Radio channel modeling: from GSM to LTE

Transcription

1 Radio channel modeling: from GSM to LTE and beyond Alain Sibille Telecom ParisTech Comelec / RFM

2 Outline Introduction: why do we need channel models? Basics Narrow band channels Wideband channels MIMO channels Multi-link channels Perspectives & conclusion Séminaire Comelec, 7 juin /56

3 Introduction: why do we need channel models? Séminaire Comelec, 7 juin /56

4 Why do we need channel models? From the operator s point of view For network planning, BST deployment As inputs to engineering rules & tools From the manufacturer s point of view For performance evaluation For device/equipment design optimization From the researcher s point of view For trying novel network architectures For evaluating novel antenna technologies Séminaire Comelec, 7 juin /56

5 Why do we need channel models? How to proceed? 1. Propagation research done by researchers 2. Extract the substance of the channel physics into something tractable 3. Standardize channel models de-facto or through official bodies: COST, IEEE, ETSI, 3GPP Séminaire Comelec, 7 juin /56

6 Why do we need channel models? Example - back to basics: GSM (operator s need) The main need: to model the attenuation (path loss) vs. distance, for «typical» environments and varying BST height Séminaire Comelec, 7 juin /56

7 Why do we need channel models? Example - back to basics: GSM (operator s need) The main need: to model the mean attenuation (path loss) vs. distance, for «typical» environments and varying BST height Séminaire Comelec, 7 juin /56

8 Why do we need channel models? Example - back to basics: GSM (operator s need) The main need: to model the mean attenuation (path loss) vs. distance, for «typical» environments and varying BST height COST-Hata model for suburban or rural environments: (db) ( MHz, 1-20 km) This is typical of an empirical path loss model Séminaire Comelec, 7 juin /56

9 Why do we need channel models? Example: MIMO WLAN - IEEE n (manufacturer s need) MIMO channels deeply involve the «space variant» characteristics of the channel (and the multi-antenna system) Séminaire Comelec, 7 juin /56

10 Why do we need channel models? Example: LTE / LTE-A (manufacturer & operator s need) A more complicated networking scheme, implying a sophistication of channel models Cell-Edge Beamforming Séminaire Comelec, 7 juin /56

11 User throughput Why do we need channel models? Example: LTE / LTE-A (manufacturer & operator s need) A more complicated networking scheme, implying a sophistication of channel models Relaying Distance to BS Relay Séminaire Comelec, 7 juin /56

12 Why do we need channel models? Example: LTE / LTE-A (manufacturer & operator s need) A more complicated networking scheme, implying a sophistication of channel models Coordinated Multipoint Tx and Rx Séminaire Comelec, 7 juin /56

13 Why do we need channel models? Example: UWB RFID (researcher s game: ongoing) In every wireless communications research project, one needs to make channel measurement campaigns! Séminaire Comelec, 7 juin /56

14 Why do we need channel models? Trend Vehicular Multi-user WINNER LTE IEEE n MIMO channel Directional channel Hiperlan Body Area Networks IEEE IEEE UMTS IEEE a Ultra Wide Band GSM Impulse response Path loss COST 207 COST 231 COST 259 COST 273 COST 2100 COST IC1004 Séminaire Comelec, 7 juin /56

15 Basics Séminaire Comelec, 7 juin /56

16 Basics Propagation channel Tx signal Rx signal Transmission channel The transmission channel comprises antennas and all objects contributing or hampering propagation between input/output ports The propagation channel excludes the antennas and expresses all wave propagation phenomena between Tx and Rx Both channels are considered to be linear (time variant) filters characterized by their impulse response Séminaire Comelec, 7 juin /56

17 Basics Path 2 LOS (Line Of Sight) Building Direct Path Base Station Path 3 Mobile Terminal Tall Buildings Spherical waves at Tx Planar waves at Rx and in-between These are approximations! (may not be verified ) Séminaire Comelec, 7 juin /56

18 Basics NLOS Direct propagation (LOS path) Specular reflection Diffraction Diffuse scattering Refraction/transmission scatterers Séminaire Comelec, 7 juin /56

19 Basics NLOS The simple multipath based discrete channel model of the impulse response: h( t, ) i A i t i t H( t, F) i A i t exp 2 j F i t Time Delay Time Frequency At baseband, A i (t) is complex valued Séminaire Comelec, 7 juin /56

20 Basics What (may) need be modeled The multipath amplitudes The multipath angles (at Tx/Rx) The multipath delays The time dependence of these parameters (comes from Tx/Rx mobility, or mobility of the environment) For various environments, Tx/Rx distances, locations Séminaire Comelec, 7 juin /56

21 Narrowband channels Séminaire Comelec, 7 juin /56

22 Narrowband channels Received signal: r I t It cos2 F t Qt sin2 F t Assumption: c t SLOS exp jjlos Sn exp jjn t Q LOS/NLOS small scale fading t SLOS exp j LOS Sn exp j n t n n c NLOS LOS Rx S LOS behaves as a deterministic variable (DV) S n and q n, j n behaves as random variables (RV) Central-limit theorem I(t), Q(t) are non-centered, identically distributed Gaussian RV Séminaire Comelec, 7 juin /56

23 Narrowband channels LOS/NLOS small scale fading Statistics: Random part of I, Q: PDF I or Q 1 exp s 2 2 I 2 or 2s Q 2 LOS Std deviation s 0 Envelope r Rice distribution: 0 PDF exp K factor: ratio of deterministic to random mean powers K NLOS Rx I 2 2 r r S0 r S s 2s S 2s s Séminaire Comelec, 7 juin /56

24 Statistics: Narrowband channels LOS/NLOS small scale fading Rice distribution: PDF K= - db (Rayleigh) K= 3 db K= 10 db exp I 2 2 r r S0 r S s K 2s S 2s s NLOS LOS Rx Séminaire Comelec, 7 juin /56

25 Narrowband channels The three spatial scales of fading Small distance/medium distance/long distance fading (shadowing) Séminaire Comelec, 7 juin /56

26 Long distance fading Narrowband channels Path loss Friis formula (free space): G G 4 Pt Gr Gt 1 Pr LdB log dkm 20log F L Going from 1 GHz to 10 GHz 20 db higher attenuation! Pr P t r t 2 d d, 2 MHz Is this frequency dependence maintained in NLOS channels? Yes! Frequency variations roughly speaking obey F -2 in NLOS Is this distance dependence maintained in NLOS channels? No! The path loss exponent n often exceeds 2 L d d L 0 d d 0 n Séminaire Comelec, 7 juin /56

27 Narrowband channels Path loss Long distance fading Séminaire Comelec, 7 juin /56

28 Narrowband channels Shadowing Medium distance fading What is that? Shadowing! = medium distance fading = macroscopic fading Séminaire Comelec, 7 juin /56

29 Narrowband channels Shadowing Medium distance fading Gaussian in db = lognormal fading Séminaire Comelec, 7 juin /56

30 Wideband channels Séminaire Comelec, 7 juin /56

31 Wideband channels Small scale / short term / wideband fading Constructive/destructive interference between multipaths H( t, F, r) A i i t exp 2 j F t, r i i t, r t,0 i k i r 2F Path 21 Base Station Path 2 Direct Path Path i Path 3 Building k i Constructive/destructive interference Mobile Terminal r Tall Buildings Séminaire Comelec, 7 juin /56

32 Wideband channels Small scale / short term / wideband fading Constructive/destructive interference between multipaths H( t, F, r) A i i t exp 2 j F t, r i Coherence band width i t, r t,0 i k i r 2F Flat (narrowband) fading Selective (wideband) fading Frequency Séminaire Comelec, 7 juin /56

33 Wideband channels Small scale / short term / wideband fading Constructive/destructive interference between multipaths H( t, F, r) A i i t exp 2 j F t, r BW i i t, r t,0 i k i r 2F Time or space frequency Séminaire Comelec, 7 juin /56

34 Wideband channels Frequency Delay (Fourier): impulse response D1/2.BW D rms Delay Séminaire Comelec, 7 juin /56

35 Wideband channels Fourier pairs Time Doppler Delay Frequency Position Wavector Angle Séminaire Comelec, 7 juin /56

36 Wideband channels Fourier pairs x k x k exp x exp jkx sinf jk x f The directional spectrum is obtained from the spatial structure of the signal Séminaire Comelec, 7 juin /56

37 Wideband channels Angle vs. delay Street canyon 900 MHz terminal side Séminaire Comelec, 7 juin /56

38 Wideband channels Angle vs. delay Dunand & Conrat, 2008 Séminaire Comelec, 7 juin /56

39 Wideband channels Angle vs. delay Dunand & Conrat, 2008 Séminaire Comelec, 7 juin /56

40 MIMO channels Séminaire Comelec, 7 juin /56

41 Séminaire Comelec, 7 juin /56 MIMO channels The impulse response becomes a matrix! The relative variation of coefficients (with position, frequency, time) dramatically impacts the diversity/spatial multiplexing performance The multipath structure is responsible for this variation ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( NM N N M M h h h h h h h h h H

42 MIMO channels A non physical approach : based on the structure of the MIMO correlation matrix Kronecker approximation: The method lends itself to easy stochastic generation in a software tool: Statistical modeling: define statistical distributions for the coefficients of the correlation matrices 3GPP (LTE), IEEE n, IEEE Séminaire Comelec, 7 juin /56

43 MIMO channels A non physical approach : based on the structure of the MIMO correlation matrix Kronecker approximation: H UΛV min( n R, n T ) i1 H u v i i H i Séminaire Comelec, 7 juin /56

44 MIMO channels Double directional channels for MIMO channel modeling Tx Rx Statistical modeling: define statistical distributions for path amplitudes, path delays, path DoA, path DoD 3GPP, IEEE n, IEEE Séminaire Comelec, 7 juin /56

45 MIMO channels A semi-physical modeling approach : GSCM (Geometry-based Stochastic Channel Modeling) Statistical modeling: define statistical distributions for scatterers positions, characteristics COST 259, 273, 2100 Séminaire Comelec, 7 juin /56

46 MIMO channels What s that for? Example : standardization of OTA (Over The Air) test methods for MIMO terminals Séminaire Comelec, 7 juin /56

47 Multi-link channels Séminaire Comelec, 7 juin /56

48 Multi-link channels Multi-link channels are encountered in future networks Where BS can simultaneously be connected to several users for which terminals may simultaneously be connected to several BS Séminaire Comelec, 7 juin /56

49 Multi-link channels Multi-link channels are encountered in future networks Where BS can simultaneously be connected to several users for which terminals may simultaneously be connected to several BS channel modeling requires proper account of macroscopic spatial correlations Séminaire Comelec, 7 juin /56

50 Perspectives & conclusion Séminaire Comelec, 7 juin /56

51 Perspectives Antennas are part of the radio channel! Propagation channel Tx signal Transmission channel Rx signal # An instrumentation antenna (for channel measurements) A use case Séminaire Comelec, 7 juin /56

52 Perspectives Antennas are part of the radio channel! The super-antenna concept: antenna + user in near field F. Harrysson et al., COST 2100 TD , Sep Séminaire Comelec, 7 juin /56

53 Perspectives Antennas are part of the radio channel! In-body antennas: e ~50 s ~2 S/m! channel channel Embedded antenna Composite antenna-channel problem Séminaire Comelec, 7 juin /56

54 Perspectives A variety of propagation environments for new wireless use cases Séminaire Comelec, 7 juin /56

55 Conclusion The main message: radio channel modeling is a rich subject, combining propagation physics, data processing and a lucid view of systems/networks requirements The increasing sophistication of channel investigations and models stems from the complexification of wireless networks, from picocells to macrocells (even satellites) throughout It seems that researchers in this area are not yet jobless Séminaire Comelec, 7 juin /56

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