Radio Propagation Basics

Transcription

1 Radio Propagation Basics MSE, Radio-Prop, 1 The radio range is a primary requirement parameter for most wireless communication systems. therefore we repeat some basics of radio propagation here please consult the technical literature for more details (or the MSE-module TSM_SignProc) Contents Free space radio propagation Link budget Mobile Radio Channel (Multipath Channel) Conclusions Contact: ZHAW Zürcher Hochschule für angewandte Wissenschaften Prof. Dr. M. Rupf ZSN Zentrum für Signalverarbeitung und Nachrichtentechnik Technikumstrasse 9, TB 425 CH-8401 Winterthur Phone: ++41 (0) Mail: marcel.rupf@zhaw.ch Web:

2 Radio Propagation Basics MSE, Radio-Prop, 2 References [1] Christian Lüders, Mobilfunksysteme, Grundlagen, Funktionsweise, Planungsaspekte, Vogel Buchverlag, [2] Ke-Lin Du, M.N.S. Swamy, "Wireless Communication Systems«, Cambridge, Chapter 3: Channel and Propagation [3] Prof. R. Küng, Nachrichtentechnik und Mobilkommunikation NTM1-Skript, 2009, The following contributions are mostly taken from [3] with kind permission from Prof. R. Küng. [4] Prof. R. Küng, "Radio Propagation: Teil I und II, MSE-documents of M. Rupf, week 1.

3 Radio Propagation in Free Space MSE, Radio-Prop, 3 Spherical radio propagation transmit power surface of sphere with radius d of effective area A e power density at distance d received power is proportional to 1/d 2 p(d) Pt 4π d 2 P r p(d) A e Pt A 4π d e 2

4 Antenna Gain MSE, Radio-Prop, 4 Practical antennas do not radiate isotropically. They have a Gain G [dbi] as a result of concentrating the radiation into an angle of the space. Antennas have the same gain G in Rx- and Tx-direction 4π G θ θ 1 2 4π A Gr 2 λ horizontal or vertical radiation pattern area comparison on unit sphere: for small angles θ 1 and θ 2 (i.e. small aperture θ 1 θ 2 ) from Maxwell for reception: e where the wavelength effective antenna area A e = (λ / (θ 1 θ 2 )) 2 = function of the wavelength λ λ c f

5 Kathrein Kathrein Example: Patch Antenna MSE, Radio-Prop, 5 GSM900 uplink: MHz, downlink: MHz half power beam with 65 antenna gain of 12.5 dbi large size" GSM1800 uplink: MHz, downlink: MHz half power beam with 65 antenna gain of 11.5 dbi "small size"

6 Equivalent Isotropically Radiated Power MSE, Radio-Prop, 6 G = 0 dbi Same field strength! G = 30 dbi EIRP P t G t IC- and Device-suppliers typically limit P t Frequency regulators limit EIRP (or ERP) Example: 2.4 GHz ISM band, ETSI EN EIRP -10 dbw (100 mw) For wide band modulations other than FHSS the maximum EIRP spectral density 10 mw per MHz.

7 Free space radio propagation MSE, Radio-Prop, 7 Tx-power Tx-antenna gain P (d) r PtG tgr 2 2 (4 ) d 2 Rx-antenna gain wave length Rx-power distance G t propagation factor λ π d G r Tx Rx P t d P r EIRP

8 Free space radio propagation MSE, Radio-Prop, 8 Logarithmic description P [dbm] r P [dbm] G [dbi] PL t t path [db] G [dbi] r Path Loss: PL path [db] 10 log PL fs [db] = 32.4 db + 20 log 10 (f/mhz) + 20 log 10 (d/km) 10 4πd λ Two convenient free space path loss formulas 2 where λ = c / f PL fs [db] = 32.4 db + 20 log 10 (f/ghz) + 20 log 10 (d/m) Free space path loss PL fs increases with 20 db / distance-decade (10x), 6 db / distance-octave (2x) with increasing frequency

9 Link budget MSE, Radio-Prop, 9 1. Set up the link budget losses in Txantenna system Tx-antenne gain G t PL path (d) Rx-antenna gain G r losses in Rxantenna system Tx L t L r Rx P t d P r Tx-power EIRP Rx-power P r [dbm] = P t [dbm] L t [db] + G t [dbi] PL path [db] + G r [dbi] L r [db] EIRP [dbm]

10 Link budget MSE, Radio-Prop, Take the minimum required P r, i.e. the Rx-sensitivity (data sheet) or determine it theoretically as follows P r [dbm] 174 [dbm] + 10 log 10 (B / Hz) + F tot [db] + SNR min [db] noise floor K Rx noise figure self noise of Rx thermal noise power in Rx-bandwidth B (bandwidth B is often limited by regulation, standards etc.) SNR min = function (modulation, FEC, target BER, margin, etc.) 3. Determine maximum radio range d max - combine (1) and (2) and determine max. tolerable path loss PL path - compute d max from the radio propagation model The larger the bandwidth or the higher the data rate, the smaller the radio range.

11 Modulation: BER versus SNR MSE, Radio-Prop, 11 Q QAM16 I for a BER = 10-5 on an AWGN channel the required SNR is: 9.5 db with QPSK (2 Bits/Symbol) 14.5 db with QAM16 (4 Bits/Symbol) 19.5 db with QAM64 (6 Bits/Symbol) the bandwidth B 1/T symbol

12 Forward-Error-Correction (FEC) MSE, Radio-Prop, 12 BPSK data transmission over an AWGN-channel uncoded R=1/2, 64 states soft-decision R=1/2, 4 states soft-decision G = 5.25 db G = 3.75 db coding rate R=1/2 means that 2 code bits are transmitted per info bit coding gain G = 4-6 db with R=1/2 convolutional coding with 4 to 256 states

13 Forward-Error-Correction (FEC) MSE, Radio-Prop, 13 Performance of convolutional codes with 64 states and different rates BPSK Coding Gain G only slightly improves for Coding Rates R < 0.5 G = 5.25 db up to 2-3 db more coding gain with Turbo codes or LDPC codes Coding Rates: Source: Qualcomm, Datenblatt Q0256

14 maximum Path Loss (limits radio range) antenna gains (cable) losses Link budget in db MSE, Radio-Prop, 14 EIRP (regulation limit) Tx-power P t [dbm] G t L t G r L r SNR min F tot Rx-sensitivity = minimum required Rx-Power Pr min minimum signal-to-noise-ratio for target BER Noise Figure, self noise of Rx thermal noise power in Rx-bandwidth B 10 log 10 (B) noise floor K (physical limit)

15 Example: Reading Range of UHF RFID MSE, Radio-Prop, 15 UHF RFID Reader (EPC Gen 2) d max? Tag max. Path Loss 49 db f = 868 MHz EIRP = 33 dbm (2 W) P r -16 dbm Reasonable assumption P r -16 dbm (25 µw) for tag powering (limits the reading range!) Link Budget => Path loss free space PL fs => (optimistic) reading range d max P r = EIRP PL fs -16 dbm => PL fs 49 db PL fs = log 10 (0.868) + 20 log 10 (d max /m) = 49 => d max 8 m

16 Mobile Radio Channel (Multipath Channel) MSE, Radio-Prop, 16 many reflections, diffractions and scatterings at the same time Tx, Rx or obstacles are moving! Reflexion Streuung Beugung (line-of-sight path) Absorption

17 Simple 2-Path-Model MSE, Radio-Prop, 17 In practice: many reflections! but often, one dominates observable in: connections over lakes, radio on planes, factory buildings, metallic fassades P for d < d 0 : P r ~ 1/d 2 as LOS, free space (-20 db / decade) for d > d 0 : P r ~ 1/d 4 (-40 db / decade) d 0 4 h t h r / λ P r P t G t G r 2 ht h 4 d 2 r

18 Exponent n Model MSE, Radio-Prop, 18 Theoretical and practical investigations show P r ~ 1/d n but adaptation of the exponent n in the path loss Path loss [db] for d d 0 : thereof LOS until breakpoint distance d 0 : PL PL path fs (d (d) PL 0 free space fs (d 4π d ) 10 log λ 0 ) 10 n log( 0 2 d d 0 ) P r [db] environment d 0 Indoor Office 1 m Outdoor Urban Outdoor Rural 10 m 100 m Example: n = 3.8 d 0 log-scale

19 Example: Radio Range of Bluetooth smart MSE, Radio-Prop, 19 Bluetooth Low Energy in GHz ISM-band Some reasonable assumptions EIRP = 0 dbm (1 mw): P t = 3 dbm, G t = -3 dbi Rx-sensitivity of BLE-chip (e.g. nrf51822): P r -93 dbm, G r = -3 dbi (BT4.0 specification requires a sensitivity of at least -70 dbm) Wireless Sensor Simple exponent n=4 (indoor) propagation model (breakpoint-distance d 0 = 1 m) Link Budget d max? P r = EIRP PL path + G r -93 dbm => PL path 90 db Exponent n=4 model: PL path = PL fs (d 0 ) + 10 n log 10 (d max /d 0 ) = 90 db Free space path 1m: PL fs (d 0 =1m) = log 10 (2.44) 40 db Radio Range: 40 log 10 (d max /1m) = (90 40) = 50 => d max = 10 50/40 18 m

20 Empirical Model MSE, Radio-Prop, 20 General form: PL 50 [db] = A + 10 n * log 10 ( d [km] ) COST 231 model Hata model for mobile radio under the following constraints: f = 1500 MHz 2500 MHz, ht = 30 m 200 m, hr = 1 m 10 m d = 1 km 20 km h t PL 50 h r Median value of the attenuation d PL log(f c) log(h t ) a(h r) ( log(h t ))log(d) a( h ) (1.1 log( f ) 0.7) h (1.56 log( f ) 0.8) r c correction factor for medium and small cities r A c 10 n h t = 30m => n = db / (distance) Decade

21 Empirical Model MSE, Radio-Prop, 21 Propagation measurements end of 1960 by Okumura in Japan Derivation of formulas from measurements by Hata suitable for first overview (macro cells only) Area of validity Frequency f: MHz Tx height h t : m Rx height h r : m Distance d: km Assumptions: h r ~ 1.5 m, log = log 10, d [km], f [MHz], PL 50 [db], h t [m] PL 50,urban = log(f) log(h t ) C + [ log(h t )] log(d) correction factor C (other terms cf. [2]) medium/small cities: C = (1.1 log(f)-0.7) h r log(f) if h r = 1.5 m suburban: C = 2 (log(f/28))

22 Fading Effects MSE, Radio-Prop, 22 Long-term fading because of shadowing dependent of area contour between Tx and Rx also called large-scale fading or log-normal fading Short-term fading because of multipath communication because of reflections near Tx and Rx also called small-scale fading or Rayleigh-Fading P r [linear] variation caused by Rayleigh fading (distance λ/2... λ) global mean distance d [lin] variation caused by shadowing (distance >> λ)

23 Long Term Fading MSE, Radio-Prop, 23 Global mean (Rx-power P r ) ~ 1/d n, e.g. n = 3.5 but level fluctuations on a length scale from m (outdoor) because of various shadowing situations Local mean (P r ) = global mean (P r ) L L [db] log-normal fading: L L [db] is normal distributed [log-scale] mean = 0 db Standard deviation σ [db] different for differend areas => Dense urban: 7 db => Urban, suburban: 6 db => Rural, open, forest: 5 db [Pr in db Median-value] σ => 90% location-time-probability (98% averaged over the whole cell) 10%

24 Short Term Fading MSE, Radio-Prop, 24 Fluctuation over small displacements absolute value of vector sum of the statistically independent, normal distributed multipath signal components is Rayleigh-distributed Link budget modification: [log-scale] 20 db (up to 40 db) without countermeasures

25 Example: Radio Range of GSM Source: Siemens 1999 (Russian town, voice) MSE, Radio-Prop, 25 Comments to downlink Downlink Uplink Comments to uplink Tx BS-power (40W) 46 dbm 33 dbm max. Tx-power mobile PA Back Off -1 db -- Total Combiner + Diplexer -3 db -- cable loss BS -3 db 0 db Cable loss mobile Antenna gain 15 dbi 0 dbi Antenna gain mobile EIRP 54 dbm 33 dbm EIRP max. path loss 122 db 122 db Okumura-Hata, urban, 915 MHz, h BS = 30 m => d max = 0.74 km Received power outdoor -68 dbm -89 dbm Temporary sum Location-Time-Probability 98% averaged over the cell (90% on cell boarder) -8 db -8 db Location-Time-Probability urban area Loss building -20 db -20 db Building loss Interference -3 db -3 db Interference Body loss -3 db -3 db Body loss Antenna gain mobile 0 dbi 15 dbi Antenna gain BS -- 0 db no Tower Mounted Amp. Cable loss mobile 0 db -3 db Cable loss BS 4 db Diversity gain Rx dynamic sensitivity -102 dbm -107 dbm dynamic sensitivity

26 Multipath Channel: Time Variance MSE, Radio-Prop, 26 Impulse Response Delay Spread (UHF-Indoor radio channel) 8 1 (LOS) 3 Fourier Transformation Frequency Response Coherence Bandwidth B c (over which the channel is flat ) B c B c ~ 1 / Delay Spread

27 Delay-Spread and Coherence Bandwidth MSE, Radio-Prop, 27 second moment τ 1 τ 2 τ 3 τ 4 Coherence bandwidth B c 1 / (2π σ t ) Flat Fading if all signal frequencies fade similarily - if the signal bandwidth B < B c, where B 1/T symbol - if the RMS-Delay-Spread σ t < T symbol / 2π - if there is no Inter-Symbol-Interference (ISI)

28 Example UMTS Evaluation Channel ETSI, 1997 MSE, Radio-Prop, 28 Typical Mobile Radio Channels = 2.51µs = 20 µs NLOS-Path LOS-Path distance NLOS distance LOS = 6 km (20 µs delay)

29 Coherence Time and Doppler Spread MSE, Radio-Prop, 29 Coherence Time T c measures how fast the channel changes in time due to movements of Tx, Rx and/or refecting objects a large T c corresponds to a slow channel fluctuation T c = 0.4 / f m max. Doppler-frequency f m = v / λ B c T c

30 Classification Small-Scale Fading MSE, Radio-Prop, 30 Delay no ISI ISI Doppler

31 Flat Fading MSE, Radio-Prop, 31 signal bandwidth B < channel coherence bandwidth B c or, equivalently, RMS delay spread σ t < T symbol / 6 => no ISI Tx t T symbol Rx1 LOS NLOS t t multipath signals subtract (deep fade) => Rayleigh-Fading antenna diversity Rx2 LOS NLOS t t multipath signals add up

32 Frequency selective fading MSE, Radio-Prop, 32 signal bandwidth B > channel coherence bandwidth B c or, equivalently, RMS delay spread σ t > T symbol => ISI Tx t T sym Rx LOS NLOS ISI t t Counter Measures: costly channel equalizer (GSM-soultion), or, spread-spectrum RAKE receiver (UMTS-solution), or, OFDM (state-of-the-art)

33 Countermeasures against Multipath Fading MSE, Radio-Prop, 33 B bb B c B sb For low-rate, smallband systems with B sb < B c (flat fading, no ISI) - antenna-diversity For high-rate, broadband systems with B bb > B c (freq. selective fading, ISI) - costly channel equalizer as in GSM - spread-spectrum RAKE-Rx as in UMTS (to receive on each of the delayed pathes) - state-of-the-art: OFDM-modulation as in LTE, WiFi a,g,n, DAB, DVB-T, using N parallel data streams on N subchannels each with bandwidth B broad /N < B c

34 Orthogonal Frequency Division Multiplexing MSE, Radio-Prop, 34 The channel allows a useful bandwidth of B c max. symbol rate OFDM: parallel transmission on many channels each with bandwidth < B c Example: Digital Audio Broadcasting (DAB), Transmission Mode I - N = 1536 Subchannels each with bandwidth 1 khz, all QPSK-modulated - B c 1 khz => RMS-Delay-Spread 1/6 ms => multipath distance difference 50 km (DAB- cells are large!)

35 Some Consequences to Mobile Radio (I) MSE, Radio-Prop, 35 The higher the frequency f, the larger the path loss, the smaller the coverage area (cell size). e.g. 900 MHz e.g MHz WLAN 2.4 GHz BTS/AP BTS/AP WLAN 5 GHz If a wireless system uses Frequency Division Duplexing (FDD), (UL-) uplink-frequencies are arranged below (DL-) downlink-frequencies. the Mobile has less Tx-power than the Base Station! GSM UL 124 x 200 khz GSM DL 124 x 200 khz f / MHz GSM BTS

36 Some Consequences to Mobile Radio (II) MSE, Radio-Prop, 36 The propagation loss depends on the environment free space: -20 db / (distance) decade terrestrial mobile radio: about -35 db / (distance) decade The higher the frequency, the smaller the antenna size (for a given antenna gain G) it can be shown that effective antenna area A e = (λ / aperture θ) 2 Example: multi-gigabit WiFi ad 60 GHz chipsets exploit the short-carrier wavelength by incorporating antennas directly on chip or in-package. Example: 24 GHz, 8 patch doppler Radar module K-LC1a, RFbeam microwave GmbH, St. Gallen.

37 Some Consequences to Mobile Radio (III) MSE, Radio-Prop, 37 The higher the antenna height h t, the smaller the path loss, the larger the coverage area (cell size). if diversity is used in a mobile radio Base Station, the Rx-antenna is usually placed above the Tx-antenna on the antenna mast. Rx1 Rx1/Tx TMA Tx ~ 10λ ~ 3m (900 MHz) TMA Rx2 Rx2 antennacables Diplexer Rx1 Tx Rx2 Combiner Power Amp (BTS)-Controller Tx Rx1 Rx2 Combiner Power Amp (BTS)-Controller TMA: Tower Mounted Amplifier

38 Some Consequences to Mobile Radio (IV) MSE, Radio-Prop, 38 Signal bandwidth B and Signal-to-Noise-Ratio (SNR) limit the Rx-sensitivity and the radio range the higher the data rate (because of large bandwidth B, large modulation alphabet and/or high coding rate), the less sensitive is the Rx and, thus, the smaller is the radio range Example: WLAN IEEE g supports 54 Mbps with QAM64 modulation and coding R=3/4 on short distances and 6 Mbps with BPSK modulation and coding R=1/2 on «longer» distances The larger the cell is, the larger is typically the RMS-Delay-Spread and the smaller is the coherence bandwidth B c the coherence bandwidth limits the symbol rate if ISI is avoided e.g. by using OFDM All modern high-rate, broadband wireless systems working in a multipath environment use OFDM-modulation parallel transmission on many narrow-band channels each with bandwidth < B c