Seyed Mohamad Alavi, Chi Zhou, Yu Cheng Department of Electrical and Computer Engineering Illinois Institute of Technology, Chicago, IL, USA ICC 2009

Outline  Introduction  System model  Reduced Complexity Proposed Model  Performance Evaluation  Conclusions

Introduction  The orthogonal frequency division multiple access, also known as Multiuser-OFDM, is a class of multiple access schemes for the 4 th generation wireless networks.  OFDMA is immune to intersymbol interference and frequency selective fading as it divides the frequency band into a group of orthogonal subcarriers

Introduction  The combination of OFDMA with adaptive modulation and coding (AMC) and dynamic power allocation is of great prominence in the design of future broadband radio systems 64-QAM 16-QAM QPSK 64-QAM QPSK

Introduction  Radio Resource Allocation problems are usually divided into two classes:  Margin Adaptive (MA) problem  minimizing total transmission power while satisfying QoS requirements of each user  Rate Adaptive (RA) problem  maximize throughput in a system subject to a constraint on maximum total transmission power, while satisfying each user’s QoS requirements

Introduction  To formulize the resource allocation problem with constraints on rate, BER, power and delay requirements  To propose a heuristic algorithm that is superior to the linearized algorithm in terms of complexity, but with a little lower capacity.

System model  Assume that the base station has perfect channel estimation which is made known to the transmitter via a dedicated feedback channel

System model Bit loading values number of bits per symbol that can be carried by modulation scheme, m Number of time slotNumber of subcarrierNumber of user

System model rate requirement transmission power

System model delay requirement

Reduced Complexity Proposed Model  Step 1  Determine the number of subcarriers assigned to each user  Step 2  Assign the subcarriers to each user based on rate requirement.  Step 3  Allocate the time slots to different users based on delay requirement.  Step 4  Solve the optimization problem with the only constraint on power

Reduced Complexity Proposed Model  A. Step 1-Number of subcarriers per user Rate requirement Delay requirement

Reduced Complexity Proposed Model  A. Step 1-Number of subcarriers per user total number of subcarriers Unallocated subcarriers

Reduced Complexity Proposed Model  B. Step 2-Subcarrier assignment  all subcarriers will be sorted in descending order for all users  If there is any unsatisfied user, subcarrier replacement is done with the most satisfied user. This process will be finished when all users required data rate is satisfied.

Reduced Complexity Proposed Model  C. Step 3- providing user delay requirement

Reduced Complexity Proposed Model  D. Step 4-power allocation  In this step the optimization problem with only a constraint on maximum power allocation assigns the power of each user on its specified subcarrier.

Performance Evaluation  Implemented using Matlab  Frequency selective multipath channel model  Eight independent Rayleigh multipaths  Maximum Doppler shift of 30 Hz is assumed  The channel information is sampled every 0.5 ms to update the subchannel and power allocation

Performance Evaluation  The possible modulation schemes that can be used, are BPSK, QPSK rectangular 16-QAM and 64-QAM, U = {0,1,2,4,6}  Maximum number of Users are chosen from the set of K = {4, 8, 12, 16}  total number of subcarriers are selected from the set of N = {8, 16, 24, 32}  K and N are chosen somehow that always K < N

Performance Evaluation  Computational complexity comparison

Performance Evaluation  Total capacity versus number of users

Conclusions  In this paper, we have proposed a linear optimization formulation that considers delay in addition to rate requirement.  It is shown through simulation that that the proposed heuristic method performs better than the previous models in terms of significantly decreasing the computational complexity, and yet achieving almost same total capacity.

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