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WT Week 2

Radio Frequency I

tldr - Antenna & Modulation Techniques

Modulation

The process of altering a carrier wave in order to use it to convey information

Analog Modulation

Amplitude Modulation (AM)

The process of varying the amplitude of the carrier signal accordingly with the amplitude of message signal

Frequency Modulation (FM)

The process of varying the frequency of the carrier signal accordingly with the amplitude of the message signal

Phase Modulation (PM)

The process of varying the phase of the carrier signal accordingly with the amplitude of the message signal

Digital Modulation

Amplitude-Shift Keying (ASK)

The process of varying the amplitude of the carrier signal in proportion to the message signal

Basic form of ASK is On-Off Keying (OOK), where absence of carrier is 0 and presence of carrier is 1
Used in wireless LAN and Bluetooth
Multiple amplitude levels can be used to encode groups of bits

Frequency-Shift Keying (FSK)

The process of varying the frequency of the carrier signal accordingly with the message signal

Output signal varies between 2 predetermined frequencies, logic 1 and logic 0

Phase-Shift Keying (PSK)

The process of shifting the phase of the carrier signal accordingly with the message signal

Normally < 8 phases so that phase signals are more distinct
Constellation diagrams are used to represent the PSK scheme used

Examples

Binary PSK (BPSK)

Simplest form of PSK
2-phase PSK

Quadrature PSK (QPSK)

Each phase represents a 2-bit group: 00, 01, 10, 11
4-phase PSK

Constellation Diagrams

The image shows the constellation diagrams for a 4-PSK scheme and an 8-PSK scheme respectively.

Quadrature Amplitude Modulation (QAM)

The combination of 2-level ASK and QPSK
QPSK provides the 4 points in the constellation diagram for each amplitude (lines)
2-level ASK provides the 2 amplitudes in the constellation diagram (circles)
Each point represents 3 bits (e.g. 001)
- 1st 2 bits represent the phase (00)
- 3rd bit represents the amplitude (1)

Radio Frequency (RF) Components

Transmitter

Generates alternating current (AC) signal upon receiving digital data
AC signal oscillates 2.4 billion times per second for 2.4 GHz transmissions
Modifies AC signal by using a modulation technique to encode data into signal
Transmits modulated AC signal to antenna

Antenna

A passive conductor
Relies on power source (transmitter / receiver)
Collects AC signal received from transmitter and converts electricity into RF waves
RF waves are radiated away at the transmission end
Another antenna receives the RF waves
Converts RF waves into AC electric signals and passes it to the receiver

Receiver

Receives AC signal from antenna
Demodulates AC signal into digital data

Properties of Antenna

Signal Strength

Loss

Signal energy decreased

RF signal received is weaker
Signal strength affected by
- Distance
- Noise
- Absorption
- Reflection
- Refraction
- Diffraction
- Scattering

Gain

Signal energy increased

Signal energy focused in one direction
Signal is put through an amplifier

Absorption

Signal is absorbed if it does not bounce off, move around or pass through an object
2.4 GHz signal passed through a brick & concrete wall will be 1/16 the original power (Fun fact)

Reflection

Signals may bounce in an unintended direction when it hits a smooth object
Causes serious performance issues in WIFI
Multiple reflected signals degrade the signal’s strength and quality

Diffraction

The bending and spreading of signals around an obstruction
Creates RF shadows, the spot behind the obstruction
RF shadow may become a dead zone of coverage
Thus, selecting antenna locations is important!

Refraction

Signals passing through mediums with different densities can cause the direction of the wave to change
Commonly occurs as a result of atmospheric conditions

Size of an antenna

Length should be a multiple of the wavelength, such as 1/2 or 1/4

Formula to find length of antenna — Length of antenna

Where

λ is the wavelength (m)
x is a multiple (etc. 1/2 and 1/4)

Example

Find a suitable length of an antenna for a signal frequency of 100MHz. Assume that the speed of light 3 x 10^8 m/s.

Power Ratio

Decibel

Relative measurement used in calculations of gain and loss, to indicate the strength of a signal

Usually expressed as the logarithmic ratio of the strength of a transmitted signal to the strength of the original signal

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