Wireless Channel Modeling (Modeling, Simulation, and Mitigation)

Transcription

1 Wireless Channel Modeling (Modeling, Simulation, and Mitigation) Dr. Syed Junaid Nawaz Assistant Proessor Department o Electrical Engineering COMSATS Institute o Inormation Technology Islamabad, Paistan. Courtesy o, Dr. Noor M. Khan (MA Jinnah University, Paistan) Dr. M. N. Patwary(Staordshire University, UK) Dr Muhammad Ali Imran (University o Surrey, UK) Our irst model y=hx+n Channel Source Destination Junaid Nawaz, CIIT Islamabad. 1

2 Shannon s Wireless Communication System Message Signal Channel code word Modulated Transmitted Signal Source Source Encoder Channel Encoder Modulator Estimate o Message signal Estimate o channel code word Wireless Channel User Source Decoder Channel Decoder Demodulator Received Signal Source Coding Junaid Nawaz, CIIT Islamabad.

3 Source Coding Source Coding [Analog and Digital Communication B.P. Lathi] Junaid Nawaz, CIIT Islamabad. 3

4 Shannon s Wireless Communication System Message Signal Channel code word Modulated Transmitted Signal Source Source Encoder Channel Encoder Modulator Estimate o Message signal Estimate o channel code word Wireless Channel User Source Decoder Channel Decoder Demodulator Received Signal Baseband? Modulation? Communications taes place at [ c -W/, c +W/] Processing occurs at baseband [-W/, W/] Junaid Nawaz, CIIT Islamabad. 4

5 Modulation/Sampling in baseband Variations in the received signal with the mobility Junaid Nawaz, CIIT Islamabad. 5

6 Scales o variation Short-term ading Area 1 Area Log-normal shadowing Transmitter Wireless Multipath Channel 16 db 3sec Real Measurements Courtesy Pro D Tse and Qualcom Junaid Nawaz, CIIT Islamabad. 6

7 Large and Small Scale Propagation Models Large scale propagation models: T-R separation distances are large Main propagation mechanism: relections Attenuation o signal strength due to power loss along distance traveled: shadowing Distribution o power loss in dbs: Log-Normal dx (t) = β (γ X (t))dt + δ dw n n n n n t 6 Log-Normal shadowing model 5 4 Fluctuations around a slowly 3 Large Scale Fading varying mean X n (t) [db s] dχ n (t) = α n (γ n χ n (t))dt + σ n sqrt(χ n ) dw t 5 4 χ n (t) 3 1 Small Scale Fading time [us] Large and Small Scale Propagation Models Small scale propagation: T-R separation distances are small Heavily populated, urban areas Main propagation mechanism: scattering Multiple copies o transmitted signal arriving at the transmitted via dierent paths and at dierent time-delays, add vectotrially at the receiver: ading dx n (t) = β n (γ n X n (t))dt + δ n dw t 6 Distribution o signal attenuation coeicient: Rayleigh, Ricean. Short-term ading model Rapid and severe signal luctuations around a slowly varying mean X n (t) [db s] χ n (t) dχ n (t) = α n (γ n χ n (t))dt + σ n sqrt(χ n ) dw t Large Scale Fading Small Scale Fading time [us] Junaid Nawaz, CIIT Islamabad. 7

8 Scales o variation Increasing separation between the receiver and transmitter Slow Medium Fast An illustration rom Goldsmith: Not realistic data Log-Distance Path Loss Model d Pd ( db) Pd 10n log 0 d 0 Path loss exponent n= n=3 n= n=3 Junaid Nawaz, CIIT Islamabad. 8

9 Log-Distance Path Loss Model Path loss exponent Environment n Free space Urban area cellular.7 to 3.5 Shadowed urban cell 3 to 5 In building LOS 1.6 to 1.8 Obstructed in building 4 to 6 Obstructed in actories to 3 Log-Distance Path Loss Model Junaid Nawaz, CIIT Islamabad. 9

10 Log-Normal Shadowing Model d P ( db) P 10nlog Xˆ d d0 d 0 Random variable X has the log-normal distribution, i ln(x) has the normal distribution. n=3 n= Empirical Models or Propagation Losses to Environment Oumura: Empirical study o path loss in Japanese cities. Useul or planning urban cellular systems. Includes correction actors or rural and suburban areas. Urban macrocells 1-100m, requencies GHz, BS antenna m high; Hata: similar to Oumura, but simpliied GHz band COST 31: Hata model extended by European study to GHz Junaid Nawaz, CIIT Islamabad. 10

11 Empirical Models or Propagation Losses to Environment HATA-OKUMURA Model Large cell coverage (distances up to 100 m), Frequency up to the GHz band. PL = log ( ) log (ht) - a (hm) + [ log (ht)] log (d) db a (hm) = [1.1 log( ) - 0.7] hm - [1.56 log( ) - 0.8] db or midsize city a(hm) - correction actor or mobile unit antenna height (db) = 1500 MHz, ht = 40 m, hm = 1.5 m Junaid Nawaz, CIIT Islamabad. 11

12 Geometry o rootop diraction and shadowing Multi-Path Fading Channel Base Station Mobile Station Junaid Nawaz, CIIT Islamabad. 1

13 Multipath Channel s Impulse response Multipath Channel s Impulse response Time delays in multipath signals causes ISI. Severe ISI requires complex equalizers. Junaid Nawaz, CIIT Islamabad. 13

14 Multi-path Propagation Multi-path smears or spreads out the signal delay spread Causes inter-symbol intererence limits the maximum symbol rate Base Station Mobile Station Transmitted Symbol Received Symbol t Delay Spread Base Station Space Mobile Station Transmitted Symbol Time Received Symbol t Junaid Nawaz, CIIT Islamabad. 14

15 Inter-symbol Intererence Transmitted Symbol o Interest Received Symbol o Interest t Transmitted Symbols Received Symbols t Mobile Channel Parameters Time delay spread Coherence Bandwidth -> ISI Doppler Spread Coherence Time -> Unstable channel Flat ading Frequency selective ading Fast ading Slow ading Junaid Nawaz, CIIT Islamabad. 15

16 Average Delay Spread Average delay spread τ τ a a τ P (τ P (τ )τ ) ) P (τ 1 ) P (τ Multi-path Proile (Discrete) P (τ 0 ) P (τ ) ) P (τ t τ 0 0 τ τ 1 τ RMS Delay Spread (Discrete) RMS delay spread σ τ τ τ τ a a τ P(τ )τ P(τ ) Junaid Nawaz, CIIT Islamabad. 16

17 Average Delay Spread (Continuous Delay Proile) Average delay spread Representative delay unctions τ τ 0 0 t P ( t ) dt P ( t ) dt exp uniorm 1 P( t) e P( t) t 0 t d and zero elsewhere Measurements Type o Delay Spread d Environment (s) Open area <0. Suburban area 0.5 Urban area 3 Junaid Nawaz, CIIT Islamabad. 17

18 Coherence Bandwidth Coherence bandwidth B c is a range o requencies over which the channel can be considered lat passes all spectral components with approximately equal gain and liner phase Bandwidth where the correlation unction R T () or signal envelopes is high, or in other words the approximate maximum bandwidth over which two requencies o a signal are liely to experience correlated amplitude ading. Thereore two sinusoidal signals with requencies that are arther apart than the coherence bandwidth will ade independently. Coherence Bandwidth I R T () is approximately 90% 1 B C 50 I R T () is approximately 50% B C 5 An exact relationship between coherence bandwidth & delay spread does not exist 1 R T () correlation between responses at dierent requencies. Junaid Nawaz, CIIT Islamabad. 18

19 Need o Equalization I a transmitted signal s bandwidth is greater than the 50% coherence bandwidth, then the channel is requency selective An equalizer (adaptive tapped delay ilter) will be needed in the receiver Flat-ading channels do not require equalization Doppler Shit c broadening rom c to ( c + m ) d c v c v v BS1 Junaid Nawaz, CIIT Islamabad. 19

20 Relativistic Doppler Frequency The observed requency is c v 1 c v 1 c d c c v c where the relative velocity v is positive i the source is approaching and negative i receding. c - carrier req., c-speed o light, d -Doppler shit Doppler Spread & Coherence Time Describes the time varying nature o the channel in a local area Doppler Spread B D, is a measure o the spectral broadening caused by the time rate o change c broadening rom ( c - m ) to ( c + m ) I the base-band signal bandwidth is much greater than B D, the eects o Doppler spread are negligible at the receiver Junaid Nawaz, CIIT Islamabad. 0

21 Coherence Time Coherence Time is the time domain dual o Doppler spread Doppler spread and coherence time are inversely proportional T C = 1/ m Statistical measure o the time duration over which the channel impulse response is invariant Coherence Time I the coherence time is deined as the time over which the correlation unction is above 0.5, then T C 9 16 m Rule o thumb or modern digital communication deines TC as the geometric mean o two expressions or TC or dierent correlation unctions T C 9 16 m Junaid Nawaz, CIIT Islamabad. 1

22 Inter-symbol Intererence For no Inter-symbol Intererence the transmission rate R or a digital transmission is limited by delay spread and is represented by: R < 1/ ; I R >1/ Inter-symbol Intererence (ISI) occurs Need or ISI removal measures (Equalizers) Types o Small-Scale Fading Junaid Nawaz, CIIT Islamabad.

23 Small-Scale Fading (Frequency selectivity) Small-Scale Fading (Based on time delay spread) Frequency Flat Fading 1. BW o signal < Coherence BW o channel. Delay Spread < Symbol period Frequency Selective Fading 1. BW o signal > Coherence BW o channel. Delay Spread > Symbol period Small-Scale Fading (Time Variability) Small-Scale Fading (Based on Doppler spread) Fast Fading 1. High Doppler spread. Coherence time < Symbol period 3. Channel variations aster then base-band signal variations Slow Fading 1. Low Doppler spread. Coherence time > Symbol period 3. Channel variations slower then base-band signal variations Junaid Nawaz, CIIT Islamabad. 3

24 Frequency Flat Fading I the mobile radio channel has a constant gain and linear phase over a bandwidth greater than the bandwidth o the transmitted signal - the received signal will undergo lat ading Please, observe that the ading is lat (or requency selective) depending on the signal bandwidth relative to the channel coherence bandwidth. Frequency Selective Fading I the mobile radio channel has a constant gain and linear phase over a coherence bandwidth, smaller than the bandwidth o the transmitted signal - the received signal will undergo requency selective ading Again, the signal bandwidth is wider then the channel coherence bandwidth, causing one or more areas o attenuation o the signal within the signal bandwidth Junaid Nawaz, CIIT Islamabad. 4

25 Example 1. B S << B C & T S >> Classiy the requency selective behavior o the channel??? Example. B S > B C & T S < Classiy the requency selective behavior o the channel??? Junaid Nawaz, CIIT Islamabad. 5

26 Time Fast Fading The channel impulse response changes rapidly within the symbol duration - coherence time < symbol period T S > T c and B S < B D Channel speciies as a ast or slow ading channel does not speciy whether the channel is lat ading or requency selective ading Time Slow Fading The channel impulse response changes at a rate much slower than the transmitted base-band signal. Doppler spread is much less than the bandwidth o the base-band signal T S << T c and B S >> B D Velocity o the MS and the base-band signaling determines whether a signal undergoes ast or slow ading Junaid Nawaz, CIIT Islamabad. 6

27 Summary Fast and slow ading deal with the relationship between the (time) rate o change in the channel and the transmitted signal, NOT with propagation path loss models Typical Cellular Mobile Environment Remote Dominant Relectors/Scatterers (Inluential) Medium-Distance Dominant Relectors/Scatterers (Inluential) Elevated BS Antenna MS Scatterers local to BS (Non-inluential) Scatterers local to MS (Inluential) Junaid Nawaz, CIIT Islamabad. 7

28 Fading Fading: The intererence between two or more versions o the transmitted signal which arrive at the receiver at slightly dierent times Multipaths: Above mentioned versions o the transmitted signal Fading (Continued) Delay Spread Coherence Bandwidth Frequency separation at which two requency components o Tx signal undergo independent attenuations Doppler Spread Coherence Time Time separation at which two time signal undergo independent attenuations components o Tx Junaid Nawaz, CIIT Islamabad. 8

29 Fading (Continued) Bandwidth B c Flat in Time and Selective in Frequency Selective in both Time and Frequency Flat in Time and Frequency T c Flat in Frequency and Selective in Time Time Fading in Brie Flat ading B S B C Multi path time delay Frequency selective ading B S B C Fading Doppler spread Fast ading Slow ading T S T C T S T C Junaid Nawaz, CIIT Islamabad. 9

30 Fading in Brie Large Doppler Spread Time-Selective Fading Large Delay Spread Frequency-Selective Fading Large Angle Spread Space-Selective Fading Fading in Brie Junaid Nawaz, CIIT Islamabad. 30

31 Fading in Brie Fading in Brie What is the correlation between received signals that are spaced in requency Δ=1-?? Bc Tc What is the correlation between received signals that are spaced in delay Δt=t1-t?? Junaid Nawaz, CIIT Islamabad. 31

32 Fading in Brie What is the correlation between received signals that are spaced in requency Δ=1-?? How can be measured? 1) Transmitting a pair o sinusoids separated in requencies by Δ ) Cross correlating the complex spectra o the two separately received signals 3) Repeat the process by increasing the separation Δ. Coherence bandwidth represents a requency range over which a signal's requency components have a strong potential or amplitude correlation. Fading in Brie What is the correlation between received signals sent at dierent time, with dierence Δt=t1-t?? How can be measured? 1) Transmitting a sinusoid at time t1 and send the signal again at time t, Δt=t-t1 ) Cross correlating the channel responses o the two signals received at dierent times 3) Repeat the process by increasing the dierence Δt. Coherence time is a measure o the expected time duration over which the channel's response is essentially invariant. Junaid Nawaz, CIIT Islamabad. 3

33 Example (Power delay proile) P r () 0 db -10 db -0 db -30 db 4.38 µs 1.37 µs (µs) _ (1)(5) (0.1)(1) (0.1)() (0.01)(0) 4. 38s [ ] _ (1)(5) (0.1)(1) (0.1)() (0.01)(0) 1.07s [ ] 1.07(4.38) 1. 37s RMS Delay Spread: Typical values Delay spread is a good measure o Multipath Manhattan San Francisco Suburban Oice building Oice building 1 10ns 50ns 150ns 500ns 1µs µs 5µs 10µs 5µs 3m 15m 45m 150m 300m 600m 3Km 7.5Km Junaid Nawaz, CIIT Islamabad. 33

34 Fading in Brie Quiz 1 Solution Junaid Nawaz, CIIT Islamabad. 34

35 Question No. 1 (Part a) B S << B C & T S >> Frequency Flat Channel. Question No. 1 (Part b) B S > B C & T S < Frequency Selective Channel. Junaid Nawaz, CIIT Islamabad. 35

36 Question No. Write the disturbances caused by the spread o the channel in a) Time Domain: Frequency Selectivity Inter Symbol Intererence BER Perormance Degradation Question No. Write the disturbances caused by the spread o the channel in b) Frequency Domain: Time Selectivity/Variability Inter Carrier Intererence BER Perormance Degradation PLL Failure. Junaid Nawaz, CIIT Islamabad. 36

37 Question No. 3 What is the reason or selecting microwave window or wireless communications? Reasonable antenna size. Good propagation characteristics. Support non-los communications. Rich Scattering. Reasonable path loss. Low galaxy and atmospheric noises. Microwave Question No. 4 For the power delay proile shown in the igure below, a) Find the rms delay spread o the channel. b) Calculate the 55 % correlation bandwidths. c) For a mobile travelling with a speed o 40m/s, receiving the signal at the carrier requency o 800 MHz through the channel, calculate the time over which the channel appears stationary. d) Classiy behavior o the channel as slow or ast ading? Junaid Nawaz, CIIT Islamabad. 37

38 Question No. 4 (a) τ a a τ P (τ )τ P (τ ) _ (0.1)(1) (1.0)() (0.001)(3) s [ ] τ a a τ P(τ )τ P(τ ) _ (0.1)(1) (1.0)() (0.001)(3) s [ ] σ τ τ τ (1.9101) 0.89s Question No. 4 (b) B C 1 5 B C B C BC KHz Junaid Nawaz, CIIT Islamabad. 38

39 Question No. 4 (c) m v Hz m c 8 c T c Tc sec 16 m 16 ( ) Question No. 4 (d) T s sec 6 F 810 s c Tc sec T c T s Slow Fading Channel Junaid Nawaz, CIIT Islamabad. 39

40 Fast Fading When? The channel impulse response changes rapidly within the symbol period o the transmitted signal. What? The Doppler Spread causes requency dispersion which leads to signal distortion. Doppler Spread The Doppler eect (in addition to the ading eect) renders the received pulse to be time-varying The State Transitions are determined rom the dynamics o the ading channel (Fading Correlation Function or The Doppler Spectrum) Junaid Nawaz, CIIT Islamabad. 40

41 Doppler Spread (Continued) y Scattering Point Incoming multipath MS α v : carrier requency c: speed o light v: mobile speed Line o Sight x α: Angle o motion with incoming multipath BS Doppler Spread (Continued) d vcos c : carrier requency c: speed o light v: mobile speed α: Angle o motion with incoming multipath Junaid Nawaz, CIIT Islamabad. 41

42 Junaid Nawaz, CIIT Islamabad. 4 Doppler Shit c broadening rom c to ( c + m ) v c v c m cos m D Doppler Spread m cos ; - Uniormly distributed (0,) ( ) ( ) cos m S S d d sin 1 ) ( m S cos 1 sin m cos 1 1 ) ( m m S c v c m 1 ( ) S

43 Doppler Spectrum The incident received power at the MS depends on the power gain o the antenna and the polarization used. S( ) A 1 ( / ) m Research Projects Criteria or selection o a paper or presentation, 1. It should have been published in reputed journals (e.g., IEEE transactions) in an issue not older than Conerence papers are not acceptable, except the lagship conerences o IEEE societies. 3. It should be clearly related to wireless channel modeling or any wireless communication environment and on any o its aspects. Proposal Submission Deadline: 7 April 016 Junaid Nawaz, CIIT Islamabad. 43

44 Doppler Spectrum The incident received power at the MS depends on the power gain o the antenna and the polarization used. S( ) S( ) 1 ( / ) m m S( ) 1 1 ( / ) m m S( ) A 1 ( / ) m Spatial Channel Modeling Geometry based stochastic channel modeling Junaid Nawaz, CIIT Islamabad. 44

45 Spatial Channel Modeling [Ertel et al] Observing at BS Spatial distribution unction o scatterers represented in in Cartesian coordinates Spatial Channel Modeling Restricting radial parameter r b to positive values, it becomes: Junaid Nawaz, CIIT Islamabad. 45

46 Spatial Channel Modeling PDF o AoA observed at BS: Angle-o-Arrival Statistics (USM). Uniormly distributed scatterers conined within a circle centered at MS. [Petrus et al] AoA: Distribution o power w.r.t. physical angles o arriving multipath signals. Junaid Nawaz, CIIT Islamabad. 46

47 Angle-o-Arrival Statistics (GSM). Gaussian distributed scatterers in the vicinity o MS, with mean at location o MS. [Khan et al] AoA: Distribution o power w.r.t. physical angles o arriving multipath signals. Spatial Channel Modeling Junaid Nawaz, CIIT Islamabad. 47

48 Spatial Channel Modeling Spatial Channel Modeling Junaid Nawaz, CIIT Islamabad. 48

49 Spatial Channel Modeling Spatial Channel Modeling Uniormly distributed scattering points within a certain region R A with area A Joint AoA and ToA observed at BS Joint AoA and ToA observed at MS Junaid Nawaz, CIIT Islamabad. 49

50 Joint AoA and ToA observed at MS side. [ertel et al] Spatial Channel Models Junaid Nawaz, CIIT Islamabad. 50