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Issues in Wireless Physical Layer
A. Chockalingam
Assistant Professor
Indian Institute of Science, Bangalore-12
[email protected]
http://ece.iisc.ernet.in/~achockal
Outline
RF
Spectrum Issues
Wireless
Channel Characteristics
Combating Fading
– Diversity Techniques
– Transmit Diversity
Multiple
Access
Power Control
Co-channel Interference
Ultra Wideband Techniques
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
2
Radio Frequency Spectrum
Communication through electromagnetic
wave propagation
Frequency Spectrum
– Certain ranges of frequency
Only certain frequency spectra are usable
–
–
–
–
Limitations of atmospheric propagation effects
Technology/Device limitations
Regulatory issues
Safety hazards
Demand
for spectrum far exceeds supply
– Efficient use of RF spectrum is important
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
3
RF Spectrum - Some Current systems
900 MHz Cellular Band
– GSM: 890 - 915 MHz Uplink; 935 - 960 MHz Downlink
– IS-54: 824 - 849 MHz Uplink; 869 - 894 MHz Downlink
– PDC: 810 - 820 MHz and 1429 - 1453 MHz Uplink
940 - 960 MHz and 1477 - 1501 MHz Uplink
– IS-95: 824 - 844 MHz Uplink; 869 - 889 MHz Downlink
1800 MHz PCS Band
– 1850 - 1910 MHz Uplink; 1930 - 1960 MHz Downlink
– DECT: 1880 - 1900 MHz
C, Ku, L and S-Bands for SATCOM
– C-band: 5.9 - 6.2 GHz Uplink; 3.7- 4.2 GHz: Downlink
– Ku-band: 14 GHz Uplink; 12 GHz Downlink
– L-band: 1.61 - 1.6265 GHz; S-band: 2.4835 - 2.5 GHz
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
4
Unlicensed Radio Spectrum
Carrier
wavelength: 33 cm
26 MHz
902
MHz
928
MHz
• Wireless LANs
• Cordless phones
Dr. A. Chockalingam
2.4
GHz
12 cm
5 cm
83.5 MHz
200 MHz
2.4835
GHz
• 802.11b
• Bluetooth
• Microwave Oven
Dept of ECE, IISc, Bangalore
5.15
GHz
5.35
MHz
• 802.11a
5
RF Spectrum
Some forward looking developments
– 300 MHz BW in the 5 GHz band made available
to stimulate Wireless LAN technologies and use
– Ultra wideband (UBW) technology
– 60 GHz band for high-speed, short-range
communications
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
6
Physical Layer Tasks
Wireless systems need to overcome one or
more of the following distortions:
– AWGN (receiver thermal noise)
– Receiver carrier frequency and phase offset
– Receiver timing offset
– Delay spread
– Fading (without or with LOS component)
– Co-channel and adjacent interference (CCI, ACI)
– Nonlinear distortion, intermodulation, impulse
noise
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
7
Motivation for PHY Layer Advances
Increase channel capacity (spectral
efficiency) - higher average bit rate
Increase Erlang Capacity - more users per
square area
Increase reliability
Reduce Tx power
Increase range
Increase coverage
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
8
PHY Layer Advances
Erlang
Capacity
Spatial Multiplexing
Sectorisation
Variable Bit-Rate
OFDM
Link
Adaptation Transmit Diversity
Space-Time Coding
Voice Activity
Detection
Frequency
Hopping
Spectral
Efficiency
Transmit Diversity
Receive Diversity
Smart Beam-forming
Interference
Suppression
Turbo Coding
DS-CDMA
Fixed Beamforming
Power Control
Multi-user Detection
Dynamic Channel
Selection
Dr. A. Chockalingam
Range
(Power Efficiency)
Dept of ECE, IISc, Bangalore
9
Wireless Channel Characteristics
Free-space
Transmission
GR )
( GT
PT
d
Rx
Tx
PR
Dr. A. Chockalingam
PR
PT GT GR
4d
Dept of ECE, IISc, Bangalore
2
10
Mobile Radio Channel
Characterized
by
– Free space (distance) loss
– Long-term fading (shadowing)
– Short-term fading (multipath fading)
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
11
Mobile Radio Channel
Short Term
Fading
0.1 - 1 m
(10 - 100 msecs)
Received
Power
Distance Loss
Long Term
Fading
10 - 100 m
(1 - 10 secs)
Distance, d
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
12
Distance Loss
In line-of-sight AWGN channels
(AWGN: Additive White
– distance loss d , d : distance between Tx and Rx
– loss exponent
is 2 (i.e., 20 dB/decade loss)
2
Gaussian Noise)
In urban mobile radio channels
– loss exponent
varies between 2.5 to 5.5
– 40 dB/decade loss (typ)
Rx Signal power
(Based on field measurements)
40 dB
Slowly varying compared
to carrier wavelength
40 dB
10 m
Fwd & Rev links impacted
in the same way Dept of ECE, IISc, Bangalore
Dr. A. Chockalingam
40 dB/decade
100 m
1 km
d
13
Shadowing
Signals are blocked by obstacles (e.g., bridges
buildings, trees, etc)
Shadow loss variation - typ
log-normally distributed
(Std Dev of distribution: 4 to 12 dB)
Slowly varying compared
to carrier wavelength
Fwd & Rev links impacted
in the same waybri
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
14
Multipath Propagation
Base
Station
n
r (t ) i s(t i )
i 1
Tx. signal
Path 1
Channel
Path 2
Impulse
Response
h(t )
Path n
Mobile
1
2
3
1 2
3
Rx. signal
n
n
t
Frequency
Response
H( f )
f
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
15
Multipath (Short term) Fading
Time-varying impulse response
h( ; t ) i (t )e
j 2f c i ( t )
i 1
(t i (t ))
Fluctuations in received signal amplitude (typically
Rayleigh distributed)
Time spread
Doppler Spread
Fade variations are fast
Rev link fading independent
Signal
Strength
Rev link fade
Fwd link fade
of Fwd link fading
time
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
16
Key Multipath Parameters
Delay
/ Frequency Characterization
– Delay spread, Tm
– Coherence BW, Bc
Time
variations
– Coherence time, Tc
– Doppler BW, Bd
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
17
Delay Spread / Coherence BW
h( ; t )
c (1, 2 ; t ) E h (1; t )h( 2 ; t t
Autocorrelation function of
If we let
t 0 , c ( ;0) gives the average
power output of the channel as a function of
Autocorrelation
s (t )
c ( )
r (t )
t
Tm :
BC :
Tm
t
Max. Delay Spread
Coherence Bandwidth
Dr. A. Chockalingam
FT
c ( )
FT Pair
Dept of ECE, IISc, Bangalore
c ( f )
c ( f )
1
Bc
Tm
18
f
Delay / Frequency Characterization
Delay Spread (Tm )
– range of differential delay between different paths
–
–
–
–
jitter in Rx time of the signal, long echoes
results in Inter-Symbol Interference (ISI).
Need equalization to combat ISI (in unspread systems)
Provides “time Diversity” in spread systems (RAKE
Combining in CDMA)
Coherence
BW ( Bc )
– BW over which fade remains constant or have
strong amplitude correlation
1
Bc
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
Tm
19
Delay / Frequency Characterization
– Frequency non-selective fading
» Coherence BW > Signal BW
W
Bc W
f
Bc
– Frequency selective fading
» Coherence BW < Signal BW:
W
Bc W
Bc
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
f
20
Time Variations
Coherence Time (Tc )
– Time over which fade remains constant or have
strong amplitude correlation
– Coherence time > symbol time : Slow fading
– Coherence time < symbol time : Fast fading
Doppler
BW ( Bd )
– frequency shift on the carrier frequency due to
relative motion between Tx and Rx
– depends on user velocity and carrier wavelength
1
Bd
Tc
Dr. A. Chockalingam
Note:
Dept of ECE, IISc, Bangalore
21
Doppler Bandwidth
Bd
v
For f 900MHz,
v : mobile velocity
c
: carrier wavelength
f
f : carrier frequency
v 60Km/h, 0.33m
Bd 50Hz
Dr. A. Chockalingam
• Larger Doppler Bandwidth necessitates
• Larger power control control update
rates in CDMA
• Faster converging algorithms when
adaptive receivers are employed
Dept of ECE, IISc, Bangalore
22
Effect of Fading
1
0 .1
pe
Fading
0.01
AWGN
0.001
0.0001
Eb
N0
Non-fading AWGN Channel:
Fading Channel:
Dr. A. Chockalingam
pe
pe falls exponentially with increasing SNR
falls linearly with increasing SNR
Dept of ECE, IISc, Bangalore
23
Combating Fading Effects
– Diversity techniques
» Provide the receiver with multiple fade replicas of
the same information bearing signal
» Assume L independent diversity branches
» If
p denote the probability that the instantaneous
SNR is below a given threshold on a particular
diversity branch
» Then, the probability that the the instantaneous
SNR is below the same threshold on
branches is
Dr. A. Chockalingam
L diversity
pL
Dept of ECE, IISc, Bangalore
24
SISO to MIMO
– Single Input Single Output (SISO)
» LOS point-to-point links
– Single Input Multiple Output (SIMO)
» Receiver diversity
– Multiple Input Single Output (MISO)
» Transmit diversity
» Space time transmission
– Multiple Input Multiple Output (MIMO)
» Multiple transmitting and multiple receiving
antennas
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
25
Receive Diversity Techniques
– Several methods by which receive diversity
can be achieved include
» Space diversity
» Time diversity (coding/interleaving can be viewed
as a efficient way of time diversity)
» Frequency diversity (multiple channels separated
by more than the coherence BW)
» Multipath diversity (obtained by resolving
multipath components at different delays)
» Angle/Direction diversity (directional antennas)
» Macro diversity
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
26
Receive Diversity Combining
– Method by which signals from different
diversity branches are combined
» Predetection Combining
» Postdetection combining
» With ideal coherent detection there is no difference
between pre- and postdetection combining
» With differentially coherent detection, there is a
slight difference in performance
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
27
Receive Diversity Combining
– Maximal Ratio Combining (MRC)
L
For BPSK:
rk
l 1
L
r xk
(l ) l
k
k
l 1
(l ) 2
k
L
nk( l ) k( l )
l 1
– Equal Gain Combining (EGC)
L
rk r
l 1
(l )
k
L
xk
l 1
L
(l )
k
nk( l ) k(l )
– Selection Combining (SC)
rk xk ak nk
where
l 1
ak max k(1) ,k2 ,...,k( L)
– Generalized Selection Combining (GSC)
– Switch and Stay Combining (SSC)
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
28
Diversity Performance
1
0 .1
pe
Fading (L=1)
0.01
0.001
L=2
AWGN
L=4
L=3
0.0001
Average SNR
• Diversity gain is maximum when the diversity branches are
uncorrelated.
• Correlation between diversity branches reduces diversity gain
• Diversity gain is greater for Raleigh fading than for Ricean
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
29
Transmit Diversity
– Issue: Receive diversity at the mobile is
difficult because of space limitations
– Using multiple transmit antennas at the
base station with a single receive at the
mobile can give same diversity benefits
– Tx. Diversity schemes
» with feedback from the mobile
» without feedback from the mobile
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
30
Transmit Diversity
s (k 1), s(k )
Tx
h11
h21
r (k 1), r (k )
Rx
s (k ), s(k 1)
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
31
Spatial Multiplexing
• Use N Tx antennas and M Rx antennas (N < M)
by sending N symbols at a time
r1 (k )
s1 (k )
r2 (k )
Tx
s2 ( k )
Dr. A. Chockalingam
H 32
Channel Matrix
Dept of ECE, IISc, Bangalore
Rx
r3 (k )
32
Co-channel Interference
Frequencies
reused in different cells to
increase capacity
Reuse Distance: D
– Minimum distance between cells using
same frequencies
Cell
Radius: R
Reuse
Ratio: D
R
Dr. A. Chockalingam
R
Dept of ECE, IISc, Bangalore
D
R
33
Co-channel Interference
S/I : Signal-to-Interference Ratio
For same size cells, co-channel interference (CCI)
becomes a function of R and D
D
Increasing R reduces CCI
S
I
S
L
Ii
i 1
:
R
L
(
D
)
i
( D / R)
L
3N
L
i 1
path loss exponent (=4 typ)
L : No. of co-channel cells
S/I required = 18 dB (typ) => cluster size N > 6.49
For 7-cell reuse (N = 7), S/I = 18.7 dB
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
34
Co-Channel Interference
– In FDMA/TDMA CCI determines the reuse
distance
– In CDMA, CCI affects the number of users
supported by a BS
– CCI can be reduced by
» Sectorization
» Power Control
» Discontinuous Transmission
» Frequency Hopping
» Multiuser detection
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
35
Multiple Access
– FDMA
» AMPS
– TDMA
» GSM, EDGE, DECT, PHS
– CDMA
» IS-95, WCDMA, cdma2000
– OFDM (can be viewed as a spectrally efficient FDMA)
» 802.11a, 802.11g, HiperLAN, 802.16
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
36
OFDM
Tones
Carriers
Power
Frequency
Time-slots
Dr. A. Chockalingam
Time
Dept of ECE, IISc, Bangalore
37
DS-CDMA vs OFDM
Tx. signal
Rx. signal
Channel
Impulse
Response
h(t )
1
CDMA attempts to exploit
“time-diversity” through
RAKE receiver
2
3
1 2
3
n
n
t
Frequency
Response
OFDM attempts to exploit
“frequency-diversity” by
frequency slicing
H( f )
f
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
38
RAKE Receiver
H*(f)
Carrier
y1
L-Parallel
Demodulators
90
y2
Y
yL
H*(f)
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
39
RAKE Finger
nTc
Np
H*(f)
ˆ l cosˆl
1
Carrier
Initial timing
from searcher
Pilot Seq
Tracking Loop
(Early-Late Gate)
cu( I )
Pilot
Sequence
Despreader
nTc
1
cu( I )
c (pI )
90
N
)
c (Q
p
Np
H*(f)
Dr. A. Chockalingam
1
Dept of ECE, IISc, Bangalore
ˆ l c sin ˆl
0 l
40
Power Control
To combat the effect of fading, shadowing and
distance losses
Transmit only the minimum required power to
achieve a target link performance (e..g, FER)
– Minimizes interference
– Increases battery life
FL Power Control
– To send enough power to reach users at cell edge
RL
Power Control
– To overcome “near-far” problem in DS-CDMA
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
41
Power Control
Types of Power Control
– Open Loop Power Control
– Closed loop Power Control
Open Loop Power Control (on FL)
– Channel state on the FL is estimated by mobile
– RL Transmit power made proportional to FL channel Loss
– Works well if FL and RL are highly correlated
» which is generally true for slowly varying distance and
shadow losses
» but not true with fast multipath Rayleigh fading
– So open loop power control can effectively compensate for
distance and shadow losses, and not for multipath fading
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
42
Power Control
Closed Loop Power Control (on RL)
– Base station measures the received power
– Compares it with the desired received power (target
Eb/No)
– Sends up or down command to mobile asking it to
increase or decrease the transmit power
– Must be performed fast enough a rate (approx. 10
times the max. Doppler BW) to track multipath
fading
– Propagation and processing delays are critical to
loop performance
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
43
Ultra wideband (UBW) Techniques
Impulse
Radio Tx (Marconi’s century old radio tx)
has now emerged under the banner `ultrawideband
Reason:
– mature digital techniques
– practicality low power impulse radio communications
UWB
– Tx and Rx of ultra-short (sub-nanosecs) electromagnetic
energy impulses (or monocycles with few zero crossings)
FCC’s
definition of UWB:
– BW’s greater than 1.5 GHz or
– or BW’s greater than 25% of the center frequency
measured at 10 dB down points
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
44
UWB
Modern UWB radio is characterized by
– very low effective radiated power (sub-mW
range)
– extremely low power spectral densities and
wide bandwidths (> 1GHz)
– EIRP < -41.25 dBm/MHz, with restrictions in
bands below 960 MHz, between 1.99 and 10.6
GHz
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
45
UWB
Ways
of generating signals having UWB
characteristics
– TM-UWB
» Time modulated impulse stream
– DS-UWB
» continuous streams of PN-coded impulses (resemble
CDMA signaling)
» employ a chip rate commensurate with the emission
center frequency
– TRD-UWB
» employs impulse pairs that are differentially polarity
encoded by the data
Dr. A. Chockalingam
Dept of ECE, IISc, Bangalore
46
UWB Capabilities
High
spatial capacity
High
channel capacity and scalability
Robust
Very
multipath performance
low transmit power
Location
Dr. A. Chockalingam
awareness and tracking
Dept of ECE, IISc, Bangalore
47