Wireless Communication FundamentalsWireless Communication Fundamentals

David TipperAssociate ProfessorAssociate Professor

Department of Information Science and Telecommunications

University of PittsburghTelcomTelcom 2700 Slides 22700 Slides 2

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Wireless NetworksWireless Networks

– Wireless Wide Area Networks (WWANs)• Cellular Networks :

– GSM, cdmaone (IS-95), UMTS, cdma2000 EVDO

• Satellite Networks: – Iridium, Globalstar, GPS, etc.

– Wireless Metro Area Networks (WMANs)• IEEE 802.16 WiMAX

– Wireless Local Area Networks (WLANs)• IEEE 802.11, a, b, g, etc. (infrastructure, ad hoc, sensor)

– Wireless Personal Area Networks (WPANs)• IEEE 802.15 (Bluetooth), IrDa, Zigbee, etc.

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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 a signal?– How to clean up the signal in order to have good quality?– How to deal with limited data rate and limited bandwidth?

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Typical Wireless Communication System

Source Source Encoder

ChannelEncoder Modulator

Destination Source Decoder

ChannelDecoder

Demod-ulator

Channel

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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 source info– Example: In English, you may encode the alphabet “e” with fewer bits

than you would “q” using a vocoder• 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 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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Signals

• Signal - physical representation of data• Mathematically, a signal is represented as a

function of time – or can be expressed as a function of frequency

• Any electromagnetic signal can be shown to consist of a collection of sinusoids at different amplitudes, frequencies, and phases (Fourier Series or Transform)

• Communication systems perform the tasks of– Signal generation– Signal transmission– Signal reception– Signal detection

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Terminology

• Consider a periodic signal (e.g., a sine wave)• Period (T) - amount of time it takes for one repetition of

the signalT = 1/frequency = 1/f

• Phase (φ) - measure of the relative position in time within a single period of the signal

• Wavelength (λ) - distance occupied by a single cycle of the signal– Or, the distance between two points of corresponding phase of

two consecutive cycles• For electromagnetic waves in air or free space, λ = c/f

where c is the speed of light = 3 x 108 m/sec

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Consider a Sinusoid

• General sine wave– s(t) = A cos(2πft + φ)

• Next slide shows the effect of varying each of the three parameters– A = 1, f = 1 Hz, φ = 0 => T = 1s– Increased peak amplitude; A=2– Increased frequency; f = 2 => T = ½– Phase shift; φ = π/4 radians (45 degrees)

• Note: 2π radians = 360° = 1 period

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The sinusoid – Acos(2πft +φ)

-1 0 1 2 3 4-2

-1

0

1

2

-1 0 1 2 3 4-2

-1

0

1

2

-1 0 1 2 3 4-2

-1

0

1

2

-1 0 1 2 3 4-2

-1

0

1

2

cos(2cos(2ππtt)) cos(2cos(2ππ ×× 2 2 ×× tt))

2 2 ×× cos(2cos(2ππtt)) cos(2cos(2ππtt + + ππ/4)/4)

Am

plitu

de

time

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Frequency-Domain Concepts

• Frequencies measured by number of cycles per second – unit is Hertz– 5 KHz 5000 times per second

• Spectrum - range of frequencies that a signal contains

• Absolute bandwidth - width of the spectrum of a signal

• Effective bandwidth (or just bandwidth) - narrow band of frequencies that most of the signal’s energy is contained in

• Example: Human Voice – absolute bandwidth 0-20 KHz, effective bandwidth 50 – 4000 Hz.

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Frequencies for Communication

• VLF = Very Low Frequency UHF = Ultra High Frequency• LF = Low Frequency SHF = Super High Frequency• MF = Medium Frequency EHF = Extra High Frequency• HF = High Frequency UV = Ultraviolet Light• VHF = Very High Frequency

• Frequency and wavelength: λ = c/f• Wavelength λ, speed of light c ≅ 3x108m/s, frequency f in Hz

1 Mm300 Hz

10 km30 kHz

100 m3 MHz

1 m300 MHz

10 mm30 GHz

100 μm3 THz

1 μm300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted pair

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Radio Frequency Bands

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Licensed Vs. Unlicensed

More worldwide optionsHigher barriers for entrance

Coverage and quality inconsistentBetter coverage and quality

Fast RolloutGuaranteed access

UnlicensedLicensed

• Licensed Spectrum– need to buy right to use spectrum allocation in a specific geographic

location from the government (e.g., AM/FM radio) – Prevents interference – licensee can control signal quality

• Unlicensed spectrum – Anyone can operate in the spectrum (e.g. ISM band for WLANs) but must

maintain proper behavior in spectrum (max power level and frequency leakage, etc.)

– Can have interference problems

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Frequency Allocations

Europe USA Japan

WWANs Licensed

Cellular: 453-457MHz, 463-467 MHz; PCS: 890-915 MHz, 935-960 MHz; 1710-1785 MHz, 1805-1880 MHz 3G: 1920-1996 MHz 2110-2186 MHz

Cellular 824-849 MHz, 869-894 MHz; PCS 1850-1910 MHz, 1930-1990 MHz;

Cellular 810-826 MHz, 940-956 MHz; 1429-1465 MHz, 1477-1513 MHz 3G 1918.1-1980 MHz 2110-2170 MHz

WMANs Licensed Unlicensed

IEEE 802.16 3.4-3.6 GHz SAME as WLANs

IEEE 802.16 2.5 – 2.6 GHz, 2.7-2.9GHz Same as WLANs

IEEE 802.16 4.8-5 GHz Same as WLANS

WLANs Unlicensed

IEEE 802.11 2400-2483 MHz 5.7-5.825 GHz HIPERLAN 1 5176-5270 MHz

IEEE 802.11 2400-2483 MHz (b, g) 5.7 – 5.825 GHz (a)

IEEE 802.11 2471-2497 MHz (b, g) 5.7-5.825 GHz (a)

WPANs Unlicensed

IEEE 802.15 2400-2483 MHz

IEEE 802.15 2400-2483 MHz

IEEE 802.15 2471-2497 MHz

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What is Signal Propagation?

• Signal Propagation describes how a radio signal is 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 transmitters (i.e., base stations/access points)

be placed– What transmit powers should be used– What frequency channels need be assigned to a transmitter– 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 received power

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

reflection scattering diffractionshadowing

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Signal Propagation

• Effects are similar indoors and out• Several paths from Tx to Rx

– Different delays, phases and amplitudes

– Add motion – makes it very complicated

• Termed a multi-path propagationenvironment

• Difficult to look at all of the effects in a composite way

• In practice – Ray Tracing Approach:

Breakdown phenomena into different categories use physics model for each path

– Use empirical based models

Reflection

Scattering

Transmission Diffraction

Tx

Rx

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Multipath Propagation

signal at sendersignal at receiver

• Signal can take many different paths between sender and receiverdue 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• Limits the data rate on the channel

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Effects of mobility

• Channel characteristics change over time and location – signal paths change– different delay variations of

different signal parts– different phases of signal parts

• Results in quick changes in the power received

• Called short term or fast fading• Results in sudden burst of

errors on the channel limits the goodput of the channel. short term fading

long termfading

t

power

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The Radio Channel

• 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

• Consider the first one only – two and three impact physical and link layer and will be studied later.

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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/Term Fading effects)– Path loss (long term fading)– Shadow fading

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Signal Propagation Ranges

distance

sender

transmission

detection

interference

• 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

• Power in dBm = log10 (Power in W / 1 mW) – Values relative to 1 W are expressed in dBW

• Power in dBW = log10 (Power in W / 1 W) – Other values are simply expressed in dB (i.e., Gains of

Antennas, loss due to obstacles, etc.)

• 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• Note 3 dB implies doubling/halving power

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Free Space Loss Model

• Assumptions– Transmitter and receiver are in free open 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 (dB) – Pr (dB)

d

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A simple explanation of free space loss

• 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 = 4πd2

– The “power density” on this sphere is

Pt/ 4πd2

• Isotropic receive antenna– Captures power equal to the density

times the area of the antenna– Ideal area of antenna is

Aant = λ2/4π• The received power is:

Pr = Pt/ 4πd2 × λ2/4π = Pt λ2/(4πd)2

d

Pt λ2/(4πd)2

Pr = Pt / Lp

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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:

• Where d is in meters• 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)

22

2

)4( dPP t

r πλ

=

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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 at 880 MHz 1km 91.29 dB 10Km 111.29 dB

• Note that higher the frequency the greater the path loss for a fixed distance Distance 880 MHz 1960MHz1km 91.29 dB 98.25 dBthus 7 dB greater path loss for PCS band compared to cellular band in the US

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Example

• Consider Design of a Point-to-Point link connecting LANs in separate buildings across a freeway– Distance .25 mile– Line of Sight (LOS)

communication – Spectrum Unlicensed – using

802.11b at 2.4GHz– Maximum transmit power of

802.11 AP is Pt = 24dBm – The minimum received signal

strength (RSS) for 11 Mbps operation is -80 dBm

– Will the signal strength be adequate for communication?

• Given LOS is available can approximate propagation with Free Space Model as follows

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Example

• Example – Distance .25 mile ~ 400m– Receiver Sensitivity Threshold = - 80dBm

• The Received Power Pr is given by Pr = Pt - Path LossPr = Pt - 21.98 + 20 log10 (λ) – 20 log10 (d)

= 24 – 21.98 + 20log10 (3x108/2.4x109) – 20 log10 (400)= 24 -21.98 -18.06 -52.04= 24 – 92.08 = -68.08

Pr is well above the required -80 dBm for communication at the maximum data rate – so link should work fine

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Cell/Radio Footprint

• The Cell is the area covered by a single transmitter• Path loss model roughly determines the size of cell

RSS

distance

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0 500 1000 1500 2000 2500 3000-70

-60

-50

-40

-30

-20

-10

0

10

distance from Tx in m

Pr in

dB

m

Example

Pt = 5 Wf = 900 MHzλ = 0.333 m

Can use model to predict coverage area of a base station

If we require-60dbm

RSS

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Path Loss Models

• Path Loss Models are 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

• Simple characterization: PL = 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 with distance

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Path Loss Model cont.• Can be written in terms of received power:

Pr = K Pt d-α

• α is called the “path-loss” coefficient• K depends on the frequency used • α depends on several factors and is often obtained

empirically• - Dense shadowed urban α = 4 to 5.5

– Shadowed Urban area α = 3 to 4.5– Suburban area α = 2.7 to 3.5– Free Space α = 2

• More complicated models based on curve fitting to measurements. These models allow for some site dependent parameters (e.g., antenna heights, indoor vs. outdoor, etc.).

• Consider examples – Okumura –Hata (outdoor cellular band model)– JTC (indoor WLAN band model)

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Okumura-Hata Model

• Okumura collected measurement data ( in Tokyo) and plotted a set of curves for path loss in urban areas– 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 dWhere 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

a(hre) is a correction factor for different environments for fc > 400 MHz and large city

a(hre) = 3.2 (log [11.75 hre])2 – 4.97 dBCan approximate a(hre) with a constant C where C = -2 dense

urban, -5 urban, -10 suburban, -26 rural

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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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Shadow Fading

• Shadowing occurs when line of site is blocked

• Modeled by a random signal component Xσ

• Pr = Pt – Lp +Xσ

• Measurement studies show that Xσ can be modeled with a lognormal distribution normal in db with mean = zero and standard deviation σ db

• Thus at the “designed cell edge” only 50% of the locations have adequate RSS

• Since Xσ can be modeled in db as normally distributed with mean = zero and standard deviation σ db σ determines the behavior

d

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How shadow fading affects system design

• Typical values for σ are rural 3 db, Suburban 6 db, urban 8 db, dense urban 10 db.

• Since X is normal in db Pr is normalPr = Pt – Lp +Xσ

• Prob {Pr (d) > T } can be found from a normal distribution table with mean Pr and σ

• In order to make at least Y% of the locations have adequate RSS

– Reduce cell size– Increase transmit power– Make the receiver more sensitive

d

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Example of Shadow Calculations

• The path loss of a system is given by– Lp = 47 + 40 log10 d – 20 log10 hb– hb = 10m, Pt = 0.5 W, receiver sensitivity = -100 dBm– What is the cell radius?

• Pt = 10 log10500 = 27 dBm• The permissible path loss is 27-(-100) = 127 dBm• 20 log10hb = 20 log1010 = 20 dB• 127 = 47 + 40 log10d – 20 => d = 316m• But the real path loss at any location is

– 127 + X where X is a random variable representing shadowing – Negative X = better RSS; Positive X = worse RSS

• If the shadow fading component is normally distributed with mean zero and standard deviation of 6 dB. What should be the shadow margin to have acceptable RSS in 90% of the locations at the cell edge?

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Example again• Let X be the shadow fading component

– X = N(0,6)– We need to find F such that P{X > F } = 0.1

• We need to solve Q(F/σ) = 0.1• Use tables or software• In this example F = 7.69 dB

– Increase transmit power to 27 + 7.69 = 34.69 dBm = 3 W

– Make the receiver sensitivity -107.69 dBm– Reduce the cell size to 203.1 m

• In practice use .9 or .95 quantile vales to determine the Shadow Margin SM

• SM is the amount of extra path loss added to the path loss budget to account for shadowing

.9 SM = 1.282 σ

.95 SM = 1.654 σ

-10 -8 -6 -4 -2 0 2 4 6 8 100.01

0.02

0.03

0.04

0.05

0.06

0.07

10%

Fading Margin

F

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The JTC Indoor Path Loss Model

Similar to Okumura – Hata model in cellular (curve fitting to measure values used to set up model

• A is an environment dependent fixed loss factor (dB)• B is the distance dependent loss coefficient,• d is separation distance between the base station

and portable, in meters• Lf is a floor/wall penetration loss factor (dB)• n is the number of floors/walls between the access

point and mobile terminal• Xσ is a shadowing term due to non-line of sight

σXnLdBAL fTotal +++= )()(log 10

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JTC Model (Continued)

Environment Residential Office Commercial

A (dB) 38 38 38

B 28 30 22

Lf(n) (dB) 4n 15 + 4(n-1) 6 + 3(n-1)

Log Normal ShadowingStd. Dev. (dB)

8 10 10

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JTC Model (Continued)

• Example Consider an AP on the first floor of a three story house.The distance to a third floor home office is approximately 8 metersIf the AP operates at a power level of .05 W using the JTC model determine the path loss and received signal strength in the office area

Using the JTC model with residential parameter set

Ltotal = A + B log10 (d) + Lf (n) + 8 = 38 + 28 log10 (8) + 4x2 +8 = 79.28 dB

Power received = Pr = Pt - Ltotal = 16.98 dbm – 79.28 dB = -62.29 dBm

Pr is more than adequate.

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Cell Coverage modeling

• Simple path loss model based on environment used as first cut for planning cell locations

• Refine with measurements to parameterize model • Alternately use ray tracing: approximate the radio

propagation by means of geometrical optics-consider line of sight path, reflection effects, diffraction etc.

• CAD deployment tools widely used to provide prediction of coverage and plan/tune the network

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Cellular CAD ToolsCellular CAD Tools

• Use GIS terrain data base, along with vehicle traffic/population density overlays and propagation models

• Output map with cell coverage at various signal levels and interference values– To plan out cell coverage area, cell placement, handoff

areas, interference level frequency assignment

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Use GIS mapsUse GIS maps

• This shows possible location of cell site and possible location of users where signal strength prediction is desired

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Outdoor ModelOutdoor Model

CAD Toolsprovide a variety of

propagation models: free

space, Okumura-Hata, etc.

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Typical City pattern

Microcell diamondRadiation pattern

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Ray Tracing ModeRay Tracing Mode

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Indoor ModelsIndoor Models

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Cellular CAD Tools

• CAD tool – first cut cell site placement, augmented by extensive measurements to refine model and tune location and antenna placement/type

Temporary cell

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Signal strength prediction for Indoor WLANS

• Motorola LAN Planner

• Lucent: WiSE tool • Given

building/space to be covered and parameters of building and AP –predicts signal coverage

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Site Survey Tools

Software to measure signal strength and recording in order to construct a coverage map of structure – must drive/walk around structure to gather dataNOKIA site survey tool, Ekahau Site Survey, Motorola LAN survey, etc.

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Typical Wireless Communication System

Source Source Encoder

ChannelEncoder Modulator

Destination Source Decoder

ChannelDecoder

Demod-ulator

Channel

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Antennas

• Antenna – Converts analog signals into electromagnetic radiation as

efficiently as possible in the direction required• Radiation pattern

– Way in which energy propagates in as a function of direction• Any conductor or can serve as an antenna

– Use materials that result in efficient radiation

Thin Dipole Biconical Dipole Loop Parabolic Reflector

Microstrip Horn Antenna

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Radiation lobes

• Ideal antenna– Gain = 1 over a certain angle– Gain = 0 over the rest of the

directions• Real antenna

– Radiates power in unwanted directions

– Has one or more main lobes and many sidelobes

• Antenna Beamwidth– The beamwidth is the angle of

coverage where the radiated energy is 3 dB down from the peak of the beam (half-power)

• Front-to-Back Ratio– The ratio of the power in the main

lobe to the power in the lobe created at the back of the antenna

Mainlobe

Sidelobe

Backlobe

3 dB Beamwidth

3 dB

IdealAntenna

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Antenna Gain• The “gain” of an antenna in a given direction is the ratio of the

power density produced by it in that direction divided by the power density that would be produced by a reference antenna in the same direction

• Two types of reference antennas are generally used– Isotropic antenna: gain is given in dBi– Half-wave dipole antenna: gain is given in dBd

• Manufacturers often use dBi in their marketing– To show a slightly higher gain ☺

• dBi = dBd + 2.15 dB 0 dBi

0 dBd

5 dBd = 7.15 dBiIsotropicDipole

Other

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Antenna Gains

Directional antennaFocused beam – high gain

Omni-directional signal radiates in all directions equally – low gain

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Antennas

• Two factors influence the size and shape of an antenna

• The frequency of the RF signal– A low frequency signal needs a larger

antenna • The gain desired

– A high-gain antenna needs a larger antenna and more focused beam than a low-gain antenna

– Antenna gain adds into path loss calculations

• Directional antennas can be created using antenna arrays or horn/dish elements

450 Beamwidth19 dBd Gain

Panel Antenna

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Cellular Antennas

Cells are typically sectored into 3 parts each having 1200

sector of the cell to cover

1 transmit antenna in middle of each sector face

2 receive antenna at edge of sector face on the tower.

This is done to provide antenna diversity – it combats fast fading – as only 1 antenna will likely be in fade at any point in time. Can get 3-5 dB gain in the system

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Antenna Examples

Monopole Omnidirectional Panel Array of

dipoles for sectored

cell

Grid ReflectorAntenna

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Link Budget

• Used to plan useful radio coverage of link/cells– Relates transmit power, path losses, margins,

interference, etc. – Used to find max allowable path loss on each link– Typical Factors in Link Budget

• Transmit Power, • Antenna Gain, Diversity Gain, • Receiver Sensitivity• Shadow Margin, Interference Margin, • Vehicle Penetration Loss, Body Loss, Building Penetration,

etc.. (Typical values from measurements used)

– Gains are added, Losses are subtracted – must balance

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Link BudgetLink Up Down

TX Power 30dbm 30dbm

Antenna Gain 3 5

Antenna Diversity Gain 5 X

Shadow Margin 10 10

Body Attenuation 2 2

Vehicle Penetration 5 5

Receiver Sensitivity -105 -90

Path Loss Budget 126 db 108 db

Typical Cellular System Downlink Limited!