Ece401 Lecture 15
Transcript of Ece401 Lecture 15
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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]