Hyungjoon Song

Compressive Feedback Based on Sparse Approximation for Multiuser MIMO Systems

In multiuser multiple-input-multiple-output (MU-MIMO) systems, limited feedback schemes using a vector quantization codebook (VQC) for partial channel state information (CSI) can enable a base station (BS) to enhance the sum throughput at low feedback rates. However, the VQC has a limited throughput gain, although it requires a great deal of memory and complexity as the feedback rate increases. Therefore, we propose a high-precision feedback technique by using sparse approximation and compression called ¿compressive feedback (CF).¿ By using the sparse approximation with a unitary codebook and the compression via a ¿good¿ compression matrix, CF achieves greater sum throughput than VQC. Our analysis of the maximum achievable sum rate verifies the superiority of CF over VQC.

Joint Doppler-Frequency Diversity for OFDM Systems Using Hybrid Interference Cancellation in Time-Varying Multipath Fading Channels

In time-varying fading channels, orthogonal frequency division multiplexing systems suffer severe performance degradation caused by interchannel interference (ICI). We utilize this ICI as a source of diversity by using a new ICI-cancellation algorithm and Wiener filter. The proposed algorithm can support joint Doppler-frequency diversity gain. In addition, both the computational complexity and total detection time are reduced.

An Improved Location Tracking Algorithm with Velocity Estimation in Cellular Radio Networks

The location tracking of a mobile station is required to support the location-based services and the radio resource management. The improved location tracking algorithm which uses the Kalman filter with the velocity renovation process is proposed. The velocity renovation process consists of a velocity estimator and a direction finder. By this process, the proposed algorithm can use accurately estimated velocity in the location estimation. The proposed algorithm reduces more than 66% of the location estimation error and shortens the transient time of estimation compared to the Kalman filtering method in M. Hellebrandt and R. Mathar, (1999). The performance of the proposed algorithm is verified in the aspect of the outage probability and the location error ratio.

A Power Division Reuse Partitioning Scheme with Half Frequency Reuse Factor for OFDMA Downlink Systems

In multicell OFDMA systems, throughput is severely degraded by other cell interference (OCI), especially at cell edge. To satisfy the performance requirement, OCI must be mitigated effectively. Several schemes have been proposed to handle this problem by coordinating the way to allocate spectrum resource to each cell. Since the allocation of spectrum has effects on OCI mitigation and the spectral efficiency, an wise strategy to consider those two factors is required. In this paper, we propose a new scheme named as power division reuse partitioning scheme (PDRPS). PDRPS balances OCI mitigation and spectral efficiency by adopting interference cancellation (IC) scheme. The simulation results show that PDRPS has more throughput and lower outage probability than the conventional schemes.

A new asymptotic analysis of throughput enhancement from selection diversity using a high SNR approach in multiuser systems

This letter presents our study of throughput enhancement achieved by multiuser diversity (MUDiv) in a multiuser system using a new asymptotic approach. The MUDiv gain is evaluated by deriving a new asymptotic formula with a closed form, which is flexible to the number of MUDiv orders. To do this, we simplify a Puiseux series instead of a extreme value theory used in the previous researches. The formula shows that the MUDiv gain is independent of the signal-to-noise ratio, and it is applied to analyze proportional fair scheduling. Finally, this analysis is verified using Monte-Carlo simulations of scheduling algorithms.

Line Encoding for Multiuser MIMO Downlink

We investigate nonlinear precoding techniques for a multiuser multiple-input-multiple-output (MU-MIMO) downlink. When assuming full knowledge of the channel state information (CSI) at the transmitter, a vector perturbation (VP), based on sphere encoding, is a promising precoding scheme that approaches sum capacity while using a simple receiver. However, its encoding complexity can exponentially increase with the number of transmit antennas. This paper proposes a new VP scheme based on line encoding (LE), which provides very low complexity as well as competent throughput performance. For this purpose, the proposed LE scheme finds the smallest set of candidate vectors, including the optimal perturbation vector with high probability.

Robust Channel Estimation in Multicell OFDM(A) Downlink Systems With Propagation Delay

On estimating channel information in multicell OFDM(A) system, the received pilot signal is interfered by other cells signals. In this paper, we propose a new channel estimation technique based on orthogonal pilot structures in multicell OFDM(A) system. The proposed pilot structure in time domain has zero padding of the channel length to minimize the noise enhancement and use orthogonal sequences to eliminate other cells interference. Especially, we design orthogonal sequences to give robustness to propagation delay. Simulation results confirm that mean square error (MSE) performance of the proposed channel estimation is better than that of the conventional methods in multicell environment.

A Vector Perturbation Technique Based on Eigenvalue Normalization for Multi-User MIMO Downlink

We investigate a non-linear precoding techniques for multi-user multiple-input multiple-output (MU-MIMO) down- link. When assuming full knowledge of the channel state informa- tion at the transmitter, a vector perturbation (VP) is a promising precoding scheme that approaches sum capacity and has simple receiver. However, its encoding is nondeterministic polynomial time (NP)-hard problem. In the MU-MIMO downlink, this encoding problem becomes harder because the proper user selection must be considered. In this paper, we propose a new vector perturbation scheme based on eigenvalue normalization (VPEVN), which converts the NP-hard problem into a channel condition-based finite integer problem, resulting in a reduction of encoding complexity. In addition, it can select a good user set by simple maximum norm criterion owing to its orthogonalization effect. Index Terms—MIMO broadcast channel, multi-user MIMO, precoding, vector perturbation, eigen decomposition

Dirty Paper Coding Aided Multihop Cellular Networks: Architecture and Resource Allocation Framework

Most existing works on orthogonal frequency division multiple access (OFDMA) multihop cellular networks have assumed half-duplex relay, which suffers from spectral inefficiency since it requires additional resources for relaying. In order to alleviate this half-duplex loss, a new relay transmission scheme using dirty paper coding (DPC) is proposed. By alternately forwarding the messages to two mobile stations (MSs) via two fixed relay stations (FRSs) for each subcarrier, the base station (BS) predicts and pre-cancels the inter-relay interference between FRSs in advance. This proposed DPC-aided transmission scheme matches well with the nature of the quasistatic fading channel between FRSs. We also propose a two step resource allocation algorithm to select the appropriate MSs and to maximize the achievable sum rate. Numerical results demonstrate that the achievable sum rate of the proposed DPC-aided multihop cellular networks is significantly increased by utilizing the multi-user and multi-relay diversity gain as well as pre-cancelling the inter-relay interference between FRSs.

An Equalization Technique based on QR Decomposition for DSTBC-OFDM Systems with Distributed Carrier Frequency Offsets

In the distributed space-time block coded (DSTBC) orthogonal frequency division multiplexing (OFDM) system (DSTBC-OFDM), there may exist distributed carrier frequency offsets (CFOs) by sharing the transmission antennas belonging to different nodes. In this paper, the signal model of the DSTBC-OFDM system with distributed CFOs is presented. Then we propose a new equalization technique via the modification of the QR decomposition (MQRD) in order to eliminate ICI and ISI which are caused by CFOs. The proposed MQRD scheme guarantees transmit diversity gain. Several kinds of equalization techniques are introduced and compared in terms of bit error rate (BER) performance and transmit diversity order. Simulation results show that the proposed method outperforms conventional equalization techniques and achieves equal transmit diversity order with perfectly synchronized DSTBC-OFDM system.