Tokuro Kubo

A highly efficient adaptive digital predistortion amplifier for IMT-2000 base stations

We have developed the worlds most compact, energy-efficient transmitter amplifier for IMT-2000 base station systems that uses an adaptive digital predistorter (DPD). The power efficiency is improved by reducing the output back-off (the margin from the saturation point) of power amplifiers and by suppressing the resulting increase in nonlinear distortion with the DPD. This method uses a predistortion technique that estimates the distortion characteristics of amplifiers by using the least mean square (LMS) algorithm, and it adds a complementary signal to the digital baseband signal with a reverse-characteristic of the distortion. The power amplifiers we have developed achieve twice the power efficiency of conventional types. In this paper, we describe the DPD equipment and the experimental results of a DPD transmitter prototype. Currently, IMT-2000 base station systems equipped with the DPD are being shipped worldwide.

Simulated tolerance of loop parameters of Cartesian feedback amplifier for pi /4-shifted QPSK transmitter

Evaluates Cartesian feedback amplifier performance for a mobile terminal for a digital cellular system using pi /4-shifted QPSK. To determine the out-of-band emission, the authors calculated spectra in peak-hold mode based on RCR Standard STD-27 on Japan Digital Cellular Telecommunication System. They calculated the tolerance of the feedback amplifiers loop parameters, assuming TDMA system parameters based on the RCR specifications. Since the specifications for out-of-band emission of -45 dB at 50 kHz and -60 dB at 100 kHz from the band center were satisfied, the Cartesian feedback amplifier is applicable to a TDMA transmitter and is easily implemented for practical applications.<<ETX>>

A coefficients extraction algorithm for LUT-based Hammerstein predistorters

There are many digital baseband predistorters, which are used to overcome the nonlinearity and memory effect of power amplifier for wideband signals. Look-up table-based Hammerstein predistorter is one of them and has remarkable advantages comparing with the others. In this paper, an iterative algorithm is proposed for extracting the coefficients of the look-up table-based Hammerstein predistorter. An ADS simulation testbed is built to verify the predistorter and algorithm, and the simulation results demonstrate the good performance of our algorithm.

A Novel Carrier Frequency Offset Estimation Method for IEEE 802.16E System

A new carrier frequency offset estimation method is proposed for OFDM based IEEE 802.16E system, with good tolerance to long-delay-spread multipath fading and time synchronization error. New designed correlation method utilizing approximate symmetry of the preamble can overcome the negative influence of imperfect periodicity caused by virtual subcarriers of preamble symbol. Simulations show that the proposed method can be more effective than CP based method in practical multipath fading channel, in terms of root mean square of estimation error. And the proposed method can be easily generalized for other OFDM based systems with inter- training symbol subcarrier spacing other than 3.

Wireless network technologies toward 5G

Fifth generation mobile communications (5G) are expected to accommodate rapidly increasing mobile traffic aiming at the realization of a “Hyper Connected World” in which all people and surrounding things are connected and information is exchanged between them, and to play the role of a basis of the Internet of everything. A lot of research projects have started focusing on the research of 5G since around 2013, and some agreements are being formed about the vision and the candidate technologies of 5G. The 5G wireless network is expected to become a “Heterogeneous Network” where new wireless access technologies incompatible with 4G and the wireless access technologies for unlicensed band are incorporated with the enhanced technology of 4G (e.g. IMT-advanced). This paper introduces the vision and technology trends of 5G, shows key directions of the research and development of 5G in the future, and introduces some of our studies on 5G.

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>>

Downlink Non-Orthogonal Multiple Access (NOMA) Constellation Rotation

The non-orthogonal multiple access (NOMA) technique or more generally the non-orthogonal principle is a crucial component for the next generation cellular system. For two-user downlink NOMA, the constellations of near and far users are superposed for transmission, and maximum likelihood (ML) or successive interference cancellation (SIC) receiver is used for reception. In this paper, we aim to further enhance the linklevel performance for NOMA with ML receiver. A method of NOMA constellation rotation is proposed where the constellations are respectively rotated before superposition so as to exploit signal space diversity. The symbol error rate (SER) upper bounds for both near and far users are derived and used for rotation angle optimization. Within this framework, we show the best we can do in terms of enhancing the performance for a particular user (either near or far). Simulation results have verified the effectiveness of our proposed method. It can be observed that there is performance gain for one user while almost no negative impact on the other user.

Space division full-duplex scheme with joint scheduling and precoding design

In-band full-duplex, allowing simultaneous transmission and reception on the same device over the same frequency, can double the spectral efficiency theoretically. However, the severe self-interference challenges the physical layer design. Thus, to simplify the interference cancellation (IC), the space division full-duplex (SDFD) is considered, where two spatial-separated transmission points (TPs), e.g., base stations, simultaneously serve the same cell in downlink (DL) and uplink (UL), respectively. In this paper, assuming the TP has digital-domain IC capability only, we study the SDFD scheme in heterogeneous network, i.e., a small cell (named SDFD-cell) is served as SDFD via macro TP (MTP) in DL and small TP (STP) in UL. Due to the inter-TP interference, the capacity of SDFD-cell is restricted by the digital-domain IC. Moreover, SDFD degrades the performance of other non-SDFD-cells. Therefore, we propose a joint scheduling and precoding design algorithm. The scheduling scheme selects a user equipment (UE) pair in the SDFD-cell for DL and UL transmissions as proportional fairness metric. Then, an optimal precoder is designed for MTP to reduce the interference in both SDFD-cell and non-SDFD-cells. The simulation results verify that the proposed algorithm can significantly improve the capacity of the SDFD-cell with limited capacity degradation in non-SDFD-cells.

Synchronous OBS solution to support TDM and data services in OBS ring network

We suggest a new protocol for Optical-Burst-Switched (OBS) ring network to support synchronous services (SONET/SDH, named TDM services) and asynchronous services (data services) expediently. In the new protocol, we use different bandwidth assignment methods for the two kinds of services. For synchronous services, we use fixed bandwidth assignment to broadcast the TDM slots to all remained nodes in optical domain; for asynchronous services, we use dynamic bandwidth assignment by OBS technologies. The data channel is frame based with 125μs frame period. In each 125μs frame, there are several reserved time slots for TDM services, and the remained time block can be used to transport data services. The main features of this protocol: 1) the bandwidth ratio of TDM and data services can be adjusted flexibly basing on demand; 2) the TDM services can be transported with guaranteed QoS; 3) the data services can be transported in data burst with statistic multiplexing profit; 4) we can transport TDM and data services in same data channel with OBS technology; 5) the total bandwidth utilization is about 70%.