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By Rehab Oct 2025
1stLecture Contents Signal Types. General block diagram of digital communication. 1. 2.
Lecture overview îSignals îDigital signal îAnalog signal îPeriodic & Aperiodic signals îSome network terminologies îTypes of analog signals îTransmission Impairments
Signals îTo be transmitted, data must be transformed to electromagnetic signals. îData can be either digital or analog. îAnalog refers to something that is continuous– a set of specific points of data and all possible points between. e.g. human voice îDigital refers to something that is discrete – a set of specific points of data with no other points in between. E.g. digital data. îTypes of Signals Ø Digital Signal Ø Analog signal
Digital & Analog Signals îAnalog Signal Ø It is a continuous wave form that changes smoothly over time. Ø As the wave moves from value A to value B,it passes through and includes an infinite number of values along its path. îDigital Signal Ø A digital signal is discrete. Ø It can have only a limited number of defined values, often as simple as 1 and 0. Ø The transition of a digital signal from value to value is instantaneous, like a light being switched on and off.
Digital & Analog Signals Vertical axis: value or strength of a signal Horizontal axis: Passage of time
Periodic & Aperiodic Signals îBoth analog and digital signals can be of two forms: ØPeriodic ØAperiodic(non-periodic ) Note:- In data communication, we commonly use periodic analog signals and aperiodic digital signals.
Periodic Signal îA signal is a Periodic signal if it completes a pattern within a measurable time frame, called a period. îA period is defined as the amount of time required to complete one full cycle. îThe completion of one full pattern is called a cycle. îThe duration of a period represented by T may be different for each signal but it is constant for any given periodic signal.
Periodic Analog Signal Period( T )
Aperiodic Signals îAn Aperiodic signal changes without exhibiting a pattern or cycle that repeats over time. îAperiodic signal can be decomposed in to infinite number of periodic signals.
Types of analog signals îSimple Analog signals(sine wave) îComposite Signals
Simple analog signal / sine wave îThe sine wave is the most fundamental form of a periodic analog signal. îVisually a simple oscillating curve, its change over the course of a cycle is smooth and consistent, a continuous, rolling flow. îSine wave can be described by three characteristics: Ø Amplitude Ø Period Or frequency Ø Phase
Amplitude îThe amplitude of a signal is the value of the signal at any point on the wave. îIt is equal to the vertical distance from a given point on the wave form to the horizontal axis. îThe maximum amplitude of a sine wave is equal to the highest value it reaches on the vertical axis. îAmplitude is measured in either, volts ,amperes or watts.
Frequency îFrequency is the number of cycles/Periods in one second. îFrequency is the relationship of a signal to time. îFrequency is the measure of the rate of change. îElectromagnetic signals are oscillating wave forms that is they fluctuate continuously and predictably above and below a mean energy level. îThe rate at which a sine wave moves from its lowest to highest level is its frequency. îRate of change of signal with respect to time. îThe unit of frequency is Hertz.
High & low Frequency îIf the value of a signal changes over a very short span of time, its frequency is high. îIf it changes over a long span of time, its frequency is low. îIf a signal does not change and it maintains a constant voltage level the entire time it is active the signal is having 0 Hz frequency. îIf a signal changes instantaneously, then its frequency is infinite.
Period îPeriod refers to the amount of time (in seconds) a signal needs to complete one cycle . îPeriod and Frequency are inverse of each other. ØT=1/f Øf=1/T
Phase îThe term phase describes the position of the waveform relative to time zero. îIf we think of the wave as something that can be shifted backward or forward along the time axis, phase describes the amount of that shift. îIt actually indicates the status of the first cycle. îPhase is measured in degrees or radians.
Time and frequency domains îThe time-domain plot shows changes in signal amplitude with respect to time(it is an amplitude versus time plot). îPhase and frequency are not explicitly measured on a time-domain. îA frequency domain-plot shows the relationship between amplitude and frequency.
Composite Signals îMany useful wave forms do not change in a single smooth curve between a minimum and maximum amplitude. îThey jump, slide, wobble, spike and dip. îAs along as any irregularities are consistent, cycle after cycle, a signal is still periodic and logically must be describable in the same terms used for sine waves. îAny periodic signal, no matter how complex can be decomposed in to a collection of sine waves,each having a measurable amplitude,frequency and phase.
Frequency spectrum & Bandwidth îThe frequency spectrum of a signal is the collection of all the component frequencies it contains and is shown using a frequency domain graph. îThe frequency spectrum of a signal is the combination of all sine wave signals that make up that signal. îThe bandwidth of a signal is the width of the frequency spectrum. îBand width refers to the range of component frequencies and frequency spectrum refers to the elements within that range. îThe bandwidth is the difference between the highest and the lowest frequency within the range of frequencies the signal support.
Bit rate and bit interval îThe bit interval is the time required to send one single bit. îThe bit rate is the number of bit intervals / bits per second. îThe unit for bit rate is bits per second(bps).
Through put îThe throughput is the measurement of how fast data can pass through an entity(such as a point or a network). îIf we consider this entity as a wall through which bits pass, throughput is the number of bits that can pass this wall in one second.
Propagation Speed & Propagation Time î Propagation Speed measures the distance a signal or a bit can travel through a medium in one second. î The propagation speed of electromagnetic signals depends on the medium and on the frequency of the signal. î Propagation time measures the time required for a signal (or a bit) to travel from one point of the transmission medium to another. î The propagation time is calculated by dividing the distance by the propagation speed. î Propagation Time = Distance/Propagation speed
Wave length îWave length is the distance a simple signal can travel in one period.
Attenuation îAttenuation means loss of energy. îWhen a signal simple or complex travels through a medium it loses some of its energy so that it can overcome the resistance of the medium. îThat’s why a wire carrying electrical signals gets warm. îThis problem is called attenuation. îTo compensate for this loss, amplifiers are used to amplify the signal.
Distortion îDistortion means that a signal changes its form or shape. îDistortion occurs in a composite signal,made of different frequencies. îEach signal component has its own propagation speed through the medium and therefore its own delay in arriving at the destination.
Noise î The external energy that corrupts a signal. î Several types of noise may corrupt the signal. î Thermal noise is the random motion of electrons in a wire that creates an extra signal not originally sent by the transmitter. î Induced noise comes from sources such as motors and appliances. These devices act as sending antenna and the transmission medium acts as a receiving antenna. î Crosstalk is the effect of one wire on the other. î Impulse noise is a spike(a signal with high energy in a very short period of time) that comes from power lines, lightening and so on.
Class will Follow This Diagram Channel coding Symbol mapping TX filter DAC Upconversi on Info bits Code d bits IQ symbols RF Analog baseban d Passband Channel Sync Down- conversion RX filter and ADC Equalizatio n Channe l decodin g Symbol demod 42
What is Digital Communications? Transmission of digital data through a channel Analog waveform Digital data 011000101… Digital data 011000101… Transmitter Channel Receiver 43
Digital Communications is Everywhere! Systems vary with Scales Data rates Channel media, Ranges, … 44
What do Communications Theorists Do? Try to make communication: Reliable, Fast, Cheap, … Basic tools in this class: Look at point-to-point links. Model transmission and reception as a statistical estimation problem. Develop mathematical methods for good communication 45
