Into to Communication Systems Chap 2

8/13/2019 Into to Communication Systems Chap 2

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What is MOBILE COMMUNICATION??

•

What’s Mobility ???• Does it really mean anything on a go ???

• What’s Communication ???



MOBILE COMMUNICATION • Mobility in real terms is the effectiveness

of same equipment & services irrespective of an area (geographical

location).

• Communication means interaction oftwo entities or devices .



What is CELL & BTS

• Cell is a geographical area in which a clear radio Signal from

one BTS can be sensed.

• Base Transceiver Station is tall tower with antennas at the top &

necessary electronics to generate & manipulate the signals.



Diff b/w Cellular & Non Cellular

Communication

• In Non Cellular (e.g. radio) Communication a single high

power Transmitter used to cover a wide range of area.

• Cellular Communication network is composed of number of

small cel ls . The reasons are

1. radio signals at the frequencies used for cellular Comm travel

only a few ki lometers from the point at which they are

transmitted.

2. They travel more or less equal distances in all directions;

hence, the area around it where a radio signal can be received

is typically approximately circular .



Diff b/w Cellular & Non Cellular

Communication cont’d • If the network designer wants to cover a large area, then he must

have a number of transmitters positioned so that when one gets tothe edge of the first cell there is a second cell overlapping slightly,

providing radio signal.

• Hence the construction of the network is a series of approximately

circular cells





What are the motivations for this?

The aim is to use spectrum or bandwidth efficiently .

Bandwidth allocation for voice channels (and data) is

limited .

Each cell allows one to use a number of radiochannels.

Adjacent cells use different frequencies.

Each cell has a control channel.

A call started in one zone has to be re-initiated whenmoving to a new zone because the call will bedropped .





Why Cells Theoretically made Hexagons ??

• The actual radio coverage is known as the

footprint .

• It is determined from the field measurements or

propagation prediction model.

• The real footprint is amorphous in nature.



Area of a Hexagon

• What is the Area of hexagon in terms of R???

• Hint:

s = R

s ┴ h R

10



Frequency Reuse

• Each cell is assigned a part of the availablefrequency spectrum.

• Cellular radio systems offer the possibility ofusing the same part of the frequency spectrummore than once.

• This is called frequency reuse.

•Cells with identical channel frequencies are calledco-channel cells.

• The co-channel cells have to be sufficientlyseparated to avoid interference.



Frequency Reuse Cntd.......

• The distance between

these co-channel cells is

achieved by the creation

of a cluster of cells.

• The cells which

collectively use the

complete set of

available frequencies iscalled a cluster.



Frequency Reuse Cntd....... • A cellular system which has a total of ―n‖ duplex

channels

• If each cell is allocated a group of nc channels (nc < n)

• if the ―n‖ channels are divided among N cells

• the total number of available radio channels can be

n = nc * N

• If a cluster is replicated M times within the systemC = M * nc *N = M * n

• The capacity of cellular system is directly proportional

to the number of times a cluster is replicated 13




The factor N is called the cluster size and is typicallyequal to 4,7,12.

The value for N is a function of how much interferencea mobile or base station can tolerate while maintaining

a sufficient quality of communication. If the cluster size N is reduced, more clusters are

required to cover a given area and hence more capacity.

Large cluster size indicates that the co-channel cells

are located far away from each other, hence lessinterference.

So cluster size N is a compromise between capacity &interference.






• How many cells per cluster ?

•

What is the distance between cells with thesame frequency ?




• the hexagon geometry has exactly six equidistant neighbors and that the lines joiningthe centers of any co- channel cell areseparated by multiples of 60 degrees.

• there are only certain cluster sizes and celllayouts which are possible.

• In order to tessellate, the cluster size N should

satisfyN = i 2 + ij + j 2

• where i greater than or equal to j.


17







* not valid selections.



Frequency Reuse Cntd.....



• An important parameter denoting the amount of frequency

reuse in a certain area is called frequency reuse distance Ru.

• Ru is defined as the ratio of the reuse distance, D, between the

centers of the nearest co-channel cells and the cell radius, R.

Frequency Reuse Distance



• D = R + 2 R sin 30o + R or 3 R

• Hence D / R = 3.

•

{ Note that D / R = ( 3 N )

½

or 3 }

Prove by yourself ! ! ! ! ! !

Frequency Reuse Distance Cntd....



• Co -channel interference depends on D / R.• D - distance between cells using the same

frequency.

• R - radius of each cell.

• As D / R increases, co-channel interference

decreases.

• How many co-channel interfer ing

cells are there in f irst tier?????

Co-Channel Interference



• Always 6 in the first tier e.g. N = 3

Co-Channel Interference Cntd....






• The ratio of carrier to interference power is given by

• C / I = carrier power / interference power

• = C / { I1 + I2 + I3 + I4 + I5 + I6 }

• since there are 6 interfering cells in the first tier.

• The carrier power is proportional { R } – a

• where a = propagation path loss slope determined bythe actual terrain environment. It varies between 2 and5 . A value of 4 is usually assumed for a.

• Hence for a fully developed system,• C / I = { R }

– a/ { 6 D

-a}

• if we assume that all distances DI are the same.

• C / I = 1 / { 6 ( D / R ) – a

}




Effect of Imperfect Site Location What is the C/I in thiscase ??

Hint:

Use the samemethod as did in ideal

case.



Here is the answer

• C/I = R -4

/ {2 (D – R)-4

+ 2 (D)-4

+ 2 (D + R)-4

}

• = 1 /{ 2 (D/R – 1)-4

+ 2 (D/R)-4

+ 2 (D/R + 1)-4

}

• For N=7, D/R = (3.N)½ = 4.5826.

• C / I = (10 log10 54) or 17 dB i.e. lower than 18

dB.

• If all the distances are equal to D - R, then

• C/I = R -4

/ 6 (D – R) -4

or 28 i.e. 14.5 dB



To further test your patience!!!

1) Obtain C/I for case where three neighbours

are at (D+R) & other three are at (D-R)

Now to really add your agony

2) Obtain C/I for the case where only oneneighbour in first tier is at (D+R), all other areat (D-R).



Handover / Handoff Mechanism

• Handover, also known as

handoff, is a process to switch anongoing call from one cell to theadjacent cell as a mobile userapproaches the cell boundary.

• Handover is an automatic

process, if the signal strengthfalls below a threshold level.

• It is not noticed by the user because it happens veryquickly — within 200 to 300 ms

• The need for a handover may be caused by radio, operation

and management (O&M), or by traffic.

• The main reasons are low signal level or high error rate.



A handover is performed in three stages.

1. The mobile station (MS) continuously gathersinformation of the received signal level of the basestation (BS) with which it is connected, and of allother BTSs it can detect.

2. This information is then averaged to filter out fast-fading effects. The averaged data is then passed onto the decision algorithm, which decides if it willrequest a handover to another station.

3. When it decides to do so, handover is executed by both the old BS and the MS, resulting in aconnection to the new BS.

Handover Cntd.......



•To prevent handover resulting fromtemporary fluctuations in the received

signal level, the measurements must be

averaged.• Longer averaging lengths give more reliable

handover decisions, but also result in longer

handover delays.• So it’s a bit of a trade-off between handover

rate & handover delay

Handover Cntd.......



Dwell Time

The time over which a call may be maintainedwithin a cell, without handoff, is called thedwell time .

Even when a mobile user is stationary, ambient

motion in the vicinity of the base station andthe mobile can produce fading.

thus even a stationary subscriber may have a

random and finite dwell time.the statistics of dwell time vary greatly,

depending on the speed of the user and thetype of radio coverage.



Mobile Assisted HandOff (MAHO)

In first generation analog cellular system, signal strength

measurement are made by base station and supervised by MSC.

Each base station constantly monitors the signal strengths of all of its

reverse voice channels to determine the relative location of each

mobile user with respect to base station tower.

In addition to measuring the RSSI of calls in progress within a cell, aspare receiver in each base station, called the locator receiver, is

used to scan and determine signal strengths of mobile users which are

in neighboring cells.

The locator receiver is controlled by the MSC and is used to monitorthe signal strength of users in neighboring cells which appear to be in

need of handoff and reports all RSSI values to the MSC.

Based on the locator receiver signal strength information from each

base station, the MSC decides if a handoff is necessary or not.



In today’s second generation system, handoff decisions are mobileassisted.

In mobile assisted handover (MAHO), every mobile station measures

the received power from the surrounding base stations and continually

reports the result of these measurements to the serving base station.

A handoff is initiated when the power received from the base stationof a neighboring cell begins to exceed the power received from the

current base station by a certain level or for a certain period of time.

The MAHO method enables the call to be handed over between the

base stations at much faster rate than in first generation analogsystems since the handoff measurements are made by each mobile and

the MSC no longer constantly monitors signal strength.

MAHO is practically suited for microcell environments where

handoffs are more frequent.

Mobile Assisted HandOff (MAHO)



Umbrella Cell Approach

problems arise when design for a wide range of mobilevelocities.

High speed vehicles pass through the coverage regionof a cell within a matter of seconds, whereas pedestrianusers may never need a handoff during a call.

MSC can quickly become burdened if high speed usersare constantly being passed between very small cells.

In practice it is difficult for cellular service providers toobtain new physical cell site locations in urban

areas.(non technical barriers)By using different antenna heights and different power

levels, it is possible to provide ―large‖ and ―small‖ cellswhich are co-located at a single location. Thistechnique is called the umbrella cell approach



It provides large area coverage to high speed

users.

And provides small area coverage to userstraveling at low speeds.

The umbrella cell approach ensures that the

number of handoffs is minimized for high

speed users and provides additional microcell

channels for pedestrian users.

Umbrella Cell Approach Cntd….



Umbrella Cell Approach Cntd….



IMPROVING CAPACITY &

COVERAGE IN CELLULAR SYSTEM



Cell Splitting

• Total number of voice channels = C• There are N cells per cluster.

• Hence the number of voice channels per cell = C / N

•With traffic growth within a cell, all capacity will beused up. Further growth is only possible by

1. increasing the number of voice channels in the cell, or

2. revising the cell boundaries so that the area formerly

regarded as a single cell can now contain several cells.

• This latter process is called cell spli tting.



•When a cell becomes congested it divides thecell into smaller cell.

• By this way the subdivided cell has its own base station with small antenna size & lowtransmitted power.

• cell has smaller radius and the new smaller cellcalled the micro cell should be installed

between the existing cells.• The capacity increases due to the additional

number of channel per unit area.

Cell Splitting Cntd....






• Cell splitting allows asystem to grow by

replacing large cell with

smaller, while notupsetting the channel

allocation scheme

required maintaining the

minimum number of co-channel reuse ratio.




Cell Sectoring

• Sectoring increase s the SIR so that the cluster size may be reduced.

• In this approach, first the SIR is improved using thedirectional antennas, then capacity improvement isachieved by reducing the number of cells in the cluster,

thus increasing the frequency reuse.

C S i C



• When sectoring is employed, the channels

used in a particular cell are broken down into

sectored groups and are used only within

particular sector.

• Assuming seven-cell reuse, for the case of 120

degrees sectors.

• the number of interferers in the first tier is

reduced from six to two.

• How come ????

Cell Sectoring Cntd….

C ll S i C d



Cell Sectoring Cntd…..

C ll S i C d



•

The reduction in interference increases SIR• which allows us to reduce the cluster size.

• Hence capacity is increased.

For example:If N= 7 with 120 degree sectoring, the SIR

increases to 24.5 dB.

Can you Prove it ????




Answer for dullards !!!

• The worst case carrier to interference ratio isgiven by

• C/I = R -4

/ { (D + 0.7R)-4

+ (D)-4

}

• = 1 / { (D/R + 0.7)-4 + (D/R)-4 }• For N=7, D/R = (3.N)

½ = (21)

½ .

Hence

• C/I = 1 / { ((21)½ + 0.7)-4 + ((21)½ )-4 }• or C/I = 24.5 dB.



• So far we have shown that sectoring improves

SIR.

• Now if we prove that reducing cluster size

with sectoring can increase the capacity, our

job will be done.

•

But who wil l bell the cat ????




Take Some pain Guyzzz !!!!

• Calculate C/I for the case of 120 degreessectoring with N=4

• Calculate C/I for 60 degrees sectoring with

N = 7• Last but not the least

• Measure C/I for N= 4 & N= 12 with 60

degrees sectoring• Give your overall comments about the

combination giving best results in terms ofcapacity & Interference.



Micro Cell Zone Concept

• Do you really believe cell sectoring is an

ultimate solution ???

• Do you think of any disadvantage it contains???



The two main disadvantages of sectoring are

1. It increases inter-cell handovers

2. The trunking eff iciency is decreased

Micro Cell Zone Concept covers both points


Mi C ll Z C t



I n this scheme

Sectors are converted to zones.

mobile travels from one zone to zone to otherzone within the cell, it retains the same

channel.

Thus unlike in sectoring, a handoff is notrequired at the MSC.

In this way a given channel is active only inthe particular zone in which the mobile istraveling


Mi C ll Z C t



while the cell maintains a particular coverage

radius , the co-channel interference in the cellular

system is reduced because

1. A large central base station is replaced byseveral lower powered transmitter on theedges of the of the cell.

2. Decrease co-channel interference improves

the signal quality and also leads to anincrease in capacity without the degradationin trucking eff iciency


Mi C ll Z C t






Near Far Effect

• As name suggests, it’s the effect of nearuser’s transmission on far user.

• If both using adjacent channels, the effect is pronounced.

• If near user’s power is more, the effect is

even more prominent.



The receiver will not be able to decode far

user mainly because

1. Filter are ideally not very sharp .2. Signal from far user deteriorates enough

when reaches BTS.

So what’s the solution ?????

Near Far Effect Cntd....



Two proposed solutions are

1. Deployment of POWER CONTROL .

2. Proper distribution of Channel frequencies(adjacent channels should be separated far

enough).


Near Far Effect Cntd






Cell

A1

Cell

B1

Cell

C1

Cell

D1

Cell

A2

Cell

B2

Cell

C2

Cell

D2

Cell

A3

Cell

B3

Cell

C3

Cell

D3

1 2 3 4 5 6 7 8 9 10 11 12

13 14 15 16 17 18 19 20 21 22 23 24

25 26 27 28 29 30 31 32 33 34 35 36

Cell

A1

Cell

B1

Cell

C1

Cell

A2

Cell

B2

Cell

C2

Cell

A3

Cell

B3

Cell

C3

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18

19 20 21 22 23 24 25 26 27

28 29 30 31 32 33 34 35 36


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Into to Communication Systems Chap 2

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