Digital Wireless Communication Basics:

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Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 1
Digital Wireless
Communication Basics:
Overview of basic concepts
Wired Vs. Wireless Communication
Wireless
Wired
Each cable is a different channel One media (cable) shared by all
Signal attenuation is low
High signal attenuation
No interference
High interference
noise; co-channel interference; adjacent
channel interference
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 2
Why go wireless ?
Advantages

Sometimes it is impossible/impractical to lay cables

User mobility

Cost
Limitations

Bandwidth

Power

Security
EM Spectrum
ν
Propagation characteristics are different in each frequency band
UV
1 MHz
1 kHz
1 GHz
1 THz
1 PHz
1 EHz
infrared
visible
X rays
Gamma rays
LF
HF
VHF
UHF
SHF
EHF
MF
A
M

r
a
d
i
o
S
/
W

r
a
d
i
o
F
M

r
a
d
i
o
T
V
T
V
c
e
l
l
u
l
a
r
ν
902 – 928 Mhz
2.4 – 2.4835 Ghz
5.725 – 5.785 Ghz
ISM band
λ
30kHz 300kHz
3MHz
30MHz
300MHz
30GHz 300GHz
10km
1km
100m
10m
1m
10cm
1cm
100mm
3GHz
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 3
Unlicensed Radio Spectrum
902 Mhz
928 Mhz
26 Mhz
83.5 Mhz
125 Mhz
2.4 Ghz
2.4835 Ghz
5.725 Ghz
5.850 Ghz
cordless phones
baby monitors
WaveLan
802.11b+g+n
Bluetooth
Microwave oven
802.11a+n
λ
33cm
12cm
5cm
Understanding wireless communication
• How does signal propagate ?
• How much attenuation take place ?
• How does signal look like at the receiver ?
Tx
Rx
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 4
Radio Propagation
Three basic propagation mechanisms
• Propagation effects depend on not only on the specific portion of
spectrum used for transmission, but also on the bandwidth (or
spectral occupancy) of the signal being transmitted
• Spatial separation of Tx-Rx
Reflection
λ << D
Diffraction
λ

D
Scattering
λ
>>
D
Propagation in the “Real World”
a wave
can
Rain drop
reflect
reflect
penetrate
penetrate
bend
be absorbed
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 5
Propagation
And, the higher frequencies will
usually encounter more “loss”
in “real world” situations
(again, smaller cells?;
more base stations?)
The Cluttered World of
Radio Waves
walls
hallways
windows
trees
vehicles
rain
hills
girders
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 6
Exercise

Selection of the spectrum is one of the important part
of the network design
 What are the trade-off factors for the spectrum selection?

If you select lower frequency
– Good for ____(Use case), reason _______
– Bad/Difficult for _____(Use case), reason ______

If you select higher frequency
– Good for ____(Use case), reason _______
– Bad/Difficult for _____(Use case), reason ______
Evaluating Frequencies

50 MHz- Good for range outdoors (antenna size,
bending and penetrating), no foliage problems. “Sees”
metallic building structures, doesn’t pass through
windows or down corridors, needs large antenna (2
meter). TV?

450 MHz to 2 GHz - Good compromise for cellular-
type systems. Antenna small, but big enough for
outdoor range. Minor foliage effects. OK for windows
walls and corridors. (450 might be best, but ...) (Range
issue for 2 GHz systems- more bases)

5-20 GHz- Antenna too small for range. Foliage and
rain effects. Indoor microcells? Point-to-point?
Satellites to ground stations?
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 7
Summary of Path Loss
in Propagation
Understanding RF Propagation
Goals
1.Estimate radio coverage area
2.Estimate link performance
3.Estimate network design parameters
1.Transmitters and their location
2.Transmit power
3.Antenna type
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 8
Interesting Scenarios
At which locations will
correct reception take
place?
A
B
C
D
Antenna Basics
Isotropic
Dipole
High gain
directional
isotropic
ldirectiona
P
P
G =
0 dB
i
2.2 dB
i
14 dB
i
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 9
Antenna performance

half-power beam width

Sample calculation
 Parabolic antenna for sat
com
3dB
Beam Width
][
][70
mD
m
BW
λ
×
=
Sample calculation

You have 1.8m antenna for satellite communication

The antenna receive and transmit the signal in Ku
band (UL 14GHz, DL 12GHz) and also can be used
in C band (UL 6GHz, DL 4GHz)

Calculate the half beam power width (angle)
 Ku band ______ deg
 C band ______ deg

Compare with Yagi-antenna BW for terrestrial TV
service
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 10
Free Space Propagation Model
P
T
P
R
d
2
/
2
4
mW
d
P
P
T
Di
π
=
Isotropic power
density
2
4 d
GP
P
TT
D
π
=
Power density along
the direction of
maximum radiation
eff
TT
R
A
d
GP
P
2

=
π
λ
4
2
=
G
A
eff
2






4
=
d
GGPP
RTTR
π
λ
Power received by
Antenna
effDR
APP
=
Predict received signal
strength when the transmitter
and receiver have a clear
line-of-sight path between them
Also known
as Friis free
space formula
Path Loss (relative measure)
P
t
P
R
2






4
=
d
GG
P
P
RT
T
R
π
λ
)log20log205.32()()(
1010
fdGG
P
P
dBRdBT
dB
T
R
++−+=








2
3
)(
10*57.0
df
GG
P
P
RT
T
R

=
f is in MHz
d is in Km
Path Loss represents signal attenuation
(measured on dB) between the effective
transmitted power and the receive power
(excluding antenna gains)
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 11
Path Loss (Example)
P
t
P
R
50 W
= 47 dBm
Assume that antennas are isotropic.
Calculate receive power (in dBm) at free
space distance of 100m from the antenna.
What is P
R
at 10Km?
dB
P
P
dB
T
R
5.71−=








)log20log205.32()()(
1010
fdGG
P
P
dBRdBT
dB
T
R
++−+=








dB
P
P
dB
T
R
5.111−=








)900log201.0log205.32(00
1010
++−+=








dB
T
R
P
P
-20 (for d = 0.1)
59
20 (for d = 10)
dBmP
dBmR
5.245.7147)(

=
−=
dBmP
dBmR
5.645.11147)(

=

=
Path Loss (another example)
Path Loss Vs. Distance
0
20
40
60
80
100
120
140
160
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Distance (Km)
Path Loss (dB)
2.4 GHz
5 GHz
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 12
Path Loss (another example)
Path Loss Vs. Distance
60
70
80
90
100
110
120
130
140
150
0.01 0.1 1 10 100
Distance (Km) Log Scale
Path Loss (dB)
2.4 GHz
5 GHz
Radio propagation: path loss
P
t
P
r
P
r
near field
path loss = 10 log (4πr
2
/λ) r ≤ 8m
= 58.3 + 10 log (
r
3.3
/8) r > 8m
r
path loss in 2.4 Ghz band
near field
far field
r
2

r
≤ 8m
r
> 8m
r
3.3

Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 13
Basics of Small Scale
Fading: Towards choice
of PHY
Basic Questions
Desert
Metro
Street
Indoor
What will happen if the transmitter
- changes transmit power ?
- changes frequency ?
- operates at higher speed ?
What will happen if
the receiver moves?
What will happen if we conduct
this experiment in different types
of environments?
Channel effects
Effect of mobility
Transmit power, data rate,
signal bandwidth, frequency
tradeoff
T
x
R
x
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 14
Review of basic concepts

Channel Impulse response

Power delay profile

Inter Symbol Interference

Coherence bandwidth

Coherence time
Channel Impulse Response
)(tx
Channel
)(ty
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 15
Power delay Profile
Received Signal Level (dBm)
-105
-100
-95
-90
-90
0
50
100
150
200
250
300
350
400
450
Excess Delay (ns)
RMS Delay Spread (σ
τ
) = 46.4 ns
Mean Excess delay (τ) = 45 ns
Maximum Excess delay < 10 dB = 110 ns
Noise threshold
Example (Power delay profile)
-30 dB
-20 dB
-10 dB
0 dB
0 1 2 5
P
r
(τ)
(µs)
τ
=
+++
+++
= sμ
τ
38.4
]11.01.001.0[
)0)(01.0()2)(1.0()1)(1.0()5)(1(
_
=
+++
+++
=
2
2222
_
2
07.21
]11.01.001.0[
)0)(01.0()2)(1.0()1)(1.0()5)(1(

τ
=−= sμσ
τ
37.1)38.4(07.21
2
1.37 µs
4.38 µs
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 16
RMS Delay Spread: Typical values
10ns
50ns
150ns
1µs
2µs
5µs
10µs
25µs
500ns
Office building 1
San Francisco
Manhattan
Suburban
Office building 2
Delay spread is a good measure of Multipath
3m 15m 45m 150m 300m 600m 3Km 7.5Km
Inter Symbol Interference
-30 dB
-20 dB
-10 dB
0 dB
0 1 2 5
P
r
(τ)
(µs)
τ
1.37 µs
4.38 µs
0 1 2 5 (µs)
Symbol time
4.38
σ
τ
Symbol time > 10*
σ
τ
--- No equalization required
Symbol time < 10*
σ
τ
--- Equalization will be required to
deal with ISI
In the above example, symbol time should be more than 14µs to avoid ISI.
This means that link speed must be less than 70Kbps (approx)
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 17
Coherence Bandwidth
)(tx
Time domain view
High correlation of amplitude
between two different freq.
components
Range of freq over
which response is flat
B
c
σ
τ
delay spread
)( fX
Freq. domain view
Doppler Shift
λ
θ
cosv
f =Δ
v
θ
Doppler shift
Example
- Carrier frequency f
c
= 1850 MHz (i.e. λ = 16.2 cm)
- Vehicle speed v = 60 mph = 26.82 m/s
- If the vehicle is moving directly towards the transmitter
- If the vehicle is moving perpendicular to the angle of arrival of the
transmitted signal
Hzf 165
162.0
82.26
==Δ
0
=
Δ
f
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 18
Small scale fading
Multi path time delay
Doppler spread
Flat fading
Frequency selective fading
Slow fading
Fast fading
fading
PHY Layer Design Choices ?

Required Data Rates
 Determines channel : frequency selective or flat fading; fast
or slow fading

Required QoS at the PHY: bit-error-rate (BER),
packet-error-rate (PER), Frame-error-rate (FER)
 May be determined by application needs (higher layers)
 Affected by Interference and Noise levels

PHY layer choices include selection of
 Modulation/Demodulation
 Techniques to mitigate fading: diversity, equalization, OFDM,
MIMO
 Techniques to mitigate interference (if necessary)
 Error correction Coding
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 19
Exercise

Consider a low earth orbiting satellite network system
design. It would have multipath and Doppler shift
effect
 Compare the link environment difference between terrestrial
cellar network and low earth satellite network (e.g. orbit
altitude 100km and 1000km)

Fading, Pass loss, Tracking, Delay, etc.
 Hint: you have to consider the relative speed between
satellite and the terminal on the earth
 You can set any assumption, such as

Number of the satellite

Terminal size, mobility

Use case

Etc.
Back up
Advanced Internet Technology
IV
Jan 8th, 2008
Digital Wireless
Communication Basics 20
Path loss in dB
1 μW
d2
10 W
source
d1
1 mW
10
-3
10
1
10
-6
Power
dB = 10 log (----)
P
1
P
2
Path loss from source to d2 = 70dB
1,000 times
40 dB
30 dB
10,000 times
dBm( absolute measure of power)
1 μW
d2
10 W
source
d1
1 mW
+ 10,000 times
- 1,000 times
= 40 dBm
= 0 dBm
10
-3
10
1
10
-6
Power
dBm= 10 log (-------)
P
1
1mW
= -30 dBm