1. QoS Aware Adaptive Subcarrier Allocation in OFDMA Systems
Mustafa Ergen & Sinem Coleri
University of California Berkeley

2. Introduction Motivation
Orthogonal Frequency Division Multiple Access(OFDMA)
OFDMA System
Resource Allocation Problem
Algorithms
Optimal
Suboptimal
Simulation
Conclusion
The analysis, Research and consultancy (ARC) group forecasts that the fixed wireless deployments in both homes and business will reach 28 million by 2005.
comiitee broadband wireless access based on OFDMA.

3. Motivation Broadband Wireless Access OFDM OFDMA
Ex: IEEE , Wireless MAN
OFDM
Eliminates InterSymbol Interference
OFDMA
BWA is alternative to DSL technologies
Physical layer should mitigate non LOS environments in indoor.
GHz
802.16a 2-11GHz

4. OFDM Diagram

5. Multiuser OFDM OFDM-TDMA OFDM-FDMA OFDMA OFDM-TDMA OFDM-FDMA OFDMA
Subcarrier
Time
OFDM-TDMA
Multiuser OFDM
OFDM-TDMA
OFDM-FDMA
OFDMA
OFDM-FDMA
Time
Subcarrier
OFDMA
Time …
User 1
User 2
User 3 …
Subcarrier

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
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.

7. Resource Allocation Assumptions: Base station knows the channel
Base station informs the mobiles for allocation
subcarrier
Base
Station
user

8. System oCoS=Ptotal for downlink oCoS=Pu for uplink Application Network
rQoS=[rR,rBER]
oQoS=[oR,oBER,oCoS]
Resource Allocation
[User x Subcarrier]
Physical Layer

9. OFDMA

10. Resource Allocation RATE: [12 6 6 8 ] BER: [1e-2 1e-2 1e-4 1e-4] QoS
[ ]
BER:
[1e-2 1e-2 1e-4 1e-4]
QoS
Channel
Resource Allocation
Subcarrier
User
64-QAM
16-QAM
4-QAM

11. Notation

12. Optimal Integer Programming Subcarrier User Pc1 Subcarrier User Pc2

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 Allocation
Hungarian algorithm
Optimal, very complex
LP approximation to IP problem
Close to optimal
Bit Loading
Subcarrier
User
Subcarrier
User
Subcarrier
User

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.

18. Iterative Algorithm Fair Selection(FS) Greedy Release(GR)
Horizontal Swaping(HS)
Vertical Swaping(VS)

19. Iterative Algorithm Start HORIZONTAL SWAP VERTICAL SWAP FAIR SELECTION
Ptotal<Pmax
GREEDY
RELEASE
Start
Modulation--
HORIZONTAL SWAP
VERTICAL SWAP
ASSIGNMENT ITERATION

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

25. Instantaneous Average bit SNR vs Time
Iterative Algorithm improves its Average Bit SNR by the time.

26. Conclusion OFDMA Resource Allocation Optimal Algorithms
Broadband Wireless Access
Resource Allocation
Channel Information
QoS Requirement
Optimal Algorithms
complex
Iterative Algorithms