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8/13/2019 Capacity of CDMA With Multiple Cells
1/23
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