Sung-Nam Hong

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.

FQAM : A modulation scheme for beyond 4G cellular wireless communication systems

We analyze the performance of bit interleaved coded modulation (BICM) and coded modulation (CM) systems with frequency and quadrature amplitude modulation (FQAM), which is a combination of frequency shift keying (FSK) and quadrature amplitude modulation (QAM). Numerical results show that for modest code rates in low signal-to-noise ratio regions, the normalized throughputs of FQAM based CM systems are close to the theoretical channel capacity limit compared to that of QAM and FSK based CM systems. Also, we analyze the performance of downlink cellular LTE systems using FQAM and compare it with that using QAM, especially for cell-edge users. Numerical results show that, unlike QAM, the statistical distribution of inter-cell interference (ICI) incurred with FQAM highly deviates from the Gaussian distribution and has a heavier tail. The results also show that due to the non-Gaussian nature of the ICI incurred with FQAM, the transmission rate of a cell-edge user using FQAM is significantly higher than that using QAM.

Performance of multitone-frequency and quadrature-amplitude modulation over Rayleigh fading channels

Multitone-frequency and quadrature-amplitude modulation (MFQAM) is a combination of multitone frequency-shift keying and quadrature-amplitude modulation (QAM). In this study, the authors consider MFQAM with active subcarriers grouped in balance, where the active subcarriers are divided into a specified number of groups and the same QAM symbol is conveyed by every subcarrier in each group. This grouping may make MFQAM have frequency diversity in itself. They first derive the joint probability density function of the correlator outputs of an MFQAM receiver. They then compute the channel capacity of a coded modulation (CM) system employing MFQAM over Rayleigh fading channels. As a result, they demonstrate that for a moderate code rate between 1/3 and 2/3, its normalised throughput is close to the channel capacity limit in the low signal-to-noise ratio region, contrary to a CM system employing the conventional modulation. They also evaluate the frame error rate (FER) performance of a CM system employing MFQAM when a non-binary error-correcting code is employed. Numerical results demonstrate that the normalised throughput of the CM system computed via the FER shows a similar trend to that obtained via the channel capacity.

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.