Mobile Communication 3

8/2/2019 Mobile Communication 3

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Lecture 3

The Cellular Concept-System Design

Fundamentals



Cellular concept replaces a single, high powertransmitter (large cell) with many low powertransmitters (small cells)

Each providing coverage to only a small portion ofthe service area

Each base station allocated a portion of the totalnumber of channels available

New base stations can be added to increase theradio capacity without increase in radio spectrum

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Service area is divided into a small geographicalarea called a cell

Base stations in adjacent cells are assigned

different groups of radio channels so thatinterference is minimum

Base station antennas are designed to achievedesired coverage within a particular cell

The design process of selecting & allocatingchannel groups for all of the cellular base stationswithin a system is called fr equency r euse or fr equency

planning.

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Hexagonal cell shape is conceptual & simplisticmodel because it permits easy and manageableanalysis of a cellular system

Actual radio coverage of a cell is called thefootprint

By using hexagon geometry, the fewest

number of cells can cover a geographic region

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When using hexagonal shape cells, there aretwo types of cells:

Center-excited cells: base station transmitters aredepicted as being in the center of the cell

Edge-excited cells: base station transmitters are

depicted as on three of the six cell vertices

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Consider a cellular systems:Total number of duplex channels available for use = S

Number of channels allocated to each cell = k, where k<S

Total number of cells among which S channels are divided= N, then

Total number of available radio channels:

S = kN

The N cells which collectively use the complete set ofavailable frequencies is called a cluster.

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If a cluster is replicated M times within thesystem, thentotal number of duplex channels = C (Capacity) is

given by: C = MkN = MS

N is called the cluster size and is typically equal to 4, 7,or 12

Larger cluster size, weaker co-channel interference

Small cluster size indicates co-channels cells arelocated much closer together.

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Figure 3.2 Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., I = 3, j = 2). (Adapted from [Oet83] © IEEE.)



Frequency reuse factor is given by 1/N, sinceeach cell is only assigned 1/N of the totalavailable channels

The geometry of hexagons such that thenumber of cells per cluster, N, can only havethe values which satisfy the equation

N = i2 + ij+ j2

where i and j are non negative integers

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Can be classified as:1. Fixed CA:

each cell is allocated a predetermined set of voicechannels.

Any call attempt within the cell can only be served byunused channels in that particular cell

If all channels are occupied the call is blocked and thesubscriber does not receive service

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orrowing strategy: cell is allowed to borrow channelsfrom neighboring cells supervised by the MSC

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2. Dynamic CA Voice channels are not allocated permanently Each time a call is made, the serving BS requests a channel

from MSC

MSC only allocates unused frequency in the cell or anyother neighboring cells to avoid co-channel interference It reduces the likelihood of blocking, increasing trunking

capacity of the system Also requires MSC to collect real-time data on channel

occupancy, traffic distribution and radio signal strength

indications (RSSI

) of all channels continuously DCA increases the storage and computational load on thesystem but provides advantage of increased channelutilization and decreases probability of a blocked call.

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When a mobile moves into a different cell while aconversation is in progress, MSC automaticallytransfers the call to a new channel belonging to thebe BS ² called handoff

Must be performed successfully and asinfrequently as possible, and be imperceptible tothe users

Margin is given by = P r handoff ² P r min usable

If is too large ² unnecessary handoffs, burden onMSC

If is too small - insufficient time to completehandoff before a call is lost due to weak signalconditions

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BS monitors the signal level for a certain periodof time before a handoff is initiated to becertain that the mobile is actually moving away

from the servingB

S Dwell time: over which a call may be

maintained within a cell, without handoff

MAHO: Mobile assisted handoff ² every

mobile station measures received power fromsurrounding base station & continually reportsthe results to serving BS

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Intersystem handoff: if mobile moves from onecellular system to a different system controlledby a different MSC

Issues: a local call may become a long distancecall, compatibility b/w two MSCs must bedetermined

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Guard channel concept: a fraction of the totalavailable channels in a cell is reservedexclusively for handoff requests

Advantage: efficient spectrum utilization whendynamic channel assignment is used

Disadvantage: reduces total carried traffic

Queuing of handoff requests: to decreaseprobability of forced termination

Tradeoff b/w decrease in probability of forcedtermination and total carried traffic

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Umbrella cell approach: use different antennaheights and power levels to provide ´largeµ and´smallµ cells co-located at a single location

Provides large coverage areas to high speed usersand small area coverage to users traveling at lowspeed Cell dragging: results from pedestrian usersthat move at a very slow and the average signalstrength does not decay rapidly.

This creates a potential interference and trafficmanagement problem

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Mobile Communication 3