HW2_mobile personal communication

7/25/2019 HW2_mobile personal communication

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Personal & Mobile Communications

Fall 2015

HW Assignment - 2

1. Explain te relationsip bet!een cluster si"e# capacit$# an% co-cannel intererence'

Ans!er( Each cellular base station is allocated a group of radio channels to be usedwithin a small geographic area called cell. N cells which collectively use the complete set ofavailable frequencies is called a cluster.

The capacity of a cellular system is directly proportional to the number oftimes a cluster is replicated in a fixed service area.

Frequency reuse implies that in a given coverage area, there are several cellsthat use the same set of frequencies. These cells are called co-channel cells and interference

between signals from these cells is called co-channel interference. Thereby, as the cluster sie N increases, the capacity and co-channel

interference both decreases.

2' Wat are te possible solutions to increase capacit$ o cellular s$stems)

Ans!er( !s the demand for wireless service increases, the number of channelsassigned to a cell eventually becomes insufficient to support the required number of users. !tthis point, cellular design techniques are needed to provide more channels per unit coveragearea.

! few of the possible solutions to increase capacity of cellular systems are"cell splitting, sectoring and coverage one approaches. #ell splitting allows an orderly growthof the cellular system. $ectoring uses directional antennas to further control the interferenceand frequency reuse of channels. %hile cell splitting increases the number of base stations inorder to increase capacity, sectoring and one microcells rely on base station antenna

placements to improve capacity by reducing co-channel interference.

*' W$ %o !e use exagonal cellular structure instea% o using oter structure)

Ans!er( &n wireless communication systems, the cellular concept plays an importantrole in solving the problem of spectral congestion and capacity. The cellular concept isobtained by replacing a large cell with many small cells.

!s the demand for services increases, the number of cells must be increased

to improve the user capacity. &n wireless communication, we use the hexagonal concept tomodel coverage because of the following reasons" 'exagonal model cover an entire areawithout overlapping( it has the largest area of the above mentioned geometries( 'exagonalsystem can be considered as the semi-realistic model.

Normally the power transmitted by the base station is in all directions )*mni-direction+. !ccording to this, if we use a circular structure, the adacent cells may interferewith each other causing co-channel interference. *ther structures lie triangle, rectangle canalso be used, but they cover less area when compared to circular structures. The hexagonalstructure is nearest to the circular structure and covers maximum area as compared to othergeometrical shapes.

+' ,s co-cannel intererence a unction o transmitte% po!er rom cell site) Explain'Ans!er(


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%hen the sie of each cell is practically equal or roughly same, the co-channelinterference is independent of transmitted power from the base station and becomes thefunction of radius of cell and the resue factor /.

0 1 /2 1 345

&s called the co-channel interference reduction factor or reuse factor is measure of ofco-channel interference.

*nce the sie of each cell is different, the transmitted power of one cell interfereswith the signals of the adacent cells and causes co-channel interference.

5' iscuss o! to re%uce CC, !it sectori"ation'

Ans!er( $ectoriation is briefly described as a way to cut down equipment costs in acellular networ. %hen applied to clusters of cells sectoriation also reduces co-channelinterference.

This is because the power radiated bacward from a directional base station

antenna is minimal and interfering with adacent cells is reduced. The number of channels isdirectly proportional to the number of cells.

.' iscuss !$# in o/erlai% s$stems# te mobiles near base stations are assigne% to cannels

rom smaller cluster si"e# !ile te mobiles a!a$ rom base stations are assigne% larger

cluster si"e'

Ans!er(The method for allocating resource is done by F#! )Fixed #hannel !ssignment+in which fixed channels is assigned to each cell according to its present cluster sie,

but it do nnot adapt to changing traffic conditions and user distributions. $uch fixedallocation of resource in cell may result in blocing in some cells, while others may havelarge number of spare channels which leads to poor resource utiliation.

To solve this problem channel borrowing method also could be inadequate. &n orderto avoid such problem /#!)/ynamic #hannel !llocation+ is used to achieve high spectrumefficiency in cellular systems by using different cluster sies. &n this method mobile far from6$ are provided with channels with smaller reuse distance )i.e from small cluster sie+ andvice versa. This scheme allows less complex systems to perform efficiently without need foradaptive modulation and power rearrangements.

' iscuss !$ AC, is a problem' Ho! can !e re%uce AC,)

Ans!er( !dacent-channel interference )!#&+ is the interference caused by extraneouspower from a signal in an adacent channel. !#& may be caused by inadequate filtering,improper tuningor poor frequency control.

!dacent channel interference occurs more frequently in small cell clustersand heavily used cells. !#& is also caused by imperfect receiver filtering of the adacentchannels signal and lea into the pass band. The problem with !#& is severe if the adacentchannel user is transmitting very close to the subscriber receiver attempting to receive a basestation signal on the desired channel. This is nown as 7near-far effect8. !#& can be reduced by careful designing of band pass filter at the receiverend, by using proper modulation schemes that have low out of band radiation, by maintaining

proper channel interleaving by assigning adacent channels to different cells and by the use ofmultiplexing schemes in time and frequency to separate the uplin and downlin.

' a , te call arri/ing rate is 10 calls per minute' A/erage call ol%ing time is * minutes'Calculate traic intensit$'
https://en.wikipedia.org/wiki/Tuner_(radio)https://en.wikipedia.org/wiki/Tuner_(radio)https://en.wikipedia.org/wiki/Tuner_(radio)


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b eine te 34' ,s it better to increase or %ecrease it)

c Plot te 34 as a unction o number o cannels or ixe% oere% traic intensit$'

Ans!er(

a 9iven" #all arriving rate : 1 ;

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HW2_mobile personal communication