Haelyong Kim

OFDM Channel Estimation for the Amply-and-Forward Cooperative Channel

OFDM can significantly reduce the receiver complexity by using one-tap equalizer in frequency selective channels, and it can be enhanced the capacity and diversity by using cooperative scheme. However, high data rate transmission with OFDM requires the channel state information (CSI), and therefore OFDM channel estimation for cooperative communication can be a challenging problem. In this paper, we investigate the OFDM channel estimation suitable for the amply-and-forward (AF) cooperative channel. Cramer-Rao lower bound (CRB) of the estimation is derived, and optimal preamble conditions that minimize the CRB with several power constraints are proposed. Moreover, minimum variance unbiased (MVU) estimator that achieves the CRB is introduced for the cooperative channel. Simulations with analytical results confirm the performance of our methods.

Effects of Antenna Correlation on Spatial Diversity and Multiuser Diversity

This paper investigates the effects of antenna correlations on spatial diversity and multiuser diversity. Using an upper bound on achievable capacity from order statistic theory, we quantify the interactions between spatial correlation, spatial diversity and multiuser diversity. Our theoretical analysis and simulation results demonstrate a positive influence of spatial correlations on achievable capacity of spatial diversity techniques combined with multiuser diversity, while it has been known that spatial diversity limits a multiuser diversity gain without spatial correlation.

Cooperative Diversity System Achieving Full-Rate and Full-Diversity by Constellation Rotation

We introduce and analyze an enhanced cooperative diversity system where the source, relay, and destination are equipped with single antenna, respectively. A time-division multiple-access (TDMA) based cooperative protocol is considered. This protocol has full data rate because the source transmits two symbols during two time slots. However, the protocol does not obtain full diversity gain, because each column of effective channel matrix has a different distribution. We propose the constellation rotation scheme to solve the problem. Also, we analyze the diversity gain of the proposed system with the maximum-likelihood (ML) receiver by deriving the pairwise error probability (PEP). The analysis proves that the system applying the constellation rotation scheme achieves a full diversity. Also, we verify the superiority of the proposed scheme from the simulations.

Improved User Scheduling Algorithms for Codebook Based MIMO Precoding Schemes

In this paper, we deal with user scheduling algorithms for a codebook based precoding of a multiuser multiple-input multiple-output (MIMO) downlink system considered in IEEE 802.16m and 3rd Generation Partnership Project Long-Term Evolution (3GPP LTE) standardization. We especially focus on per-user unitary rate control (PU2RC) and semi-orthogonal user selection schemes. Also, we examine the disadvantage of the codebook based precoding with these schemes -- a loss of sum-capacity when the codebook size is larger than the number of total users who request the communication service to a base station. Finally, we propose two user scheduling algorithms to improve the sum-capacity when the number of total users is smaller than the codebook size. First, we introduce feedback information retransmission algorithm in which each user sends a new codeword again depending on a request of the base station. Second, we address the adaptive feedback algorithm where users transmit one or two codeword indices called preferred matrix index (PMI) depending on their channel condition. The improvement of the sum-capacity is verified through the simulation.

Performance of Multiuser Transmit Diversity in Spatially Correlated Channels

This paper investigates the performance of an Alamouti coding based transmit diversity scheme with multiuser max-scheduling in spatially correlated channels. An exact closed form of ergodic capacity is derived, and bit error rate (BER) analysis is performed with a log likelihood radio (LLR) approach for two transmit antennas case. Our analytical and numerical results verify a positive effect of spatial correlation on multiuser diversity in terms of ergodic capacity. We show, however, that spatial correlation is beneficial in terms of BER until a certain signal-to-noise ratio (SNR) level while it hurts BER as SNR increases compared with ergodic capacity.

Asymptotic Sum Rate in Spatially Correlated Two-Cell Channel With User Scheduling

Coordinated beamforming (CBF) provides improved sum rate with less overhead and computational complexity, and its theoretical performance analysis is mostly finished in a spatially uncorrelated environment. In this paper, we investigate the effect of spatial correlation on the sum rate of a two-cell multiuser downlink channel. We analyze the scaling laws of the sum rate with a large number of users. To improve the performance, different roles are assigned to the two cells: One is a master cell, and the other is a slave cell. The base station (BS) of the master cell requests the interference cancellation to the slave cell, and the BS of the slave cell performs precoding to cancel the interference influencing over the master cell, although the achievable rate of the slave cell is decreased. The decision of the master and slave cells can be affected by the number of users and the degree of spatial correlations. From computer-based simulations, we show that our proposed algorithm can achieve a higher sum rate than other previous approaches with lower overhead.

Adaptive Rotation Based Full-Diversity Full-Rate Scheme with Channel State Information

Open-loop full-diversity full-rate (FDFR) code can achieve both high data rate and good reliability at the expense of significantly increased decoding complexity. To mitigate the decoding complexity, low-complexity FDFR code was designed for closed-loop system, combining the preceding and constellation rotation with channel state information (CSI) at the transmitter. Since conventional closed loop FDFR encoding procedure is similar to that of open loop FDFR code, however, it suffers from latency problem due to multiple-symbol-periods encoding and decoding process. Also, the rotation matrix is fixed irrespective of temporal channel response. In this paper, we propose simple FDFR scheme which performs single-symbol- period encoding and decoding procedure. Moreover, we improve error performance of closed-loop FDFR code by exploiting a adaptive rotation matrix corresponding to temporal channel state, especially the singular values of channel. The adaptive rotation matrix construction aims at maximizing the minimum Euclidean distance among received signals. Simulation results support the validity of the proposed FDFR scheme.

An efficient channel tracking method for OFDM based high mobility wireless multimedia system

Preamble based channel estimation is widely used in OFDM based wireless multimedia system. However, the channel response varies significantly during one transmission packet due to high mobility. We propose new channel estimation method that tracks the channel variation along the time. We take advantage of the pilots occupied for the phase tracking to catch the variation of the channel responses with slightly modified pilot structure. For the accurate channel tracking, the variation trend of the channel is examined by correlation analysis. In here, we see the linear channel variation and so we use the linear interpolation for channel tracking. Without loss of data rate, the method reduces the error floor of bit error rate (BER) dramatically. Simulation results demonstrate the capability of the proposed channel tracking scheme even though the Doppler shift is very large. Moreover, the proposed method shows more competitive performance in convolutional coded systems used in many wireless multimedia application like IEEE 802.11a.