Chương 1: Giới thiệu tổng quanvề hệ thống viễn thông

PGS.TS. Hoà Vaên KhöôngBMVT, ÑHBK Tp.HCM

Email: khuong.hovan@yahoo.ca

Heä thoáng thoâng tin

• Khoái phaùt coù chöùc naêng xöû lyù tín hieäu tin töùc vaø cung caápvaøo moâi tröôøng thoâng tin moät tín hieäu coù daïng thöùc phuøhôïp vôùi ñaëc tính cuûa moâi tröôøng, vôùi ñieàu kieän laø noäidung cuûa tin töùc ñöôïc truyeàn ñi laø khoâng thay ñoåi.

• Khoái phaùt coù theå goàm caùc phaàn maõ hoùa, ñieàu cheá vaøkhueách ñaïi phaùt.

Heä thoáng thoâng tin

• Moâi tröôøng thoâng tin laø moät moâi tröôøng vaät lyù cuï theå, cho pheùp chuyeån taûi tín hieäu töø nôi phaùt ñeán nôi thu. Moâitröôøng thoâng tin coù theå döôùi daïng höõu tuyeán (daây daãnñieän song haønh, daây cable tín hieäu, sôïi quang,...) hoaëc coùtheå döôùi daïng voâ tuyeán (khoâng gian töï do, chaân khoâng, moâi tröôøng chaát loûng,...).

• Moâi tröôøng thoâng tin coù ñaëc tính gaây suy hao coâng suaáttín hieäu vaø gaây treã pha tín hieäu khi truyeàn tin. Cöï lythoâng tin caøng lôùn thì ñoä suy hao vaø treã pha caøng nhieàu.

Heä thoáng thoâng tin

• Khoái thu coù chöùc naêng thu nhaän tín hieäu tin töùc töø moâi tröôøngthoâng tin, taùi taïo laïi tin töùc ñeå cung caáp ñeán nôi nhaän tin.

• Khoái thu coù theå goàm caùc phaàn khueách ñaïi tín hieäu ñieän (ñeå buøtröø ñoä suy hao treân moâi tröôøng thoâng tin), giaûi ñieàu cheá vaø giaûimaõ hoùa (ñeå khoâi phuïc laïi tin töùc goác ban ñaàu ôû nôi phaùt), khoáichoïn loïc keânh thoâng tin (ñeå choïn löïa ñuùng tín hieäu töø nguoàn tin maø ta muoán thu nhaän, trong khi moâi tröôøng thoâng tin coù theå ñöôïcsöû duïng truyeàn tin ñoàng thôøi cho nhieàu nguoàn tin khaùc nhau). .

Heä thoáng thoâng tin

• Moät loaïi tín hieäu phuï nhöng luoân luoân xuaát hieän vaø toàn taïi trong baátkyø heä thoáng thoâng tin naøo ñöôïc theå hieän bôûi khoái nhieãu, can nhieãuvaø caùc taùc nhaân gaây meùo daïng. Ñaây laø caùc tín hieäu maø chuùng takhoâng mong muoán nhaän ñöôïc taïi nôi thu trong quaù trình truyeàn tin. Chuùng coù theå xuaát hieän trong moâi tröôøng thoâng tin döôùi daïng nhieãucoäng hoaëc nhieãu nhaân.

• Do tính chaát suy hao cuûa moâi tröôøng thoâng tin, tín hieäu tin töùc maø tamuoán truyeàn ñi coù theå bò suy hao coâng suaát ñeán möùc bò xen laãn vôùicaùc tín hieäu nhieãu trong moâi tröôøng hoaëc taïi nôi thu. Luùc naøy, quaùtrình thoâng tin laø thaát baïi, nôi nhaän tin khoâng theå taùi taïo laïi tin töùc töønguoàn phaùt tin nöõa.

Taùc ñoäng• Nhieãu laø caùc tín hieäu khoâng mong muoán, xuaát

hieän moät caùch ngaãu nhieân trong moâi tröôøngthoâng tin hoaëc töø caùc phaàn töû, linh kieän cuûa thieátbò.

• Nhieãu coäng coù theå ñöôïc loaïi boû hoaëc giaûm thieåuaûnh höôûng nhôø caùc boä loïc taàn soá, caùc boä xöû lyùngöôõng taïi nôi thu.

• Ñoái vôùi nhieãu nhaân, quaù trình xöû lyù phöùc taïp hônnhieàu, thöôøng phaûi söû duïng caùc thuaät toaùn thöû-vaø-saïi (chaúng haïn, thuaät toaùn logic môø, maïngneural, chuoãi Markov,...).

Taùc ñoäng - Nhiễu AWGN

( )( )−

−σ=

πσ

x m

Xp x e

2

222

1

2

( )∞ λ

−= λ

π ∫kQ k e d

2

212

Nhieãu nhieät (thermal noise)

Nhieãu nhieät xuaát hieän do caùc chuyeån ñoängngaãu nhieân cuûa caùc phaàn töû mang ñieän(electron hoaëc loã) trong moâi tröôøng truyeàndaãn

Moät ñieän trôû R taïi nhieät ñoä τ seõ phaùt sinh ñieäntheá nhieãu v(t) coù giaù trò phaân boá Gauss vaøphöông sai laø

( )v

kv R

hπ τ

= σ = ⋅2

2 2 23

k = 1,37×10–23 [J/oK] laø haèng soáBoltzmann h = 6,62×10–34 [J.s] laø haèng soá Plank

Maät ñoä phoå coâng suaát cuûa nhieãunhieät: Gv(f) = 2Rkτ [V2/Hz] Nguoàn doøng ñieän coù maät ñoä phoå coâng suaát

( ) ( )vi

G f kG fRRτ

= =22

Taùc ñoäng• Can nhieãu laø nhieãu gaây ra bôûi caùc taùc nhaân

chuû quan cuûa con ngöôøi, chaúng haïn, nhieãu do tín hieäu töø nguoàn phaùt khaùc, nhieãu do nguoàncung caáp coâng suaát, nhieãu do caùc thieát bò phuïtrôï,...

• Can nhieãu xuaát hieän ôû caùc daûi taàn soá khaùcvôùi daûi taàn soá muoán thu, coù theå ñöôïc loaïi boûdeã daøng nhôø caùc pheùp loïc taàn soá thoângthöôøng. Tuy nhieân, can nhieãu cuøng daûi taànraát khoù ñöôïc loaïi tröø, ngöôøi ta phaûi duøng caùcpheùp maõ hoùa nguoàn phuø hôïp.

Taùc ñoäng• Taùc nhaân gaây meùo daïng tín hieäu thöôøng

xaûy ra do caùc phaàn töû, linh kieän trongthieát bò khoâng coù ñaëc tính tuyeán tính.

• Tuy nhieân, ñieåm khaùc bieät giöõa taùc nhaânmeùo daïng naøy vôùi nhieãu, can nhieãu laø söïmeùo daïng chæ xaûy ra khi coù tín hieäuphaùt.

• Söï meùo daïng coù theå ñöôïc khaéc phuïc nhôøcaùc boä söûa daïng (equalizer) trong heäthoáng thoâng tin.

Kieåu truyeàn

• Ñôn coâng (simplex). • Song coâng (full-duplex).• Baùn song coâng (half-duplex).

Moâ hình heä thoáng thoâng tin

• Tin töùc laø yeáu toá trung taâm cuûa moät heäthoáng thoâng tin. Muïc tieâu cuûa heä thoánglaø chuyeån taûi vaø taùi laäp laïi tin töùc taïi nôinhaän tin sao cho noäi dung cuûa tin töùc laøkhoâng ñoåi so vôùi nôi phaùt hoaëc coù theåhieåu ñöôïc, chaáp nhaän ñöôïc.

• Töông töï vaø daïng soá.

The fundamental limitations when designing a communication system are the noise and the bandwidth:

• There is always thermal noise (due to the random motion of charged particles at temperatures above absolute zero), which is the main problem when the transmission distance increases.

• Every communication system has a finite bandwidth. The bandwidth is the main problem when the transmission speed is increased because they are directly proportional to each other.Channel capacity is

where B is the bandwidth and S/N is the Signal-to-Noise Ratio (SNR) of the channel. This relationship is also known as Shannon law.

Physical Limitations

The modulation involves two waveforms:A modulating signal that represents the message, and a carrier wave that suits the particularapplication.Here are some examples of amplitude modulation using a sinusoidal and a pulse train as carriers. The message signal can be seen in the envelope of the modulated signal. In the receiver, the message can be retrieved using demodulation. a) Modulating signal; b) Sinusoidal

carrier with amplitude modulation; c) Pulse-train carrier with amplitudemodulation.

Modulation

In general, the carrier frequency is much higher than the highest frequency component of the modulating signal. In this case, the spectrum of the modulated signal consists of a band of frequency components clustered around the carrier frequency. Therefore, the modulation produces frequency translation.

• Modulation Benefits:– Modulation for efficient transmissions

The efficiency of any transmission method depends on the frequency of the signal being transmitted. For example, efficient line-of-sight radio propagation requires antennas whose physical dimensions are at least 1/10 of the signal wavelength. E.g., unmodulated transmission of an audio signal at 100Hz would require 300 km long antenna, while modulated transmission at 100MHz allows a practical antenna size of about 1 m.

– Modulation for frequency assignmentFor example, since each radio/TV station has a different assigned carrier frequency, the desired signal can be separated from the others by filtering. Radio frequencies are allocated by international agreements.

– MultiplexingSeveral signals can be combined for simultaneous transmission on one channel if, e.g. the carrier frequencies are different (frequency division multiplexing).

– Modulation to overcome hardware limitationsThe design of a communication system may be constrained by the cost and availability of hardware, hardware whose performance often depends on the frequencies involved. Modulation permits the designer to place a signal in some frequency range that avoids hardware limitations.

Coding is a processing of message signal for improving digital communication. Decoding is the inverse operation.– Channel coding (a technique used to introduce controlled

redundancy to improve the performance reliability in a noisy channel).

– Source coding (a technique that reduces the redundancy in the signal to achieve more efficiency).

Examples: 1. ASCII-code: coding of the alphanumerical characters to binary data. 2. Transmission capacity can be improved by sending 2M level symbols

that represent binary code words of length M (source coding). 3. By appending extra check digits to each binary code word we can

detect or correct most of the errors in the receiver (channel coding).

Coding

Classification of RF Applications

Wireless Communication Standards (1)

Wireless Communication Standards (2)

Wireless Communication Standards (3)

Wireless Communication Standards (4)

Wireless Communication Systems (1)

Wireless Communication Systems (2)

Wireless Communication Systems (3)

Frequency Band in Communication Systems (1)

Frequency Band in Communication Systems (2)

Microwave frequency allocations according to IEEE