MSc Mobile Computing

mashpeemoveΚινητά – Ασύρματες Τεχνολογίες

24 Νοε 2013 (πριν από 4 χρόνια και 1 μήνα)

62 εμφανίσεις

MSc Mobile Computing

Systems 1

Professor Rolando Carrasco

BSc(Hons), PhD, CEng, FIEE

R.Carrasco@wlv.ac.uk

2

Introduction (1)


Mobile Computing Systems


Types of wireless communication systems


Cellular System


Intra
-
cell/Inter
-
cell operation


Frequency Re
-
use


Channel Assignment Strategies


Handoffs


Interference and System Capacity


Types of Interference


Capacity/Interference Relation


Improving Capacity in Cellular Systems

3

Introduction (2)


Multiple Access in the Mobile Environment


Frequency Division Multiple Access (FDMA)


Time Division Multiple Access (TDMA)


Capacity and Interference for FDMA and TDMA


Commercial Applications for FDMA & TDMA


Spread Spectrum Multiple Access Techniques, Code
Division Multiple Access (CDMA)


General Concepts and Characteristics


Capacity and Interference in CDMA


Other Multiple Access Techniques

4

Mobile Computing Systems


Rely on radio transmission as the final link between
terminals


Finite resource, spectrum available is strictly limited


Multipath propagation, fading & interference


Terminals ability to move, complicates the system


The term mobile:


Any radio terminal, that can be moved during operation


Radio terminal that is attached to a high speed platform
(cellular telephone inside a vehicle)


The term portable:


A radio terminal that can be hand
-
held & used at
walking speed


5

Radio Transmission Impairments

MSC

Fixed Networks

PSTN, ISDN

BISDN,...

6

Mobile Computing Systems


Mobiles users communicate
through fixed base stations
(BS)


BSs are controlled by the
radio network controller
(RNC in 3G)


RNC allows the system to
contact the fixed backbone
network



Forward link

Reverse link

Base Station

Mobile Station


RNC

Fixed
Networks

7

Types of wireless communication
systems


type of transmission used is evaluated:


Analogue: transmitting unbroken electromagnetic
waves, closely correspond to the waveforms produced
by the original sounds.


Digital:convert information (e. g. voice or data) into a
series of coded pulses, transmitted at a fast rate.


Depending on the direction of the transmission and
the simultaneity of the communication


Simplex: communication in one direction only.


Half
-
duplex: two
-
way communication over the same
radio channel. A user can only transmit or receive (no
simultaneity).


Full
-
duplex: two simultaneous but separate channels to
achieve a two
-
way communication

8

Types of wireless communication
systems


A

final

classification

can

be

made

depending

on

the

type

of

service

provided
:


Paging systems


Cordless


Adhoc


Cellular

9

Paging Systems


Systems that send brief messages (numeric,
alphanumeric or voice) to a subscriber.


message is called a page and is sent in one
direction only (simplex transmission)


messages are broadcasted to inform the
subscriber about the attempts made by other
users to contact them or to receive news
headlines, faxes or other types of information.


10


Paging

Terminal

Paging

Terminal

Paging

Terminal

Paging

Control

Center

Landline link

Satellite link

Landline link

City 1

City 2

City N

An example Paging System

11

Cordless


Fully duplex, using a radio channel to connect a
portable handset to a dedicated Base Station.


Connected to fixed network via a specific telephone
number


Connection over short distances


1G cordless = few metres


2G cordless, DECT = few hundred metres


Public

Switched

Telephone

Network

(PSTN)

Fixed

Port

(Base

Station)

Wireless

Link

Cordless
Handset

12

Mobile Station
Fixed
Networks
Mobile Adhoc Network (MANET)



Mobiles communicate
bouncing off each other.


They are not fixed to
using a BS to connect to
fixed network


Useful in disaster
situations

13


PSTN, ISDN

ATM,
Internet...

RNC 2

RNC 1

1

1

2

2

3

3

4

4

5

5

6

6

7

7

Cellular Mobile Communication
system


The cellular
concept is a
system level idea
where many low
power transmitters
replace a single
high power
transmitter,
covering a large
geographic area,
each covering a
portion of the
service area called
a cell

14

Cellular Concept

MSC

Fixed Networks

PSTN, ISDN

BISDN,...

15

Frequency Re
-
use


Each BS is allocated a different set of carrier
frequencies


Each cell has a usable bandwidth associated with
these carriers


No. of carrier frequencies available is limited


It is therefore necessary to re
-
use the available
frequencies many times in order to provide sufficient
channels for the required demand


This process is called
frequency re
-
use


All the cells with a different set of frequencies form a
cluster

16

Frequency Reuse & Channel
Assignment

Frequency reuse concept.

Cells with the same letter use the same set of frequencies.

B

A

D

C

E

G

F

B

A

D

C

E

G

F

B

A

D

C

E

G

F

Co
-
Channel Interference

Fixed Channel Allocation


Dynamic Channel Allocation

Centralised or Distributed

A b
5

f
4

d
6

b
1
-
b
4

d
1
-
d
5

C

e2
-
e
6

G

f
1
-
f
3

A

b
1
-
b
3

D

C

E

G

F

A

b6

D

C

E

G

F

17

Channel Assignment Strategies


The way the channels are assigned inside a
cell affects the performance of the system


especially when a change of BSs occurs



Fixed Channel Allocation Schemes (FCA)


Dynamic Channel Allocation (DCA)


Hybrid Channel Allocation (HCA)

18

Fixed Channel Allocation (FCA)


channels are divided in sets


allocated to a group of cells & reassigned to other groups,



according to some reuse pattern


Different considerations are taken before the assignment




of the channels (i.e. signal quality, distance between BSs,



traffic per BS)


they are fixed (i.e. a cell can not use channels that are not



assigned to it)


assignment of frequency sets to cells when the system




is designed & does not change unless restructured


Any call attempt within the cell can only be served by the



unused channels in that cell


If all the channels in that cell are busy, the service is blocked


simple method but does not adapt to changing traffic conditions


introduction of new BSs supposes frequency reassignment for the complete system


BS
1

BS
3

BS
4

BS
2

10 Channels

10 Channels

10 Channels

10 Channels

19

Dynamic Channel Allocation (DCA)

BS
1

BS
3

BS
4

BS
2

40

Channels


Channels are placed in a pool


assigned to new calls depending on the carrier to



interference ratio (CIR) and other criteria.


Each time a call is made the serving base station requests




a channel from the RNC


The switch then allocates a channel to the requested cell



following an algorithm that takes into account the




likelihood

of future blocking within the cell


the frequency of use of the candidate channel


the reuse distance of the channel, and other cost functions.


The RNC only allocates a given frequency if that frequency is



not presently in use in the cell or any other cell which falls within the
minimum restricted distance of frequency reuse to avoid interference


reduces the likelihood of blocking, which increases the trunking capacity of
the system, since all the available channels in a market are accessible to all
of the cells

20

Dynamic Channel Allocation (DCA)


Require the RNC to collect real
-
time data on


channel occupancy


traffic distribution


radio signal strength indications (RSSI) of all channels on a
continuous basis


This increases the storage and computational load on the
system but provides the advantage of increased channel
utilisation and decreased probability of a blocked call


Allocation of channels is more complex since additional
information is needed, but is also more flexible to traffic changes
(i.e. non
-
uniform traffic).


21

Hybrid Channel Allocation (HCA)


a combination of both FCA and DCA


some channels are pre
-
assigned


others are shared dynamically


One of these approaches is based on the principal of
borrowing channels from a neighbouring cell when its
own channels are occupied


Known as the borrowing strategy


RNC supervises such borrowing procedures &
ensures that the borrowing of a channel does not
disrupt or interfere with any of the calls in progress in
the donor cell

22

Allocation Comparison


FCA better for high uniform traffic loads


Max reusability of channels is always achieved


DCA performs better for non
-
uniform traffic loads


allocation of channels is flexible


FCA schemes behave like a no. of small groups of
servers


DCA provides a way of making these small groups of
servers behave like a larger server, which is more
efficient.


FCA call must always be handed off into another
channel


same channel is not available in adjacent cells.


DCA the same channel can be used if interference
does not occur.

23

Allocation Comparison


variations in traffic that are typical of microcells are
not well handled in FCA.


DCA techniques perform better in microcells


Implementation complexity of DCA is higher than
FCA.


FCA:each cell has a number of channels and the
channel selection is made independently


DCA: the knowledge of occupied channels in other
cells is necessary (i.e. heavy signalling load).


A great deal of processing power to determine optimal
allocations is also required.


24

Allocation Control


Centralised fashion


channels are assigned by a central controller, usually
the RNC


Distributed fashion


Channels are selected either by the local BS or by the
mobile


BS control: BSs keep info about current available channels in
its vicinity.


Updated by exchanging data between BSs. In a mobile control
system the mobile chooses the channel based in its local CIR
measurements (i.e. lower complexity but less efficiency).


FCA is suitable for a centralised control system.


DCA is applicable to a centralised or decentralised
control system