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Capacity Analysis of Cellular
CDMA Systems
Abdullah Abu Romeh
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Capacity Analysis of
Cellular CDMA Systems Outline:
Introduction
Reverse & forward link capacity analysis Erlang Capacity
Capacity-Coverage Tradeoff
Effect of Soft Handoff
Capacity-Coverage Tradeoff with Soft Handoff
Capacity of UMTS systems
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Introduction
Capacity of a CDMA system is interference limited
Assumptions
Users are power controlled by the BS All BS's require the same power
Power control is exercised by the BS
corresponding to maximum pilot signal SIR based admission policy
Users are are uniformly distributed in each cell
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Reverse Link Capacity
Single Cell (Single User Detection):
SIR seen at the BS:
where:S: power of the received signal per userN: number of users in the cell: Background noise
Equivalent to:E
b
N0
=S /R
N1S /W/W
SIR=
S
N1S
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Reverse Link Capacity
Single Cell Capacity:
For multi-cell systems, BS suffers from intra-cell as
well as inter-cell interference
where, I: intra-cell interference (stochastic)
N=1W/R
Eb /N
0
S
Eb
N0
= W/RN1I /S/s
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To find the capacity we need the distribution of I
Depends on the attenuation due to large scale
variations (path loss and shadow fading) G = ,
For a user at distance rm from his BS and r0 from
the BS under consideration:
10/10
r4
Reverse Link Capacity
: N0,2
I
S=10
0/10
r0
4
rm
4
10m/10
=rm
r0
4
100m /101
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Reverse Link Capacity
Glihousen et al.: On the capacity of a cellular CDMA system
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Reverse Link Capacity
Utilizing the voice activity:
where is Bernoulli()
Calculate the capacity based on BER for adequate
performance: P(BER
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Reverse Link Capacity
Glihousen et al.: On the capacity of a cellular CDMA system
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Forward Link Capacity
In most systems, the reverse link capacity is the
limiting factor due to the limited power available for
the subscribers Power control is also exercised in the forward link:
Subscriber sends the power received from its BS
and the total interference
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Forward Link Capacity
The ith subscriber SNR can be lower bounded by
where: is the fraction of the total site power devoted to
users (excluding pilot)
iis the fraction of power devoted to the ith
subscriberis the total power available from BS under
consideration
E
b
N0
i
iST
1/R
[ j=1
k
STji]/W
ST1
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Forward Link Capacity
Glihousen et al.: On the capacity of a cellular CDMA system
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Erlang Capacity
Def: The average traffic load in terms of average
number of users requesting service resulting in a
certain blocking probability Blocking Probability: the probability that a new user
will find all channels busy and hence be denied
service Condition: P( )
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Reverse Link Erlang Capacity
Simple Case:
a) constant number of users NU in every sector,
b) each user transmits continuously,
c) users require the same Eb/I0
Condition for no blocking:
NuEbR1fN0WI0W
NuW/R
Eb/ I0.1
1f
f: ratio of intra-cell interference tointer-cell interference
= N0/I0
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Practical case:
a) Number of active calls is a Poisson random
variable with mean /b) each user is gated on with probability and off
with probability 1- (voice activity)
c) each user's received energy-to-interference ratio isvaried according to propagation conditions
Reverse Link Erlang Capacity
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Condition for no blocking:
and so:
where (stochastic)
Reverse Link Erlang Capacity
i=1
k
iE biR j
othercells
i=1
k
i j Ebi j RN0WI0W
P{Z=i=1
k
ii j
other cells
i=1
k
i j i
j W/R
1}=Pblocking
i=Ebi /I0
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The statistics of depends on the power control
mechanism
Field trials with all cells fully loaded show that iswell modeled as log-normal
Chernoff pound for the outage probability can't be
obtained because the moment generating functionof doesn't converge
Reverse Link Erlang Capacity
i
i
i
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Reverse Link Erlang Capacity
Viterbi & Viterbi: Erlang Capacity of Power Controlled CDMA System
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Using Central Limit theorem for Z we get:
Reverse Link Erlang Capacity
P{Z=i=1
k
ii j
other cells
i=1
k
i j i
j W/R
1}=Pblocking
PblockingQ [AEZ
VarZ]
=1W/RFB,
1 fexpm
B=Q
1 Pblocking2expm
A
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Reverse Link Erlang Capacity
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Reverse Link Erlang Capacity
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Cell Coverage: maximum distance that a given user
of interest can be from the base station and still
have a reliable received signal strength at the basestation
An accurate prediction of cell coverage as a
function of user capacity is essential in CDMAnetwork design and deployment
Capacity-Coverage Tradeoff
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Capacity-Coverage Tradeoff
As the number of users in the cell increases, the
interference seen by each user increases
Each user has to increase his transmitted power inorder to acheive the desired SNR
For a given upper limit on the transmit power, the
coverage of a cell is inversely proportional to thenumber of users in it
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Analysis:
Case I: Deterministic number of users in the cell
Case II: Random number of users in the cell
Capacity-Coverage Tradeoff
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To account for coverage, we need to include the
probability that the power required from the
subscriber to achieve a certain SNR is greater thanthe maximum power possible (power limited)
P(outage) = P(blocking) +P(req power>Smax|no blocking)
Capacity-Coverage Tradeoff I
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Outage occurs when a user's SNR is less than the
minimum required by the BS for a certain amount of
time resulting in service degradation and call drop
whereis the SNR required by the BS for the jth user
and
Capacity-Coverage Tradeoff I
Pblock=P{Sj/R
i :i j
iSi
WN0I
jx}=PAout
jx
jx= j
target j
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Let be the required received power to obtainSo, we have
The above equation has feasible solutions when
and
S jx j
x
j
x=S jx /R
i : i j
i Six
WN
0I
i=1
k R ixi
WR ixi
1
Capacity-Coverage Tradeoff I
PAout =P{i=1
k R ixi
WR ixi
1}
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With no limit on the maximum transmitted power,
the maximum number of users admitted in the cell
is called Pole capacity (kpole)
Capacity-Coverage Tradeoff I
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Let Boutbe the event that the power control
equations have feasible solutions but greater than
the maximum possible transmitted power
P(out) = P(Aout) + (1 - P(Aout)) P(Bout|Aout')
Capacity-Coverage Tradeoff I
PBout=PStransSmax
Strans=S1PLdZ1
Pout=PAout[1PAout]PS1xPLdZ1Smax l Aout
c
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The maximum outage probability occurs at the
edge of the cell, so:
After some approximations and computations:
where
Capacity-Coverage Tradeoff I
pm=PA
out[1P A
out]PS
1
xPL Rcell
Z1S
maxl A
out
c
logRcell=
1
K2 [SmaxK1mS k S2
k z2
Q
1
pmPAk
1PAk ]
PL d=K1K2 log d
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Capacity-Coverage Tradeoff I
Veeravalli & Sendonaris: Coverage-Capcity tradeoff in cellular CDMA systems
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To design cell coverages and capacities to match
projected traffic densities in the network, it will be
reasonable to model the number of usersrequesting service as a random variable depending
on the admission policy and offered traffic
For number of users modeled as Poisson, we getthe following tradeoff curve
Capacity-Coverage Tradeoff II
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Capacity-Coverage Tradeoff II
Veeravalli & Sendonaris: Coverage-Capcity tradeoff in cellular CDMA systems
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Soft Handoff
Soft Handoff: a technique whereby mobile units in
transition between one cell and its neighbor transmit
to and receive the same signal from both basestations simultaneously (two-cell handoff)
Soft handoff increases cell coverage and reverse link
capacity compared to hard handoff
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Coverage:
For hard handoff:
where is the power added by the user to over
path loss
For soft handoff:
Soft Handoff
PBoutlA
out
c =P100 /10r0
41 /
PBoutlA
out
c =Pmin 100/10r0
4 ,101/10r1
41/
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Soft Handoff
Viterbi et al.: Soft Handoff extends CDMA cell coverage and increases reverse link capacity
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Capacity:
Analyze the capacity in terms of the ratio f
Path loss standard deviation vs f:
Soft Handoff
0 2 4 6 8 10 12
0
2
4
6
8
10
12
14
16
18
20
i d ff
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Capacity-Coverage Tradeoffwith soft handoff
Veeravalli & Sendonaris: Coverage-Capcity tradeoff in cellular CDMA systems with soft handoff
C i C T d ff
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Similar analysis to soft handoff with:
Aout: the event that all BSs connected don't have
a feasible solution Bout: the event that all BSs require power greater
than the maximum transmitted
Capacity-Coverage Tradeoffwith soft handoff
C it C T d ff
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Capacity-Coverage Tradeoffwith soft handoff
Veeravalli & Sendonaris: Coverage-Capcity tradeoff in cellular CDMA systems with soft handoff
For ard link Capacit of
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Forward link Capacity ofUMTS
Reverse link Capacity of
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Reverse link Capacity ofUMTS
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Conclusions
Capacity of CDMA systems can be improved by
decreasing the interference
Reverse link is the capacity bottleneck for 2G
whereas for 3G it is the forward
Coverage and capacity are inter-related in cellular
CDMA systems
Soft handoff increases the capacity and coverage
compared to hard handoff
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Questions?
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