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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
Cellular Communication Systems 2 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
References
Jochen Schiller: Mobile Communications (German and English), 2nd edition, Addison-
Wesley, 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 3 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 4 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Frequencies for communication (spectrum)
VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency
Frequency and wave length:
λ = c / f wave length λ, speed of light c ≅ 300 x 106 m/s, frequency f
1 Mm 300 Hz
10 km 30 kHz
100 m 3 MHz
1 m 300 MHz
10 mm 30 GHz
100 µm 3 THz
1 µm 300 THz
visible light VLF LF MF HF VHF UHF SHF EHF infrared UV
optical transmission coax cable twisted pair
GSM, DECT, UMTS, WLAN
Cellular Communication Systems 5 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Electromagnetic Spectrum
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
o
Cellular Communication Systems 6 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 7 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
Cellular Phones (licensed)
GSM 450-457, 479-486/460-467,489-496, 890-915/935-960, 1710-1785/1805-1880 UMTS (FDD) 1920-1980, 2110-2190 UMTS (TDD) 1900-1920, 2020-2025
AMPS, TDMA, CDMA 824-849, 869-894 TDMA, CDMA, GSM 1850-1910, 1930-1990
PDC 810-826, 940-956, 1429-1465, 1477-1513
Cordless Phones (un-licensed)
CT1+ 885-887, 930-932 CT2 864-868 DECT 1880-1900
PACS 1850-1910, 1930-1990 PACS-UB 1910-1930
PHS 1895-1918 JCT 254-380
Wireless LANs (un-licensed)
IEEE 802.11 b 2400-2483 802.11a/HIPERLAN 2 5150-5350, 5470-5725
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
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
o
Cellular Communication Systems 8 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
UMTS Frequency Bands (FDD mode only)
Operating Band
Frequency Band
UL Frequencies 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
o
Cellular Communication Systems 9 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
(16,37 Mrd. DM Mobilcom; returned)
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. €)
In 2000, the UMTS frequency bands were auctioned in Germany. 6 operators won 10 MHz each, for total 50 B€
o
Cellular Communication Systems 10 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Basic Lower Layer Model for Wireless Transmission Transmit direction Receive direction Data link layer – media access – fragmentation – reassembly
– frame error protection – frame error detection
– multiplexing – demultiplex Physical layer – encryption – decryption
– coding, forward error protection
Digital Signal
Processing – decoding, bit error correction
– interleaving – deinterleaving – modulation – demodulation
– D/A conversion, signal generation
– A/D conversion; (signal equalization)
– transmit – receive
Wireless Channel (path loss)
– Intersymbol-Interference (distortion of own signal) – Intercell-Interference (multiple users) – Intracell-Interference (multiple users) –Thermal Noise
Cellular Communication Systems 11 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
frequency phase shift
Cellular Communication Systems 12 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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!)
Signal representations
f [Hz]
A [V]
ϕ
I= M cos ϕ
Q = M sin ϕ
ϕ
A [V]
t[s]
amplitude (time domain)
frequency spectrum (frequency domain)
phase state diagram (amplitude M and phase ϕ in polar coordinates)
Cellular Communication Systems 13 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Fourier representation of periodic signals
)2cos()2sin(21)(
11nftbnftactg
nn
nn ππ ∑∑
∞
=
∞
=
++=
1
0
1
0 t t
ideal periodic signal real composition (based on harmonics)
Every periodic signal g(t) can be constructed by
Cellular Communication Systems 14 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
reflection scattering diffraction shadowing
Cellular Communication Systems 15 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Radio Propagation: Received Power due to Pathloss
1m 10m 100m Ideal line-of sight (d-2): 1 1:100 1:10000
Realistic propagation 1 1:3000 to 1:10 Mio to (d-3.5…4): 1:10000 1:100 Mio 35-40
dB 35-40
dB
Cellular Communication Systems 16 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction
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
Multipath propagation
signal at sender signal at receiver
Delayed signal rec’d via longer path
Signal received by direct path
Cellular Communication Systems 17 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 obstacles further away
slow changes in the average power received (long-term fading or slow fading)
short-term fading
long-term fading
t
power
Cellular Communication Systems 18 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Fast Fading
simulation showing time and frequency dependency of Rayleigh fading (model for urban environments)
V = 110km/h 900MHz
Cellular Communication Systems 19 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Signal propagation ranges
distance
sender
transmission
detection
interference
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 20 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Interference
Cellular Communication Systems 21 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Carrier to Interference Ratio (CIR, C/I)
(Uplink Situation)
Ratio of Carrier-to-Interference power at the receiver
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)
NICCIRj +
=∑
Cellular Communication Systems 22 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 23 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 24 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
o
Cellular Communication Systems 25 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
o
Cellular Communication Systems 26 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Link Capacity for Various Rate-Controlled Technologies
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
-15 -10 -5 0 5 10 15 20 0
1
2
3
4
5
6
required SNR (dB)
achi
evab
le ra
te (b
ps/H
z)
Shannon bound Shannon bound with 3dB margin
(3GPP2) EV-DO (IEEE) 802.16
(3GPP) HSDPA
o
Cellular Communication Systems 27 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
Antennas: isotropic radiator
z y
x
z
y x ideal isotropic radiator
Cellular Communication Systems 28 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
Example: Radiation pattern of a simple Hertzian dipole
Gain: maximum power in the direction of the main lobe compared to the 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
simple dipole
λ/4 λ/2
Cellular Communication Systems 29 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 (narrow directed beam) or base stations for cellular networks (sectorized cells)
directed antenna
sectorized antenna
Cellular Communication Systems 30 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Antenna
downtilt
3-sectorized
Cellular Communication Systems 31 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Real world propagation examples
Cellular Communication Systems 32 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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/4 λ/2 λ/4
ground plane
λ/2 λ/2
+
λ/2
Cellular Communication Systems 33 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
Important: guard spaces needed!
s2
s3
s1
Multiplexing
f
t
c
k2 k3 k4 k5 k6 k1
f
t
c
f
t
c
channels ki
Cellular Communication Systems 34 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
Disadvantages: waste of bandwidth
if the traffic is distributed unevenly
inflexible guard space
k2 k3 k4 k5 k6 k1
f
t
c
Cellular Communication Systems 35 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
f
t
c
k2 k3 k4 k5 k6 k1
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
Cellular Communication Systems 36 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
f
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
t
c
k2 k3 k4 k5 k6 k1
Cellular Communication Systems 37 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Code multiplex
Each channel has a unique code All channels use the same spectrum at the same time Advantages:
bandwidth efficient no coordination and synchronization
necessary good protection against interference and
tapping Disadvantages:
complex receivers (signal regeneration) Implemented using spread spectrum technology
k2 k3 k4 k5 k6 k1
f
t
c
Cellular Communication Systems 38 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 39 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Cellular systems: Frequency planning I Frequency reuse only with a certain distance between the base stations Typical (hexagon) model: reuse-3 cluster: reuse-7 cluster:
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
f4 f5
f1 f3
f2
f6
f7
f4 f5
f1 f3
f2
f6
f7
f4 f5
f1 f3
f2
f6
f7 f2
f1 f3
f2
f1 f3
f2
f1 f3
Cellular Communication Systems 40 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Cellular systems: frequency planning II
f1 f2
f3 f2
f1
f1
f2
f3 f2
f3 f1
f2 f1
f3 f3
f3 f3
f3
f4 f5
f1 f3
f2
f6
f7
f3 f2
f4 f5
f1 f3
f5 f6
f7 f2
f2
f1 f1 f1 f2 f3
f2 f3
f2 f3 h1
h2 h3 g1
g2 g3
h1 h2 h3 g1
g2 g3
g1 g2 g3
3 cell cluster
7 cell cluster
3 cell cluster with 3 sector antennas
Cellular Communication Systems 41 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
Side effects: coexistence of several signals without dynamic coordination tap-proof
Alternatives:
Direct Sequence (UMTS) Frequency Hopping (slow FH: GSM, fast FH: Bluetooth)
detection at receiver
interference spread signal
signal (despreaded)
spread interference
f f
power power
Cellular Communication Systems 42 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Effects of spreading and interference
dP/df
f
i) narrow band signal
dP/df
f
ii) spreaded signal (broadband signal)
sender
dP/df
f
iii) addition of interference
dP/df
f
iv) despreaded signal
receiver f
v) application of bandpass filter
user signal broadband interference narrowband interference
dP/df
Cellular Communication Systems 43 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Spreading and frequency selective fading
frequency
channel quality
1 2 3
4
5 6
narrow band signal
guard space
2 2
2 2
2
frequency
channel quality
1
spread spectrum
narrowband interference without spread spectrum
spread spectrum to limit narrowband interference
Cellular Communication Systems 44 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
user data
chipping sequence
resulting signal
0 1
0 1 1 0 1 0 1 0 1 0 0 1 1 1
XOR
0 1 1 0 0 1 0 1 1 0 1 0 0 1
=
tb
tc
tb: bit period tc: chip period
Cellular Communication Systems 45 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
DSSS (Direct Sequence Spread Spectrum) II
X user data
chipping sequence
modulator
radio carrier
spread spectrum signal
transmit signal
transmitter
demodulator
received signal
radio carrier
X
chipping sequence
lowpass filtered signal
receiver
integrator
products
decision data
sampled sums
correlator
o
Cellular Communication Systems 46 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 47 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Modulation and demodulation
synchronization decision
digital data analog
demodulation
radio carrier
analog baseband signal
101101001 radio receiver
digital modulation
digital data analog
modulation
radio carrier
analog baseband signal
101101001 radio transmitter
o
Cellular Communication Systems 48 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Digital modulation
Modulation of digital signals known as Shift Keying
Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference
Frequency Shift Keying (FSK):
needs larger bandwidth
Phase Shift Keying (PSK): more complex robust against interference
1 0 1
t
1 0 1
t
1 0 1
t
Cellular Communication Systems 49 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 50 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Example of MSK
data
even bits
odd bits
1 1 1 1 0 0 0
t
low frequency
high frequency
MSK signal
bit
even 0 1 0 1
odd 0 0 1 1
signal h l l h value - - + +
h: high frequency l: low frequency +: original signal -: inverted signal
No phase shifts!
Transformation scheme
Cellular Communication Systems 51 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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):
2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to BPSK more complex used in UMTS and EDGE (8-PSK) often also transmission of relative, not absolute phase shift:
DQPSK - Differential QPSK (IS-136, PHS) Puls filtering of baseband to avoid sudden phase shifts => reduce bandwidth of modulated signal
Q
I 0 1
Q
I
11
01
10
00
Cellular Communication Systems 52 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
0000
0001
0011
1000
Q
I
0010
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
Cellular Communication Systems 54 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 55 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
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
Motivation - hidden and exposed terminals
B A C
B A C
Cellular Communication Systems 56 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
Severe problem for CDMA-networks – precise power control needed!
Motivation - near and far terminals
A B C
Cellular Communication Systems 57 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 58 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 59 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Duplex modes
Frequency Division Duplex (FDD)
Separate frequency bands for up- and downlink
+ separation of uplink and downlink interference
- no support for asymmetric traffic
Examples: UMTS, GSM, IS-95, AMPS
Fd
Fu
Td Tu
Td Tu
Time Division Duplex (TDD)
Separation of up- and downlink traffic on time axis
+ support for asymmetric traffic
- mix of uplink and downlink interference on single band
Examples: DECT, UMTS (TDD)
Cellular Communication Systems 60 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
FDD/FDMA - general scheme, example GSM
f
t
124
1
124
1
20
200 kHz
890.2
935.2
915
960
Cellular Communication Systems 61 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
TDD/TDMA - general scheme, example DECT
1 2 3 11 12 1 2 3 11 12
t downlink uplink
417 µs
Cellular Communication Systems 62 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Mechanism random, distributed (no central arbiter), time-multiplex Slotted Aloha additionally uses time-slots, sending must always start at
slot boundaries Aloha
Slotted Aloha
Aloha/slotted aloha
sender A
sender B
sender C
collision
sender A
sender B
sender C
collision
t
t
Cellular Communication Systems 63 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 64 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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
Aloha reserved Aloha reserved Aloha reserved Aloha
collision
t
Cellular Communication Systems 65 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 66 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
CDMA Principle
Code 0
Code 1
Code 2
Σ
data 0
data 1
data 2
Code 0
Code 1
Code 2
data 0
data 1
data 2
sender (base station) receiver (terminal)
Transmission via air interface
Cellular Communication Systems 67 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
CDMA by example
Source 2
Source 1
data stream A & B
Code 2
Code 1
spreading
Source 2 spread
Source 1 spread
spreaded signal
Cellular Communication Systems 68 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
CDMA by example
Sum of Sources Spread
+
overlay of signals
Sum of Sources Spread + Noise
transmission and distortion (noise and interference)
Despread Source 2
Despread Source 1
decoding and despreading
Cellular Communication Systems 69 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 70 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
CDMA on signal level I
data A
key A
signal A
data ⊕ key
key sequence A
Real systems use much longer keys resulting in a larger distance between single code words in code space
1 0 1
1 0 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1 0 1 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0
Ad
Ak
As
Cellular Communication Systems 71 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
CDMA on signal level II
signal A
data B
key B key
sequence B
signal B
As + Bs
data ⊕ key
1 0 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
Bd
Bk
Bs
As
1 0 -1
Cellular Communication Systems 72 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
CDMA on signal level III
Ak
(As + Bs) * Ak
integrator output
comparator output
As + Bs
data A
1 0 1
1 0 1 Ad
1 0 -1
1
-1
1 0 -1
Cellular Communication Systems 73 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
CDMA on signal level IV
integrator output
comparator output
Bk
(As + Bs) * Bk
As + Bs
data B
1 0 0
1 0 0 Bd
1 0 -1 1
-1 1 0 -1
Cellular Communication Systems 74 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
comparator output
CDMA on signal level V
wrong key K
integrator output
(As + Bs) * K
As + Bs
(0) (0) ?
Assumptions orthogonality of keys neglectance of noise no differences in signal level => precise power control
1 0 -1 1
-1
1 0 -1
Cellular Communication Systems 75 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Comparison SDMA/TDMA/FDMA/CDMA
Approach SDMA TDMA FDMA CDMA Idea segment space into
cells/sectors segment sending time into disjoint time-slots, demand driven or fixed patterns
segment the frequency band into disjoint sub-bands
spread the spectrum using orthogonal codes
Terminals only one terminal can be active in one cell/one sector
all terminals are active for short periods of time on the same frequency
every terminal has its own frequency, uninterrupted
all terminals can be active at the same place at the same moment, uninterrupted
Signal separation
cell structure, directed antennas
synchronization in the time domain
filtering in the frequency domain
code plus special receivers
Advantages very simple, increases capacity per km²
established, fully digital, flexible
simple, established, robust
flexible, less frequency planning needed, soft handover
Dis-advantages
inflexible, antennas typically fixed
guard space needed (multipath propagation), synchronization difficult
inflexible, frequencies are a scarce resource
complex receivers, needs more complicated power control for senders
Comment only in combination with TDMA, FDMA or CDMA useful
standard in fixed networks, together with FDMA/SDMA used in many mobile networks
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
Basic Functions in Mobile Systems
Location management Handover Roaming Authentication (see later)
Cellular Communication Systems 77 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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 78 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
RA
RA
RA RA
RA
RA RA
RA
RA
Location Update
Location Update
Location Update
Location Update
Location Update
Location Management
The issue: Compromise between minimizing the area where
to search for a mobile minimizing the number of
location updates
Solution 1: Large paging area
Solution 2: Small paging area
Paging Signalling Cost
Paging Area Update Signalling Cost
TOTAL Signalling Cost
∑ ∑ +
=
Cellular Communication Systems 79 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
Handover
The problem: Change the cell while communicating
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
Link
qua
lity
Link to cell 1 Link to cell 2 time
cell 1
cell 2
Handover margin (avoid ping-pong effect)
cell 1 cell 2
Cellular Communication Systems 80 Andreas Mitschele-Thiel, Jens Mückenheim October 2013
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