Transcript Mobile Access to Interworking and ATM

MSc Mobile Computing
Systems 1
Professor Rolando Carrasco
BSc(Hons), PhD, CEng, FIEE
[email protected]
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Introduction (1)
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Mobile Computing Systems
Types of wireless communication systems
– Cellular System
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Intra-cell/Inter-cell operation
Frequency Re-use
Channel Assignment Strategies
Handoffs
– Interference and System Capacity
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Types of Interference
Capacity/Interference Relation
Improving Capacity in Cellular Systems
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Introduction (2)
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Multiple Access in the Mobile Environment
– Frequency Division Multiple Access (FDMA)
– Time Division Multiple Access (TDMA)
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Capacity and Interference for FDMA and TDMA
Commercial Applications for FDMA & TDMA
– Spread Spectrum Multiple Access Techniques, Code
Division Multiple Access (CDMA)
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General Concepts and Characteristics
Capacity and Interference in CDMA
– Other Multiple Access Techniques
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Mobile Computing Systems
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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 Impairments
PSTN, ISDN
BISDN,...
Fixed Networks
MSC
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Mobile Computing Systems
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Mobiles users communicate
through fixed base stations
(BS)
 BSs are controlled by the
radio network controller RNC
(RNC in 3G)
 RNC allows the system to
contact the fixed backbone
network
Fixed
Networks
Forward link
Reverse link
Mobile Station
Base Station
Types of wireless communication
systems
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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
Types of wireless communication
systems
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A final classification can be made depending
on the type of service provided:
– Paging systems
– Cordless
– Adhoc
– Cellular
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Paging Systems
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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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An example Paging System
City 1
Landline link
Paging
Terminal
City 2
Paging
Control
Center
Landline link
Paging
Terminal
City N
Paging
Terminal
Satellite link
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Cordless
Public
Switched
Telephone
Network
(PSTN)
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Fixed
Port
(Base
Station)
Wireless
Link
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
Cordless
Handset
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Mobile Adhoc Network (MANET)
Fixed
Networks
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Mobile Station
Mobiles communicate
bouncing off each other.
They are not fixed to
using a BS to connect to
fixed network
Useful in disaster
situations
Cellular Mobile Communication
system
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PSTN, ISDN
ATM,
Internet...
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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
RNC 1
RNC 2
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1
4
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3
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1
2
4
3
2
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Cellular Concept
PSTN, ISDN
BISDN,...
Fixed Networks
MSC
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Frequency Re-use
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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
Frequency Reuse & Channel
Assignment
b1B
-b4
G
G
Co-Channel Interference
C
C
A b5
A
f 4 d6
f1F
-f3
G
G
G
G
A
C
C
A
D
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Dynamic Channel Allocation
B
b6
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Fixed Channel Allocation
d1D
-d5
e2E
-e6
b1B
-b3
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F
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D
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Frequency reuse concept.
Cells with the same letter use the same set of frequencies.
Centralised or Distributed
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Channel Assignment Strategies
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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)
10 Channels
10 Channels
– channels are divided in sets
– allocated to a group of cells & reassigned to other groups,
according to some reuse pattern
BS1
BS2
– Different considerations are taken before the assignment
of the channels (i.e. signal quality, distance between BSs,
traffic per BS)
10 Channels
10 Channels
– they are fixed (i.e. a cell can not use channels that are not
assigned to it)
BS3
BS4
– 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
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Dynamic Channel Allocation (DCA)
– Channels are placed in a pool
– assigned to new calls depending on the carrier to
interference ratio (CIR) and other criteria.
BS1
– 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
likelihoodof future blocking within the cell
BS
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the frequency of use of the candidate channel
the reuse distance of the channel, and other cost functions.
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Channels
BS2
BS4
– 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)
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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)
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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 Comparison
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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 Comparison
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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 Control
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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
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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).
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FCA is suitable for a centralised control system.
DCA is applicable to a centralised or decentralised
control system