WLAN Topic 2 Introduction to RF Fundamentals

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Topic 2 Introduction to RF

Fundamentals 1 hour

Definition of RF

• Radio Frequency are high frequency AC signals, that may travel along a conductor and then radiated into air via an antenna

• After being radiation, RF propagates in straight line in defined direction

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Ground Wave Propagation

• Ground waves propagation follows the contour of the earth and can propagation considerable distances, well over the visual horizon

• This effect is found in the frequencies up to 2 MHz

• AM radio is an example of ground wave communication

Line-of-Sight Propagation

• Above 30 MHz, neither ground nor sky wave propagation operate, and communication must be by the line of sight

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Free-Space Loss

• If we assume RX antenna to be isotropic, then

TXRX Pd

P

2

4

Attenuation between two isotropic antennas in free space is (free-space loss):

2

4

ddL free

Free-Space Loss

• Received power, with antennas GTX and GRX (also known as Friis’ Law):

TXRXTXTX

free

TXRXRX GG

dPP

dL

GGdP

2

4

dBRXdBTXdBTX

dBRXdBfreedBTXdBTXdBRX

Gd

GP

GdLGPdP

2

10

4log10

Valid in the far field only

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Free-Space Loss

• Near field and far field are propagation regions of the time-varying EM energy (or RF)

• The Rayleigh distance

22 aR

Ld

where La is the largest dimension of the antenna

2

2

R

a

d

L

28

2

rd

rL

R

a

Propagation Phenomena

Reflection and Transmission

• When RF hits an object whose dimension is larger than the wavelength of the wave

• Caused by earth, building, walls, doors, …

• Reflections of the primary signal from many objects is known as multipath

• Multipath may degrade or cancel the primary signal at the receiver

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Reflection and Transmission • Total transmission coefficient

• Total reflection coefficient

with the electrical length of the wall

ja

ja

eRR

eTTT

2

21

21

1

ja

ja

e

e2

21

2

21

1

tlayerd cos2

1

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for distance greater than

The power profile



• For the following scenario

• The power at the receiver:

2

2

d

hhGGPdP RXTX

RXTXTXRX

RXTX

break

hhd

4


• Christiaan Huygens states

that every point of a wave front may be considered the source of secondary wavelets that spread out in all directions with a speed equivalent to the speed of propagation of the waves

• It is like when light passes through an aperture, it does not create a perfect image, the edges actually spread out

Diffraction – Huygen’s Principle When the energy goes through the aperture, every point of the waves within the aperture can be viewed as creating circular waves

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• Compute diffraction loss for each screen separately and

add the losses

Diffraction – Epstein-Peterson’s Method


• Compute diffraction loss for each screen separately and

add the losses

Diffraction – Epstein-Peterson’s Method

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Scattering

Link Budget

• Link budgets show how different components and propagation processes influence the available SNR

• Link budgets can used to compute, e.g., required transmit power, possible range of a system, or required receiver sensitivity

• Link budgets can be most easily setup using logarithmic power units (dB)

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Link Budget

Transmit power Received power Feeder loss Feeder loss

Noise Sources

• For any data transmission event, the received signal will consist of the transmitted signal, modified by the various distortions imposed by the transmission system, plus additional unwanted signals that are inserted somewhere transmission and reception

• These unwanted signals are referred to as noise

• Noise may be divided into four categories

• Thermal noise

• Inter-modulation noise

• Crosstalk

• Impulse noise

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Noise Sources

• The noise situation in a receiver depends on several noise sources

Noise Sources

Man-Made Noise

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Noise Sources

Receiver Noise – Equivalent Noise Source

• The noise situation in a receiver depends on several noise sources

Noise Sources

Receiver Noise – Equivalent Noise Source • The power spectral density of a noise source is usually given

in one of the following three ways

• Directly [W/Hz] Ns

• Noise temperature [Kelvin] Ts

• Noise factor Fs

• The relation between the three is

where k is the Boltzmann’s constant (1.38 10–23 W/Hz) and T0 is the room temperature (290 K or 17C)

0TkFkTN sss

Noise factor is sometimes given in dB which is known as noise figure

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• Example:

• Due to a definition of noise factor (in this case) as the ratio of noise powers on the output versus on the input, when a resistor in room temperature (T0 = 290 K) generates the input noise, the PSD of the equivalent noise source (place at the input) becomes

Noise Sources

W/Hz1 0TFkNsys

Do not use dB value Equivalent noise temperature

Noise Sources

Receiver Noise – Equivalent Noise Source • Antenna example:

• Power spectral density of antenna noise is

and its noise factor/noise figure is

W/Hz 6.196W/Hz1021.2

16001038.1

20

23

dB

kTN aa

dBTTF aa 42.742.5290/1600/ 0

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Noise Sources

W/Hz1 0TFkNsys

Do not use dB value Equivalent noise temperature


• Example:

• Due to a definition of noise factor (in this case) as the ratio of noise powers on the output versus on the input, when a resistor in room temperature (T0 = 290 K) generates the input noise, the PSD of the equivalent noise source (place at the input) becomes

Receiver Noise – Several Noise Sources

Noise Sources

022

011

1

1

TFkN

TFkN

kTN aa

Documents

WLAN Topic 2 Introduction to RF Fundamentals