chap 2

CHAPTER 2

CELLULAR CONCEPTS

Engr. Naveed Jan

1

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 of two entities or devices .

3

What is BTS• A base transceiver station (BTS) is a piece of equipment that facilitates wireless

communication between user equipment (UE) and a network.

• A BTS site normally have a tall tower with antennas at the top & necessary electronics to generate & manipulate the signals.

4

A typical BTS tower which holds the antenna. The tower is quite widely misinterpreted as the BTS itself. The shelter which houses the actual BTS can also be seen

An actual BTS device

What is CELL• Cell is a geographical area in which a clear radio Signal from one BTS can be sensed.

5

GSM Cell Types

• Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average rooftop level.

• Micro cells are cells whose antenna height is under average rooftop level; they are typically used in urban areas.

• Picocells are small cells whose coverage diameter is a few dozen metres; they are mainly used indoors.

• Femtocells are cells designed for use in residential or small business environments and connect to the service provider’s network via a broadband internet connection

6

Diff b/w Cellular & Radio (non cellular) Communication

• In RADIO Communication a single high power Transmitter used to cover a wide range of area.

• Cellular Communication network is composed of number of small cells. The reasons are

1. radio signals at the frequencies used for cellular Comm travel only a few kilometers 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.

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Diff b/w Cellular & Non Cellular Communication

• If the network designer wants to cover a large area, then he must have a number of transmitters positioned so that when one gets to the 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

8

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 radio channels. Adjacent cells use different frequencies. Each cell has a control channel. A call started in one zone has to be re-initiated when

moving to a new zone because the call will be dropped.

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Why Cells are Theoretically made Hexagons ?

• While it might seem natural to choose a circle but adjacent circles can’t be overlaid without

1. Leaving gaps.2. Or creating overlapping regions.• Thus when considering geometric shapes, triangle,

square & hexagon can cover an entire region. • Hexagons are preferred because1. It closely approximates a circular radiation pattern2. Among the three, hexagon has the largest area. Hence

fewer number of cells can cover entire region.

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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.

12

Area of a Hexagon

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

s = R s ┴ h

R

13

Frequency Reuse

• Each cell is assigned a part of the available frequency spectrum.

• Cellular radio systems offer the possibility of using the same part of the frequency spectrum more than once.

• This is called frequency reuse.• Cells with identical channel frequencies are called

co-channel cells.• The co-channel cells have to be sufficiently

separated to avoid interference.14

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 is called a cluster.

15


• 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 system

C = M * nc *N = M * n

• The capacity of cellular system is directly proportional to the number of times a cluster is replicated 16

Frequency Reuse Cntd....... The factor N is called the cluster size and is typically

equal to 4,7,12. The value for N is a function of how much interference

a 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 less interference.

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

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18

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

• there are only certain cluster sizes and cell layouts which are possible.

• In order to tessellate, the cluster size N should satisfy

N = i2 + ij + j2

• where i greater than or equal to j.


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20

• How many cells per cluster ???

• What is the distance between cells with the same frequency ???


21


* not valid selections.22

Frequency Reuse Cntd.....

23

• 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

24

• From the cell geometry prove that

• D / R =

Prove by yourself !

Frequency Reuse Distance Cntd....

25

N3

D

R

• 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 interfering cells are there in first tier?????

Co-Channel Interference

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

Co-Channel Interference Cntd....

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• 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 by the actual terrain environment. It varies between 2 and 5 . 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}


29

Effect of Imperfect Site Location

What is the C/I in this case ??

Hint: Use the same method as did in ideal case.

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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

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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 to your agony

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

32

Handover / Handoff Mechanism• Handover, also known as

handoff, is a process to switch an ongoing call from one cell to the adjacent cell as a mobile user approaches the cell boundary.

• Handover is an automatic process, if the signal strength falls below a threshold level.

• It is not noticed by the user because it happens very quickly—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.

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Handover is the process of transferring a call from one base station to another as the user moves. Handovers should be fast (so overlaps of areas of cells can be minimised); reliable (dropped calls are more frustrating than blocking - some channels can be reserved for handovers)

.

Note the slight gap in threshold levels: this prevents too many handovers.

Time

Rec

eive

d S

ign

al

Le

vel

Level at ‘A’ and ‘B’

Handover +ve threshold

Handover -ve threshold

Minimum acceptablequality level

Call Termination

user moving

A B

Time

Rec

eive

d S

ign

al

Le

vel

Level at ‘A’ and ‘B’

Handover +ve threshold

Handover -ve threshold

Minimum acceptablequality level

Call Termination

user moving

A B

Handover / Handoff Mechanism

A handover is performed in three stages. 1. The mobile station (MS) continuously gathers

information of the received signal level of the base station (BS) with which it is connected, and of all other BTSs it can detect.

2. This information is then averaged to filter out fast-fading effects. The averaged data is then passed on to the decision algorithm, which decides if it will request 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 a connection to the new BS.

Handover Cntd.......

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• To prevent handover resulting from temporary 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.......

36

Dwell TimeThe time over which a call may be maintained

within a cell, without handoff, is called the dwell time.

Even when a mobile user is stationary, ambient motion in the vicinity of the base station and the 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 the type of radio coverage.

37

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, a spare 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 monitor the 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.

38

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

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 station of 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 analog systems 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)

39

Umbrella Cell Approach

problems arise when design for a wide range of mobile velocities.

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

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

In practice it is difficult for cellular service providers to obtain 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” cells which are co-located at a single location. This technique is called the umbrella cell approach

40

It provides large area coverage to high speed users.

And provides small area coverage to users traveling at low speeds.

The umbrella cell approach ensures that the number of handoffs are minimized for high speed users and provides additional microcell channels for pedestrian users.

Umbrella Cell Approach Cntd….

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Umbrella Cell Approach Cntd….

42

IMPROVING CAPACITY & COVERAGE IN CELLULAR SYSTEM

43

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 be used 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 splitting.

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

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

• cell has smaller radius and the new smaller cell called 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....

45


46

Original Cell distribution Cell Distribution following the splitting of the cell labeled A.


• After cell splitting, the new small cells are reassigned new frequencies that do not cause co‐channel interference with adjacent cells as shown in the figure.

• In addition, the power transmitted in the small cells is reduced compared to the power transmitted in the large cells as it would require much less power to cover the cell compared to the large cells

• In addition to the advantage of having a higher network capacity due to cell splitting, the reduced transmitted power, especially by the mobile phone, is another major advantages because it increases the battery life of these mobile phones.

47

• Cell splitting allows a system to grow by replacing large cell with smaller, while not upsetting the channel allocation scheme required maintaining the minimum number of co-channel reuse ratio.

• The main disadvantage of cell splitting is that it requires the construction of new towers, which is very costly.


48

Cell Sectoring

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

• In this approach, first the SIR is improved using the directional antennas, then capacity improvement is achieved by reducing the number of cells in the cluster, thus increasing the frequency reuse.

49

• 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….

50


51

No sectorization Sectorization implemented


52

• 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 ????

Cell Sectoring Cntd…..

53

Answer for dullards !!!

• The worst case carrier to interference ratio is given 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.

54

• 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 will bell the cat ????

Cell Sectoring Cntd…..

55

Take Some pain Guyzzz !!!!

• Calculate C/I for the case of 120 degrees sectoring 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 of capacity & Interference.

56

Micro Cell Zone Concept

• Do you really believe cell sectoring is an ultimate solution ???

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

57

The two main disadvantages of sectoring are1. It increases intra-cell handovers

2. The trunking efficiency is decreased

Micro Cell Zone Concept covers both points


58

In this scheme Sectors are converted to zones. mobile travels from one zone to other zone

within the cell, retains the same channel. Thus unlike in sectoring, a handoff is not

required at the MSC. In this way a given channel is active only in

the particular zone in which the mobile is traveling


59

while the cell maintains a particular coverage radius, the co-channel interference in the

cellular system is reduced because1.A large central base station is replaced by

several lower powered transmitter on the edges of the of the cell.

2.Decrease co-channel interference improves the signal quality and also leads to an increase in capacity without the degradation in trucking efficiency


60


61

Near Far Effect

• As name suggests, it’s the effect of near user’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.

62

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....

63

Two proposed solutions are1. Deployment of POWER CONTROL.

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


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65

CellA1

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 1213 14 15 16 17 18 19 20 21 22 23 2425 26 27 28 29 30 31 32 33 34 35 36

CellA1

CellB1

CellC1

CellA2

CellB2

CellC2

CellA3

CellB3

CellC3

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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chap 2

Documents

circular cells

cell cell

adjacent cells

micro cells

number of small cells

cellular communication

fewer number of cells

gsm cell typesmacro