8/14/2019 mobile Wireless Transmission
1/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.1
Mobile Communications
Chapter 2: Wireless TransmissionFrequenciesSignals
AntennaSignal propagation
MultiplexingSpread spectrum
ModulationCellular systems
8/14/2019 mobile Wireless Transmission
2/41
8/14/2019 mobile Wireless Transmission
3/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.3
Frequencies for mobile communication
VHF-/UHF-ranges for mobile radio
simple, small antenna for carsdeterministic propagation characteristics, reliable connections
SHF and higher for directed radio links, satellite communicationsmall antenna, focusinglarge bandwidth available
Wireless LANs use frequencies in UHF to SHF spectrumsome systems planned up to EHFlimitations due to absorption by water and oxygen molecules(resonance frequencies)
weather dependent fading, signal loss caused by heavy rainfall etc.
8/14/2019 mobile Wireless Transmission
4/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.4
Frequencies and regulations
ITU-R holds auctions for new frequencies, manages frequency bandsworldwide (WRC, World Radio Conferences)
Europe USA Japan
Cellular Phones
GSM 450-457, 479 -486/460 -467,489 -496, 890 -915/935 -960,1710 -1785/1805 -1880UMTS (FDD) 1920 -
1980, 2110 -2190UMTS (TDD) 1900 -1920, 2020 -2025
AMPS , TDMA , CDMA 824 -849,869 -894TDMA , CDMA , GSM 1850 -1910,1930 -1990
PDC 810 -826,940 -956,1429 -1465,1477 -1513
CordlessPhones
CT1+ 885 -887, 930 -932CT2864 -868DECT1880 -1900
PACS 1850 -1910, 1930 -1990PACS -UB 1910 -1930
PHS 1895 -1918JCT 254 -380
WirelessLANs
IEEE 802.112400 -2483HIPERLAN 25150 -5350, 5470 -5725
902 -928IEEE 802.112400 -24835150 -5350, 5725 -5825
IEEE 802.11 2471 -24975150 -5250
Others RF-Control27, 128, 418, 433,868
RF-Control315, 915
RF-Control 426, 868
8/14/2019 mobile Wireless Transmission
5/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.5
Signals I
physical representation of datafunction of time and locationsignal parameters: parameters representing the value of dataclassification
continuous time/discrete timecontinuous values/discrete values
analog signal = continuous time and continuous valuesdigital signal = discrete time and discrete values
signal parameters of periodic signals:period T, frequency f=1/T, amplitude A, phase shift
sine wave as special periodic signal for a carrier:
s(t) = A t sin(2 f tt + t)
8/14/2019 mobile Wireless Transmission
6/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.6
Fourier representation of periodic signals
)2cos()2sin(2
1)(
11
nft bnft act g n
nn
n
=
=
++=
1
0
1
0
t t
ideal periodic signalreal composition(based on harmonics)
8/14/2019 mobile Wireless Transmission
7/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.7
Different representations of signalsamplitude (amplitude domain)frequency spectrum (frequency domain)
phase state diagram (amplitude M and phase in polar coordinates)
Composed signals transferred into frequency domain using Fourier transformationDigital signals need
infinite frequencies for perfect transmissionmodulation with a carrier frequency for transmission (analog signal!)
Signals II
f [Hz]
A [V]
I= M cos
Q = M sin
A [V]
t[s]
8/14/2019 mobile Wireless Transmission
8/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.8
Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission
Isotropic radiator: equal radiation in all directions (threedimensional) - only a theoretical reference antennaReal antennas always have directive effects (vertically and/or horizontally)Radiation pattern: measurement of radiation around an antenna
Antennas: isotropic radiator
zy
x
z
y x idealisotropicradiator
8/14/2019 mobile Wireless Transmission
9/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.9
Antennas: simple dipoles
Real antennas are not isotropic radiators but, e.g., dipoles with lengths /4 on car roofs or /2 as Hertzian dipole
shape of antenna proportional to wavelength
Example: Radiation pattern of a simple Hertzian dipole
Gain: maximum power in the direction of the main lobe compared tothe power of an isotropic radiator (with the same average power)
side view (xy-plane)
x
y
side view (yz-plane)
z
y
top view (xz-plane)
x
z
simpledipole
/4
/2
8/14/2019 mobile Wireless Transmission
10/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.10
Antennas: directed and sectorized
side view (xy-plane)
x
y
side view (yz-plane)
z
y
top view (xz-plane)
x
z
top view, 3 sector
x
z
top view, 6 sector
x
z
Often used for microwave connections or base stations for mobile phones(e.g., radio coverage of a valley)
directedantenna
sectorizedantenna
8/14/2019 mobile Wireless Transmission
11/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.11
Antennas: diversity
Grouping of 2 or more antennas
multi-element antenna arraysAntenna diversity
switched diversity, selection diversityreceiver chooses antenna with largest output
diversity combining
combine output power to produce gaincophasing needed to avoid cancellation
+
/4 /2
/4
ground plane
/2 /
2
+
/2
8/14/2019 mobile Wireless Transmission
12/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.12
Signal propagation ranges
distance
sender
transmission
detection
interference
Transmission rangecommunication possiblelow error rate
Detection rangedetection of the signalpossibleno communicationpossible
Interference rangesignal may not bedetectedsignal adds to thebackground noise
8/14/2019 mobile Wireless Transmission
13/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.13
Signal propagation
Propagation in free space always like light (straight line)Receiving power proportional to 1/d
(d = distance between sender and receiver)Receiving power additionally influenced by
fading (frequency dependent)shadowingreflection at large obstacles
refraction depending on the density of a mediumscattering at small obstaclesdiffraction at edges
reflection scattering diffractionshadowing refraction
8/14/2019 mobile Wireless Transmission
14/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.14
Real world example
8/14/2019 mobile Wireless Transmission
15/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.15
Signal can take many different paths between sender and receiver due toreflection, scattering, diffraction
Time dispersion: signal is dispersed over timeinterference with neighbor symbols, Inter Symbol Interference (ISI)
The signal reaches a receiver directly and phase shifteddistorted signal depending on the phases of the different parts
Multipath propagation
signal at sender signal at receiver
LOS pulsesmultipathpulses
8/14/2019 mobile Wireless Transmission
16/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.16
Effects of mobility
Channel characteristics change over time and locationsignal paths changedifferent delay variations of different signal partsdifferent phases of signal parts
quick changes in the power received (short term fading)
Additional changes indistance to sender obstacles further away
slow changes in the average power received (long term fading)
short term fading
long termfading
t
power
8/14/2019 mobile Wireless Transmission
17/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.17
Multiplexing in 4 dimensionsspace (s i)
time (t)frequency (f)code (c)
Goal: multiple useof a shared medium
Important: guard spaces needed!
s 2
s 3
s1
Multiplexing
f
t
c
k2 k3 k4 k5 k6k1
f
t
c
f
t
c
channels k i
8/14/2019 mobile Wireless Transmission
18/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.18
Frequency multiplex
Separation of the whole spectrum into smaller frequency bands
A channel gets a certain band of the spectrum for the whole timeAdvantages:
no dynamic coordinationnecessaryworks also for analog signals
Disadvantages:waste of bandwidthif the traffic isdistributed unevenlyinflexibleguard spaces
k2 k3 k4 k5 k6k1
f
t
c
8/14/2019 mobile Wireless Transmission
19/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.19
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 evenfor many users
Disadvantages:precisesynchronizationnecessary
8/14/2019 mobile Wireless Transmission
20/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.20
f
Time and frequency multiplex
Combination of both methodsA channel gets a certain frequency band for a certain amount of timeExample: GSMAdvantages:
better protection againsttappingprotection against frequencyselective interferencehigher data rates compared tocode multiplex
but: precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
8/14/2019 mobile Wireless Transmission
21/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.21
Code multiplex
Each channel has a unique code
All channels use the same spectrumat the same time
Advantages:bandwidth efficient
no coordination and synchronizationnecessarygood protection against interference andtapping
Disadvantages:lower user data ratesmore complex signal regeneration
Implemented using spread spectrumtechnology
k2
k3
k4
k5
k6
k1
f
t
c
8/14/2019 mobile Wireless Transmission
22/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.22
Modulation
Digital modulationdigital data is translated into an analog signal (baseband)ASK, FSK, PSK - main focus in this chapter differences in spectral efficiency, power efficiency, robustness
Analog modulationshifts center frequency of baseband signal up to the radio carrier
Motivationsmaller antennas (e.g., /4)Frequency Division Multiplexingmedium characteristics
Basic schemes
Amplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)
8/14/2019 mobile Wireless Transmission
23/41
8/14/2019 mobile Wireless Transmission
24/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.24
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
8/14/2019 mobile Wireless Transmission
25/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.25
Advanced Frequency Shift Keying
bandwidth needed for FSK depends on the distance betweenthe carrier frequencies
special pre-computation avoids sudden phase shiftsMSK (Minimum Shift Keying)
bit separated into even and odd bits, the duration of each bit isdoubleddepending 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 other Equivalent to offset QPSK
even higher bandwidth efficiency using a Gaussian low-passfilter GMSK (Gaussian MSK), used in GSM
8/14/2019 mobile Wireless Transmission
26/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.26
Example of MSK
data
even bits
odd bits
1 1 1 1 000
t
lowfrequency
highfrequency
MSKsignal
bit
even 0 1 0 1odd 0 0 1 1
signal h n n hvalue - - + +
h: high frequencyn: low frequency+: original signal-: inverted signal
No phase shifts!
8/14/2019 mobile Wireless Transmission
27/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.27
Advanced Phase Shift Keying
BPSK (Binary Phase Shift Keying):bit value 0: sine wave
bit 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 toBPSKmore complex
Often also transmission of relative, notabsolute phase shift: DQPSK -Differential QPSK (IS-136, PHS)
11 10 00 01
Q
I01
Q
I
11
01
10
00
A
t
8/14/2019 mobile Wireless Transmission
28/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.28
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation (QAM): combines amplitude andphase modulationit is possible to code n bits using one symbol2 n discrete levels, n=2 identical to QPSKbit error rate increases with n, but less errors compared tocomparable PSK schemes
Example: 16-QAM (4 bits = 1 symbol)Symbols 0011 and 0001 have the same phase ,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
a
8/14/2019 mobile Wireless Transmission
29/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.29
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 entire
64QAM 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
00 0010 01 0101
8/14/2019 mobile Wireless Transmission
30/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.30
Spread spectrum technology
Problem of radio transmission: frequency dependent fading can wipe outnarrow band signals for duration of the interference
Solution: spread the narrow band signal into a broad band signal using aspecial codeprotection 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
8/14/2019 mobile Wireless Transmission
31/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.31
Effects of spreading and interference
dP/df
f
i)
dP/df
f
ii)
sender
dP/df
f
iii)
dP/df
f
iv)
receiver f
v)
user signalbroadband interferencenarrowband interference
dP/df
8/14/2019 mobile Wireless Transmission
32/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.32
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
8/14/2019 mobile Wireless Transmission
33/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.33
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 selectivefadingin cellular networks
base stations can use thesame frequency rangeseveral base stations candetect and recover the signalsoft handover
Disadvantagesprecise power control necessary
user data
chippingsequence
resulting
signal
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
8/14/2019 mobile Wireless Transmission
34/41
8/14/2019 mobile Wireless Transmission
35/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.35
FHSS (Frequency Hopping Spread Spectrum) I
Discrete changes of carrier frequencysequence of frequency changes determined via pseudo random number
sequenceTwo versions
Fast Hopping:several frequencies per user bitSlow Hopping:
several user bits per frequencyAdvantages
frequency selective fading and interference limited to short periodsimple implementationuses only small portion of spectrum at any time
Disadvantagesnot as robust as DSSSsimpler to detect
( d )
8/14/2019 mobile Wireless Transmission
36/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.36
FHSS (Frequency Hopping Spread Spectrum) II
user data
slowhopping(3 bits/hop)
fasthopping
(3 hops/bit)
0 1
tb
0 1 1 t
f
f 1
f 2
f 3
t
td
f
f 1
f 2
f 3
t
td
tb: bit period t d: dwell time
FHSS (F H i S d S ) III
8/14/2019 mobile Wireless Transmission
37/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.37
FHSS (Frequency Hopping Spread Spectrum) III
modulator user data
hoppingsequence
modulator
narrowbandsignal
spreadtransmit
signal
transmitter
receivedsignal
receiver
demodulator data
frequencysynthesizer
hoppingsequence
demodulator
frequencysynthesizer
narrowbandsignal
C ll
8/14/2019 mobile Wireless Transmission
38/41
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 2.38
Cell structure
Implements space division multiplex: base station covers a certaintransmission area (cell)
Mobile stations communicate only via the base station
Advantages of cell structures:higher capacity, higher number of usersless transmission power neededmore robust, decentralizedbase station deals with interference, transmission area etc. locally
Problems:fixed network needed for the base stations
handover (changing from one cell to another) necessaryinterference with other cells
Cell sizes from some 100 m in cities to, e.g., 35 km on the country side(GSM) - even less for higher frequencies
8/14/2019 mobile Wireless Transmission
39/41
8/14/2019 mobile Wireless Transmission
40/41
C ll b thi
8/14/2019 mobile Wireless Transmission
41/41
Cell breathing
CDM systems: cell size depends on current loadAdditional traffic appears as noise to other usersIf the noise level is too high users drop out of cells