Basic of RF

1. RF fundamentals
Created by Shahanawaz Mansuri

2. Communication
The term communications refers to the sending, receiving and processing
of information by electric means.
Channel:
The function of the channel is to provide a physical connection between
the transmitter and receiver. It may be wired or wireless
Input Transducer:
It converts the information to be transmitted into its electrical equivalent.
Ex. The microphone converts spoken words into audio signals.

3. Communication
Transmitter:
To transmit the message signal over a communication channel we need to
modify it into a suitable form for efficient transmission over the channel.
Modification of message signal is achieved be means of a process called
modulation.
This process involves varying some parameters of carrier wave in
accordance with signal in such a way that the spectrum of modulated
wave matches the assigned bandwidth
Receiver:
The function of the receiver is to process the received signal so as to
produce an estimate of original message signal.
The receiver is required to re-create the original message signal from the
transmitted signal after propagation through the channel. The re-creation
is accomplished by using a process called demodulation

4. Communication
Output Transducer:
After the received signal is restored to its electrical equivalent as it was
before it was modulated, it is converted back to its original form using
output transducer. Ex. The loudspeaker converts the electrical signals back
to audio signals.
So finally communication is by creating electromagnetic waves at a source
and being able to pick up those electromagnetic waves at a particular
destination. These electromagnetic waves travel through the air at near
the speed of light. The wavelength of an electromagnetic signal is
inversely proportional to the frequency; the higher the frequency, the
shorter the wavelength.

5. Communication
Frequency is measured in Hertz (cycles per second) and radio frequencies
are measured in kilohertz (KHz or thousands of cycles per second),
megahertz (MHz or millions of cycles per second) and gigahertz (GHz or
billions of cycles per second). Higher frequencies result in shorter
wavelengths. The wavelength for a 900 MHz device is longer than that of a
2.4 GHz device.
In general, signals with longer wavelengths travel a greater distance and
penetrate through, and around objects better than signals with shorter
wavelengths.
Now Question is that what do you mean by
Modulation/Demodulation..???
And why we require modulation………..???

6. Need for Modulation
1. Modulation for efficient transmission
The efficiency of any particular transmission method depends upon the
frequency of the signal being transmitted.
Typically, efficient line-of-sight radio propagation requires antennas
whose physical dimensions are at least 1/10 of the signal‘s wavelength.
Unmodulated transmission of an audio signal containing frequency
components down to 100 Hz would thus require for antennas 300 km
long. Modulated transmission at 100 MHz, as in FM broadcasting, allows a
practical antenna size of about one meter.
2. Modulation for overcome hardware limitations
The cost and availability of hardware is the constrained of communication
system which is frequency dependent. Modulation permits the designer
to place a signal in some frequency range that avoids hardware
limitations. Hardware costs and complications are minimized by keeping
the fractional bandwidth within 1-10 %.

7. Need for Modulation
3. Modulation for frequency assignment
When you tune a radio or television set to a particular station, you are
selecting one of the many signals being received at that time. Since each
station has a different assigned carrier frequency, the desired signal can
be separated from the others by filtering. Due to modulation it is possible
to modulate different sound signals with different frequency carriers
thereby create the modulated signals that occupy different slots of the
frequency spectrum and avoid a jumble of signals.
4. Increases the range of communication
The frequency of base band signal is low. Therefore it cannot travel over a
long distance. When such signals are transmitted, they get heavily
attenuated. The attenuation of the signal reduces with increase in
frequency of transmitted signal and they travel larger distance. The
modulation process up shifts the frequency of the signal to be
transmitted. Therefore it increases the range of the communication.

8. Modulation
Modulation is a process of mixing a signal with a sinusoid of high
frequency to produce a new signal. This new signal will have certain
benefits of an un-modulated signal, especially during transmission. If we
look at a general function for a sinusoid:
f(t) = A sin(ωt + φ)
We can see that this sinusoid has 3 parameters that affect the shape of
the graph.
A: the magnitude or amplitude of the sinusoid,
ω: the frequency, and the last term,
φ: the phase angle.
All 3 parameters can be altered to transmit data.

9. Modulation
The high frequency sinusoidal signal that is used in the modulation is
known as the carrier signal, or simply "the carrier". The signal that is used
in modulating the carrier signal (or sinusoidal signal) is known as the data
signal or the message signal.
A simple sinusoidal carrier contains no information of its own. In other
words we can say that modulation is used because the some data signals
are not always suitable for direct transmission, but the modulated signal
may be more suitable.
So how many types of modulation is available…..?????
You know the answer………….we have just read it….
Guess????????????

10. Amplitude Modulation
Amplitude modulation:
A type of modulation where the amplitude of the carrier signal is
modulated (changed) in proportion to the message signal while the
frequency and phase are kept constant.

11. Frequency Modulation
Frequency modulation:
A type of modulation where the frequency of the carrier signal is
modulated (changed) in proportion to the message signal while the
amplitude and phase are kept constant.

12. Phase Modulation
Phase modulation:
A type of modulation where the phase of the carrier signal is modulated
(changed) in proportion to the message signal while the amplitude and
frequency are kept constant.

13. Demodulation
Demodulation is the reverse process of modulation. It is the process of
recovering original modulating signal from modulated waveform. The
process of Demodulation is also called as Detection.

14. Digital Modulation
So far we have studied about analog modulation. However, Digital
signals have become very important in both wired and wireless
communication.
The main difference between analog modulation and digital
modulation is in the manner that they transmit data. With analog
modulation, the input needs to be in the analog format, while digital
modulation needs the data in a digital format.
Because of the differences in the input signal, the output signal is also
quite different. In analog modulation, any value between the
maximum and minimum is considered to be valid. It is not so with
digital modulation as only two values are considered valid; one value
to represent “1” and another to represent “0.” All other values are
considered noise and are rejected.
Modulation of digital signals is known as Shift Keying

15. Digital Modulation
FREQUENCY SHIFT KEYING (FSK):
In Frequency shift keying, we change the frequency of the carrier wave.
Bit 0 is represented by a specific frequency, and bit 1 is represented by a
different frequency.
In the figure below frequency used for bit 1 is higher than frequency used
for bit 0.

16. Digital Modulation
AMPLITUDE SHIFT KEYING (ASK)
In amplitude shift keying, the amplitude of the carrier signal is varied to
create signal elements. Both frequency and phase remain constant while
the amplitude changes.
Bit 1 is transmitted by a carrier of one particular amplitude.
To transmit Bit 0 we change the amplitude keeping the frequency is kept
constant.

17. Digital Communication
PHASE SHIFT KEYING (PSK)
In this method, the phase of a transmitted signal is varied to convey
information. Both amplitude and frequency remain constant as the phase
changes.
If the phase of the wave does not change, then the signal state stays the
same (low or high). If the phase of the wave changes by 180 degrees, that
is, if the phase reverses, then the signal state changes (from low to high or
from high to low)

18. Multiplexing
What is mean by multiplexing???
Lets first understand the way of communication system….
Simplex System
A technology that allows only one-way communication from a transmitter
to a receiver ƒExamples: FM radio, Pagers, TV, One-way AMR systems
Half-duplex Systems
Operation mode of a communication system in which each end can
transmit and receive, but not simultaneously. ƒNote: The communication
is bidirectional over the same frequency, but unidirectional for the
duration of a message. The devices need to be transceivers. Applies to
most TDD and TDMA systems. ƒExamples: Walkie-talkie, wireless keyboard
mouse

19. Multiplexing
Full-duplex Systems
Systems in which each end can transmit and receive simultaneously ƒ
Typically two frequencies are used to set up the communication channel.
Each frequency is used solely for either transmitting or receiving. Applies
to Frequency Division Duplex (FDD) systems. ƒExample: Cellular phones,
satellite communication
Now…………..what is Multiplexing………????
Simultaneous transmission of multiple messages over a channel is called
multiplexing. There are 2 types of multiplexing.
1. Frequency division multiplexing.
2. Time division multiplexing.

20. FREQUENCY DIVISION MULTIPLEXING
(FDM):
FDM uses analogue modulation system, whereas TDM uses pulse
modulation system.
The RF (radio frequency) channel is split into several smaller sub-channels.
For example, one 12.5kHz wide narrowband FM channel that previously
carried only one conversation becomes two 6.25kHz sub-channels, each
capable of carrying a separate conversation.

21. FREQUENCY DIVISION MULTIPLEXING
(FDM):
This technique has been around for decades and is used with either
analog or digital radios. A ‘telephone style conversation’ can be set up if
one sub-channel is used to transmit and one to receive.
Drawback:
The more sub-channels you try to fit in to the original channel, the more
likely that the users will suffer interference on the call. This is because the
reduced channel spacing makes it harder to filter only the intended sub-
channel and reject all the others at the receiver.

22. TIME DIVISION MULTIPLE ACCESS
(TDMA)
Instead of splitting the original RF channel in to two RF sub-channels, it is
instead split into timeslots. The transmitted RF frequency is identical in
each slot, but each slot is still capable of carrying a separate conversation.
Again, a ‘telephone-style conversation’ can be set up, if certain slots are
used for transmit and others for receive.

23. CODE DIVISIONAL MULTIPLE ACCESS
(CDMA)
Instead of splitting the RF channel in to sub-channels or time slots, each
slot has a unique code. Unlike FDMA, the transmitted RF frequency is the
same in each slot, and unlike TDMA, the slots are transmitted
simultaneously. In the diagram, the channel is split in to four code slots.

24. CODE DIVISIONAL MULTIPLE ACCESS
(CDMA)
Each slot is still capable of carrying a separate conversation because the
receiver only reconstructs information sent from a transmitter with the
same code.
Drawback:
As transmissions on the same frequency with different codes are still
received and decoded but simply re-appear as noise. This means the
greater the number of users, the higher the noise level on the system,
which of course can affect coverage.

25. GSM
Now question is that, GSM uses what TDMA, FDMA or CDMA…????
GSM (Global System for Mobile communications) is an open, digital
cellular technology used for transmitting mobile voice and data services.
GSM differs from first generation wireless systems in that it uses digital
technology and Time Division Multiple Access (TDMA) transmission
methods. GSM is a circuit-switched system that divides each 200kHz
channel into eight 25kHz time-slots. GSM operates in the 900MHz and
1.8GHz bands in Europe.

26. How range is determined
Link Budget?????
A link budget is accounting of all of the gains and losses from the
transmitter, through the medium (free space, cable, waveguide, fiber, etc.)
to the receiver in a telecommunication system.
A simple link budget equation looks like this:
Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses
(dB)
dB - Decibels
Decibels are logarithmic units that are often used to represent RF power.
To convert between milliWatts (mW) and decibels (dB), use the following
conversion equations (P = power):
(mW to dBm) PdBm = 10 * Log10(PmW)
(dBm to mW) PmW = 10^(PdBm/10)

27. How range is determined
The strength of the electromagnetic radio wave received at some location
is a function of: (a) the strength of the original transmitted signal, (b) the
performance of the antennas at the transmitter and receiver, (c) the
wavelength corresponding to the frequency of operation, and (d) the
distance separating the transmitter and receiver.
PR = power received (watts)
PT = power transmitted (watts)
GT = gain of transmit antenna (scalar)
GR = gain of receive antenna (scalar)
λ = wavelength (metric or English)
d = distance separating transmitter and receiver (metric or English)
And exponent for environmental conditions

28. RF in real word environmental
• Multipath wave propagation
When a wave leaves the antenna, it travels in all directions. “Multi-path”
describes the situation in which the wave is modified by its propagation through
the environment, before arriving at the receiver. These waves incident on the
receiver's antenna are categorized into four types:
Direct waves - waves which travel
on a line-of-sight path.
Reflected waves - waves which
bounce off smooth surfaces that are
much greater than one wavelength
in size for the specific operating freq.
Diffracted waves - waves which are
Bent around sharp corners
Scattered waves - waves which
bounce off objects or features on a rough surface that are much smaller than a
wavelength in size

29. RF in real word environmental
Waves which are diffracted, reflected, or scattered experience changes in
magnitude and phase, additional to what naturally occurs to a direct
wave. These variations in magnitude and phase are caused by
(1) Absorption of some of the wave’s energy by the reflecting surface,
(2) The phase change caused by a reflection, and
(3) The differences in length of the paths travelled by the various waves.
These multi-path waves arrive from many angles and directions,
causing them to sum with various magnitudes and phases at the
receiver.
As a result, the composite wave at the receiver can be either greater than
or less than what would be produced by the direct wave alone. This
means that some waves add to the direct signal and some subtract from
it. The worst case could be total cancellation, yielding no detectable signal
at all!

30. RF in real word environmental
• Loss as a Function of Distance
Since radio waves behave much like sound waves, we can draw a parallel
here. As one is distanced further and further from a sound's source, the
sound one hears is weaker. The same is true for radio waves.
Doubling the distance separating the transmitter and receiver reduces the
received signal strength by 6dB, whereas increasing the separation
distance by a factor of 10 reduces the received signal strength by 20dB.
• Attenuation from obstacles
In addition to the detrimental affects of multi-path travel and loss due to
separation distance, RF waves are also attenuated when they pass
through obstacles. These measured attenuation values are for common
building materials

31. RF in real word environmental
• Antenna Losses
Antennas are an integral part of an RF communication system. For a
receiver, antennas convert the electromagnetic energy of a radio wave in
space into a voltage/current that can be processed by the receiver. For the
transmitter, the reverse occurs – an antenna converts the voltage/current
signal generated by a transmitter into a radio wave that travels through
space. In this way, antennas are a radio's link to the “outside world”. It
means the size of a properly designed antenna is related directly to the
frequency on which it operates.
Antenna efficiency represents the losses of antenna. Means, “The ratio of
the total power radiated by an antenna to the net power accepted by the
antenna from the connected transmitter”.

32. RF in real word environmental
Summing These Losses
we see that an RF wave can experience significant degradation as it travels
through a real-world environment, and that these influences must be
accounted for during the design phase if a communication system has any
chance of performing to realistic expectations

33. Methods to Improve Range
Optimize RF energy transfer
According to the Maximum Power Transfer Theorem, the amount of RF
power that reaches the antenna from the radio’s output depends
primarily on how well the impedances of the transmitter’s output and the
antenna are matched. i.e. from the source (the transmitter’s output) to
the load (the radiating antenna). Impedance matching is achieved using
passive L-C networks.
Focus on the Antenna Situation
Size: larger antennas capture/radiate more RF energy. To improve RF-link
performance, then, the largest antenna possible (properly designed for
the frequency of operation) should be used.
Efficiency: “radiation resistance” needs to be appropriately large in order
for an antenna to be effective.

34. Methods to Improve Range
From DC circuit theory, Ohm’s law states that the current through a
resistance produces a voltage (V = IR). Likewise with antennas, output
current from the transmitter passing through the “radiation resistance” of
an antenna produces a voltage. This voltage is converted on the antenna
to an electromagnetic wave which propagates through space. Antennas
which are at least 1/4-wavelength for the operating frequency have
“appropriately large” radiation resistances. A fair range of values
“appropriately large” for an antenna’s radiation resistance is 35Ω to 100
Ω.
Polarization: The electromagnetic waves which antennas radiate and
receive have two components – an electric field and a magnetic field.
These two fields travel through space at right angles (90°) to one another.
The orientation of the electric field designates the “polarization” of the
wave radiated by that antenna.

35. Methods to Improve Range
Radio waves are polarized horizontally, vertically, or circularly. To
maximize energy transfer from the transmitter to the receiver, the
antenna polarizations need to match, regardless of which polarization is
chosen.
Near-by Objects: antennas should be kept away from nearby metal or
other electrically conducting objects. Their presence distorts the radiation
pattern from an antenna.
Employ signal processing
First, methods which broadens the bandwidth of signal i.e. “Spread
spectrum”.
Second, approach for improving range is through “diversity” in the
receiving system.
Finally acknowledgement of transmission by the receiver.