basics of modulation · •modulation is the process of varying one or more properties of a...

BASICS OF MODULATIONOverview of different modulation techniques

WEEK 3BY : ALI MUSTAFA

CONTENTS

• Modulation• Modulation Techniques

– Analog Modulation– Digital Modulation

• Demodulation and Detection• Factors Affecting Choice of Modulation• Modulation Trade-Offs• Comparisons of Digital Modem• Modem Block Diagram

MODULATION

• Modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted.

– The three key parameters of a periodic waveform are its amplitude (volume), its phase (timing) and its frequency (pitch).

• Modulation of a sine waveform is used to transform a baseband message into a passband signal, for example low-frequency audio signal into a radio-frequency signal (RF signal)

MODULATION

• The technique of superimposing the message signal on the carrier is known as modulation.

• That is, modulation is the process by which a parameter of one signal (carrier) is varied in proportion to the second signal (message signal).

• Let m(t) = message (or information) signalc(t) = carrier signals(t) = modulated signal (transmitted signal)

m(t) s(t)

Modulating Modulated

c(t) Carrier

Modulator

MODULATION

• The carrier c(t) is a pure sinusoidal signal generally given as

• Examination of c(t) indicate that there are 3 parameters which may be varied:– Amplitude– Frequency – Phase

• These parameters can be varied in Analog or Digital form. When varied in digital form, it is referred to as “Shifting and Keying”

MODULATION TECHNIQUES

• Difference between Analog and Digital

• There are three parts to a communication system.– The information, also called the baseband– The medium– The carrier

• Information can be divided into two forms, digital or analog.

• Analog signal is considered continuous. Its signal amplitude can take on any number of values between the signal maximum and minimum.


• Voice is analog and can take any number of volume levels between its dynamic-range which is the range of volumes your vocal cords can produce.

• Digital devices convert analog voice to a digital signal by process of sampling and quantization. The analog signal is first sampled and then quantized in levels and then each level is converted to a binary number.


• Analog Modulation

• The aim of analog modulation is to transfer an analog baseband signal, for example an audio or TV signal, over an analog bandpass channel at a different frequency (over a limited radio frequency band or cable TV network channel).

• Using the message signal to vary amplitude, frequency, phase leads to three basic types of analog modulation schemes respectively known as– Amplitude Modulation– Frequency Modulation– Phase Modulation

• These types of modulation are carrier/continuous wave modulation. • Frequency and Phase Modulation are also known as Angle Modulation.

AMPLITUDE MODULATION

• Amplitude Modulation (AM) is a technique used in electronic communication for transmitting information using radio waves in which the amplitude of the carrier wave is varied in accordance with the amplitude of the information signal.

• Amplitude Modulation is used whenever a shift in the frequency components of a given signal is desired.– E.g., transmitting voice signal (3 kHz) through electromagnetic wave

requires that 3 kHz be raised several orders of magnitude before transmission.

• AM radio broadcast transmissions contain two signals of primary importance to the user: the carrier signal and the audio signal, or the program signal.


In AM, the voltage (amplitude) of the carrier is varied by the incoming signal. In this example, the modulating wave implies an analog signal.

FREQUENCY MODULATION

• Frequency Modulation (FM) is a method of transmitting information by varying the frequency of the carrier wave in accordance with the amplitude of the input signal, the amplitude of the carrier remaining unchanged.

• FM is widely used for broadcasting music and speech, two-way radio systems, magnetic tape-recording systems and some video-transmission systems.

• In radio systems, FM with sufficient bandwidth provides an advantage in cancelling naturally-occurring noise.


A signal modifies the frequency of a carrier in FM In FM, the frequency of the carrier wave is varied by the incoming signal. In this example, the modulating wave implies an analog signal.


Difference between Amplitude and Frequency Modulation

PHASE MODULATION

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

• PM is not very widely used for radio transmissions, because it tends to require more complex receiving hardware and there can be uncertainty problems in determining whether the signal has changed phase by +180 deg or -180 deg.


Difference between Amplitude, Frequency and Phase Modulation


• Digital Modulation

• The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, for example over the public switched telephone network (where a bandpass filter limits the frequency range to between 300 and 3400 Hz), or over a limited radio frequency band.

• The purpose of digital modulation is to convert an information-bearing discrete-time symbol into a continuous-time waveform.

• The objective of a digital communication system is to transport digital data between two or more nodes.


• In radio communications this is usually achieved by adjusting a physical characteristic of a sinusoidal carrier, either the frequency, phase, amplitude or a combination of thereof.

• This is performed in real systems with a modulator at the transmitting end to impose the physical change to the carrier and a demodulator at the receiving end to detect the resultant modulation on reception.

• Digital modulation methods can be considered as digital-to-analog conversion and the corresponding demodulation or detection as analog-to-digital conversion.


• Symbols, Bits and Bauds

• A symbol is quite apart from a bit in concept although both can be represented by sinusoidal or wave functions.

• Where bit is the unit of information, the symbol is a unit of transmission energy. It is the representation of the bit that the medium transmits to convey the information.

• Imagine bits as widgets, and symbols as boxes in which the widgets travel on a truck. We can have one widget per box or we can have more.

• Packing of widgets (bits) per box (symbols) is what modulation is all about.


In communications, the analog signal shape, by pre-agreed convention, stands for a certain number of bits and is called a symbol.

Digital information travels on analog carrier.


• Bit Error Rate (BER): Better accuracy of the transmitted digital signal is measured by BER. Simply put Bit Error Rate is:

The number of Error Bits• BER= ----------------------------

The total number of Bits

• A lower Bit Error Rate implies that the signal has been more accurately transmitted and demodulated.

• A Bit Error Rate of one error in 10,000 Bits transmitted is quite normal for modulated signals. After error correction is applied, the Error further falls down to one part in 100,000 Million Bits.


• To help transmit digital bits over any significant distances, different modulation schemes have been devised.

• The requirements for broadcasting a satellite signal would be rather than different from the requirements for transmitting a digital signal over a cable network.

• The most fundamental digital modulation techniques are based on keying.

• Keying is a family of modulation forms where the modulating signal takes one of two (or more) values at all times. The goal of keying is to transmit a digital signal over an analog channel.


• There are three basic types of digital modulation techniques known as:– Amplitude-Shift Keying (ASK)– Frequency-Shift Keying (FSK)– Phase-Shift Keying (PSK)

• Amplitude-Shift Keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.– Amplitude of the carrier is switched between two or more levels according to the digital

data.

MODULATION TECHNIQUES• In ASK, the amplitude of the carrier is changed in response to information and all else is

kept fixed.

• Bit 1 is transmitted by a carrier of one particular amplitude. To transmit 0, we change the amplitude keeping the frequency constant.

• On-Off Keying (OOK) is a special form of ASK, where one of the amplitudes is zero as shown below.

Baseband Information Sequence - 0010110010 Binary ASK (OOK) Signal


• Frequency-Shift Keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave.

• The simplest FSK is binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s) information.

• With this scheme the “1” is called the mark frequency and the “0” is called the space frequency.

• In FSK, the instantaneous frequency of the carrier is switched between 2 or more levels according to the baseband digital data.


• Phase-Shift Keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave).

• Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary digits.

• Usually each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase.


• The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data.

• This requires the receiver to be able to compare the phase of the received signal to a reference signal – such a system is termed as coherent (CPSK).

• In PSK, the phase of the carrier signal is switched between 2 or more values in response to the baseband digital data.


• In PSK, we change the phase of the sinusoidal carrier to indicate information. Phase in this context is the starting angle at which the sinusoid starts.

• To transmit 0, we shift the phase of the sinusoid by 180 degrees. Phase shift represents the change in the state of the information in this case.


For digital signals, PSK uses two phases for 0 and 1 as in this example.

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basics of modulation · •modulation is the process of varying one or more properties of a...

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