This paper proposes a new simultaneous carrier and bit-timing recovery (CBR) scheme for offset quadrature phase shift keying (O-QPSK) for agile acquisition over satellite communication channels. The proposed simultaneous CBR scheme employs a preamble shared for the carrier and bit-timing recover, which has a specific bit-pattern designed so that its baseband signal alternates between two adjacent decision points at the symbol rate. Using the preamble, the proposed simultaneous CBR scheme estimates the carrier phase and the bit-timing, simultaneously and independently, by open-loop approach. For comparison, this paper also describes the performance and configuration of a joint carrier and bit-timing recovery scheme, which is expanded for O-QPSK from the one conventionally proposed for QPSK. This paper demonstrates with simulation results that the proposed simultaneous CBR scheme significantly improves the agility of acquisition: a mere 30-symbol preamble is sufficient for low-Eb/No channels typical of satellite communication systems. The proposed CBR scheme is also advantageous from the viewpoint of digital implementation: it processes at 2 samples/symbol and eliminates an analog voltage control clock (VCC). The proposed simultaneous CBR scheme is a strong candidate for TDMA systems that require the high data-transmission and frequency utilization efficiency.

This paper presents a new Frequency Shift Keying (FSK) demodulation method using the Short Time-Discrete Fourier Transform (ST-DFT) analysis to combat large frequency offset with time variation in low earth orbit (LEO) satellite communications systems. This demodulation method can demodulate the received signal only by searching for the instantaneous spectrum energy peaks without complicated carrier recovery. In addition, it is insensitive to the signal-to-noise ratio (SNR) degradation caused by the excessively wide bandwidth of the receiver front-end band pass filter. Furthermore, the ST-DFT analysis combined with a differential encoding scheme gives FSK demodulation method a potential robustness against large and fast time-varying frequency offset.

M-ary orthogonal keying (MOK) systems under carrier frequency offset (CFO) are investigated. It is shown that spurious signals are introduced by the offset frequency components of spectrum after multiplication in correlation detection process, and some conditions on robust orthogonal signal sets are derived. Walsh function sets are found to be very weak against CFO, since they produce large spurious signals. As robust orthogonal signal sets against CFO, the rows of circulant Hadamard matrices are proposed and their error performanses are evaluated. The results show that they are good M-ary orthogonal signal sets in the presence of CFO.

The performance of an M-ary spread-spectrum multiple-access (M-ary/SSMA) scheme in the presence of carrier frequency offset is discussed in this paper. The influence of carrier frequency offset on the non-coherent reception of M-ary/SSMA signals is examined and it is shown that the carrier frequency offset degrades the performance remarkably, yet. this influence has a distinctive property. Making use of this property, we propose a new M-ary/SSMA scheme that can mitigate the influence of the carrier frequency offset. The scheme is based on the assignment of two distinctive Hadamard codes to in-phase and quadrature components of the transmitted signal. The effect of simultaneous transmission is evaluated in terms of bit-error-rate performance with the carrier frequency offset. As the result, it is observed that the satisfactory bit-error-rate performance can be achieved in the presence of carrier frequendy offset.

Orthogonal frequency division multiplexing (OFDM) has been receiving a lot of attention in the field of broadcasting because of its ruggedness under multipath environments. One of important issues to realize high quality reception of OFDM signals is to correct frequency and timing offsets between the transmitter and receiver so that orthogonality of the carriers can be maintained. This paper discusses a frequency and timing period acquisition technique for OFDM systems. A new offset estimation technique is introduced that detects both the frequency and timing peirod offsets at the same time by using only one pilot symbol with its suitable frequency assignment. A pseudo noise (PN) sequence is also introduced to assign these frequencies of the pilot symbol so that the frequency acquisition range can be widened. Numerical examples are given to show the estimate variances of the proposed frequency and timing period estimator over both additive white Gaussian noise (AWGN) and multipath fading channels. Also the bit error rate (BER) performance for an open loop acquisition system is examined.

This paper proposes a half-chip offset QPSK (Quadrature Phase Shift Keying) modulation CDMA (Code Division Multiple Access) scheme to allow the simple differential detection while realizing a compact spectrum in nonlinear channels for wireless LAN systems. The experimental results show the proposed scheme achieves excellent Pe (probability of error) performances in ACI (adjacent channel interference) and CCI (co-channel interference) environments. Moreover, by employing time diversity and high-coding-gain FEC (Forward Error Correction), the half-chip offset QPSK-CDMA scheme realizes an improvement of 3.0 dB (in terms of Eb/No at a Pe of 105) in Rician fading environments with a Doppler frequency fD of 10 Hz and a delay spread of 40 nsec.

A dynamic measurement method of static R/W skew caused by MR-Inductive element alignment error is discussed. A geometrical model for the displacement between a write gap and a read gap on a swing arm is formulated. A measurement method wthich employs read-after-write procedure on disk surfaces in a drive is proposed. A simulation results is reported. It contributes to increase the track densities of HDDs using MR/Inductive heads in the region of more than 200 TPMM (5000 TPI).

This paper discusses the performance of non-coherent reception for M-ary spread-spectrum (M-ary/SS) signals in the presence of carrier frequency offset. In general, the M-ary/SS scheme is expected to be of higher spectral efficiency than the conventional DS/SS schemes, but its performance may be degraded by the carrier frequency offset. We, therefore, analyze the effect of carrier frequency offset on the performance of the non-coherent M-ary/SS system with orthogonal modulation using a set of sequences generated by the Hadamard matrix. As a result of the analysis, it has been found that the carrier frequency offset may cause a great deal of degradation in the performance, and that its effect has a distinctive property which is due to the characteristic of Hadamard matrix, at the same time. Making use of this property, we propose two schemes that can mitigate the effect of carrier frequency offset: one is based on choise of the code sequences, the other is on the error correcting code. The effectiveness of the schemes is evaluated in the terms of symbol-error-rates through analysis and computer simulation.

The performance of coded multi-pulse pulse position modulation (MPPM) consisting of m slots and 2 pulses, denoted as (m, 2) MPPM, with imperfect slot synchronization is analyzed. Convolutional codes and Reed-Solomon (RS) codes are employed for (m, 2) MPPM, and the bit error probability of coded (m, 2) MPPM in the presence of the timing offset is derived. In each coded (m, 2) MPPM, we compare the performance of some different code rate systems. Moreover, we compare the performance of both systems at the same information bit rate. It is shown that in both coded systems, the performance of code rate-1/2 coded (m, 2) MPPM is the best when the timing offset is small. Wheji the timing offset is somewhat large, however, uncoded (m, 2) MPPM is shown to perform better than coded (m, 2) MPPM. Further, convolutional coded (m, 2) MPPM with the constraint length k7 is shown to perform better than RS coded (m, 2) MPPM for the same code rate.

This study shows that using the direct offset integration technique (DOIT) and additional positive feedback (APF) in a high-Tc dc superconducting quantum interference device (SQUID) improves the effective flux-to-voltage transfer function and reduces the flux noise of a magnetometer, thus improving the magnetic field noise. The effective flux-to-voltage transfer function and the flux noise with APF were measured at different values of the positive feedback parameter βa, which depends on the resistance of the APF circuit. These quantities were also compared between conditions with and without APF. This investigation showed that a βa condition the most suitable for minimizing the flux noise of a magnetometer with APF exists and that it is βa=0.77. The effective flux-to-voltage transfer function with APF is about three times what it is without APF (93 µV/Φ0 vs. 32 µV/Φ0). The magnetic field noise of a magnetometer with APF is improved by a factor of about 3 (242 fT/Hz vs. 738 fT/Hz).

This paper describes reverse modulation carrier recovery with a tank-limiter for Offset QPSK (OQPSK) burst signals. Acquisition performance is discussed taking into account hardware implementation errors in the carrier recovery circuit. The results indicate hardware implementation errors cause a significant recovered carrier phase error during BTR (Bit Timing Recovery) of OQPSK burst signals. A phase error reduction technique by modifying the BTR code for OQPSK burst signals is proposed to improve the acquisition performance. Computer simulation and hardware experiments confirmed its improvement. The performance of a prototype OQPSK burst demodulator using the proposed carrier recovery scheme is also presented.

Circuit design techniques for linearizing adaptively biased differential pairs are described. An emitter-and source-coupled pair is adaptively biased by a squaring circuit to linearize its transconductance, one of whose inputs is divided by resistors. An input signal for a differential pair or a squaring circuit is set to an adequate amplitude by a resistive divider without sacrificing linearity. Therefore, a differential pair is biased by the output current of a squaring circuit and they are coupled directly. There are three design techniques for squaring circuits. One is the transistor-size unbalance technique. Another is the bias offset technique. A third is the multitail technique. The bipolar and MOS squaring circuits discussed in this paper were proposed by the author previously, and consist of transistor-pairs with different transistor size (i.e., the emitter areas or gate W/L values are different), transistor-pairs with the same bias offset, or a multitail cell(i.e., a triple-tail cell or quadritail cell). Several kinds of squaring circuits consisting of such transistor-pairs are applied to produce the quadratic bias currents for compensating the nonlinearity of an emitter-and source-coupled pair. Therefore, four circuits using emitter-coupled pairs with adaptive-biasing current and four circuits using source-coupled pairs with adaptive-biasing current are proposed and analyzed in depth. Furthermore, a circuit configuration for low voltage operation is also introduced and verified with bipolar transistor-arrays on a breadboard.

This paper discusses a CMOS differential-difference amplifier circuit suitable for low voltage operation. A new multiple weighted input transconductor circuit structure is suggested to be use in DDA implementation. The proposed DDA can be employed in several analog/digital systems to improve their parameters. Selected examples of the proposed transconductor/DDA applications are also discussed.

The performance of multi-pulse pulse position modulation (MPPM) consisting of m slots and 2 pulses, denoted as (m, 2) MPPM, with imperfect slot synchronization is analyzed. The word error probability of (m, 2) MPPM in the presence of timing offset is analyzed, and the optimum symbol sets of (m, 2) MPPM minimizing the symbol error probability are assigned. When an unassigned symbol is detected, the receiver decodes the unassigned symbol as one of the assigned symbols having the highest probability of transition from the assigned symbol to the unassigned symbol. The bit error probability of (m, 2) MPPM in the presence of the timing offset is analyzed, and the bit error probability of (m, 2) MPPM is compared with that of PPM for the same transmission bandwidth and the same transmission rate. Moreover, the bit error probability of (m, 2) MPPM synchronized by a phase-locked loop (PLL) is also analyzed. It is shown that a word with two continuous pulses has better performance than a word with two separate pulses. It is also shown that when the timing offset occurs, and when the slot clock is synchronized by a PLL, (m, 2) MPPM performs better than PPM because (m, 2) MPPM has the optimum assigned symbols, and can decode detected words more correctly than PPM.

For IF direct sampling phase detection method (IFSM) which realizes the arithmetical operations with digital filters by direct A/D (Analog to Digital) conversion of IF (Intermediate Frequency) signal, the method to eliminate DC offset is proposed and developed by using the gate array. A principle of the proposed method and the results of the measurement are shown.

This paper deals with study results on the effect of nonlinear amplification in the CDMA system using offset-QPSK signals bandlimited with a square-root cosine roll-off filter. As a result of the study, it is shown that the nonlinear amplification does not affect bit error rate performance with reasonable out-of-band emission characteristics when the roll-off factor of the transmit filter is one.

It has been reported that the efficiency of a low voltage power supply is improved by replacing diodes in an output-stage with synchronous rectifiers (SR). A SR consists of a bipolar junction transistor with a low-saturation voltage and a current transformer. Although the SR has low offset-voltage, its reverse recovery characteristic is usually poor. In this paper, an RCD circuit which improves the reverse recovery characteristic of the SR is proposed. This circuit is simple, and it is composed of a diode, a capacitor and a resistor. The analysis and the experimental results of the SR with the proposed RCD circuit are presented. The optimum design of the RCD to improve the reverse recovery characteristic of SR is discussed.

A novel coupling configuration consisting of a tapered slab waveguide with a wedge-shaped nonlinear cladding is proposed. Coupling characteristics for TE waves are analyzed by means of the beam propagation method. The proposed configuration is less sensitive to the offset between coupled waveguides than is the configuration with a homogeneous non-linear cladding.