Jae-won Kim

Areal capacity limit on the growth of small cell density in heterogeneous networks

We consider heterogeneous network (HetNet) systems comprising of two different types of base-stations (BSs): macro and small BSs. In the previous studies for HetNet, it is observed that the distribution of signal to interference plus noise ratio (SINR) is independent of the BS density. This implies that network throughput increases linearly with the number of BSs. In this paper, however, we have identified key practical factors which degrade the SINR distribution for dense HetNet. On the contrary to the previous studies, limited network throughput is verified by evaluating the areal capacity with respect to the density of BS under the practical channel model. The observation is made regardless of the deployment scenario whether the two types of BSs share the same frequency band or not. Based on the analysis and the simulation results provided in this paper, we present principal guidelines for cell deployment and system operation to improve the areal capacity of dense HetNet systems.

A New Codebook Structure for Enhanced Multi-User MIMO Transmission in mmWave Hybrid-Beamforming System

A multi-user hybrid-beamforming (H-BF) system consisting of both analog beamforming and digital beamforming is considered for millimeter-wave (mmWave) channels. In the hybrid-beamforming system, analog beams are first allocated to scheduled users and then each user reports channel state information of the effective channel, i.e. channel between transmit and receive digital paths. Since amplitudes of the effective channel elements can largely be different from each other due to the analog beamforming which has a certain directivity, the conventional feedback scheme, e.g. precoder matrix indicator (PMI) in long-term evolution (LTE), that does not reflect this channel characteristic may not work well. In this paper, we propose a new feedback scheme for multi-user H-BF systems in mmWave channels. The proposed feedback scheme jointly utilizes two types of codebooks: 1) a direction of the H-BF channel; and 2) amplitudes of the channel elements. In numerical results, it is shown that the proposed feedback scheme largely improves the average throughput performance, especially when the analog beamwidth is narrow.

Spatio-Temporal Video Transcoder for Streaming over Mobile Communications Networks

Video transcoding technique is an efficient mechanism to deliver visual contents to a variety of users who have different network conditions or terminal devices with different display capabilities. In this paper, we propose two types of transcoding methods for adapting the bitrate of streaming video to the bandwidth of the transmission channel; spatial resolution reduction (SRR) transcoding and temporal resolution reduction (TRR) transcoding. The two transcoding methods are alternatively operated according to the requirements of users. Experimental results show that the proposed transcoding methods can preserve image quality while transcoding to the low bitrate.

An Adaptive Adjacent Cell Interference Mitigation Method for Eigen-Beamforming Transmission in Downlink Cellular Systems

EB(Eigen-Beamforming) has widely been applied to MIMO(Multiple-Input Multiple-Output) systems to form beams which maximize the effective signal-to-interference plus noise ratio(SINR) of the receiver using the singular value decomposition(SVD) of the MIMO channel. However, the signal detection performance for the mobile station near the cell boundary is severely degraded and the transmission efficiency decreases due to the influence of the interference signal from the adjacent cells. In this paper, we propose an adaptive interference mitigation method for the EB transmission, and evaluate the reception performance. In particular, a reception strategy which adaptively utilizes optimal combining(OC) and minimum mean-squared error for Intercell spatial demultiplexing(MMSE-lSD) is proposed, and the reception performance is investigated in terms of the effective SINR and system capacity. For the average system capacity, the proposed adaptive reception demonstrates the performance enhancement compared to the conventional EB reception using the receiver beamforming vector, and up to 2 bps/Hz performance gain is achieved for mobile station located at the cell edge.