CS 647 2.1
CS647: Advanced Topics in Wireless Networks
Basics of Wireless TransmissionPart II
Drs. Baruch Awerbuch & Amitabh MishraComputer Science Department
Johns Hopkins University
CS 647 2.2
For a circular reflector antennaG = η ( π D / λ )2
η = net efficiency (depends on the electric field distribution over the antenna aperture, losses such as ohmic heating , typically 0.55)D = diameter, thus,
G = η (π D f /c )2, c = λ f (c is speed of light)
Example:
Antenna with diameter = 2 m, frequency = 6 GHz, wavelength = 0.05 mG = 39.4 dB
Frequency = 14 GHz, same diameter, wavelength = 0.021 mG = 46.9 dB
* Higher the frequency, higher the gain for the same size antenna
Antenna Gain
CS 647 2.3
Definition of path loss LP :
Path Loss in Free-space:
Lf = (4π d/λ) 2 = (4π f cd/c )2
where fc is the carrier frequency
This shows greater the fc , more is the loss.
,r
tP P
PL =
),(log20)(log2045.32)( 1010 kmdMHzfdBL cPF ++=
Path Loss (Free-space)
CS 647 2.4
Example of Path Loss (Free-space)
Path Loss in Free-space
70
80
90
100
110
120
130
0 5 10 15 20 25 30
Distance d (km)
Path
Los
s L f
(dB
)
fc=150MHzfc=200MHzfc=400MHzfc=800MHzfc=1000MHzfc=1500MHz
CS 647 2.5
Land Propagation
The received signal power:
L is the propagation loss in the channel, i.e.,L = LP LS LF
LPGGP trt
r =
Path loss
Fast fadingSlow fading (Shadowing)
CS 647 2.6
Slow Fading (Long-term
fading)
Propagation Loss
Signal Strength
(dB)
Distance
Path Loss
Fast Fading (Short-term
fading)
CS 647 2.7
Path Loss (Land Propagation)
Simplest Formula:
Lp = A d-α
where A and α: propagation constantsd : distance between transmitter and receiverα : value of 3 ~ 4 in typical urban area
CS 647 2.8
Path Loss (Urban, Suburban and Open areas)
Urban area:
where
Suburban area:
Open area:
[ ][ ] )(log)(log55.69.44
)()(log82.13)(log16.2655.69)(
1010
1010
kmdmhmhmhMHzfdBL
b
mbcPU
−+−−+= α
[ ][ ] [ ]
[ ][ ]
≥−
≤−
−−−
=citymediumsmallfor
MHzfformh
MHzfformh
cityelforMHzfmhMHzf
mh
cm
cm
cmc
m &,400,97.4)(75.11log2.3
200,1.1)(54.1log29.8
arg,8.0)(log56.1)(7.0)(log1.1
)(2
10
210
1010
α
4.528
)(log2)()(2
10 −
−=
MHzfdBLdBL cPUPS
[ ] 94.40)(log33.18)(log78.4)()( 102
10 −+−= MHzfMHzfdBLdBL ccPUPO
CS 647 2.9
Path Loss
Path loss in decreasing order:Urban area (large city) Urban area (medium and small city)Suburban areaOpen area
CS 647 2.10
Example of Path Loss (Urban Area: Large City)
Path Loss in Urban Area in Large City
100
110
120
130
140
150
160
170
180
0 10 20 30
Distance d (km)
Path
Los
s L p
u(d
B) fc=200MHz
fc=400MHzfc=800MHzfc=1000MHzfc=1500MHzfc=150MHz
CS 647 2.11
Example of Path Loss (Urban Area: Medium and Small Cities)
Path Loss in Urban Area for Small & Medium Cities
100110
120130
140150
160170
180
0 10 20 30
Distance d (km)
Path
Los
s Lp
u(d
B)
fc=150MHz
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz
CS 647 2.12
Example of Path Loss (Suburban Area)
Path Loss in Suburban Area
90
100
110
120
130
140
150
160
170
0 5 10 15 20 25 30
Distance d (km)
Path
Los
s Lp
s(d
B)
fc=150MHz
fc=200MHz
fc=400MHz
fc=800MHz
fc=1000MHz
fc=1500MHz
CS 647 2.13
Example of Path Loss (Open Area)
Path Loss in Open Area
8090
100
110120
130
140
150
0 5 10 15 20 25 30
Distance d (km)
Path
Los
s L p
o(d
B)
fc=150MHzfc=200MHzfc=400MHzfc=800MHzfc=1000MHzfc=1500MHz
CS 647 2.14
Slow Fading
The long-term variation in the mean level is known as slow fading (shadowing or log-normal fading). This fading caused by shadowing.Log-normal distribution:- The pdf of the received signal level is given in decibels by
where M is the true received signal level m in decibels, i.e., M=10log10m, is the area average signal level, i.e., the mean of M,
σ is the standard deviation in decibels
( )( )2
2212
M M
p M e σ
πσ
−−
=
M
CS 647 2.15
Log-normal Distribution
M
2σ
M
p(M)
The pdf of the received signal level
CS 647 2.16
Fast Fading
The signal from the transmitter may be reflected from objects such as hills, buildings, or vehicles.
When MS far from BS, the envelope distribution of received signal is Rayleigh distribution. The pdf is
where σ is the standard deviation and r is the envelope of fading signal.
Middle value rm of envelope signal within sample range to be satisfied by
We have rm = 1.777
( ) 0,22
22 >=
−rerrp
rσ
σ
.5.0)( =≤ mrrPσ
CS 647 2.17
Rayleigh Distribution
The pdf of the envelope variation
r2 4 6 8 10
P(r)
0
0.2
0.4
0.6
0.8
1.0
σ=1
σ=2
σ=3
CS 647 2.18
Fast Fading (Continued)
When MS is close to BS, the envelope distribution of received signal is Rician distribution. The pdf is
where σ is the standard deviation,I0(x) is the zero-order Bessel function of the first kind,α is the amplitude of the direct signal
( ) 0,02
22
22
≥
=
+−
rrIerrpr
σα
σσα
CS 647 2.19
Rician Distribution
r
Pdfp
(r)
r 86420
0.6
0.5
0.4
0.3
0.2
0.1
0
α = 2α = 1α= 0 (Rayleigh)
σ = 1
α = 3
The pdf of the envelope variation
CS 647 2.20
Characteristics of Instantaneous Amplitude
Level Crossing Rate:Average number of times per second that the signal envelope crosses the level in positive going direction.
Fading Rate:Number of times signal envelope crosses middle value in positive going direction per unit time.
Depth of Fading:Ratio of mean square value and minimum value of fading signal.
Fading Duration:Time for which signal is below given threshold.
CS 647 2.21
Doppler ShiftDoppler Effect: When a wave source and a receiver are moving towards each other, the frequency of the received signal will not be the same as the source.
When they are moving toward each other, the frequency of the received signal is higher than the source.When they are opposing each other, the frequency decreases.
Thus, the frequency of the received signal is
where fC is the frequency of source carrier, & fD is the Doppler frequency.
Doppler Shift in frequency:
where v is the moving speed, λ is the wavelength of carrier.
θλcosvf D =
DCR fff −=
Moving speed v
MS
Signal
θ
CS 647 2.22
Delay Spread
When a signal propagates from a transmitter to a receiver, signal suffers one or more reflections.
This forces signal to follow different paths.
Each path has different path length, so the time of arrival for each path is different.
This effect which spreads out the signal is called “Delay Spread”.
CS 647 2.23
Moving Speed Effect
Time
V1 V2 V3 V4
Sign
al st
reng
th
CS 647 2.24
Delay Spread
Delay
Sign
al S
tren
gth
The signals from close by reflectors
The signals from intermediate reflectors
The signals from far away reflectors
CS 647 2.25
Intersymbol Interference (ISI)
Caused by time delayed multipath signals
Has impact on burst error rate of channel
Second multipath is delayed and is received during next symbol
For low bit-error-rate (BER)
R (digital transmission rate) limited by delay spread τd.
d
Rτ21
<
CS 647 2.26
Intersymbol Interference (ISI)
Time
Time
Time
Transmission signal
Received signal (short delay)
Received signal (long delay)
1
0
1
Propagation time Delayed signals
CS 647 2.27
Coherence Bandwidth
Coherence bandwidth Bc:Represents correlation between 2 fading signal envelopes at frequencies f1 and f2.Is a function of delay spread.Two frequencies that are larger than coherence bandwidth fade independently.Concept useful in diversity reception
Multiple copies of same message are sent using different frequencies.
CS 647 2.28
Cochannel Interference
Cells having the same frequency interfere with each other.rd is the desired signalru is the interfering undesired signal
β is the protection ratio for which rd ≤ βru (so that the signals interfere the least)
If P(rd ≤ βru ) is the probability that rd ≤ βru ,Cochannel probability Pco = P(rd ≤ βru )
CS 647 2.29
Multiplexing in 4 dimensionsspace (si)time (t)frequency (f)code (c)
Goal: multiple use of a shared medium
Important: guard spaces needed!
s2
s3
s1
Multiplexing
f
t
c
k2 k3 k4 k5 k6k1
f
t
c
f
t
c
channels ki
CS 647 2.30
Frequency multiplex
Separation of the whole spectrum into smaller frequency bandsA channel gets a certain band of the spectrum for the whole timeAdvantages:no dynamic coordination necessaryworks also for analog signals
Disadvantages:waste of bandwidth if the traffic is distributed unevenlyinflexibleguard spaces
k2 k3 k4 k5 k6k1
f
t
c
CS 647 2.31
f
t
c
k2 k3 k4 k5 k6k1
Time multiplex
A channel gets the whole spectrum for a certain amount of time
Advantages:only one carrier in themedium at any timethroughput high even for many users
Disadvantages:precise synchronization necessary
CS 647 2.32
f
Time and frequency multiplex
Combination of both methodsA channel gets a certain frequency band for a certain amount of timeExample: GSM
Advantages:better protection against tappingprotection against frequency selective interferencehigher data rates compared tocode multiplex
but: precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
CS 647 2.33
Code multiplex
Each channel has a unique code
All channels use the same spectrum at the same timeAdvantages:
bandwidth efficientno coordination and synchronization necessarygood protection against interference and tapping
Disadvantages:lower user data ratesmore complex signal regeneration
Implemented using spread spectrum technology
k2 k3 k4 k5 k6k1
f
t
c
CS 647 2.34
Modulation
Digital modulationdigital data is translated into an analog signal (baseband)ASK, FSK, PSK - main focus in this chapterdifferences in spectral efficiency, power efficiency, robustness
Analog modulationshifts center frequency of baseband signal up to the radio carrier
Motivationsmaller antennas (e.g., λ/4)Frequency Division Multiplexingmedium characteristics
Basic schemesAmplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)
CS 647 2.35
Modulation and demodulation
synchronizationdecision
digitaldataanalog
demodulation
radiocarrier
analogbasebandsignal
101101001 radio receiver
digitalmodulation
digitaldata analog
modulation
radiocarrier
analogbasebandsignal
101101001 radio transmitter
CS 647 2.36
Digital modulation
Modulation of digital signals known as Shift KeyingAmplitude Shift Keying (ASK):
very simplelow bandwidth requirementsvery susceptible to interference
Frequency Shift Keying (FSK):needs larger bandwidth
Phase Shift Keying (PSK):more complexrobust against interference
1 0 1
t
1 0 1
t
1 0 1
t
CS 647 2.37
Advanced Frequency Shift Keying
bandwidth needed for FSK depends on the distance between the carrier frequenciesspecial pre-computation avoids sudden phase shifts
MSK (Minimum Shift Keying)bit separated into even and odd bits, the duration of each bit is doubled depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosenthe frequency of one carrier is twice the frequency of the otherEquivalent to offset QPSK
even higher bandwidth efficiency using a Gaussian low-pass filter GMSK (Gaussian MSK), used in GSM
CS 647 2.38
Example of MSK
data
even bits
odd bits
1 1 1 1 000
t
low frequency
highfrequency
MSKsignal
bit
even 0 1 0 1
odd 0 0 1 1
signal h n n hvalue - - + +
h: high frequencyn: low frequency+: original signal-: inverted signal
No phase shifts!
CS 647 2.39
Advanced Phase Shift Keying
BPSK (Binary Phase Shift Keying):bit value 0: sine wavebit value 1: inverted sine wavevery simple PSKlow spectral efficiencyrobust, used e.g. in satellite systems
QPSK (Quadrature Phase Shift Keying):
2 bits coded as one symbolsymbol determines shift of sine waveneeds less bandwidth compared to BPSKmore complex
Often also transmission of relative, not absolute phase shift: DQPSK -Differential QPSK (IS-136, PHS) 11 10 00 01
Q
I01
Q
I
11
01
10
00
A
t
CS 647 2.40
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulationit is possible to code n bits using one symbol2n discrete levels, n=2 identical to QPSKbit error rate increases with n, but less errors compared to comparable PSK schemes
Example: 16-QAM (4 bits = 1 symbol)Symbols 0011 and 0001 have the same
phase φ, but different amplitude a. 0000 and 1000 have different phase, but same amplitude.
used in standard 9600 bit/s modems
0000
0001
0011
1000
Q
I
0010
φ
a
CS 647 2.41
Hierarchical Modulation
DVB-T modulates two separate data streams onto a single DVB-T streamHigh Priority (HP) embedded within a Low Priority (LP) streamMulti carrier system, about 2000 or 8000 carriersQPSK, 16 QAM, 64QAMExample: 64QAM
good reception: resolve the entire64QAM constellationpoor reception, mobile reception: resolve only QPSK portion6 bit per QAM symbol, 2 mostsignificant determine QPSKHP service coded in QPSK (2 bit), LP uses remaining 4 bit
Q
I
00
10
000010 010101
CS 647 2.42
Spread spectrum technology
Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interferenceSolution: spread the narrow band signal into a broad band signal using a special code
protection against narrow band interference
protection against narrowband interference
Side effects:coexistence of several signals without dynamic coordinationtap-proof
Alternatives: Direct Sequence, Frequency Hopping
detection atreceiver
interference spread signal
signal
spreadinterference
f f
power power
CS 647 2.43
Effects of spreading and interference
dP/df
fi)
dP/df
fii)
sender
dP/df
fiii)
dP/df
fiv)
receiverf
v)
user signalbroadband interferencenarrowband interference
dP/df
CS 647 2.44
Spreading and frequency selective fading
frequency
channelquality
1 23
4
5 6
narrow bandsignal
guard space
22
22
2
frequency
channelquality
1
spreadspectrum
narrowband channels
spread spectrum channels
CS 647 2.45
DSSS (Direct Sequence Spread Spectrum) I
XOR of the signal with pseudo-random number (chipping sequence)
many chips per bit (e.g., 128) result in higher bandwidth of the signalAdvantages
reduces frequency selective fadingin cellular networks
base stations can use the same frequency rangeseveral base stations can detect and recover the signalsoft handover
Disadvantagesprecise power control necessary
user data
chipping sequence
resultingsignal
0 1
0 1 1 0 1 0 1 01 0 0 1 11
XOR
0 1 1 0 0 1 0 11 0 1 0 01
=
tb
tc
tb: bit periodtc: chip period
CS 647 2.46
DSSS (Direct Sequence Spread Spectrum) II
Xuser data
chippingsequence
modulator
radiocarrier
spreadspectrumsignal
transmitsignal
transmitter
demodulator
receivedsignal
radiocarrier
X
chippingsequence
lowpassfilteredsignal
receiver
integrator
products
decisiondata
sampledsums
correlator
CS 647 2.47
FHSS (Frequency Hopping Spread Spectrum) I
Discrete changes of carrier frequencysequence of frequency changes determined via pseudo random number sequence
Two versionsFast Hopping: several frequencies per user bitSlow Hopping: several user bits per frequency
Advantagesfrequency selective fading and interference limited to short periodsimple implementationuses only small portion of spectrum at any time
Disadvantagesnot as robust as DSSSsimpler to detect
CS 647 2.48
FHSS (Frequency Hopping Spread Spectrum) II
user data
slowhopping(3 bits/hop)
fasthopping(3 hops/bit)
0 1
tb
0 1 1 t
f
f1
f2
f3
t
td
f
f1
f2
f3
t
td
tb: bit period td: dwell time
CS 647 2.49
FHSS (Frequency Hopping Spread Spectrum) III
modulatoruser data
hoppingsequence
modulator
narrowbandsignal
spreadtransmitsignal
transmitter
receivedsignal
receiver
demodulatordata
frequencysynthesizer
hoppingsequence
demodulator
frequencysynthesizer
narrowbandsignal