Jongpil Seo

A modified two-way relaying method with a maximum ratio transmission

We present a modified two way relaying method exploiting a maximum ratio transmission. The proposed relaying method attains SNR gain using a maximum ratio transmission, and is feasible to implement in relay systems owing to the absence of need for channel estimation at the relay station. Simulation results show that the modified relaying method provides higher SNR gain than that of a conventional two phase system with multiple antennas.

Equalizer Mode Selection Method for Improving Bit Error Performance of Underwater Acoustic Communication Systems

The linear and decision-feedback equalization can mitigate time-varying intersymbol interference (ISI) caused by time-varying multipath propagation for underwater acoustic channels. The perfect elimination of interference components, however, is difficult using the linear equalization and the decision feedback equalizer has an error propagation problem. To overcome these shortcomings, this paper proposes an equalizer mode selection method using training sequences. The proposed method selects an equalization mode corresponding to the signal-to-noise ratio (SNR). If the SNR is low, the proposed system operates the linear equalizer for preventing the error propagation and if the SNR is high, the decision feedback equalizer for eliminating the residual ISI. Therefore, the proposed method can improve the error performance compared to the conventional equalizers. The computer simulation shows the proposed method improves the bit error performance using practical underwater channels responses acquired from the sea experiment.

Macrocell Protection Interference Alignment in Two-Tier Downlink Heterogeneous Networks

Conventional interference alignment (IA) has been developed to mitigate interference problems for the coexistence of picocells and macrocells. This paper proposes a macrocell protection interference alignment (MCP-IA) in two-tier MIMO downlink heterogeneous networks. The proposed method aligns the interference of the macro user equipment (UE) and mitigates the interference of the pico-UEs with a minimum mean squared error interference rejection combining (MMSE-IRC) receiver. Compared to the conventional IA, the proposed MCP-IA provides an additional array gain obtained by the precoder design of the macro BS and a diversity gain achieved by signal space selections. The degrees of freedom (DoF) of the proposed MCP-IA are equal to or greater than that of the conventional IA and are derived theoretically. Link level simulations show the link capacity and the DoF of the macro UE, and also exhibit the proposed MCP-IA attaining additional array gain and diversity gain. The system level simulation illustrates that the proposed method prevents the interference of the macro UE completely and preserves the throughput of the pico-UE irrespective of the number of picocells. For antenna configuration, the system level simulation demonstrates that the proposed MCP-IA throughput of the macro UE is not affected by the number of picocells and that the proposed MCP-IA throughput of the picocells approaches that of single-user MIMO (SU-MIMO) with a 3% loss.

Orthogonal Reference Vectors Selection Method of Subspace Interference Alignment

This paper proposes theorthogonal reference vectors selection method of the subspace interference alignment. The proposed method selects multiple orthogonal reference vectors instead of using one reference vector for all users at the same time. The proposed scheme selects a reference vector which maximizes a sum-rate for a certain cell, generates orthogonal vectors to the previous selected vector and selects the one of orthogonal vectors whose sum rate is maximized for each cell. Larger channel gain and sum-rate than the previous method can be obtained by selection degree of freedom. The computer simulation demonstrates the proposed method gives higher sum-rate compared with that of the previous reference vector selection method.

Array antenna calibration method using internal loop-back

This paper proposes an array antenna calibration method for array antenna systems. The proposed method calibrates the amplitude and the phase of RF systems using internal coupler and switches without aids of external calibration systems. Computer simulation demonstrates that the proposed method corrects beamforming angles.

QoS based precoding scheme in K-user MIMO interference channels

A QoS based precoding scheme with partial CSIT and simple computation in K-user MIMO interference channel is proposed. A transmitter-receiver pair for interference alignment (IA) is selected by a given criterion such as QoS or required data rate, etc., and others are performed by random beamforming and whitening matched filtering. Global channel knowledge is no longer needed at all transmitters and computational complexity is significantly reduced for transmitter and receiver design. Numerical results show that the sum capacity of the proposed scheme is reasonable compared to that of perfect IA.

Opportunistic user selection for interference alignment

An opportunistic user selection method for interference alignment is proposed. The proposed scheme selects a space between desired signal and interference spaces according to received signals from all base stations. An additional selective diversity gain is obtained in addition to a multi-user diversity gain. Simulation examples show that the proposed method achieves higher sum capacity than that of conventional opportunistic interference alignment.

Selective Subspace Interference Alignment for Cognitive Radio Systems

This paper presents a selective interference alignment for cognitive radio system with spectrum leasing. The proposed method selects users who cause severe interference to other basestations that have required SINR. Since few users are selected to apply subspace interference alignment, the total complexity of the system is not high compared with that of the system who utilizes subspace interference alignment to all users. In addition, all users can transmit without considering interference. The computer simulation shows the proposed method exhibits 350% throughput enhancement at a two cell case, and 400% increase at a three cell case.

A Relay and Transmission Mode Selection Scheme to Enhance the Bit Error Rate Performance in Relay Systems

In a cooperative communication system with a source node and multiple relays equipping single antenna and a destination node equipping multiple antennas, the selective cooperative spatial multiplexing scheme can obtain spatial multiplexing gain and additional selection diversity gain. But it can degrade a bit error rate performance because some received symbols forwarded from particular relays may be lost by attenuation due to path-loss. We propose a relay and transmission mode selection scheme which selects minimum number of multiple relays having the channel capacity larger than a given data rate and transmission mode which switches spatial multiplexing and spatial diversity mode in cooperation phase to enhance the bit error rate performance. The proposed scheme achieves 1.5~2dB gain at the low SNR range compared with the conventional scheme by obtaining additional spatial diversity gain.

Selection diversity of subspace interference alignment in uplink systems

This paper proposes a reference vector selection diversity method of subspace interference alignment systems to obtain higher sum rate capacity. The proposed method calculates sum-rate capacities and selects the best reference vector to increase the total sum rate. Simulation result shows that the proposed method increases the sum-rate compared with a previous method.