Multiuser OFDM with Adaptive Subcarrier, Bit and Power Allocation (IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 17, NO. 10, OCTOBER 1999)

Outline 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 -magnitude of the channel gain Assume =1 for all users required received power Then the transmitted power:

Problem Formulation

Bit Allocation Algorithm for Single User Greedy Algorithm back

Multiuser Subcarrier and Bit Allocation Algorithm 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 Let be a real number and introduce another variable

Lagrangian Optimization Using the Lagrangian Multipliers to solve the problem

Lagrangian Optimization

Optimal Solution

Optimal Solution (Cont) The nth subcarrier are selected by the kth user if To satisfy the bit rate requirement, increase =>H decrease => will increase the pr to become 1 to transmit data However, optimal may be not an interger

Multiuser Adaptive OFDM (MAO) Satisfy the constraint: 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 The required power for supporting c bits/symbol at a given BER Pe:

Simulation Results

Conclusions 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