CELL SPLITTING
2) Cell Splitting ± Cell splitting is the process of subdividing a congested cell into smaller cells such that each smaller cell has its own base station with R educed antenna height and duced transmitter powe ower. R educ It increases the capacity of a cellular system since number of times channels are reused increases. 4/3/2010
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CELL SPLITTING
The
increased number of cells would increase the number of clusters over the coverage region, which again would increase the number of chan channe nells and thus thus capa capaccity ity. The
distance between co-channel cells also reduces to half (D=D/2) as the cell radius is reduced to half (R =R /2). /2). Thus the co-channel reuse ratio (Q=D/R ) remains same. 3 4/3/2010
C
D
E
D
G B
E F
D
C
A
F F
C
G
E
C B D E
G
B
G
F Fig: Cell splitting
Splitting ± Static, Dynamic Dynamic 4/3/2010
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3)
Sectoring ±
The
coco-chan chann nel int interf erferen rence in a cel cellular ular syst system em may be redu reducced by replacing a single omn omni-directional antenna at the base station by seve severa rall dire direct ctio iona nall ante antenn nnas as radi radiat atin ing g withi ithin n spec specif ifie ied d sect sector ors. s. A cell is normally partitioned in three 120 sectors or six 60 sectors. A given cell will receive interference and transmit with only a fraction of the available co-ch o-chaannel cells. In the the sectori oring sch scheme eme, the co-channe nnel interference is reduced and thus sys system capacity is impr improv oved ed.. Co-c o-chann annel interf terfer ereence is redu reduce ced d beca becau use the the numbe umber r of inte interf rfer erer er gets gets redu reduce ced. d. r
2
2 1
1
1 2
2
6
3
3
5
3
1
4
6
(a)
(b)
Fig: (a) 120 sector and (b) 60 sector r
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r
r
6
5
3 4
SECTORING
The sectoring is done by replacing a single omni-directional ante an tenn nna a with with 3 dire directi ction onal al an ante tenn nnas as (120 (120O sectoring sectoring)) or with with 6 dire direct ctio iona nall an ante tenn nnas as (60 (60O sectoring)
In this scheme, each cell is divided into 3 or 6 sectors.
Each Each se sect ctor or us use es a dire direct ctiion onal al an ante ten nna at the the BS an and d is assi as sign gne ed a set of chan chann nels. els.
The number of channels in each sector is the number of chan ann nels in a cell divided by the the number of sector tors.
The amount of CCI is als also reduced by the number of sector tors.
Reduced Tx Power .. Bty life more 4/3/2010
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IMPROVING CC IN CELLULAR SYSTEMS
Aim: To provide more channels per unit coverage co verage area Sectoring uses directional antennas to further control the the inte interf rfer eren ence ce an and d freq freque uenc ncy y reus reuse e of chann channel els. s. Exam Exampl ples es:: Omni Omni,, 120 120O
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IMPROVING CC IN CELLULAR SYSTEMS
Aim: To provide more channels per unit coverage co verage area Sectoring uses directional antennas to further control the the inte interf rfer eren ence ce an and d freq freque uenc ncy y reus reuse e of chann channel els. s. Examples: 60O and 90O
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Handover or HANDOFF
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BASICs
Higher
priority is given to ongoing connections than new call attempts.
A channel remains allocated to a mobile user until, either its call is completed in the cell, or it crosses the cell boundary, boundary, requiring a new channel frequency to continue.
This
procedure that transfers an ongoing call from one cell to another is called Han andover dover or HANDOFF.
Handover
That
attempt that finds all channels occupied in the Target cell will be forced to terminate unsuccessfully. is not desirable. desirable.
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Call Handover Handover procedure may measurement such as ±
i. ii. iii. iv.
be
based
on
the
Signal strength of Current and Target CELLs Bit error rate Traffic load (Heavily Heavily / Lightly Lightly Loaded CELLs) Carrier to interference interference ratio etc. etc.
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Point A, B, C or D Where should Handoff take Handoff take place ? at Point 4/3/2010
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Rx SS vs Threshold Value
The
effect of the thres reshold depends on its value comp ompared to the signal strengths of the two base stations at the point at which they are equal.
If the threshold is
Higher
than this value, say
T 1,
the handover occurs at
pos p ositi ition on A. If the threshold is Lower than this value, say
T 2,
the mobile will delay
hand handov over er unti untill the the curr curren entt sign signal al leve levell cros crosse sess the the thre thresh shol old d at position B. In the case of T 3, the delay may be so long that the mobile drifts far into the new cell ( Position D). and
may
result
This
in
reduces the qualit ality y of the com communi unicati cation on link link a
dropped
FAILU AILUR R E *
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call or HANDOFF
How to prevent
ping-pong
effect
elativ ivee Relat
sign signal al stre streng ngth th from from both both BTSs BTSs allows a hand over
Only nly if the new base station is the current one.
fficiently sufficiently
user
to
stronger than
In this case the handover will occur at point C. This
technique prevents the so-called ping-pong effect , the
repeated handover between two base stations caused by rapid fluctuations in the received signal strengths from both base stations.
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Relative signal strength and Threshold Handover
to a new BTS is done only only if the current signal Strength drops below elow a th thrreshold and the target BTS is stronger than the current one by a given
HO Margin.
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Many any input parameters can be considered for Hand-Off 1. Distance from base station -- defined as very near, near, near near,
medium, far and very far defined as very very high, high, high, high, 2. Signal strength difference -- defined medium, low and very low of mobile user -- defined as very low, low, low, low, high and very high elocity y 3.Velocit
handover ver,, Wait, Be care careful, ful, Handover state - No hando Handover, Sure handover hando ver 4/3/2010
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Umbrella cell approach approach to accommodate wide range of velocities -
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