MSc Mobile ComputingMSc Mobile ComputingSystems 1Systems 1

Professor Rolando Carrasco

BSc(Hons), PhD, CEng, FIEE

R.Carrasco@wlv.ac.uk

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Introduction (1)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

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Introduction (2)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

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Mobile Computing SystemsMobile 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

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Radio Transmission ImpairmentsRadio Transmission Impairments

MSCFixed Networks

PSTN, ISDNBISDN,...

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Mobile Computing SystemsMobile 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

F o r w a r d l i n k

R e v e r s e l i n k

B a s e S t a t i o n

M o b i l e S t a t i o n

R N C

F i x e d N e t w o r k s

7Types of wireless communication Types of wireless communication systemssystems

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

8Types of wireless communication Types of wireless communication systemssystems

A final classification can be made depending on the type of service provided:

– Paging systems

– Cordless

– Adhoc

– Cellular

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Paging SystemsPaging 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.

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Paging Terminal

Paging Terminal

Paging Terminal

Paging Control Center

Landline link

Satellite link

Landline link

City 1

City 2

City N

An example Paging SystemAn example Paging System

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CordlessCordless

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

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M o b i l e S t a t i o n

F i x e dN e t w o r k s

Mobile Adhoc Network (MANET)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

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P S T N , I S D N A T M , I n t e r n e t . . .

R N C 2 R N C 1

1

1

2

2

3

3

4

4

5

5

6

6

7

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Cellular Mobile Communication Cellular Mobile Communication systemsystem

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

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Cellular ConceptCellular Concept

MSCFixed Networks

PSTN, ISDNBISDN,...

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Frequency Re-useFrequency 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

16Frequency Reuse & Channel Frequency Reuse & Channel AssignmentAssignment

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 b5

f4 d6

b1-b4

d1-d5

C

e2-e6

G

f1-f3

A

b1-b3

D

C

E

G

F A

b6

D

C

E

G

F

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Channel Assignment StrategiesChannel 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)

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Fixed Channel Allocation (FCA)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

BS1

BS3 BS4

BS2

10 Channels

10 Channels 10 Channels

10 Channels

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Dynamic Channel Allocation (DCA)Dynamic Channel Allocation (DCA)

BS1

BS3 BS4

BS240Channels

– 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

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Dynamic Channel Allocation (DCA)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).

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Hybrid Channel Allocation (HCA)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

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Allocation ComparisonAllocation 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.

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Allocation ComparisonAllocation 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.

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Allocation ControlAllocation 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