Wireless Transmission

부산대학교 정보컴퓨터공학부

김종덕 (kimjd@pusan.ac.kr)

강의의 목표

Transmission Impairments

Transmission 과정에서 발생하는 다양한 신호 품질 저하(Impairment)의 원인을

살펴본다.

Logarithmic Unit인 Decibel(dB)과

Decibel로 신호 품질을 표현하는 SNR을 이해한다.

Wireless Propagation

Frequency Reuse의 필요성, Cellular Architecture를 이해한다.

무선 전송 시의 신호 품질 저하 요소인 Path Loss, Slow Fading, Fast Fading을 이

해한다.

무선의 다중 경로 전송 (Multi-Path Propagation)의 특성을 이해한다.

WIRELESS TRANSMISSION AND IMPAIRMENTS

Transmission Impairments

Signal received may differ from signal transmitted

Analog - Degradation of Signal Quality, Metric : SNR (Signal to Noise Ratio)

Digital - Bit Errors, Metric : BER (Bit Error Rate)

Caused by

Attenuation and attenuation distortion

Delay distortion

Noise

AttenuationNoise

Delay Distortion

Attenuation and Delay Distortion

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

Additional signals inserted between transmitter and receiver

Thermal noise (White noise)

Due to thermal agitation of electrons; Can’t be eliminated

Uniformly distributed

Inter-modulation

Happens when different frequencies share the same transmission medium.

Produces sum, difference of multiples of original frequencies.

Crosstalk

Unwanted coupling between signal paths. Often heard on phone lines. Can

occur due to electrical coupling between nearby twisted pair lines.

Impulse

Irregular pulses or spikes

e.g. External electromagnetic interference

Short duration, High amplitude

Combined Effects

Signal to Noise Ratio (SNR)

Important parameter in determining the performance of a

transmission system.

Measured in decibel or dB to express ratio of two power, voltage or current

levels. A relative, not absolute measure.

A high SNR means high quality signal reception:

Power of Signal

An electronic signal can be represented as a voltage 𝑣 𝑡 or a current 𝑖 𝑡

with instantaneous power 𝑝(𝑡) across a resistor defined by

𝑝 𝑡 =𝑣2 𝑡

ℜor 𝑝 𝑡 = 𝑖2 𝑡 ⋅ ℜ

• In communication system, power is often normalized by assuming ℜ to be 1 .

For a signal 𝑥(𝑡), its power can be defined by 𝑝 𝑡 = 𝑥 𝑡 2

𝑆𝑁𝑅𝑑𝐵 = 10 ⋅ log10𝑆

𝑁𝑆 = 𝑠𝑖𝑔𝑛𝑎𝑙 𝑝𝑜𝑤𝑒𝑟, 𝑁 = 𝑛𝑜𝑖𝑠𝑒 𝑝𝑜𝑤𝑒𝑟

Signal to Noise Ratio (SNR)

0.5 -3.01031 02 3.010310 10100 201000 30

N

SdB

SNR

receivedtransmitlossPPP

Power loss:

(in Watts)

Power loss:

(in dB)

𝑆𝑁𝑅𝑑𝐵 = 10 ⋅ log10𝑆

𝑁

𝑆

𝑁= 100.1⋅𝑆𝑁𝑅𝑑𝐵

𝑃𝑙𝑜𝑠𝑠 = 𝑃𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡 − 𝑃𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑

𝑃𝑙𝑜𝑠𝑠𝑑𝐵 = 10 ⋅ log10

𝑃𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡

𝑃𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑= 𝑃𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡

𝑑𝐵 − 𝑃𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑𝑑𝐵

Wireless Transmission

Unguided media, Transmission and reception via Antenna

Antenna and radiation pattern

Directional

• Focused beam, Careful Alignment Required

Omni-Directional

• Signal Spreads in All Directions, Can be received by many antenna

Gain & Radiation Pattern ?

Wired vs. Wireless

Scarce Resources, i.e. Bandwidth

주파수 재사용 (Frequency Reuse, Cellular Architecture)

Less reliable communications

Large Scale and Small Scale Fading

Path-loss, Shadowing, Multipath, Interference and Noise

Time-Varying Environments

Mobility

Broadcast Nature of Channel

Less Secure Environment

Cellular Structure - Concept

Cellular 구조란 무엇이고 왜 사용하는가?

Cellular Phone과 Cordless Phone은 어떻게 다른가?

Figure 14.1 Cellular Geometries를 보고 두 Pattern을 비교하라.

Cellular Structure – Frequency Reuse

오른쪽 그림에서 Frequency

Reuse Factor가 4라는 것의 의

미는?

CDMA의 경우 Frequency

Reuse Factor가 1이라고 한다.

설명하라.

CDMA 강의에서 다룰 것임

Wireless Propagation

Signal travels along 3 routes

Ground Wave

• Follows contour of earth

• Up to 2MHz

• AM radio

Sky Wave

• Amateur radio, BBC world service,

Voice of America

• Signal reflected from ionosphere

layer of upper atmosphere

Line of Sight

• Above 30Mhz

• May be further than optical line of

sight due to refraction

Radio Propagation Mechanisms

Reflection

When the propagating radio wave hits an object which is very

large compared to its wavelength (such as the surface of the Earth,

or tall buildings), the wave gets reflected by that object. Reflection

causes a phase shift of 180 degrees between the incident and the

reflected rays.

Diffraction

This propagation effect is undergone by a wave when it hits an

impenetrable object. The wave bends at the edges of the object,

thereby propagating in different directions. This phenomenon is

termed as diffraction. The dimensions of the object causing

diffraction are comparable to the wavelength of the wave being

diffracted. The bending causes the wave to reach places behind

the object which generally cannot be reached by the line-of-sight

transmission. The amount of diffraction is frequency dependent,

with the lower frequency waves diffracting more.

Scattering

When the wave travels through a medium, which contains many

objects with dimensions small when compared to its wavelength,

scattering occurs. The wave gets scattered into several weaker

outgoing signals, In practice, objects such as street signs, lamp

posts, and foliage cause scattering.

Refraction

Refraction is the change in direction of a wave due to a change in

its velocity. This is most commonly seen when a wave passes from

one medium to another. Refraction of light is the most commonly

seen example, but any type of wave can refract when it interacts

with a medium, for example when sound waves pass from one

medium into another

Wireless Transmission Impairments

Path Loss

Signal disperses with distance

Free Space Model

Outdoor Model / Indoor Model

Fading

Fluctuations in signal strength

Large Scale/Slow Fading

• Shadowing

• Due to obstacles such as

Buildings

Small Scale/Fast Fading

• Multi-path Interference

• Rayleigh / Ricean Model

Interference

Adjacent Channel Interference

Enough Guard Band

Co-Channel Interference

(Narrow-band Interference)

• Between signals using same

frequency the frequency

reuse of Cellular System

Doppler Shift

Path Loss and Fading

Path Loss Model

Classification

Empirical model

• Based on measurement data, simple (few parameters), use statistical properties, not very accurate

Deterministic model

• Site-specific, require enormous number of geometric information about the cite, very important

computational effort, accurate

Semi-empirical (deterministic) model

• Based on empirical + deterministic aspects

Fundamental Model - Free Space Path Loss Model

The expression for FSPL actually encapsulates two effects

• Firstly, the spreading out of electromagnetic energy in free space is determined by

the inverse square law.

• The second effect is that of the receiving antenna's aperture, which describes how

well an antenna can pick up power from an incoming electromagnetic wave.

𝐿𝑓𝑟𝑒𝑒 =𝑃𝑇𝑃𝑅

=4𝜋𝑑

𝜆

2

Multi-Path Propagation

Is it Good or Bad ? Interference vs. Diversity

SPREAD SPECTRUM AND CDM, OFDM

강의의 목표

Spread Spectrum의 기본 개념과 원리를 이해한다.

Spread Spectrum의 장, 단점을 설명할 수 있다.

Code Orthogonal 의 의미를 이해하고 이를 활용한 Multiple Access

기법, 즉 CDMA를 설명할 수 있다.

Spread Spectrum과 CDMA의 차이를 설명할 수 있다.

Spread Spectrum Concept

1 0

1010 1010

Code

Direct Sequence Spread Spectrum (DSSS)

1100 Pseudo Random

1 1 0 1

0110 0011 0101

1100 Pseudo Random0110 0011 0101

time

Easy to jam or intercept …

1 0 1 1 0 1Sender

Code

1010 1010

Receiver

frequency

frequency

Waste of Bandwidth…-_-

FHSS

[Q1] FHSS 는 무엇의 약자인가? Hopping의 의미는?

[Q2] Hopping 순서는 어떻게 결정하나?

DSSS

[Q3] DSSS는 무엇의 약자인가?

[Q4] Fig 9.6에서 A가 0이고 PN=1010일 때 나머지를 그려라.

0

1010

PN & Spread Spectrum

[Q5] PN은 무엇의 약자인가?

[Q6] Pseudo Random과 Random은 어떻게 다른가?

[Q7] PN과 통신 보안과의 연관 관계를 설명하라.

[Q8] Spread Spectrum은 우리말로 어떻게 표현하는가? 그리고 이

기법의 대가(비용)는 무엇인가?

CDM (Code Division Multiplexing)

Multiplexing Technique used with spread spectrum

Orthogonal Code

Transmitter

CODE AReceiver #1

Receiver#2

CODE A

CODE B

Orthogonal to Code A

동일 코드 사용 시원래 정보 복원

송신자가 사용한 것과 다른(직교) 코드 사용 시 해당 신호는 0 (제거됨. 마치 존재하지 않는 것과 같음)

CADA SA CASA DA

CBSA 0

CB(CA(DA)) = 0, CA(CB(DB)) = 0,CA(CA(DA)+CB(DB)) = DA, CB(CA(DA)+CB(DB)) = DB

CDM (Code Division Multiplexing)

time

1 1 1 1

1-11-1 1-11-1 11-1-1

1 1 1 1

-111-1 -1-111 -11-11

1-11-1 1-11-1 11-1-1 -111-1 -1-111 -11-11

-1 -1

-1 -1

11-1-1 11-1-1 1-11-1 11-1-1 1-11-1 -1-111

1 1 1 1

1-11-1 1-11-1 11-1-1 -111-1 -1-111 -11-11

-1 -1

+=0

Orthogonal Code

Transmitter

Receiver #1

+=4 +=-4

time

time

time

time

time

(1-11-1) (11-1-1)=

Receiver #2

CDM (Code Division Multiplexing)

1 1

1-11-1 1-11-1 11-1-1

-1

Transmitter

1

-1

1

Data for A

11-1-1 11-1-1 1-11-1

1-11-1 1-11-1 11-1-1

Receiver A

+=4 +=4

+=-4

Receiver B

11-1-1 11-1-1 1-11-1

+=4 +=4

+=-4

With orthogonal codes, we can sharea channel with others at same timewithout interference.

+

Combined signal

Data for B

CDM Concept

[Q9] pp. 319~pp. 320(9th Edition 기준)를 참조하여 다음 표를 채워라.

Transmission from A, B and C, receiver attempts to recover A’s transmission

A’s Code=< 1 -1 -1 1 -1 1>, B’s Code=<1 1 -1 -1 1 1>, C’s Code=<1 1 -1 1 1 -1>

Sum(A’s Code * B’s Code) = ?

A (data bit = 1)

B (data bit = 1)

C (data bit = 1)

Combined Signal

Receiver Codeword

Multiplication

CDM

[Q10] Orthogonal이란 단어의 뜻은? Orthogonal Code의 의미는?

Orthogonal Code는 무한히 만들 수 있는가?

[Q11] CDM은 시간 및 주파수 측면에서 보았을 때 다른 사용자들과

중첩되는가? CDM은 무엇의 약자인가? CDM을 사용할 경우 인접한

Cell에서 동일 주파수를 사용하여도 되나?

[Q12] Spread Spectrum을 사용하는 이유 (이점) 3가지를 들라.

Benefits of Spread Spectrum

Immunity from various noise and multipath distortion

Including jamming

Can hide/encrypt signals

Only receiver who knows spreading code can retrieve signal

Several users can share same higher bandwidth with little

interference

Code division multiplexing (CDM)

Code division multiple access (CDMA)

고속 무선 통신과 문제점

고속 무선 통신 기술 필요 {+ 이동 환경}

데이터 전송 속도 증가 Symbol 길이를 짧게

Noise, Multi-Path Interference 등의 영향에 취약

1 0 11Multipath-Signal

01 1 1 0 1 1 0 0 0 1 1

1

1 1 0 1 0 1 1 0 0 0 1 1

1 0 1

Burst Noise

Burst Noise

Burst Noise

Orthogonal Frequency Division Multiplexing

Frequency

Frequency

Frequency

인접한 주파수 채널간의 간섭을줄이기 위한 Guard Band 등으로주파수 효율이 떨어지잖아…

Symbol의 길이를 늘이기 위해 하나의 채널에서 고속으로 보내는 대신여러 개의 저속 (부)채널들로 나누어 보내는 것은 어떨까?(Serial Parallel)

1

2

3

4

Orthogonal Frequency Division Multiplexing

특정 주파수 간(Orthogonal Frequency)에는 주파수 대역이 겹쳐도겹치는 다른 신호 요소를 제거하고 자신의 신호만 추출할 수 있다네…

Frequency

Frequency

OFDM은 차세대 무선 이동 통신 기술로 주목 받고 있음

MIMO

Multiple Input Multiple Output

Pronounced as “my mo”

Use of multiple transmitters and receivers using multiple antenna

Diversity

Directional Antenna

Directional communication

Less Energy in the wrong direction

• Better Spatial reuse and less

multipath

More Energy in the right direction

• Longer ranges more robust links

Antenna Model

Typically, 2 operation mode

Omni mode / Directional Mode

Directional Antenna Type

Switched Antenna : Select One

Steerable/Steered Antenna

Adaptive Array AntennaA B

X

Y

A B

X

Y

Omni-Directional Antenna

Directional Antenna

Red nodes cannot communicate presently

Not Possible using Omni