EE442 Spring 2017 Lecture 12 - Sonoma State UniversityFrequency Shift Keying (FSK) ... the same,...
Transcript of EE442 Spring 2017 Lecture 12 - Sonoma State UniversityFrequency Shift Keying (FSK) ... the same,...
Digital Carrier Systems
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EE442 – Spring 2017 Lecture 12
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Digital Carrier Systems
So far we have studied baseband digital signals; that is, the modulating signal m(t) had not been frequency shifted.
However, for wireless and satellite communications we must use higher frequencies to transmit and receive communication signals.
Now we require a modulator and a demodulator – together they make 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 of these under the heading of analog modulation.
Lathi & Ding Section 7.8
pp. 423 - 427
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Example of Amplitude Shift Keying (ASK)
( )cosASK Cm t t
This is binary amplitude shift keying.
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Example of Multilevel ASK w/ Two 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.
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Band Limiting Softens the Edges of ASK Waveforms
http://www.slideshare.net/Zeolite27/dc-ppt-final
You can see the correlation between ASK and analog AM because the amplitude of the modulated signal is proportional to m(t).
m(t)
This is more realistic case for actual ASK communication systems. In fact, all waveforms are softened by bandwidth limitations.
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Now for 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
Special case: on-off keying (OOK)
0
I
Q
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Expressing PSK as I and Q Components
cos forPSK C C k b b bA t kT t kT T
For PSK we can write,
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
a t b t kT t kT T
This is in polar form (I and Q)
For binary PSK we have k = 0 or This is 2-QAM but we don’t generally use this terminology for binary PSK. Note: Quadrature amplitude modulation (QAM) is a mixture of both amplitude modulation and phase modulation.
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Binary PSK (BPSK) Transmitter and Receiver
Carrier
cos(ct) Balanced
Modulator Amplifier BPF
LPF NRZ Data input
PSK
BPSK Modulator:
LPF S&H
+
cos(ct)
PSK d(t)
Comparator
Binary data output
( ) cos[2 ( )] cos[ ( )]Cr t B t t B t
BPSK Demodulator:
Sample at center of symbol
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Binary PSK (BPSK) Received Waveforms
Without noise With noise
After Lawrence Burns, “Digital Modulation and Demodulation,” Chapter 4 in RF and Microwave Circuit Design for Wireless Communications, edited by Lawrence E. Larson, Artech House Publishers, 1996. Pages 99 to 233. Lawrence Burns was an engineer at 3COM.
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Frequency Shift Keying (FSK)
In frequency shift keying each digital symbol has its own unique carrier signal frequency for encoding it. The signal amplitude and phase remain the same, only the frequency is varied. In the figure binary frequency shift keying is illustrated.
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FSK Modulation and Demodulation
FSK Modulator:
Voltage Controlled oscillator
Amplifier BPF
NRZ Data input
FSK
RF Output
Vcontrol
t
Amplifier LPF BPF
Frequency Discriminator
FSK + m(t)
Comparator
Binary data output
FSK Demodulator:
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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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Comparing PSDs For Binary ASK, PSK and FSK
Lathi & Ding Section 7.8.2
pp. 427
FSK
PSK
ASK
Pow
er s
pec
tral
den
sity
[w
atts
/Hz)
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BPSK Waveforms and Noise
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Quadrature Phase Shift Keying (QPSK)
Sometimes this is known as quadri-phase PSK, 4-PSK, or 4-QAM. QPSK uses four points on the constellation diagram, equi-spaced around a circle. With four phases, QPSK can encode two bits per symbol,
Q
I
I = -1; Q = -1 I = +1; Q = -1
I = +1; Q = +1 I = -1; Q = +1
cos sinQPSK C CI t Q t
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i(t)
q(t)
Digital I/Q Modulation Anticipating our coverage of digital communication systems
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Simple QPSK Modulator
Brute-force 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
Amplifier BPF
LPF
NRZ Data input
PSK
LPF
Serial-to -Parallel Parser
cos( )Ct
sin( )Ct
I
Q t
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Basic Building Block: Quadrature Modulator
cos( )Ct
sin( )Ct
I
Q
I and Q can be either analog or
digital signals
2 2
1
( ) cos( ( ))
( )where ( ) tan
( )
Ct I Q t t
Q tt
I t
( )t
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QPSK Time Domain Waveforms
QPSK
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Data Demultiplexer (Serial to Parallel) For QPSK
Demodulator uses three D-type flip-flops and is driven by clock and clock/2.
Q
I
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QPSK Demodulator
C/R = clock/carrier recovery STR = symbol timing recovery
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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 definition Definition: 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.
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Bits/Symbol and Symbol Rates
Modulation Bits 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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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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Bandwidth Efficiency (aka Spectral Efficiency)
Given: Eb = energy per bit Rb = bit rate (bits/second) B = bandwidth of baseband signal N0 = noise spectral density (watts/Hz) N = noise power = N0B
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/s B = 200 kHz, thus
Bandwidth efficiency = 1.35 bits/sec/Hz
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Circuit Switched Networks vs. Packet-Switched Network
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Circuit-Switched Network
Telephone Switch
Telephone Switch
Telephone Switch
Telephone Switch
Telephone Switch
Telephone Switch
Telephone Switch
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 and each addressed
Packets sequentially reassembled
to reveal message
= Packet
Router or 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.
This network
Is vulnerable.
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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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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 Data IP 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 & congestion control, retransmission, & uses acknowledgements. UDP does not use these because it is focused only upon sending packets.
UDP
TCP and UDP Analogies:
Post Office Verifies delivery Registered
Letter
TCP
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Layer Pictorial View of Protocol Data Unit Entity
Application Data or
Message
Transport Segments
Internet or Network
Packets or Datagrams
Network Access
Frames
Data
Data Transport
Header
Data Transport
Header Network Header
Data Transport
Header Network Header
Frame Header
Frame Trailer
Protocol
SMTP HTTP, DNS
TCP UDP
IP
Ethernet Modem
FDDI
Number of segments 1
Bits transmitted over channel medium
TCP/IP Protocol Architecture Model