Digital Carrier Systems

1

EE 442 – Spring SemesterLecture 12

1 0 1 1 0 1

1 0 0 1 0 1

(4 states)

ON-Off Keying

2

Digital Carrier Systems

In the last lecture we studied “baseband” digital signals; that is, the

modulating signal m(t) have not been frequency shifted.

However, for wireless and satellite communications we must use higherfrequencies to transmit and receive communication signals.

Now we require a modulator and a demodulator – together they form a “modem.”

There are two basic forms of carrier modulation – they are (1) amplitude modulation and (2) angle modulation (phase and frequency modulation).We have already studied both under the heading of analog modulation.

3

Example: Amplitude Shift Keying (ASK)

( )( )cosASK Cm t t =

This is binary amplitude shift keying (BASK).

m(t)

4

Example of Multilevel ASK with 2-Bit Coding

http://www.tmatlantic.com/encyclopedia/index.php?ELEMENT_ID=10420

This is multilevel amplitude shift keying.

Symbols 00, 01, 10 & 11 translate into four amplitude levels.

Bit 1

Bit 2

5

Band Limiting Softens the Edges of ASK Waveforms

http://www.slideshare.net/Zeolite27/dc-ppt-final

Notice the similarity between ASK and analog AM because the amplitude

of the modulated signal is proportional to m(t).

m(t)

This is the more realistic case for ASK communication systems.In fact, all waveforms are softened by bandwidth limitations

of transceiver equipment and the channel itself.

time

6

Next: Phase Shift Keying (PSK)

Angle modulation gives rise to both phase modulation and frequency modulation.

Starting with phase modulation; this is generally known as “phase shift keying.”

http://electronicdesign.com/communications/understanding-modern-digital-modulation-techniques

m(kTb) = +1

m(kTb) = -1

Example:

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Constellation Diagram For PSK

( )cos( )C CA m t t =cos( ) for ( ) 1C C bA t m kT = +

cos( ) for ( ) 1C C bA t m kT + = −

PSK

We can also express as I and Q components.

Q

I

A special case: on-off keying (OOK)

0

I

Q

OOK

8

Expressing PSK in I and Q Components

( )cos forPSK C C k b b bA t kT t kT T = + +

For PSK we can write,

This is in polar form (I and Q)

For binary PSK we have k = 0 or radians – there is no Q component.

This is 2-QAM, but we don’t use this terminology for binary PSK.

Note: Quadrature amplitude modulation (QAM) is a mixture of bothamplitude modulation and phase modulation.

( ) ( )

( )

cos( )cos sin( )sin

Therefore,

cos sin( ) for

PSK C k C C k C

PSK k C k C b b b

A t A t

I t Q t kT t kT T

= −

= + +

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Binary PSK (BPSK) Transmitter and Receiver

Carrier

cos(ct)Balanced

Modulator AmplifierBPF

LPFNRZ Datainput

PSK

BPSK Modulator:

LPF S&H

+

cos(ct)

PSKd(t)

Comparator

Binary dataoutput

( ) cos[2 ( )] cos[ ( )]Cr t B t t B t + +

BPSK Demodulator:

Sample atcenter ofsymbol

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Binary PSK (BPSK) Waveforms at the Receiver

Without noise With noise

After Lawrence Burns, “Digital Modulation and Demodulation,” Chapter 4in RF and Microwave Circuit Design for Wireless Communications, editedby Lawrence E. Larson, Artech House Publishers, 1996. Pages 99 to 233.Lawrence Burns was an engineer at 3COM.

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BPSK Waveforms and Noise

Sampled data points:

Constellation Diagram

12

https://www.cdt21.com/resources/Modulation/modulation_FSK.asp

Frequency Shift Keying (FSK):

Continuous-Phase Frequency Shift Keying (CPFSK):

aka MSK

Bit 0

Bit 1

Bit 0

Bit 1

1 0 1 0

Frequency Shift Keying (FSK)

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Minimum shift keying (MSK) is a form frequency modulation based on a system called continuous-phase frequency-shift keying (CPFSK).MSK advantages are (1) better spectral efficiency as compared to other modes, and (2) it allows power amplifiers to operate in saturation enabling higher levels of efficiency.

Minimum Shift Keying (MSK)

https://www.electronics-notes.com/articles/radio/modulation/what-is-msk-minimum-shift-keying.php

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Minimum Shift Keying (MSK)

( )

( )

0

0 0

1 1

1 2

0

1

For Bit 0 (frequency ) we write,

2( ) cos 2 over interval 0

For Bit 1 (frequency ) we write,

2( ) cos 2 over interval 0

For orthogonality we require ( ) ( ) 0

Define the f

Tb

bb

b

bb

b

f

Es t f t t T

T

f

Es t f t t T

T

s t s t dt

=

=

=

0 1

1 2

1requency separation:

2

is the minimum separation to ensure orthogonality to hold.

The carrier frequency is defined to be 2

C C

b

f f fT

f ff f

= − =

+=

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MSK Modulation and Demodulation

MSK Modulator:

VoltageControlledOscillator

AmplifierBPF

NRZ Datainput

FSK

RFOutput

Vcontrol

t

Amplifier LPFBPF

FrequencyDiscriminator

FSK+ m(t)

Comparator

Binary dataoutput

MSK Demodulator:

VCO

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Gaussian Minimum Shift Keying (GMSK)

Gaussian Minimum Shift Keying (GMSK) modulation is a modified

version of the Minimum Shift Keying (MSK) modulation where the

phase is further filtered through a Gaussian filter to smooth the

transitions from one point to the next in the constellation of states.

A Gaussian filter’s impulse response is a Gaussian function (or an

approximation to it, because a true Gaussian response is physically

unrealizable). Gaussian filters have no overshoot in responding to a

step function input and minimize the rise and fall times of pulses.

http://home.scarlet.be/~pc030062/extra_info/MSK%20-%20GMSK.pdf

GMSKwaveform

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Gaussian Minimum Shift Keying (GMSK)

Advantages:

(1) Constant amplitude (better noise resistance)(2) Spectrally efficient (spectrum falls as 4th power of frequency)(3) Good Bit Error Rate performance(4) Self-synchronizing capability

Applications:

(1) Automatic Identification System in maritime navigation(2) Bluetooth headsets(3) Standard GSM cellular (usage is > 40% of cell phones in world)

Uses BTb = 0.3 for Gaussian filter giving 99% of transmission power in bandwidth of 250 kHz. The GSMbandwidth per channel is 200 kHz and GSM transmitsat 270 kb/sec (limited by ISI problems). B = 81.3 kHzand Tb = 3.7 microseconds.

Tradeoff: Smaller BTb product gives faster spectrum falloff at expense of longer pulse duration leading to increased ISI.

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https://en.wikipedia.org/wiki/Minimum-shift_keying#/media/File:GMSK_PSD.png

Comparing Spectrums of MSK with GMSK (BTb = 0.5 and 0.3)

MSK

Spectral efficiency = 1.35 bps/Hz

19https://www.electronics-notes.com/articles/radio/modulation/what-is-gmsk-gaussian-minimum-shift-keying.php

Generating GMSK Signals

GMSK modulator using VCO

GMSK modulator using an I-Q modulator

I

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Second Generation (2G) Mobile Phones

http://en.wikipedia.org/wiki/Global_System_for_Mobile_Communications

❑ Second Generation (2G) introduced in early 1990s to replace 1G

❑ Digital (rather than analog) transmission of voice

❑ Designed for circuit-switched networks (voice centric standard)

❑ Uses 900 MHz and 1800 MHz frequency bands

❑ Dominant 2G phone standard: GSM (Global System for Mobile)

❑ Introduced SMS (aka “text messaging”)

❑ GSM uses SIM card containing user ID (Subscriber Identity Module)

❑ Began in Europe; rapidly expanded around the World

http://sharingmythoughts-

ben.blogspot.com/2010/09/histor

y-of-mobile-phone.html

Nokia 2G

Mobile

Phones

GSM is still

widest used

cell phone

standard

Worldwide!

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Comparing PSDs For Binary ASK, PSK and FSK

FSK

PSK

ASK

Pow

er s

pec

tral

den

sity

[w

atts

/Hz)

Figure 7.32 in Lathi & Ding, Modern Digital and Analog Communication Systems. 4th ed., 2009.

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Multilevel Frequency Shift Keying (FSK)

This animation shows frequency shift keying of the sinusoidal carrier signal. A two-digit code modulates the carrier signal frequency into four frequencies

Symbol Binary code Frequency

“0” 00 4 kHz

“1” 01 3 kHz

“2” 10 2 kHz

“3” 11 1 kHz

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Quadrature Phase Shift Keying (QPSK)

Sometimes this is known as quadrature-phase PSK, 4-PSK, or 4-QAM. QPSK uses a circle of four points on the constellation diagram, equi-spaced around a circle. With four states, QPSK can encode two bits per symbol,

Q

I

I = -1; Q = -1 I = +1; Q = -1

I = +1; Q = +1I = -1; Q = +1

( ) ( )cos sinQPSK C CI t Q t = +

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i(t)

q(t)

Digital I/Q ModulationAnticipating our coverage of digital communication systems

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Simple QPSK Modulator

QPSK modulator using delay lines to set phase delay:

+45

+135

-135

-45

Delay lines (depend upon fC)

Switch Decoder and Driver

RF Input RF Output

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Widely-Used QPSK Modulator

QPSK Modulator

AmplifierBPF

LPF

NRZ Datainput

PSK

LPF

Serial-to-ParallelParser

cos( )Ct

sin( )Ct

I

Qt

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Basic Building Block: Quadrature Modulator

cos( )Ct

sin( )Ct

I

Q

I and Q can beeither analog or

digital signals

2 2

1

( ) cos( ( ))

( )where ( ) tan

( )

Ct I Q t t

Q tt

I t

−

= + +

=

( )t

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Data Demultiplexer (Serial to Parallel) For QPSK

Demodulator uses three D-type flip-flops and is driven by both clockand one-half clock rates.

Q

I

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QPSK Demodulator

C/R = clock/carrier recovery

STR = symbol timing recovery

PSK(t)

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M-ary Signaling With Quadrature Amplitude Modulation (QAM)

Quadrature Amplitude Modulation, QAM is a form of modulation that is a combination of phase modulation and amplitude modulation. The QAM scheme represents bits as points in a quadrant grid know as a constellation map.

16-ary QAM

APSK definitionDefinition: Amplitude and Phase-Shift Keying, APSK, is a digital modulation scheme that uses both the amplitude and the phase changes of on the carrier signal to provide the data transport mechanism for the information. Also called QAM.

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Number-Bases in M-ary Constellations

Variants of QAM are also used for many wireless and cellular technology applications. In addition, 64-QAM and 256-QAM are commonly used in digital cable television and cable modem applications. In the US, 64-QAM and 256-QAM are the mandated modulation schemes for digital cable as standardized by the SCTE in the standard ANSI/SCTE 07 2000.

(aka 4-QAM)

SCTE = Society of Cable Telecommunications Engineers

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Bits/Symbol and Symbol Rates

ModulationBits per Symbol

Symbol Rate

BPSK 1 1 bit rate

QPSK 2 1/2 bit rate

8-PSK 3 1/3 bit rate

16-QAM 4 1/4 bit rate

32-QAM 5 1/5 bit rate

64-QAM 6 1/6 bit rate

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http://farhek.com/jd/i1t1154/up-to/7i45u1/

Greater Number of States Leads to Greater Demand Upon Communication System

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Bit Error Rate versus Energy/Noise Ratio

0/ ( )bE N dB

energy per bit-to-noise power ratio

BER = Bit Error Rate

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Signal-to-Noise Ratio vs. Energy/Bit-to-Noise Ratio

In analog and digital communications, signal-to-noise ratio, usually written S/Nor SNR, is a measure of signal strength relative to background noise strength. The ratio is usually expressed in decibels (dB) and equals 10log10[S/N].

Another metric that is often more useful in digital systems is the energy perbit-to-noise power ratio, denoted by Eb/N0.

Define: Rb = bit rate (in bits per second)S = total signal power (watts)Eb = energy per bit (in joules/bit)N = total noise power (over entire bandwidth B in Hz)N0 = noise spectral density (N = N0B where B = bandwidth)

Then,

Increasing the data rate Rb increases the SNR. However, in general it also increases the noise in the denominator, which lowers the SNR.

= = = 0

and andb b bb

b b

E R ES SE SNR

R N R N N B

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WiFi systems use two primary radio transmission techniques.

802.11b (≤ 11 Mbps) − The 802.11b radio link uses a direct sequence spread spectrum technique (DSSS) called complementary coded keying (CCK). The bit stream is processed and then modulated using Quadrature Phase Shift Keying (QPSK).

802.11a and 802.11g (≤ 54 Mbps) − The 802.11a and g systems use 64-channel orthogonal frequency division multiplexing (OFDM). The transmitter encodes the bit streams onto 64 subcarriers using Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), or one of two levels of Quadrature Amplitude Modulation (16-QAM, or 64-QAM).

What Modulation Schemes Does Wi-Fi Use?

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Circuit Switched Networks vs. Packet-Switched Network

https://www.photonics.com/Articles/Toward_Optical_Packet_Switching/a24582

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Circuit-Switched Network

TelephoneSwitch

TelephoneSwitch

TelephoneSwitch

TelephoneSwitch

TelephoneSwitch

TelephoneSwitch

TelephoneSwitch

Many paths are possible, but only one is selected per

call.

Once a connection is established, this

connection is maintained until call

is terminated.

Caller

= Dedicated connection (point-to-point)

Subscriber lines(or local loops)

Trunks(links between

Exchanges)

Central Office

Central Office

Central Office

PSTN = public switched telephone network

Full Duplex

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Packet Switched Network

Internet

Many paths possible for a single message as packets are routed to

the destination.

Packets are routed according to the best path available at the

time.

Receiver(destination)

Sender(source)

Message broken into packets andeach addressed

Packets sequentiallyreassembled

to revealmessage

= Packet

Routeror Switch

(Data Packet or “Datagram”)

Large array of routers and data links.

Packet route

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Network Organization

Centralized Network Decentralized Network

(e.g., PSTN)

Distributed Network

(e.g., Internet)

In 1962, Paul Baran (RAND Corp.) envisioned a network of unmanned nodes using intelligent switches to route data node to node to their final destinations. Baran called this "hot-potato routing" or distributed communications. This was implemented in ARPANET which became the Internet.

Concept of hardened networks to deal with disasters.

A networkof routers

A highly vulnerablenetwork

41After Kurose & Ross, Computer Networking, 5th Edition, Addison-Wesley, New York, 2010.

National & Global ISPs

Local orRegional

ISP

MobileCellular

NetworkWireless

HomeNetwork

Corporate orUniversityNetwork

HomeNetwork

Example:

sonic.net

Example:

Keysight Technologies

Example:

Verizon

Example:

Sprint

ModemPacket Switch or Router

WirelessRouter

Cell Phone

Cellular

Base Station

VoIP Phone

Host

Mobile Laptop Server

Key:

A “representative” section of the Internet:

(ISP = Internet Service Provider)

Internet is a “Network of Networks”

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Feature Circuit Switching Packet Switching

Dedicated Path Yes

No

Path FormationPath dedicated for one conversation

Route is established on a per packet basis of the conversation using datagram (or per conversation with virtual circuit)

Delay Call setup delay Packet transmission delay (call setup delay for virtual circuit)

Bandwidth Type Fixed Bandwidth Dynamic bandwidth

Overload EffectsStops call establishment

Increases packet delay (can block call establishment and increase packet delay with virtual circuit)

Comparing Packet Switching to Circuit Switching

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Packet-Switched Network Operation

• Adaptive routing – routers chose the best path by examining traffic loading along available paths. Routers create a “routing table” for the packet travel.

• All users share the same network resources.

• Packet-switching is more efficient than circuit-switching in networks when data is bursty (i.e., variable delays interspersed with periods of data transmission). More “efficient” means a better utilization of the network resources.

This is an example of

“bursty” data

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Layer Pictorial View of Protocol Data Unit Entity

ApplicationData or

Message

Transport Segments

Internet or Network

Packets orDatagrams

Network Access

Frames

Data

DataTransport

Header

DataTransport

HeaderNetwork Header

DataTransport

HeaderNetwork Header

Frame Header

Frame Trailer

Protocol

SMTPHTTP, DNS

TCPUDP

IP

EthernetModem

FDDI

Number of segments 1

Bits transmitted over channel medium

TCP/IP Protocol Architecture Model

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An Internet Packet and its Headers

• In IPv4, each packet is restricted to 1,500 bytes of data (i.e., payload)

• Each packet consists of the application data and headers

• The headers contain control and routing information such as:

– Source IP address and destination IP address

– Packet numbering for reconstruction at destination

• Every computer on the Internet has the TCP/IP program. The client/server model is used on the Internet.

• TCP (Transmission Control Protocol) puts the data or message into packets at the source and reassembles the data or message at the destination

• IP (Internet Protocol) does the packet addressing for the routing over the Internet

Application DataIP header TCP/UDP header

Internet Packet

The rules that govern communication – any form – are called “protocols.”

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TCP versus UDP Transmission

TCP is “reliable” because it has flow & congestioncontrol, retransmission, &uses acknowledgements.

UDP does not use these because it is focused onlyupon sending packets.

UDP

TCP and UDP Analogies:

Post OfficeVerifies deliveryRegistered

Letter

TCP

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IPv6

How do you say 340,282,366,920,938,463,463,374,607,431,768,211,456?

It is 340 undecillion, 282 decillion, 366 nonillion, 920 octillion, 938

septillion, 463 sextillion, 463 quadrillion, 607 trillion, 431 billion,

768 million, 211 thousand, 456.

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▪ Uses computer-based technologies

▪ Highly interconnected but distributed in practice

▪ Networks interconnected thru gateways and access points (i.e., routers, link-layer switches, etc.)

▪ Each network stands on its own (TCP/IP protocols do not dictate internal changes to networks)

▪ TCP/IP is independent of the data type or the transport medium

▪ TCP/IP protocols are not proprietary (all can freely use it)

▪ Effectively little regulation applies to Internet (in fact, users of the Internet generally oppose regulation)

▪ There is NO direct global control of the Internet

Internet Attributes

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Questions

50

Bandwidth Efficiency (aka Spectral Efficiency)

Given: Eb = energy per bit (joules or ergs)Rb = bit rate (bits/second)B = bandwidth of baseband signal (Hz)N0 = noise spectral density (watts/Hz)N = noise power = N0B (watts)

Therefore, EbRb = total signal power

We define the Bandwidth Use Efficiency as

In general,

bits/second

HzbR

B

2log 1b b bR E R

B NB

= +

Example:GSM Digital Cellular

Data rate = 270 kb/sB = 200 kHz, thus

Bandwidth efficiency =1.35 bits/sec/Hz

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http://wiki.ucalgary.ca/page/Courses/Computer_Science/CPSC_441.W2014/Chapter_1:_Computer_Networks_and_The_Internet.html