04 Basic Radio

8/6/2019 04 Basic Radio

http://slidepdf.com/reader/full/04-basic-radio 1/18

Basic Radio

Chapter 2

This chapter is designed to provide the student with an overviewof basic radio concepts.

OBJECTIVES:Upon completion of this chapter the student will be able to:

• discuss some general properties of electromagnetic waves

• describe how radio waves are generated• describe how information is superimposed on radio waves

• discuss radio wave propagation and attenuation

• understand the pathloss concept

• understand the origin of some signal variations, i.e. fadings



Cell Planning Overview

EN/LZT 123 3313 R2C

l l aa n n k kB B

iio o n n a a l l l l y y

t t n n e e t t

n n I I



2 Basic Radio

EN/LZT 123 3313 R2C – i –

2 Basic Radio

Table of Contents

Topic Page

WAVES ................................................................................................13

GENERATION OF RADIO WAVES .....................................................15SUPERIMPOSING INFORMATION ON RADIO WAVES....................18

AIR INTERFACE DATA.......................................................................19FREQUENCY SPECTRUM ......................................................................................... 19

DUPLEX DISTANCE.................................................................................................... 21

CHANNEL SEPARATION............................................................................................ 21

ACCESS METHOD AND TRANSMISSION RATE...................................................... 21

RADIO WAVE PROPAGATION...........................................................22SIGNAL VARIATIONS.........................................................................25




– ii – EN/LZT 123 3313 R2C

l l

aa n n k kB B

iio o n n a a l l l l y y

t t n n e e t t

n n I I



2 Basic Radio

EN/LZT 123 3313 R2C – 13 –

WAVES

There are many seemingly different types of electromagneticwaves. They include radio waves, infrared rays, light, x-rays andgamma rays among others. Radio waves are one type of electromagnetic radiation. They are typically generated asdisturbances sent out by oscillating charges on a transmittingantenna whereas e.g. gamma rays are generated when excitednuclei decay by emission of radiation. Regardless of its origin,an electro-magnetic wave is comprised of oscillating electric andmagnetic fields. For a simple, traveling, plane wave the electricand magnetic fields are perpendicular to each other and also tothe direction of propagation. Waves which can be described by

simple sinusoidal functions, as the one in Figure 2-1, areconveniently characterized by their wavelength, λ, the length of one cycle of oscillation, or equivalently with its frequency, I , thetwo related via the speed of propagation, F, as:

λ ⋅ = I F

In vacuum F ≈3⋅108 ms -1 for all electromagnetic waves.

Magnetic Field

Electric Field

H

E90°

D i r e c t i o n o f T r a v e l

Figure 2-1 An electromagnetic plane wave “frozen” in time




– 14 – EN/LZT 123 3313 R2C

Note that if the electric field is “pointing” strictly in onedirection, as in Figure 2-1, the wave is said to be linearlypolarized. For example, if the electric field is in the “horizontal”

direction, the wave is horizontally polarized.

Two very important features of the electromagnetic wave areworth noting. First, it is a transverse wave because the vibratingquantity, e.g. the electric field, is directed perpendicular to thedirection of propagation. Second, the wave requires no mediumfor its transmission, i.e. it is capable of transporting energy evenin the absence of any material.

Propagation properties are different across the frequencyspectrum. Radio waves fall in the frequency spectrum between 3

Hz and 3000 GHz. This part of the spectrum is divided intotwelve bands, shown in Figure 2-2 below. Only the UHF band isconsidered from now on, since properties of UHF waves andfrequency allocations have made this the mobile telephonyfrequency band.

FREQUENCY CLASSIFICATION DESIGNATION3 - 30 Hz30 - 300 Hz300 - 3000 Hz3 - 30 kHz

Extremely Low FrequencyVoice FrequencyVery-Low Frequency

ELFVFVLF

30 - 300 kHz Low Frequency LF300 - 3000 kHz Medium Frequency MF3 - 30 MHz High Frequency HF30 - 300 MHz Very High Frequency VHF300 - 3000 MHz Ultra High Frequency UHF3 - 30 GHz Super High Frequency SHF30 - 300 GHz300 - 3000 GHz

Extremely High Frequency EHF

Figure 2-2. Frequency spectrum bands



2 Basic Radio

EN/LZT 123 3313 R2C – 15 –

GENERATION OF RADIO WAVES

High frequency radio waves are typically generated byoscillating charges on a transmitting antenna. In the case of aradio station, the antenna is often simply a long wire, a dipole,fed by an alternating voltage/current source, i.e. charges areplaced on the antenna by the alternating voltage source. We canthink of the electric field as being disturbances sent out by thedipole source and the frequency of the oscillating electric field(the electromagnetic wave) is the same as the frequency of thesource.

Each antenna has a unique radiation pattern. This pattern can be

represented graphically by plotting the received, time-averagedpower, as a function of angle with respect to the direction of maximum power, in a log-polar diagram. The pattern isrepresentative of the antenna’s performance in a testenvironment. However, it only applies to the free-spaceenvironment in which the test measurement takes place. Uponinstallation, the pattern becomes more complex due to factorsaffecting propagation in the reality. Thus the real effectivenessof any antenna is measured in the field.

An isotropic antenna is a completely non-directional antennathat radiates equally in all directions. Since all practical antennasexhibit some degree of directivity, the isotropic antenna existsonly as a mathematical concept. The isotropic antenna can beused as a reference to specify the gain of a practical antenna (seethe appendix for a general discussion on gain/loss andlogarithmic units). The gain of an antenna referencedisotropically is the ratio between the power required in thepractical antenna and the power required in an isotropic antennato achieve the same field strength in the desired direction of themeasured practical antenna. Directive gain in relation to an

isotropic antenna is expressed in units of “dBi”.A half-wave dipole antenna may also be used as a gain referencefor practical antennas. The half-wave dipole is a straightconductor cut to one-half of the electrical wavelength with theradio frequency signal fed to the middle of the conductor.Figure 2-3 illustrates the radiation pattern of the half-wavedipole which normally is referred to as a dipole. Whereas theisotropic antenna’s three dimensional radiation pattern isspherical, the dipole antenna’s three dimensional pattern isshaped like a donut.




– 16 – EN/LZT 123 3313 R2C

Directive gain in relation to a dipole is expressed in units of “dBd”. For a dipole and an isotropic antenna with the sameinput power, the energy is more concentrated in certain

directions by the dipole. The difference in directive gainbetween the dipole and the isotropic antenna is 2.15 dB.Figure 2-4 illustrates the differences, in gain, between theisotropic, dipole and practical antenna. The vertical patternshown in Figure 2-4 for the practical antenna is that of adirectional antenna.

3-D VIEWOF DIPOLE PATTERN

VERTICAL VIEWOF DIPOLE PATTERN

HORIZONTAL VIEWOF DIPOLE PATTERN(DIPOLE IN CENTER)

Figure 2-3 Dipole radiation pattern

IDEAL ISOTROPIC RADIATOR

THEORETICAL HALFWAVE DIPOLE

PRACTICAL ANTENNA

dBd

dBi2.15dBi = dBd + 2.15

2.15dB

Figure 2-4 Gain comparison



2 Basic Radio

EN/LZT 123 3313 R2C – 17 –

45 degrees

-45 degrees

0dB-5-10 5 10 0 10 degreesat 3dB down

VERTICAL PATTERN

HORIZONTAL PATTERN

0dB

5

10

-5

-10

0 degrees

315 degrees 45 degrees

3dB down 3dB down

3dB down

3dB down

60 degreesat 3dB down

Figure 2-5. Vertical and horizontal antenna patterns for a “real” antenna

When choosing an antenna for a specific application, themanufacturer’s data sheet must be consulted. The data sheetcontains information including antenna gain, beamwidth(vertical and horizontal) and graphs showing the vertical and

horizontal patterns. Examples of the graphs normally found in adata sheet are shown in Figure 2-5. The patterns displayed arethose of a directional antenna. The antenna’s gain isapproximately 15 dBd.




– 18 – EN/LZT 123 3313 R2C

The beamwidth, B, is defined as the opening angle between thepoints where the radiated power is 3 dB lower than in the maindirection (Figure 2-6). Both the horizontal and vertical

beamwidths are found using the 3 dB down points, alternativelyreferred to as half-power points.

) B

Antenna lobe

Max gain-3 dB

Main direction

Max gain-3 dB

Figure 2-6 Definition of beamwidth

SUPERIMPOSING INFORMATION ON RADIO WAVES

Information is seldom transmitted in the same frequency rangeas it was generated. The reason is that if, as an example, wewant to broadcast a 2 kHz signal; the antenna would have to be75 km long (half a wavelength). However, by translating thesignal to a much higher frequency band, e.g. the UHF band of cellular telephony, antenna sizes drop to a few decimeters. Thisis also a prerequisite for having numerous “channels”simultaneously. Frequency translation is implemented bymodulating the amplitude, frequency or phase of a so called

carrier wave in accordance with the wave form of the wantedsignal. Several modulation schemes exist, e.g. amplitudemodulation, common for analog radio signals, and phasemodulation. Any modulation scheme increases the carrierbandwidth and hence is a limit on the capacity of the frequencyband available. Since the bandwidth of the carrier increases if the bit rate increases, a high carrier frequency is necessary toobtain many different “channels”. The cell planner can not of course chose modulation technique, but the consequences of thesystem choice are very important, since carrier bandwidth andcarrier separation affects e.g. interference properties. Wavepropagation also behaves differently in different frequencybands.



2 Basic Radio

EN/LZT 123 3313 R2C – 19 –

The modulation technique used in GSM 900 is called GaussianMinimum Shift Keying (GMSK). This narrow-band digitalmodulation technique is based on phase shifting. That is, bits are

represented by continuous positive or negative phase shifts. Bychanging the phase continuously, sharp discontinuities areavoided, thus narrowing the bandwidth of the modulated carrier.GMSK modulation also involves filtering the incoming bitstream with a Gaussian filter to obtain a more narrow bandwidthof the modulated carrier. In fact the full width at half maximumof the carrier becomes 162 kHz, corresponding nicely to the 200kHz carrier separation.

Transmitting the information on the air interface in digitizedform has an advantage over analogue techniques in that sincechannel coding protects bits, the signal is less sensitive toperturbations. In addition, it enables Time Division MultipleAccess (TDMA) which means that one carrier frequency can beused for several connections. Each connection uses only oneparticular time slot (out of the eight available in GSM). This hasthe advantage that the mobile is released fromtransmitting/receiving continuously and can perform e.g.measurements on neighboring cells. One main advantage withTDMA is the fact that it enables Mobile Assisted Hand Over(MAHO) which is essential for effective connection control.

AIR INTERFACE DATA

Below is a summary of some important air interface data forGSM 900, GSM 1800 and GSM 1900.

FREQUENCY SPECTRUM Different frequency bands are used for GSM 900, GSM 1800and GSM 1900 (cf. Figure 1-3). In some countries, operatorsapply for the available frequencies. In other countries, (e.g.,United States) operators purchase frequency bands at auctions.




– 20 – EN/LZT 123 3313 R2C

In December of 1994, the Federal Communications Commission(FCC) auctioned “broadband” licenses to prospective operatorsoffering personal communications services. Each operator owns

the rights to the licenses for a period of ten years. The UnitedStates is divided into 51 regions or Major Trading Areas (MTA)and 493 Basic Trading Areas (BTA). The FCC issued twoGSM 1900 licenses for each MTA and four for each BTA. OneMTA can be geographically as large as a state, while one BTAcan be compared in size to a large city. BTAs are designed foruse in major metropolitan areas.

The FCC has specified the frequency range and output power.The frequency band is divided into six frequency blocks(Figure 2-7): three duplex blocks A, B, and C (90 MHz totalspectrum bandwidth) and three other duplex blocks D, E, and F(30 MHz total spectrum bandwidth).

1850 1910 1930 1990

A D B E F C A D B E F CUnlicensed

MTAsUplink

MTAsDownlink

A,B 2 x 15 MHz MTAC, 2 x 15 MHz BTAD, E, F 2 x 5 MHz BTA

60 MHz 60 MHz

Figure 2-7. Spectrum allocation for GSM 1900 in United States. 140 MHz for GSM 1900; 120 MHz licensed and 20 MHz unlicensed



2 Basic Radio

EN/LZT 123 3313 R2C – 21 –

DUPLEX DISTANCE

The distance between the uplink and downlink frequencies isknown as duplex distance. The duplex distance is different forthe different frequency bands, see figure below:

Standard GSM 900 GSM 1800 GSM 1900Duplex dist. 45 MHz 95 MHz 80 MHz

Figure 2-8. Duplex differences for different frequency bands

CHANNEL SEPARATION

The distance between adjacent frequencies on the uplink or thedownlink is called channel separation. The channel separation is200 kHz, regardless of the standard chosen from the onesmentioned above. This separation is needed to reduceinterference from one carrier to another neighboring frequency.

ACCESS METHOD AND TRANSMISSION RATE

GSM has chosen the Time Division Multiple Access (TDMA)concept for access. In GSM, there are eight TDMA time slots

per frame, see Figure 2-9. Each time slot is 0.577 ms long andhas room for 156.25 bits (148 bits of information and a 8.25 bitslong guard period) yielding a bit rate on the air interface of 270.8 kbit/s.

0 1 2 3 4 5 6 7

4.615 ms

3 57 1 26 1 57 3

Burst148 Bits

156.25 Bits0.577 ms

DataTrainingData

Figure 2-9. Basic TDMA frame, timeslot, and burst structures




– 22 – EN/LZT 123 3313 R2C

RADIO WAVE PROPAGATION

In this course we are primarily interested in the transmission loss

between two antennas, the transmitter/emitter and the receiver.Many factors including absorption, refraction, reflection,diffraction and scattering affect the wave propagation. However,in free space an electromagnetic wave travels indefinitely if unimpeded. This does not mean that there are no transmissionlosses as we will see in this first, simple model, where isotropicemission from the transmitter and line of sight between the twoantennas separated by a distance, G , in free space are assumed(Figure 2-10).

Figure 2-10. Radio wave propagation in free space

Since an isotropic antenna by definition distributes the emittedpower, 3 W , equally in all directions; the power density, 6 U , (powerper area unit) decreases as

6 3

G U

W =4 2

when the irradiated area, πG , at distance G , increases.

If the transmitting antenna has a gain, * W , it means that it isconcentrating the radiation towards the receiver. The powerdensity at the receiving antenna increases with a factorproportional to * W , i.e.

6 3 *

G U

W W=4 2

The power received by the receiving antenna, 3 U , is proportional

to the effective area, $U

, of that antenna, i.e. 3 6 $

U U U

= ⋅



2 Basic Radio

EN/LZT 123 3313 R2C – 23 –

It can be shown that the effective area of an antenna isproportional to the antenna gain, * U , and the square of thewavelength, λ, of the radio wave involved, i.e.

$*

U

U =λ 2

4

and hence the received power becomes

( ) 3

3 * *

G U

W W U =λ

π

2

24.

The transmission loss can be calculated as the ratio between the

transmitted power and recieved power, i.e.

( )ORVV

3 3

G * *

W

U W U

= = 4 2

2

π

λ .

Radio engineers work with the logaritmic unit dB so thetransmission loss, / , then becomes

( )( )

( ) ( ) / ORVVG

* * G

* * W U

U W

= =

=

− −

log log log log logπ

λ

π

λ

Radio engineers treat the antenna gains, 10log( * U ) and10log ( * W ), separately so what in the literature is given as thepathloss, / S , is only the term 20log( πG λ). To make it clearer,the pathloss in free space is

/G

S

=

20

4log

π

λ .

Note that the wavelength dependency of the pathloss does notcorrespond to losses in free space as such. It is a consequence of the finite effective receiver area.

This expression is fairly general. The only thing which changeswhen we improve our models is the expression for the pathloss.The antenna gain is normally given in dB(i), i.e. as 10log( * ),where gain means a reduction of the total transmission loss, / ,between a transmitting and receiving antenna.

This model helps us to understand the most important features

of radio wave propagation. That is, the received power decreaseswhen the distance between the antennas increases and the




– 24 – EN/LZT 123 3313 R2C

transmission loss increases when the wavelength decrease (oralternatively when the frequency increases).

For cell planning it is very important to be able to estimate thesignal strengths in all parts of the area to be covered, i.e. topredict the pathloss. The model described in this section can beused as a first approximation. However, more complicatedmodels exist. Improvements can be made by:

• taking into account the fact that radio waves are reflectedtowards the earth’s surface (the conductivity of the earth isthus an important parameter).

• accounting for transmission losses due to obstructions in theline of sight.

• taking into account the finite radius of curvature of the earth.

• accounting for the topographical variations in a real case aswell as the different attenuation properties of differentterrain types such as forests, urban areas, etc.



2 Basic Radio

EN/LZT 123 3313 R2C – 25 –

In fact, the best models used are semi-empirical, i.e. based onmeasurements of pathloss/attenuation in various terrains. Theuse of such models are motivated by the fact that radio

propagation can not be measured everywhere. However, if measurements are taken in typical environments, the parametersof the model can be fine tuned so that the model is as good aspossible for that particular type of terrain.

SIGNAL VARIATIONS

The models described in the previous section can be used toestimate the average signal level, called the “global mean” at thereceiving antenna. However, a radio signal envelope iscomposed of a fast fading signal, super-imposed on a slowfading signal (Figure 2-8). These fading signals are the result of obstructions and reflections. They yield a signal which is thesum of a possibly weak, direct, line-of-sight signal and severalindirect or reflected signals.

The fast fading signal (peak-to-peak distance ≈ λ /2) is usuallypresent during radio communication due to the fact that themobile antenna is lower than the surrounding structures such as

trees and buildings. These act as reflectors. The resulting signalconsists of several waves with various amplitudes and phases.Sometimes these almost completely cancel out each other. Thiscan lead to a signal level below the receiver sensitivity. In openfields where a direct wave is dominating, this type of fading isless noticeable.

Short-term fading is Rayleigh distributed with respect to thesignal voltage. It is, therefore, often called Rayleigh fading. Thistype of fading affects the signal quality and therefore somemeasures must be taken to counter it.

The first and most simple solution is to use more power at thetransmitter(s), thus, providing a fading margin. Another way toreduce the harm done by Rayleigh fading is to use spacediversity, which reduces the number of deep fading dips.Diversity means that two signals are received which haveslightly different “histories” and therefore the “best” can beused. With the efficient space diversity system used by Ericsson,voice quality is much improved.




– 26 – EN/LZT 123 3313 R2C

SS at Rx-antenna

DistanceVariations due toShadowing

Global means

Variations due toRayleigh fading

Figure 2-11 Short-term (fast) and long-term (slow) fading

The signal variation received, if we smooth out the short-termfading, is called the “local mean”. Its power, often called thelocal average power, expressed in a logarithmic scale, isnormally distributed. Therefore, this slow fading is called “log-normal fading”. If we drive through a flat desert without anyobstructions, the signal varies slowly with distance. However, innormal cases the signal path is obstructed.

Obstructions near the mobile such as buildings, bridges andtrees cause a rapid change of the local mean (in the range of fiveto fifty meters), while topographical obstructions cause a slowersignal variation. Because log-normal fading reduces the averagestrength received, the total coverage from the transmitter isreduced. To combat this, a fading margin must be used. Log-normal fading also makes problems generated by multi-pathreflections more severe since the direct beam is weakened by theobstructing object.

Phases between various reflected waves are different. This is

due to the fact that they propagate over different distances orequivalently use different times to reach the receiver. This timedispersion can cause particular problems if the phase differencebetween the reflected waves is very large. For GSM 900, a largephase difference is on the order of several thousands of wavelengths, i.e. one kilometer or more. In this case differentwaves added together in the receiver carry information aboutdifferent symbols (bits). If the direct wave is weak, andconsequently the reflected waves are relatively strong, it can bedifficult to determine which symbol (bit) was transmitted. Thisintersymbol interference is discussed along with otherinterferences in more detail in chapter ³1RPLQDO &HOOSODQ´.

04 Basic Radio

Documents

Transcript of 04 Basic Radio