Mobile Communication Systems (3+0)- Lecture 2

EE-451 Mobile Communication Systems (3+0)

Lecture 2

Radio Propagation: Large-scale Path Loss and Shadowing

Lec Moiz Ahmed Pirkani

EE-451 MILITARY COLLEGE OF SIGNALS- NUST

Lecture Outline

Introducing radio waves and propagation

Discuss different propagation models and mechanisms

Introducing some common propagation models both for outdoor and indoor environments

References Goldsmith Ch 2

Rappaport Ch 4

Haykin Ch 2

Learn how to estimate noise in a system Rappaport Appendix B

Haykin Ch 2.8


Mobile Communication Systems (3+0)

Introduction to Radio Waves (I)

Why antenna radiates? Radiation occurs whenever a current flows through a

wire with a certain frequency Electric and magnetic field

Transmission line theories



Introduction to Radio Waves (II)



Antennas Many different types

Passive device i.e. no gain

Isotropic antenna Hypothetical lossless antenna having equal radiation in all direction

The reference of 0dBi

Realistic antennas Has a maximum gain larger than 0dBi

Doesnt mean it is active, but is directional such that in some direction, the power is larger than in other directions

Gain at a particular direction

Introduction to Radio Waves (III)



When a signal is injected into the antenna Radio wave is generated and propagates through the

wireless channel

The received signal can be severely distorted

Introduction to Radio Waves (IV) Three level model

Path loss Models the signal attenuation in large transmitter-receiver (T-R)

separation Generally, attenuation increases when T-R increases Caused by the wave propagation through free space

Shadowing Models the signal power at same T-R separation but different

locations The signal variation in a circular loci Caused by change of environment in different locations

Multipath fading Models the rapid variation within a distance of few wavelengths Caused by constructive or destructive interference resulted from

multiple arrival paths



Free Space Propagation Model (I) Consider a radio wave with power Pt from an isotropic antenna

At a distance d, the power flux density (power per unit area) is

The power Pr captured by an antenna with effective area Ae is:

For isotropic receive antenna

Hence, the received power for isotropic antenna is:

Power attenuates in a squared rate on distance and frequency



Free Space Propagation Model (II) Now consider realistic antennas

Transmit antenna with gain Gt When no direction is specified for the gain, the maximum is used

Receive antenna with gain Gr

Maximum antenna gain

Hence, the received power for realistic antennas is

Also known as the Friis equation

The path loss is defined as



Free Space Propagation Model (III)

Assumptions Receiver at far-field d>>

Plain wave model can be used (E, H & propagation direction are orthogonal)

The max beam of the Tx antenna points to the max beam of the Rx antenna

Both Gt and Gr are at max

Free space propagation

No obstacles or reflectors, not even the ground!

Reference distance d0 A known received power reference point

Could be measured or predicted value

Received power can be written as:



Quiz The ground transmitter for a low earth orbit (LEO)

satellite is 1000km away from the satellite. The carrier frequency is 1.5GHz, and the transmission power is 10dBW. The antenna gain of the transmitter is 15dBi and the receiver is 2dBi. Calculate the received power in free space propagation model in dBm.



FSL- Limitations for Mobile Communications Free space loss factor shows that more power is lost at higher frequencies. It is

evident that for antennas with specified gains defined by the antenna structure, transmitter or receiver architectures,

The energy transfer (ratio between received and transmit power) will be highest at lower frequencies.

In the design of antenna structures for mobile communications, we observe that mobile phones operate at less than 2 GHz (GSM900, GSM 1800, LTE1800).

More frequency spectrum that can provide higher data rates is available at higher frequencies, but the associated path loss will not enable quality reception.

Recently developed USA standards for cellular communications, Verizon Wireless (4G) operates at the frequency band of 700MHz to ensure that considerable quality of reception may be achieved at larger distances thereby enhancing their coverage area.

Another important inference that can be derived from the Friis Transmission equation is that antennas that are made to operate in the higher microwave or millimeter wave range cannot be used for communication at larger distances because of the path loss that is incurred during the transmission.

Since the path loss is very high, so only point-to-point communication is possible. This occurs when the receiver and transmitter are in the vicinity of each other.



Noise (I) System performance is controlled by signal-to-noise

ratio (SNR) Received signal power can be estimated from the models

Noise must be separately calculated

Thermal noise

k = 1.38x10-23 J/K (Boltzmanns constant)

T0 = 290K (room temperature)

B = bandwidth

N0 = Noise power spectral density

What is the room temperature noise spectral density? N0 = -174dBm/Hz



Noise (II) Noise figure

The ratio increase on noise power at the output of the device

Noise figure measures the additional noise generated by the device If device is noiseless (F=1), the input noise is only amplified

Noise figure is usually expressed in dB

The smaller the noise figure (close to 0dB), the lower the noise

Equivalent noise temperature Te Noise generated by the device can be considered as additional thermal

noise Te K

At room temperature, input noise = kT0B:



Noise (III) Output noise power

Two cascaded devices At input of device 2, the noise power density = G1F1N0

Output of device 2 = Gain x (input noise + additional noise)



Noise (IV) Cascaded system

Overall noise figure

System equivalent temperature

Antenna is always considered to have unity gain Remember that antenna gain is the ratio of max strength/isotropic?

Hence, a system with antenna is

Noise is significantly reduced if the first device has high gain but low noise

Importance of low noise amplifier (LNA)!



Signal to Noise Ratio The received wireless signal might be very small

Could be in the order of 10-11W

How could we detect such signal?

Performance of communication system is governed by the signal to noise ratio (SNR) If the noise is even smaller, the received signal can be detected

Thats why LNA is very important in wireless communication systems!

SNR calculation SNR after the RF devices (e.g., antenna, amplifiers, mixer etc)

Output received power is also amplified!



Tutorial Question For the previously considered LEO satellite, the antenna of

the receiver has a noise figure of 3dB. The low noise amplifier (LNA) has a noise figure of 0.5dB and a gain of 10dB. The overall system noise figure is 10dB and an overall system gain of 20dB. The system has a bandwidth of 1MHz. Calculate the noise figure of the satellite, and the received SNR (assume shadow facing space temperature = 120K).



Propagation Mechanism Three basic propagation mechanism

Reflection

Diffraction

Scattering



Reflections (I) Ground reflection (Two-Ray) model

Made use of the theories on reflection of radio waves ht: height of Tx

hr: height of Rx

d: T-R separation

ETOT: total E-field

ELOS: line-of-sight E-field

Er: reflected E-field

i: incident angle

0: reflected angle



Reflections (II) The total received power

PR-LOS is the received power from the LOS path

Assuming ddd, and < 0.3rad such that sin( /2)= /2 (i.e. d>>hthr)

Plan-Earth propagation equation

Frequency independent

Inverse fourth-power law

Antenna height dependent



Tutorial Question The base station of a GSM 900 system (200kHz BW) has a

height of 10m and is 10km away from the mobile user, who has a height of 2m. The antenna gain of the base station and the mobile are 2 and 3dBi respectively. The transmit power is 1dBW.

Calculate the received power with ground reflection



Diffraction (I) The phenomenon that radio signal can propagate around

curved surface or sharp-edged obstacles Huygens principle

Each point on a wave front acts as a secondary point source

Diffraction is caused by propagation of secondary wavelets into the shadowed area

Excess path length The wave that bends around

the obstacle will travel with a longer distance

The excess path length will lead to a phase difference between the arrival paths

Could have constructive or destructive interference



Diffraction (II) Fresnel zones

Successive regions where secondary waves have a n/2 excess path length

These successive zones provides constructive and destructive interference alternately

The centre region will have all signals in-phase, leading to constructive interference

The next region will have all signals out-of-phase, leading to destructive interference



Diffraction (III) Diffraction is affected by frequency and obstacle location

Higher freq, less diffraction

If the first Fresnel zone is unobstructed, diffraction effect can be neglected i.e. Free space propagation if the first Fresnel zone is clear



Scattering The phenomenon when a radio signal hits an object and the

reflected waves are spread out Unlike reflection, where the reflected wave is (theoretically) in one

direction

Occurs when the object is small e.g. lamp post, trees, etc

Or when the large reflective object has a rough surface Affects the reflection coefficient

Considered rough when surface protuberance larger than

Difficult to have a generic model as the effect is heavily dependent on the object



Path Loss Exponent (I) Path loss exponent

An average path loss exponent is used to model the propagation loss in large T-R separation

Mean path loss at distance d

where d0 is a reference distance and is the mean path loss at d0

If is not specified, it is usually taken as free-space path loss at a distance of 1m

Received power at distance d

Pr(d0) is the received power at the reference distance



Path Loss Exponent (II) Empirically measured path loss exponent (Rappaport)



Tutorial Question For the previous case with ground reflection, assuming free space

propagation within the first 10m, what is the path loss exponent?



Shadowing Models the signal power at different location but same T-R separation

Location dependent

Studies have shown that this effect can be modelled as a log-normal distribution Gaussian distributed in dB

Path loss at distance d in dB

is the mean path loss at distance d, which is modelled by the effect described previously

X is the log-normal shadowing effect with zero mean and variance

Hence, PL is a random variable with mean and variance



Outdoor Channel Model (I) Prediction model

Estimate the received power using the theories

e.g. Durkins model User first build a terrain map

Model the signal using the theories of path loss, reflection, diffraction, scattering, and shadowing

Only models the geographical terrain but no man made obstacles, such as buildings etc.

Too slow

Site specific



Outdoor Channel Model (II) Empirical model

Model the channel from extensive measured results

Okurmura-Hata model

Valid for 150MHz to 1500MHz

In urban area

L50 is the 50-th percentile (median) value of propagation loss in dB

a(hr) is the correction factor for mobile antenna height (in dB) Area dependent

Small to medium sized city

For large city



Outdoor Channel Model (III) In suburban areas, the formula can be modified by

In rural areas

PCS extension to Hata model (COST-231 model) Extend Hata model to 2GHz

CM=0dB for medium sized city and suburban, =3dB for metropolitan areas

Restricted to: Frequency: 1.5 to 2GHz

Tx height: 30 to 200m

Rx height: 1 to 10m

Distance: 1km to 20km

Many other models (e.g., COST 231 Walfisch-Ikegami model)



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Mobile Communication Systems (3+0)- Lecture 2