There have recently been more and more reports on CMOS integrated circuits operating at terahertz (≥ 0.1THz) frequencies. However, design environments and techniques are not as well established as for RF CMOS circuits. This paper reviews recent progress made by the authors in terahertz CMOS design for low-power and high-speed wireless communication, including device characterization and modeling techniques. Low-power high-speed wireless data transfer at 11Gb/s and 19pJ/bit and a 7-pJ/bit ultra-low-power transceiver chipset are presented.

Conventional localization techniques such as triangulation and multilateration are not reliable in non-line-of-sight (NLOS) environments such as dense urban areas. Although fingerprint-based localization techniques have been proposed to solve this problem, we may face difficulties because we do not know the parameters of the illegal radio when creating the fingerprint database. This paper proposes a novel technique to localize illegal radios in an urban environment by interpolating the channel impulse responses stored as fingerprints in a database. The proposed interpolation technique consists of interpolation in the bandwidth (delay), frequency and spatial domains. A localization algorithm that minimizes the squared error criterion is employed in this paper, and the proposed technique is evaluated through Monte Carlo simulations using location fingerprints obtained from ray-tracing simulations. Results show that utilizing an interpolated fingerprint database is advantageous in such scenarios.

The technique of partial transmit sequences (PTS) is effective in reducing the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals. However, the conventional PTS (CPTS) scheme has high computation complexity because it needs several inverse fast Fourier transform (IFFT) units and an optimization process to find the candidate signal with the lowest PAPR. In this paper, we propose a new low-complexity PTS scheme for OFDM systems, in which a hybrid subblock partition method (SPM) is used to reduce the complexity that results from the IFFT computations and the optimization process. Also, the PAPR reduction performance of the proposed PTS scheme is further enhanced by multiplying a selected subblock with a predefined phase rotation vector to form a new subblock. The time-domain signal of the new subblock can be obtained simply by performing a circularly-shift-left operation on the IFFT output of the selected subblock. Computer simulations show that the proposed PTS scheme achieves a PAPR reduction performance close to that of the CPTS scheme with the pseudo-random SPM, but with much lower computation complexity.

A low-complexity time-frequency multiplex estimator and low-complexity equalizer transceiver design are proposed to combat the problems of RF impairment associated with zero-IF transceiver of multi-carrier systems. Moreover, the proposed preambles can estimate the transmitter (TX) in-phase and quadrature-phase (IQ) imbalance, carrier frequency offset (CFO), and channel impulse response parameters. The proposed system has two parts. First, all parameters of the impairments are estimated by the designed time-frequency multiplex estimator. Second, the estimated parameters are used to compensate the above problems and detect the transmitted signal with low complexity. Simulation results confirm that the proposed estimator performs reliably with respect to IQ imbalance, CFO, and multipath fading channel effects.

New classes of zero-difference balanced (ZDB) functions derived from Fermat quotients are proposed in this letter. Based on the new ZDB functions, some applications, such as the construction of optimal frequency hopping sequences set and perfect difference systems of sets, are introduced.

We have developed a capacitance sensor of frequency modulation for integrated touchpanels using amorphous In-Sn-Zn-O (α-ITZO) thin-film transistors (TFTs). This capacitance sensor consists of a ring oscillator, whose one stage is replaced by a reset transistor, sensing transistor, and sensing electrode. The sensing electrode is prepared as one terminal to form a sensing capacitor when the other terminal is added by a finger. The ring oscillator consists of pseudo CMOS inverters. We confirm that the oscillation frequency changes when the other terminal is added. This result suggests that this capacitance sensor can be applied to integrated touchpanels on flatpanel displays.

The pilot contamination is a serious problem which hinders the capacity increasing in the massive MIMO system. Similar to Fractional Frequency Reuse (FFR) in the OFDMA system, Fractional Pilot Reuse (FPR) is proposed for the massive MIMO system. The FPR can be further classified as the strict FPR and soft FPR. Meanwhile, the detailed FPR schemes with pilot assignment and the mathematical models are provided. With FPR, the capacity and the transmission quality can be improved with metrics such as the higher Signal to Interference and Noise Ratio (SINR) of the pilots, the higher coverage probability, and the higher system capacity.

This letter presents a two-stage method to extend the degenerate unmixing estimation technique (DUET) for reverberant speech separation. First, frequency-bin-wise attenuation and delay parameters are introduced and estimated by online update rules, to handle early reflections. Next, a mask reestimation algorithm based on the precedence effect is developed to detect and fix the errors on binary masks caused by late reflections. Experimental results demonstrate that the proposed method improves separation performance significantly.

In this paper, we propose a transmit multi-block frequency-domain equalization (MB-FDE) for frequency-domain space-time block coded joint transmit/receive diversity (FD-STBC-JTRD). Noting that a STBC codeword consists of multiple coded blocks, the transmit MB-FDE uses the multiple transmit FDE weight matrices, each associated with each coded block. Both single-carrier (SC) transmission and orthogonal frequency-division multiplexing (OFDM) transmission are considered. For SC transmission, the transmit MB-FDE weight matrices are jointly optimized so as to minimize the mean square error (MSE) between the transmit signal before STBC encoding and the received signal after STBC decoding. For OFDM transmission, they are jointly optimized so as to maximize the received signal-to-noise power ratio (SNR) after STBC decoding. We show by theoretical analysis that the proposed transmit MB-FDE can achieve 1/RSTBC times higher received SNR than the conventional transmit single-block FDE (SB-FDE), where RSTBC represents the code rate of STBC. It is confirmed by computer simulation that, when more than 2 receive antennas are used, MB-FDE can always achieve better BER performance than SB-FDE irrespective of the number of transmit antennas, and the channel frequency-selectivity.

This paper proposes a voice activity detection (VAD) algorithm based on an energy related feature of the frequency modulation of harmonics. A multi-resolution spectro-temporal analysis framework, which was developed to extract texture features of the audio signal from its Fourier spectrogram, is used to extract frequency modulation features of the speech signal. The proposed algorithm labels the voice active segments of the speech signal by comparing the energy related feature of the frequency modulation of harmonics with a threshold. Then, the proposed VAD is implemented on one of Texas Instruments (TI) digital signal processor (DSP) platforms for real-time operation. Simulations conducted on the DSP platform demonstrate the proposed VAD performs significantly better than three standard VADs, ITU-T G.729B, ETSI AMR1 and AMR2, in non-stationary noise in terms of the receiver operating characteristic (ROC) curves and the recognition rates from a practical distributed speech recognition (DSR) system.

The theoretically minimum length of a signal for fundamental frequency estimation in a noisy environment is discussed. Assuming that the noise is additive white Gaussian, it is known that a Cramér-Rao lower bound (CRLB) is given by the length and other parameters of the signal. In this paper, we define the minimum length as the length whose CRLB is less than or equal to the specific variance for any parameters of the signal. The specific variance is allowable variance of the estimate within an application of fundamental frequency estimation. By reformulating the CRLB with respect to the initial phase of the signal, the algorithms for determining the minimum length are proposed. In addition, we develop the methods of deciding the specific variance for general fundamental frequency estimation and pitch estimation. Simulation results in terms of both the fundamental frequency estimation and the pitch estimation show the validity of our approach.

In this paper, we present a new electro-optic (EO) probing system based on heterodyne detection. The use of a recirculating frequency shifter allows to expand the bandwidth of the system far beyond what is attainable with a conventional heterodyne EO set-up. The performance for the frequencies up to 50GHz is analysed to forecast the viability of the system up to the THz range.

We investigate a quantization error improvement technique using a dual rail configuration for optical quantization. Our proposed optical quantization uses intensity-to-wavelength conversion based on soliton self-frequency shift and spectral compression based on self-phase modulation. However, some unfavorable input peak power regions exist due to stagnations of wavelength shift or distortions of spectral compression. These phenomena could induce a serious quantization error and degrade the effective number of bit (ENOB). In this work, we propose a quantization error improvement technique which can make up for the unfavorable input peak power regions. We experimentally verify the quantization error improvement effect by the proposed technique in 6 bit optical quantization. The estimated ENOB is improved from 5.35 bit to 5.66 bit. In addition, we examine the XPM influence between counter-propagating pulses at high sampling rate. Experimental results and numerical simulation show that the XPM influence is negligible under ∼40 GS/s conditions.

In order to tackle the rapidly increasing traffic, the 5th generation (5G) mobile communication system will introduce small cells using higher frequency bands with wider bandwidth to achieve super high bit rate transmission of several tens of Gbps. Massive multiple input multiple output (MIMO) beamforming (BF) is promising as one of the technologies that can compensate for larger path-loss in the higher frequency bands. Joint analog fixed BF and digital precoding have been proposed to reduce the cost of a Massive MIMO transceiver. However, the conventional scheme assumes the transmission of a few streams using well-known codebook-based precoding as the digital precoding, and both a selection method of the fixed BF weights and a digital precoder design, which are suitable for super high bit rate transmission using multiple streams, have not been studied. This paper proposes a joint fixed BF and CSI-based precoding (called FBCP) scheme for the 5G Massive MIMO systems. FBCP first selects the analog fixed BF weights based on a maximum total received power criterion, and then it calculates an eigenmode (EM) precoding matrix by exploiting CSI. This paper targets a 5G system achieving over 20Gbps in the 20GHz band as one example. Throughput performances of the Massive MIMO using the proposed FBCP are evaluated by link level simulations using adaptive modulation and coding and it is shown that the proposed FBCP with the optimum number of selected beams (baseband chains) can use higher level modulation, up to 256QAM, and higher coding rates and achieve throughputs close to 30Gbps while the cost and complexity can be reduced compared with the fully digital Massive MIMO.

A re-configurable wavelength de-multiplexer for wave-length-division-multiplexed (WDM) radio-over-fiber (RoF) systems, which is specially designed for delivering frequency-modulated continuous-wave (FM-CW) signals, is newly developed. The principle and characteristics of the developed de-multiplexer are described in detail. Then the de-multiplexing performances of 4-channel WDM 32-GHz-band, 8-channel WDM 48-GHz-band, and 5-channel WDM 96-GHz-band FM-CW RoF signals are evaluated with the de-multiplexer.

Open-loop (OL) transmit diversity is more subject to the influence of channel estimation error than closed-loop (CL) transmit diversity, although it has the merit of providing better performance in fast Doppler frequency environments because it doesn't require a feedback signal. This paper proposes an OL transmit diversity scheme combined with intra-subframe frequency hopping (FH) and iterative decision-feedback channel estimation (DFCE) in a shared channel for discrete Fourier transform (DFT)-precoded orthogonal frequency division multiple access (OFDMA). We apply intra-subframe FH to OL transmit diversity to mitigate the reduction in the diversity gain under high fading correlation conditions among antennas and iterative DFCE to improve the channel estimation accuracy. Computer simulation results show that the required average received signal-to-noise power ratio at the average block error rate (BLER) of 10-2 of the space-time block code (STBC) with intra-subframe FH is reduced to within approximately 0.8dB compared to codebook-based CL transmit diversity when using iterative DFCE at the maximum Doppler frequency of fD =5.55Hz. Moreover, it is shown that STBC with intra-subframe FH and iterative DFCE achieves much better BLER performance compared to CL transmit diversity when fD is higher than approximately 30Hz since the tracking ability of the latter degrades due to the fast fading variation in its feedback loop.

In this paper, a threshold-based I-Q diversity combining scheme for ultra-high frequency (UHF) radio frequency identification (RFID) readers with a quadrature receiver is proposed in the aspect of improving the tag detection performance. In addition, the performance of the proposed scheme is evaluated as the closed-form expressions. In particular, its statistical characteristics are detailed and its performance is compared to that of conventional schemes over independent and identically distributed Rician fading conditions in terms of average signal-to-noise ratio (SNR), bit error rate (BER), and the average number of required combining process. Numerical results indicate that the proposed scheme enables processing power control through threshold control while meeting the required quality of service compared to conventional schemes.

An electro-optic (EO) modulator integrated with the microwave planar circuit directly formed on a LiNbO3 (LN) substrate for low frequency-chirp performance and compact configuration is introduced. Frequency chirp of EO intensity modulators was investigated and a dual-electrode Mach-Zehnder (MZ) modulator combined with a microwave rat-race (RR) circuit was considered for the low-chirp modulation. The RR circuit, which operates as a 180-degree hybrid, was designed on a z-cut LN substrate to create two modulation signals of the same amplitude in anti-phase with each other from a single input signal. Output ports of the RR were connected to the modulation electrodes on the substrate. The two signals of the equal amplitude drive two phase modulation parts of the modulator so that the symmetric interference are realized to obtain intensity modulation of low frequency-chirp. The modulator was designed and fabricated on a single LN substrate for around 10 GHz modulation frequencies and 1550 nm light wavelength. The chirp parameters were measured to be less than 0.2 in the frequency range between 8 and 12 GHz. By compensating imbalance of the light power splitting in the waveguide MZ interferometer the chirp could be reduced even more.

Terahertz (THz) band is an attractive candidate for future broadband (> 10 Gb/s) wireless backhaul and fronthaul. THz transmitter employing optical frequency comb can provide high quality THz carrier, and is useful to the future broadband THz communication systems based on coherent transmission technique. To realize coherent transmission, high quality carrier generation is essential and it is important to evaluate the signal quality of a THz transmitter. In this paper, we derive error vector magnitude (EVM) including optical impairments (optical amplifier noise, laser phase noise, optical crosstalk and IQ imbalance of optical modulator) of the optical frequency comb based transmitter. The calculated EVM is in good agreement with simulated one, and practical requirements for optical impairment are indicated. The analysis will be useful in the design of THz transmission systems employing an optical frequency comb.

Passive SONAR receives a mixed form of signal that is a combination of continuous and discrete line-component spectrum signals. The conventional algorithms, DEMON and LOFAR, respectively target each type of signal, but do not consider the other type of signal also present in the practical environment. Thus when features from two types of signals are presented at the same time, analysis results may cause confusion. In this paper, we propose an integrated analysis algorithm for underwater signals using the modulation spectrogram domain. The proposed domain presents the visual difference between the different types of signal, and therefore can prevent confusion that would otherwise be feasible. Moreover, the proposed algorithm is more efficient than multiband DEMON in terms of computation complexity, while in colored ambient noise environment, it has similar performance to conventional DEMON and LOFAR. We prove the validity of the proposed algorithm through the relevant experiments with synthesized signal and actual underwater recordings.