Basics of Wireless and Mobile
Communications
Wireless Transmission
 Frequencies
 Signals
 Antenna
 Signal propagation
 Multiplexing
 Modulation
 Spread spectrum
 Cellular systems
Media Access Schemes
 Motivation
 SDMA, FDMA, TDMA, CDMA
 Comparison
Basic Functions in Mobile Systems
 Location management
 Handover
 Roaming
References


Jochen Schiller: Mobile Communications (German and English), 2nd edition, AddisonWesley, 2003 (most of the material covered in this chapter is based on the book)
Holma, Toskala: WCDMA for UMTS. 3rd edition, Wiley, 2004
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
2
Mobile Communication Systems – the Issues:
What does it require?
 Provide telecommunition services
 voice
(conversation, messaging)
 data (fax, SMS/MMS, internet)
 video (conversation, streaming, broadcast)
anywhere  coverage
 anytime  ubiquitous connectivity, reachability
 wireless  without cord/wire
 mobile  in motion, on the move (terrestrial)
 secure  integrity, identity, privacy, authenticity,
non-repudiation
 reliable  guaranteed quality of service

Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
3
Frequencies for communication (spectrum)
twisted
pair
GSM, DECT,
UMTS, WLAN
coax cable
1 Mm
300 Hz
10 km
30 kHz
VLF
LF
100 m
3 MHz
MF
1m
300 MHz
HF
VLF = Very Low Frequency
LF = Low Frequency
MF = Medium Frequency
HF = High Frequency
VHF = Very High Frequency
VHF
UHF
10 mm
30 GHz
SHF
EHF
optical transmission
100 µm
3 THz
infrared
1 µm
300 THz
visible light UV
UHF = Ultra High Frequency
SHF = Super High Frequency
EHF = Extra High Frequency
UV = Ultraviolet Light
Frequency and wave length:
λ = c/f
wave length λ, speed of light c ≅ 300 x 106 m/s, frequency f
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
4
Electromagnetic Spectrum
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
100 MHz:
UKW Radio, VHF TV
400 MHz:
UHF TV
450 MHz:
C-Netz
900 MHz:
GSM900
1800 MHz:
GSM1800
1900 MHz:
DECT
2000 MHz:
UMTS (3G)
2400 MHz:
WLAN, Bluetooth
2450 MHz:
Mikrowellenherd
3500 MHz:
WiMax
October 2013
5 o
Frequencies for mobile communication
VHF-/UHF-ranges for mobile radio
 simple, small antennas
 good propagation characteristics (limited reflections, small path loss,
penetration of walls)
 typically used for radio & TV (terrestrial+satellite) broadcast,
wireless telecommunication (cordless/mobile phone)
SHF and higher for directed radio links, satellite communications
 small antenna, strong focus
 larger bandwidth available
 no penetration of walls
Mobile systems and wireless LANs use frequencies in UHF to SHF spectrum
 systems planned up to EHF
 limitations due to absorption by water and oxygen molecules (resonance
frequencies)
weather dependent fading, signal loss caused by heavy rainfall etc.
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
6
Frequencies and regulations
ITU-R holds auctions for new frequencies, manages frequency bands worldwide
(WRC, World Radio Conferences)
Examples of assigned frequency bands (in MHz):
Europe
USA
Japan
GSM 450-457, 479486/460-467,489-496,
890-915/935-960,
1710-1785/1805-1880
UMTS (FDD) 19201980, 2110-2190
UMTS (TDD) 19001920, 2020-2025
CT1+ 885-887, 930932
CT2
864-868
DECT
1880-1900
IEEE 802.11 b
2400-2483
802.11a/HIPERLAN 2
5150-5350, 5470-5725
AMPS, TDMA, CDMA
824-849,
869-894
TDMA, CDMA, GSM
1850-1910,
1930-1990
PDC
810-826,
940-956,
1429-1465,
1477-1513
PACS 1850-1910, 19301990
PACS-UB 1910-1930
PHS
1895-1918
JCT
254-380
902-928
IEEE 802.11
2400-2483
5150-5350, 5725-5825
IEEE 802.11
2471-2497
5150-5250
Others
RF-Control
27, 128, 418, 433, 868
RF-Control
315, 915
RF-Control
426, 868
WiMax
(IEEE
802.16,
licensed)
2.3GHz, 2.5GHz and
3.5GHz
2.3GHz, 2.5GHz and
3.5GHz
2.3GHz, 2.5GHz
and 3.5GHz
Cellular
Phones
(licensed)
Cordless
Phones
(unlicensed)
Wireless
LANs
(unlicensed)
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
Abbreviations:
AMPS Advanced Mobile Phone
System
CDMA Code Division Multiple
Access
CT Cordless Telephone
DECT Digital Enhanced
Cordless
Telecommunications
GSM Global System for Mobile
Communications
HIPERLAN High-Performance
LAN
IEEE Institute of Electrical and
Electronics Engineers
JCT Japanese Cordless
Telephone
NMT Nordic Mobile Telephone
PACS Personal Access
Communications System
PACS-UB PACS- Unlicensed
Band
PDC Pacific Digital Cellular
PHS Personal Handyphone
System
TDMA Time Division Multiple
Access
WiMAX Worldwide
Interoperability for
Microwave Access
October 2013
7 o
UMTS Frequency Bands (FDD mode only)
Operating
Band
Frequency UL Frequencies
Band
UE transmit
(MHz)
DL Frequencies
UE receive
(MHz)
Typically used in
region ...
I
2100
1920 - 1980
2110 - 2170
EU, Asia
II
1900
1850 - 1910
1930 - 1990
America
III
1800
1710 - 1785
1805 - 1880
EU (future use)
IV
1700
1710 - 1755
2110 - 2155
Japan
V
850
824 - 849
869 - 894
America, Australia,
Brazil
VI
800
830 - 840
875 - 885
Japan
VII
2600
2500 - 2570
2620 - 2690
„Extension Band“
VIII
900
880 - 915
925 - 960
EU (future use)
IX
1800
1749.9 - 1784.9
1844.9 - 1879.9
Japan
X
1700
1710 - 1770
2110 - 2170
America/US
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
8 o
UMTS Frequency Bands (FDD mode only), Germany
Operator
Uplink (MHz)
Downlink (MHz)
Carriers
Auction Price
Vodafone
1920,3 – 1930,2
2110,3 – 2120,2
2x10 MHz
16,47 Mrd. DM (8,42
Mrd. €)
Currently
spare
1930,2 – 1940,1
2120,2 – 2130,1
2x10 MHz
16,45 Mrd. DM Group
3G
(Marke Quam)
E-Plus
1940,1 – 1950,0
2130,1 – 2140,0
2x10 MHz
16,42 Mrd. DM (8,39
Mrd. €)
Currently
spare
1950,0 – 1959,9
2140,0 – 2149,9
2x10 MHz
O2
1959,9 – 1969,8
2149,9 – 2159,8
2x10 MHz
16,52 Mrd. DM (8,45
Mrd. €)
T-Mobile
1969,8 – 1979,7
2159,8 – 2169,7
2x10 MHz
16,58 Mrd. DM (8,48
Mrd. €)
(16,37 Mrd. DM
Mobilcom; returned)
In 2000, the UMTS frequency bands were auctioned in Germany.
6 operators won 10 MHz each, for total 50 B€
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
9 o
Basic Lower Layer Model for Wireless Transmission
Transmit direction
Data link layer – media access
– fragmentation
– frame error
protection
– multiplexing
Physical layer – encryption
– coding,
forward error
protection
– interleaving
– modulation
– D/A conversion,
signal generation
– transmit
Receive direction
– reassembly
Digital
Signal
Processing
– frame error detection
– demultiplex
– decryption
– decoding,
bit error correction
– deinterleaving
– demodulation
– A/D conversion;
(signal equalization)
– receive
Wireless Channel
(path loss)
Cellular Communication Systems
– IntersymbolInterference (distortion
of own signal)
– Intercell-Interference
(multiple users)
– Intracell-Interference
(multiple users)
–Thermal
Noise
Andreas Mitschele-Thiel,
Jens Mückenheim
October 2013
10
Signals in general





physical representation of data
function of time and location
signal parameters: parameters representing the value of data
classification
 continuous time/discrete time
 continuous values/discrete values
 analog signal = continuous time and continuous values
 digital signal = discrete time and discrete values
signal parameters of periodic signals:
period T, frequency f=1/T, amplitude A, phase shift ϕ
 sine wave as special periodic signal for a carrier:
s(t) = At sin(2 π ft t + ϕt)
amplitude
Cellular Communication Systems
frequency
Andreas Mitschele-Thiel, Jens Mückenheim
phase shift
October 2013
11
Signal representations
amplitude
(time domain)
frequency spectrum
(frequency domain)
A [V]
phase state diagram
(amplitude M and phase ϕ
in polar coordinates)
Q = M sin ϕ
A [V]
t[s]
ϕ
I= M cos ϕ
ϕ


f [Hz]
Composed signals transferred into frequency domain using Fourier
transformation
Digital signals need
 infinite frequencies for perfect transmission
 modulation with a carrier frequency for transmission (analog signal!)
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
12
Fourier representation of periodic signals
Every periodic signal g(t) can be constructed by
∞
∞
1
g (t ) = c + ∑ an sin( 2πnft ) + ∑ bn cos(2πnft )
2
n =1
n =1
1
1
0
0
t
ideal periodic signal
Cellular Communication Systems
t
real composition
(based on harmonics)
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
13
Signal propagation
Propagation in free space always like light (straight line, line of sight)
Receiving power proportional to
1/d² (ideal),
1/dα (α=3...4 realistically)
(d = distance between sender and receiver)
Receiving power additionally influenced by
 fading (frequency dependent)
 shadowing
 reflection at large obstacles
 scattering at small obstacles
 diffraction at edges
shadowing
Cellular Communication Systems
reflection
scattering
Andreas Mitschele-Thiel, Jens Mückenheim
diffraction
October 2013
14
Radio Propagation: Received Power due to Pathloss
1m
10m
100m
(d-2):
1
1:100
1:10000
Realistic propagation
1
1:3000 to
1:10000
Ideal line-of sight
(d-3.5…4):
Cellular Communication Systems
35-40
dB
Andreas Mitschele-Thiel, Jens Mückenheim
35-40
dB
October 2013
1:10 Mio to
1:100 Mio
15
Multipath propagation
Signal can take many different paths between sender and receiver due to reflection,
scattering, diffraction
signal at sender
signal at receiver
Time dispersion: signal is dispersed over time
 interference with “neighbor” symbols, Inter Symbol Interference (ISI)
The signal reaches a receiver directly and phase shifted
 distorted signal depending on the phases of the different parts
Delayed signal rec’d
via longer path
Signal received
by direct path
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
16
Effects of mobility – Fading
Channel characteristics change over time and location
 signal paths change
 different delay variations of different signal parts (frequencies)
 different phases of signal parts
 quick changes in the power received (short-term fading or fast fading)
Additional changes in
 distance to sender
power
 obstacles further away
 slow changes in the average power
received (long-term fading or slow fading)
long-term
fading
short-term fading
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
t
17
Fast Fading

simulation showing time and frequency dependency of Rayleigh fading
(model for urban environments)
V = 110km/h 900MHz
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
18
Signal propagation ranges



Transmission range
 communication possible
 low error rate
Detection range
 detection of the signal
possible
 no communication
possible
Interference range
 signal may not be
detected
 signal adds to the
background noise
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
sender
transmission
distance
detection
interference
October 2013
19
Interference
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
20
Carrier to Interference Ratio (CIR, C/I)
(Uplink Situation)

Ratio of Carrier-to-Interference
power at the receiver
C
CIR =
∑Ij + N


The minimum required CIR
depends on the system
and the signal processing potential
of the receiver technology
Typical in GSM:
C/I=15dB (Factor 32)
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
21
Range limited systems (lack of coverage)




Mobile stations located far away from
BS (at cell border or even beyond the
coverage zone)
C at the receiver is too low, because
the path loss between sender and
receiver is too high
C/I is too low
No signal reception
possible
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
22
Interference limited systems (lack of capacity)

Mobile station is within coverage zone
C is sufficient, but too much
interference I at the receiver

C/I is too low


No more resources /
capacity left
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
23
Information Theory: Channel Capacity (1)





Bandwidth limited Additive White
Gaussian Noise (AWGN) channel
Gaussian codebooks
Single transmit antenna
Single receive antenna (SISO)
Shannon (1950):
Channel Capacity <=
Maximum mutual information between
sink and source
Signal-to-noise ratio SNR
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
24 o
Information Theory: Channel Capacity (2)



For S/N >>1 (high signal-to-noise ratio), approximate
Observation: Bandwidth and S/N are reciproke to each other
This means:
 With low bandwidth very high data rate is possible provided
 S/N is high enough
 Example: higher order modulation schemes
 With high noise (low S/N) data communication is possible if
 bandwidth is large
 Example: spread spectrum
 Shannon channel capacity has been seen as a “unreachable”
theoretical limit, for a long time. However:
 Turbo coding (1993) pushes practical systems up to 0.5 dB to
Shannon channel bandwidth
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
25 o
Link Capacity for Various Rate-Controlled Technologies
6
Shannon bound
Shannon bound with 3dB margin
(3GPP) HSDPA
(3GPP2) EV-DO
(IEEE) 802.16
achievable rate (bps/Hz)
5
4
3
2
1
0
-15


-10
-5
0
5
required SNR (dB)
10
15
20
The link capacity of current systems is quickly approaching the Shannon limit (within a factor of two).
Future improvements in spectral efficiency will focus on intelligent antenna techniques and/or coordination
between base stations.
Link performance of OFDM & 3G systems are similar and
approaching the (physical) Shannon bound
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
26 o
Antennas: isotropic radiator




Radiation and reception of electromagnetic waves, coupling of wires to
space for radio transmission
Isotropic radiator: equal radiation in all directions (three dimensional) only a theoretical reference antenna
Real antennas always have directive effects (vertically and/or
horizontally)
Radiation pattern: measurement of radiation around an antenna
y
z
z
y
ideal
isotropic
radiator
x
x
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
27
Antennas: simple dipoles
Real antennas are not isotropic radiators but, e.g.
 dipoles with lengths λ/4 on car roofs or
 λ/2 as Hertzian dipole
 shape of antenna proportional to wavelength
λ/4
λ/2
Example: Radiation pattern of a simple Hertzian dipole
y
y
x
side view (xy-plane)
z
z
side view (yz-plane)
x
simple
dipole
top view (xz-plane)
Gain: maximum power in the direction of the main lobe compared to the power
of an isotropic radiator (with the same average power)
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
28
Antennas: directed and sectorized
Often used for microwave connections (narrow directed beam)
or base stations for cellular networks (sectorized cells)
y
y
z
x
z
side view (xy-plane)
x
side view (yz-plane)
top view (xz-plane)
z
z
Cellular Communication Systems
sectorized
antenna
x
x
top view, 3 sector
directed
antenna
top view, 6 sector
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
29
Antenna
3-sectorized
downtilt
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
30
Real world propagation examples
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
31
Antennas: diversity
Grouping of 2 or more antennas
 multi-element antenna arrays
Antenna diversity
 switched diversity, selection diversity


receiver chooses antenna with largest output
diversity combining


combine output power to produce gain
cophasing needed to avoid cancellation
λ/2
λ/4
λ/2
+
λ/4
λ/2
λ/2
+
ground plane
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
32
Multiplexing
Goal: multiple use of a shared medium
Multiplexing in 4 dimensions
 space (si)
 time (t)
 frequency (f)
 code (c)
Multiple use is possible,
if resource (channel)
is different
in at least one dimension
channels ki
k1
k2
k3
k4
k5
k6
c
c
t
t
s1
f
s2
f
c
t
s3
f
Important: guard spaces needed!
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
33
Frequency multiplex
Separation of the whole spectrum into smaller frequency bands
A channel gets a certain band of the spectrum for the whole time
Advantages:


no dynamic coordination needed
applicable to analog signals
k1
k2
k3
k4
k5
k6
c
Disadvantages:
 waste of bandwidth
if the traffic is
distributed unevenly
 inflexible
 guard space
f
t
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
34
Time multiplex
A channel gets the whole spectrum for a certain amount of time
Advantages:
 only one carrier in the
medium at any time
 throughput high even
for many users
Disadvantages:
 precise synchronization
needed
k1
k2
k3
k4
k5
k6
c
f
t
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
35
Time and frequency multiplex
Combination of both methods
A channel gets a certain frequency band for a certain amount of time
Example: GSM (frequency hopping)
Advantages:
 some (weak) protection against
tapping
 protection against frequency
selective interference
but: precise coordination required
k1
k2
k3
k4
k5
k6
c
f
t
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
36
Code multiplex
Each channel has a unique code
All channels use the same spectrum
at the same time
k1
k2
k3
Advantages:
 bandwidth efficient
 no coordination and synchronization
necessary
 good protection against interference and
tapping
k4
k5
k6
c
f
Disadvantages:
 complex receivers (signal regeneration)
Implemented using spread spectrum technology
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
t
October 2013
37
Cellular systems: Space Division Multiplex
Cell structure implements space division multiplex:
 base station covers a certain transmission area (cell)
Mobile stations communicate only via the base station
Advantages of cell structures:
 higher capacity, higher number of users
 less transmission power needed
 more robust, decentralized
 base station deals with interference, transmission area, etc. locally
Disadvantages:
 fixed network needed for the base stations
 handover (changing from one cell to another) necessary
 interference with other cells
Cell sizes vary from 10s of meters in urban areas to many km in rural areas (e.g.
maximum of 35 km radius in GSM)
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
38
Cellular systems: Frequency planning I
Frequency reuse only with a certain distance between the base stations
Typical (hexagon) model:
f5
f4
f1
reuse-3 cluster:
f1
reuse-7 cluster:
f3
f3
f2
f1
f3
f2
f4
f6
f1
f3
f2
f7
f2
f5
f1
f6
f3
f5
f4
f7
f2
f6
f1
f3
f7
f2
Other regular pattern: reuse-19
 the frequency reuse pattern determines the experienced CIR
Fixed frequency assignment:
 certain frequencies are assigned to a certain cell
 problem: different traffic load in different cells
Dynamic frequency assignment:
 base station chooses frequencies depending on the frequencies already used in
neighbor cells
 more capacity in cells with more traffic
 assignment can also be based on interference measurements
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
39
Cellular systems: frequency planning II
f3
f3
f2
f1
f2
f1
f3
f2
f1
f3
f2
3 cell cluster
f2
f1
f1
f3
f3
f3
f3
f2
f3
f5
7 cell cluster
f4
f2
f6
f1
f3
f7
f6
Cellular Communication Systems
f5
f4
f1
f3
f2
f2
f2
f2
f1
f
f1 f
f3
h
h 1 f3
3
h1 2
h1 2
g2 h3 g2 h3
g2
g1
g1
g1
g3
g3
g3
f7
f5
f2
3 cell cluster
with 3 sector antennas
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
40
Spread spectrum technology:
Problem of radio transmission: frequency dependent fading can wipe out narrow
band signals for duration of the interference
Solution: spread the narrow band signal into a broad band signal using a special
code
⇒ protection against narrow band interference
power
interference
power
spread
signal
signal (despreaded)
spread
interference
detection at
receiver
f
Side effects:
 coexistence of several signals without dynamic coordination
 tap-proof
f
Alternatives:
 Direct Sequence (UMTS)
 Frequency Hopping (slow FH: GSM, fast FH: Bluetooth)
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
41
Effects of spreading and interference
i) narrow band signal
ii) spreaded signal (broadband signal)
dP/df
dP/df
user signal
broadband interference
narrowband interference
f
f
sender
iii) addition of
interference
iv) despreaded
signal
dP/df
v) application of
bandpass filter
dP/df
dP/df
f
f
f
receiver
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
42
Spreading and frequency selective fading
channel
quality
1
2
5
3
6
narrowband interference
without spread spectrum
4
frequency
narrow band
signal
guard space
channel
quality
1
spread
spectrum
Cellular Communication Systems
2
2
2
2
2
spread spectrum to limit
narrowband interference
frequency
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
43
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 signal
Advantages
 reduces frequency selective
fading
 in cellular networks



base stations can use the
same frequency range
several base stations can
detect and recover the signal
soft handover
Disadvantages
 precise power control needed
tb
user data
0
1
XOR
tc
chipping
sequence
01101010110101
=
resulting
signal
01101011001010
tb: bit period
tc: chip period
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
44
DSSS (Direct Sequence Spread Spectrum) II
spread
spectrum
signal
user data
X
transmit
signal
modulator
chipping
sequence
radio
carrier
transmitter
correlator
lowpass
filtered
signal
received
signal
demodulator
radio
carrier
products
sampled
sums
data
X
integrator
decision
chipping
sequence
receiver
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
45 o
Modulation
“The shaping of a (baseband) signal to convey information”.
Basic schemes
 Amplitude Modulation (AM)
 Frequency Modulation (FM)
 Phase Modulation (PM)
Digital modulation
 digital data is translated into an analog signal (baseband)
 ASK, FSK, PSK
 differences in spectral efficiency, power efficiency, robustness
Motivation for modulation
 smaller antennas (e.g., λ/4)
 medium characteristics
 Frequency Division Multiplexing
 spectrum availability
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
46
Modulation and demodulation
digital
data
101101001
digital
modulation
analog
baseband
signal
analog
modulation
radio transmitter
radio
carrier
analog
demodulation
analog
baseband
signal
synchronization
decision
digital
data
101101001
radio receiver
radio
carrier
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
47 o
Digital modulation
Modulation of digital signals known as Shift Keying
Amplitude Shift Keying (ASK):
 very simple
 low bandwidth requirements
 very susceptible to interference
1
0
1
t
1
0
1
Frequency Shift Keying (FSK):
 needs larger bandwidth
Phase Shift Keying (PSK):
 more complex
 robust against interference
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
t
1
0
1
t
October 2013
48
Advanced Frequency Shift Keying

bandwidth needed for FSK depends on the distance between the carrier
frequencies
Idea:
 special pre-computation avoids sudden phase shifts
 MSK (Minimum Shift Keying)
MSK technique:
 bit stream is 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 chosen
 the frequency of one carrier is twice the frequency of the other, eliminating
abrupt phase changes

even higher bandwidth efficiency using a Gaussian low-pass filter
 GMSK (Gaussian MSK), used for GSM and DECT
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
49
Example of MSK
1
0
1
1
0
1
0
data
Transformation
scheme
even bits
bit
odd bits
low
frequency
even
0101
odd
0011
signal
value
h l l h
- - ++
h: high frequency
l: low frequency
+: original signal
-: inverted signal
high
frequency
MSK
signal
t
No phase shifts!
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
50
Advanced Phase Shift Keying
BPSK (Binary Phase Shift Keying):
 bit value 0: sine wave
 bit value 1: inverted sine wave
 very simple PSK
 low spectral efficiency
 robust, used e.g. in satellite systems
QPSK (Quadrature Phase Shift Keying):






Q
1
10
0
Q
2 bits coded as one symbol
symbol determines shift of sine wave
needs less bandwidth compared to BPSK
00
more complex
used in UMTS and EDGE (8-PSK)
often also transmission of relative, not absolute phase shift:
DQPSK - Differential QPSK (IS-136, PHS)
I
11
I
01
Puls filtering of baseband to avoid sudden phase shifts
=> reduce bandwidth of modulated signal
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
51
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation (QAM)
 combines amplitude and phase modulation
 it is possible to code n bits using one symbol
 2n discrete levels: e.g. 16-QAM, 64-QAM
n=2: 4-QAM identical to QPSK
 bit error rate increases with n, but less errors compared to comparable
PSK schemes
Example: 16-QAM (1 symbol = 16 levels = 4 bits)
Symbols 0011 and 0001 have the same phase, but different amplitude
0000 and 1000 have different phase, but same amplitude
also: 64-QAM (1 symbol = 64 levels = 6 bits)
QAM is used in
 UMTS HSDPA (16-QAM)
 UMTS LTE (64-QAM)
 standard 9600 bit/s modems
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
Q
0010
0011
0001
0000
I
1000
October 2013
52
Media Access Schemes
(Recap from Advanced Mobile Communication Networks course)





Motivation
 limits of CSMA/CD
 hidden and exposed terminals
 near-far problem
TDD vs. FDD
TDMA
 Aloha, slotted Aloha
 Demand Assigned Multiple Access (DAMA)
CDMA theory and practice
Comparison
Media Access: Motivation
The problem: multiple users compete for a common, shared resource (medium)
Can we apply media access methods from fixed networks?
Example CSMA/CD
 Carrier Sense Multiple Access with Collision Detection (IEEE 802.3)
 send as soon as the medium is free (carrier sensing – CS)
 listen to the medium, if a collision occurs stop transmission and jam
(collision detection – CD)
Problems in wireless networks
 signal strength decreases (at least) proportional to the square of the
distance
 the sender would apply CS and CD, but the collisions happen at the
receiver
 it might be the case that a sender cannot “hear” the collision, i.e., CD
does not work
 furthermore, CS might not work if, e.g., a terminal is “hidden”
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
54
Motivation - hidden and exposed terminals
Hidden terminals
 A sends to B, C cannot receive A
 C wants to send to B, C senses a “free” medium -> CS fails
 collision at B: A cannot detect the collision -> CD fails
 A is “hidden” for C
Exposed terminals




A
B
C
B sends to A, C wants to send to another terminal (not A or B)
C has to wait, CS signals a medium in use
but A is outside the radio range of C, therefore waiting is not
necessary
C is “exposed” to B
A
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
B
C
October 2013
55
Motivation - near and far terminals
Terminals A and B send, C receives
 signal strength decreases proportional to the square of the distance
 the signal of terminal B therefore drowns out A’s signal
 C cannot receive A
A
B
C
Severe problem for CDMA-networks – precise power control needed!
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
56
Access methods SDMA/FDMA/TDMA
SDMA (Space Division Multiple Access)
 segment space into sectors, use directed antennas
 cell structure
FDMA (Frequency Division Multiple Access)
 assign a certain frequency to a transmission channel between a sender
and a receiver
 permanent (e.g., radio broadcast), slow hopping (e.g. GSM), fast
hopping (FHSS, Frequency Hopping Spread Spectrum)
TDMA (Time Division Multiple Access)
 assign the fixed sending frequency to a transmission channel between a
sender and a receiver for a certain amount of time
The multiplexing schemes presented previously are now used to control
medium access!
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
57
Communication link types
Each terminal needs an uplink and a downlink
Types of communication links:



Simplex
 unidirectional link transmission
Half Duplex
 Bi-directional (but not simultaneous)
Duplex
 simultaneous bi-directional link transmission, two types:
 Frequency division duplexing (FDD)
 Time division duplexing (TDD)
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
58
Duplex modes
Td
Tu
Td
Fd
Tu
Fu
Frequency Division Duplex (FDD)
Time Division Duplex (TDD)
Separate frequency bands for up- and
downlink
Separation of up- and downlink
traffic on time axis
+ separation of uplink and downlink
interference
+ support for asymmetric traffic
- no support for asymmetric traffic
- mix of uplink and downlink
interference on single band
Examples: UMTS, GSM, IS-95, AMPS
Examples: DECT, UMTS (TDD)
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
59
FDD/FDMA - general scheme, example GSM
f
960
935.2
124
200 kHz
1
20
915
890.2
124
1
t
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
60
TDD/TDMA - general scheme, example DECT
417 µs
1 2 3
11 12 1 2 3
downlink
Cellular Communication Systems
11 12
uplink
Andreas Mitschele-Thiel, Jens Mückenheim
t
October 2013
61
Aloha/slotted aloha
Mechanism
 random, distributed (no central arbiter), time-multiplex
 Slotted Aloha additionally uses time-slots, sending must always start at
slot boundaries
collision
Aloha
sender A
sender B
sender C
t
Slotted Aloha
collision
sender A
sender B
sender C
t
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
62
DAMA - Demand Assigned Multiple Access
Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson
distribution for packet arrival and packet length)
Reservation can increase efficiency to 80%
 a sender reserves a future time-slot
 sending within this reserved time-slot is possible without collision
 reservation also causes higher delays
 typical scheme for satellite links
 application to packet data, e.g. in GPRS and UMTS
Examples for reservation algorithms:



Explicit Reservation (Reservation-ALOHA)
Implicit Reservation (PRMA)
Reservation-TDMA
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
63
Access method DAMA: Explicit Reservation
Explicit Reservation (Reservation Aloha):
Two modes:
 ALOHA mode for reservation:
competition for small reservation slots, collisions possible
 reserved mode for data transmission within successful reserved slots
(no collisions possible)
synchronisation: it is important for all stations to keep the reservation list
consistent at any point in time and, therefore, all stations have to synchronize
from time to time
collision
Aloha
reserved
Cellular Communication Systems
Aloha
reserved
Aloha
Andreas Mitschele-Thiel, Jens Mückenheim
reserved
October 2013
Aloha
t
64
Access method CDMA
CDMA (Code Division Multiple Access)
 all terminals send on the same frequency probably at the same time and
can use the whole bandwidth of the transmission channel
 each sender has a unique random number, the sender XORs the signal with
this random number
 the receiver can “tune” into this signal if it knows the pseudo random
number, tuning is done via a correlation function
Advantages:
 all terminals can use the same frequency, less planning needed
 huge code space (e.g. 232) compared to frequency space
 interference (e.g. white noise) is not coded
 forward error correction and encryption can be easily integrated
Disadvantages:
 higher complexity of a receiver (receiver cannot just listen into the medium
and start receiving if there is a signal)
 all signals should have the same strength at a receiver (power control)
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
65
CDMA Principle
sender (base station)
receiver (terminal)
Code 0
Code 0
data 0
data 0
Code 1
Code 1
Σ
data 1
Transmission via
air interface
data 1
Code 2
Code 2
data 2
data 2
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
66
CDMA by example
data stream A & B
spreading
spreaded signal
Source 1
Code 1
Source 1 spread
Source 2
Code 2
Source 2 spread
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
67
CDMA by example
Despread Source 1
Sum of Sources Spread
Sum of Sources Spread + Noise
decoding
and
despreading
+
Despread Source 2
overlay of signals
Cellular Communication Systems
transmission and
distortion (noise
and interference)
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
68
CDMA in theory
Sender A

sends Ad = 1, key Ak = 010011 (assign: „0“= -1, „1“= +1)

sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)
Sender B

sends Bd = 0, key Bk = 110101 (assign: „0“= -1, „1“= +1)

sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)
Both signals superimpose in space

interference neglected (noise etc.)

As + Bs = (-2, 0, 0, -2, +2, 0)
Receiver wants to receive signal from sender A


apply key Ak bitwise (inner product)

Ae = (-2, 0, 0, -2, +2, 0) • Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6

result greater than 0, therefore, original bit was „1“
receiving B

Be = (-2, 0, 0, -2, +2, 0) • Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. „0“
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
69
CDMA on signal level I
data A
1
0
Ad
1
key A
key
sequence A
data ⊕ key
0
1
0
1
0
0
1
0
0
0
1
0
1
1
0
0
1
1
1
0
1
0
1
1
1
0
0
0
1
0
0
0
1
1
0
0
Ak
As
signal A
Real systems use much longer keys resulting in a larger distance
between single code words in code space
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
70
CDMA on signal level II
As
signal A
data B
key B
key
sequence B
data ⊕ key
1
0
Bd
0
0
0
0
1 1 0
1 0
1 0
0
0
0
1 0
1
1
1
1
1
1
0 0 1
1 0
1 0
0
0
0
1 0
1
1
1
Bk
Bs
signal B
1
0
As + Bs
Cellular Communication Systems
-1
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
71
CDMA on signal level III
data A
1
0
1
Ad
1
As + B s
0
-1
1
Ak
-1
1
(As + Bs)
* Ak
0
-1
integrator
output
comparator
output
Cellular Communication Systems
1
0
Andreas Mitschele-Thiel, Jens Mückenheim
1
October 2013
72
CDMA on signal level IV
data B
1
0
0
Bd
1
As + B s
0
-1
1
Bk
-1
1
(As + Bs)
* Bk
0
-1
integrator
output
comparator
output
Cellular Communication Systems
1
0
Andreas Mitschele-Thiel, Jens Mückenheim
0
October 2013
73
CDMA on signal level V
1
As + B s
0
-1
1
wrong
key K
-1
1
(As + Bs)
*K
0
-1
integrator
output
comparator
output
(0)
(0)
?
Assumptions
 orthogonality of keys
 neglectance of noise
 no differences in signal level => precise power control
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
74
Comparison SDMA/TDMA/FDMA/CDMA
Approach
Idea
segment space into
cells/sectors
Terminals
only one terminal can
be active in one
cell/one sector
Signal
separation
cell structure, directed
antennas
segment sending
time into disjoint
time-slots, demand
driven or fixed
patterns
all terminals are
active for short
periods of time on
the same frequency
synchronization in
the time domain
Advantages very simple, increases established, fully
capacity per km²
inflexible, antennas
Disadvantages typically fixed
Comment
only in combination
with TDMA, FDMA or
CDMA useful
Cellular Communication Systems
digital, flexible
guard space
needed (multipath
propagation),
synchronization
difficult
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
CDMA
FDMA
TDMA
SDMA
segment the
frequency band into
disjoint sub-bands
spread the spectrum
using orthogonal codes
every terminal has its all terminals can be active
at the same place at the
own frequency,
same moment,
uninterrupted
uninterrupted
code plus special
filtering in the
receivers
frequency domain
inflexible,
frequencies are a
scarce resource
flexible, less frequency
planning needed, soft
handover
complex receivers, needs
more complicated power
control for senders
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
still faces some problems,
higher complexity,
lowered expectations; will
be integrated with
TDMA/FDMA
simple, established,
robust
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
75
Basic Functions in Mobile Systems




Location management
Handover
Roaming
Authentication (see later)
Location Management
The problem:
locate a mobile user from the network side (mobile-terminated call)
Two extreme solutions:


Mobile registers with each visited cell
(e.g. direct call to the hotel room to reach a person)
– signaling traffic to register mobile when cell is changed
– network has to maintain location information about each mobile
+ low signaling load to page mobile (i.e. in one cell only)
Page mobile using a network- or worldwide broadcast message
(e.g. broadcast on TV or radio to contact a person)
– heavy signaling load to page the mobile (i.e. in all cells)
+ no signaling traffic while mobile is idle
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
77
Location Management
The issue: Compromise between
 minimizing the area where
to search for a mobile
 minimizing the number of
location updates
TOTAL
Signalling Cost
Solution 1:
Large paging area
Solution 2:
Small paging area
RA
RA
Location
RA
Update
RA
RA
Location
RA
Update
RA
RA
RA
=
Paging
∑ Signalling
Cost
Area Update
+∑ Paging
Signalling Cost
Location
Update
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
Location
Update
Location
Update
October 2013
78
Handover
The problem:
Change the cell while
communicating
cell 2
cell 1
Link quality
Reasons for handover:
 Quality of radio link
deteriorates
 Communication in other cell
requires less radio resources
 Supported radius is
exceeded (e.g. Timing
advance in GSM)
 Overload in current cell
 Maintenance
cell 1
Handover margin
(avoid ping-pong
effect)
cell 2
Link to cell 1
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
Link to cell 2
time
79
Roaming
The problem:
Use a network not subscribed to
Roaming agreement needed between network operators to exchange
information concerning:
 Authentication
 Authorisation
 Accounting
Examples of roaming agreements:
 Use networks abroad
 Use of T-Mobile network by O2 (E2) subscribers in area with no O2 coverage
Cellular Communication Systems
Andreas Mitschele-Thiel, Jens Mückenheim
October 2013
80