Multiuser OFDM with Adaptive
Subcarrier, Bit and
Power Allocation
(IEEE JOURNAL ON SELECTED AREAS IN
COMMUNICATIONS, VOL. 17, NO. 10, OCTOBER 1999)
Outline
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Motivation for Multiuser OFDM
Bit Allocation Algorithm for Single User
Multiuser Subcarrier and Bit Allocation
Algorithm
Simulation Results
Conclusions
References
Motivation


Orthogonal Frequency Division Multiplexing
(OFDM) provides a transmission scheme to avoid
frequency selective fading in a wide band wireless
communications.
How to allocate different user to use different
subcarriers with adaptive optimal bit (modulation)
and power becomes an interesting problem in a
wireless multiuser environment.
Motivation
Multiple Access Schemes
System Model
Problem Formulation
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 k ,n-magnitude of the channel gain
Assume N=1
o for all users
f k (c )required received power
Then the transmitted power:
Pk ,n 
f k (c
k ,n
 k2,n
)
Problem Formulation
P
*
T
 min
Ck ,n D
N
K
 
n 1
k 1
f k ( ck ,n )

2
k ,n
Bit Allocation Algorithm for Single User

Greedy Algorithm
N
P  min 
*
T
Cn
D
N
n 1
R   cn
f(c )
n
 n2
Initializa tion :
for all n, let cn  0 and Pn  [ f (1)  f (0)] /  n2
Bit Assignment Iterations :
Repeat the following R times :

n  arg min
n 1
n
Pn ;
c   c   1;
n
n
P  [ f (c   1)  f (c  )] /  2 ;
c
n
n
n
End;
{cn }nN1 is the final bit allocation solution.
back
Multiuser Subcarrier and Bit Allocation
Algorithm
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As users can not share the same subcarrier,
this dependency makes the greedy
algorithm a non-optimal solution.
Some relaxations on previous problem
have to be made.
to
Relaxzation of the Constraints

] a real number and introduce
Let c k ,n  [ o , Mbe
another variable k ,n[ 0,1]
PT 
N
min
K
 
C k , n [ o , M ] n 1
 k , n[ 0,1]
k 1
 k ,n f k ( c
k ,n
)
 k2,n
rk ,n  ck ,n  k ,n
Lagrangian Optimization
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Using the Lagrangian Multipliers to solve
the problem
Lagrangian Optimization
 k ,n  0
Lagrangian Optimization
 k ,n  0
Optimal Solution
Optimal Solution (Cont)
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The nth subcarrier are selected by the kth user if
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To satisfy the bit rate requirement, increase
 =>H decrease =>  will increase the pr to
become 1 to transmit data
However, optimal ck ,nmay be not an interger
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Multiuser Adaptive OFDM (MAO)
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Satisfy the constraint:
PT  min
N
 
[ o , M ]
n 1
 k , n[ 0,1]
Ck , n
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K
k 1
 k ,n f k ( c
k ,n
)
 k2,n
Using the optimal method to decide the
proper carrier for a specific user.
Then use the single user bits allocation
algorithm to satisfy its data rate requiremet.
Note that
to
Simulation Results
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The required power for supporting c
bits/symbol at a given BER Pe:
Simulation Results
Simulation Results
Simulation Results
Simulation Results
Simulation Results
Conclusions
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The proposed MAO outperforms than the
conventional MA techniques in an OFDM system.
Quantify the performance improvement in terms
of SNR, BER and Outage Pr.
References

Multiuser OFDM with Adaptive Subcarrier, Bit,
and Power Allocation (IEEE JOURNAL ON
SELECTED AREAS IN COMMUNICATIONS,
VOL. 17, NO. 10, OCTOBER 1999)

Digital Communication, Prokis, 4th edition

Wireless Communications, Rappaport,2nd edition