Mobile Communications Chapter 2: Wireless Transmission

2.1Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013

Mobile CommunicationsChapter 2: Wireless Transmission• Frequencies• Signals, antennas, signal propagation, MIMO• Multiplexing, Cognitive Radio• Spread spectrum, modulation• Cellular systems

2.2

Wireless communications

• The physical media – Radio Spectrum

– There is one finite range of frequencies over which radio waves can exist – this is the Radio Spectrum

– Spectrum is divided into bands for use in different systems, so Wi-Fi uses a different band to GSM, etc.

– Spectrum is (mostly) regulated to ensure fair access

Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013


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• = c/f • wave length , speed of light c 3x108m/s, frequency f

1 Mm300 Hz

10 km30 kHz

100 m3 MHz

1 m300 MHz

10 mm30 GHz

100 m3 THz

1 m300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted pair


Example 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, beam forming• large bandwidth available

• Wireless LANs use frequencies in UHF to SHF range• 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.


Frequencies and regulations• In general: ITU-R holds auctions for new frequencies, manages frequency bands

worldwide (WRC, World Radio Conferences)

Examples Europe USA JapanCellular networks

GSM 880-915, 925-960, 1710-1785, 1805-1880UMTS 1920-1980, 2110-2170LTE 791-821, 832-862, 2500-2690

AMPS, TDMA, CDMA, GSM 824-849, 869-894TDMA, CDMA, GSM, UMTS 1850-1910, 1930-1990

PDC, FOMA 810-888, 893-958PDC 1429-1453, 1477-1501FOMA 1920-1980, 2110-2170

Cordless phones

CT1+ 885-887, 930-932CT2 864-868DECT 1880-1900

PACS 1850-1910, 1930-1990PACS-UB 1910-1930

PHS 1895-1918JCT 245-380

Wireless LANs 802.11b/g 2412-2472

802.11b/g 2412-2462

802.11b 2412-2484802.11g 2412-2472

Other RF systems

27, 128, 418, 433, 868

315, 915 426, 868


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

2.7

Signals I

• signal parameters of periodic signals: period T: time to complete a wave. frequency f=1/T :no of waves generated per second amplitude A : strength of the signal phase shift :where the wave starts and stops

• sine wave as special periodic signal for a carrier:

s(t) = At sin(2 ft t + t)

• These factors are transformed into the exactly required signal by Fourier transforms.

Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013


Fourier representation of periodic signals

1

0

1

0t t

ideal periodic signal real composition(based on harmonics)

• It is easy to isolate/ separate signals with different frequencies using filters

)2cos()2sin(21)(

11

nftbnftactgn

nn

n


Signals II• Different representations of signals

• amplitude (amplitude domain)• frequency spectrum (frequency domain)• phase state diagram (amplitude M and phase 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!)

f [Hz]

A [V]

I= M cos

Q = M sin

A [V]

t[s]


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 do not produce radiate signals in equal

power in all directions. They always have directive effects (vertically and/or horizontally)

• Radiation pattern: measurement of radiation around an antenna

zy

x

z

y x idealisotropicradiator


Antennas: simple dipoles• Real antennas are not isotropic radiators but, e.g., dipoles

with lengths /4 on car roofs or /2 as Hertzian dipole shape of antenna proportional to wavelength

• Example: Radiation pattern of a simple Hertzian dipole

• Omni-Directional Antennas are wasteful in areas where obstacles occur (e.g. valleys)

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

simpledipole

/4 /2


• Directional antennas reshape the signal to point towards a target, e.g. an open street• Placing directional antennas together can be used to form

cellular reuse patterns

• Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power)

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

• Directed antennas are typically used in cellular systems, several directed antennas combined on a single pole to construct Sectorized antennas.


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

directedantenna

sectorizedantenna


Antennas: diversity• Multi-element antenna arrays: 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

+

/4/2/4

ground plane

/2/2

+

/2


• Smart antennas use signal processing software to adapt to conditions –e.g. following a moving receiver (known as beam forming), these are some way off commercially

• E.g. wireless devices direct the electro magnetic radiation away from human body toward a base station to reduce the absorbed radiation.


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

• Wireless is less predictable since it has to travel in unpredictable substances – air, dust, rain, bricks

distance

sender

transmission

detection

interference


Signal propagation: Path loss (attenuation)• In free space signals propagate as light in a straight line

(independently of their frequency).• If a straight line exists between a sender and a receiver it

is called line-of-sight (LOS) • Receiving power proportional to 1/d² in vacuum (free

space loss) – much more in real environments(d = distance between sender and receiver)

• Situation becomes worse if there is any matter between sender and receiver especially for long distances• Atmosphere heavily influences satellite transmission• Mobile phone systems are influenced by weather condition

as heavy rain which can absorb much of the radiated energy


Signal propagation: Path loss (attenuation)• Radio waves can penetrate objects depending on

frequency. The lower the frequency, the better the penetration• Low frequencies perform better in denser materials• High frequencies can get blocked by, e.g. Trees

• Radio waves can exhibit three fundamental propagation behaviours depending on their frequencies:• Ground wave (<2 MHz): follow the earth surface and can

propagate long distances – submarine communication• Sky wave (2-30 MHz): These short waves are reflected at the

ionosphere. Waves can bounce back and forth between the earth surface and the ionosphere, travelling around the world – International broadcast and amateur radio

• Line-of-sight (>30 MHz): These waves follow a straight line of sight – mobile phone systems, satellite systems


Signal propagation• Propagation in free space always like light (straight line)• Receiving power proportional to 1/d² in vacuum – much more in

real environments, e.g., d3.5…d4

(d = distance between sender and receiver)• Receiving power additionally influenced by• fading (frequency dependent)• shadowing• reflection at large obstacles• refraction depending on the density of a medium• scattering at small obstacles• diffraction at edges

reflection scattering diffractionshadowing refraction


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

signal at sendersignal at receiver

LOS pulsesmultipathpulses

LOS(line-of-sight)


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)short term fading

long termfading

t

power


Multiplexing• Multiplexing describes how several users can share a

medium with minimum or no interference• It is concerned with sharing the frequency range amongst

the users• Bands are split into channels• Four main ways of assigning channels

• Space Division Multiplexing (SDM) : allocate according to location

• Time Division Multiplexing (TDM): allocate according to units of time

• Frequency Division Multiplexing (FDM): allocate according to the frequencies

• Code Division Multiplexing (CDM) : allocate according to access codes

• Guard Space: gaps between allocations


• Multiplexing in 4 dimensions• space (si)• time (t)• frequency (f)• code (c)

• Goal: multiple use of a shared medium

• Important: guard spaces needed!

Multiplexing

s2

s3

s1f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki


Space Division Multiplexing (SDM)

• Space Division • This is the basis of

frequency reuse• Each physical space is

assigned channels• Spaces that don’t overlap

can have the same channels assigned to them

• Example: FM radio stations in different countries

s2

s3

s1f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki


Frequency Division Multiplexing (FDM)• Separation of the whole spectrum into smaller non

overlapping frequency bands (guard spaces are needed)

• A channel gets a certain band of the spectrum for the whole time – receiver has to tune to the sender frequency

• Advantages• no dynamic coordination

necessary • works also for analog signals

• Disadvantages• waste of bandwidth

if the traffic is distributed unevenly

• inflexible

k2 k3 k4 k5 k6k1

f

t

c


f

t

c

k2 k3 k4 k5 k6k1

Time Division Multiplexing (TDM)

• A channel gets the whole spectrum for a certain amount of time

• Guard spaces (time gaps) are needed• Advantages

• only one carrier in themedium at any time

• throughput high even for many users

• Disadvantages• precise clock

synchronization necessary


f

Time and frequency multiplexing

• 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

• but: precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1


Code Division Multiplexing (CDM)

• Code Division• Instead of splitting the channel, the receiver is told

which channel to access according to a pseudo-random code that is synchronised with the sender

• The code changes frequently• Security: unless you know the code it is (almost)

impossible to lock onto the signals• Interference: reduced as the code space is huge• Complexity: very high


Code multiplexing• 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

• precise power control required• more complex signal regeneration

• Implemented using spread spectrum technology

k2 k3 k4 k5 k6k1

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., /4)• Frequency Division Multiplexing• medium characteristics

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


Modulation and demodulation

synchronizationdecision

digitaldataanalog

demodulation

radiocarrier

analogbasebandsignal

101101001 radio receiver

digitalmodulation

digitaldata analog

modulation

radiocarrier

analogbasebandsignal

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• more error resilience than AM

• Phase Shift Keying (PSK):• more complex• robust against interference

1 0 1

t

1 0 1

t

1 0 1

t

2.33

Analog modulation

• Impress an information-bearing analog waveform onto a carrier waveform for transmission


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

detection atreceiver

interference spread signal

signal

spreadinterference

f f

power power


Effects of spreading and interference

dP/df

fi)

dP/df

fii)

senderdP/df

fiii)

dP/df

fiv)

receiver fv)

user signalbroadband interferencenarrowband interference

dP/df


Spreading and frequency selective fading

frequency

channelquality

1 23

4

5 6

narrow bandsignal

guard space

22222

frequency

channelquality

1

spreadspectrum

narrowband channels

spread spectrum channels

2.37

Spread spectrum problems

• Spread spectrum problems:• Increased complexity of receivers• Raising background noise

• Spread spectrum can be achieved in two different ways: • Direct Sequence• Frequency Hopping

2.38

Spread Spectrum – Direct Sequence Spread Spectrum (DSSS)

• Each bit in original signal is represented by multiple bits in the transmitted signal

• Spreading code spreads signal across a wider frequency band

• 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

2.39

DSSS• Chipping sequence appears like

noise, to others• Spreading factor S = tb /tc• If the original signal needs a

bandwidth w, the resulting signal needs s*w

• The exact codes are optimised for wireless• E.g. for Wi-Fi 10110111000

(Barker code)• For civil application

spreading code between 10 and 100

• For military application the spreading code is up to 10,000

user data

chipping sequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit periodtc: chip period

2.40

DSSS

• New modulation process:• Sender: Chipping Digital Mod. Analog Mod.• Receiver: Demod. Chipping Integrator Decision?

• At the receiver after the XOR operation (despreading), an integrator adds all these products, then a decision is taken for each bit period

• Even if some of the chips of the spreading code are affected by noise, the receiver may recognize the sequence and take a correct decision regarding the received message bit.

2.41

DSSS (Direct Sequence Spread Spectrum)

Xuser data

chippingsequence

modulator

radiocarrier

spreadspectrum

signaltransmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

chippingsequence

lowpassfilteredsignal

receiver

integrator

products

decisiondata

sampledsums

correlator


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


FHSS (Frequency Hopping Spread Spectrum) II

user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

tb

0 1 1 t

f

f1

f2

f3

t

td

f

f1

f2

f3

t

td

tb: bit period td: dwell time


FHSS (Frequency Hopping Spread Spectrum) III

modulatoruser data

hoppingsequence

modulator

narrowbandsignal spread

transmitsignal

transmitter

receivedsignal

receiver

demodulatordata

frequencysynthesizer

hoppingsequence

demodulator

frequencysynthesizer

narrowbandsignal

2.45

Resource Management

• Radio Resource Management• Channel Access• Channel Assignment

• Power Management• Mobility Management

• Location Management• Handoff/Handover: the term handover or

handoff refers to the process of transferring an ongoing call or data session from one channel to another

• Example: The cellular System

2.46

The cellular system: cell structure• Channel allocation: Implements space division

multiplexing (SDM)• base station covers a certain transmission area (cell)• Cellular concept: channel reuse across the network

prevents interference, improves the likelihood of a good signal in each 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

• Expensive• fixed network needed for the base stations• handover (changing from one cell to another) necessary• interference with other cells

f4

f5

f1f3

f2

f6

f7

f3f2

f4

f5

f1

2.47

Frequency planning

• Frequency reuse only with a certain distance between the base stations

• Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

• Cells are combined in clusters• All cells within a cluster use disjointed sets of

frequencies• The transmission power of a sender has to

be limited to avoid interference • Standard model using 7 frequencies• To reduce interference further, sectorized antennas

can be used especially for larger cell radii

f4

f5

f1f3

f2

f6

f7

f3f2

f4

f5

f1

2.48

Frequency planning

f1

f2

f3f2

f1

f1

f2

f3f2

f3

f1

f2f1

f3f3

f3f3

f3

f4

f5

f1f3

f2

f6

f7

f3f2

f4

f5

f1f3

f5f6

f7f2

f2

f1f1 f1

f2

f3

f2

f3

f2

f3h1

h2

h3g1

g2

g3

h1

h2

h3g1

g2

g3g1

g2

g3

3 cell cluster

7 cell cluster

3 cell clusterwith 3 sector antennas

2.49

Radio resource management• Channel Allocation

• Channel Allocation is required to optimise frequency reuse • Fixed Channel Allocation• Dynamic Channel Allocation• Hybrid Channel Allocation

1

2 4

56

7 2

13 6

7

3

Frequency Reuse

2.50

Radio resource management : Channel allocation• Fixed Channel Allocation (FCA)

• Permanent or semi-permanent allocation• Certain frequencies are assigned to a certain cell• Problem: different traffic load in different cells• Methods:

• Simple: all cells have same number of channels• Non-uniform: optimise usage

according to expected traffic• Borrowing: channels can be

reassigned if underused (BCA)

1

2 4

56

7 2

13 6

7

3

Frequency Reuse

2.51

Radio resource management• Dynamic Channel Allocation (DCA)

• Gives control to base stations / switches to adapt• Channels are assigned as needed, not in advance• Base station chooses frequencies depending on the

frequency already used in neighbour cells• Channels are returned when user has finished• More capacity in cells with more traffic• Assignment can also be based on interference

measurements• Affecting factors include:

• Blocking probability• Usage patterns and reuse distance• Current channel measurement

2.52

Radio resource management

• Hybrid Channel Allocation (HCA)• Fixed schemes are not flexible enough• Dynamic schemes are too complex / difficult• Hybrid Schemes:

• Split resources into pools of fixed and dynamic channels

• Assign core of fixed channels then allocate rest dynamically

• Altering the ratio may optimise the system• E.g. produce the lowest blocking rate

2.53

Radio resource management• Overlapping Cells

• Cells are naturally overlap (ideal shape is circular)

• System may push some users into adjacent cells

• Cost: increased handoff rate

• Handoff• Two types of channel assignment: new calls,

handoff• New calls have lower priority than handoff

calls• QoS Channel Access Control should favour

handoff over new

2.54

Radio resource management

• Macrocell/Microcell Overlay• Smaller cells increases frequency of handoff• Overlaying large cells on top of small ones:

• Fast moving terminals are assigned channels in Macrocells

• Slow moving terminals can use microcells• Overlap can be used to handoff during congestion• Increases the capacity (area)• But, increases the complexity


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

2.56

Conclusion

• Radio spectrum is regulated• Characteristics of the signal depends upon its

frequency• Sine wave is the basic signal for radio

communication• Signal attenuates while moving in free space. • Signals affected by obstacles and mobility pattern• Multiplexing schemes are needed to share the

medium between different signals. A guard space is needed with all multiplexing schemes.

• Both digital and analogue modulation schemes are required for radio communication

• Spread spectrum is needed to prevent narrow band interference

Mobile Communications Chapter 2: Wireless Transmission

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Transcript of Mobile Communications Chapter 2: Wireless Transmission