Wireless Communications

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Wireless Communications

Engineering Basics

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Outline

•

Wireless Signals and Wireless Transmission

•

Electromagnetic waves

•

Complex numbers recap/tutorial

•

Time domain and frequency domain

•

The relation between input and output

•

Representing signals with complex exponentials

•

Antennas

•

Logarithmic representation of quantities

Wireless Communications - Engineering Basics

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WIRELESS SIGNALS AND WIRELESS TRANSMISSION

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Signals



Physical representation of data



Signal parameters: Parameters whose value or value curve represent the data



Classification by properties:



time-continuous or time-discrete



value continuous or discrete



Analog signal = time and value continuous



Digital signal = time and value discrete



In the following we consider signals as a function s(t) in time t



Here we a considering wireless signals represented by parameters of electromagnetic waves

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Problem: Wireless = Analog

0110 1001 1000 1010

Transmitter Receiver

0110 1001 1000 1010

Definition: Transmitter + Receiver = Transceiver

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Passband Transmission Principle

0110 1001 1000 1010

Transmitter Receiver

0110 1001 1000 1010 Carrier wave with

carrier frequency f

Amplitude Frequency Phase

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Terminology

1011

Bit(s) Symbol

Modulation

Demodulation

symbol rate:

number of symbols per second

data rate:

number of bits per seconds

N-ary modulation scheme: number of different symbols!

i.e., this can convey log(N) Bits per symbol

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

Different representations of a signal:



Amplitude spectrum (amplitude over time)



Frequency spectrum (amplitude or phase over frequency)



Phase state diagram (amplitude M and phase angle φ are plotted in polar coordinates)



Compound signals can be split into frequency components by Fourier transformation



Digital signals have rectangular edges



in the frequency spectrum infinite bandwidth



transmission requires modulation to analog carrier signals

Preview: we will look into some details about

f [Hz]

A [V]



I = M cos φ (In-phase) Q = M sin φ (Quadrature)

 A [V]

t[s]

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Frequency ranges used for communication

VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency (long wave radio) SHF = Super High Frequency MF = Medium Frequency (medium wave radio) EHF = Extra High Frequency HF = High Frequency (short wave-Radio) UV = ultraviolet light

VHF = Very High Frequency (UKW-Radio)

1 Mm 300 Hz

10 km 30 kHz

100 m 3 MHz

1 m 300 MHz

10 mm 30 GHz

100 m 3 THz

1 m 300 THz

visible light

VLF LF MF HF VHF UHF SHF EHF Infra red UV

optical transmission wave guide

coax cable twisted

pair

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Frequencies and regulations

The ITU-R regularly organizes conferences to negotiate and manage the frequency bands

(WRC, World Radio Conferences)

Examples of operating frequencies in mobile communications:

E u r o p e U S A J a p a n

C e llu la r P h o n e s

G S M 4 5 0 -4 5 7 , 4 7 9 - 4 8 6 /4 6 0 -4 6 7 ,4 8 9 - 4 9 6 , 8 9 0 -9 1 5 /9 3 5 - 9 6 0 ,

1 7 1 0 -1 7 8 5 /1 8 0 5 - 1 8 8 0

U M T S (F D D ) 1 9 2 0 - 1 9 8 0 , 2 1 1 0 -2 1 9 0 U M T S (T D D ) 1 9 0 0 - 1 9 2 0 , 2 0 2 0 -2 0 2 5

A M P S, T D M A, C D M A 8 2 4 -8 4 9 ,

8 6 9 -8 9 4

T D M A, C D M A, G S M 1 8 5 0 -1 9 1 0 ,

1 9 3 0 -1 9 9 0

P D C 8 1 0 -8 2 6 , 9 4 0 -9 5 6 , 1 4 2 9 -1 4 6 5 , 1 4 7 7 -1 5 1 3

C o r d le s s P h o n e s

C T 1 + 8 8 5 -8 8 7 , 9 3 0 - 9 3 2

C T 2 8 6 4 -8 6 8 D E C T 1 8 8 0 -1 9 0 0

P A C S 1 8 5 0 -1 9 1 0 , 1 9 3 0 - 1 9 9 0

P A C S -U B 1 9 1 0 -1 9 3 0

P H S 1 8 9 5 -1 9 1 8 J C T

2 5 4 -3 8 0

W ir e le s s L A N s

IE E E 8 0 2 .1 1 2 4 0 0 -2 4 8 3 H IP E R L A N 2 5 1 5 0 -5 3 5 0 , 5 4 7 0 - 5 7 2 5

9 0 2 -9 2 8 IE E E 8 0 2 .1 1 2 4 0 0 -2 4 8 3

5 1 5 0 -5 3 5 0 , 5 7 2 5 -5 8 2 5

IE E E 8 0 2 .1 1 2 4 7 1 -2 4 9 7 5 1 5 0 -5 2 5 0

O th e r s R F -C o n tr o l

2 7 , 1 2 8 , 4 1 8 , 4 3 3 , 8 6 8

R F -C o n tr o l 3 1 5 , 9 1 5

R F -C o n tr o l 4 2 6 , 8 6 8

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ELECTROMAGNETIC WAVES

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Principle

E-Field M-Field

Image source: http://de.wikipedia.org/wiki/Elektromagnetische_Welle

Side note:

• Fraunhofer distance

• near field

• far field

• (Maxwell's equations)

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Characterization of an electromagnetic wave

Time representation of the

E-field

Wave length

distance

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Far Field and Fraunhofer Distance

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rappaport02wireless: 4.2

far field

near field When are we located in the far field?

This is related to:

• largest physical linear dimension D of the transmitter antenna

• carrier wavelength λ

• Fraunhofer distance d_f

To be in the far field the distance d to the transmitter antenna must satisfy:

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Far Field and Fraunhofer Distance

rappaport02wireless: 4.2

The expressions d >> D and d >> λ are somewhat vague.

Consider an example

• 12 cm WLAN antenna

• Carrier frequency 2,4 GHz

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Free space propagation and an omnidirectional radiator

Wave front Fraunhofer distance

Near field Far Field

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We will discuss later: transmitted power per square meter on the wave front

s

1m 1m

Power P:

spherical surface

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Remark: Energy and Power

1 m

Gravitation:

9,81 m/s^2 Weight:

102 g

Force (Newton)

Energy (Joule)

Power (Watts)

1 sec

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Remark: Voltage, Current and Power

Voltage [U in V]

Current [I in A]

Power [P in W]

Resistance [R in ]

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ANTENNAS

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Operating Principle of Antennas

Antenna: electrical conductor or system of conductors used either for radiating or for collecting electromagnetic energy

Isotropic radiator (isotropic antenna)

Idealized antenna which radiates power in all directions equally Real antennas do not perform equally well in all directions

Receiving antenna captures a constant area of the energy distributed over the sphere centered at the sender antenna

Reciprocity: characteristics are essentially the same whether the an antenna is sending or receiving electromagnetic energy

stallings05wireless: 5.1

Transmit

antenna Receive

antenna 1

Receive antenna 2

Image: lecture slides

„Mobilkommunikation“, Prof. Dr. Holger Karl

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Example of a Real Antenna

Conductor

Conductor Gap

Image: http://www.elektronik-

kompendium.de/sites/kom/0810171.htm

Image: http://de.wikipedia.org/

wiki/Dipolantenne

Resonant Circuit

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Electric Field E Magnetic Field H

How differs a half-wave dipole from an isotropic radiator?

Half-Wave Dipole (Hertz Antenna)

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Radiation Pattern

Image: http://en.wikipedia.org/

wiki/Radiation_pattern

Example: radiation pattern of a half-wave dipole

x y

z y

x z

• a common way to characterize the performance of an antenna

• due to reciprocity: radiation pattern characterizes both transmission and reception performance

• when an antenna is used for reception, the

radiation pattern becomes a reception pattern

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The size of the pattern does not matter

What is important is the relative distance from the antenna position in each direction. The relative distance characterizes the relative power in that direction compared to other

directions.

Examples

Difference between two directions A and B for an isotropic radiator?

In which direction A will a directed radiator radiate with half of the power than in direction B?

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Beam Width

The angle within which the power radiated by the antenna is at least half of what it is in the most preferred direction

Beispiel

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Antenna Gain

Power output in a particular direction compared to the power output produced in any direction by a perfect isotropic antenna.

(i.e. total area of both radiation patterns of the isotropic antenna and the considered one are the same)

Example: what is the antenna gain into the strongest direction?

(Note: an increase of power in one direction means a lowering of power

into another one; antenna gain does not mean amplification of the total

power!)

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Effective Area (1)

Consider the amount PFD [watt/m²] (power flux density) of power passing through a unit area of one square meter.

Consider an antenna oriented with the axis of maximum sensitivity toward the source. Let the antenna deliver P_o watts to the receiver.

The effective area A_e is defines as:

Basically it expresses the size of the area oriented perpendicular to the direction of an incoming electromagnetic wave which would intercept P_owatt (i.e. the power intercepted by the considered antenna).

Transmit antenna

Receive antenna

Image: lecture slides

„Mobilkommunikation, Prof. Dr. Holger Karl

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Effective Area (2)

Without further details: the effective area is of course related to the physical size and type of the antenna. (how depends on the antenna type)

Antenna gain G and effective area A_e are related. Let  be the wave length. We have:

Note: we considered an antenna oriented with the axis of maximum sensitivity toward the source. The concept can of course be generalized to any antenna orientation.

Example of antenna gains and effective areas for different antenna types

Type of Antenna Effective Area A_e[m²] Antenna Gain G

into the strongest direction

Isotropic ² / (4 π) 1

Half‐wave dipole 1.64 ² / (4 π) 1.64

Parabolic with face area A (see next) 0.56 A 7 A / ²

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What is the beam width?

What is the antenna gain in an arbitrary direction?

Quiz: radiation pattern of an isotropic antenna?

x y

z y

x

z

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Antenna examples:

quarter wave antenna (Marconi antenna)

Image source: http://en.wikibooks.org/wiki/

Communication_Systems/Antennas

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Surface acts as a „mirror“ for the lambda/4 radiator (example: radio antenna in the roof of a car)

Image source: Jochen Schiller,

„Mobilkommunikation“, 2te überarbeitete Auflage, 2003

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Antenna examples:

inverted‐F antenna (IFA) of a TmoteSky node

Where is the antenna?

Such an antenna is also called a PCB antenna (printed circuit board antenna)

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Antenna examples:

radiation pattern from the TmoteSky data sheet

Horizontal mounting Vertical mounting

Image source of the radiation patterns: Tmote Sky Datasheet (2/6/2006)

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Antenna examples: parabolic reflective antenna

x y

Focus

same length

Directrix

Parabola construction Reflective property

x

y

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Antenna examples:

radiation pattern of a parabolic reflective antenna

x y

z y

x

z

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Antenna beamwidths for various parabolic reflective antenna diameters at frequency f=12GHz

Antenna diameter (m) Beam width (in degree)

0,5 3,5

0,75 2,33

1,0 1,75

1,5 1,166

2,0 0,875

2,5 0,7

5,0 0,35

Parabolic reflective antennas always have a beam with >0. In practice the focus is not one single idealized point. Note: the larger the antenna diameter the more tightly directional is the beam.

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Physical size of an antenna

For the parabolic reflecting antenna the antenna size is the diameter parabolic reflector

For the considered lambda/x antenna the antenna size is proportional to the utilized wave length

The size of the example antenna of the TmoteSky node (more precisely the height of th “ ground plane” ) is approximately 3,125cm and is ¼ of the wave length

(lambda/4 antenna).

Which frequency band is probably used?

Put oversimplified for antennas in communication systems: the higher the utilized frequency the smaller the required antenna size

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More about antenna types

• This was a small example selection of antenna types: a list of many more elementary antenna types can be found here:

http://www.antenna‐theory.com/antennas/main.php

• Moreover elementary antenna types can be used to build

more complex ones: see next...

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Antennas: directional and with sectors

Seitenansicht (xy-Ebene) x y

Seitenansicht (yz-Ebene) z y

von oben (xz-Ebene) x z

von oben, 3 Sektoren x z

von oben, 6 Sektoren x z

Frequently used antenna types for direct microwave connections and base stations for mobile networks (for example, covering of valleys and street canyons)

directed antenna

sector antenna

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side view (xy plane) side view (yz plane) top view (xz plane)

top view, 3 sectors top view, 6 sectors

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Antenna diversity



Grouping of 2 or more antennas



Antenna array with several elements antenna diversity



Switching / selection

 Receiver selects the antenna with the best reception



Combining

 Combining of antennas for better reception

 Phase adaptation to avoid cancellation

+

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ground plane

/2

+

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MIMO

Multiple-Input Multiple-Output

 Use of several antennas at receiver and transmitter

 Increased data rates and transmission range without additional transmit power or bandwidth via higher spectral efficiency, higher link robustness, reduced fading

Examples

 IEEE 802.11n, LTE, HSPA+, …

Functions

 “Beamforming”: emit the same signal from all antennas to maximize signal power at receiver antenna (and beamforming at the receiver side also possible; reduces interference)

 Spatial multiplexing: split high-rate signal into multiple lower rate streams and transmit over different antennas

sender

receiver t₁

t₂ t₃

Time of flight t₂=t₁+d₂ t₃=t₁+d₃

1 2

3

Sending time 1: t₀

2: t₀-d₂ 3: t₀-d₃

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LOGARITHMIC

REPRESENTATION OF QUANTITIES

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Remark: dB?

Logarithmic representation of the relation of two power or two energy quantities of the same unit.

By example: for P

₁

and P

₂

the relation P

₂

/ P

₁

is expressed in dB by:

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Note: What is dBm?

Logarithmic expression of power in mW Conversion



P mW  x dBm



x dBm  P mW

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Examples (from wikipedia)

dBm level Power Notes

80 dBm 100 kW Typical transmission power of a FM radio station

60 dBm 1 kW = 1000 W Typical RF power inside a microwave oven

36 dBm 4 W Typical maximum output power for a Citizens' band radio station (27 MHz) in many countries 30 dBm 1 W = 1000 mW Typical RF leakage from a microwave oven - Maximum output power for DCS 1800 MHz mobile

phone

27 dBm 500 mW Typical cellular phone transmission power

21 dBm 125 mW Maximum output from a UMTS/3G mobile phone (Power class 4 mobiles) 20 dBm 100 mW Bluetooth Class 1 radio, 100 m range (maximum output power from unlicensed FM transmitter)

4 dBm 2.5 mW Bluetooth Class 2 radio, 10 m range

0 dBm 1.0 mW =

1000 µW Bluetooth standard (Class 3) radio, 1 m range

−70 dBm 100 pW Typical range (−60 to −80 dBm) of Wireless signal over a network

−111 dBm 0.008 pW Thermal noise floor for commercial GPS signal bandwidth (2 MHz)

−127.5 dB

m 0.000178 pW Typical received signal power from a GPS satellite

−174 dBm 0.000004 fW Thermal noise floor for 1 Hz bandwidth

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