In order to fulfill the demand of increasing data transmission rate, usage of THz band is attracting much attention. To overcome the high propagation loss in THz band, access points (APs) are supposed to be densely placed. In order to control radio resource usage of APs as well as supporting applications that require device positioning, it is highly desirable that symbol timings of the APs are synchronized. In this paper, as an alternative approach to use a synchronization master, network synchronization nor the synchronization using global navigation satellite system (GNSS), over-the-air (OTA) peer AP synchronization is proposed, where a pair of peer APs adjusts their symbol timings according to measured time of arrival (ToA) of each other's signal. By performing the peer AP synchronization with other surrounding peer APs, a whole group of APs can form a synchronized cluster. The results of computer simulations show that the maximum symbol timing difference of APs can be kept less than 10 ps for the case that the symbol duration is 100 ps, with which centimeter level device positioning can be supported.

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.

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 PD's building blocks can no longer be ignored. Such distortions are important factors that define a PD's 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 PD's 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.

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.

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

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.

InGaAsP/InP buried heterostructure (BH) lasers emitting at 1.3 μm have continued to operate stably for more than3.3 \times 10^{4}h (3.8 years) at 50-60°C and at an output power of 5 mW/facet. A statistically estimated median lifetime exceeds 106at 50°C. A relatively low activation energy of 0.32 eV is obtained for slow degradation. The saturable behavior of the aging characteristics is observed in many of the lasers. This mode is explained by the increased leakage current through the buried regions, and can be eliminated by electroluminescence (EL) mode aging at high temperature and current. Distributed feedback (DFB) lasers emitting at 1.55 μm are also subjected to accelerated aging at 60°C with a 3 mW/facet output after EL-mode aging. These DFB lasers demonstrate stable aging characteristics, for more than 2000 h of operating time being currently achieved.

Optoelectronic monolithic integration is preferable for multigigabit transmission systems because of small waveform degradation due to interconnect parasitics in optoelectronic hybrid integration. Implementation of a wavelength division multiplexing subsystem on the optoelectronic integrated chip will further increase the transmission capacity in optical fibers. A five-wavelength integrated distributed feedback laser array with 50 Å lasing wavelength separation has been developed. Multiquantum-wells are promising as a basic structure in future optoelectronic integration. They show high potential in low threshold current laser diodes, high-frequency modulators, and high-speed detectors.