Transcript No Slide Title

RADIO ENVIRONMENT
• Path Loss
• Shadow Fading
• Multipath
• Interference
• Infrared Versus Radio
8C32810.71-Cimini-7/98
RADIO ENVIRONMENT
• Path Loss
• Shadow Fading
• Multipath
8C32810.83-Cimini-7/98
Limit the Bit Rate
and/or Coverage
PATH LOSS MODEL
• Different, often complicated, models are
used for different environments.
• A simple model for path loss, L, is
L=
Pr
Pt
=K
1
f da
2
where Pr is the local mean received signal
power, Pt is the transmitted power, d is the
transmitter-receiver distance, f is frequency,
and K is a transmission constant.
The path loss exponent a = 2 in free space;
2  a  4 in typical environments.
7C29822.011-Cimini-9/97
PATH LOSS LIMITATIONS
• The received signal-to-noise power ratio, SNR, is
SNR =
Pr
KPt
1
=
•
Pn
da
NoB
where No is the one-sided noise power spectral
density and B is the signal bandwidth.
• Given the performance requirement SNR SNRo,
the path loss imposes limits on the bit rate and the
signal coverage.
KPt
B a
or d 
d NoSNRo
8C32810.12-Cimini-7/98
(
KPt
NoBSNRo
1/a
)
SHADOW FADING
• The received signal is shadowed by
obstructions such as hills and buildings.
• This results in variations in the local mean
received signal power,
Pr (dB) = Pr (dB) + Gs
where Gs ~ N(0, ss2 ), 4  ss  10 dB.
• Implications
– nonuniform coverage
– increases the required transmit power
8C32810.84-Cimini-7/98
COVERAGE AREA
• For a desired received power Pr0 the coverage
area C defines the percentage of the cell with
received power P Pr0.
P = Pr0
R
For P(R) = Pr0
C=
1
1 + exp
2
a
7C29822.66-Cimini-9/97
( b12 )(1 - erf ( 1b))], b =
[
ss
4
8
12
2
77% 67%
63%
4
85% 77%
71%
6
90% 83%
77%
10a log e
ss 2
MULTIPATH
Received
Power
t
Delay Spread
h(t) =
Si aiejq d(t-ti)
i
• Constructive and Destructive Interference
of Arriving Rays
10
0 dB With Respect
-10 to RMS Value
-20
-30
0.5l
8C32810.85-Cimini-7/98
0
0.5
0
10
1
t, in seconds
20
x, in wavelength
1.5
30
FLAT FADING
• The delay spread is small compared to the
symbol period.
• The received signal envelope, r, follows a
Rayleigh or Rician distribution.
Pr (dB) = Pr (dB) + Gs + 20 log r
Received
Signal
Power
(dB)
path loss
shadow fading
Rayleigh fading
log (distance)
• Implications
– increases the required transmit power
– causes bursts of errors
8C32810.86-Cimini-7/98
DOPPLER SPREAD
• A measure of the spectral broadening caused by
the channel time variation.
fD 
v
l
Example: 900 MHz, 60 mph, fD = 80 Hz
5 GHz, 5 mph, fD = 37 Hz
• Implications
– signal amplitude and phase decorrelate after
a time period ~ 1/fD
8C32810.87-Cimini-7/98
Received
Power
DELAY SPREAD
TIME DOMAIN INTERPRETATION
Two-ray model
t = rms delay spread
2t
Delay
t small
Channel Input
1
0
T
1
T
Channel Output
0
T
2T
0
T
2T
2T
t large
T
t
• T small
• t large
negligible intersymbol interference
significant intersymbol interference,
T which causes an irreducible error floor
8C32810.88-Cimini-7/98
DELAY SPREAD
FREQUENCY DOMAIN INTERPRETATION
H(f)
Bs = signal bandwidth  1/T
Bs
1
2t
t
• T small
•
8C32810.89-Cimini-7/98
t
T
large
flat fading
frequency-selective fading
f
PHYSICAL LAYER ISSUES
• Link Performance Measures
• Modulation Tradeoffs
• Flat Fading Countermeasures
• Delay Spread Countermeasures
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LINK PERFORMANCE MEASURES
PROBABILITY OF BIT ERROR
• The probability of bit error, Pb, in a radio
environment is a random variable.
– average Pb, Pb
– Pr [Pb > Pbtarget] D outage, Pout
=
• Typically only one of these measures is
useful, depending on the Doppler frequency
and the bit rate.
8C32810.92-Cimini-7/98
GOALS OF
MODULATION TECHNIQUES
• High Bit Rate
• High Spectral Efficiency
• High Power Efficiency
• Low-Cost/Low-Power Implementation
• Robustness to Impairments
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DIGITAL MODULATION
• Any modulated signal can be represented as
s(t) = A(t) cos [wct + f(t)]
amplitude
phase or frequency
s(t) = A(t) cos f(t) cos wct
in-phase
- A(t) sin f(t) sin wct
quadrature
• Linear versus nonlinear modulation  impact
on spectral efficiency
• Constant envelope versus non-constant
envelope hardware implications with impact
on power efficiency
8C32810.94-Cimini-7/98
DEMODULATION
• Coherent detection requires a coherent phase
reference.
– difficult to obtain in a rapidly fading
environment
– increases receiver complexity
• Differential detection uses the previous symbol
for the reference signal.
– eliminates need for coherent reference
– entails loss in power efficiency (up to 3 dB)
– Doppler causes irreducible error floor,
typically small for high bit rates
8C32810.133-Cimini-7/98
BIT ERROR PROBABILITY
AWGN CHANNEL
10-1
5
2
10-2
5
For Pb = 10-3
2
BPSK 6.5 dB
QPSK 6.5 dB
DBPSK ~8 dB
DQPSK ~9 dB
DBPSK
10-3
BPSK, QPSK
5
Pb
2
DQPSK
10-4
5
2
10-5
5
2
10-6
0
2
4
6
8
10
12
14
gb, SNR/bit, dB
• QPSK is more spectrally efficient than BPSK with the
same performance.
• M-PSK, for M>4, is more spectrally efficient but requires
more SNR per bit.
• There is ~3 dB power penalty for differential
detection.
8C32810.99-Cimini-7/98
BIT ERROR PROBABILITY
FADING CHANNEL
1
5
2
10-1
5
2
10-2
5
Pb
DBPSK
2
BPSK
10-3
5
AWGN
2
10-4
5
2
10-5
0
5
10
15
20
25
30
35
gb, SNR/bit, dB
• Pb is inversely proportion to the average SNR per bit.
• Transmission in a fading environment requires about
18 dB more power for Pb = 10-3.
8C32810.100-Cimini-7/98
BIT ERROR PROBABILITY
EFFECTS OF DOPPLER SPREAD
• Doppler causes an irreducible error floor when differential
detection is used decorrelation of reference signal.
100
QPSK
DQPSK
10 -1
Rayleigh Fading
10 -2
-3
Pb 10
fDT=0.003
No Fading
10 -4
0.002
0.001
10 -5
0
10 -6
0
10
20
30
40
50
60
gb, SNR/bit, dB
• The irreducible Pb depends on the data rate and the Doppler.
For fD = 80 Hz,
data rate
T
Pbfloor
10 kbps
10-4s
3x10-4
100 kbps
10-5s
3x10-6
1 Mbps
10-6s
3x10-8
The implication is that Doppler is not an issue for high-speed
wireless data.
[M. D. Yacoub, Foundations of Mobile Radio Engineering , CRC Press, 1993]
8C32810.101-Cimini-7/98
BIT ERROR PROBABILITY
EFFECTS OF DELAY SPREAD
• ISI causes an irreducible error floor.
10-1
Irreducible Pb
Coherent Detection
+ BPSK
QPSK
OQPSK Modulation
x MSK
x
10-2
x
x
+
+
x
+
10-3
+
x
+
10-4
10-2
•
10-1
100
rms delay spread t
=
symbol period
T
The rms delay spread imposes a limit on the maximum bit rate
in a multipath environment.
For example, for QPSK,
t
Maximum Bit Rate
Mobile (rural)
25 msec
8 kbps
Mobile (city)
2.5 msec
80 kbps
Microcells
500 nsec
400 kbps
Large Building
100 nsec
2 Mbps
[J. C.-I. Chuang, "The Effects of Time Delay Spread on Portable Radio
Communications Channels with Digital Modulation," IEEE JSAC, June 1987]
8C32810.102-Cimini-7/98
SUMMARY OF
MODULATION ISSUES
• Tradeoffs
– linear versus nonlinear modulation
– constant envelope versus non-constant
envelope
– coherent versus differential detection
– power efficiency versus spectral efficiency
• Limitations
– flat fading
– doppler
– delay spread
8C32810.103-Cimini-7/98
HOW DO WE OVERCOME THE
LIMITATIONS IMPOSED BY THE
RADIO CHANNEL?
• Flat Fading Countermeasures
– Fade Margin
– Diversity
– Coding and Interleaving
– Adaptive Techniques
• Delay Spread Countermeasures
– Equalization
– Multicarrier
– Spread Spectrum
– Antenna Solutions
8C32810.104-Cimini-7/98
DIVERSITY
• Independent signal paths have a low probability
Received Signal Power
(dBm)
of experiencing deep fades simultaneously.
0
-20
-40
-60
-80
-100
0
4
8
12
16
d
The chance that two deep fades
occur simultaneously is rare.
• The basic concept is to send the same
information over independently fading radio
• Independent fading paths can be achieved by
separating the signal in time, frequency, space,
polarization, etc.
8C32810.105-Cimini-7/98
DIVERSITY COMBINING TECHNIQUES
• • •
a1
a2
a3
aM
Combiner
Output
• Selection Combining: picks the branch with the
highest SNR.
• Equal-Gain Combining: all branches are coherently
combined with equal weights.
• Maximal-Ratio Combining: all branches are coherently
combined with weights which depend on the branch
SNR.
8C32810.106-Cimini-7/98
DIVERSITY PERFORMANCE
• There is dramatic improvement even with two-branch
selection combining.
– 10 dB reduction in required SNR for 1% outage 
less transmitted power or higher bit rates or larger
coverage area
Pb
10-1
5
Pout
99.99
99.9
99.5
98.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
Maximal
Ratio
Combining
2
10-2
5
M=2
20.0
2
10-3
10.0
M=1
5
5.0
2
2.0
10-4
5
1.0
Maximal
Ratio
Equal
Gain
0.5
M=2
Selection
0.2
0.1
2
10-5
5
0.05
M=4
2
0.02
10-6
5
10
15
20 25 30
gb, SNR/bit, dB
35
40
0.01
-40
-30
-20
10log
(
-10
1
margin
0
)
• The output SNR with Maximal-Ratio Combining improves
linearly with the number of diversity branches, M  the
complexity becomes prohibitive.
7C29822.014-Cimini-9/97
10
CODING AND INTERLEAVING
• The basic principle is to spread the burst errors
over many code words.
1 codeword
read
into
interleaver
by rows
1
2
3
4
5
6
7
8
9
10
11
12
read
out
by
columns
1,5,9,2,6,10,3,7,
11,4,8,12
channel
reads out
by rows
1, 2 ,3,4,5, 6 ,7,8,
9, 10 ,11,12
1
2
3
4
5
6
7
8
9
10
11
12
1,5,9, 2 , 6 ,10 ,3,7,11,
4,8,12
reads in
by rows
• The required interleaver size can be large if the
relative fading rate is slow, as is usually the case
for high-speed data. For example, fD = 10 Hz,
bit rate = 10 Mb/s, error burst = 330,000 bits.
delay and complexity
8C32819.16-Cimini-7/98
ADVANCED CODING TECHNIQUES
• Trellis Codes
– reduce Pb without bandwidth expansion
through joint design of the channel code
and signal constellation
– can be designed with “built-in” time diversity
• Turbo Codes
– exhibit enormous coding gains
– interleaving inherent to code design
– very complex with large delays
– not well-understood for fading channels
8C32810.19-Cimini-7/98
ADAPTIVE MODULATION
TRANSMITTER
Adaptive
Modulation
and Coding
RECEIVER
Power
Control
Channel
noise
Demodulation
and Decoding
+
Channel
Estimate
Delay
FEEDBACK CHANNEL
• Power and/or data rate adapted at transmitter to
channel conditions
• Potential for large increase in spectral efficiency
• Can be combined with adaptive compression
– requires reliable feedback channel and accurate
channel estimation
– increases transmitter and receiver complexity
8C32810.22-Cimini-7/98
DELAY SPREAD COUNTERMEASURES
• Signal Processing
– at the receiver, to alleviate the problems
caused by delay spread (equalization)
– at the transmitter, to make the signal less
sensitive to delay spread (multicarrier,
spread spectrum)
• Antenna Solutions
– change the environment to reduce, or
eliminate, the delay spread (distributed
antenna system, small cells, directive
antennas)
7C29822.024-Cimini-9/97
EQUALIZER TYPES AND STRUCTURES
The goal of equalization is to cancel the ISI
or, equivalently, to flatten the frequency response.
Equalizer
Types
Linear
Nonlinear
DFE
ML Symbol
Detector
MLSE
Structures
Transversal
Lattice
Transversal
Lattice
Transversal
Channel
Estimator
[J. G. Proakis, "Adaptive Equalization for TDMA Digital Mobile Radio,"
IEEE Trans. on Veh. Tech. , May 1991]
8C32810.107-Cimini-7/98
EQUALIZER ISSUES FOR
HIGH-SPEED WIRELESS DATA
• The number of required equalizer taps, N, is proportional
to the delay spread.
• The equalizer taps must be adapted at the highest
Doppler rate.
– The length and periodicity of the training sequence
impacts the spectral efficiency.
– There is a tradeoff between speed of convergence
and complexity.
Algorithms
(for DFE)
Number of
Multiply
Operations
Least Mean
Square (LMS)
2N + 1
Kalman
Recursive Least
Squares (RLS)
2.5N2 + 4.5N
Square Root
Fast Kalman
7C29822.026-Cimini-9/97
Convergence
Advantages
Disadvantages
Low computational
complexity
Slow convergence,
depends on
channel
~N
Fast convergence,
good tracking ability
High
computational
complexity
1.5N2 + 6.5N
~N
Better stability
than Kalman
High
computational
complexity
20N + 5
~N
Fast convergence
and good tracking
Could be
unstable
~10-100N
EQUALIZER PERFORMANCE
BPSK
1
10 Mbps
5
10 -1
10 -2
1
10
Pb 10 -3
10 -4
10 -5
.1
5
.1
1
no equalizer
DFE
10 -6
25
30
35
40
45
50
SNR (dB)
BPSK
1
16 Mbps
8
4,16
10 -1
1
8
.1
.1,4
1
Pout 10 -2
10 -3
no equalizer
DFE
10 -4
1
10-4
10-8
Target Pb
10-12
• Pahlavan has shown that, for 30-meter cells (t = 50 ns), 20 Mb/s
can be achieved using a DFE with 3 forward taps and 3 feedback taps.
[K. Pahlavan, S. J. Howard, and T. A. Sexton, "Decision Feedback Equalization
of the Indoor Radio Channel," IEEE Trans. on Commun., January 1993]
8C32810.110-Cimini-7/98
MULTICARRIER MODULATION
• The transmission bandwidth is divided into many
narrow subchannels which are transmitted in
parallel.
• Ideally, each subchannel is narrow enough so
that the fading it experiences is flat  no ISI.
Transmitter
R/N b/s
R/N b/s
R/N b/s
QAM
filter
QAM
filter
QAM
filter
d0(t)
f0
d1(t)
RF
D(t)
f1
d N-1(t)
fN-1
Bandlimited
signals
f0
Receiver
f2
f1
filter
f0
RF
filter
f1
f0
QAM
f1
filter
fN-1
8C32810.111-Cimini-7/98
QAM
QAM
fN-1
WHAT TO DO WITH
BAD SUBCHANNELS?
• Coding Across Subchannels  works best
with large delay spread
• Frequency Equalization  requires accurate
channel estimation
• Adaptive Loading  requires reliable
feedback channel and accurate channel
estimation
8C32810.114-Cimini-7/98
SPREAD SPECTRUM
• Spread spectrum increases the transmit signal
bandwidth to reduce the effects of flat fading,
ISI and interference.
• SS is used in all wireless LAN products in the ISM
band
– required for operation with reasonable power
levels
– minimal performance impact on other systems
– IEEE 802.11 standard
• There are two SS methods: direct sequence and
frequency hopping.
– Direct sequence multiplies the data sequence
by a faster chip sequence.
– Frequency hopping varies the carrier
frequency by the same chip sequence.
8C32810.117-Cimini-7/98
DIRECT SEQUENCE
SPREAD SPECTRUM
Interference
Data
(T)
Modulator
Carrier
Recovery
Channel
Spreading
(PN) Code
Tc << T
Data
(T)
Demod
Spreading
(PN) Code
Synch
Transmitter
Receiver
Narrowband
Filter
Narrowband
Interference
Data Signal
with Spreading
Modulated
Data
8C32810.117-Cimini-7/98
Original
Data Signal
ISI
Receiver
Input
Other
SS Users
Other
SS Users
Demodulator
Filtering
ISI
RAKE RECEIVER
sc(t)
Received
Signal
sc(t-Tc)
Coherent
Combiner
Data
Output
Demodulator
sc(t-2Tc)
•
•
•
sc(t-TM)
• When the chip time is much less than the rms delay spread,
each branch has independent fading  equivalent to
diversity combining.
• When the chip time is greater than the rms delay spread,
the paths cannot be resolved  no diversity gain.
8C32810.119-Cimini-7/98
PERFORMANCE OF RAKE RECEIVER
FADING CHANNEL
0.5
DPSK
10-1
Rayleigh
10-2
Pb
10-3
RAKE
AWGN
10-4
10-5
8C32810.27-Cimini-7/98
0
5
10
gb, SNR/bit, dB
15
SPREAD SPECTRUM ISSUES
FOR HIGH-SPEED WIRELESS DATA
• Hardware Complexity
– synchronization
– high processing speeds for high
bit rates
– RAKE receiver
• High Required Bandwidth to Accommodate
Spreading
Spread spectrum is difficult at
high bit rates and not really
needed.
8C32810.120-Cimini-7/98
ANTENNA SOLUTIONS
Goal: Reduce (or eliminate) delay spread
• Distributed Antenna System
• Very Small Cells  antenna in every room
• Sectorization
• Directive Antennas/Beam Steering
Omnidirectional
120
90
60
150
180
0
330
210
7C29822.028-Cimini-9/97
120
30
240
270
Sectorized
300
90
Directive
60
150
120
30
180
0
330
210
240
270
300
90
60
150
30
180
0
330
210
240
270
300
SUMMARY OF COUNTERMEASURES
• Diversity
• Coding and Interleaving
• Adaptive Techniques
• Equalization
• Multicarrier
• Spread Spectrum
• Antenna Solutions
These techniques can be combined.
8C32810.123-Cimini-7/98
COMBINED EQUALIZATION AND
SECTORED ANTENNAS
1
Pt = 100 mW
Rb = 20 Mbps
Omni
.1
Pout
Omni+DFE
.01
Sector
Sec+DFE
.001
.0001
20
40
60
Square room length (meter)
1
30mx30m
Omni+DFE
.1
Pout
Omni
Sector
.01
.001
.0001
Sec+DFE
0
10
20
30
Rb (Mbps)
40
50
[G. Yang and K. Pahlavan, "Comparative Performance Evaluation of Sector
Antenna and DFE Systems in Indoor Radio Channels," Proc. of ICC '92]
8C32810.122-Cimini-7/98