EM Waves

Basics

Modulation

Multiple Access

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Basics:

Wavelength: Amplitude: Frequency: Velocity:

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Example of Amplitude Modulation:

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Why do you need a carrier signal? We have to modulate the original low frequency radio

signal or wave: Higher frequency waves transmit better. To allow multiple signals to be transmitted at the same

time without interference. So we modulate the different signals onto different carrier waves, and we ALL could talk at the same time. Each specific carrier frequency is called a channel.

Restrictions on the antenna size: the length of an antenna is based on the length of the wave it was designed to transmit. Usually the length of the antenna is a half of the wave length.

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Example: (bit extreme!)

Determine the length of an antenna for a 60 Hz (lowest range of human hearing) radio signal travelling at a speed of 3 x 105

km/second We first have to calculate the wavelength of

this wave, because it is related to the length of the antenna.

Frequency = velocity/wavelength

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Length of antenna:

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Different modulations:

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Conclusion:

When we talk about modulation, we are talking about a minimum of two waves, the signal and the carrier.

Certain properties of the carrier waveform are modified (modulated) to represent the signal waveform.

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The Electromagnetic Spectrum

The electromagnetic (EM) spectrum is the range of all types of EM radiation.

Radiation is energy that travels and spreads out as it goes – the visible light that comes from a lamp in your house and the radio waves that come from a radio station are two types of electromagnetic radiation.

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EM Spectrum:

The other types of EM radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultraviolet light, X-rays and gamma-rays.

You know more about the electromagnetic spectrum than you may think. The image below shows where you might encounter each portion of the EM spectrum in your day-to-day life.

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Mobile networks: (page 119-120)

To improve efficiency and quality of digital communications, two technologies have been developed:  TDMA CDMA

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TDMA:

a channel is divided into time slots. Each time slot is assigned to a different user. The transmitter transmits information for all time slots at the same frequency and the receivers receive all time slots but only ‘listen’ to the time slot they have been allocated. Example: A channel is divided into six slots. A

user gets two slots: one for transmission and one for reception. This means each channel can accommodate 3 calls.

CDMA:

it assigns a unique code to each signal and then combines all the signals together in a single large channel. The receiver receives the integrated signal and uses the same code to process (decodes) the desired signal.

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