Modulation Types & TechniquesToday vast amounts of information are communicated using radio communications systems. Both analogue radio communications systems, and digital or data radio communications links are used.

However one of the fundamental aspects of any radio communications transmission system is modulation, or the way in which the information is superimposed on the radio carrier.

In order that a steady radio signal or "radio carrier" can carry information it must be changed or modulated in one way so that the information can be conveyed from one place to another.

There are very many ways in which a radio carrier can be modulated to carry a signal, each having its own advantages and disadvantages. The choice of modulation have a great impact on the radio communications system. Some forms are better suited to one kind of traffic whereas other forms of modulation will be more applicable in other instances. Choosing the correct form of modulation is akey decision in any radio communications system design.

Basic types of modulationThere are three main ways in which a radio communications or RF signal can be modulated:

• Amplitude modulation, AM: As the name implies, this form of modulation involves

modulating the amplitude or intensity of the signal.

Amplitude modulation was the first form of modulation to be used to broadcast sound, and although other forms of modulation are being increasingly used, amplitude modulation is still in widespread use. Read more about Amplitude Modulation.

• Frequency modulation, FM: This form of modulation varies the frequency in line with the

modulating signal.

Frequency modulation has the advantage that, as amplitude variations do not carry any information on the signal, it can be limited within the receiver to remove signal strength variations and noise. As a result is form of modulation has been used for many applications including high quality analogue sound broadcasting.

• Phase modulation, PM: As the name indicates, phase modulation varies the phase of the

carrier in line with the modulating signal.

Phase modulation and frequency modulation have many similarities and are linked - one is

the differential of the other. However phase modulation lends itself to data transmissions, and as a result its use has grown rapidly over recent years.

Each type of modulation has its own advantages and disadvantages, and accordingly they are all used in different radio communications applications.

In addition to the three main basic forms of modulation or modulation techniques, there are many variants of each type. Again these modulation techniques are used in a variety of applications, some for analogue applications, and others for digital applications.

Angle ModulationAngle modulation is a name given to forms of modulation that are based on altering the angle or phase of a sinusoidal carrier. Using angle modulation there is no change in the amplitude of the carrier.

The two forms of modulation that fall into the angle modulation category are frequency modulation and phase modulation.

Both types of angle modulation, namely frequency modulation and phase modulation are linked because frequency is the derivative of phase, i.e. frequency is the rate of change of phase.

Another way of looking at the link between the two types of modulation is that a frequency modulated signal can be generated by first integrating the modulating waveform and then using the result as the input to a phase modulator. Conversely, a phase modulated signal can be generated by first differentiating the modulating signal and then using the result as the input to a frequency modulator.

Modulation combinationsIt is possible to use forms of modulation that combine both amplitude and angle modulation components. In this way enhancements in performance can be gained.

• Quadrature amplitude modulation, QAM: Using this form of information amplitude and

phase information are sued to carry the signal. Data is modulated onto In-phase and Quadrature elements of the signal: I & Q and the constellation forms a number of points in the two planes.

• Amplitude & Phase Sift Keying, APSK: Using APSK, the constellation can be arranged to

optimise the peak to average power ratio and fewer amplitude levels cab be set when compared to QAM. This enables RF power amplifiers to operate more efficiently.

Signal bandwidthOne key element of any signal is the bandwidth it occupies. This is important because it defines the channel bandwidth required, and hence the number of channels that can be accommodated within a given segment of radio spectrum. With pressure on the radio spectrum increasing, the radio signal bandwidth is an important feature of any type of radio emission or transmission.

The bandwidth is governed by two major features:

• The type of modulation Some forms of modulation use their bandwidth more effectively

than others. Accordingly where spectrum usage is of importance, this alone may dictate the choice of modulation.

• The bandwidth of the modulating signal: A law called Shannon's law determines the

minimum bandwidth through which a signal can be transmitted. In general, the wider the bandwidth of the modulating signal, the wider the bandwidth required.

Modulating signal typeApart from the form of modulation itself the type of signal being used to modulate the carrier also has a bearing on the signal. Analogue and data are two very different forms of modulating signal and need to be treated differently. While different formats of actual modulation may be used, the type of signal being applied via the modulator also have a bearing on the signal.

Signals for high quality stereo broadcasting will be treated differently to signals that provide digital telemetry for example. As a result, it is often important to know the signal type that needs to be carried by the RF carrier.

In order to easily describe the different types of radio emissions or transmissions, the ITU, International Telecommunications Union has defined a series of codes that easily define a radio transmission or modulation format.

These ITU radio emission designations are widely used in the definition of the types of radio transmission that are used within different portions of the spectrum and in other areas.

These ITU radio emission designations define the signal - the type of modulation, bandwidth and the type of information being carried. As such the type of radio emission, or transmission is defined and not the transmitter or the system that is used.

The ITU designation system was agreed at the 1979 World Administrative Radio Conference (WARC 79), and superseded a previous system which has now completely fallen out of use.

Radio emission types designation formatThe ITU designations for the different types of radio emissions follows a standard format. This enables anyone using the system to quickly identify the parameters of the particular transmission. Although not all elements of the system may be used every time, it has been developed so that thereis no ambiguity whatever part of the system for describing the types of radio emission is used.

The system has the following format:

BBBB 123 45

Where: BBBB are characters that define the bandwidth

Character "1" is a letter indicating the type of modulation Character "2" is a digit that indicates the type of modulating signal Character "3" is a letter indicating the type of information being transmitted Character "4" is an optional letter indicating the practical details of the transmitted information Character "5" is an optional letter indicating details about any multiplexing, if used.

Tables for the different characters 1 to 5 are given below.

Bandwidth designatorThe bandwidth designator has the format of three digits that express the significant figures, and a letter used for the decimal point.

The letters used are: H: indicates hertz k: indicates kilohertz M: indicates Megahertz G: indicates Gigahertz

Examples may include 200H for a 200 Hz bandwidth transmission, 6K00 for a 6 kHz bandwidth, and 1M25 for a 1.25 MHz wide transmission, etc..

Character 1 - type of modulationThis character describes the format for the modulation itself. It provides information about the way in which the signal is superimposed onto the carrier.

Letterindicator

Details

A Double sideband, DSB, including DSB full carrier, i.e. amplitude modulation

B Independent sideband, i.e. two sidebands present, each carrying different information

C Vestigial sideband

D Combination of AM and FM or PM, either simultaneously or in a pre-established sequence

F Frequency modulation, FM G Phase modulation, PM H Single sideband full carrier J Single sideband suppressed carrier, SSBSC K Pulse amplitude modulation, PAM L Pulse width modulation, PWM M Pulse position modulation, PPM N Un-modulated carrier P Series of pulses without modulation

Q Sequence of pulses, phase or frequency modulation within each pulse

R Single sideband with reduced or variable level carrier V Combination of pulse modulation methods W Combination of any of above X cases not covered by the above definitions It is worth noting that frequency modulation and phase modulation may also be referred to by the generic term: "angle modulation."

Character 2 - type of modulating signalThis character of the ITU designations for radio emissions details the characteristics of the modulating signal. It provides information including whether the modulation is analogue or digital and whether there is one channel of information or more being carried.

Letterindicator

Details

0 No modulating signal

1 One channel containing digital information without the useof modulating sub-carriers (excludes time division multiplex)

2 One channel containing digital information with the use of a modulating sub-carrier (excludes time division multiplex)

3 One channel containing analogue information 7 More than one channel containing digital information 8 More than one channel containing analogue information 9 Combination of analogue and digital channels X cases not covered by the above

Character 3 - type of transmitted informationThis character in the ITU designation of radio emissions details the type of information being carried. It provides some insight into the use and the way in which the information may be decoded.

Letterindicator

Details

A Telegraphy for aural reception - e.g. Morse code B Telegraphy for automatic reception, i.e. machine decoded C Facsimile D Data transmission, telemetry or command

E Telephony, i.e. voice or music intended for human listening(including sound broadcasting)

F Video - television W Any combination of above X None of above

Character 4 - details of informationThis character provides some insight into the format of the information - its coding and therefore the requirements for decoding he information once it has been demodulated.

Letterindicator

Details

A Two condition code - elements vary in quantity and duration

B Two condition code - elements fixed in quantity and duration

C Two condition code - elements vary in quantity and duration - error correction included

D Four-condition code in which each condition represents a signal element (or one or more bits)

E Multi-condition code in which each condition represents a signal element (of one or more bits)

F Multi condition code - one character represented by one or more conditions

G Monophonic broadcast quality sound H Stereophonic or quadraphonic broadcast quality sound

J Commercial, non-broadcast, quality sound (but excluding K & L below)

K Sound of commercial quality with the use of frequency inversion and/or band-splitting employed

L Sound of commercial quality with independent FM signals to control the level of the demodulated signal, e.g. pilot tones used to control demodulation process

M Monochrome images or video N Full colour images or video W Combination of the above X Cases not covered by the above descriptions

Character 5 - details of multiplexingIncreasingly radio channels are used to carry more than one stream of information, or they may be required to share the channel with other users or streams of information. This character in the ITU designation of radio transmissions provides information about any multiplexing.

Letterindicator

Details

C Code-division multiplex (including code expansion techniques such as direct sequence spread spectrum)

F Frequency-division multiplex N None used T Time-division multiplex W Combination of frequency division and time division X Other types of multiplexingNone of above