| In multiuser OFDM systems, performance gain in terms of reduction in total power consumption is made possible if subcarrier-and-bit allocation algorithm is used to exploit the time-frequency selectivity. However, most of these work are focused on developing heuristic algorithms, and no attention has been paid to quantify the average achievable multiuser diversity gain. In some of the recent developed techniques, such as cyclic delay diversity (CDD), spatial diversity is intentionally introduced to OFDM signal through the use of multiple antennas each with different cyclic delays. This basically transformed a multiple-input system into a single-input system with increased frequency selectivity, i.e., spatial diversity is transformed into frequency diversity, so that multiple users can exploit adaptively among the time-frequency bins to achieve diversity gain. This gives rise to a question of how much spatial diversity can be brought into the signals and whether there is a limit on the gain we can achieve. The objective of this paper is to show that it is possible to quantify the achievable multiuser diversity gains in terms of the channel parameters such as spectral flatness measurement, channel delay spread and subcarrier correlation coefficient. Our example shows that there exists a maximum allowable diversity gain that can be brought into the OFDM signals, and beyond this point performance gain is degraded.
In this paper, we propose an approach to quantify the achievable multiuser diversity gain when applying optimal subcarrier-and-bit allocation algorithm. We first define spectral flatness measurement (SFM) which is used to measure the spectral flatness of the channel frequency response, and its technical manifestation is explained. The statistical distributions of SFM under different number of multipath and power delay profile are presented. We then formulate a subcarrier allocation problem to minimize the total power consumption in a multiuser single-class OFDM system. The total transmit power is minimized with the fulfillment of data rate requirement of each user. Simulation results are presented, in which the normalized average power is obtained as a function of channel delay spread or subcarrier correlation coefficient. Furthermore, the relationships between the total system power and SFM, channel delay spread and subcarrier correlation coefficient are obtained.
Our results show that for a given multipath channel, the amount of gain achievable through subcarrier-and-bit allocation increases with the instantaneous SFM of the channel and the RMS delay spread. We have shown that there is a clear relationship between the optimal total power and the channel delay spread as well as the SFM. Hence we provide a quantitative justification on the potential performance gain achievable through exploiting the multiuser diversity. Furthermore, our approach could be useful for the investigations on the achievable gain in the CDD OFDM system. |