Mobile Communications by Schiller�Chapter 2: Wireless Transmission

• Frequencies
• Signals
• Antenna
• Signal propagation

​

Mobile Communication: Wireless Transmission

2.0.1

• Multiplexing
• Spread spectrum
• Modulation
• Cellular systems

Frequencies for communication

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 ≅ 3x10⁸m/s, frequency f

Mobile Communication: Wireless Transmission

2.1.1

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

Frequencies for mobile communication

• VHF-/UHF-ranges for mobile radio
• simple, small antenna for cars
• deterministic propagation characteristics, reliable connections
• SHF and higher for directed radio links, satellite communication
• small antenna, focussing
• large bandwidth available
• Wireless LANs use frequencies in UHF to SHF spectrum
• some 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.

​

Mobile Communication: Wireless Transmission

2.2.1

Frequencies and regulations

ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)

Mobile Communication: Wireless Transmission

2.3.1

Signals I

• 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) = A_t sin(2 π f_t t + ϕ_t)

Mobile Communication: Wireless Transmission

2.4.1

Fourier representation of periodic signals

Mobile Communication: Wireless Transmission

1

0

1

0

t

t

ideal periodic signal

real composition

(based on harmonics)

2.5.1

Signals II

• Different representations of signals
• amplitude (amplitude domain)
• frequency spectrum (frequency domain)
• phase state diagram (amplitude M and phase ϕ in polar coordinates)

​

​

​

​

​

• 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!)

Mobile Communication: Wireless Transmission

f [Hz]

A [V]

ϕ

I= M cos ϕ

Q = M sin ϕ

ϕ

A [V]

t[s]

2.6.1

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

​

Mobile Communication: Wireless Transmission

2.7.1

z

y

x

z

y

x

ideal

isotropic

radiator

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)

​

Mobile Communication: Wireless Transmission

2.8.1

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

simple

dipole

λ/4

Antennas: directed and sectorized

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

Mobile Communication: Wireless Transmission

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

2.9.1

top view, 3 sector

x

z

top view, 6 sector

x

z

directed

antenna

sectorized

antenna

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

​

Mobile Communication: Wireless Transmission

2.10.1

+

λ/4

λ/2

λ/4

ground plane

λ/2

λ/2

+

λ/2

Types of Antennas:

1. Wire antennas
2. Aperture antennas
3. Reflector antennas
4. Lens antennas
5. Microstrip antennas
6. Array antennas

​

Mobile Communication: Wireless Transmission

Smart antennas

Smart antenna system combines

1. Antenna array technology with
2. DSP algorithms to make the antenna system smart

Mobile Communication: Wireless Transmission

Types of Smart Antenna Systems

1. Switched-Beam: A finite number of fixed, predefined patterns

2. Adaptive Array: A theoretically infinite number of patterns (scenario-based) that are adjusted in real time according to the spatial changes of SOIs and SNOIs.

Mobile Communication: Wireless Transmission

Switched-Beam

Mobile Communication: Wireless Transmission

Adaptive Array

Mobile Communication: Wireless Transmission

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

​

Mobile Communication: Wireless Transmission

distance

sender

transmission

detection

interference

2.11.1

Signal propagation

Propagation in free space always like light (straight line)

Receiving power proportional to 1/d² �(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

Mobile Communication: Wireless Transmission

reflection

scattering

diffraction

shadowing

2.12.1

Multipath propagation

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

Mobile Communication: Wireless Transmission

signal at sender

signal at receiver

2.13.1

Effects of mobility

Channel characteristics change over time and location

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

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

​

Additional changes in

• distance to sender
• obstacles further away

🡺 slow changes in the average power �received (long term fading)

Mobile Communication: Wireless Transmission

short term fading

long term

fading

2.14.1

t

power

Multiplexing

Multiplexing in 4 dimensions

• space (s_i)
• time (t)
• frequency (f)
• code (c)

​

Goal: multiple use �of a shared medium

​

Important: guard spaces needed!

Mobile Communication: Wireless Transmission

s₂

s₃

s₁

f

t

c

k₂

k₃

k₄

k₅

k₆

k₁

f

t

c

f

t

c

channels k_i

2.15.1

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 �necessary
• works also for analog signals

​

Disadvantages:

• waste of bandwidth �if the traffic is �distributed unevenly
• inflexible
• guard spaces

​

Mobile Communication: Wireless Transmission

k₂

k₃

k₄

k₅

k₆

k₁

f

t

c

2.16.1

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 �necessary

Mobile Communication: Wireless Transmission

f

t

c

k₂

k₃

k₄

k₅

k₆

k₁

2.17.1

Time and frequency multiplex

Combination of both methods

A channel gets a certain frequency band for a certain amount of time

Example: GSM

Advantages:

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

but: precise coordination�required

Mobile Communication: Wireless Transmission

f

t

c

k₂

k₃

k₄

k₅

k₆

k₁

2.18.1

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:

• lower user data rates
• more complex signal regeneration

Implemented using spread spectrum technology

Mobile Communication: Wireless Transmission

2.19.1

k₂

k₃

k₄

k₅

k₆

k₁

f

t

c

Modulation

Digital modulation

• 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

• shifts center frequency of baseband signal up to the radio carrier

Motivation

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

Basic schemes

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

Mobile Communication: Wireless Transmission

2.20.1

Modulation and demodulation

Mobile Communication: Wireless Transmission

synchronization

decision

digital

data

analog

demodulation

radio

carrier

analog

baseband

signal

101101001

radio receiver

2.21.1

digital

modulation

digital

data

analog

modulation

radio

carrier

analog

baseband

signal

101101001

radio transmitter

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

Mobile Communication: Wireless Transmission

2.22.1

1

0

1

t

1

0

1

t

1

0

1

t

Advanced Frequency Shift Keying (MSK)

In a first step, data bits are separated into even and odd bits, the duration of each bit being doubled.

The scheme also uses two frequencies: f1, the lower frequency, and f2, the higher frequency, with f2 = 2f1.

Mobile Communication: Wireless Transmission

2.23.1

Rules of MSK

According to the following scheme, the lower or higher frequency is chosen (either inverted or non-inverted) to generate the MSK signal:

● if the even and the odd bit are both 0, then the higher frequency f2 is inverted (i.e., f2 is used with a phase shift of 180°);

● if the even bit is 1, the odd bit 0, then the lower frequency f1 is inverted. This is the case, e.g., in the fifth to seventh columns of Figure (Example of MSK),

● if the even bit is 0 and the odd bit is 1, as in columns 1 to 3, f1 is taken without changing the phase,

● if both bits are 1 then the original f2 is taken.

​

A high frequency is always chosen if even and odd bits are equal. The signal is inverted if the odd bit equals 0. This scheme avoids all phase shifts in the resulting MSK signal.

Mobile Communication: Wireless Transmission

Example of MSK

Mobile Communication: Wireless Transmission

Example of MSK

Mobile Communication: Wireless Transmission

2.24.1

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 n n h�value - - + +

h: high frequency

n: low frequency

+: original signal

-: inverted signal

No phase shifts!

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

Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PACS, PHS)

​

Mobile Communication: Wireless Transmission

11

10

00

01

2.25.1

A

t

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, n=2 identical to QPSK
• bit error rate increases with n, but less errors compared to comparable PSK schemes

​

Example: 16-QAM (4 bits = 1 symbol)

Symbols 0011 and 0001 have the same phase, but different amplitude. 0000 and 1000 have different phase, but same amplitude.

🡺 used in standard 9600 bit/s modems

Mobile Communication: Wireless Transmission

0000

0001

0011

1000

2.26.1

Q

I

0010

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

​

​

​

​

protection against narrowband interference

Side effects:

• coexistence of several signals without dynamic coordination
• tap-proof

Alternatives: Direct Sequence, Frequency Hopping

​

Mobile Communication: Wireless Transmission

detection at

receiver

interference

spread signal

signal

spread

interference

2.27.1

f

f

power

power

Effects of spreading and interference

Mobile Communication: Wireless Transmission

P

f

i)

P

f

ii)

sender

P

f

iii)

P

f

iv)

receiver

f

v)

user signal

broadband interference

narrowband interference

2.28.1

P

Spreading and frequency selective fading

Mobile Communication: Wireless Transmission

frequency

channel�quality

1

2

3

4

5

6

narrow band�signal

guard space

2.29.1

narrowband channels

spread spectrum channels

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 necessary

Mobile Communication: Wireless Transmission

2.30.1

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

=

t_b

t_c

t_b: bit period

t_c: chip period

DSSS (Direct Sequence Spread Spectrum) II

Mobile Communication: Wireless Transmission

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

2.31.1

FHSS (Frequency Hopping Spread Spectrum) I

Discrete changes of carrier frequency

• sequence of frequency changes determined via pseudo random number sequence

Two versions

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

Advantages

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

Disadvantages

• not as robust as DSSS
• simpler to detect

​

Mobile Communication: Wireless Transmission

2.32.1

FHSS (Frequency Hopping Spread Spectrum) II

Mobile Communication: Wireless Transmission

user data

slow

hopping

(3 bits/hop)

fast

hopping

(3 hops/bit)

0

1

t_b

0

1

1

t

f

f₁

f₂

f₃

t

t_d

f

f₁

f₂

f₃

t

t_d

t_b: bit period t_d: dwell time

2.33.1

FHSS (Frequency Hopping Spread Spectrum) III

Mobile Communication: Wireless Transmission

modulator

user data

hopping

sequence

modulator

narrowband

signal

spread

transmit

signal

transmitter

received

signal

receiver

demodulator

data

frequency

synthesizer

hopping

sequence

demodulator

frequency

synthesizer

narrowband

signal

2.34.1

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

Problems:

• 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

Mobile Communication: Wireless Transmission

2.35.1

Frequency planning I

Frequency reuse only with a certain distance between the base stations

Standard model using 7 frequencies:

​

​

​

​

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

Mobile Communication: Wireless Transmission

2.36.1

Frequency planning II

Mobile Communication: Wireless Transmission

f₄

f₅

f₁

f₃

f₂

f₆

f₇

f₃

f₂

f₄

f₅

f₁

f₃

f₅

f₆

f₇

f₂

f₂

2.37.1

3 cell cluster

7 cell cluster

3 cell cluster

with 3 sector antennas