chanel allocation
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Channel Allocation in Cellular SystemChannel Allocation in Cellular System
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Channel Allocation StrategiesChannel Allocation 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 (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 reusepattern
Different considerations are taken before the assignment of the channels (i.e. signalquality, 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 notchange unless restructured
Any call attempt within the cell can only be served by the unused channels in thatcell
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 Channel
10 Channel
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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 othercriteria.
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 inuse in the cell or any other cell which falls within the minimum restricted distance offrequency 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 ofborrowing channels from a neighbouring cell when itsown channels are occupied
Known as the borrowing strategy RNC supervises such borrowing procedures &
ensures that the borrowing of a channel does notdisrupt or interfere with any of the calls in progress inthe 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 behavelike 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 notwell 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 thechannel selection is made independently
DCA: the knowledge of occupied channels in other cellsis necessary (i.e. heavy signalling load).
A great deal of processing power to determine optimalallocations 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 mobileBS control: BSs keep info about current available channels in its vicinity.
Updated by exchanging data between BSs. In a mobile control system the mobilechooses the channel based in its local CIR measurements (i.e. lower complexitybut less efficiency).
FCA is suitable for a centralised control system.
DCA is applicable to a centralised or decentralised control system
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Multiple Access ProtocolsMultiple Access Protocols
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)
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Code
Frequency
Time
Channel
1
Channel
2
Channel3
ChannelN
Frequency Division Multiple AccessFrequency Division Multiple Access
(FDMA)(FDMA)
F D M l lF Di i i M lti l A
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12Frequency Division Multiple AccessFrequency Division Multiple Access(FDMA)(FDMA)
Divide spectrum into frequency bins Each host sends in a pre-determined frequency
bin Out-of-band reservation mechanism (FCC)
Also called Frequency Division Multiplexing (FDM) Example: AM/FM radio, TV
AM500-1700 KHz FM88-108 MHz SatelliteGHz range
Freq.
(Hz)
Host 1 Host 2 Host 3
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Code
Frequency
Time
Channel 1
Channel2
Channel3
Channel N
Tim
eSl
ots
Time Division Multiple AccessTime Division Multiple Access
(TDMA)(TDMA)
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Time Division Multiple Access (TDMA)Time Division Multiple Access (TDMA)
Divide time into multiple slots Each host sends in a pre-determined slot Out-of-band reservation mechanism Compare to Time Division Multiplexing(TDM)
2 3 11 2 3 1 2 3
Host 1
Host 2
Host 3
1
2
3
Not Eth.
Router/
Mux
1
2
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TDMATDMA
Several TDMA schemes have been studied for
the third generation air interface
The frame length is 4.615 ms and it can consist of
64 1/64 time slots of length 7216 1/16 time slots of length 288
Downlink Uplink
72 s288
sSwitching point between
uplink and downlink
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An example of TDMAn example of TDM
Ethernet uses a protocol called CSMA/CD
CarrierSense Multiple Access with Collision
Detection
When a node wants to broadcast, it checks whetherany other node is broadcasting (senses the carrier).
A node broadcasts when no other node is
broadcasting. Otherwise, it tries later at a random
interval.
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Hidden Terminal ProblemHidden Terminal Problem
Wireless transmission is usually short range.
Even if the medium is free near the
transmitter, it may not be free near the
intended receiver.
AC
B
Collision at
B
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Exposed Terminal ProblemExposed Terminal Problem
Even if the medium is busy near the
transmitter, it may be free near the intended
receiver.
A B C D
C cannot transmit because B is transmitting.
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Message Loss due to CollisionMessage Loss due to Collision
Using CSMA in wireless medium results in message
loss and requires retransmission of lost messages.
A node spends much more energy while receiving ortransmitting messages. Hence, retransmission wastes
a lot of energy.
The other alternative is to use a reservation based
TDM protocol.
21Demand Assignment Multiple AccessDemand Assignment Multiple Access
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21Demand Assignment Multiple AccessDemand Assignment Multiple Access
(DAMA)(DAMA)
In a DAMA protocol, nodes first reserve slotswhich they intend to use for broadcasting.
Each round of broadcast is preceded by areservation round.
DAMA protocols are widely used in satellite
communication and increasingly being usedin wireless networking.
22Code Division Multiple AccessCode Division Multiple Access
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Macro diversity
Code
Frequency
Time
Channel 1
Channel 2
Channel 3
Channel N
Code Division Multiple AccessCode Division Multiple Access
(CDMA)(CDMA)
Power Control
MAI (Multi-access Interference)
Cancellation
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Code Division Multiple Access (CDMA)Code Division Multiple Access (CDMA)
Use multiple orthogonal codes to partition a rangeof spectrum
Each host sends using a pre-determined code Also called spread spectrum
Two forms spread spectrum: Direct-Sequence Spread Spectrum DSSS
Chipping sequences spread the signals spectrum CDMA is often used as synonym for DSSS
Examples: 802.11b, cell Frequency-hopping spread spectrum FHSS
Example: Bluetooth Advantage: simple, but not as efficient
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Code Division Multiple Access (CDMA) (2)Code Division Multiple Access (CDMA) (2)
Frequency hopping example
BluetoothHost 1s Code: 1342,
Host 2s Code: 3214,
Host 3s Code: 4123
Note that all 3 codes are orthogonal: at each instant in
time, each host is on a different frequency
Host 1
Host 2
Host 3
F1 F2 F3
Freq (Hz)
F4
F1 F2 F3
, F1, F3, F4, F2, F1, F3, F4, F2,
Possible
hopping
sequence
F4
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