RRC Vehicular Communications Part II Radio Channel Characterisation

Transcription

1 RRC Vehicular Communications Part II Radio Channel Characterisation Roberto Verdone Slides are provided as supporting tool, they are not a textbook!

2 Outline 1. Fundamentals of Radio Propagation 2. Large Scale phenomena 3. Small Scale phenomena a) Wideband characterisation b) Narrowband characterisation 4. The Narrowband Mobile Radio Channel The scope of this lecture block is to introduce the basics of radio channel characterisation that will be useful to follow the link level analysis performed in this course.

3 1. Fundamentals of Radio Propagation Prof. Roberto Verdone

4 Fundamentals of Radio Propagation transmit power d receive power T Pt radio space Pr R transmit antenna system receive antenna system gain G t efficiency h t gain G r efficiency h r

5 Fundamentals of Radio Propagation Radio Waves

6 Fundamentals of Radio Propagation Radio Waves Radio Communication is* the transmission, emission, reception of signs, signals, writings, images, sounds or information of whatever nature, making use of electromagnetic waves x λ E y H d z * MIN. SV. EC DIP. COM. IT. Piano Naz. di ripartiz. delle frequenze Glossario

7 Fundamentals of Radio Propagation Radio Waves Phase speed in clear sky c = m/s = f λ Power density in free space p(d) = 0.5 E m 2 / 377 = Pt Gt h t / 4 p d 2 W/m 2 Received power is Pr = p(d) Gr h r / 4 p / λ 2 W VLF LF MF HF VHF UHF SHF EHF f [ MHz ] 0,03 0, l [ m ]

8 Inquiry Based Session Is a GPRS link (Bc = 200 KHz, fc = 1800 MHz) affected by the presence of a vehicle? Is it affected by rain drops? How large should be the antenna system on a vehicle using 3G (fc = 2000 MHz)? Does the link between two vehicles suffer from the ground reflected path?

9 Fundamentals of Radio Propagation Radio Waves Phase speed in clear sky c = m/s = f λ Power density in free space p(d) = 0.5 E m 2 / 377 = Pt Gt h t / 4 p d 2 W/m 2 Received power is Pr = p(d) Gr h r / 4 p / λ 2 W Waves tend to interact with objects of size equal to or larger than l VLF LF MF HF VHF UHF SHF EHF f [ MHz ] 0,03 0, l [ m ]

10 Fundamentals of Radio Propagation Radio Waves Phase speed in clear sky c = m/s = f λ Power density in free space p(d) = 0.5 E m 2 / 377 = Pt Gt h t / 4 p d 2 W/m 2 Received power is Pr = p(d) Gr h r / 4 p / λ 2 W Waves tend to interact with objects of size equal to or larger than l Efficient antennas have size close to l VLF LF MF HF VHF UHF SHF EHF f [ MHz ] 0,03 0, l [ m ]

11 Fundamentals of Radio Propagation Radio Waves Phase speed in clear sky c = m/s = f λ Power density in free space p(d) = 0.5 E m 2 / 377 = Pt Gt h t / 4 p d 2 W/m 2 Received power is Pr = p(d) Gr h r / 4 p / λ 2 W Waves tend to interact with objects of size equal to or larger than l Efficient antennas have size close to l Propagation is almost free space if first Fresnel elipsoid is free from obstacles ( r = sqrt(dλ/2) ) VLF LF MF HF VHF UHF SHF EHF f [ MHz ] 0,03 0, l [ m ]

12 Fundamentals of Radio Propagation Radio Waves 3 > f [ MHz ] l [ m ] > 100 Ground Waves 3 < f [ MHz ] < < l [ m ] < 100 Sky Waves 30 < f [ MHz ] 10 > l [ m ] Space Waves Ground waves Sky waves Space waves Near-optical propagation VLF LF MF HF VHF UHF SHF EHF f [ MHz ] 0,03 0, l [ m ]

13 Fundamentals of Radio Propagation Radio Frequency Spectrum

14 Fundamentals of Radio Propagation Radio Frequency Spectrum VLF LF MF HF VHF UHF SHF EHF f [ MHz ] 0,03 0, l [ m ] Frequency band assignments to services are: 1) Requested by industry alliances, standardisation bodies 2) Negotiated within and recommended by ITU-R 3) Regulated on a country basis by National Authorities 4) Released to operators / users

15 Fundamentals of Radio Propagation Radio Frequency Spectrum ISM Bands: Licence - Exempt in Most Countries VLF LF MF HF VHF UHF SHF EHF f [ MHz ] 0,03 0, l [ m ] MHz RFid MHz RFid MHz Proprietary Radios MHz Proprietary Radios, LoRa, GHz Proprietary Radios, Bluetooth, WiFi, Zigbee, GHz WiFi,

16 Fundamentals of Radio Propagation Radio Unpredictable Channel

17 Fundamentals of Radio Propagation Radio Unpredictable Channel d T Pt radio space Pr R transmit power receive power If radio space is uniform, isotropic, perfect dielectric, without obstacles, Pr = Pt Gt Gr / Aio Aio = ( 4 p d / l ) 2 [Friis, 1945] Otherwise Aio replaced by Aim = h * d b * x where h is constant, x is r.v.

18 Fundamentals of Radio Propagation Radio Unpredictable Channel Channel Filtering d Pt Pr T R BLER [log] Pr = Pt Gt Gr / Aim BLER* Receiver Power [dbm] d -b b > 2 d -2 Transmission Range Prmin SNR Prmin R R distance

19 Fundamentals of Radio Propagation Radio Unpredictable Channel Channel Filtering d Pt Pr T R b = 2 b > 2 R d R

20 Fundamentals of Radio Propagation Radio Unpredictable Channel Channel Loss Fluctuations d T Pt Pr R Receiver Power outage events Prmin time

21 Fundamentals of Radio Propagation Radio Unpredictable Channel Channel Loss Fluctuations d T Pt Pr R R R d

22 Fundamentals of Radio Propagation Radio Unpredictable Channel d T Pt radio space Pr R Pr = k d -b x

23 2. Large Scale Phenomena Prof. Roberto Verdone

24 Large Scale: Shadowing Tx Humans, vehicles, hills, trees, Rx Narrowband characterisation d = 1 km x

25 Large Scale Tx Humans, vehicles, hills, trees, Rx Shadowing effect: isotropic attenuation is the product of several terms: Aim = A1 A2 An Aim [db] = A1 [db] + A2 [db] + + An [db] If n is large, and central limit theorem assumptions hold, then Aim [db] is Gaussian distributed. Standard deviation from 4 to 12 depending on environment

26 Received power [dbm] Prof. Roberto Verdone Large Scale Shadowing is usually log-normally distributed, for both mobile/stationary applications Autocorrelation function is R(d) = R 0 exp [ - d/d 0 ] [Gudmunson s Model] d 0 = metres Large scale fluctuations Link Reciprocal High frequency coherence Low spatial coherence distance

27 3. Small Scale Phenomena Prof. Roberto Verdone

28 Small Scale: Fading Tx Rx Narrowband characterisation Wideband characterisation Transmitted Power time Received Power delay

29 Small Scale: Wideband Characterisation Power Delay Profile: P h (t) = Int [ h(t, t) 2 ] Mean Delay: T m = Int [ t P h (t) ] / P r Root Mean Square Delay Spread: PDP s t = [(( Int t 2 P h (t) ) / P r ) T m 2 ] delay

30 Small Scale: Wideband Characterisation To be compared to symbol time.

31 Small Scale: Two Path Model Tx Rx t s t = t / 2 Fp(f, t) 2 fc notch frequency Math. derivation 1 / t

32 Small Scale: Two Path Model Tx Rx t t s t = t / 2 Fp(f, t) 2 fc Pr t time frequency

33 Small Scale: Two Path Model Tx Rx t s t = t / 2 Fp(f, t) 2 fc Bc frequency 1 / t

34 Small Scale: Two Path Model Tx Rx t s t = t / 2 BLER [log], one path only BLER [log], two paths SNR [db] Bc > 1 / t (with rake receiver) Bc 1 / t Error Floor Bc << 1 / t SNR [db]

35 Small Scale: Wideband Characterisation Rs t << 1 Signal strength (random) attenuation Rs t ~ 1 or larger signal distortion t represents delay spread of the channel impulse response

36 Small Scale: Narrowband (Mobile) Characterisation Tx Rx Speed v

37 Received power [dbm] Prof. Roberto Verdone Small Scale: Narrowband (Mobile) Characterisation m Large Scale fluctuations Small Scale fluctuations Rayleigh/Rice/ distributed envelope wavelengths (l / 2 under typical urban conditions) distance

38 Received power [dbm] Prof. Roberto Verdone Small Scale: Narrowband (Mobile) Characterisation time = distance / speed s Pedestrian User Large Scale fluctuations Small Scale fluctuations s Speed: 1 m/s GSM frequency band time

39 Received power [dbm] Prof. Roberto Verdone Small Scale: Narrowband (Mobile) Characterisation time = distance / speed 1 10 s Vehicular User (Urban) Large Scale fluctuations Small Scale fluctuations s Speed: 10 m/s GSM frequency band time

40 Received power [dbm] Prof. Roberto Verdone Small Scale: Narrowband (Mobile) Characterisation time = distance / speed s High Speed (e.g. trains) Large Scale fluctuations Small Scale fluctuations s Speed: 80m/s LTE frequency band (800 MHz or 2600 MHz) time

41 Received power [dbm] Prof. Roberto Verdone Small Scale: Narrowband (Mobile) Characterisation Small Scale fluctuations Non Link Reciprocal (for long ranges) Low frequency coherence Low spatial coherence time

42 Small Scale: Doppler Effect Spectrum Transmitted Carrier fo Frequency Tx Rx Speed v Spectrum fo fo + Df Doppler shift Frequency Received Carrier Df is proportional to v

43 Small Scale: Doppler Spectrum Spectrum Transmitted Carrier fo Frequency Tx Rx Speed v Spectrum fo - Df fo fo + Df Frequency Received Carrier Df is proportional to v/l

44 Small Scale: Doppler Spectrum Spectrum fo - Df fo fo + Df Frequency BLER [log], no Doppler BLER [log], with Doppler Bc v / l Bc >> v / l Error Floor SNR [db] SNR [db]

45 Small Scale: Narrowband (Stationary) Characterisation Do not use previous model for v tending to zero; the speed of obstacles has to be considered The Doppler spectrum might change (typically bell-shaped) The fluctuation rate depends on speed of scatterers The statistics of channel fluctuations might change

46 Small Scale: Narrowband Characterisation Channel fluctuations are fast or slow depending on the user speed. In any case, one has to define what Fast or Slow means with respect to the figures considered e.g. The quality of human oriented communications: it is known that human perception is not sensitive to fluctuations faster than a few Hertz, Hence, link quality has to be averaged over fast fluctuations where fast means with a frequency larger than a few Hertz (for instance, fading for pedestrian or vehicular speeds in the GSM frequency band)

47 Radio Channel Characterisation in a nutshell The main phenomena characterising the mobile channel can be categorised as: Large Scale Small Scale shadowing due to obstruction along the entire path fading due to multipath caused by reflections, diffractions, over objects in the vicinity of receiver (scatterers) Channel characterisation can be Narrowband Wideband Single carrier is transmitted: link budget is the issue flat or time selective fading (small scale) shadowing (large scale) Short pulse is transmitted: multipath delay is the issue frequency selective fading (small scale) We focus mainly on link budget related issues (i.e. narrowband characterisation)

48 Radio Channel Characterisation in a nutshell Frequency bands from 400 MHz to 4 GHz Fading Shadowing Coherence Time T coh ms 1 10 s Coherence Band B coh MHz MHz Coherence Space S coh m m

49 Inquiry Based Session Does GPRS (Bc = 200 KHz, fc = 1800 MHz) suffer from frequency selective fading? Does WiFi (Bc = 22 MHz, fc = 2.4 GHz) suffer from frequency selective fading? Does GPRS (Bc = 200 KHz, fc = 1800 MHz) suffer from time selective fading?

50 4. The Narrowband Mobile Radio Channel

51 Received power [dbm] Prof. Roberto Verdone The Narrowband Mobile Radio Channel Prm(d) x distance Pr(x) = Prm(d) m 2 (x) r 2 (x) = Pra(x) r 2 (x) Pr(x) short term average power (over few carrier cycles) Pra(x) = Prm(d) m 2 (x) long term average power (over fading: few wavelenghts) Prm(d) median power (distance dependent component)

52 The Narrowband Mobile Radio Channel Pr(x) = Prm(d) m 2 (x) r 2 (x) Model Assumptions Shadowing: Pra(x) [dbm] = Prm(d) [dbm] + s(x) [db] m 2 (x) is assumed to be log-normal distributed: s(x) is Gaussian, zero mean, std dev. s Fading: r(x) is assumed to be Rayleigh distributed (worst case), i.e. f(x) = r 2 (x) is negative exponentially distributed: pdf(f) = [1 / X] exp (- f / X) u(f) X = E[r(x) 2 ] =1 u(f) step function Median Power: Prm(d) = k d -b