Chi-Woo Lim

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.

A Class of Structured LDPC Codes Over GF(q) for Efficient Encoding

In this paper we present a class of structured LDPC codes over GF(q) which are suitable for efficient encoding. We derive the conditions under which the Phi matrix defined by Richardson & Urbanke in (2001) is an identity matrix over GF(q). If the parity-check matrix satisfies the derived conditions, the inversion of the Phi matrix can be removed in encoding process so that the computational complexity grows linearly with the code length. Simulation results show that efficiently encodable structured LDPC codes have no performance degradation due to the constraint on their design structure, compared with randomly constructed LDPC codes.

Perpendicular Magnetic Recording Process Using Finite-Element Micromagnetic Simulation

The perpendicular magnetic recording process of a shielded-pole-type head was studied using the finite-element micromagnetic simulation. When the return and write pole are sufficiently close to each other, 40 nm in our simulation, the effective recording field is tilting at an angle of around 30deg. The tilted recording field leads to a more efficient writing since the recording media switching field is reduced according to Stoner-Wohlfarth model. This means that the shielded-pole-type head can be used to write higher coercivity recording media and allow for higher recording density and better thermal stability

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.

Perpendicular magnetic recording process simulation using finite element micromagnetic

In this paper, finite element modelling is used to simulate the perpendicular magnetic recording process with micromagnetic considerations. The simulation consists of a full-sized shielded-type head with current coil, a voronoi granular media and soft magnetic underlayer.

High density magnetic random access memory using a pair of asymmetrical cell

Magnetic random access memory (MRAM) is one of the most promising candidates to provide energy efficient and non-volatile memory. We present a new MRAM design using a pair of asymmetrical cells. The pair cells have parallelogram shaped to induce broken shape magnetic anisotropy along the short axis of the cell. One important advantage of the pair cells is that they can share the same Word and Bit lines intersection in the MRAM architecture. This means that the number of electrical current lines can be reduced and leading to higher recording density. The new concept is successfully demonstrated using finite element micromagnetic simulation.