Transmit Power Adaptation for Multiuser OFDM Systems

Jiho Jang, Student Member, IEEE, and Kwang Bok Lee, Member, IEEE

IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 21, NO. 2, FEBRUARY 2003

學生：黃亮穎 學號： 96325510

Outline

• Introduction• System Model• Transmit Power Adaptation• Number Results• Conclusion• Reference

Introduction

• In order to reduce the computational complexity in calculating water-filling level in the proposed transmit power adaptation method

• Transmit power is equally

System Model

• Signal-to-interference-plus noise ratio (SINR)

• Assuming that QAM modulation and ideal phase detection are used as in [11]

• we replace the average signal-to-noise ratio (SNR) in the [11,eq. (17)] with the SINR (2)

• The total data rate of the multiuser OFDM system may be represented by

• is appropriately adjusted by the transmit power adaptation method

Transmit Power Adaptation

• Considering a downlink transmission ， the constraint of total transmit power is written as

• we divide the problem into two steps– subcarrier assignment for users– power allocation for subcarriers

• For the first step– we assign a subcarrier to a set of users to maximize

the feasible data rate for that subcarrier

• Theorem 1:– subcarrier should be assigned to only one user who

has the best channel gain for that subcarrier. proof ： See the Appendix

• For the second step– Theorem 1, FDMA system with dynamic subcarrier allocation

• The total data rate (5) and the total transmit power constraint (6) may be rewritten as

• we define the Lagrangian as

where is a Lagrange multiplier

• transmit power should be allocated as

• The proposed equal power allocation strategy may be represented by

Simulation

• Rayleigh fading• Average SNR define • Fixed bandwidth B • BER = 10-3

• AWGN capacity• Obtain average data rate

Number Results

Conclusion• The two proposed transmit power adaptation schemes significantly increases with the number of users and is much

greater than that for the conventional FDMA-like transmit power adaptation methods.

Reference• [8] C. Y. Wong, R. S. Cheng, K. B. Letaief, and R. D. Murch, “Multiuser OFDM with

adaptive subcarrier, bit and power allocation,” IEEE J. Select. Areas Commun., vol. 17, pp. 1747–1758, Oct. 1999.

• [9] W. Rhee and J. M. Cioffi, “Increase in capacity of multiuser OFDM system using dynamic subchannel allocation,” in Proc. IEEE Vehicular Technology Conf. (VTC 2000), Tokyo, Japan, May 2000, pp. 1085–1089.

• [11] A. J. Goldsmith and S.-G. Chua, “Variable-rate variable-power MQAM for fading channels,” IEEE Trans. Commun., vol. 45, pp. 1218–1230,Oct. 1997.