Making it work: Radiowave propagation 1 Chapter 4 - Making It Work Multiple Access Radiowave Propagation Signal Processing The Network

Making it work: Radiowave propagation 2 Radiowave Propagation Multipath- radiowaves can reach mobile user by many paths

Making it work: Radiowave propagation 3 Signal strength Signal varies in Fast fading – due to multipath fading Medium fading – due to geographical features or ground cover Slow fading – due to power fall-off with distance Signal varies in Fast fading – due to multipath fading Medium fading – due to geographical features or ground cover Slow fading – due to power fall-off with distance Signal varies in Fast fading – due to multipath fading Medium fading – due to geographical features or ground cover Slow fading – due to power fall-off with distance

Making it work: Radiowave propagation 4 Multipath fading Signals from different paths may add or cancel User in a 'multipath environment' or a fading environment

Making it work: Radiowave propagation 5 Cell planning BT CellNet UK coverage

Making it work: Radiowave propagation 6 Cell planning Problem 1To establish edge of cell to enable placement of main base stations - do calculations using simple propagation models - do measurements and derive simple equations 2To predict signal level within cell to discover if ‘fill-ins’ are needed - do difficult ray tracing models using reflection, diffraction, etc - do measurements

Making it work: Radiowave propagation 7 Slow Signal Reduction -Propagation in Free Space Free space loss equation P d = P.G 1.G 2.( /4. .d) 2 where P d = power received P = power transmitted G 1,2 = antenna gains = wavelength d = distance between antennas

Making it work: Radiowave propagation 8 Putting the loss factor (4. .d / ) 2 in dBs L dB = log 10 f MHz + 20.log 10 d km So that P r = P t + G 1 + G 2 - L dB Assuming a receiver noise, N r, and that a signal to noise ratio of S is required. Then P > S + N r + L – G 1 – G 2 Example, Find P for S = 20dB, N r = -120dBm, G 1 = G 2 = -3dBi, f = 150MHz, d = 1km Answer L = 76dB and P = -18dBm

Making it work: Radiowave propagation 9 Slow Signal Reduction - Propagation over ground

Making it work: Radiowave propagation 10 Direct wave: E d = A.exp(-j.k.r 0 )/4. .r 0 (1) Ground reflected wave: E r = A. .exp(-j.k.r 1 )/4. .r 1 (2) where A is a constant that contains antenna gains and transmit power level and  is the ground reflection coefficient.

Making it work: Radiowave propagation 11 Total received wave: E tot = E d + E r (3) Substituting from eqn (1) and (2) E tot = A. exp(-j.k.r 0 )/4. .r 0. [ 1 + .exp(-j.k.(r 1 - r 0 )).r 0 /r 1 ] (4) or E tot = E d. [ 1 + .exp(-j.k.(r 1 - r 0 )).r 0 /r 1 ] (5)

Making it work: Radiowave propagation 12 If d >> h T, h R, as is usually the case, then R 0 /R 1  1 and expression (5) simplifies to: E tot = E d. [ 1 + .exp(-j.k.(R 1 - R 0 )) ] (6) Now for low angle incidence on the ground .exp(j.  ) = -1

Making it work: Radiowave propagation 13 Furthermore, for d >> h T, h R, we have: and (8) (7) 2 ) 2 ( 1

Making it work: Radiowave propagation 14 Using  = - 1 and eqn (8), we can see that the square bracket in eqn (6) becomes (9)

Making it work: Radiowave propagation 15 Thus putting eqn (9) into eqn (5) E tot = 2.E d.sin(k.h t.h r /d)(10) or in power terms P tot = 4.P d. sin 2 (k.h t.h r /d)(11) Now P d = P.G 1.G 2.( /4. .d) 2 Thus P tot = 4.P.G 1.G 2.( /4. .d) 2. sin 2 (2. .h t.h r /.d)(12)

Making it work: Radiowave propagation 16 It can be seen that for grazing incidence d >> h T, h R and thus sin 2 (2. .h t.h r /.d) = (2. .h t.h r /.d) 2 and P tot = P.G 1.G 2.(h t.h r /d 2 ) 2 (13) Note that free space signal  1/d 2 plane earth signal  1/d 4

Making it work: Radiowave propagation 17 Loss factor is L dB = 40 log 10 d – 20 log 10 (h t.h r ) So that P r = P t + G 1 + G 2 - L dB Example, Find P for S = 20dB, N r = -120dBm, G 1 = G 2 = -3dBi, f = 150MHz, d =25km h t.h r = 100m 2 (high base station and handheld receiver) Answer P = 16 watts

Making it work: Radiowave propagation 18 To improve accuracy, include land usage factor 0 < L < 1 terrain height difference between tx and rx, H So L dB = 40 log 10 d – 20 log 10 (h t.h r ) f MHz / L – 0.34.H Example, Example, Find P for S = 20dB, N r = -120dBm, G 1 = G 2 = -3dBi, f = 150MHz, d =25km h t.h r = 100m 2, L = 0.3, H = 50m Answer P = 125W

Making it work: Radiowave propagation 19 Range of applicability of the two-ray model Good for VHF band or above (>30MHz) At high frequencies (when wavelength ~ roughness ) reflection coefficients not accurate reflection is diffuse At long range (>25km) earth not flat but spherical

Making it work: Radiowave propagation 20 Fast and medium fading -ray tracing methods Find ray paths including Reflections Diffractions Combinations of the two Pictures taken from

Making it work: Radiowave propagation 21 Diffraction Ray is scattered by any edge Shadow region Illuminated region

Making it work: Radiowave propagation 22 Result from commercial modelling tool Direct ray only

Making it work: Radiowave propagation 23 Direct + 2 reflections + 1 diffraction Direct + 1 reflection

Making it work: Radiowave propagation 24 Direct + 6 reflections + diffraction + double diffraction + diffraction/reflection + diffraction/2 reflections

Making it work: Radiowave propagation 25

Making it work: Radiowave propagation 26

Making it work: Radiowave propagation 27 How to model propagation losses? expressions based on analytical results parameters determined by lots of measurements

Making it work: Radiowave propagation 28 How to model propagation losses? Simple model. Free space loss P r = P t.G t.G r. ( /4  d) 2 or putting loss factor (4  d / ) 2 in dBs L dB = log 10 f MHz + 10.v.log 10 d km (so that P r = P t + G t + G r – L dB ) where v = 2

Making it work: Radiowave propagation 29 How to model propagation losses? Simple model. Plane earth loss P r = P t.G t.G r. ( /4  d) 2.sin 2 (2  h t. h r / d) = P t.G t.G r. (h t. h r /d 2 ) 2 or putting loss factor in dBs L dB = 10.v.log 10 d – 20.log 10 (h t.h r ) (so that P r = P t + G t + G r – L dB ) where v = 4

Making it work: Radiowave propagation 30 How to model propagation losses? Simple model. In many cases of communications 2 < v < 4 Lower values of v correspond to rural or sub-urban areas Higher values of v correspond to urban areas

Making it work: Radiowave propagation 31 How to model propagation losses? Simple model. Fig 2.7 shankar

Making it work: Radiowave propagation 32 How to model propagation losses? Hata’s model. For urban areas L dB = log 10 f MHz + (44.9 – 6.55.log 10 h b ).log 10 d log 10 h b – a(h m ) whered = separation in km, (must be > 1km) h b, h m = base and mobile antenna heights in m a(h m ) = mobile antenna height correction factor

Making it work: Radiowave propagation 33 How to model propagation losses? Hata’s model. For large cities a(h m ) = 3.2[log 10 (11.75.h m )] 2 – 4.97f > 400MHz For small and medium cities a(h m ) = [1.1.log 10 f – 0.7].h m – [1.56.log 10 f-0.8]

Making it work: Radiowave propagation 34 How to model propagation losses? Hata’s model. For suburban areas L dB = L p – 2[log 10 (f MHz /28)] 2 whereL p = loss for small to medium cities (from previous expression)

Making it work: Radiowave propagation 35 How to model propagation losses? Hata’s model. For rural areas L dB = L p – 4.78.[log 10 f MHz ] log 10 f MHz – whereL p = loss for small to medium cities (from previous expression)

Making it work: Radiowave propagation 36 Results - Note that large and small to medium loss different by only 1dB

Making it work: Radiowave propagation 37 How to model propagation losses? Hata’s model. Received power given by P r (d) (dBm) = P t – P loss (d) where P loss (d) = L dB (dB) for a given d from above expressions

Making it work: Radiowave propagation 38 How to model propagation losses? Hata’s model. We know that from simple model P r (d)  (1/d) v At a distance d ref P loss (d ref )  10.v.log 10 (d ref ) and P loss (d)  10.v.log 10 (d) so that v = [P loss (d) - P loss (d ref )] / 10.[log 10 (d) - log 10 (d ref )]

Making it work: Radiowave propagation 39 How to model propagation losses? Hata’s model. Examples of value of v For d > 5km large cityv = 4.05 small to medium city v = 4.04 suburbsv = 3.3 ruralv = 2.11

Making it work: Radiowave propagation 40 Cell Planning BT CellNet UK coverage

Making it work: Radiowave propagation 41 Network Planning Microwave links to mobile switching centres

Making it work: Radiowave propagation 42 The Multipath Environment Propagation mechanisms Diffraction Multiple diffraction Reflection Vertex diffraction Scattered paths with long delays

Making it work: Radiowave propagation 43 The Multipath Environment Signal varies in Fast fading – due to multipath fading Medium fading – due to geographical features or ground cover Slow fading – due to power fall-off with distance

Making it work: Radiowave propagation 44 threshold Movement creates fading important to know statistics of fading to optimally design system

Making it work: Radiowave propagation 45 The Multipath Environment-Fading Urban Channels Rayleigh probability density function describes short term fading if mobile moves characteristic of deep urban environments

Making it work: Radiowave propagation 46 To create a suitable statistical model, assume No direct ray Many (>10) approximately equal amplitude reflected/diffracted rays Rays have random phase and angle of arrival, with uniform arrival angle distribution 0 <  < 360  uniform arrival phase distribution 0 <  < 360 

Making it work: Radiowave propagation 47 where a = received signal envelope  2 = variance, (  = standard deviation) and2  2 = mean square value This is a Rayleigh statistical model Then probability of received signal envelope, a, is

Making it work: Radiowave propagation 48 f(a) a Characteristics Zero probability of zero signal Zero probability of infinite signal Peak value at  Non symmetrical shape

Making it work: Radiowave propagation 49 threshold Movement creates fading system will have threshold above which signal will be detectable; below it will be lost Key parameters outage probability level crossing rate average duration of fades All needed to choose best bit rates, word lengths and coding schemes

Making it work: Radiowave propagation 50 The outage probability is the probability that the signal level will be below the threshold level, a thresh. Outage Probability

Making it work: Radiowave propagation 51 Example If average signal is 100  W, what is probability of outage, if a thesh = 50  W. Now remember that 2  2 = mean square envelope value = c x average power and also a 2 thresh = square threshold envelope value = c x threshold power So P out = [1 – exp(-50/100)] = Outage Probability

Making it work: Radiowave propagation 52 Level crossing rate and average duration of fades Rate of positive (or negative) going crossings and average time spent below threshold in fades must be quantified

Making it work: Radiowave propagation 53 Level crossing rate To find rate, need to know joint probability of signal being at given level, a, and at a given slope (or rate of change of signal), da/dt. Assuming that these are uncorreleated, then then where Not able to prove in scope of course

Making it work: Radiowave propagation 54 Level crossing rate Note N R is dependent on velocity (by f m ) and envelope level. Result:- N R /f m is number of crossings per wavelength, Peaks when a is on a RMS value and low elsewhere

Making it work: Radiowave propagation 55 Average duration of fades Average duration of fades is average period of fade below threshold, that is ave. of τ 1, τ 2, τ 3, etc. It is given by the outage probability / level crossing rate.

Making it work: Radiowave propagation 56 Average duration of fades where

Making it work: Radiowave propagation 57 Calculation Assume, f m = 100Hz, (fast car) ρ = 1 (signal envelope at RMS value), so exp(1) = 2.72

Making it work: Radiowave propagation 58 Other fading models - Rician Rician probability density function describes short term fading if mobile moves characteristic of suburban and rural environments Same assumptions as Rayleigh, with some direct ray f(a) = (a/  2 ).exp[-(a 2 + A 0 2 )/2  2 ].I[aA 0 /  2 ] where A 0 is amplitude of direct ray Statistics for Non-Urban Cases

Making it work: Radiowave propagation 59 Rician characterised by K K(dB) = 10log 10 [A 0 2 /2  2 ] For K = -  Rician becomes Rayleigh, with increasing direct ray K increases and for very large K Rician tends to Gaussian f(a) a K = 0.8dB K = 6dB K = 14dB

Making it work: Radiowave propagation 60 Lognormal probability distribution describes case when multiple scattering of single ray occurs f(P) = (1/  (2  2 P 2 )).exp[-ln 2 (P/P 0 )/2  2 ] f(P) P

Making it work: Radiowave propagation 61 The Multipath Environment - Dispersion Rays arriving at different times result in pulse broadening or time dispersion Transmitted pulse Received pulses and envelope Effect is to produce Inter symbol interference

Making it work: Radiowave propagation 62 The Multipath Environment - Dispersion Rms delay spread,  d, is a measure of the broadening. Thus channel bandwidth is given by B c = 1/(5  d ) If B c > B m channel is flat fading ( no ISI ) and if B c < B m channel is frequency selective ( ISI occurs) where B m is the message bandwidth

Making it work: Radiowave propagation 63 The Multipath Environment - Dispersion If mobile is moving then repetitive fading will take place Assume two rays coming from 0  and 180 . Interference will produce a standing wave with /2 wavelength Fading rate, R = 2v/ where v = velocity of mobile Example, freq = 100MHz, v = 34mph = 15m/s, R = 10Hz

Making it work: Radiowave propagation 64 The Multipath Environment - Dispersion If mobile is moving then frequency dispersion will take place Rays will be received from all directions and each will experience a Doppler shift of f = (v/ ).cos  where  = angle of arrival (0  <  < 360  ) and when  = 0 , f = f m, the maximum shift, = v/ Assume that the frequency seen by the mobile is f = f c + f m cos  where f c = the carrier frequency

Making it work: Radiowave propagation 65 The Multipath Environment - Dispersion Now to preserve power the power spectral density must equal the power arrival density so S(f).df = P(  ).d  Assuming equal arrival probability from all angles, then S(f) = d  /df Now d  /df = -1/(f m.sin  ) = -(1/f m ) /  (1 – cos 2  ) So S(f) = -(1/f m ) /  [1 – (f - f c ) 2 /f m 2 ]

Making it work: Radiowave propagation 66 The Multipath Environment - Dispersion S(f) f -f m fmfm Received frequencies will be smeared over range from –f m to f m Channel coherence time given by T c = 9/16  f m Pulse duration is T p then if T p < T c no pulse distortion, channel has slow fading if T p > T c distortion occurs, channel has fast fading

Making it work: Radiowave propagation 67 Diversity Basic Principle : if two or more independent samples of a random process are taken then these samples will fade in an uncorrelated manner. Diversity Methods Frequency - unacceptable as it would increase spectrum congestion. Polarisation - possible but depends on degree of depolarisation in scattering process. Field - E and H field may be uncorrelated but antenna design may be hard. Space - best method, but needs > antenna spacing. - OK at VHF on vehicles and at > 900 MHz on handsets

Making it work: Radiowave propagation 68 Diversity Can be done at base station or mobile but normally at base station to keep cost of handsets down

Making it work: Radiowave propagation 69 Key concept is sampling of multipath waveform at two points or creation of two uncorrelated waveforms in multipath environment

Making it work: Radiowave propagation 70 Base station diversity (mainly down-link) two antennas create two uncorrelated multipath field environments at mobile Multipath scattering area Handset diversity (mainly down-link) two antennas sample multipath field environments at two uncorrelated points

Making it work: Radiowave propagation 71 Typical diversity base station antennas (a) USA, (b) UK, (c) Japan

Making it work: Radiowave propagation 72 How to combine signals from multiple antennas in a diversity system (a)Switching Simple Cheap Least effective Improvement in SNR So for M = 2 D(M) = 1.5

Making it work: Radiowave propagation 73 (b) Cophasing and summing Better performance But requires phase shifters Improvement in SNR So for M = 2 D(M) = 1.8

Making it work: Radiowave propagation 74 (c) Maximal ratio combining Best performance But requires phase shifters and variable gain amps Improvement in SNR So for M = 2 D(M) = 2.0

Making it work: Radiowave propagation 75 Switching strategies for diversity systems switch and stay (until threshold is dropped below). switch and examine (and keep switching if other SNR is below threshold). selection diversity (selected best SNR)

Making it work: Radiowave propagation 76