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

    12

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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

    19

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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.

    20

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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 wide