Variation of power distribution caused by splices is calculated as a function of numbers of splice points and misalignment. The calculated power distribution is verified by experiment. Calculation shows that mode conversion caused by splice makes power distribution approach to steady-state. The numbers of splice points required to make power distribution stationary are calculated as a function of misalignment. Pulse broadenning in the fiber transmission line is calculated for the case that fibers are mode conversion free. Pulse broadenning rate becomes proportional to the square root of transmission distance after optical pulse passes many splice points. These calculated results show that mode conversion caused by splice has similar tendency as that caused by longitudinal inhomogeneity of waveguide.

Estimation is done for the offset, tilt and end separation loss of grade index fiber connectors with arbitrary modal power distribution using the modal evaluation technique instead of geometrical treatment as reported by D. Gloge. Comparison of the results with experiments shows that it provides a reasonable description and is useful in designing optical fiber connectors and splices.

This paper describes the first realization of the APSK system with a honeycomb constellation (HC) for high capacity digital radio links. Partial Gray coding method to improve receiver sensitivity is also described. It is shown that the 68 APSK modulation can increase receiver sensitivity by 0.7 dB compared with a 64 QAM. Possibility of the system with the HC using the present state of the art is confirmed by theoretical estimation of tolerances for modem impairments. Techniques necessary to realize a system with the HC are also described. Using the techniques, the 68 APSK modem with the HC was experimentally fabricated. The modem we fabricated has a transmission capacity of 90 Mbps within the FCC-authorized bandwidth of 20 MHz in the 4 GHz band. The BER performance and signature for multipath fading have been measured. It is confirmed that the signature of the 68 APSK is almost the same as that of the conventional 64 QAM. The signature improved using 5 tap transversal equalizer correspons to an outage of 2 seconds per year per hop.

Lower mode excitation was confirmed for the coupling of semiconductor lasers into multimode fibers with tapered hemispherical ends, by measuring modal power distribution. Judging from the measured power distribution, 2 dB lower transmission loss can be expected for tapered hemispherical end coupling than for flat-end fiber coupling.

To reduce laborious tasks of the phase determination, our previous paper has proposed a method to derive phase reference for two-tone intermodulation distortion (IMD) measurement of a power amplifier (PA) by using small-signal S-parameters. Since the method is applicable to low output power level, this paper proposes an iterative process to extend the applicable power level up to 1-dB compression. The iterative process is based on extraction of linear response: the principle of the extraction is described theoretically by using an accurate model of the PA with memory effect. Measurement of two-tone IMD is made for a GaN FET PA. Validity of the iteration is confirmed as convergence of the extracted linear response to that given by the product of S21 and input signal. Measured results also show validity of the physical model of the memory effect provided by Vuolevi et al. because beat frequency dependences of IMD's are accurately fitted by bias impedances at even order harmonics of envelope frequency. The PA is characterized by using measured results and the third and fifth order inverses of the PA are designed. Improvement of IMD is theoretically confirmed by using the inverses as predistorters.

This paper describes the error probability of the second order BAM estimated by a computer simulation and an analytical calculation method. The computer simulation suggests that the iterations to retrieve a library pattern almost converge within four times and the difference between once and twice is much larger than that between twice and four times. The error probability at the output of the second iteration is estimated by the analytical method. The effect of the noise bits is also estimated using the analytical method. The BAM with larger n is more robust for the noise. For example, the noise bits of 0.15n cause almost no degradation of the error probability when n is larger than 100. If the error probability of 10-4 is allowable, the capacity of the second order BAM can be increased by about 40% in the presence of 0.15n noise bits when n is larger than 500.

In this letter, an analysis of bit error probability of 4-state soft decision Viterbi decoding is presented. The bit error probability of recursive systematic convolutional code is also derived.

Non-linear behavioral models play a key role in designing digital pre-distorters (DPDs) for non-linear power amplifiers (NLPAs). In general, more complex behavioral models have better capability, but they should be converted into simpler versions to assist implementation. In this paper, a conversion from a complex fifth order inverse of a parallel Wiener (PRW) model to a simpler memory polynomial (MP) model is developed by using frequency domain expressions. In the developed conversion, parameters of the converted MP model are calculated from those of original fifth order inverse and frequency domain statistics of the transmit signal. Since the frequency domain statistics of the transmit signal can be precalculated, the developed conversion is deterministic, unlike the conventional conversion that identifies a converted model from lengthy input and output data. Computer simulations are conducted to confirm that conversion error is sufficiently small and the converted MP model offers equivalent pre-distortion to the original fifth order inverse.

This paper describes the details of the iteration process used to determine the transfer functions of linear time-invariant (LTI) circuits causing the memory effect of power amplifier (PA). An outline of the method is reported in our work presented at ICCS2012. The accuracy of the method is improved by using cross-correlation spectra at three signal levels, and its validity is confirmed by a computer simulation. The method can be applied to online updating of PAs operating in mobile communication systems. The updating is realized separately from the fast varying nonlinear coefficients. The possibility of updating with a short interval is indirectly shown for the nonlinear coefficients using a procedure similar to that of memoryless PAs. For PAs characterized by the method, this paper also describes the inverses that cancel the nonlinear distortion with minimum complexity. The validity of the inverse is confirmed by a computer simulation on the power spectrum of the PA for orthogonal frequency-division multiplexing (OFDM) signals with 500 subcarriers. The simulated spectra show that the fifth order or higher inverses are effective in keeping adjacent channel leakage power ratio (ACLR) lower than -60dB at the practical signal level. Improvements in the error vector magnitude (EVM) due to the inverse were also confirmed by reductions of gain and phase variations under varying envelope conditions.

This paper proposes a method for calculating transfer function of transmission lines composed of combinations of multimode fibers with arbitrary index profiles. Bandwidths of graded-index fiber transmission lines are estimated taking into account optical equalization, mode conversion and deviation of index profiles from the power-law. Optical equalization greatly improves bandwidths for power-law fibers. As for those which deviate from power-law profile, perturbation of index profiles with low transversal spatial frequency significantly decreases the bandwidth of graded-index fibers. It is important to eliminate the low spatial frequency perturbation in index profiles to provide fibers with wide transmission bandwidths. Bandwidths of transmission lines are estimated when the phase of sinusoidal perturbation in the index profile varies randomly between the fibers at the sending and receiving ends. Calculations suggest that a 100 MHz bandwidth can be expected at a fiber length of 100 km, when deviation from the power-law profile is controlled to within 0.5% of relative index difference between the on-axis and cladding.

A 5 GHz GaAs Monolithic astable multivibrator type voltage controlled oscillator has been developed. The monolithic oscillator uses 2 µm long self aligned TiW-silicide gate MESFETs, and GaAs Shottky diodes for capacitance. Good agreement between the experiment and calculations for oscillation frequency characteristics versus control voltage, is obtained by assuming donor density in the FET active layer to be a Gaussian distribution. This oscillator is useful for monolithic front ends and phase-locked oscillators used in microwave signal processing. X band oscillation frequency will be able to obtained with 1 µm long gate FET and low loss resonance inductors.

Since power penalty of multilevel QAM system is caused by many simultaneously existing impairments, it is important to know whether the sum of the penalties caused by each impairment coincides with the power penalty caused by the actual channel condition or not. To examine the combined effect caused by plural impairments, theoretical power penalty is estimated in detail when any two impairments among the typical seven exist simultaneously. The excess penalty is defined as deviation of the power penalty from the sum of penalties caused by each impairment. Calculation of the excess penalty shows that there are four categories for combinations of impairments. It is shown that there is a typical category of combinations which includes more than half of all possible combinations. Calculated excess penalties are very close for all combinations within this category. A simple algebraic equation is given to approximate the excess penalty for the category. The excess penalties of other categories will be shown and their characteristics will be discussed in detail.

This paper describes theoretical estimation of power efficiency improvement by adopting amplitude and phase modulation (APSK) with a honeycomb constellation (HC) instead of multilevel QAM modulation. Nonlinear distortion caused by a power amplifier is considered in the estimation, and nonlinearity of the amplifier is approximated by third and fifth order nonlinearity. To eliminated the difficulty in carrier reconstruction, a pilot carrier injection method is assumed for the APSK with the HC. However injecting the carrier reduces the power efficiency improvement, so dependence of power efficiency on the injected carrier level is estimated, theoretically. SNR of the recovered carrier as a function of the pilot carrier level is also estimated, experimentally. From these two estimations, an optimum pilot carrier level is determined for a 64 APSK system wit the HC. The possibility of reducing maximum available power of the amplifier by 2.0 dB is confirmed at the optimum pilot carrier level that corresponds to an offset of 1/4 data space in constellation. At the optimized level, SNR of the recovered carrier is 40 dB, which guarantees satisfactory operation of the system.

This paper describes a newly-developed 4 GHz 135 Mbps 256 QAM system with a rolloff factor of 20%, which can attain a spectrum efficiency of 6.75 bps/Hz. The key techniques are theoretically investigated to realize this system. It was predicted theoretically that the simultaneous incorporation of 7-tap transversal equalizers (TEQL) and a recursive slope equalizer (SEQL) would be required as countermeasure for multipath fading. The 256 QAM system was designed considering the results of the theoretical investigation. Excellent BER performance was obtained with the aid of forward error correction and pilot carrier injection. Since remarkable improvement in the signature was obtained by the simultaneous user of TEQL and SEQL, the 256 QAM system with a very low rolloff factor is promising.

This paper describes feasibility of a proposed fixed wireless access system with CDMA technology. The system adopts a primary modulation of 16 QAM and the same frequency allocation in all cells to improve spectral efficiency. The system capacity is 1 Gbps per cell within 120 MHz bandwidth. The number of available orthogonal codes corresponds to the orthogonal code length in the system. All subscribers can attain an error free condition with output power control in the presence of inter-cell interference. The following two items are considered to examine the proposed system feasibility. 1) A test modem is fabricated, and a back-to-back modem BER performance is measured. An inter-symbol interference (ISI) level of the modem is estimated with the measured performance. 2) A computer simulation of down-link power control is carried out considering inter-cell interference and impairment factors of the power control such as intra-sector interference caused by the ISI and limited ranges of total and relative output power controls. The simulation results show that the proposed system would be feasible because the obtained power penalties caused by the above impairment factors are negligible.

This paper describes relation between the number of library pairs and error probability to have all the pairs as fixed points for second-order bidirectional associative memory (BAM). To estimate accurate error probability, three methods have been compared; (a) Gaussian approximation, (b) characteristic function method, and (c) Hermite Gaussian approximation (proposed by this paper). Comparison shows that Gaussian approximation is valid for the larger numbers of neurons in both two layers than 1000. While Hermite Gaussian approximation is applicable for the larger number of neurons than 30 when Hermite polynomials up to 8th are considered. Capacity of second-order BAM at the fixed error probability is estimated as the function of the number of neurons.

This paper describes a time-domain expression based on the physical model of power amplifiers where electric memory effect is considered to be caused by even-order nonlinearity and bias impedance. It is demonstrated that the time-domain expression is consistent with the general memory polynomial reported by D.R. Morgan et al. To confirm validity of the physical model, a simple method is proposed to measure amplitude and phase of IMD by two tone test: the phase is extracted from measured small signal S-parameters of the amplifier under test. The method is applied to a GaN FET amplifier under condition that memory effect is enhanced by applying inductive cable for DC supply. Frequency dependent IMD is fitted by a parallel connection of L, C, and R: it has been confirmed that the frequency dependence of IMD is given by the bias impedance at even order harmonics of envelope frequency. The frequency dependence assures the validity of the physical model as well as the time-domain expression.

This letter proposes a peak power reduction method that optimizes sub-carrier phases of an OFDM signal. The proposed method doesn't require side information transmission and original signal regeneration, which are required in conventional peak power reduction methods with phase optimization, since the optimized phases are distributed as jitter around the original phases before optimization. The iterative PTS (partial transmit sequences) algorithm with a restricted phase control range is used for the jitter injection: the phase optimization process is repeated with widening the control range. A computer simulation is carried out to estimate the proposed method performance. The results show that the proposed method can reduce the peak power by 4 dB when the power penalty caused by phase jitter is only 0.2 dB.

This paper describes a method to design a predistorter (PD) for a GaN-FET power amplifier (PA) by using nonlinear parameters extracted from measured IMD which has asymmetrical peaks peculiar to a memory effect with a second-order lag. While, computationally efficient equations have been reported by C. Rey et al. for the memory effect with a first-order lag. Their equations are extended to be applicable to the memory effect with the second-order lag. The extension provides a recursive algorithm for cancellation signals of the PD each of which updating is made by using signals in only two sampling points. The algorithm is equivalent to a memory depth of two in computational efficiency. The required times for multiplications and additions are counted for the updating of all the cancellation signals and it is confirmed that the algorithm reduces computational intensity lower than half of a memory polynomial in recent papers. A computer simulation has clarified that the PD improves the adjacent channel leakage power ratio (ACLR) of OFDM signals with several hundred subcarriers corresponding to 4G mobile radio communications. It has been confirmed that a fifth-order PD is effective up to a higher power level close to 1 dB compression. The improvement of error vector magnitude (EVM) by the PD is also simulated for OFDM signals of which the subcarrier channels are modulated by 16 QAM.