Cellular Mobile Communication Systems Lecture 2 Engr Shahryar

Cellular Mobile Communication Systems Lecture 2 Engr. Shahryar Saleem Assistant Professor Department of Telecom Engineering University of Engineering and Technology Taxila TI -1011 1

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 TI -1011 2

Typical Wireless Communication System TI -1011 3

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 the channel may introduce • Modulator – Converts the encoded bits into a signal suitable for transmission over the channel • Antenna – A transducer for converting guided signals in a transmission line into electromagnetic radiation in an unbounded medium or vice versa • Channel – Carries the signal, but will usually distort it • Receiver – reverses the operations TI -1011 4

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 a base station – How are handoff decision algorithms affected… • Propagation in free open space like light rays • In general make analogy to light and sound waves TI -1011 5

Signal Propagation • Received signal strength (RSS) influenced by – Fading – signal weakens with distance - proportional to 1/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 TI -1011 6

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 TI -1011 7

Multipath Propagation • Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction • Time dispersion: signal is dispersed over time Ø interference with “neighbor” symbols, Inter Symbol Interference (ISI) • The signal reaches a receiver directly and phase shifted Ø distorted signal depending on the phases of the different parts TI -1011 8

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 TI -1011 9

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. TI -1011 10

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. TI -1011 11

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 TI -1011 12

Communication Issues and Radio Propagation TI -1011 13

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) TI -1011 14

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 TI -1011 15

Decibels • Power (signal strength) is expressed in decibels (d. B) for ease of calculation – Values relative to 1 m. W are expressed in d. Bm – Values relative to 1 W are expressed in d. BW – Other values are simply expressed in d. B • Example 1: Express 2 W in d. Bm and d. BW – d. Bm: 10 log 10 (2 W / 1 m. W) = 10 log 10(2000) = 33 – d. BW: 10 log 10 (2 W / 1 W) = 10 log 10(2) = 3 d. BW d. Bm • In general d. Bm value = 30 + d. BW value TI -1011 16

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 strength and the transmitted signal strength PL = Pt (d. B) – Pr (d. B) TI -1011 17

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 d. B, we have: Pr (d. Bm)= Pt (d. Bm) - 21. 98 + 20 log 10 (λ) – 20 log 10 (d) Path Loss = PL = Pt – Pr = 21. 98 - 20 log 10(λ) + 20 log 10 (d) TI -1011 18

Free Space Propagation • Notice that factor of 10 increase in distance => 20 d. B increase in path loss (20 d. B/decade) Distance d= 1 km d= 10 Km Path Loss @ 880 MHz PL= 91. 29 d. B PL= 111. 29 d. B • Note that higher the frequency the greater the path loss for a fixed distance Distance 1 km PL @ 880 MHz PL @ 1960 MHz 91. 29 d. B 98. 25 d. B • Thus 7 d. B greater path loss for PCS band compared to cellular band TI -1011 19

Example Can use model to predict coverage area of a base station TI -1011 20

A Simple Explanation of Free Space Propagation • • Isotropic transmit antenna – Radiates signal equally in all directions Assume a point source – At a distance d from the transmitter, the area of the sphere enclosing the Tx is A = 4πd 2 – The “power density” on this sphere is Pt / 4πd 2 Isotropic receive antenna – Captures power equal to the density times the area of the antenna – Ideal area of antenna is Aant = λ 2/4π The received power is: Pr = Pt / 4πd 2 × λ 2/4π = Pt λ 2/(4πd)2 TI -1011 21

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 λ 2/(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 TI -1011 22

Two-Ray Model for Mobile Radio Environment Where; d 1= line of sight path d 2= ground reflected paths ht= Height of the transmitter hr= Height of the receiver TI -1011 23

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 TI -1011 24

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 d. B increase in path loss (40 d. B/decade) • The Received Power can be increased by raising the heights of the transmit and receive antenna TI -1011 25

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 TI -1011 The diffraction loss Ld (d. B) is approximated by 26

Diffraction Example TI -1011 27

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 TI -1011 28

Environment Based Path Loss Model • Basic characterization: LP = L 0 + 10α log 10(d) – L 0 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 TI -1011 29

Shadow Fading • The signal strength for the same distance from the TX and RX is different for different locations depending upon the environment • LP= L 0 + 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= L 0 + 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 TI -1011 30

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= L 0 + 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 TI -1011 31

Path Loss for Macrocellular Areas Okumura-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 curves Lp = 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 d. B • See Table 2. 1 in textbook for other cases TI -1011 32

Example of Hata’s Model • Consider the case where hre = 2 m, receiver antenna’s height hte = 100 m, transmitter antenna’s height fc = 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 d. B • If the maximum allowed path loss is 120 d. B, what distance can the signal travel? • – Lp = 120 = 118. 14 + 31. 8 log d => d =10(1. 86/31. 8) = 1. 14 km TI -1011 33

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 d. B in dense urban; -5 d. B in urban; -10 d. B in suburban; -17 d. B 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 200 m), hre is mobile antenna height (between 1 and 10 m) TI -1011 34

Path Loss Models for Microcellular Areas • • Area of the microcell spans from 1 m 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 TI -1011 35

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 TI -1011 36

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 30 m to 100 m • 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 TI -1011 37

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=L 0 + n. F+ 10 log (d) F is the signal attenuation provided by each floor L 0 is the path loss at first meter, L 0 = 10 log (Pt) – 10 log (P 0) 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=10 d. B and 16 d. B TI -1011 38

JTC Model Lp= A + Lf (n) + B log (d) + X • A is an environment dependent fixed loss factor (d. B) • B is the distance dependent loss coefficient • d is separation distance between the base station and portable, in meters • Lf is a floor penetration loss factor (d. B) • n is the number of floors between the access point and mobile terminal • Xσ is a shadowing term TI -1011 39

JTC Model (cont. ) TI -1011 40

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 d. B associated with that partition d = distance between transmitter and receiver point in meter X = the shadow fading L 0 = the path loss at the first meter, computed by • where d 0 = 1 m. • f = operating frequency of the transmitter TI -1011 41

Partition Loss Model TI -1011 42

THE END TI -1011 43