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8/21/2019 Communication system overview.pdf
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High Speed Commun icat ion Circu i ts Lectu re 1 Commun icat ion Systems Overv iew
Profs. Hae-Seung Lee and Michael H. Perrott
Massachusetts Institute of Technology
February 1, 2005
Copyright © 2005 by H.-S. Lee and M. H. Perrott
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Modu lat ion Techniques
Amplitude Modulation (AM)
-Standard AM
-Double-sideband (DSB)
-Single-sideband (SSB)
-Quadrature Amplitude Modulation (QAM)
Constant Envelope Modulation
-Phase Modulation (PM)-Frequency Modulation (FM)
Multiple Access
-FDMA
-TDMA-CDMA
Ultra Wide Band (UWB)
-Pulse-OFDM
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Ampl i tude Modulat ion (Transm it ter)
Vary the amplitude of a sine wave at carrier frequency f oaccording to a baseband modulation signal
DC component of baseband modulation signal influencestransmit signal and receiver possibilities
- DC value greater than signal amplitude shown above Allows simple envelope detector for receiver
Strong carrier frequency tone is transmitted(wasted power)
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Frequency Domain View of Standard AM Transm it ter
Baseband signal x’(t) has a nonzero DC component
Causes impulse to appear at DC in baseband signal
- Transmitter output has an impulse at the carrier frequency- This component is fixed in frequency and phase, so
carries no information (waste of transmit power)
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Zero DC Value (DSB o r ‘Suppressed Carr ier ’)
Envelope of modulated sine wave no longer correspondsdirectly to the baseband signal
- Envelope instead follows the absolute value of thebaseband waveform, negative value of the baseband inputproduces 180o phase shift in carrier
- Envelope detector can no longer be used for receiver The carrier frequency tone that carries no information is
removed: less transmit power required for sametransmitter SNR (compared to standard AM)
2cos(2πf ot)
y(t)
Transmitter Output
x(t)
0
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DSB Spectra
Impulse in DC portion of baseband signal is now gone
- Transmitter output now is now free from having an impulse atthe carrier frequency: more power efficient
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Accompany ing Receiver (Coherent Detect ion)
Works regardless of DC value of baseband signal
Requires receiver local oscillator to be accurately aligned in
phase and frequency to carrier
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Frequency Domain View of DSB Receiver (Coherent)
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Impact o f Phase Misalignment in Receiver LocalOsci l lator
Worst case is when receiver LO and carrier frequency are
phase shifted 90 degrees with respect to each other - Desired baseband signal is not recovered
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Impact of 90 Degree Phase Misal ignment (Freq. Domain View )
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SSB (Sing le-Sideband )
The upper sideband (USB) and the lower sideband(LSB) are symmetric, so they contain the sameinformation
Standard AM is neither power efficient nor bandwidthefficient
The DSB improves power efficiency, but still takes up
twice the necessary bandwidth Most baseband signals have no DC or very low
frequency components
One of the sidebands can be removed at the IF or RF
stage (much easier to filter in the IF stage)
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SSB Spectra
One of the sidebands is removed by sideband filter or
phase shift techniues
- Signal bandwidth is reduced 2x: more bandwidth efficient
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Quadrature Modu lat ion (QAM)
Takes advantage of coherent receiver’s sensitivity to phasealignment with transmitter local oscillator
- We essentially have two orthogonal transmission channels (Iand Q) available
- Transmit two independent baseband signals (I and Q) onto twosine waves in quadrature at transmitter
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Accompanying Receiver
Demodulate using two sine waves in quadrature at receiver - Must align receiver LO signals in frequency and phase to
transmitter LO signals
Proper alignment allows I and Q signals to be recovered as shown
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Impact of 90 Degree Phase Misal ignm ent
I and Q channels are swapped at receiver if its LO signal is90 degrees out of phase with transmitter
- However, no information is lost!- Can use baseband signal processing to extract I/Q signals
despite phase offset between transmitter and receiver
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Simp li f ied View
For discussion to follow, assume that
- Transmitter and receiver phases are aligned- Lowpass filters in receiver are ideal- Transmit and receive I/Q signals are the same except for scalefactor
In reality
-RF channel adds distortion, causes fading
- Signal processing in baseband DSP used to correct problems
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Analog Modu lat ion
I/Q signals take on a continuous range of values (as viewed in
the time domain) Used for AM/FM radios, television (non-HDTV), and the first cell
phones
Newer systems typically employ digital modulation instead
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Digi tal Modulat ion
I/Q signals take on discrete values at discrete time instantscorresponding to digital data
- Receiver samples I/Q channels Uses decision boundaries to evaluate value of data at each time
instant
I/Q signals may be binary or multi-bit
- Multi-bit shown above
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Advantages of Digi tal Modu lat ion
Allows information to be “packetized”
- Can compress information in time and efficiently sendas packets through network
- In contrast, analog modulation requires connectionsthat are continuously available Inefficient use of radio channel if there is “dead time” in
information flow
Allows error correction to be achieved
- Less sensitivity to radio channel imperfections Enables compression of information
- More efficient use of channel Supports a wide variety of information content
- Voice, text and email messages, video can all berepresented as digital bit streams
C t l l t i Di f M l t i b i t Q d t Di i t l
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Cons tel lat ion Diagram of Mul t i-b i t Quadrature Digi talModulat ion (2-b it examp le)
We can view I/Q values at sample instants on a two-
dimensional coordinate system Decision boundaries mark up regions corresponding
to different data values
Gray coding used to minimize number of bit errorsthat occur if wrong decision is made due to noise
Amplitudes I and Q are encodedIn 2-bit digital values
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Impact o f Noise on Constel lat ion Diagram
Sampled data values no longer land in exact same locationacross all sample instants
Decision boundaries remain fixed
Significant noise causes bit errors to be made (channel
SNR determines maximum number of bits)
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Cons tant Envelope Modu lat ion
H.-S. Lee & M.H. Perro tt MIT OCW
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The Issue o f Power Eff ic iency
Power amp dominates power consumption for many wirelesssystems
- Linear power amps more power consuming than nonlinear ones
Constant-envelope modulation allows nonlinear power amp- Lower power consumption possible
Baseband to RF
Modulation
Power Amp
Transmitter
Output
Baseband
Input
Variable-Envelope Modulation Constant-Envelope Modulation
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Simpl i f ied Implementat ion for Constant-Envelope
Constant-envelope modulation limited to phase and
frequency modulation methods Can achieve both phase and frequency modulation with
ideal VCO
-Use as model for analysis purposes
- Note: phase modulation nearly impossible with practical VCO
Baseband to RF Modulation Power Amp
Transmitter
Output
BasebandInput
Constant-Envelope Modulation
TransmitFilter
E l C t l l t i Di f Ph
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Example Cons tel lat ion Diagram for PhaseModulat ion
DecisionBoundaries I
Q
DecisionBoundaries
000
001
011
110111
100
101
010
I/Q signals must always combine such that amplituderemains constant
- Limits constellation points to a circle in I/Q plane
- Draw decision boundaries about different phase regions
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Trans i t ion ing Between Constel lat ion Poin ts
DecisionBoundaries I
Q
DecisionBoundaries
000
001
011
110111
100
101
010
Constant-envelope requirement forces transitions toallows occur along circle that constellation points sit on
- I/Q filtering cannot be done independently!
- Significantly impacts output spectrum
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Mult ip le Access Techniques
H.-S. Lee & M.H. Perro tt MIT OCW
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The Issue of Mu lt ip le Access
Want to allow communication between many differentusers
Freespace spectrum is a shared resource
- Must be partitioned between users Can partition in either time, frequency, or through
“orthogonal coding” (or nearly orthogonal coding) of
data signals
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Frequency-Divis ion Mult ip le Access (FDMA)
Place users into different frequency channels
Two different methods of dealing with transmit/receive of
a given user
- Frequency-division duplexing- Time-division duplexing
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Frequency-Div is ion Dup lexing (Ful l -duplex)
Transmitter RXTX
f
Duplexer Antenna
RX
TX
Receiver TransmitBandReceive
Band
Separate frequency channels into transmit and receive bands
Allows simultaneous transmission and reception
- Isolation of receiver from transmitter achieved with duplexer - Cannot communicate directly between users, only between handsets and
base station
Advantage: isolates users
Disadvantages:
-duplexer has high insertion loss (i.e. attenuates signals passingthrough it)
-takes up twice the bandwidth
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Time-Div is ion Duplexing (Hal f -duplex)
Use any desired frequency channel for transmitter andreceiver
Send transmit and receive signals at different times
Allows communication directly between users (not
necessarily desirable) Advantage: switch has low insertion loss relative to
duplexer
Disadvantage: receiver more sensitive to transmittedsignals from other users
Transmitter
Switch Antenna
RX
TX
Receiver
switchcontrol
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Time-Divis ion Mult ip le Access (TDMA)
Place users into different time slots
- A given time slot repeats according to time frame period Often combined with FDMA
- Allows many users to occupy the same frequencychannel
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Channel Part i t ion ing Using (Nearly) “Orthogonal Cod ing”
Consider two correlation cases- Two independent random Bernoulli sequences
Result is a random Bernoulli sequence
-Same Bernoulli sequence
Result is 1 or -1, depending on relative polarity
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Code-Divis ion Mult ip le Access (CDMA)
y1(t)x1(t)
PN1(t)
y2(t)x2(t)
PN2(t)
y(t)
SeparateTransmitters
Transmit SignalsCombine
in Freespace
Td
Tc
Assign a unique code sequence to each transmitter
Data values are encoded in transmitter output stream by
varying the polarity of the transmitter code sequence- Each pulse in data sequence has period Td Individual pulses represent binary data values
- Each pulse in code sequence has period Tc Individual pulses are called “chips”
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Receiver Selects Desired Transm it ter Through Its Code
Receiver correlates its input with desired transmitter code- Data from desired transmitter restored- Data from other transmitter(s) remains randomized
y1(t)x1(t)
PN1(t)
y2(t)x2(t)
PN2(t)
x(t) y(t)
Separate
Transmitters
Transmit Signals
Combine
in Freespace
Receiver
(Desired Channel = 1)
PN1(t)
Lowpassr(t)
F D i Vi f Ch i V D t S
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Frequency Domain View of Ch ip Vs Data Sequences
tt
Tc
t
data(t) p(t) x(t)
Sdata(f)
f 0
*1
-1Tc
1
-1
1
Sx(f)|P(f)|2
f 1/Tc0
Tc
Td
1/Tdf
1/Tc0
Tc
Td
1/Td
ttTd
t
data(t) p(t) x(t)
*1
-1
1
-1
1
Td
Data and chip sequences operate on different time scales
- Associated spectra have different width and height
F D i Vi f CDMA
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Frequency Domain View of CDMA
CDMA transmitters broaden data spectra by encoding it
onto chip sequences (‘spread-spectrum’) CDMA receiver correlates with desired transmitter code
- Spectra of desired channel reverts to its original width- Spectra of undesired channel remains broad
Can be “mostly” filtered out by lowpass
r(t)
Sy1(f)
f 1/Tc0
Tc
Sx(f)
f 1/Tc0
Tc
Td
1/Td
Sx1(f)
f 0
Td
1/Td
Sy2(f)
f 1/Tc0
Tc
Sx2(f)
f 0
Td
1/Td
y1(t)x1(t)
PN1(t)
Transmitter 1
y2(t)x2(t)
PN2(t)
Transmitter 2y(t)
PN1(t)
Lowpass
Sx1(f)
UWB (Ult Wid b d )
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UWB (Ultra-Wideband)
Pictures Courtesy of R. Blazquez, et. al.
Extreme case of spread-spectrum communication
Takes advantage of Shannon’s theorem :
-data rate goes up proportionally to bandwidth but degrades only
logarithmically with SNR.
Very low energy emission per Hz
UWB Standards
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FCC recently allowed 3.1-10.6 GHz for UWB
Two separate IEEE standards are under development
Pictures Courtesy of R. Blazquez, et. al.
Figure by MIT OCW.
Distance
500Kb
1m 10m 100m
5Mb
50Mb
500MbWLAN
Locationing/Tagging
Wireless USB
and Multimedia
UWB Approaches
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UWB Approaches
Pictures Courtesy of F. R. Lee, et. al.
Pulsed UWB: Marconi invented it! It’s a form of TDMA
OFDM UWB: Utilizes knowledge base of narrowbandsystems. Strong jammers can be avoided.
Pulsed UWB
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Pulsed UWB
Data encoded in impulse train
Multipath can be exploited
No narrowband filters (RF or baseband) needed intranceivers
Extremely tight time-synchronization is essentialPictures Courtesy of R. Blazquez, et. al.
OFDM UWB
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OFDM UWB
Can take advantage of wealth of knowledge innarrowband communication
Involves the usual blocks of narrowband systems –
filters, LNA’s etc. Bandwidth of each channel is much wider: filtering is
easier than narrowband systems
SNR requirement in each channel is much lower thannarrowband systems
More digital processing than pulsed OFDM
Strong jammers can be avoided