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Modulation schemes

The selection of appropriate modulation schemes for an application are usually dependent on (but not necessarily limited to) the following factors:

1.required range of propagation;

2.frequency of operation and propagation properties;

3.spectral efficiency;

4.equipment complexity, reliability, size weight and cost;

5.required data or voice throughput rate;

6.regulatory constraints

Modulation schemes

Single sideband (SSB) modulation

With DSB-AM, it is found that the upper sideband is a perfect reflection of the lower sideband; i.e. the same information is carried by both. By removing one of the sidebands and by removing the carrier, therefore, no information is lost and the SNR (signal-to-noise ratio) increases for a given transmit power as all the transmitted energy is concentrated into the information signal.

Frequency modulation

Frequency modulation (FM) conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant). In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal.

Phase modulation

Phase modulation (PM) is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.

An example of phase modulation. The left diagram shows the modulating signal superimposed on the carrier wave. The right diagram shows the resulting phase-modulated signal.

Digital modulation

In modulation, an analog carrier signal is modulated by a digital bit stream. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to- digital conversion.

On-off keying (OOK)

On-off keying (OOK) is a type of modulation that represents digital data as the presence or absence of a carrier wave. In its simplest form, the presence of a carrier for a specific duration represents a binary one, while its absence for the same duration represents a binary zero.

On-off keying is most commonly used to transmit Morse code over radio frequencies (referred to as continuous wave operation), although in principle any digital encoding scheme may be used. OOK has been used to transfer data between computers, for example.

Chart of the Morse code letters and numerals

Morse code is a character encoding for transmitting telegraphic information, using standardized sequences of short and long elements to represent the letters, numerals, punctuation and special characters of a given message. The short and long elements can be formed by sounds, marks, or pulses, in on-off keying and are commonly known as "dots" and "dashes" .

Amplitude-shift keying

Amplitude-shift keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.

The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal), keeping frequency and phase constant. The level of amplitude can be used to represent binary logic 0s and 1s. In the modulated signal, logic 0 is represented by the absence of a carrier.

Like AM, ASK is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes in PSTN, etc. Both ASK modulation and demodulation processes are relatively inexpensive. The ASK technique is also commonly used to transmit digital data over optical fibre. For LED transmitters, binary 1 is represented by a short pulse of light and binary 0 by the absence of light.