Michiharu Nakamura

Channel estimation of multiple transmit antennas for OFDM systems with cyclic delay preamble

A simple and efficient channel estimation technique for multiple input multiple output (MIMO) and orthogonal frequency division multiplexing (OFDM) system (MIMO-OFDM) with cyclic delay preamble is proposed in this paper. Based on the properties of the discrete Fourier transform (DFT), the delay in the time domain introduces the phase rotations in the frequency domain, and vice versa. The window function in the time domain is presented to separate and estimate channel transfer function for different transmit antennas. The time domain window can be considered as a filter in the frequency domain, thus the improved performance can be obtained by the proposed method. Several window functions are investigated and compared with minimum mean squared error (MMSE) filter for various delay spread channels. Simulation results are presented to show the good performance of the proposed method for two transmit and one receive antenna configuration. The proposed window method is not sensitive to channel statistics and significantly reduces the complexity of channel estimation for cyclic delay preamble.

A new digital predistorter linearizer for wide band signals

This paper presents a new method of digital adaptive predistortion for linearization of high-power amplifiers (HPAs) that mitigates linearization performance deterioration due to bandwidth limitation of feedback path. With increases of signal bandwidth, the effects of limited bandwidth of the predistorter feedback path cannot be ignored. In the proposed algorithm, adaptation is derived from direct learning for the lookup table (LUT) gains. According to the proposed approach, when the LUT contents are adapted according to an LMS algorithm, several other LUT entries adjacent to the currently processed entry are also updated. However, the level of adaption for adjacent entries is less than for the currently processed entry. This level is defined by the preselected window weight function. The proposed algorithm makes it possible to design a low-complexity, memoryless adaptive predistortion system with narrowband feedback path to compensate for the nonlinearity of an HPA.

High Performance Space Frequency Interleaved MIMO-OFDM Eigenmode Transmission Systems

Spatial multiplexing with linear preceding is a simple technique to increase system capacity and to exploit the high spectral efficiency provided by multiple-input multiple- output (MIMO) systems, and it can be easily implemented over frequency-selective MIMO channels by using orthogonal frequency division multiplexing (OFDM). The optimal transmitter preceding matrix is obtained by singular value decomposition (SVD) of the channel matrix. The eigenmodes of the channel matrix exhibited frequency selectivity, and the signal to noise ratio associated with each of the eigenmodes is directly proportional to its eigenvalue. Space frequency interleaver aims to achieve space and frequency diversity. Based on these observations, this paper proposes a weighted soft decision Viterbi decoding by using the eigenvalues of the channel matrix, to implement high performance space frequency interleaved MIMO-OFDM eigenmode transmission systems. Our simulation results confirm that this technique with low complexity improves error rate performance considerably.

Controlling TCP ACK transmission for throughput improvement in LTE-Advanced Pro

The 3rd Generation Partnership Project (3GPP) is standardizing LTE-Advanced (LTE-A) Pro. One of the technical challenges is the reduction of the uplink latency, which is larger than the downlink latency, aiming at improving the Transmission Control Protocol (TCP) throughput. A study item called Latency Reduction (LATRED) has been agreed to standardize fast uplink access schemes. The proposed solutions include shortening of the subframe length and fast uplink resource allocation to mobile terminals for the uplink data transmissions. The reason for this is that speeding up the data processing of the layer 1 and the layer 2 protocols reduces the TCP Round Trip Time (RTT) and hence the downlink TCP throughput can be improved. However, the asymmetric bandwidth of the radio link has not been taken into account. The asymmetry tends to cause the uplink congestion and uplink resource shortage for TCP ACK transmission, which has a negative impact on the TCP RTT. In this paper, we propose methods to improve the TCP throughput focusing on the TCP ACK filtering in the LTE-A Pro uplink.

VISA MIMO OFDM transmit scheme for wireless communications

The use of multiple antennas at the transmitter and the receiver results in enormous capacity increase. It has been well known that a spectral efficiency near the Shannon bound can be achieved in multi-input multi-output (MIMO) systems. However, such capacity is attained with penalty of tremendous receiver complexity. Time domain VISA (virtual subcarrier assignment) method has been first proposed for OFDM (orthogonal frequency division multiplexing) signal by Hara et al. (2003, 2004). Principle of the VISA structure is to deliberately color OFDM signals by different virtual subcarrier positions in the spectrum to enable adaptive array of antennas (AAA) to accept OFDM signal selectively. Due to the promising results, we extend the VISA concept, in this paper, into the frequency domain processing for MIMO (multi-input multi-output) transmission. To illustrate the performance of the proposed scheme, simulation results will be presented for QAM modulations.

Adaptive Linearization through Narrowband Feedback

An adaptive predistorter (PD) is an effective technique to compensate for the nonlinear distortion of power amplifiers. To enable broadband communications, the bandwidth of communication systems has had to continuously increase. With these increases in bandwidth, frequency distortions due to limitations in the bandwidth of the PDs building blocks can no longer be ignored. Such distortions are important factors that define a PDs linearization performance. In this study, we investigated the adaptive PD performance degradation caused by bandwidth restrictions in the feedback (FB) path. Employing a newly proposed graphical approach which we confirmed by computer simulations. We show that achieving PD linearization with narrowband FB strongly depends on the PDs architecture and the FB path bandwidth. Finally, we make practical recommendations regarding the implementation of adaptive PDs having memoryless and memory polynomial architectures that depend on the available FB path bandwidth.

Controlling TCP ACK transmission: Impact of discard ratio on performance in LTE-Advanced Pro

Recent research interest in cellular communication systems focuses on the design of the radio access networks (RAN) considering Transmission Control Protocol (TCP) behavior. However, less attention has been paid to the specific transmission method of TCP ACK. The authors have been studying methods including TCP ACK filtering that can well work with the link layer protocols in LTE-Advanced Pro (LTE-A Pro), where the TCP ACKs can be intentionally discarded. Basic methods of TCP ACK filtering have been investigated and it has been clarified that there is a method that can accomplish good TCP throughput with a reduced number of TCP ACKs. In this paper, we further study TCP ACK filtering and develop new methods, which are generalized methods of TCP ACK filtering. The novel feature of our new method is that the TCP ACK filtering can be controlled by a “discard ratio” parameter. The performance is evaluated by computer simulations. It is observed that the TCP throughput remains good even when the discard ratio is set to large values. In addition, it is found that there exists a value of the “discard ratio which can minimize the number of TCP ACKs while maintaining good TCP throughput.

New Digital Predistorter Architecture for Signals with High Peak-to-Average Power Ratios

Two closely related non-linear distortions are introduced to the frontend of a transmitting multicarrier signal: a clipping noise due to a high peak-to-average-power-ratio (PAPR) reduction and intermodulation distortions from the nonlinearity of high power amplifiers (HPAs). In recent years, there has been growing interest in various PAPR reduction and linearization techniques using predistortion schemes. Although these two impairments are somewhat interdependent, most designs deal with them separately as it relates to problem formulation, optimization objectives and implementation issues. The overall performance may not be maximized even if each of them is optimized under their own criteria due to possible conflicts of interest. In this paper, we present a new predistorter architecture that jointly optimizes PAPR reduction and digital predistortion, and achieves better transmitting signal quality for desired levels of PAPR. During the optimization, only the clipping noise was employed, so the proposed predistorter became an independent PAPR-specific reduction scheme, i.e., it is robust, and it can therefore operate with various standard and non-standard PAPR reduction solutions.

Weighted Viterbi decoding for MIMO-OFDM systems with linear precoding

In this paper, a weighted Viterbi decoding by using the eigenvalues of the channel matrix is proposed to improve error rate performance for multiple-input multiple-output (MIMO) and orthogonal frequency division multiplexing (OFDM) systems with linear precoding. The optimal transmitter precoding matrix is obtained from the singular value decomposition (SVD) of the channel matrix. The proposed method exploit the features of precoding based on the SVD, in which the eigenvalues are defined as data reliability metric and we incorporate this information in the soft decision Viterbi decoding. Simulation results confirm that this technique improves error rate performance significantly