Hiroyuki Yashima

Optical CDMA with time hopping and power control for multimedia networks

Optical code-division multiple access (OCDMA) for high-speed multimedia transmission is proposed and its performance is investigated. The proposed system introduces time hopping to vary transmission bit rate and power control to control transmission signal performance, which allows various signals with different desired rate and performance. An expression for the bit error rate (BER) of the proposed system is derived and numerical results are shown. An optical power selector (OPS) coupled with a hard limiter are also proposed to improve system performance. The OPS is a device to transmit the instantaneous maximum optical power among input signals. The OPS combined with the optical hard limiter reduces the interference from simultaneous users which results in an improved BER. BER is theoretically derived and numerical results for some parameters are shown. We show that the proposed system significantly improves BER and is an attractive scheme for future multimedia networks.

Performance enhancement of the 2-D wavelength-hopping/time-spreading synchronous OCDM system using a heterodyne detection receiver and PPM signaling

Feature Issue on Optical Code Division Multiple AccessTwo-dimensional wavelength-hopping/time-spreading synchronous optical code-division multiplexing systems using a heterodyne detection receiver and pulse-position modulation (PPM) signaling are investigated. Prime sequences are used as signature code for both wavelength hopping and time spreading. Performance of the proposed systems is theoretically analyzed taking into account various kinds of noise and interference, including multiple access interference, optical beating interference, crosstalk, and receiver noise. We also compare the performance of the proposed systems with the conventional ones using ON-OFF keying signaling. Numerical results show the improvement in both number of users and optical power gain with a proper selection of PPM word length M.

Spectral-amplitude-encoding optical-code-division-multiplexing system with a heterodyne detection receiver for broadband optical multiple-access networks

Feature Issue on Optical Access Networks (OAN) We propose a spectral-amplitude-encoding optical-code-division-multiplexing (SAE∕OCDM) system with a heterodyne detection receiver for broadband optical multiple-access networks (OMANs). The performance of the proposed system is theoretically analyzed, taking into account various types of noise and interference, including multiple-access interference (MAI), cross talk, optical beating interference (OBI), and receiver noise. Analytical results show that the proposed system offers significant improvement in terms of receiver sensitivity and system capacity (number of users) compared with conventional direct-detection systems. We also discuss conditions necessary for a heterodyne detection receiver to work properly in a SAE∕OCDM system. In addition, the system performance is analyzed with several signature code sets including m-sequence, Hadamard, and modified quadratic congruence (MQC) code sets. It is found that signature code sets with higher weight, such as m-sequence and Hadamard code sets, are preferred in a SAE∕OCDM system with a heterodyne detection receiver.

A study of driving assistance system based on a fusion network of inter-vehicle communication and in-vehicle external sensors

This paper describes the evaluation on the safety performance of a driving assistance system based on a fusion network of inter-vehicle communication and in-vehicle external sensors in which the other vehicles running information acquired by in-vehicle external sensors and own running information acquired by internal-sensors. The safety performance is evaluated by microscopic traffic simulator in a mixed environment of the three types vehicles(system equipped vehicle with communication and sensor devices, system equipped vehicle with only communication devices, and vehicle without driving assistance system). The result of simulation indicates that when the half of vehicle which has communication function equips an external sensing function, the safety performance of the driving assistance system in the fusion network of inter-vehicle communication and in-vehicle external sensors rises up than that in the conventional inter-vehicle network which only shares the in-vehicle running information. Especially, it greatly improves the safety of the vehicle with only communications equipment.

Performance evaluation of vehicle cooperative driving assistance systems that uses forward obstruction detecting sensors and inver-vehicle communication

This paper describes a evaluation of the safety performance of a vehicle cooperative driving assistance system that uses forward obstruction detecting sensors. The information acquisition rate is defined as the ratio of number of information acquisition vehicles to number of vehicles of and others in surrounding that exists in constant range to have centered on the subject vehicle and evaluated it by microscopic traffic simulation. Furthermore safety performances of the driving assistance system have been evaluated by microscopic traffic simulator from the viewpoint of a road administrator and drivers. Results of simulation have been indicated that the system that uses only the forward obstruction detecting sensor achieves the same safety-performance as the combinations system by improving penetration from 10 to 30 points.

Microscopic traffic simulator including driver recognition error model for evaluation of driving assistance systems

This paper describes a microscopic traffic simulator considering driver recognition errors for evaluation of driving assistance systems. The simulator including the model of both the driverpsilas delay and the driver recognition error that cause the collision between vehicles has been developed. The validity of the lane changing frequency of the simulator is verified. Moreover, the average accident interval evaluated by the simulation is compared with the average accident interval in the real traffic.

LLR calculation for iterative decoding on fading channels using Padé approximation

In this paper, approximate calculation of channel log likelihood ratio (LLR) for wireless channels using Padé approximation is devised. Due to the lack of knowledge of the channel state information of a wireless fading channel, such as uncorrelated flat Rayleigh fading channels, calculation of exact LLR for these channels is quite complicated for a practical implementation. The previous work, LLR calculation using the Taylor approximation, quickly becomes inaccurate as the channel output leaves some derivative point. To overcome this problem, we devise a new LLR approximation using Padé approximation, which expresses the original function by rational form of two polynomials with the same total number of coefficients of the Taylor series, accelerates the Taylor approximation, is devised. By applying the proposed approximation to the iterative decoding and the low-density parity-check codes, we show the effectiveness of the proposed methods by simulation results and analysis based on the density evolution.

Improvement of Coherent Ultrashort Light Pulse CDMA Communication Systems with Distinct 4-Level m-Sequences

Optical fiber communications require multiple-access schemes to access a shared channel among multiple users. The coherent ultrashort light pulse code-division multiple-access (CDMA) system is one such scheme, and it also offers asynchronous-access communication. This system usually employs 2-level, i.e., binary, m-sequences as signature codes because of their low correlation. If the number of active users is greater than the length of the m-sequence, i.e., code length, distinct m-sequences are used. However, the distinct 2-level m-sequences do not exhibit low correlation, resulting in performance degradation. We therefore propose a coherent ultrashort light pulse CDMA communication system with distinct 4-level, i.e., quaternary, m-sequences to improve system performance when the number of users is greater than the code length. We created the 4-level m-sequences from 2-level m-sequences, and assess the correlation of the 4-level m-sequences. We also theoretically derive the bit error rate (BER) of the proposed system taking into account multiple-access interference (MAI), beat noise, amplified spontaneous emission (ASE), shot noise, and thermal noise. The numerical results show that BER for distinct 4-level m-sequences is more than an order of magnitude smaller than that for distinct 2-level m-sequences. BER is limited by MAI and beat noise when the power of the received signal is high, otherwise BER is limited by ASE, shot noise, and thermal noise.

Simulation of all-optical NOR gate using single quantum-dot SOA and optical filter

We propose an ultrafast all optical NOR gate for 160 Gb/s return-to-zero Gaussian data signals using a single quantum-dot semiconductor optical amplifier (QD-SOA) and optical filter (OF). By conducting numerical simulations, we investigate and evaluate the impacts of probe wavelength and power of the involved signals on the extinction ratio (ER) and Q2-factor. Results show that the proposed NOR gate can be achieved both with logical correctness and high quality when the specified conditions are satisfied.

Constellation shaping for non-uniform signals in bit-interleaved coded modulation combined with multi-stage decoding

A signal shaping for the bit-interleaved coded modulation (BICM) is considered. Previous work on signal shaped BICM (SBICM) has two problems: (i) It requires an optimization for the linear factors of superposition modulation. The optimization is valid around target SNR. (ii) The binary labeling of signal constellation is not Gray code. To overcome these problems, non-uniform signal labeling for the input signal with Gaussian distribution is considered. We implement the reduction of peak-to-average power ratio (PAPR) by clipping for the largest signal points. From the numerical results, we show that achievable rate of the proposed modulation is larger than that of the previous modulation of SBICM for wide range of SNR, while keeping the expansion of PAPR small. Moreover it is quite close to the coded modulation capacity for wide range of SNR, especially in some SNR region, it is larger than the coded modulation capacity.