Capacity of CDMA With Multiple Cells

8/13/2019 Capacity of CDMA With Multiple Cells

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Capacity of CDMAwith

Multiple Cells

PREPARED BY

PAWAN KUMAR THAKUR

ME IT-13-613


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Contents

1. CDMA

2. Capacity of Cellular Systems

3. Capacity of Cellular CDMA

4. Compare single cell CDMA with Multiple cell CDMA

5. Capacity of CDMA with Multiple Cells(i) Frequency reuse factor, f

(ii) Impact of Propagation Pathloss on

Frequency reuse of CDMA systems

(iii) Weighing Factor6. Conclusions

7. References


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CDMA

It stands for Code Division Multiple Access

CDMA: is a SPREAD SPECTRUM technology,

which spreads the information contained in aparticular signal of interest over a much greaterbandwidth than the original signal.

This is achieved by multiplying the signal by avery large bandwidth signal called the spreadingsignal.


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Features

1. All The users in CDMA share the same carrierfrequency and may transmit simultaneously

2. Each user has its own pseudorandom codeword

which is approximately orthogonal to all other

code words.

3. The receiver needs to know this codeword used by thetransmitter and it performs time correlation to detectonly the specific desired signal.


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Features

4. The near-farproblem occurs at a CDMA receiver if an

undesired user has a high detected power as compared to

the desired user. To combat the near-farproblem a power

control is used in most CDMA implementations.

5. Capacity: There is no absolute limit on the number of

users in CDMA. But as the number of users in the CDMA

system increases the system performance decreases &vice-versa.


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Capacity of Cellular Systems

Channel Capacity: of a radio system is defined asthe maximum number of channels or users thatcan be provided in a fixed frequency band.

Spectrum efficiency of a wireless system isdetermined by its radio capacity, whichdepends on the required

Carrier-to-interference ratio (C/I)

channel band-width (Bc).


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Interference

Interference in a cellular system can be due to thesurrounding base stations or due to the surroundingsubscriber units.

That is there are two kinds of interferences:

1. Forward channel interference: This is the interference

due to the surrounding co-channel base stations.

2. Reverse channel interference: This is the interferencedue to the surrounding subscriber units.


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Capacity of Cellular CDMA

There is no absolute limit on the capacity of CDMA, but

there is a tradeoff between the capacity and performance.

The capacity of CDMA is interference limited, which is

unlike that TDMA and FDMA, which are bandwidth limited.So to increase the capacity of CDMA we should

reduce the interference.

INTERFERENCE REDUCING TECHNIQUES:1. Using Multisectorized antenna

2. Operating in discontinues transmission mode


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Interference Reduction Technique

1. Using Multisectorized antenna: In this technique wereduce the interference by using a directional antennawhich spatially isolates the users by receiving signalsfrom only a fraction of the current users.

2. Discontinues Transmission Mode(DTX): This techniquetakes advantage of the intermittent(happening atirregular intervals) nature of speech.Here the

transmitter is turned off during the period of silence.


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)/()1(

/

/ 0 SN

RW

NEb

Where is the background noise,

Bis the bandwidth,

Ris the information bit rate

With the application of the sectorization, DTX techniques

the Bit energy to noise density ratio

)/()1(

// 0

SN

RWNEb

That is Reducesthe interference term from (N-1) to

This Increasesthe bit energy ratio by almost a factor of 8

)1( N

If the voice activity factor is , then the average bit-energy to noise

density ratio, without applying-DTX is


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Single Cell CDMA vs Multicell CDMA

Single cell Multicell

1. The base stations are 1. All the base stations

independent are interconnected by themobile switching center(MSC)

2. The weighting factors of all 2. The weighting factors ofthe users are equal all the users are not equal

3. In the forward link no power 3. We need power control

control is required. Since for in both the forward and

a subscriber any interference reverse links.

caused by the other subscriber

remains at the same level as

desired signal.


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Capacity of CDMA With Multiple Cells

In CDMA cellular system each base station can onlycontrol the transmit power of each of its own in-cellusers, but it can not control the power of users in theneighboring cells.

These neighboring users add to the noise floor anddecrease the capacity of the particular cell of interest.


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Frequency reuse factor

Frequency reuse factor (f): is determined by the amount ofout-of-cell interference. It is defined as

Where:-

N0: is the total interference power received from

the N-1 users

Ui: is the number of users in the i th adjacent cellN

ai: is the average received power from users in the

adjacent cells

iaiiNUN

Nf

0

0


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Frequency reuse efficiency

Frequency reuse efficiency (F):is the percentage of

frequency reuse factor

F= f x 100%

Average received power (Nai):is defined as follows

j i

ij

aiU

NN

Where NiJis the power received at the base station of interest

from the j th user in the i th cell.

Each adjacent cell may have different number of users and hence

may receive different levels of interference. Therefore Nai

is

different for each cell user


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Impact of propagation path loss on the

frequency reuse

Recursive geometric technique:This technique is alsocalled concentric circle cellular geometry. It considersthat

1. All cells have equal geographic area.

2. Cell of interest is circular and located at the center of

all the surrounding cells.

3. Interference cells are wedged shaped and are arrangedin layers around the center cell of interest.


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Concentric circle cellular geometry


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1. R: is the radius of the cell of interest

2. d0: is the minimum distance, such that usersin the center are located no closer than thisdistance

3.d: is the distance from the base station at

which all the users in the center cell of interestare located, such that

Rdd 0


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First layer of adjacent interfering cells is found on

Second layer is located on

The i th interfering layer is located on

The area A of the cell of interest is

RdR 3

RdR 53

RidRi )12()12(

220

2 RdRA


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If A1denotes the entire area of the region. If each cell have

the same area A, then there should be M1cells that each span

a particular angle ( ), such that:

AMRRA 122

1 )3(

1

1

2

M

Solving the above equations gives M1=8 and0

1 45

In general:

Ai = MiA = i.MiA = i.8A 1i

ii 4

11


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Sectors

Interfering layers can be broken into two

sublayers, an inner sublayer which is on

02)12( diRdRi

outer sublayer which is onRiddiR )12(2 0

Partitioning of layers provide two sectors within each Wedge

shaped cell in a given layer

Inner sector: Which contain a small fraction of area ofthe cell and hence fewer users

Outer sector: Which contain much greater fraction of

the area of the cell and hence more users


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Weighing Factors

Weighing Factors: are used to redistribute the users in the

inner and outer sectors of an adjacent cell, since there is a

wide range of user distributions in the interfering layers.

If K is the user density then the number of users (U) within the center cell is given be

U=KA.

In the first surrounding layer, the inner and outer sectors of

each cell have areas given by

Inner sector:

8/38/))2((/ 2211 ARMA in


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Outer sector:

8/58/)2()3((/ 2211 ARMA out

For each first layer cell to possess U=KA users, weighing factors

for the user densities within the inner (W1in) and outer (W1out)

sectors may be applied such that

111111 /)(/)( MAKWMAKWKAU outoutinin

)8/58/3( 11 outin WWKAKAU


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Conditions

Optimistic conditions: for frequency reuse (or upper

bound) is seen when W1in=1 and W1out=1, then 3/8 of the

users will be in the inner sectors and 5/8 of the users will

be in the outer sector and will offer smaller levels of interf-

erence to the center cell.

Worst case:In this case all the users in each of the first

layer cells would be located in the inner sector. The weigh

factors in this case are W1in=8/3 and W1out=0

Capacity of CDMA With Multiple Cells

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