Post on 26-May-2020
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