Joontae Kim

Hidden Pilot Based Precoder Design for MIMO-OFDM Systems

In order to improve bandwidth efficiency and bit error rate (BER) performance, a new hidden pilot scheme using a precoder is proposed for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. We show that the inevitable data interference of the hidden pilot, which degrades the performance of channel estimation, can be reduced successfully by the precoder design for each antenna with the aid of an iterative scheme. We also show that frequency diversity gain can be achieved due to the spreading effect of the precoder. Computer simulations are presented in which the proposed scheme is compared with conventional methods with respect to channel estimation, BER and bandwidth efficiency.

Mitigation of transmit correlation for TIMO spatial multiplexing through phase rotation

In MIMO wireless communications, spatial multiplexing can be employed to achieve a high data rate. However, its performance can be degraded by spatial correlation between the transmit antennae. Hence, a simple closed-form linear precoder is proposed to improve performance when two transmit antennae are exploited at the transmitter. The proposed precoder rotates the constellation by a certain phase angle, which can be easily obtained from the correlation matrix of the channel. The simulation results show that the proposed algorithm can achieve near- optimal performance with remarkably reduced computational complexity due to the closed-form solution.

Constrained Codebook Design for a MISO Beamforming System in a Rician Channel

This study presents a limited feedback codebook design for a multiple-input single-output (MISO) beamforming system in a Rician fading channel. To support the Rician fading channel, the physical characteristics of the channel are exploited in designing the codebook by Grassmannian line packing (GLP). To this end, a portion of the total codewords is allocated for supporting the line-of-sight (LOS) channel. By a proposed design procedure, it is possible to design a codebook which can increase the system capacity in a large K-factor region without any capacity loss in a Rayleigh fading channel. The simulation results show that the overall capacity can be remarkably increased by the proposed codebook.

Simple phase rotation for two-input multile-output spatial multiplexing in transmit correlated channels

In MIMO wireless communications, spatial multiplexing can be employed to achieve high data rate. However, its performance can be degraded in the presence of spatial correlation among transmit antennas. Hence, we propose a simple closed-form linear precoder to improve the performance when two transmit antennas are exploited at the transmitter. For the proposed scheme, we consider a maximum likelihood detector (MLD) at the receiver. Our proposed precoder rotates the constellation by a certain phase which is easily obtained by the correlation matrix of the channel. Simulation results show that the proposed algorithm can achieve near- optimal performance with remarkably reduced computational complexity helped by the closed-form solution.

Closed-form interpolations for MISO-OFDM beamforming codewords

We derive a spherical linear interpolation (SLI) with a closed-form phase term. The derived SLI is compared with the geodesic interpolation. Simulation results show that both schemes are efficient and similar when the channels are highly correlated, since their objectives are equivalent in a sense of minimizing the distance between two adjacent codewords.

Maximum likelihood population estimator of RFID tags with Hadamard footprint

A novel method is proposed that can estimate the tag population in Radio Frequency Identification systems by using a Hadamard code for the sub-preamble of the tag response. We formulate the maximum likelihood estimator for the tag population using a given number of the observed footprints. In simulations, the proposed estimator performs considerably better than the conventional schemes.

MIMO precoding in the ad hoc networks

In this paper, multi-user multi-input multi-output (MIMO) precoding in the ad hoc networks is presented. The ad hoc channel model is considered and the leakage based concept extended centralized systems to distributed networks. The conventional scheme simply use own inverse channel matrices and can be suffered a noise enhancement problem. This paper presents that the leakage based precoding provides also strong performance in the ad hoc networks. It is proven using the achievable sum rate simulations.

Derivations of decoder and beamforming vector for amplify—and—forward relays based on the Cauchy-Schwartz inequality

For multiple amplify—and—forward (AF) relays, optimal decoder and beamforming vector have been presented [3], [4]. The derivations of these optimal ones have been based on the generalized eigenvalue problem. In this work, we show the derivations using the Cauchy-Schwartz inequality. Moreover, the intimate relation between decoder and beamforming vector is analyzed.

An adaptive space-time coding based relay allocation scheme in AF cooperative systems

An adaptive relay allocation scheme for multi-user cooperative communication systems with amplify-and-forward (AF) relays is investigated. We consider maximizing system capacity while guaranteeing the QoS (Quality of Service) of each user. Proposed relay allocation scheme uses space-time coding technique. It performs well in allocating relay nodes to multiple source-destination pairs. The proposed algorithm enhances system performance without loss of diversity.

A New Channel Estimation Method Using Phase-Shifted Pilot Sequence for MIMO-OFDM Systems

In this paper, we propose a new MIMO-OFDM channel estimator with a phase-shifted pilot (PSP) sequence to enhance the performance and overcome the problems of conventional schemes. The proposed scheme overcomes conventional PSP estimators limitation of requiring a uniformly distributed pilot position and performs better by adopting a minimum mean square error criterion that cannot be applied to conventional schemes. The proposed PSP channel estimator can perfectly separate multiple channel impulse responses, even though all transmit antennas use the same pilot position. Furthermore, the simulation results show that the performance of the proposed scheme is close to lower bound of the mean square error.