Issues in Wireless Physical Layer
A. Chockalingam
Assistant Professor
Indian Institute of Science, Bangalore-12
achockal@ece.iisc.ernet.in
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
 4d 


  
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 2f 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 32
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 cosˆl

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