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Cellular Mobile Communication Systems

Lecture 2

Engr. Shahryar SaleemAssistant Professor

Department of Telecom EngineeringUniversity of Engineering and Technology

TaxilaTI -1011

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Wireless Issues• Wireless link implications

– communications channel is the air• poor quality: fading, shadowing, weather, etc.

– regulated by governments• frequency allocated, licensing, etc.

– limited bandwidth• Low bit rate, frequency planning and reuse, interference

– power limitations• Power levels regulated, must conserve mobile terminal battery life

– security issues• wireless channel is a broadcast medium!

• Wireless link implications for communications– How to send signal?– How to clean up the signal in order to have good quality– How to deal with limited bandwidth?

• Design network and increase capacity/share bandwidth in a cell

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Typical Wireless Communication System

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Components of Communication System

• Source– Produces information for transmission (e.g., voice, keypad entry, etc.)

• Source encoder– Removes the redundancies and efficiently encodes the information

• Channel encoder– Adds redundant bits to the source bits to recover from any error that thechannel may introduce

• Modulator– Converts the encoded bits into a signal suitable for transmission over thechannel

• Antenna– A transducer for converting guided signals in a transmission line intoelectromagnetic radiation in an unbounded medium or vice versa

• Channel– Carries the signal, but will usually distort it

• Receiver – reverses the operations

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What is Signal Propagation• How is a radio signal transformed from the time it leaves

a transmitter to the time it reaches the receiver• Important for the design, operation and analysis of

wireless networks– Where should base stations/access points be placed– What transmit powers should be used– What radio frequencies need be assigned to abase station– How are handoff decision algorithms affected…

• Propagation in free open space like light rays• In general make analogy to light and sound waves

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Signal Propagation• Received signal strength (RSS) influenced by

– Fading – signal weakens with distance - proportional to1/d² (d = distance between sender and receiver)

– Frequency dependent fading – signal weakens with increase in f

– Shadowing (no line of sight path)

– Reflection off of large obstacles

– Scattering at small obstacles

– Diffraction at edges

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Signal Propagation

• Effects are similar indoors

and outdoors• Several paths from Tx to Rx

– Different delays, phases

and amplitudes

– Add motion – makes it very

complicated

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Multipath Propagation• Signal can take many different paths between sender and receiver

due to reflection, scattering, diffraction

• Time dispersion: signal is dispersed over time interference with “neighbor” symbols, Inter Symbol Interference (ISI)• The signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the

different parts

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Effects of Mobility• Time Variations in Signal Strength• Channel characteristics change over time and location

– signal paths change– different delay variations of different signal parts– different phases of signal parts

• Quick changes in the power received (short term or fast fading)• Slow changes in the average power received (long term fading)• Additional changes in;

– distance to sender– obstacles further away

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Fading

• Fading refers to the Time variation of the received signal power caused by the changes in the telecommunication medium or path.

• When a signal is transmitted from a sender to the receiver multiple copies of the signal are formed due to the obstructions in the path between sender and receiver. Each signal copy will experience different;– Attenuation– Delay – Phase shift

• This can result in either constructive or destructive interference, amplifying or attenuating the signal power as seen at the receiver.

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Types of Fading

• Slow fading/ Shadowing/ Long Term Fading/ Large Scale Fading: Caused by larger movements of the mobile or obstructions within the propagation environment.

• Fast Fading/ Multipath Fading/ Short Term Fading/ Small Scale Fading: Caused by the small movements of the mobile or obstruction.

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Communication Issues and Radio Propagation

• Three main issues in radio channel– Achievable signal coverage

• What is geographic area covered by the signal• Governed by path loss

– Achievable channel rates (bps)• Governed by multipath delay spread

– Channel fluctuations – effect data rate• Governed by Doppler spread and multipath

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Communication Issues and Radio Propagation

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Coverage

• Determines– Transmit power required to provide service in a given area (link budget)– Interference from other transmitters– Number of base stations or access points that are required

• Parameters of importance (Large Scale/ long Term Fading effects)– Path loss (long term fading)– Shadow fading (No LOS)

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Signal Coverage Range

• Transmission range– communication possible– low error rate

• Detection range– detection of the signal possible– no communication possible

• Interference range– signal may not be detected– signal adds to the background noise

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Decibels

• Power (signal strength) is expressed in decibels (dB) for ease of calculation

– Values relative to 1 mW are expressed in dBm– Values relative to 1 W are expressed in dBW– Other values are simply expressed in dB

• Example 1: Express 2 W in dBm and dBW

– dBm: 10 log10 (2 W / 1 mW) = 10 log10(2000) = 33 dBm

– dBW: 10 log10 (2 W / 1 W) = 10 log10(2) = 3 dBW

• In general dBm value = 30 + dBW value

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Free Space Loss Model• Assumptions

– Transmitter and receiver are in free space– No obstructing objects in between– The earth is at an infinite distance!– The transmitted power is Pt– The received power is Pr– Isotropic antennas

• Antennas radiate and receive equally in all directions with unit gain

• The path loss is the difference between the received signal strengthand the transmitted signal strength

PL = Pt (dB) – Pr (dB)

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Free Space Loss

• Transmit power Pt

• Received power Pr

• Wavelength of the RF carrier λ = c/f

• Over a distance d the relationship between Pt and Pr is given by:

• In dB, we have:

• Pr (dBm)= Pt (dBm) - 21.98 + 20 log10 (λ) – 20 log10 (d)

• Path Loss = PL = Pt – Pr = 21.98 - 20log10(λ) + 20log10 (d)

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Free Space Propagation• Notice that factor of 10 increase in distance => 20 dB increase in

path loss (20 dB/decade)

Distance Path Loss @ 880 MHzd= 1km PL= 91.29 dBd= 10Km PL= 111.29 dB

• Note that higher the frequency the greater the path loss for a fixed distance

Distance PL @ 880 MHz PL @ 1960MHz1km 91.29 dB 98.25 dB

• Thus 7 dB greater path loss for PCS band compared to cellular band

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Example

Can use model to predict coverage area of a base station

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A Simple Explanation of Free Space Propagation

• Isotropic transmit antenna– Radiates signal equally in alldirections

• Assume a point source– At a distance d from thetransmitter, the area of thesphere enclosing the Tx is

A = 4πd2

– The “power density” on thissphere is

Pt / 4πd2

• Isotropic receive antenna– Captures power equal to thedensity times the area of theantenna– Ideal area of antenna is

Aant = λ2/4π• The received power is:

Pr = Pt / 4πd2 × λ2/4π = Pt λ2/(4πd)2

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Isotropic and Real Antennas• Isotropic antennas are “ideal” and cannot be achieved in practice

– Useful as a theoretical benchmark

• Real antennas have gains in different directions– Suppose the gain of the transmit antenna in the direction of interest is Gt and that of the receive antenna is Gr

– The free space relation is:

Pr = Pt Gt Gr λ2/(4πd)2

• The quantity Pt Gt is called the effective isotropic radiated power (EIRP)– This is the transmit power that a transmitter should use were it having an isotropic antenna

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Two-Ray Model for Mobile Radio Environment

Where;

d1= line of sight path

d2= ground reflected paths

ht= Height of the transmitter

hr= Height of the receiver

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Two-Ray Model for Mobile Radio Environment

• Using the method of images the line-of-sight path and the ground reflected path can be calculated

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Received Power for Two-Ray Model

• From the image diagram we have;

• The relationship between the transmit power and the received power is;

• Notice that factor of 10 increase in distance => 40 dB increase in path loss (40 dB/decade)

• The Received Power can be increased by raising the heights of the transmit and receive antenna

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Diffraction Loss

• Diffraction occurs when the radio path between the Tx and Rx is obstructed by surfaces that have sharp edges

• Edges act as a secondary line source• The diffraction parameter ν is

defined as

• hm is the height of the obstacle

• dt is distance transmitter-obstacle

• dr is distance receiver-obstacle

The diffraction loss Ld

(dB) is approximatedby

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Diffraction Example

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Path Loss Models• Commonly used to estimate link budgets, cell sizes and shapes,

capacity, handoff criteria etc.

• “Macroscopic” or “large scale” variation of RSS

• Path loss = loss in signal strength as a function of distance– Terrain dependent (urban, rural, mountainous), ground reflection,diffraction, etc.– Site dependent (antenna heights for example)– Frequency dependent– Line of site or not

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Environment Based Path Loss Model

• Basic characterization: LP = L0 + 10α log10(d)– L0 is termed the frequency dependent component– The parameter α is called the “path loss gradient” or exponent– The value of α determines how quickly the RSS falls

• α determined by measurements in typical environment– For example

• α = 2.5 might be used for rural area• α = 4.8 might be used for dense urban area

• Variations on this approach– Try and add more terms to the model– Directly curve fit data

• Indoor and Outdoor Models– Okumura-Hata, COST 231, JTC

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Shadow Fading

• The signal strength for the same distance from the TX and RX is different for different locations depending upon the environment

• LP= L0 + 10α log (d) provides the mean value of the received signal strength at distance ‘d’

• The variation of the signal strength around this value is known as Shadow fading or Slow fading

• The path loss equation becomes;

LP= L0 + 10α log (d) + X• Where X is the random variable whose distribution depends on the

fading component• Measurement studies show that X can be modeled with a lognormal

distribution with mean = zero and standard deviation σ db

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Fade Margin

• In order to provide adequate signal strengths to locations where transmitted signal may no reach

• Add a Fade Margin to the path loss or the received signal strength

LP= L0 + 10α log (d) + F

• Where F is the Fade Margin associated with the path loss to overcome the shadow fading effects

• Fade Margin can be applied by• – Reducing cell size• – Increasing transmit power• – Making the receiver more sensitive

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Path Loss for Macrocellular AreasOkumura-Hata Model

• Okumura collected measurement data ( in Tokyo) and plotted a set of curves for path loss in urban areas– Frequency range 100 MHz to 1,920 MHz– Identified the height of the Tx and Rx as important parameters

• Hata came up with an empirical model for Okumura’s curvesLp = 69.55 + 26.16 log fc – 13.82 log hte – a(hre) + (44.9–6.55 log hte)log d

• Where fc is in MHz, d is distance in km, and hte is the base station transmitter antenna height in meters and hre is the mobile receiver antenna height in meters

• for fc > 400 MHz and large city• a(hre) = 3.2 (log [11.75 hre])2 – 4.97 dB

• See Table 2.1 in textbook for other cases

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Example of Hata’s Model• Consider the case where

hre = 2 m, receiver antenna’s heighthte = 100 m, transmitter antenna’s heightfc = 900 MHz, carrier frequency

• Lp = 118.14 + 31.8 log d– The path loss exponent for this particular case is α = 3.18

• What is the path loss at d = 5 km?– d = 5 km Lp = 118.14 + 31.8 log 5 = 140.36 dB

• If the maximum allowed path loss is 120 dB,what distance can the signal travel?

• – Lp = 120 = 118.14 + 31.8 log d => d =10(1.86/31.8) = 1.14 km

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COST Model• Models developed by COST

– European Cooperative for Science and Technology– Collected measurement data– Plotted a set of curves for path loss in various areas around the 1900 MHz band– Developed a Hata-like model

Lp = 46.3 + 33.9 log fc – 13.82 log hte - a(hre) + (44.9 –6.55 log hte)log d + C

• C is a correction factor– C = 0 dB in dense urban; -5 dB in urban; -10 dB in suburban; -17 dB in rural

• Note: fc is in MHz (between 1500 and 2000 MHz), d is in km, hte is effective base station antenna height in meters (between 30 and 200m), hre is mobile antenna height (between 1 and 10m)

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Path Loss Models for Microcellular Areas

• Area of the microcell spans from 1m to a kilometer• Supported by below the roof top antennas mounted on lampposts• Streets acts as urban canyons• Propagation of the signal is affected by

– reflection from buildings and ground– Scattering from vehicles– Diffraction around building and rooftops

• Bertoni and others have developed empirical path-loss models similar to Okumura-Hata models

• See table 2.2 in the text book for the Path-loss models

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Path Loss Models for Microcellular Areas

• d is the distance between the mobile and the transmitter in Kilometers

• hb is the height of the base station

• hm is the height of the mobile

• fc is the centre frequency of the carrier in GHz and ranges between 0.9 - 2 GHz

• In addition other parameters are

– rh, the distance of the mobile from the last rooftop in meters

– Δhm is the height of the nearest building above the height of the receiver

– Δh is the relative height of the base station compared to the average height of the buildings

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Path Loss Models for Picocellular Indoor Areas

• Picocells correspond to radio cells covering a building or parts of a building

• Area of picocells spans from 30m to 100m• Employed for WLANs, Wireless PBX systems and PCS operating in

indoor areas• Three models for Indoor Areas

– Multifloor Attenuation Model– JTC Model => improvement to the Multifloor Attenuation Model– Partition Dependant Model

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Multifloor Attenuation Model

• Describing path loss in multistory building• Signal Attenuation by the floors is a constant independent of

distance• The path loss is;

Lp=L0 + nF+ 10 log (d)

• F is the signal attenuation provided by each floor

• L0 is the path loss at first meter, L0 = 10 log (Pt) – 10 log (P0)

• d is the distance between the Tx and the Rx in meters• n is the number of floors through which the signal passes

• For indoor measurements at 900 MHz and 1.7 GHz, F=10dB and 16 dB

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JTC Model

Lp= A + Lf (n) + B log (d) + X• A is an environment dependent fixed loss factor (dB)• B is the distance dependent loss coefficient• d is separation distance between the base stationand portable, in meters

• Lf is a floor penetration loss factor (dB)• n is the number of floors between the access pointand mobile terminal

• Xσ is a shadowing term

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JTC Model (cont.)

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Partition Loss Model• Fixing the value of the Path Loss gradient α = 2 for free space • Introducing the losses for each partition

• mtype = the number of partitions of type• wtype = the loss in dB associated with that partition• d = distance between transmitter and receiver point in meter• X = the shadow fading• L0 = the path loss at the first meter, computed by

• where d0 = 1 m.• f = operating frequency of the transmitter

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Partition Loss Model

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THE END