Introduction(Mobile Computing)

Mobile Communications Chapter 2: Wireless Transmission Frequencies q Signals q Antennas q Signal propagation q

Multiplexing q Spread spectrum q Modulation q Cellular systems q

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.1

Frequencies for communication twisted pair

coax cable

1 Mm 300 Hz

10 km 30 kHz

VLF

LF

optical transmission

100 m 3 MHz

MF

HF

VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency

1m 300 MHz

VHF

UHF

10 mm 30 GHz

SHF

100 µm 3 THz

EHF

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 ≅ 3x108m/s, frequency f Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.2

Frequencies for mobile communication q q

VLF, LF, MF HF not used for wireless VHF-/UHF-ranges for mobile radio q q

q

SHF and higher for directed radio links, satellite communication q q

q

simple, small antenna for cars deterministic propagation characteristics, reliable connections

small antenna, beam forming large bandwidth available

Wireless LANs use frequencies in UHF to SHF range q q

some systems planned up to EHF limitations due to absorption by water and oxygen molecules (resonance frequencies) l

weather dependent fading. E.g signal loss caused by heavy rain

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.3

Frequencies and regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Cellular Phones

Cordless Phones

Wireless LANs

Others

Europe

USA

Japan

GSM 450 -457, 479486/460-467,489 496, 890 -915/935 960, 1710-1785/18051880 UMTS (FDD) 19201980, 2110-2190 UMTS (TDD) 19001920, 2020-2025 CT1+ 885-887, 930932 CT2 864 -868 DECT 1880-1900 IEEE 802.11 2400-2483 HIPERLAN 2 5150-5350, 5470 5725 R F-Control 27, 128, 418, 433, 868

AMPS, TDMA, C D M A 824-849, 869-894 TDMA, C D M A, GSM 1850-1910, 1930-1990

PDC 810 -826, 940 -956, 1429-1465, 1477-1513

PACS 1850-1910, 19301990 PACS-U B 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

R F-Control 315, 915

R F-Control 426, 868

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.4

Signals I q q q q

physical representation of data function of time and location signal parameters: parameters representing the value of data classification q q q q

q

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 q

sine wave as special periodic signal for a carrier: s(t) = At sin(2 π f t t + ϕ t)

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.5

Fourier representation of periodic signals

∞ ∞ 1 g (t ) = c + ∑ an sin( 2πnft ) + ∑ bn cos( 2πnft ) 2 n =1 n =1

1

1

0

0 t

ideal periodic signal

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

t

real composition (based on harmonics)

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2.6

Fourier Transforms and Harmonics

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.7

Signals II q

Different representations of signals q q

amplitude (amplitude domain) frequency spectrum (frequency domain)

q

phase state diagram (amplitude M and phase ϕ in polar coordinates) Q = M sin ϕ

A [V]

A [V] t[s]

ϕ I= M cos ϕ

ϕ

q q

f [Hz]

Composed signals transferred into frequency domain using Fourier transformation Digital signals need q q

infinite frequencies for perfect transmission modulation with a carrier frequency for transmission (analog signal!)

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.8

Antennas: isotropic radiator q q q q

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

x

x

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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ideal isotropic radiator

2.9

Antennas: simple dipoles q

Real antennas are not isotropic radiators but, e.g., dipoles with lengths λ/4 on car roofs or λ/2 as Hertzian dipole è shape of antenna proportional to wavelength λ/4

q

λ/2

Example: Radiation pattern of a simple Hertzian dipole y

y x

side view (xy-plane)

q

z z

side view (yz-plane)

simple dipole

x 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)

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.10

Antennas: directed and sectorized Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley)

y

y

z

x

z

side view (xy-plane)

x

side view (yz-plane)

top view (xz-plane) z

z

x

x

top view, 3 sector

directed antenna

sectorized antenna

top view, 6 sector

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.11

Antennas: diversity q

Grouping of 2 or more antennas q

q

multi-element antenna arrays

Antenna diversity q

switched diversity, selection diversity l

q

receiver chooses antenna with largest output

diversity combining l

combine output power to produce gain l cophasing needed to avoid cancellation

λ/4

λ/2

λ/4

λ/2

+

λ/2

λ/2

+

ground plane

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.12

Signal propagation ranges Transmission range q q

communication possible low error rate

Detection range q q

detection of the signal possible no communication possible

Interference range q q

signal may not be detected signal adds to the background noise

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

sender transmission distance detection interference

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2.13

Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d² in vacuum – much more in real environments (d = distance between sender and receiver) Receiving power additionally influenced by q fading (frequency dependent) q shadowing q reflection at large obstacles q refraction depending on the density of a medium q scattering at small obstacles q diffraction at edges

shadowing

reflection

refraction

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

scattering

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diffraction

2.14

Real world example

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.15

Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction multipath LOS pulses pulses

signal at sender signal at receiver

Time dispersion: signal is dispersed over time è interference with “neighbor” symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted è distorted signal depending on the phases of the different parts

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.16

Effects of mobility Channel characteristics change over time and location q q q

signal paths change different delay variations of different signal parts different phases of signal parts

è quick changes in the power received (short term fading)

Additional changes in q q

distance to sender obstacles further away

long term fading

power

è slow changes in the average power

received (long term fading) t

short term fading

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.17

Multiplexing Multiplexing in 4 dimensions q q q q

channels ki

space (si) time (t) frequency (f) code (c)

k1

k2

k3

k4

k5

k6

c t

c t

Goal: multiple use of a shared medium

s1

f s2

f

c

Important: guard spaces needed!

t

s3

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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f

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 time Advantages: q no dynamic coordination necessary k1 k2 k3 k4 k5 q works also for analog signals

k6

c

f

Disadvantages: q waste of bandwidth if the traffic is distributed unevenly q inflexible q guard spaces t

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.19

Time multiplex A channel gets the whole spectrum for a certain amount of time Advantages: q only one carrier in the medium at any time q throughput high even for many users

k1

k2

k3

k4

k5

k6

c

Disadvantages: q precise synchronization necessary

f

t

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.20

Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM Advantages: q q q

better protection against tapping protection against frequency selective interference higher data rates compared to code multiplex

k1

k2

k3

k4

k6

c f

but: precise coordination required t

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

k5

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2.21

Code multiplex Each channel has a unique code k1

k2

All channels use the same spectrum at the same time Advantages: q q q

k3

k4

k5

c

bandwidth efficient no coordination and synchronization necessary good protection against interference and tapping

f

Disadvantages: q q

lower user data rates more complex signal regeneration

Implemented using spread spectrum technology Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

k6

t

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2.22

Modulation Digital modulation q q q

digital data is translated into an analog signal (baseband) ASK, FSK, PSK - main focus in this chapter differences in spectral efficiency, power efficiency, robustness

Analog modulation q

shifts center frequency of baseband signal up to the radio carrier

Motivation q q q

smaller antennas (e.g., λ/4) Frequency Division Multiplexing medium characteristics

Basic schemes q q q

Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.23

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

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.24

Digital modulation Modulation of digital signals known as Shift Keying 1 q Amplitude Shift Keying (ASK): q q q

q

1

very simple low bandwidth requirements very susceptible to interference

Frequency Shift Keying (FSK): q

0

t

1

0

1

needs larger bandwidth t

q

Phase Shift Keying (PSK): q q

1

0

1

more complex robust against interference t

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.25

Advanced Frequency Shift Keying q q q q q q

q

bandwidth needed for FSK depends on the distance between the carrier frequencies special pre-computation avoids sudden phase shifts è MSK (Minimum Shift Keying) bit separated into even and odd bits, the duration of each bit is doubled depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen the frequency of one carrier is twice the frequency of the other Equivalent to offset QPSK even higher bandwidth efficiency using a Gaussian low-pass filter è GMSK (Gaussian MSK), used in GSM

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.26

Example of MSK 1

0

1

1

0

1

0 bit

data

even

0101

even bits

odd

0011

odd bits

signal value

hnnh - - ++

low frequency

h: high frequency n: low frequency +: original signal -: inverted signal

high frequency

MSK signal

t No phase shifts!

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.27

Advanced Phase Shift Keying Q

BPSK (Binary Phase Shift Keying): q q q q q

bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral efficiency robust, used e.g. in satellite systems

1

10

I

0

Q

11

QPSK (Quadrature Phase Shift Keying): q q q q

2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to BPSK more complex

Often also transmission of relative, not absolute phase shift: DQPSK Differential QPSK (IS-136, PHS) Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

I

00

01

A

t 11

10

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01

Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation q it is possible to code n bits using one symbol q 2n discrete levels, n=2 identical to QPSK q bit error rate increases with n, but less errors compared to comparable PSK schemes Q 0010 0011

0001 0000

f a

I 1000

Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have the same phase f , but different amplitude a. 0000 and 1000 have different phase, but same amplitude. è used in standard 9600 bit/s modems

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.29

Hierarchical Modulation DVB-T modulates two separate data streams onto a single DVB-T stream q High Priority (HP) embedded within a Low Priority (LP) stream q Multi carrier system, about 2000 or 8000 carriers q QPSK, 16 QAM, 64QAM q Example: 64QAM q q q q

good reception: resolve the entire 64QAM constellation poor reception, mobile reception: resolve only QPSK portion 6 bit per QAM symbol, 2 most significant determine QPSK HP service coded in QPSK (2 bit), LP uses remaining 4 bit

Q

10 I

00 000010

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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010101

2.30

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

spread signal

signal

power

spread interference

detection at receiver

protection againstf narrowband interference

f

Side effects: q q

coexistence of several signals without dynamic coordination tap-proof

Alternatives: Direct Sequence, Frequency Hopping Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.31

Effects of spreading and interference

dP/df

dP/df

i)

user signal broadband interference narrowband interference

ii) f sender dP/df

f

dP/df

dP/df

iii)

iv) f

receiver

v) f

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

f

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Spreading and frequency selective fading channel quality

1

2

5

3

6

narrowband channels

4 frequency narrow band signal

guard space

channel quality

1

spread spectrum

2

2

2

2

2

spread spectrum channels

frequency

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.33

DSSS (Direct Sequence Spread Spectrum) I XOR of the signal with pseudo-random number (chipping sequence) q

many chips per bit (e.g., 128) result in higher bandwidth of the signal

Advantages q q

reduces frequency selective fading in cellular networks l

base stations can use the same frequency range l several base stations can detect and recover the signal l soft handover

tb user data 0

1

XOR

tc chipping sequence 01101010110101

Disadvantages q

resulting signal

precise power control necessary 01101011001010

tb: bit period tc: chip period Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

=

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2.34

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

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.35

FHSS (Frequency Hopping Spread Spectrum) I Discrete changes of carrier frequency q

sequence of frequency changes determined via pseudo random number sequence

Two versions q q

Fast Hopping: several frequencies per user bit Slow Hopping: several user bits per frequency

Advantages q q q

frequency selective fading and interference limited to short period simple implementation uses only small portion of spectrum at any time

Disadvantages q q

not as robust as DSSS simpler to detect

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.36

FHSS (Frequency Hopping Spread Spectrum) II tb user data 0

1

f

0

1

1

t

td

f3

slow hopping (3 bits/hop)

f2 f1 f

t

td

f3

fast hopping (3 hops/bit)

f2 f1 t

tb: bit period

td: dwell time

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.37

FHSS (Frequency Hopping Spread Spectrum) III

user data modulator

modulator

received signal

hopping sequence

frequency synthesizer

transmitter

hopping sequence

spread transmit signal

narrowband signal

narrowband signal data demodulator

demodulator

frequency synthesizer

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

receiver

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2.38

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: q q q q

higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally

Problems: q q q

fixed network needed for the base stations handover (changing from one cell to another) necessary interference 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 Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.39

Frequency planning I Frequency reuse only with a certain distance between the base stations Standard model using 7 frequencies: f4 f3

f5 f1 f2

f3 f6 f7

f2 f4

f5 f1

Fixed frequency assignment: q q

certain frequencies are assigned to a certain cell problem: different traffic load in different cells

Dynamic frequency assignment: q q q

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

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.40

Frequency planning II f3 f1 f2 f3

f2 f3 f1

f3 f1 f2 f3

f2 f3 f1

f3 f1 f2

f2

3 cell cluster

f4

f3

f3 f6

f5 f1 f2

f3 f6 f7 f5

f2 f4 f3

f7 f5 f1 f2

7 cell cluster

f2 f2 f2 f1 f f1 f f1 f h h 3 3 3 h1 2 h1 2 g2 h3 g2 h3 g2 g1 g1 g1 g3 g3 g3

3 cell cluster with 3 sector antennas

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.41

Cell breathing CDM systems: cell size depends on current load Additional traffic appears as noise to other users If the noise level is too high users drop out of cells

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

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2.42