1 EMLAB EM wave propagation

2 EMLAB Impulse response Time Radio Propagation : physical model 안테나에서 나온 신호는 지형지물과 반사, 투과, 산란을 거치면서 다양한 진폭과, 시간 지연을 갖는 신호들로 분리된다.

3 EMLAB 1. 장애물이 전혀 없는 공간에서 전파의 세기가 거리에 따라 감소. 2. 장애물에 의한 반사가 있는 경우 위치에 따라 센 곳도 있고 약한 곳도 생김. 3. 송신기, 수신기가 움직이는 경우 도플러 효과에 의해 주파수 바뀜.

4 EMLAB Propagation channel model Impulse response Time Spectrum Frequency Amplitude variation time spread Mathematical model

5 EMLAB Multi-path Fading Impulse response Time Flat fading : low data rate. Ex) AMPS, ASK,... Time Impulse response Inter-symbol interference Symbol Time

6 EMLAB Frequency selective Fading Multi-path Fading 의 영향 협대역 신호인 경우 영향이 작다. 광대역 신호인 경우 신호 spectrum 에 미치는 영향이 크다. Flat Fading

7 EMLAB Fading – Doppler effect

8 EMLAB 개인 휴대 통신 (PCS) 채널 할당표 채널 배치표 채널별 주파수 ■ 사업자별 할당대역 : 한국통신 프리텔 ( 주 ) - A 밴드 (1~7FA), 한솔피씨에스 ( 주 ) - B 밴드 (8~14FA), ( 주 ) 엘지텔레콤 - C 밴드 (15~21FA) 이동국 송신 ( 기지국 수신 ) 주파수기지국 송신 ( 이동국 수신 ) 주파수 FA 번 호중심주파수 FA 번 호중심주파수 FA 번 호중심주파수 FA 번 호중심주파수 FA 번 호중심주파수 FA 번 호중심주파수 (MHz)

9 EMLABOFDM Orthogonal frequency division multiplexing 대역 폭이 좁은 저속의 데이터를 모아 보내어 frequency selective fading 에 대응

10 EMLAB OFDM modulator OFDM demodulator

11 EMLAB Electromagnetic wave source 안테나에서 나온 신호는 거리 R 떨어진 지점에서 진폭이 1/R 로 줄어들고 시 간 지연이 R/c 만큼 생긴다.

12 EMLAB 반사 투과 회절 Electromagnetic wave propagation 안테나에서 나온 신호가 반사, 회절, 투과를 거치면서 진폭과 위상에 변화가 생긴다.( 위상 변 화는 시간 지연과 관련됨.)

13 EMLAB GTD : Elementary process 1) 반사 2) 투과 3) 회절 반사, 투과, 회절에서 전파의 polarization 및 phase 가 변하므로 이를 고려에 넣어 주 어야 함 (coherent wave). ( 반면에 Computer graphics 의 ray tracing 은 phase 를 고려하지 않는 incoherent addition 임.) Propagation delay 도 계산에서 고려해 줘야 delay spread 를 계산할 수 있음.

14 EMLAB Geometrical Theory of Diffraction 입사파 회절 Edge 1 Edge 2 GO(ray tracing) Diffraction

15 EMLAB 0-face Plane of Incidence 0-face n-face

16 EMLAB Simplified propagation mechanisms 1. Free space model 2. Two-ray ground reflection 3. Diffraction model

17 EMLAB Free space model (Friis formula)

18 EMLAB 2-Ray Ground Reflection For d>> h r, h t, –low angle of incidence allows the earth to act as a reflector –the reflected signal is 180  out of phase –P r  1/d 4 R T htht hrhr Phase shift! d

19 EMLAB (R= -1 인 경우 )

20 EMLAB

21 EMLAB Classification of propagation models 1.Macro cell model ① Empirical model a.Log-distance path loss b.Okumura-Hata ② Physical model a.COST231/Walfisch-Ikegami Model ③ ITU-R model 2. Micro cell model ① Empirical model a.Dual slope model ② Physical model a.Two-ray model b.Street canyon model c.Random waveguide model d.Ray tracing model ③ ITU-R model 3. Pico cell model ① Empirical model ② Physical model

22 EMLAB Propagation Models Large scale models predict behavior averaged over distances >> –Function of distance & significant environmental features, roughly frequency independent –Breaks down as distance decreases –Useful for modeling the range of a radio system and rough capacity planning Macro-cell Micro-cell Small scale (fading) models describe signal variability on a scale of –Multipath effects (phase cancellation) dominate, path attenuation considered constant –Frequency and bandwidth dependent –Focus is on modeling “Fading”: rapid change in signal over a short distance or length of time.

23 EMLAB Log-distance generalizes path loss to account for other environmental factors –Choose a d 0 in the far field. Measure PL(d 0 ) or calculate Free Space Path Loss. Take measurements and derive  empirically. Log-Distance Path Loss Model () Log-Distance Path Loss Model (Macro-cell)

24 EMLAB Okumura-Hata model (COST207) 1.150MHz~1.5GHz 에서 유용. 2. 측정을 통해 얻은 데이터임. 3.Open area, Suburban area, Urban area 로 분류함. For large cities For medium to small cities Urban areas : Suburban areas : Open areas : Macro-cell

25 EMLAB COST231-Hata model 1.1.5GHz~2GHz 로 Hata 모델 확장 2. 측정을 통해 얻은 데이터임. 3.Open area, Suburban area, Urban area 로 분류함.

26 EMLAB Walfisch-Ikegami Propagation Model L f : free space loss L msd :multiple knife edge diffraction L sd : single diffraction

27 EMLAB 27 Two-Ray model

28 EMLAB Street Canyon model

29 EMLAB Pico-cell Wall and floor factor model

30 EMLAB Materials Attenuation values for different materials

31 EMLAB Common Distributions Rayleigh fading distribution –Models a flat fading signal –Used for individual multipath components Ricean fading distribution –Used when there is a dominant signal component, e.g. LOS + weaker multipaths –parameter K (dB) defines strength of dominant component; for K=- , equivalent to Rayleigh

32 EMLAB Rayleigh Distributions

33 EMLAB Small scale (fading) models describe signal variability on a scale of –Multipath effects (phase cancellation) dominate, path attenuation considered constant –Frequency and bandwidth dependent –Focus is on modeling “Fading”: rapid change in signal over a short distance or length of time. Fading issue

34 EMLAB Motion of the receiver: Doppler shift Transmission bandwidth of signal –Compare to BW of channel Multipath propagation –Receiver sees multiple instances of signal when waves follow different paths –Very sensitive to configuration of environment Causes of fading

35 EMLAB The Multipath Channel One approach to small-scale models is to model the “Multipath Channel” –Linear time-varying function h(t,  ) Basic idea: define a filter that encapsulates the effects of multipath interference –Measure or calculate the channel impulse response (response to a short pulse at f c ): h(t,  )   t

36 EMLAB Channel Sounding “Channel sounding” is a way to measure the channel responseChannel sounding –transmit impulse, and measure the response to find h(  ). –h(  ) can then be used to model the channel response to an arbitrary signal: y(t) = x(t)  h(  ). –Problem: models the channel at single point in time; can’t account for mobility or environmental changes h(t,  )  

37 EMLAB Characterizing Fading From the impulse response we can characterize the channel: Characterizing distortion –Delay spread (  d ): how long does the channel ring from an impulse? –Coherence bandwidth (B c ): over what frequency range is the channel gain flat? –  d  1/B c In time domain, roughly corresponds to the “fidelity” of the response; sharper pulse requires wider bandtime domain

38 EMLAB Frequency domain sounding –Sweep frequency range –Compute inverse Fourier transform of response –Problems not instantaneous measurement Tradeoff between resolution (number of frequency steps) and real-time measurement (i.e. duration as short as possible) Channel Sounding – Network analyzer