelligibility primary source of error in digital data communication sharp spike of energy of 0.01 seconds duration destroy 50 bits of data being transmitted at 4800 bps A.15

16 Impulse Noise Example: effects o thermal noise and impulse noise A.16