transmission wireless

Dr.-Ing. Jean-Pierre Ebert MC-I SS07 2.1

Mobile Communications

Chapter 2: Wireless Transmission

Frequencies Signals Antenna Signal propagation

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:

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

1 Mm

300 Hz

10 km

30 kHz

100 m

3 MHz

1 m

300 MHz

10 mm

30 GHz

100 m

3 THz

1 m

300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted

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, satellitecommunication

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 rainfalletc.


Frequencies and regulations

ITU-R holds auctions for new frequencies, manages frequency bands

worldwide (WRC, World Radio Conferences) Europe USA Japan

Cellular Phones

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

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

IEEE 802.11

2400-2483 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


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 sine wave as special periodic signal for a carrier:

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


Fourier representation of periodic signals

)2cos()2sin(2

1)(

11

nftbnftactgn

n

n

n

=

=

++=

1

0

1

0

t t

ideal periodic signal real composition

(based on harmonics)


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 Fouriertransformation

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

Signals II

f [Hz]

A [V]

I= M cos

Q = M sin

A [V]

t[s]


Radiation and reception of electromagnetic waves, coupling ofwires to space for radio transmission

Isotropic radiator: equal radiation in all directions (threedimensional) - only a theoretical reference antenna

Real antennas always have directive effects (vertically and/orhorizontally)

Radiation pattern: measurement of radiation around an antenna

Antennas: isotropic radiator

zy

x

z

y x ideal

isotropic

radiator


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

Gain: maximum power in the direction of the main lobe compared tothe 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

/4 /2


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 or base stations for mobile phones

(e.g., radio coverage of a valley)

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

+

/4/2/4

ground plane

/2/2

+

/2


Signal propagation ranges

distance

sender

transmission

detection

interference

Transmission range

communication possible low error rate

Detection range

detection of the signalpossible

no communicationpossible

Interference range

signal may not bedetected

signal adds to thebackground noise


Signal propagation

Propagation in free space always like light (straight line)

Receiving power proportional to 1/d_ in vacuum much more in real environments

(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


Real world example


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) pulses become wider, Delay Spread

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

LOS pulsesmultipath

pulses


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 powerreceived (long term fading)

short term fading

long term

fading

t

power


Multiplexing in 4 dimensions

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

Goal: multiple use

of a shared medium

Important: guard spaces needed!

s2

s3

s1

Multiplexing

f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki


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 coordinationnecessary

works also for analog signals

Disadvantages:

waste of bandwidthif the traffic is

distributed unevenly

inflexible guard spaces

k2 k3 k4 k5 k6k1

f

t

c


f

t

c

k2 k3 k4 k5 k6k1

Time multiplex

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

Advantages:

only one carrier in themedium at any time

throughput high evenfor many users

Disadvantages:

precisesynchronization

necessary


f

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 againsttapping

protection against frequencyselective interference

higher data rates compared tocode multiplex

but: precise coordination

required

t

c

k2 k3 k4 k5 k6k1


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 andtapping

Disadvantages:

lower user data rates 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 carrierMotivation

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

Basic schemes

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


Modulation and demodulation

synchronization

decision

digital

dataanalog

demodulation

radio

carrier

analog

baseband

signal

101101001 radio receiver

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

1 0 1

t

1 0 1

t

1 0 1

t


Advanced Frequency Shift Keying

bandwidth needed for FSK depends on the distance betweenthe carrier frequencies

special pre-computation avoids sudden phase shifts MSK (Minimum Shift Keying)

bit separated into even and odd bits, the duration of each bit isdoubled

depending on the bit values (even, odd) the higher or lowerfrequency, 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-passfilter GMSK (Gaussian MSK), used in GSM


Example of MSK

data

even bits

odd bits

1 1 1 1 000

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 complexOften also transmission of relative, not

absolute phase shift: DQPSK -

Differential QPSK (IS-136, PHS)11 10 00 01

Q

I01

Q

I

11

01

10

00

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 a. 0000 and 1000 have

different phase, but same amplitude.

used in standard 9600 bit/s modems

0000

0001

0011

1000

Q

I

0010

_

a


Hierarchical Modulation

DVB-T modulates two separate data streams onto a single DVB-T stream

High Priority (HP) embedded within a Low Priority (LP) stream Multi carrier system, about 2000 or 8000 carriers QPSK, 16 QAM, 64QAM Example: 64QAM

good reception: resolve the entire64QAM constellation

poor reception, mobile reception:resolve only QPSK portion

6 bit per QAM symbol, 2 mostsignificant determine QPSK

HP service coded in QPSK (2 bit),LP uses remaining 4 bit

Q

I

00

10

000010 010101


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

detection at

receiver

interference spread

signal

signal

spread

interference

f f

power power


Effects of spreading and interference

dP/df

f

i)

dP/df

f

ii)

sender

dP/df

f

iii)

dP/df

f

iv)

receiverf

v)

user signal

broadband interference

narrowband interference

dP/df


Spreading and frequency selective fading

frequency

channel

quality

1 2

3

4

5 6

narrow band

signal

guard space

22

22

2

frequency

channel

quality

1

spread

spectrum

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 signalAdvantages

reduces frequency selectivefading

in cellular networks base stations can use the

same frequency range

several base stations candetect and recover the signal

soft handoverDisadvantages

precise power control necessary

user data

chipping

sequence

resulting

signal

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 period

tc: chip period


DSSS (Direct Sequence Spread Spectrum) II

X

user data

chipping

sequence

modulator

radio

carrier

spread

spectrum

signaltransmit

signal

transmitter

demodulator

received

signal

radio

carrier

X

chipping

sequence

lowpass

filtered

signal

receiver

integrator

products

decision

data

sampled

sums

correlator


FHSS (Frequency Hopping Spread Spectrum) I

Discrete changes of carrier frequency

sequence of frequency changes determined via pseudo random numbersequence

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

slow

hopping

(3 bits/hop)

fast

hopping

(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

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


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


Frequency planning I

Frequency reuse only with a certain distance between the basestations

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 frequenciesalready used in neighbor cells

more capacity in cells with more traffic assignment can also be based on interference measurements

f4

f5

f1f3

f2

f6

f7

f3f2

f4

f5

f1


Frequency planning II

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 f1f2f3

f2f3

f2f3

h1h2h3

g1g2g3

h1h2h3

g1g2g3

g1g2g3

3 cell cluster

7 cell cluster

3 cell cluster

with 3 sector antennas


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

Andreas Willig

Overview

The Cellular Concept

System Capacity

Channel Allocation

Handover

Paging / Location Update

Cellular System Fundamentals, slide 2

Andreas Willig The cellular concept

The cellular concept

cellular systems evolved from the first systems supporting wireless andmobile telephony

initially their design was focused towards telephony services, data serviceswere added later on



Some important milestones

1946: the very first analog systems for public mobile telephony areintroduced in some cities of the US

1983: the analog AMPS system for wireless telephony is introduced

1987-1991: development phase of GSM [9], a digital cellular network

2001: GPRS becomes publicly available (packet-switching over GSM)

1993: IS-95 [5] is the first commercial cellular CDMA system



Some important milestones II

1989-today: development, deployment and operation of UMTS: UMTS = Universal Mobile Telecommunications System [8] third-generation digital cellular CDMA system supporting data andvoice services

since 1999 the UMTS specification is controlled by the 3GPP (3rdGeneration Partnership Project) all UMTS specifications are publiclyavailable under

www.3gpp.org



Design Assumptions and Requirements

the available amount of spectrum is limited

a large number of users in a large geographical area must be supported,otherwise the system is not accepted

business people term this networking effect: it becomes more and more attractive for an individual to

subscribe to cellular services the more people he/she can reach this way



Design Assumptions and Requirements II

the area is subdivided into cells: each cell has at its center a base station (BS) each cell contains a number of mobile stations (MS) all communication from/to an MS is relayed through a BS, there is nopeer-to-peer communication

the BSs are interconnected and connected to fixed networks / othercellular networks through a backbone or core network and gateways

two important notions indicate the direction of communication in a cell: downlink: from the BS to a MS uplink: from the MS to a BS



Design Assumptions and Requirements III

Backbone

Interworking Function /Gateway

OtherNetworks

BS 1 BS 2

1

2

2 1



Cell Sizes and Frequency Reuse

in systems like GSM a number of channels or frequency bands is allocatedto each BS

the cell size is determined from the transmit power of the BS and thereceive threshold of the MS:

as long as an MS can decode the signal of the BS, it is inside the cellbut even outside the cell the BS signal may cause interference

the stronger the BSs tx power the larger the cell / interference area the reuse distance of a frequency f used by a BS A is the geographicaldistance where As signal only causes negligible interference, and fmay be re-used by another BS B



Cell Sizes and Frequency Reuse II

choosing a small transmit power in BSs thus:

= reduces cell sizes and reuse distances

= increases the number of users which may use the samefrequency in a given area (at places separated by at leastreuse distance), and thus increases the system capacitythe larger the system capacity the more revenue is possible

= increases the number of base stations and thus thesystem costs



Cell Sizes and Frequency Reuse III

typically network operators choose different cell sizes: large cells (macrocells) in sparsely populated rural areas small cells (microcells or picocells) in densely populated urban areas large cells overlaying small cells (umbrella cells) to support highlymobile users

network operators have to do proper cell planning [3] to: achieve full coverage of a given area accommodate the expected number of users / user densities minimize the number of base stations needed

cell planning results in locations of base stations and their cell size



Mobility and Handover

mobile users reach the boundary of a cell and move into the next cellfrom time to time

ongoing calls should be maintained when crossing cell boundaries, thecall should be handed over from the old cell to the new cell

a handover procedure involves exchange of signalling messages between: MS old BS and new BS some further network elements in the backbone, e.g. the gateway tothe fixed network



Mobility and Handover II

Gateway to PSTN



Mobility and Handover III

= the smaller the cell sizes the more handover events!

= increased signalling traffic

= since handovers take some minimum time, a too highhandover rate can lead to loss of connection

= there is a tradeoff between cell sizes and supportedmobile speeds



Overview


System Capacity

Channel Allocation

Handover



Andreas Willig System Capacity

System Capacity

in certain cellular systems the allocated spectrum is subdivided into anumber N of equal-sized frequency channels or channels

these have to be allocated to the BS such that: co-channel interference is minimized adjacent channel interference is minimized reuse distance is properly considered

a channel (or portions of it) is assigned to a MS for the duration of a call

a connected set of M base stations / cells in which each of the Nfrequencies is assigned exactly once, is called a cluster



Visualization and Modeling of Cells

case b) sectorized antennacase a) omnidirectional antenna



Visualization and Modeling of Cells A Clustering

Example

G

B

C

A

D

E

F G

B

C

A

D

E

F

G

B

C

A

D

E

F



Visualization and Modeling of Cells II

in reality: cells have no regular shape two cells typically overlap by 10% to 15% to enable handover a MS in the overlap region of two cells belongs to either cell withsome probability (soft cell boundaries)

for the hexangular cell layout only certain cluster sizes M are feasible(i.e. can create a plane tiling) these satisfy the relation:

M = i2 + i j + j2 i, j N0

solutions are M = 1, 3, 4, 7, 9, 12, 13, . . .



Estimation of System Capacity

let us make the following assumptions: we adopt the hexagonal cell model the overall number of available channels is N each cluster consists of M cells, to each cell of a cluster k frequenciesare assigned (k M = N)

each cell has radius R be D the distance between two BS using the same frequency (reusedistance)

we consider only downlink direction, co-channel interference at a MShas its source in transmissions of other BS than the current one

the path loss exponent is n, valid for the whole system area



Estimation of System Capacity II

the ratioQ =

D

Rgives the normalized reuse distance, Q is also denoted as co-channelreuse ratio

for the hexagonal cell layout one can show that:

Q =3 M



Estimation of System Capacity III

if Q is large, we have: less interference

= smaller bit-/symbol error rates= better speech quality

less channels per cell= decreased capacity

conversely, a small Q leads to higher interference and higher capacity



Estimation of System Capacity IV

let us fix one interior cell and look at its signal-to-interference ratio (SIR)as experienced by a MS at the fringe of the cell:

S

I=

Pr(R)iI Pr(Di)

where:

I is the set of all interferers, i.e. the set of all BS using the samefrequency

Di is the distance between the MS and the i-th interferer

for maintaining a good speech quality a SIR of 18 dB should be used



Estimation of System Capacity V

if we take into account that:

Pr(d) Pr(d0) (d

d0

)n

and assume d0 = 1 (in appropriate units), we have:

S

I=

RniID

ni



Estimation of System Capacity VI

if we take only the closest I0 interferers into consideration and assumethat they all have the same distance D we have

S

I=

Rn

I0 Dn=

(D

R

)n 1I0

=

(3 M

)nI0

= for larger n (e.g. in urban areas) and for fixing a minimalSIR (e.g. of 18 dB) we can decreaseM (increase numberk of frequencies per cell) and thus make smaller clusters,thus increasing capacity



Overview


System Capacity

Channel Allocation

Handover



Andreas Willig Channel Allocation

Channel Allocation

in practice the allocation of channels to cells depends on: the expected traffic load per cell / per area unit example: urban vs. rural areas

certain performance measures

= often different cells have different numbers of channelsassigned, to accommodate differences in traffic load



Call Blocking and Call Dropping

each cell i in a cellular system has a finite number ki of channels

if a MS requests a new call in a full cell, the call is blocked

if a MS moves from a neighbored cell into a full cell, the call is dropped

there are two important performance measures for a cellular system: call blocking probability call dropping probability



Call Blocking and Call Dropping II

the call blocking probability : should be low (typical target: < 1%), blocked customers are unhappy can be computed under specific assumptions (Poisson arrivals,exponential call holding times) from simple queueing theory results(Erlang loss formulas)

the call dropping probability : should be even lower, call dropping makes customers even more angry

= proper channel allocation strategies are needed to fulfillthese requirements for a given traffic load



Fixed Channel Allocation (FCA)

channels are assigned to cells / BS on a permanent basis and the BSassigns them to the MS for the duration of a call

= FCA is susceptible to call blocking / call dropping

the following constraints have to be considered in the assignment: number of available frequencies avoiding adjacent-channel interference: do not assign neighbored frequencies to a single cell do not assign neighbored frequencies to neighbored cells

avoid co-channel interference: keep a minimum distance between twocells using the same frequency

accommodate expected traffic load



Fixed Channel Allocation (FCA) II

a number of heuristic techniques have been developed to solve FCAproblems [6]

an assignment is only valid as long as the traffic load distribution doesnot change (much) otherwise the call dropping/blocking rate increasesand the allocation must be re-computed



FCA with Borrowing

if a new call or handover call arrives to a crowded cell, the BS might askits neighbor BS to borrow a channel for the call duration:

if successful, the channel is temporarily used by the accepting BS it is not used by the donating BS for the duration of the call after the call finishes the accepting BS returns the channel

still the interference constraints have to be obeyed



Dynamic Channel Allocation

several cells are grouped into a cluster

a cluster possesses a clusterhead (CH)

channel allocation is done dynamically by the CH: if a new or handover call arrives to a BS, the BS requests a channelfrom the CH and issues this to the MS

after the call finished, the channel is returned to the CH

this approach allows to explore statistical multiplexing gains!!

a CH can coordinate with neighbored CHs to minimize co-channelinterference



Overview


System Capacity

Channel Allocation

Handover



Andreas Willig Handover

Handover

a handover becomes necessary if a MS with an ongoing call: is about to leave its current cell, and is about to enter a neighbored cell

possible causes for handovers: mobility of the MS signal degradation to current BS due to moving obstacles

types of handovers (w.r.t. data, not to signalling connections!): hard handover : MS has a data connection to at most one BS soft handover : MS can communicate with several BS simultaneously



Handover II

Gateway to PSTN



Handover III

necessary actions in a hard handover: the new BS must allocate a channel and assign it to the MS the old BS must deallocate the channel if the call is going through a PSTN gateway the connection betweenthe gateway and the old BS must be re-routed to the new BS

important requirements: no noticeable degradation of speech quality during handover no actions required from the user

important questions: when is a handover initiated? who initiates the handover?



Handover Initiation

let us assume that: the BS/MS need a minimum signal power Pmin to maintain a call atan acceptable level of speech quality

if signal power drops below this level (the MS moves out of the rangeof the BS) the call is canceled

no handoff is initiated as long as the signal level is above

Pmin +

with the safety margin > 0 a handover process takes some minimum time tmin



Handover Initiation II

if is too large the handover is initiated early and unnecessarily

= increased signalling traffic

if is too small: if the mobiles speed v is so large that it moves out of the cell beforethe handover is completed (tmin) the old connection drops and thereis no chance to set up the connection to the new BS



Handover Initiation III

to determine the signal strength, measurements must be taken over atimespan sufficiently long to average over fast fading

the measurements can either be done by the BS or by the MS

in mobile-assisted handover (MAHO): the MS measures the signal strength of surrounding BS (e.g. byevaluating signal strength of specific beacon packets)

the MS reports the measurement values to its current BS or otherstations in the network

handover if another BS is significantly stronger than the current BSsuch a behavior is called hysterese: choosing the BS such that at any time the MS is connected to

the best one would likely produce many handovers



Handover Initiation IV

MAHO is used both in GSM and UMTS

GSM: hard handover execution of a handover after making the decision takes one to twoseconds

UMTS: soft handover and softer handover



Mechanisms for Call Dropping Avoidance

to avoid dropping of handover calls, the BS treats channel allocation fornew calls and handover calls differently

the guard channel concept: the BS puts aside some channels and allocates these exclusively tohandover calls

= reduced capacity= can be effectively combined with DCA, to avoid

allocating guard channels in all cells of a cluster



Mechanisms for Call Dropping Avoidance II

queueing of handover requests: the BS or CH puts handover requests into a queue as soon as an ongoing call ends or roams away, the correspondingchannel is assigned to a handover request from the queue

in this case tmin can be large, depending on the number of mobiles



Umbrella Cells

if the mobiles have vastly different speeds there is no best choice of : for small speeds the cells can be small while maintaining a smallhandover rate

for high speeds small cells would lead to a very high handover rate

solution: put slow mobiles into small cells and fast mobiles into largeumbrella cells (with higher antennas and larger tx powers)

= the cells overlap spatially, but use different channels

the decision can be made by CH from observing a MSs handover rate


Andreas Willig Paging / Location Update

References

[1] Manuel Duque-Anton. Mobilfunknetze Grundlagen, Dienste und Protokolle. Verlag Vieweg,

Braunschweig / Wiesbaden, Germany, 2002.

[2] Jose M. Hernando and F. Perez-Fontan. Introduction to Mobile Communications Engineering. ArtechHouse, Boston, 1999.

[3] Ajay R. Mishra. Fundamentals of Cellular Network Planning and Optimisation: 2G/2.5G/3G... Evolutionto 4G. John Wiley & Sons, 2004.

[4] Theodore S. Rappaport. Wireless Communications Principles and Practice. Prentice Hall, Upper

Saddle River, NJ, USA, 2002.

[5] Arthur H. M. Ross and Klein S. Gilhausen. Cdma technology and the is-95 north american standard.

In Jerry D. Gibson, editor, The Communications Handbook, pages 199212. CRC Press / IEEE Press,Boca Raton, Florida, 1996.

[6] Harilaos G. Sandalidis and Peter Stavroulakis. Heuristics for solving fixed-channel assignment problems.In Ivan Stojmenovic, editor, Handbook of Wireless Networks and Mobile Computing, pages 5170. John

Wiley & Sons, New York, 2002.

[7] Mischa Schwartz. Mobile Wireless Communications. Cambridge University Press, Cambridge, GB, 2005.


Andreas Willig Paging / Location Update

[8] B. Walke, P. Seidenberg, and M. P. Althoff. UMTS The Fundamentals. John Wiley and Sons,

Chichester, UK, 2003.

[9] Bernhard Walke. Mobile Radio Networks Networking, Protocols and Traffic Performance. John Wileyand Sons, Chichester, 2002.


transmission wireless

Documents