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Medium Access:Basics of CellularCommunication

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The Cellular Concept

Early Mobile Communications Single, high powered transmitter with an antenna mounted on

a tall tower

The Cellular Concept Replace a single high power transmitter (large cell) with many

low power transmitters (small cells) each providing coverage toonly a small portion of the service area

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Frequency Reuse

EDF

AG B C

EDF A

GB

CE

DFA

GB

C

Each base station isallocated a group radiochannels to be used withina small geographic area

Base station in adjacent

cells are assigned channelgroups which containcompletely differentchannels than neighboringcells

Cluster

ClusterCluster

A Cluster: A Group of N cells thatwhich collectively use thecomplete set of availablefrequencies

Total Number of Channels in theSystem:

C=MkN=MS

M: Number of clusters within the systemK: Number of channels per cellN: Cluster Size

S: Number of available physical channels

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Locating Co-channel Cells Number of Cells per Cluster N = i2+ij+j2, i, j are non-negative integers

To find nearest co-channel neighbor of a given cell Move i cells along any chain of hexagons

Turn 600 counter clockwise and move j cells

i=3, j=2, N=19

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Channel Assignment Strategies

Fixed (Static) Channel Assignment (FCA)

Each cell is allocated a predetermined set of channels. Any call attempt within the cell can only be served by the

unused channels in that particular cell If all channels in a cell are occupied, the call is blocked

Dynamic Channel Assignment (DCA)

Channels are not permanently assigned to cells

Each time a call request is made, the serving base station

requests a channel from the MSC

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DCA Example LOLIA

A

Cluster Size N= 7

A new call is generatedwithin cell A

Create a List of all

channel that are NOTused within the clusterthat has A in its center

From this list select thechannel with the leastinterference

LOLIA= Locally Optimized Least Interference Algorithm

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DCA Example LOLIA

B

Cluster Size N= 7

A new call is generatedwithin cell B

Create a List of all

channel that are NOTused within the clusterthat has A in its center

From this list select thechannel with the leastinterference

LOLIA= Locally Optimized Least Interference Algorithm

A

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DCA Characteristics

DCA requires the MSC to continuously collectinformation about Channel Occupancy

Traffic Distribution Radio Signal strength Indication (RSSI) of all channels

This increases storage and computational load onthe system

DCA Advantages Reduced probability of blocked calls

Better signal quality of calls (better BER performance)

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Handoff Strategies

A B

ReceivedSignal Level

Received

Signal Level

Handoff Threshold

MinimumAccepted Signal

Handoff Threshold

MinimumAccepted Signal

Improper Handoff

Proper Handoff

: Handoff Margin

Excessive Delay Small Margin Lack of available

channels

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Handoff Strategies

Handoff: A mobile moves to a different cell while call is in

progress

Handoff Process

1. Identifying new base station

2. Allocating channels within new base station

Selection of Handoff Margin:

Large Un-necessary Handoffs (processing load)

Small Insufficient time for handoff completion

H d ff i Fi G i C ll l

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Handoffs in First Generation CellularSystems

Base stations monitor

signal strengths of itssupervised channels

A spare locaterreceiver at each base

station scans anddetermines signalstrength of mobilesin neighboring cells

MSC collects locaterreceiver informationand decides ifhandoff is necessary

MSC

H d ff i S d G ti

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Handoffs in Second GenerationCellular Systems (MAHO)

Every mobile

measures the powerreceived fromsurrounding basestations and reportsthe results to serving

base station A handoff is initiated

when power receivedfrom a neighboringbase station exceedsthat of the servingbase station by acertain level or for acertain time period

MSCMobile Assisted Handoffs (MAHO):Faster Handoffs than first generationMSC no longer monitors signal strengths

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Prioritizing Handoffs

From a users point of view:

An abrupt call termination due to failed handoff is more annoying thanbeing blocked on a new call attempt

Guard Channel Concept

A fraction of the total available channels in a cell is reservedexclusively for handoff requests

Queuing Handoff Requests

There is a finite time interval between the time received signal leveldrops below the handoff threshold and the time the call is terminateddue to insufficient signal level

Handoff requests could be given queuing priority over new calls

under the circumstances of lack of available channels

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The Umbrella Cell Approach

The umbrella cell approach

Large cells and small cells locate in the same location by usingdifferent antenna heights and different power levels

Large umbrellacell for high speedtraffic

Small microcells forlow speed traffic

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Trunking & Grade of Service

Trunked Radio System:

Each user is allocated a channel on per call basis, andupon termination of the call, the previously occupiedchannel is immediately returned to the pool of availablechannels.

Grade of Service:

A measure of the ability of a user to access the trunkedsystem

GOS measures in cellular networks

Probability that a call is blocked

Probability a call experiences a delay greater than a certainqueuing time

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Traffic Intensity

Traffic intensity generated by each user: Au Erlangs

Au = HH : average duration of the call : average number of call requests per unit time

For a system containing U users and unspecified number ofchannels, Total offered traffic intensity: A Erlangs

A = UAu

In C channel trunked system, if the traffic is equally

distributed among the channels, Traffic intensity per channel: Ac Erlangs

Ac = UAu/C

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Types of Trunked Systems

Calls Blocked Cleared

Trunking SystemNo queuing provided for callrequests and calls are blocked if

no available channels

Calls Blocked Delayed

Trunking SystemQueuing is provided to hold callrequests. Calls are blocked if noavailable channels for a certaindelay

Assumptions: Calls arrive as determined by Poisson distribution Infinite number of users

Memoryless arrivals of requests : all users can request channel at anytime Probability of user occupying a channel is exponentially distributed Finite number of channels available in the trunking pool

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Erlang B Formula

C

kC

k 1

AC!Pr blocking GOS (Calls Blocked Cleared)A

k!

M/M/C/C Queuing System

Exponential Interarrival Time(Poisson Arrival Process)

C Servers and ExponentialService TimesService time

distribution

Inter-arrival timedistribution

Number ofServers

QueueSize

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Erlang B Curves

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Erlang C Formula

C

kC 1C

k 0

C-A t-

H

A

Pr delay 0 A AA C! 1

C k!

Pr delay t =Pr delay 0 Pr delay t delay 0

Pr delay t =Pr delay 0 e =GOS (Calls Blocked Delayed)

Average Delay for Calls in Queued System

D=Pr dela

Hy 0

C A

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Erlang C Curves

Pr[delay>0]