On the Achievable Diversity Gain by the Optimal Subcarrier Allocations in Multiuser OFDM System

Although adaptive subcarrier-and-bit allocation is not new in the literature, most work was dealing with instantaneous channel conditions and there is a lack of knowledge on the average multiuser diversity gain that is attainable. In our work, we demonstrate that the achievable diversity gain in a multiuser OFDM system can be quantified in terms of spectral flatness measurements (SFM), subcarrier correlation coefficients chi and RMS delay spread D_tau ⁽²⁾ . Firstly, the probability density function (PDF) of the SFM is presented. Next, the optimal solution for the adaptive subcarrier-and-bit allocation in a multiuser OFDM system is obtained, and the results are used to obtain the relationship between the transmit power and SFM, chi and D_tau ⁽²⁾, for a given number of users and subcarriers. From these relationships, we see that the average normalized power P_a can be minimized at an optimum delay spread for a certain number of subcarriers. This developed approach could be also useful for the designers to fully exploit the multiuser diversity gain of OFDM systems