Advanced Digital Communications (EE5511) MSc Module of Wireless Communication System Dr. Qiang Ni ECE, School of Eng & Design, Brunel University E-mail:
[email protected] Homepage: http://people.brunel.ac.uk/~eestqqn/ Office: Howell Building H237 Dr Qiang Ni
Brunel University
1/44 1/45
Section 3: 3: Wireless Channels and Channel Models (1)
Dr Qiang Ni
Brunel University
2/44
Antenna and Radio Propagation
Dr Qiang Ni
Brunel University
3/44
Functionality of Antenna The functionality of an antenna is to transform electromagnetic energy into electromagnetic waves (transmission side) and to transform electromagnetic waves back into electromagnetic energy (reception). Question:
Should antenna preferably be erected as high and be as long as is possible or desirable?
Dr Qiang Ni
Brunel University
4/44
Antenna Basics In the following we only present two basic types of antennas used for radio propagation. More knowledge, Recommend 2 Books: Antennas and Propagation for Wireless Communication Systems – by Simon R. Saunders Wiley, ISBN 10:0471986097(H/B)
PRACTICAL ANTENNA HANDBOOK By Joseph Carr
Dr Qiang Ni
Brunel University
5/44
Marconi Antenna (1) The most basic antenna is called "a quarter-wave vertical“ (or called Marconi Antenna). It is a quarter wavelength long and is a vertical radiator. Typical examples would be seen installed on motor vehicles for two way communications.
Technically Marconi antenna is an "isotropic radiator". This is a mythical antenna which radiates in all directions as does the light from a lamp bulb.
Dr Qiang Ni
Brunel University
6/44
Marconi Antenna (2) The quarter-wave vertical antenna is usually the simplest to construct and erect.
Dr Qiang Ni
Brunel University
7/44
Hertz Antenna (1) The half-wave dipole antenna (or called Hertz Antenna) becomes quite common where space permits. It can be erected vertically but it is more often than not erected horizontally for practical reasons.
Dr Qiang Ni
Brunel University
8/44
Hertz Antenna (2) You will note that the up- and down hand halves are merely quarter wave sections. The input impedance of this half-wave dipole example is nominally 75 ohm. Dr Qiang Ni
Brunel University
9/44
Antenna Radiation Field It is defined as the radiation that surrounds an antenna but doesn’t collapse its field back into the antenna ִ Near field and far field are two designators for antenna fields ִ The far field region begins when the distance
R>
2D 2
λ
where R = distance from the antenna (m) D = dimension of the antenna (m) λ = wavelength of the transmitted signal (m) ִ The near field will be any distance less than R
Dr Qiang Ni
Brunel University
10/44
How to calculate the wavelength ִ Definition: The distance travelled by the wave during a period of once cycle
λ=
v f
v is the velocity of the wave in meters per second and f is the frequency ִ Example: Calculate the wavelength of a 100MHz signal travel in free space. Note that the velocity of electromagnetic waves in free space is 3x108m/s. v 3 × 10 8 λ= = = 3m f 1 × 10 8 Dr Qiang Ni
Brunel University
11/44
Example Determine the distance from a parabolic reflector with diameter (D) = 4.5m to the boundary of the far-field region if the parabolic reflector is used for Ku-band transmission of a 12-GHz signal. Solution: The wavelength for a 12-GHz signal is approximately 3 × 108 λ= = 0.025 m 9 12 × 10
D = 4.5m, therefore
2 × ( 4 .5 ) 2 R> = 1620 m 0 .025
Therefore, the boundary for the far field region for this parabolic reflector is a distance greater than 1620 meters from the antenna. Dr Qiang Ni
Brunel University
12/44
Antenna Radiation Pattern ִ Radiation pattern is an indication of radiated field strength around the antenna Omnidirectional: a spherical radiation pattern Bidirectional: concentrates energy in certain directions at the expense of lower energy in other directions …
Dr Qiang Ni
Brunel University
13/44
Antenna Gain ִ Antenna Gain is a measure of how much more power in dB an antenna will radiate in a certain direction with respect to that which would be radiated by a reference antenna Expressed as dBi, if the reference antenna is an isotropic point source Expressed as dBd, if the reference antenna is an half wavelength dipole antenna ִ For example, the half-wave dipole antenna has a 2.15dB gain as compared to an isotropic radiator Dr Qiang Ni
Brunel University
14/44
Overall Damaging Effects of Wireless Channel on Signal
Dr Qiang Ni
Brunel University
15/44
Overall Channel Damaging Effects (1)
The overall damaging effects of Wireless Channel have both multiplicative impact damaging the signal - attenuation (denoted by a(t)), and additive impact damaging the signal – known as noise (denoted by n(t)) and interference (denoted by j(t)), as shown in the figure next slice
Dr Qiang Ni
Brunel University
16/44
Overall Channel Damaging Effects (2)
s(t): transmitted signal a(t): radio channel attenuation j(t): interfering signal n(t): time-varying random noise y(t): received signal Dr Qiang Ni
y(t) = a(t) * s(t) + j(t) + n(t) Brunel University
17/44
Overall Channel Damaging Effects (3) As shown in the last figure, the received signal may first be influenced by a multiplicative factor, the attenuation a(t). Actually there are two main different attenuation effects which result in an overall attenuation of the transmitted signal:
a(t)=aPL(t)*aFA(t) Where
aPL(t): attenuation of Large-scale Path Loss;
aFA(t): attenuation of Small-scale Fading and Multipath. Dr Qiang Ni
Brunel University
18/44
Large-Scale Path Loss Effects
Dr Qiang Ni
Brunel University
19/44
Path Loss (1) Path Loss is a type of deterministic effect depending only on the distance between the transmitter and receiver. It plays an important role on larger time scales (e.g. seconds or minutes), since the distance between transmitter and receiver in most situations does not change significantly on smaller time scales.
Dr Qiang Ni
Brunel University
20/44
Path Loss (2) Definition: In a communication system, path loss is the attenuation undergone by an electromagnetic wave in transit between a transmitter and receiver.
Note 1: Path loss may be due to many effects such as free-space loss, refraction, reflection, diffraction, scattering, aperture-medium, and absorption.
Note 2: Path loss usually refers to long-distance loss (km). Note 3: Path loss is usually measureded in dB (decibel). Dr Qiang Ni
Brunel University
21/44
Large-scale Propagation Models
Dr Qiang Ni
Brunel University
22/44
Large-scale Propagation Models Two Simplified Outdoor models: Free-Space Propagation model Two-Ray Propagation model
Other Outdoor Propagation models Some Indoor Propagation models Dr Qiang Ni
Brunel University
23/44
Free-Space Propagation (1) In free space, a signal suffers from propagating over a distance between two antennas assuming line of sight (LOS: no objects obstructing the path between the transmitter and receiver). It’s usually called a free-space path loss, which can be calculated using the Maxwell equations and is given by: 2
λ PR = Pt G tG r , 4 πd Dr Qiang Ni
Brunel University
24/44
Free-Space Propagation (2) Or in dB: PR PR λ [dB] = 10 log = 20 log + 10 log(Gt ) + 10 log(Gr ) Pt Pt 4 πd
where PR is the received power, Pt is the transmitted power, λ is the wavelength, Gt is the gain of the transmitter antenna and Gr is the gain of the receiver antenna (both gains in the direction of the straight line that connects the two antennas in space), d is the distance. Dr Qiang Ni
Brunel University
25/44
Free-Space Propagation (3) Further notes d = the distance between the transmitter antenna and the receiver antenna (m) Pr = power received (W) Pt = power transmitted (W) Gt = transmitting antenna gain compared to isotropic radiator (not in dB). Normally a Unit Gain is chosen in many cases, i.e. G =1 Gr = receiving antenna gain compared to isotropic radiator (not in dB)
λ
Dr Qiang Ni
= wavelength (m) Brunel University
26/44
Free-Space Propagation (4) The received power is inversely proportional to the square of the distance and the square of the frequency. Physical explanation: 1.
In free space, the radiated energy propagates equally in every direction and the wave can be seen as a sphere of increasing radius.
2. Since energy can’t be destroyed, it will be the same whatever the distance from the radiating point is. So that the total energy over the sphere is the same independent of the radius, the energy per unit surface must decrease. 3. As the surface increases with the square of the radius, so does energy per unit surface decrease at the inverse rate. Dr Qiang Ni
Brunel University
27/44
Free-Space Propagation (5) Assumes far-field (d - distance) ִ d >> D and d >> λ , where D is the largest linear dimension of the antenna λ is the carrier wavelength
No interference, no obstructions Path Loss is a measure of attenuation based only on the distance to the transmitter Free space model only valid in far-field Dr Qiang Ni
Brunel University
28/44
Example: Two λ/2 dipoles are separated by 50km. They are aligned for optimum reception. The transmitter feeds its antenna with 10W at 144MHz. Calculate the power received. Solution: The two dipoles have a gain of 2.15dB. Therefore Gt = Gr = 10(2.15/10) = 1.64
3 × 108 10W × 1.64 × 1.64 × 6 2 PG G λ 144 × 10 Pr ( d ) = t t 2 r 2 = 2 16π d 16π 2 50 × 103
(
2
)
= 2.96 × 10 −10W
Dr Qiang Ni
Brunel University
29/44
Two-Ray Propagation Model (1) Since most communications happen close to the earth surface, the scenario for free-space loss is unrealistic. The two-ray model is a simple model based on physicaloptics theory which takes into account the reflection on the earth surface. It also assumes LOS and no influence on propagation besides the earth surface. It is a useful starting point for the study of propagation for personal communications. It is often used to describe propagation over open fields. Dr Qiang Ni
Brunel University
30/44
Two-Ray Propagation Model (2) In the two-ray model, two propagation paths between the transmitter/receiver are considered: the direct wave (LOS) path, and the reflected wave path. (hTX, hRx and d are known.) Direct wave
Reflected wave hTx
hRx
path length of direct wave: path length of reflected wave: Dr Qiang Ni
2
d1 = (hT x − hR x ) + d
2
α = arctan
d 2 = d 21 + d 22 = ( hT x + hR x ) 2 + d 2 Brunel University
hTx − hRx d
Why? 31/44
Two-Ray Propagation Model (3)
After some approximation, the two-ray propagation model is simplified as the known 4th-power-law form:
h Tx h R x PR1 = Pt G t G r 2 d
2
,
Power falls off proportional to d4 and is independent of signal wavelength. Dr Qiang Ni
Brunel University
32/44
Two-Ray Propagation Model (4) The Two-Ray Ground Reflection model has been found to be reasonably accurate for predicting large-scale signal strength over distances of several kilometers for mobile radio systems that use tall towers (heights which exceed 50m), as well as for LOS microcell channels in urban environments. This model is not accurate for complicated indoor environments. Dr Qiang Ni
Brunel University
33/44
Other Outdoor Empirical Models The above 2 simplified outdoor propagation models are
attempt to predict path loss close to the Earth’s surface. However, communication often takes place over irregular terrain. Hence, the above assumptions are unrealistic: The terrain profile of a particular area needs to be taken into account for obtaining better estimates of path loss. Irregular terrain, like in cities, doesn't lend itself to simple analytical path loss models. For example, the terrain profile may vary from a simple curved Earth profile to a highly mountainous profile.
A number of propagation models were proposed to predict path loss over irregular terrain. These models are empirical. Dr Qiang Ni
Brunel University
34/44
Empirical Outdoor models Empirical path loss models based on extensive measurements. First, we’ll show the 2 most commonly used empirical outdoor models in conjunction with 900 MHz (macro) cellular systems: Hata’s mode and Lee’s model. By macro-cell we mean a cell typically on the order of tens of kilometers.
Then, we’ll list some other empirical outdoor models. Dr Qiang Ni
Brunel University
35/44
Okumura-Hata’s models (1) The Hata model is an empirical formulation of the graphical path loss data which was provided by Okumura. Hata presented the urban propagation loss as a standard formula and supplied correction Equations for Applications to other situations Carrier Frequency : 150 MHz ≤ fc ≤ 1500 MHz Base Station Height : 30m ≤ hb ≤ 200m Mobile Station Height: 1m ≤ hm ≤ 10m T-R distance : 1km ≤ d ≤ 20km Dr Qiang Ni
Brunel University
36/44
Okumura-Hata’s models (2) Lp is the path loss:
for urban area
Lp = A + B log10(d)
for suburban area
Lp = A + B log10(d) - C
for open area
Lp = A + B log10(d) - D
A = 69.55 + 26.16 log10(fc) – 13.82 log10(hb) – a(hm) B = 44.9 – 6.55 log10(hb) C = 5.4 + 2[log10(fc/28)]2 D = 40.94 + 4.78 [log10(fc)]2 – 18.33 log10(fc)
Dr Qiang Ni
Brunel University
37/44
Okumura-Hata’s models (3) When applies to small to medium cities, a(hm) = [1.1 log10(fc) – 0.7]hm – 1.56 log10(fc) – 0.8
When large cities and for fc ≤ 400 MHz: a(hm) = 8.28 [log10(1.54 hm)]2 – 1.1
When large cities and for fc ≥ 400 MHz. a(hm) = 3.2 [log10(11.75 hm)]2 – 4.97
Dr Qiang Ni
Brunel University
38/44
Lee’s models Lee’s path loss model is used to model a flat terrain. Lee’s model has been known to be more of a “North American model” than that of Hata. Received signal power in dBm is given by:
d0 β fc β µ Ω = 10 log10 µ Ω0 ( ) ( ) a0 d f µ Ω is the power at 1 mile β is path loss exponent. 0
These parameters are determined from empirical measurements Dr Qiang Ni
Brunel University
39/44
Other Empirical models (1) Okumura’s model - One of most widely used for Urban. - based on free space path loss + correction factors for urban, suburban and rural areas, irregular terrain, street orientations Sakagmi and Kuboi model - extend Okumura’s model using regression analysis of data. Ibrahim and Parsons model - equations developed to best fit data observed at London. (freq. 168-900 MHz) Dr Qiang Ni
Brunel University
40/44
Other Empirical models (2) COST231-HATA model - the COST231-Hata model extends Hata’s model for use in the 1500-2000 MHz frequency range, which does take into account parameters such as roof heights, street widths and building separation. Two Slope model - transmission distances range up to 500 m and antenna heights are less than 20 m. Longley-Rice model - point-to-point communication system in the frequency range from 40MHz to 100 GHz. Durkin’s model Walfisch and Bertoni’s model Wideband PCS Microcell model More details read book: Wireless Com: Principles & Practice Dr Qiang Ni
Brunel University
41/44
Indoor Propagation Models (1) Indoor propagation is also dominated by reflection, diffraction and scattering as outdoor, but conditions are much more variable. Specialized models for indoor propagation also exist. These factor losses within the same floor (partition losses due to walls and other materials, including furniture) or losses for propagation across floors. Losses due to the latter are adjusted by way of the floor attenuation factor (FAF). Finally sophisticated ray-tracing and site-specific modeling techniques also have been developed. Dr Qiang Ni
Brunel University
42/44
Indoor Propagation Models (2) Partition losses (same floors). Partition losses between floors. Log-distance path loss model. Ericsson Multiple Breakpoint model. Attenuation Factor model. More Details see the referencing book:
Wireless Communications: Principles & Practice (2nd Ed) Dr Qiang Ni
Brunel University
43/44
Dr Qiang Ni
Brunel University
44/44
