A modulation technique for active interference design under downlink cellular OFDMA networks

In cellular systems, the interference has been the most critical issue that practically limits the performance of overall networks. Furthermore, the distribution of the inter-cell interference (ICI) in conventional cellular networks employing orthogonal frequency-division multiple-access (OFDMA) with quadrature amplitude modulation (QAM) tends to approach a Gaussian distribution, which is known to be the worst-case distribution of the ICI as additive noise resulting in poor channel capacity. Thus, a dramatic enhancement of the channel capacity for the cellular network is expected when the ICI could be designed properly so that it has a non-Gaussian distribution. In this context, a cellular OFDMA system with a novel modulation scheme, frequency and quadrature-amplitude modulation (FQAM), is proposed in this paper. The statistical distribution of the ICI is shown to deviate far from the Gaussian distribution for the proposed FQAM-based system. Accordingly, it is shown that the non-Gaussian distribution of the ICI incurred with FQAM results in significantly improved transmission rates for the cell-edge users. Also, from the measurement results using practically implemented FQAM-based OFDMA systems, it is verified that the transmission rate for the cell-edge users could be increased significantly over the conventional QAM-based OFDMA system.

I. Introduction

The performance of wireless communication networks supporting multipoint communications has been limited by its inherent interference, hence the need for handling the critical issue appropriately. To cope with the interference in cellular networks, interference mitigation techniques including interference cancellation at the receiver/transmitter and the coordination among multiple cells (CoMP: coordination multiple-points) have been considered so far. On the other hand, there have also been a few attempts regarding the investigation into the stochastic characteristics of the interference in wireless communication networks. In [1], it was shown that the distribution of the inter-cell interference (ICI) in a downlink cellular orthogonal frequency-division multiple-access (OFDMA) network with quadrature amplitude modulation (QAM) is close to a Gaussian distribution when all available subcarriers in each cell are fully loaded. Moreover, the recent work in [2] proves that the Gaussian distributed ICI as additive noise is the worst-case with respect to the channel capacity. Thus, it is motivated that the significant increase of the transmission rate in the interference-limited environment could be expected when the ICI could be designed properly so that it has a non-Gaussian distribution.