Takeshi Takano

Cartesian feedback amplifier with soft landing

The author describe the results of several experiments conducted on a Cartesian feedback amplifier with soft landing. This amplifier uses a new method for adaptively controlling the local signal phase in a Cartesian demodulator. They tested the soft-landing Cartesian feedback (SLCFB) method in the 900 MHz band using pi /4-shifted QPSK modulation to determine its applicability for pocket terminals in Japanese digital cellular systems. They developed a smooth loop-gain control mechanism using a variable gain amplifier with an FET and a continuous phase-shifter control circuit for digital processing in the IF band. They showed that the SL-CFB method can maintain the optimum local signal phase in the demodulator. This method therefore solves the problem at applying CFB amplifiers. They adopted circuit configurations that are easy to incorporate into LSI chips so that the amplifier can be used in compact terminals. The results show that the SL-CFB amplifier is a promising candidate for developing highly efficient linear amplifiers, which are very important in digital cellular telephone terminals.<<ETX>>

Sensitivity simulation results for a direct-conversion FSK receiver

Results of computer simulation of a direct-conversion FSK (frequency-shift keyed) receiver are reported. The demodulation sensitivity depends strongly on the characteristics of the channel and postdetection filters. The sensitivity is defined as the Eb/No ratio at a bit-error rate of 10/sup -2/. The authors evaluated the dependence of the receiver sensitivity for bandwidth, section order, and filter type as parameters. Sensitivity degradation caused by the local frequency drift was determined. A phase detection technique for this type of receiver, which improves the sensitivity by 2 dB, is proposed.<<ETX>>

A Unified Approach for Weighted Viterbi Decoding in MIMO-OFDM Precoding Systems

In this paper, a unified approach for weighted Viterbi decoding is proposed to improve error rate performance in multiple-input multiple-output (MIMO) and orthogonal frequency division multiplexing (OFDM) systems with linear precoding. The MIMO-OFDM systems with limited feedback precoding, optimal linear precoding (eigenmode), and spatial multiplexing are considered as a unified framework. In the proposed method, the branch metric is weighted by using the signal to interference noise ratio (SINR) of the effective channel in soft decision Viterbi decoding. The proposed scheme can mitigate noise enhancement for the coherent zero-forcing (ZF) or minimum mean square error (MMSE) decoder in these coded systems. Two low complexity linear decoders based on ZF and MMSE are compared. Extensive simulations show that the proposed unified approach with low complexity exhibit excellent performance.

Improved output spectrum of linearized class-F power amplifier for digital cellular mobile communications

Describes a class-F power amplifier with an amplitude distortion canceller for a cellular mobile communications terminal. The output spectrum of the power amplifier was calculated for a pi /4-shifted QPSK input signal. Calculated and measured spectra were in close agreement. Out-of-band emission of the power amplifier output spectrum is subject to both AM-AM and AM-PM conversion. The effects of AM-AM conversion can be mitigated by using an amplitude linearizer; however, AM-PM conversion is not compensated. Out-of-band emission levels were calculated as a function of AM-PM conversion to provide an estimation of allowable AM-PM conversion for a class-F power amplifier module. Calculations and measurement clarified that even a class-F power amplifier module having a phase variation of 10 to 15 degrees due to AM-PM conversion transmits a spectrum that satisfies the out-of-band emission standard.<<ETX>>

Determination method for complex valued LTI transfer functions causing memory effect of power amplifier

This paper proposes a method to measure complex valued transfer functions of linear time-invariant (LTI) circuits by assuming a memory polynomial model for a power amplifier (PA). To preserve phase information, the method uses cross-correlation of the device under test (DUT) with another PA without memory effect. This paper has clarified possibility that all coefficients in the memory polynomials are directly determined by the impulse responses of the measured transfer functions. An iterative algorithm is described to obtain accurate estimates for the LTI transfer functions using the measured cross-correlation. Virtual data of the cross-correlations are generated for the assumed model of the PA. Validity of the method including the algorithm is examined by computer simulations where the input signal is assumed to be an orthogonal frequency-division multiplexing (OFDM) signal consisting of 200 subcarriers. It has been verified that the LTI transfer functions converge to the assumed ones within five iterations in the case of the blind start.

AFC circuit with fast acquisition for a TDMA cellular mobile communication system

A novel AFC (automatic frequency control) circuit for a TDMA (time-division multiple-access) digital cellular mobile communication system with pi /4-shifted QPSK (quadrature phase-shift keying) modulation is described. The AFC circuit satisfies the two contradictory requirements of fast acquisition and stable operation even under low CNR (carrier-to-noise ratio) channel conditions, simultaneously. The performance of a 40-kb/s TDMA transmission system was measured to confirm the availability of the AFC circuit. An E/sub b//N/sub 0/ penalty of the differential demodulator is compressed to less than 0.1 dB by the AFC circuit when carrier frequency deviation is within +or-1 kHz. The results obtained show that the differential detection can be applied to a receiver of the TDMA digital cellular mobile communication system when the proposed AFC circuit is implemented.<<ETX>>

Design of Predistorter Using Measured Nonlinear Characteristics of Power Amplifier with Memory Effect

This paper describes a method to design a predistorter (PD) for a power amplifier (PA) with a memory effect by using nonlinear parameters extracted from measured intermodulation distortion (IMD) for a GaN-FET amplifier. To improve the computational efficiency, the updating of a cancellation signal is provided by an iterative algorithm. It is confirmed that the proposed algorithm attains computational intensity lower than half of a memory polynomial. 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. 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.

An indoor bidirectional message paging system with a pocket-size terminal using a direct-conversion FSK modem

The authors describe an indoor bidirectional message paging system which has good frequency utilization and serviceability. The trial system adopts microcells (with a zone radius of 50 to 100 m) and half-duplex communication using the carrier-sense multiple-access with collision detection access method in the 420 MHz band with an occupied bandwidth of 12.5 kHz and a transmission power of 10 mW, according to the Japanese RCR STD-3 standard. Messages of up to 64 alphanumeric or 32 Japanese (Kanji) characters can be transmitted from a pocket-sized mobile terminal using a direct-conversion FSK (frequency-shift keying) modem. The modem enables the size of the mobile terminal to be the same as that of the conventional message pager. The basic concept of this system can also be applied to future microcell public message communication systems.<<ETX>>

Online Method of Determining the Nonlinear Characteristics of a PA with Short Updating Intervals for LTE-Advanced System

This paper proposes a method for measuring the rapidly varying parameters of a power amplifier (PA) characterized by a memory polynomial model in less time than in the symbol period of the orthogonal frequency-division multiplexing (OFDM) signal of a long-term evolution-advanced (LTE-Advanced) system. The method fulfills the dual-updating scheme that we proposed in our previous papers, which described a method of determining slowly varying parameters. We used the dependence of bandwidth on the order of nonlinearity to distinguish the contributions from different orders of nonlinearities in the frequency domain. We assumed that fast Fourier transforms (FFTs) with shorter fundamental periods than the symbol period attain shorter measuring times. We conducted a computer simulation to confirm the applicable range of the described method for varying the speed of the model parameters. We assumed sinusoidal variations for the parameters of the PA, and used the proposed method to trace the varying parameters and used the traced results in a predistorter (PD). We estimated the improved spectrum for the PD to confirm the accuracy of the determined coefficients. The simulation shows that our method traces changes accurately up to a variation in amplitude of 20%. We also confirmed that the method can trace the sinusoidal variation frequency up to half of the symbol period.

An 800 MHz Low Power Consumption Frequency Synthesizer using Intermittent Operation of a Modified PLL Circuit with Dual Loops

This paper presents a method of reducing the power consumption of the local oscillator for mobile radio communication equipments. The reduction of power consumption is attained by the intermittent stabilization of a PLL circuit in the frequency synthesizer. Whenever the PLL is turned on, there is a transient frequency fluctuation caused by a small amount of frequency and phase difference between the reference signal and the output signal of the programmable divider. Due to this frequency fluctuation, intermittent stabilization has not widely been utilized. We present a new PLL circuit with dual loops to suppress this frequency fluctuation. This new PLL circuit is used for an 800 MHz band frequency synthesizer and can suppress frequency fluctuations within 4 ppm, which is satisfactory for practical use. This frequency synthesizer reduces power consumption by about 35% when the duty ratio is 0.1.