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Ofdma 1
1. QoS Aware Adaptive
Subcarrier Allocation
in OFDMA Systems
Mustafa Ergen & Sinem Coleri
{ergen,csinem}@eecs.berkeley.edu
University of California Berkeley
6. Resource Allocation
Goals:
Dynamic subcarrier selection
Improve system performance with adaptive
modulation
More
bits transmitted in large channel gain carriers
Provide QoS
Rate
and BER
11. Notation
Transmit Power : Pkc,n =
f k (ck , n )
α k2,n
user :
k ∈ {1,..., K }
subcarrier : n ∈ {1,..., N }
assigned bit : c
∈ {0,1,..., M }
k, n
channel gain : α 2
k, n
No −1 BER
M − QAM : f (c) =
Q (
3
4
2
) (2c − 1)
12. K
min
γ k ,n ,c
N
M
∑∑∑
f ( ck , n )
k =1 n =1 c =1
subject to Rk =
α
N
2
k ,n
γ k ,n ,c for γ k ,n ,c ∈{0,1}
M
∑∑ c
k ,n
.γ k ,n ,c for all k ,
n =1 c =1
and 0 ≤
K
M
∑∑ γ
k , n ,c
Pc2
Pc3
r es U
Integer Programming
≤ 1, for all n.
Subcarrier
Subcarrier
Subcarrier
k =1 c =1
es U
Pc1
r es U
Optimal
r es U
Subcarrier
13. Motivation for Sub-optimal
Algorithms
IP is complex
Allocation should be done within the
coherence time
Time increases exponentially with the
number of constraints
14. Current Suboptimal Algorithms
2-step:
Subcarrier Allocation
Assume
the data rate for all subcarriers
Assume modulation rate is fixed
Assign the subcarriers
Bit Loading
Greedy
approach to assign the bits of user
15. Current Suboptimal Algorithms
Subcarrier
Hungarian
algorithm
Optimal, very complex
LP
approximation to IP
problem
Bit Loading
For each k , repeat the following Rk times :
n = arg min ∆Pk ,n (ck ,n )
n∈S k
c
k ,n
=c
Subcarrier
Close to optimal
r es U
r es U
Subcarrier Allocation
k ,n
+1
evaluate ∆P (c ).
k ,n
k ,n
r es U
Subcarrier
16. Problems in Current Suboptimal
Algorithms
Subcarrier assignment and bit loading are
separated
Users
with bad channels may need higher
number of subcarriers
Not iterative subcarrier assignment
17. Iterative Algorithm
Iterative algorithm based on
Assignment of bits according to highest
modulation
Finding
the best places
Distributing the assigned bits to other
subcarriers or to non-assigned subcarriers
Exchanging the subcarriers among user
pairs for power reduction.
20. Simulation Environment
Build the OFDMA system
Modulations:4-QAM,16-QAM,64-QAM
Independent Rayleigh fading channel
to each user
Number of subcarriers =128
Nodes are perfectly synchronized
21. CDF of total transmit power
without Pmax constraint
22. CDF of total transmit power with
Pmax constraint
23. Average bit SNR vs. RMS delay
spread
As RMS delay spread increases, the fading variation increases
hence higher gains are obtained by adaptive allocation
24. Average bit SNR vs. number of
users
As the number of users increases, the probability of obtaining
good channel at a subcarrier increases
The analysis, Research and consultancy (ARC) group forecasts that the fixed wireless deployments in both homes and business will reach 28 million by 2005.
802.16 comiitee broadband wireless access based on OFDMA.
BWA is alternative to DSL technologies
Physical layer should mitigate non LOS environments in indoor.
802.16 10-66GHz
802.16a 2-11GHz
Instead of assigning a fixed frequency or time slot to each user, the performance will increase. The users will not use the subcarriers that are in deep fade. The performance will increase since it is quite unlikely that this subcarriers will be in deep fade for all the other users.