Radio Propagation Achmad Ubaidillah Ms.,ST.MT

Radio Propagation Achmad Ubaidillah Ms.,ST.MT
3/10/1999 Radio Propagation Achmad Ubaidillah Ms.,ST.MT Universitas Trunojoyo – Madura Di ambil dari : CSCI 694 Lewis Girod

Outline Introduction and terminology Propagation mechanisms
3/10/1999 Outline Introduction and terminology Propagation mechanisms Propagation models 17 March 1999 Radio Propagation

What is Radio? Radio Xmitter induces E&M fields
3/10/1999 What is Radio? Radio Xmitter induces E&M fields Electrostatic field components µ 1/d3 Induction field components µ 1/d2 Radiation field components µ 1/d Radiation field has E and B component Field strength at distance d = EB µ 1/d2 Surface area of sphere centered at transmitter Start with some simplified background. The electrostatic and induction fields decay rapidly and can be ignored. 17 March 1999 Radio Propagation

General Intuition Two main factors affecting signal at receiver
3/10/1999 General Intuition Two main factors affecting signal at receiver Distance (or delay)  Path attenuation Multipath  Phase differences Green signal travels 1/2 farther than Yellow to reach receiver, who sees Red. For 2.4 GHz,  (wavelength) =12.5cm. 17 March 1999 Radio Propagation

3/10/1999 Objective Invent models to predict what the field looks like at the receiver. Attenuation, absorption, reflection, diffraction... Motion of receiver and environment… Natural and man-made radio interference... What does the field look like at the receiver? Radio propagation is complicated and highly dependent on details of the environment. Most models are statistically based or take into account a general case. Examples: probability distributions to simulate noise, 4-ray configuration to simulate propagation down a street 17 March 1999 Radio Propagation

Models are Specialized
3/10/1999 Models are Specialized Different scales Large scale (averaged over meters) Small scale (order of wavelength) Different environmental characteristics Outdoor, indoor, land, sea, space, etc. Different application areas macrocell (2km), microcell(500m), picocell Scale: The terminology may be confusing; both large and small scale models assume “far-field” region and may be applied to the same set of distances. The difference lies in the granularity of prediction provided by the model. Applications: Models are usually developed with a specific purpose in mind and are not always generally applicable because they may make assumptions that are not generally valid. For example, some models are good for outdoor systems while others are more accurate for indoor systems. Macrocell - large outdoor cells Microcell - smaller outdoor cells or large indoor systems Picocell - room or desk sized systems 17 March 1999 Radio Propagation

Outline Introduction and some terminology Propagation Mechanisms
3/10/1999 Outline Introduction and some terminology Propagation Mechanisms Propagation models 17 March 1999 Radio Propagation

Radio Propagation Mechanisms
3/10/1999 Radio Propagation Mechanisms Free Space propagation Refraction Conductors & Dielectric materials (refraction) Diffraction Fresnel zones Scattering “Clutter” is small relative to wavelength The building blocks of propagation models are these three basic ways that radio energy can interact with the environment. Most materials are neither perfect conductors nor perfect insulators but it is useful to describe the models separately. Note that conductivity and permittivity (a measure of resistance to electric fields) are frequency dependent. 17 March 1999 Radio Propagation

Free Space Propagation Model
3/10/1999 Free Space Propagation Model Received power at distance d is where Pt is the transmitter power in Watts a constant factor K depends on antenna gain, a system loss factor, and the carrier wavelength 17 March 1999 Radio Propagation

Refraction Perfect conductors reflect with no attenuation
3/10/1999 Refraction Perfect conductors reflect with no attenuation Dielectrics reflect a fraction of incident energy “Grazing angles” reflect max* Steep angles transmit max* q qr qt This is actually quite simplified; the actual characteristics depend on the polarization of the incident wave. Depending on polarization there can also be a 180 degree phase shift in the reflected component. “grazing” angles often result in “ground reflection” for large T-R distances Reflection induces 180 phase shift 17 March 1999 Radio Propagation *The exact fraction depends on the materials and frequencies involved

Diffraction Diffraction occurs when waves hit the edge of an obstacle
3/10/1999 Diffraction Diffraction occurs when waves hit the edge of an obstacle “Secondary” waves propagated into the shadowed region Excess path length results in a phase shift Fresnel zones relate phase shifts to the positions of obstacles T R 1st Fresnel zone Obstruction 17 March 1999 Radio Propagation

Contoh Gambar 17 March 1999 Radio Propagation

3/10/1999 Outline Introduction and some terminology Propagation Mechanisms Propagation models Large scale propagation models Small scale propagation (fading) models 17 March 1999 Radio Propagation

Propagation Models: Large
Large scale models predict behavior averaged over distances >>  Function of distance & significant environmental features, roughly frequency independent Breaks down as distance decreases Useful for modeling the range of a radio system and rough capacity planning 17 March 1999 Radio Propagation

Propagation Models: Small
Small scale (fading) models describe signal variability on a scale of  Multipath effects (phase cancellation) dominate, path attenuation considered constant Frequency and bandwidth dependent Focus is on modeling “Fading”: rapid change in signal over a short distance or length of time. 17 March 1999 Radio Propagation

Large Scale Models Path loss models Outdoor models Indoor models
3/10/1999 Large Scale Models Path loss models Outdoor models Indoor models 17 March 1999 Radio Propagation

3/10/1999 Path Loss Path Loss is a measure of attenuation based only on the distance to the transmitter Path loss models typically define a “close-in” point d0 and reference other points from there: Path loss.. Ratio of transmit power to receive power in decibels. “close-in” points are especially useful for gathering empirical data: measurements are made at the d_0 point and compared with measurements from farther distances. What is dB? 17 March 1999 Radio Propagation

Outdoor Models “2-Ray” Ground Reflection model
3/10/1999 Outdoor Models “2-Ray” Ground Reflection model Diffraction model for hilly terrain 17 March 1999 Radio Propagation

2-Ray Ground Reflection
3/10/1999 2-Ray Ground Reflection For d >> hrht, low angle of incidence allows the earth to act as a reflector the reflected signal is 180 out of phase Pr  1/d4 (=4) R T ht hr Phase shift! 17 March 1999 Radio Propagation

Ground Reflection 2 Intuition: ground blocks 1st Fresnel zone
3/10/1999 Ground Reflection 2 Intuition: ground blocks 1st Fresnel zone Reflection causes an instantaneous 180 phase shift Additional phase offset due to excess path length If the resulting phase is still close to 180, the gound ray will destructively interfere with the LOS ray. 180 R T ht hr p1 p0 17 March 1999 Radio Propagation

3/10/1999 Hilly Terrain Propagation can be LOS or result of diffraction over one or more ridges LOS propagation modelled with ground reflection: diffraction loss But if there is no LOS, diffraction can actually help! 17 March 1999 Radio Propagation

Indoor Path Loss Models
3/10/1999 Indoor Path Loss Models Indoor models are less generalized Environment comparatively more dynamic Significant features are physically smaller More clutter, scattering, less LOS 17 March 1999 Radio Propagation

3/10/1999 Outline Introduction and some terminology Propagation Mechanisms Propagation models Large scale propagation models Small scale propagation (fading) models 17 March 1999 Radio Propagation

Recall: Fading Models Small scale (fading) models describe signal variability on a scale of  Multipath effects (phase cancellation) dominate, path attenuation considered constant Focus is on modeling “Fading”: rapid change in signal over a short distance or length of time. 17 March 1999 Radio Propagation

Factors Influencing Fading
3/10/1999 Factors Influencing Fading Motion of the receiver: Doppler shift Transmission bandwidth of signal Compare to BW of channel Multipath propagation Receiver sees multiple instances of signal when waves follow different paths Very sensitive to configuration of environment If the tx b/w is greater than channel b/w, frequency selective fading can occur. The signal will be distorted but it may be possible to recover from fading because it will occur on a scale smaller than individual symbols. 17 March 1999 Radio Propagation

Effects of Multipath Signals
3/10/1999 Effects of Multipath Signals Rapid change in signal strength due to phase cancellation Frequency modulation due to Doppler shifts from movement of receiver/environment Echoes caused by multipath propagation delay 17 March 1999 Radio Propagation

3/10/1999 The Multipath Channel One approach to small-scale models is to model the “Multipath Channel” Linear time-varying function h(t,) Basic idea: define a filter that encapsulates the effects of multipath interference Measure or calculate the channel impulse response (response to a short pulse at fc): h(t,)  t 17 March 1999 Radio Propagation

3/10/1999 SKIP Channel Sounding “Channel sounding” is a way to measure the channel response transmit impulse, and measure the response to find h(). h() can then be used to model the channel response to an arbitrary signal: y(t) = x(t)h(). Problem: models the channel at single point in time; can’t account for mobility or environmental changes h(t,)   17 March 1999 Radio Propagation

Characterizing Fading*
*Adapted from EE535 Slides, Chugg ‘99 From the impulse response we can characterize the channel: Characterizing distortion Delay spread (d): how long does the channel ring from an impulse? Coherence bandwidth (Bc): over what frequency range is the channel gain flat? 17 March 1999 Radio Propagation

Characterizing Fading 2*
Characterizing Time-variation: How does the impulse response change with time? Coherence time (tc): for what value of  are responses at t and t+ uncorrelated? (How quickly is the channel changing) Doppler Spread (fd): How much will the spectrum of the input be spread in frequency? fd1/tc 17 March 1999 Radio Propagation

Statistical Fading Models
Fading models model the probability of a fade occurring at a particular location Used to generate an impulse response In fixed receivers, channel is slowly time-varying; the fading model is reevaluated at a rate related to motion 17 March 1999 Radio Propagation

Common Distributions Rayleigh fading distribution
3/10/1999 Common Distributions Rayleigh fading distribution Models a flat fading signal Used for individual multipath components Ricean fading distribution Used when there is a dominant signal component, e.g. LOS + weaker multipaths parameter K (dB) defines strength of dominant component; for K=-, equivalent to Rayleigh 17 March 1999 Radio Propagation

Multi-ray Multi-ray Rayleigh fading:
3/10/1999 Multi-ray Multi-ray Rayleigh fading: The Rayleigh distribution does not model multipath time delay (frequency selective) Multi-ray model is the sum of two or more independent time-delayed Rayleigh variables s(t) R1 R2  r(t)  Rappaport, Fig. 4.24, pp. 185. 17 March 1999 Radio Propagation

Saleh & Valenzuela (1987) Measured same-floor indoor characteristics
Rappaport, pp. 188 Measured same-floor indoor characteristics Found that, with a fixed receiver, indoor channel is very slowly time-varying RMS delay spread: mean 25ns, max 50ns With no LOS, path loss varied over 60dB range Model assumes a structure and models correlated multipath components. 17 March 1999 Radio Propagation

3/10/1999 References Wireless Communications: Principles and Practice, Chapters 3 and 4, T. Rappaport, Prentice Hall, 1996. Principles of Mobile Communication, Chapter 2, G. Stüber, Kluwer Academic Publishers, 1996. Slides for EE535, K. Chugg, 1999. Spread Spectrum Systems, Chapter 7, R. Dixon, Wiley, 1985 (there is a newer edition). Wideband CDMA for Third Generation Mobile Communications, Chapter 4, T. Ojanpera, R. Prasad, Artech, House 1998. Propagation Measurements and Models for Wireless Communications Channels, Andersen, Rappaport, Yoshida, IEEE Communications, January 1995. 17 March 1999 Radio Propagation

3/10/1999 The End 17 March 1999 Radio Propagation

3/10/1999 Scattering 2 hc is the critical height of a protrusion to result in scattering. RCS: ratio of power density scattered to receiver to power density incident on the scattering object Wave radiated through free space to scatterer and reradiated: 17 March 1999 Radio Propagation

Free Space 2a Free space power flux density (W/m2)
3/10/1999 Free Space 2a Free space power flux density (W/m2) power radiated over surface area of sphere where Gt is transmitter antenna gain By covering some of this area, receiver’s antenna “catches” some of this flux 17 March 1999 Radio Propagation

Free Space 2b Fraunhofer distance: d > 2D2/
3/10/1999 Free Space 2b Fraunhofer distance: d > 2D2/ Antenna gain and antenna aperture Ae is the antenna aperture, intuitively the area of the antenna perpendicular to the flux Gr is the antenna gain for a receiver. It is related to Ae. Received power (Pr) = Power flux density (Pd) * Ae 17 March 1999 Radio Propagation

Free Space 2c where L is a system loss factor
3/10/1999 Free Space 2c where L is a system loss factor Pt is the transmitter power Gt and Gr are antenna gains  is the carrier wavelength 17 March 1999 Radio Propagation

LNSM 2 PL(d)[dB] = PL(d0) +10nlog(d/d0)+ X
3/10/1999 LNSM 2 PL(d)[dB] = PL(d0) +10nlog(d/d0)+ X where X is a zero-mean Gaussian RV (dB)  and n computed from measured data, based on linear regression 17 March 1999 Radio Propagation

Ground Reflection 1.5 The power at the receiver in this model is
3/10/1999 Ground Reflection 1.5 The power at the receiver in this model is derivation calculates E field; Pr = |E|2Ae; Ae is ant. aperture The “breakpoint” at which the model changes from 1/d2 to 1/d4 is  2hthr/ where hr and ht are the receiver and transmitter antenna heights 17 March 1999 Radio Propagation

Convolution Integral Convolution is defined by this integral:
3/10/1999 Convolution Integral Convolution is defined by this integral: Indexes relevant portion of impulse response Scales past input signal 17 March 1999 Radio Propagation

Partition Losses Partition losses: same floor
3/10/1999 Partition Losses Partition losses: same floor Walls, furniture, equipment Highly dependent on type of material, frequency Hard partitions vs soft partitions hard partitions are structural soft partitions do not reach ceiling “open plan” buildings 17 March 1999 Radio Propagation

Partition Losses 2 Partition losses: between floors
3/10/1999 Partition Losses 2 Partition losses: between floors Depends on building construction, frequency “Floor attenuation factor” diminishes with successive floors typical values: 15 dB for 1st floor 6-10 dB per floor for floors 2-5 1-2 dB per floor beyond 5 floors 17 March 1999 Radio Propagation

Materials Attenuation values for different materials 17 March 1999
3/10/1999 Materials Attenuation values for different materials 17 March 1999 Radio Propagation

What does “dB” mean? dB stands for deciBel or 1/10 of a Bel
3/10/1999 What does “dB” mean? dB stands for deciBel or 1/10 of a Bel The Bel is a dimensionless unit for expressing ratios and gains on a log scale Gains add rather than multiply Easier to handle large dynamic ranges The Bel is named after Bell of telephone fame Its common use stemmed from the need to conveniently express and manipulate amplifier gains. Decibels are a convenient way to describe ratios when the range of the values will span many orders of magnitude. 17 March 1999 Radio Propagation

dB 2 Ex: Attenuation from transmitter to receiver. Useful numbers:
3/10/1999 dB 2 Ex: Attenuation from transmitter to receiver. PT=100, PR=10 attenuation is ratio of PT to PR [PT/PR]dB = 10 log(PT/PR) = 10 log(10) = 10 dB Useful numbers: [1/2]dB  -3 dB [1/1000]dB = -30 dB 17 March 1999 Radio Propagation

dB 3 dB can express ratios, but what about absolute quantities?
3/10/1999 dB 3 dB can express ratios, but what about absolute quantities? Similar units reference an absolute quantity against a defined reference. [n mW]dBm = [n/mW]dB [n W]dBW = [n/W]dB Ex: [1 mW]dBW = -30 dBW 17 March 1999 Radio Propagation

Channel Sounding 2 Several “Channel Sounding” techniques can measure the channel response directly: Direct RF pulse (we hinted at this approach) Sliding correlator Frequency domain sounding 17 March 1999 Radio Propagation

Channel Sounding 3 Direct RF Pulse
3/10/1999 Channel Sounding 3 Direct RF Pulse Xmit pulse, scope displays response at receiver Can be done with off-the-shelf hardware Problems: hard to reject noise in the channel If no LOS must trigger scope on weaker multipath component may fail to trigger lose delay and phase information 17 March 1999 Radio Propagation

Channel Sounding 4 Sliding correlator Xmit PseudoNoise sequence
3/10/1999 Channel Sounding 4 Sliding correlator Xmit PseudoNoise sequence Rcvr correlates signal with its PN generator Rcvr clock slightly slower; PN sequences slide Delayed components cause delayed correlations Good resolution, good noise rejection 17 March 1999 Radio Propagation

Channel Sounding 5 Frequency domain sounding Sweep frequency range
3/10/1999 Channel Sounding 5 Frequency domain sounding Sweep frequency range Compute inverse Fourier transform of response Problems not instantaneous measurement Tradeoff between resolution (number of frequency steps) and real-time measurement (i.e. duration as short as possible) 17 March 1999 Radio Propagation

Digression: Convolutions
3/10/1999 Digression: Convolutions The impulse response “box” notation implies the convolution operator,  Convolution operates on a signal and an impulse response to produce a new signal. The new signal is the superposition of the response to past values of the signal. Commutative, associative 17 March 1999 Radio Propagation

Convolutions 2 y(t) is the sum of scaled, time-delayed responses x(t)
3/10/1999 Convolutions 2 y(t) is the sum of scaled, time-delayed responses x(t) h(t) y(t)  = h(t) Each component of the sum is scaled by the x(t)dt at that point; in this example, the response is scaled to 0 where x(t) = 0. + y(t) 17 March 1999 Radio Propagation

Convolutions 3 Graphical method: “Flip & Slide” x(t) h(t) y(t)  =
3/10/1999 Convolutions 3 Graphical method: “Flip & Slide” x(t) h(t) y(t)  = Pairwise multiply x*h and integrate over  x() Flip & Slide: h(t-) h(t-) Flip & Slide: h(t-) h(t-) Flip & Slide: h(t-) h(t-) Flip & Slide: h(t-) h(t-) Flip & Slide: h(t-) h(t-) y(t) and Store y(t) 17 March 1999 Radio Propagation

Frequency and Time Domains
The channel impulse response is f(time) It describes the channel in the “time domain” Functions of frequency are often very useful; Space of such functions is “frequency domain” Often a particular characteristic is easier to handle in one domain or the other. 17 March 1999 Radio Propagation

Frequency Domain Functions of frequency
usually capitalized and take the parameter “f” where f is the frequency in radians/sec and the value of the function is the amplitude of the component of frequency f. Convolution in time domain translates into multiplication in the frequency domain: y(t) = x(t)h(t)  Y(f) = X(f)H(f) 17 March 1999 Radio Propagation

Frequency Domain 2 Based on Fourier theorem:
any periodic signal can be decomposed into a sum of (possibly infinite number of) cosines The Fourier Transform and inverse FT Convert between time and frequency domains. The frequency and time representations of the same signal are “duals” 17 March 1999 Radio Propagation

Flat Fading T >> d and W << BC  minimal ISI = = s(t)
3/10/1999 Flat Fading T >> d and W << BC  minimal ISI s(t) h(t,) r(t) Delay spread Time domain (convolve) t t = t Ts  Ts+ Coherence BW Freq domain (filter) = f f f fc fc fc 17 March 1999 Radio Propagation

Frequency Selective Fading
3/10/1999 Frequency Selective Fading T << d and W >> BC  ISI s(t) h(t,) r(t) Delay spread Time domain (convolve) t t = Ts  Ts Ts+ Coherence BW Freq domain (filter) = f f f fc fc fc 17 March 1999 Radio Propagation

Review Object of radio propagation models:
predict signal quality at receiver Radio propagation mechanisms Free space (1/d2) Diffraction Refraction Scattering 17 March 1999 Radio Propagation

Review 2 Factors influencing received signal
Path loss: distance, obstructions Multipath interference: phase cancellation due to excess path length and other sources of phase distortion Doppler shift Other radio interference 17 March 1999 Radio Propagation

Review 3 Approaches to Modelling
Models valid for far-field, apply to a range of distances large scale models: concerned with gross behavior as a function of distance small scale (fading) models: concerned with behavior during perturbations around a particular distance 17 March 1999 Radio Propagation

Relevance to Micronets
Micronets may require different models than most of the work featured here Smaller transmit range Likely to be near reflectors: on desk or floor. On the other hand, at smaller scales things are less smooth: “ground reflection” may turn into scattering Outdoors, throwing sensors on ground may not work. Deployable tripods? 17 March 1999 Radio Propagation

Relevance 2 Consequences of “Fading”
You can be in a place that has no signal, but where a signal can be picked up a short distance away in any direction Ability to move? Switch frequencies/antennas? Call for help moving or for more nodes to be added? If stuck, may not be worth transmitting at all Reachability topology may be completely irrelevant to location relationships 17 March 1999 Radio Propagation

Relevance 3 Relevant modelling tools:
Statistical models (Rice/Rayleigh/Log Normal) Statistical fading assumes particular dynamics, this depends on mobility of receivers and environment CAD modelling of physical environment and ray tracing approaches. For nodes in fixed positions this is only done once. 17 March 1999 Radio Propagation

Relevance 4 An approach to modelling?
Characterize wireless system interactions with different materials, compare to published data Assess the effect of mobility in environment on fixed topologies, relate to statistical models Try to determine what environmental structures and parameters are most important: Scattering vs. ground reflection? can a simple CAD model help? 17 March 1999 Radio Propagation

Radio Propagation Achmad Ubaidillah Ms.,ST.MT

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