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Mobile Communications
Part IV- Propagation Characteristics Professor Z Ghassemlooy Faculty of Engineering and Environment University of Northumbria U.K. Z. Ghassemlooy
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Contents Radiation from Antenna Propagation Model (Channel Models)
Free Space Loss Plan Earth Propagation Model Practical Models Summary Z. Ghassemlooy
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Wireless Communication System
Channel code word Source Encoder Channel Mod- ulator Message Signal Modulated Transmitted Signal Wireless Channel User Source Decoder Channel Demod- ulator Estimate of Message signal channel code word Received Signal Z. Ghassemlooy
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Basic Questions Tx: What will happen if the transmitter Channel:
changes transmit power ? changes frequency ? operates at higher speed ? Channel: What will happen if we conduct this experiment in different types of environments? Desert Metro Street Indoor Rx: What will happen if the receiver moves? Z. Ghassemlooy
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Antenna - Ideal Isotropics antenna: In free space radiates power equally in all direction. Not realizable physically d d E H Pt EM fields around a transmitting antenna , a polar coordinate d- distance directly away from the antenna. is the azimuth, or angle in the horizontal plane. is the zenith, or angle above the horizon. Distance d Area Z. Ghassemlooy
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Antenna - Real Not isotropic radiators, but always have directive effects (vertically and/or horizontally) A well defined radiation pattern measured around an antenna Patterns are visualised by drawing the set of constant-intensity surfaces Z. Ghassemlooy
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Antenna – Real - Simple Dipoles
Not isotropic radiators, e.g., dipoles with lengths /4 on car roofs or /2 as Hertzian dipole Example: Radiation pattern of a simple Hertzian dipole shape of antenna is proportional to the wavelength Largest dimension of the antenna La = /2 /4 /2 side view (xy-plane) x y side view (yz-plane) z top view (xz-plane) simple dipole Z. Ghassemlooy
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Antenna – Real - Sdirected and Sectorized
Used for microwave or base stations for mobile phones (e.g., radio coverage of a valley) side view (xy-plane) x y side view (yz-plane) z top view (xz-plane) Directed top view, 3 sector x z top view, 6 sector Sectorized Z. Ghassemlooy
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Antenna - Ideal - contd. The power density of an ideal loss-less antenna at a distance d away from the transmitting antenna is: Gt is the transmitting antenna gain The product PtGt : Equivalent Isotropic Radiation Power (EIRP) which is the power fed to a perfect isotropic antenna to get the same output power of the practical antenna in hand. Note: the area is for a sphere. W/m2 Z. Ghassemlooy
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Antenna - Ideal - contd. The strength of the signal is often defined in terms of its Electric Field Intensity E, because it is easier to measure. Pa = E2/Rm where Rm is the impedance of the medium. For free space Rm = 377 Ohms.. V/m Z. Ghassemlooy
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Antenna - Ideal - contd. The receiving antenna is characterized by its effective aperture Ae, which describes how well an antenna can pick up power from an incoming electromagnetic wave The effective aperture Ae is related to the gain Gr as follows: Ae = Pr / Pa => Ae = Gr2/4 which is the equivalent power absorbing area of the antenna. Gr is the receiving antenna gain and = c/f Z. Ghassemlooy
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Signal Propagation (Channel Models)
Z. Ghassemlooy
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Basic Digital Communication System Model
Input data Pulse generator TX filter channel x(t) Receiver A/D + Channel noise n(t) Output data HT(f) HR(f) y(t) Hc(f) Tb: Symbol period Z. Ghassemlooy
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Fourier Representation of Periodic Signals
1 1 t t ideal periodic signal real composition (based on harmonics)
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Signals II Different representations of signals
amplitude (amplitude domain) frequency spectrum (frequency domain) phase state diagram (amplitude M and phase in polar coordinates) Composite signals mapped into frequency domain using Fourier transformation Digital signals need infinite frequencies for perfect representation modulation with a carrier frequency for transmission (->analog signal!) f [Hz] A [V] I= M cos Q = M sin t[s]
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Channel Models High degree of variability (in time, space etc.)
Large signal attenuation Non-stationary, unpredictable and random Unlike wired channels it is highly dependent on the environment, time space etc. Modelling is done in a statistical fashion The location of the base station antenna has a significant effect on channel modelling Models are only an approximation of the actual signal propagation in the medium. Are used for: performance analysis simulations of mobile systems measurements in a controlled environment, to guarantee repeatability and to avoid the expensive measurements in the field. Z. Ghassemlooy
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Channel Models - Classifications
System Model - Deterministic Propagation Model- Deterministic Predicts the received signal strength at a distance from the transmitter Derived using a combination of theoretical and empirical method. Stochastic Model - Rayleigh channel Semi-empirical (Practical +Theoretical) Models Z. Ghassemlooy
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Channel Models - Linear
Finite delay Constant Linear channel h(t) H(f) y(t) = k x(t-td) x(t) No Noise Therefore for a linear channel is: Amplitude distortion Phase distortion The phase delay Describes the phase delayed experienced by each frequency component Z. Ghassemlooy
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This can be modelled as an
ideal low pass filter non-ideal low pass filter This smears the transmitted signal x(t) in time causing the effect of a symbol to spread to adjacent symbols when a sequence of symbols are transmitted. The resulting interference, intersymbol interference (ISI), degrades the error performance of the communication system. Channel Z. Ghassemlooy
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Power Delay Profile RMS Delay Spread () = 46.4 ns
Received Signal Level (dBm) -105 -100 -95 -90 50 100 150 200 250 300 350 400 450 Excess delay (ns) RMS Delay Spread () = 46.4 ns Mean Excess delay () = 45 ns Maximum Excess delay < 10 dB = 110 ns Noise threshold Z. Ghassemlooy
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Power Delay Spread - Example
0 dB -10 dB -20 dB -30 dB 1 2 5 (µs) Z. Ghassemlooy
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Channel Models – Multipath Link
The mathematical model of the multipath can be presented using the method of the impulse response used for studying linear systems. y(t) = k x(t-td) y(n)= k x(n-N) x(t) = (t) x(n)= (n) Linear channel h(t) H(f) h(n) Channel is usually modeled as Tap-Delay-Line Z. Ghassemlooy
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Channel Models – Multipath Link
Time variable multi-path channel impulse response Where a(t-) = attenuated signal Time invariant multi-path channel impulse response Each impulse response is the same or has the same statistics, then Where aie(.) = complex amplitude (i.e., magnitude and phase) of the generic received pulse. = propagation delay generic ith impulse N = number signal arriving from N path (.) = impulse signal (delta function) Z. Ghassemlooy
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Channel Models – Multipath Link
Channel transfer function Multipath Time Mostly used to denote the severity of multipath conditions. Defined as the time delay between the 1st and the last received impulses. Coherence bandwidth - on average the distance between two notches Bc ~ 1/TMP Z. Ghassemlooy
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RMS delay spread Bc Freq. domain view Time domain view
High correlation of amplitude between two different freq. components Range of freq over which response is flat Bc RMS delay spread Freq. domain view For 0.9 correlation For 0.5 correlation Z. Ghassemlooy
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Power Delay Spread - Example
0 dB -10 dB -20 dB -30 dB 1 2 5 (µs) Signal bandwidth for Analog Cellular = 30 KHz Signal bandwidth for GSM = 200 KHz Z. Ghassemlooy
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Multipath Delay With a 52 meter separation, in a LOS environment, a large ns excess delay NLOS excess delay over 423 meters extended to ns. Office building: RMS delay spread = ns Cell size (km) Max Delay Spread Pico cell 0.1 300 nn Micro cell 5 15 us Macro cell 20 40 us Z. Ghassemlooy
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Inter-symbol interference (ISI)
Channel is band limited innature Limited frequency response unlimited time response Channel has multipath Tx filter – There are two major ways to mitigate the detrimental effect of ISI. The first method is to design bandlimited transmission pulses (i.e., Nyquist pulses) which minimize the effect of ISI. Equalization BS 1 2 0 Building MU Z. Ghassemlooy
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ISI Symbol time Pr() 4.38 µs 0 dB -10 dB -20 dB -30 dB 1 2 5 (µs) 1 2 5 (µs) 4.38 Symbol time > 10* No equalization required Symbol time < 10* Equalization will be required to deal with ISI In the above example, symbol time should be more than 14µs to avoid ISI. This means that link speed must be less than 70Kbps (approx) Z. Ghassemlooy
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Propagation Path Loss The propagation path loss is LPE = LaLlf Lsf
where La is average path loss (attenuation): (1-10 km), Llf - long term fading (shadowing): 100 m ignoring variations over few wavelengths, Lsf - short term fading (multipath): over fraction of wavelength to few wavelength. Metrics (dBm, mW) [P(dBm) = 10 * log[ P(mW) ] Z. Ghassemlooy
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Propagation Path Loss – Free Space
Power received at the receiving antenna at far field only Thus the free space propagation path loss is defined as: Isotropic antenna has unity gain (G = 1) for both transmitter and receiver. Z. Ghassemlooy
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Propagation - Free Space–contd.
The difference between two received signal powers in free space is: If d2 = 2d1, the P = -6 dB i.e 6 dB/octave or 20 dB/decade Z. Ghassemlooy
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Propagation - Non-Line-of-Sight
Generally the received power can be expressed as: Pr d-v For line of sight v = 2, and the received power Pr d-2 Average v over all NLOS could be For non-line of sight with no shadowing, received power at any distance d can be expressed as: 100 m< dref < 1000 m Z. Ghassemlooy
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Propagation - Non-Line-of-Sight
Log-normal Shadowing Where Xσ: N(0,σ) Gaussian distributed random variable Z. Ghassemlooy
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Received Power for Different Value of Loss Parameter v
Distance (km) 10 20 30 40 50 -70 -80 -90 -100 -110 -120 -130 -135 Received power (dBm) v = 2, Free space v = 3 Rural areas v = 4, City and urban areas Z. Ghassemlooy
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Propagation Model- Free Space
In terms of frequency f and the free space velocity of electromagnetic wave c = 3 x 108 m/s it is: Expressing frequency in MHz and distance d in km: Z. Ghassemlooy
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Propagation Model- Free Space (non-ideal, path loss)
BS MU Non-isotropic antenna gain unity, and there are additional losses Lad , thus the power received is: d > 0 and L 0 Thus for Non-isotropic antenna the path loss is: Note: Interference margin can also be added Z. Ghassemlooy
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Propagation Model- Free Space (non-ideal, path loss)
Considering loss S due to shadowing Z. Ghassemlooy
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Propagation Model - Mechanisms
Reflection Diffraction Scattering Z. Ghassemlooy Source: P M Shankar
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Channel Model- Plan Earth Path Loss - 2 Ray Reflection
In mobile radio systems the height of both antennas (Tx. and Rx.) << d (distance of separation) hb dd dr hm Direct path (line of sight) Ground reflected path d From the geometry dd = [d2 + (hb - hm )2] Z. Ghassemlooy
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Channel Model- Plan Earth Path Loss - contd.
Using the binomial expansion Note d >> hb or hm. Similarly The path difference d = dr - dd = 2(hbhm )/d The phase difference Z. Ghassemlooy
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Channel Model- Plan Earth Path Loss– contd.
Total received power Where is the reflection coefficient. For = -1 (low angle of incident) and . Z. Ghassemlooy
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Channel Model- Plan Earth Path Loss– contd.
Therefore: Assuming that d >> hm or hb, then sin x = x for small x Thus which is 4th power law Z. Ghassemlooy
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Channel Model- Plan Earth Path Loss– contd.
Propagation path loss (mean loss) Compared with the free space = Pr = 1/ d2 In a more general form (no fading due to multipath), path attenuation is LPE increases by 40 dB each time d increases by 10 Z. Ghassemlooy
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Channel Model- Plan Earth Path Loss– contd.
Including impedance mismatch, misalignment of antennas, pointing and polarization, and absorption The power ration is: where Gt(θt,φt) = gain of the transmit antenna in the direction (θt,φt) of receive antenna. Gr(θr,φr) = gain of the receive antenna in the direction (θr,φr) of transmit antenna. Γt and Γr = reflection coefficients of the transmit and receive antennas at and ar = polarization vectors of the transmit and receive antennas α is the absorption coefficient of the intervening medium. Z. Ghassemlooy
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LOS Channel Model - Problems
Simple theoretical models do not take into account many practical factors: Rough terrain Buildings Refection Moving vehicle Shadowing Thus resulting in bad accuracy Solution: Semi- empirical Model Z. Ghassemlooy
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Sem-iempirical Model Practical models are based on combination of measurement and theory. Correction factors are introduced to account for: Terrain profile Antenna heights Building profiles Road shape/orientation Lakes, etc. Okumura model Hata model Saleh model SIRCIM model Outdoor Indoor Y. Okumura, et al, Rev. Elec. Commun. Lab., 16( 9), 1968. M. Hata, IEEE Trans. Veh. Technol., 29, pp , 1980. Z. Ghassemlooy
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Okumura Model Widely used empirical model (no analytical basis!) in macrocellular environment Predicts average (median) path loss “Accurate” within dB in urban and suburban areas Frequency range: MHz Distance: > 1 km BS antenna height: > 30 m. MU antenna height: up to 3m. Correction factors are then added. Z. Ghassemlooy
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Hata Model Consolidate Okumura’s model in standard formulas for macrocells in urban, suburban and open rural areas. Empirically derived correction factors are incorporated into the standard formula to account for: Terrain profile Antenna heights Building profiles Street shape/orientation Lakes Etc. Z. Ghassemlooy
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Hata Model – contd. The loss is given in terms of effective heights.
The starting point is an urban area. The BS antennae is mounted on tall buildings. The effective height is then estimated at km from the base of the antennae. P M Shankar Z. Ghassemlooy
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Hata Model - Limits Frequency range: 150 - 1500 MHz
Distance: 1 – 20 km BS antenna height: m MU antenna height: 1 – 10 m Z. Ghassemlooy
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Hata Model – Standard Formula for Average Path Loss for Urban Areas
Correction Factors are: Large cities Average and small cities Z. Ghassemlooy
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Hata Model – Average Path Loss for Urban Areas contd.
Carrier frequency 900 MHz, BS antenna height 150 m, MU antenna height 1.5m. Z. Ghassemlooy P M Shankar
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Hata Model – Average Path Loss for Suburban and Open Areas
Suburban Areas Open Areas Z. Ghassemlooy
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Hata Model - Average Path Loss
S. Loyka, 2003, Introduction to Mobile Communications Z. Ghassemlooy
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Improved Model Hata-Okumura model are not suitable for lower BS antenna heights (2 m), and hilly or moderate-to-heavy wooded terrain. To correct for these limitations the following model is used [1]: For a given close-in distance dref. the average path loss is: Lpl = A + 10 v log10 (d / dref) + s for d > dref, (dB) where A = 20 log10(4 π dref / λ) v is the path-loss exponent = (a – b hb + c / hb) hb is the height of the BS: between 10 m and 80 m dref = 100m and a, b, c are constants dependent on the terrain category s is representing the shadowing effect [1] V. Erceg et. al, IEEE JSAC, 17 (7), July 1999, pp Z. Ghassemlooy
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Improved Model Terrains Model Type A Type B Type C parameter
The typical value of the standard deviation for s is between 8.2 And 10.6 dB, depending on the terrain/tree density type Terrain A: The maximum path loss category is hilly terrain with moderate-to-heavy tree densities . Terrain B: Intermediate path loss condition Terrain B: The minimum path loss category which is mostly flat terrain with light tree densities Z. Ghassemlooy
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Losses Summary of penetration losses through various common building materials at 28 GHz. Both of the horn antennas have 24.5 dBi gains with 10 half power beamwidth [28]. Penetration losses for multiple indoor obstructions in an office environment at 28 GHz. Weak signals are denoted by locations where the SNR was high enough to distinguish signal from noise but not enough for the signal to be acquired, i.e. penetration losses were between 64 dB to 74 dB relative to a 5 m free space test. No signal detected denotes an outage, where penetration loss is greater than 74 dB relative to a 5 m free space test [28]. Z. Ghassemlooy
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Reflection Coefficients
Comparison of reflection coefficients for various common building materials at 28 GHz. Both of the horn antennas have 24.5 dBi gains with 10 half power beam-width. Z. Ghassemlooy
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Summary Attenuation is a result of reflection, scattering, diffraction and reflection of the signal by natural and human-made structures The received power is inversely proportional to (distance)v, where v is the loss parameter. Studied channel models and their limitations Z. Ghassemlooy
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Questions and Answers Tell me what you think about this lecture
Next lecture: Multi-path Propagation- Fading Z. Ghassemlooy
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