Frequency and Quadrature-Amplitude Modulation for Downlink Cellular OFDMA Networks

The distribution of the intercell interference (ICI) in conventional cellular networks employing orthogonal frequency-division multiple-access (OFDMA) with quadrature-amplitude modulation (QAM) tends to approach a Gaussian distribution when all available subcarriers in each cell are fully loaded. Recently, it has been also shown that the worst-case distribution of the ICI as additive noise in wireless networks with respect to the channel capacity is Gaussian. Thus, the channel capacity in cellular networks is expected to be further enhanced when the ICI could be designed properly so that it has a non-Gaussian distribution. This observation motivates us to propose, in this paper, a downlink cellular OFDMA network employing a modulation scheme called frequency and QAM (FQAM). We also derive maximum-likelihood metrics for the binary or non-binary error-correcting codes employed in the proposed network and propose their practical sub-optimal versions. Numerical results demonstrate that the distribution of the ICI in the proposed network deviates far from the Gaussian distribution. As a result, the transmission rates for the cell-edge users in the proposed network are significantly improved. In addition, the measurement results using practically implemented FQAM-based OFDMA systems verify that the transmission rates for the cell-edge users can dramatically increase, compared with the conventional QAM-based OFDMA network.

I. Introduction

THE performance of a wireless multipoint communication network has been limited by its inherent interference. To cope with the interference in cellular networks, interference mitigation techniques including interference cancellation at the receiver/transmitter as well as coordination schemes among multiple cells have been considered so far. On the other hand, there have also been a few attempts to investigate 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 worst-case additive noise in wireless networks with respect to the channel capacity has a Gaussian distribution. These observations motivate one to expect that the transmission rate in interference-dominated environments may substantially increase when a proper modulation scheme is employed instead of QAM so that the resultant ICI has a non-Gaussian distribution.