Jiwon Kang

Tomlinson-harashima precoder with tilted constellation for reducing transmission power

Tomlinson-Harashima precoder (THP) is being noted for an efficient alternative to dirty paper coder. THP increases the transmission power due to the use of the modulo operator. In this paper, we propose a method that reduces the transmission power by tilting the constellations of the transmit data symbols. The symbol error rate (SER) is improved for a fixed SNR by reducing the transmission power. Average transmission power is calculated asymptotically by using an extreme value distribution. We have shown that the transmission power can be significantly reduced when we use the tilted constellation THP. As the transmission power determines the SER slope, at high SNRs we achieve lower SER than in the case of a transmission over an AWGN channel. The tilted angle can be estimated at the receiver. An angle estimation algorithm is proposed that optimizes angle estimation error and SER.We use the approximation of the angle estimation error and SER to find an optimum number of tilted angles. By using the proposed angle estimation algorithm and the optimum number of tilted angles, the tilted constellation THP always outperforms the conventional THP algorithm.

An Efficient Multimode Quantized Precoding Technique for MIMO Wireless Systems

Multimode quantized precoding (QP) can provide full diversity gain or high capacity gain by adapting the number of substreams, as well as the precoding matrix, according to the instantaneous channel condition with low-rate feedback. Conventional multimode QP (MM-QP), however, does not consider the adaptive rate allocation among substreams; thus, it cannot have the additional gain by adaptive modulation. Furthermore, it is computationally complex since exhaustive matrix inversions are required to determine the optimal mode. In this paper, we propose an efficient MM-QP system that improves the performance of a conventional system in terms of error rate and has a lower computational complexity than the conventional system. First, we define the rate-partitioning vector as the mode and control the rate among substreams and the number of substreams according to the channel instantaneous condition. Second, to reduce the computational complexity for the receiver to determine the optimal mode, the simplified mode-selection technique using estimates of the modal metric is proposed. In the proposed mode-selection technique, the optimal mode can be obtained by several multiplication and division operations. Finally, the mode-reduction technique eliminating the less-frequently used modes is proposed, which leads to a significant reduction of the feedback information with negligible performance loss. In numerical experiments, it was verified that the proposed MM-QP system gives a better error-rate performance than the conventional system, with much less computational complexity for the same amount of feedback information.

An Opportunistic Beamforming Technique Using a Quantized Codebook

Opportunistic beamforming is a powerful technique increasing system throughput in a static channel environment. Since the opportunistic beamforming technique requires quite large user population to exploit sufficient multiuser diversity, a codebook-based opportunistic beamforming (COBF) technique is proposed. The COBF technique uses a unitary matrix, which changes with time, to induce larger and faster channel fluctuations in the static channel. Also, the COBF technique uses a codebook to provide further selection diversity to the conventional opportunistic beamforming technique to overcome the problem of limited multiuser diversity in a small population. Compared to the recently proposed multiple-pilot based opportunistic beamforming technique in I.M. Kim et al. (2005), the COBF technique reduces required number of pilots. Moreover, since the size of codebook, not the number of pilots, determines the amount of supplementary selection diversity, the system throughput can be increased without limitation from the number of pilots. Some computer simulation results show that system throughput of the COBF technique is higher than that of other conventional techniques and required number of unitary matrices, which needs to be kept as a memory at mobile stations, is only ten in a simulation environment.

A novel primer‐extension assay for the detection of a G to A mutation in the distal precore region of hepatitis B virus DNA

The roles of genetic heterogeneity of the hepatitis B virus (HBV) precore gene in the pathogenesis of HBV infection are unclear. Various methods have been used to detect nucleotide (nt) 1896 precore mutants. We established a new primer‐extension assay to facilitate the detection of these mutants. This assay is based upon the fact that there is no adenine in the distal precore region of wild‐type HBV. Polymerase chain reaction (PCR)‐amplified template DNA was denatured and annealed to the [γ‐32P]‐labelled primer. During primer extension in the presence of DNA polymerase and dCTP, dGTP, dTTP and ddATP, the reaction terminates if there is a nucleotide A. When mixtures of different ratios of wild‐type and nt 1896 precore mutants were analysed in the primer‐extension assay, correlation between the percentage known amounts and the percentage measured amounts of nt 1896 precore mutants was excellent (r2=0.9669). When the primer‐extension assay and direct sequencing were compared in hepatitis B e antigen (HBeAg)‐positive and ‐negative chronic active hepatitis B patients, the primer‐extension assay detected a greater number of nt 1896 precore mutants than direct sequencing and thus most HBV infections were found to be mixed infections. In conclusion, the primer‐extension assay is a reliable and sensitive method for the detection of nt 1896 precore mutants.

Efficient MIMO Scheduling Algorithms With a Fixed-Time Allocation Ratio

A scheduling problem that maximizes the system throughput under a constraint on the time-slot allocation ratio of users is considered in the systems having Gaussian-distributed performance metrics, which include multiple-input-multiple-output (MIMO) systems. Two schedulers, which are denoted by the control variance (CV) scheduler and the control mean (CM) scheduler, are proposed in this paper. The proposed schedulers have a beneficial characteristic wherein parameters of the schedulers are independent of channel statistics. Due to this characteristic, the parameters can be precalculated and then applied in the systems where only a limited number of desired ratios exist. For the systems that include an infinitely or extremely large number of desired ratios, parameter computation for the CV scheduler is greatly simplified by eliminating integral calculations. An approximation of the complementary error function is applied for simplification. Simulation results show that the proposed schedulers achieve nearly the maximum system throughput, and the Gaussian assumption and the approximation of the complementary error function cause a negligible error on the time-slot allocation ratio, even in MIMO systems equipped with a small number of antennas.

Performance Optimization of Tomlinson-Harashima Precoder with Tilted Constellation

An algorithm is proposed to optimize performance of a novel technique providing a great performance improvement over Tomlinson-Harashima precoder (THP), referred to as THP with tilted constellation (THP-TC). Proposed algorithm deals with a problem that THP-TC has faced with, the optimization of size of angular candidate set under a constant noise power condition, which is valid for the most applications of THP-TC including multiuser MIMO applications. As a result, proposed algorithm facilitates THP-TC to reach its best compromise between the transmit power reduction and the symbol error increase caused by the estimation process of tilted angle at the receiver.

An Improved Tomlinson-Harashima Precoder Reducing Transmission Power

Tomlinson-Harashima precoder (THP) is an efficient interference pre-canceler, however, the transmission power is increased over the symbol power due to the modulo operation. In this paper, we propose an improved THP that reduces the transmission power of the conventional THP by tilting a group of symbols by an angle toward minimizing the transmission power. The tilted angle can be either estimated by the receiver or notified by the transmitter using a feed-forward information. Assuming a large interference, the average transmission power of the proposed THP is derived by the central limit theorem and Gumbel extreme value distribution. The symbol error rate (SER) and its asymptotic behavior is also derived and compared. The more tilted angles or the smaller size of groups are used, the more power the proposed THP saves. The transmission power can even be less than the symbol power. Accordingly, in an environment that the power gain overwhelms the other losses, the SER can be less than the SER of AWGN channel.

A Scheduling Scheme under a Ratio Constraint for the Multiuser MIMO Systems

A scheduling scheme for MIMO systems which has the capability of obtaining multiuser diversity gain and controlling the link connection ratio among users simultaneously is proposed. Based on the Gaussian assumption of MIMO channel capacity which is fed back to the base station from each mobile, a Gaussian cost function with an appropriate mean and variance is used as a scheduling criteria. The mean and variance of the cost function permits the resources for the proposed scheduler to be controlled. We first consider the optimal scheduler, which maximizes overall system capacity with the desired link connection ratio (a priori desired ratio), and a method is proposed to obtain weights for the optimal scheduler using some properties of the Gaussian statistics. Although the optimal scheduler has the major impact on system capacity, the complexity associated with weight calculation is high. Thus, two types of suboptimal schedulers, weights of which are not dependent on the channel statistics, denoted as the control variance (CV) scheduler and the control mean(CM) scheduler, are proposed. The overall system capacity of the proposed schedulers, which is significantly enhanced by multiuser diversity, was evaluated analytically and verified by computer simulation. The simulation results show that the overall system capacity of the CM and the CV scheduler is above 99% that of the optimal scheduler. In addition, another result show that the proposed schedulers manage resources accurately, for a relatively small number of antennas

Codebook-based opportunistic beamforming for downlink multiple input multiple output systems

Opportunistic beamforming (OBF) is an efficient technique to exploit multiuser diversity in static channel environments. Since the OBF experiences lack of diversity when user population is low, a codebook-based opportunistic beamforming (COBF) is proposed. The COBF allows users to choose the best beam among multiple random beams. The COBF uses fixed pilots for channel estimation and generates multiple random beams by using a codebook composed of multiple fixed beams and a random matrix altering the beams with time slots. Compared with the conventional multi-beam opportunistic beamforming (MOBF), which also provides the beam selection diversity to the OBF, the number of beams in the COBF is related to the size of codebook, not the number of pilots. Accordingly, the COBF can increase the beam selection diversity without a restriction of pilot overhead. When the COBF is extended to a system that transmits multiple data streams in a time slot, the system throughput improvement over the OBF and the MOBF grows as the number of streams is increased because the pilots can be saved further.

An Efficient Multi-Mode Precoding for MIMO Systems with Full CSI

For the closed loop multiple-input-multiple-output (MIMO) systems, Fishers adaptive bit loading algorithm gives the best error performance by jointly optimizing the transmit powers, rates, and number of streams. However, its good performance comes at the cost of high and variable computational complexity for the joint optimization. In this letter, we propose an efficient multi-mode precoding algorithm using a simplified mode table. Numerical results show that the proposed algorithm provides almost the same performance as Fischers with much less computational complexity.