Charts from Stallings, modified and added to

1

Communications Systems, Signals, and Modulation

Session 2B

Nilesh Jha

Communication Systems Context Data or Source Information - entities that convey meaning, or

information If digital, it can be referred to in 1’s and 0’s, in bits If analog, it can get digitized and formatted into bits, or transduced into an electrical signal

(eg voice over a landline phone)

Signals - electric or electromagnetic representations of data Also called waveforms sometimes Voltages changing over time and traveling to a destination Frequencies from DC (0 Freq.) to beyond light (about 10^15 cycles/sec) --- Most wireless

is VHF, UHF, Microwaves and Millimeter Waves

Transmission - communication of data by the propagation and processing of signals

Transmitters and Receivers -- terms may include formatting also and the full chain, or only the RF elements

Communications System Model

InformationSource and

Source Encode

ChannelEncode

Modulate Multiple Access Transmitter

Source Decode and Information

Sink

Channel Decode

Demodulate Multiple Access Receiver

CommunicationsChannel

Message Bits

Channel Bits

Signal/WaveformRFSignal

Source Transmitter

ReceiverDestination

Also Format

Also Format

Communications Systems Elements (1) Information Source and Destination ---- Message symbols

Sources can be Analog (Voice, Video), or Digital (Computer Data, Digitized Voice, etc) Digital Sources represented by 1’s and 0’s -- may require some formatting, eg, from text Analog Sources are transformed into an analog electrical signal, or Digitized into 1’s and 0’s This source data can be multiplexed with other sources, and demultiplexed at destination

Mux/Demux could also happen at later stages in system There could be other processes like encryption, and other formatting (see next page)

Source Coding and Decoding Involves companding/compressing the analog voice or video, or digitally coding the digital

source data for data compression --- significant savings -- lossy or not lossy

Channel Coding and Decoding Critically important in wireless, it digitally inserts or modifies bits to help minimize the

effects of channel induced errors - error detection and/or error correction codes Modulate and Demodulate --- Turns bits Into Waveforms and Back

It transforms the digital sequence of 1’s and 0’s into a signal or waveform, using various means to carry the bit information

Modulation can be into baseband waveform (eg, voltage pulses representing 1’s and 0’s) or into carrier waveforms for wireless transmission

eg, Analog: AM or FM -- Digital: ASK, PSK, FSK

Communications Systems Elements (2)

Multiple Access Communications Media often shared among different signals (could be one user per signal, or more if some

users muxed into one signal -- eg, cell phone is one, T1 many) Requires physical means to distinguish the different signals A channel defines the physical path and characteristics for one or more signals

Usually the ‘physical channel’ refers to what one signal uses Sometimes ‘channel’ and ‘path’ used interchangeably --- refers to physical characteristics

Multiple Access technique refers to the mechanism used to distinguish or separate the different channels

Frequency, Time, Code or Space --- eg, FDMA (Frequency Division Multiple Access), TDMA and CDMA are common techniques in telecom and in cellular, also SDMA

Spatial separation, eg, different antennas into different sectors in a cell basestation --- SDMA

Transmitter and Receiver At end of modulation step the signal is transformed (converted) into a signal at the the carrier frequency to

be transmitted -- it often then requires some additional RF filtering and amplification and then coupling to an antenna, which radiates --- opposite in reverse at receiver

Often everything in chain except source and sink is labeled transmitter and receiver, or transceiver ISO Layer 2 (Data Link) and Layer 3 (network) Processing --- Often part of the source and

sink process, as part of formatting --- may include packet and frame formatting In cellular, ‘Radio Frames’ can be prepared after source coding and before modulation

Electromagnetic Signal or Waveform

Function of time Variation in time is what carries the information

--- the temporal view Can also be expressed as a function of

frequency Signal can be thought of as consisting of

components at different frequencies --- the spectral components of the signal -- the frequency view

Time-Domain Concepts Analog signal - signal intensity varies in a smooth

fashion over time No breaks or discontinuities in the signal

Digital signal - signal intensity maintains a constant level for some period of time and then changes to another constant level

Periodic signal - analog or digital signal pattern that repeats over time s(t +T ) = s(t )

where T is the period of the signal

Time-Domain Concepts Aperiodic signal - analog or digital signal

pattern that doesn't repeat over time Peak amplitude (A) - maximum value or

strength of the signal over time; typically measured in volts --- also, simply amplitude

Frequency (f ) Rate, in cycles per second, or Hertz (Hz) at

which the signal repeats

Time-Domain Concepts Period (T ) - amount of time it takes for one

repetition of the signal T = 1/f

Phase () - measure of the relative position in time within a single period of a signal

Wavelength () - distance occupied by a single cycle of the signal Or, the distance between two points of corresponding

phase for two consecutive cycles

Sine Wave Parameters General sine wave

s(t ) = A sin(2ft + ) Figure 2.3 shows the effect of varying each of the three

parameters (a) A = 1, f = 1 Hz, = 0; thus T = 1s (b) Reduced peak amplitude; A=0.5 (c) Increased frequency; f = 2, thus T = ½ (d) Phase shift; eg, = /4 radians (45 degrees)

note: 2 radians = 360° = 1 period Sometimes cosines are used --- simply sines offset by 90

degrees sin(+90 degrees)=cos

Sine Wave Parameters

Time vs. Distance An EM signal travels over space, and at each point in space

it varies over time --- c=/T (distance over time); So c=f* , since f=1/T

When the horizontal axis is time, as in Figure 2.3, graphs display the value of a signal at a given point in space as a function of time

With the horizontal axis in space, graphs display the value of a signal at a given point in time as a function of distance --- ie, as it travels At a particular instant of time, the intensity of the signal varies as

a function of distance from the source

Frequency-Domain Concepts Fundamental frequency - when all frequency

components of a signal are integer multiples of one frequency, it’s referred to as the fundamental frequency

Spectrum - range of frequencies that a signal contains Absolute bandwidth - width of the spectrum of a

signal Effective bandwidth (or just bandwidth) - band of

frequencies that most of the signal’s energy is contained in

Frequency-Domain Concepts Any electromagnetic signal can be shown to

consist of a collection (technically: a sum) of periodic analog signals (sine waves) at different amplitudes, frequencies, and phases

This is Fourier analysis s(t)=Sum over all k ((amplitude of freq. sub

k)*(sine wave at freq. sub k))

Relationship between Data Rate and Bandwidth

The greater the bandwidth, the higher the information-carrying capacity

Note Signals have limited bandwidth

the signal generation and transmission system will limit the bandwidth --- components can only pass up to some frequency

Normally, the greater the bandwidth the greater the complexity of the circuit and the cost

HOWEVER, limiting the bandwidth creates distortions One may create large bandwidth signals with lots of information, but as

they are passed through the transmission circuits they may be further band limited --- this can cause distortion

Examples of Analog and Digital

Information Sources or Data Analog

Video Audio

Digital Text Integers

Analog Signals A continuously varying electromagnetic wave that

may be propagated over a variety of media, depending on frequency

Examples of media: Copper wire media (twisted pair and coaxial cable) Fiber optic cable Atmosphere or space propagation --- Wireless

Analog signals can carry/propagate analog and digital data Created and formed through modulation

Digital Signals (Stallings) Stallings: a sequence of voltage pulses that may be transmitted over a

copper wire medium --- baseband More often (not Stallings) the term is used to refer to either analog or

digital signal which carries digital data Over the air the signal is always analog (or continuous), but if it carried digital

data it is often labeled as a digital signal On wires it is cheaper than analog signaling, eg, ethernet Less susceptible to noise interference Suffer more from attenuation (Stallings) Digital signals can carry analog and digital data

through wires-- has DC and low frequencies (baseband)

Digital Signals, Modulation and Transmission

Note: Many people refer to the term ‘digital signals’ whenever it carries digital data, even if it has really been modulated into an analog signal (sometimes called carrier modulation, or modulation into a carrier)

Many refer to the above as digital transmission Usually digital modulation refers to modulation of

digital data into an analog signal Even if the digital data is a representation of voice or video

The term ‘analog data’ is sometimes NOT used --- eg, for voice sound waves it is usually referred to as the voice sound signal

In next 2 Stallings charts anything on the left is supposed to be ‘data’

Analog Signaling

Digital Signaling

Reasons for Choosing Data and Signal Combinations

Digital data, digital signal Equipment for encoding is less expensive than digital-to-analog equipment --- used for wired comms, eg,

LAN

Analog data, digital signal Conversion permits use of modern digital transmission and switching equipment --- eg, digitized voice over

wires Sometimes used to refer to digitized voice or digitized video, as a digital signal, later transmitted by

modulating a carrier with it (see last subbullet at bottom)

Digital data, analog signal Some transmission media will only propagate analog signals Examples include optical fiber and digital wireless transmission

Analog data, analog signal voice over phone line, FM radio, TV and analog wireless

Note: Digital signal from one step can be passed through a next step, an analog modulation, to be transmitted

eg, voice is digitized in digital cell phones and then modulated into an analog signal for transmission