Optical code division multiplexing (OCDM) is the other class of multiplexing techniques than time division multiplexing (TDM), wavelength division multiplexing (WDM) and space division multiplexing (SDM). OCDM has been proposed in mid '70s. It has been long since OCDM remains outside the mainstream of research community of optical communications, however, possible scarcity of the wavelength resource in future photonic networks, the simple access protocol as well as versatility of optical codes motivate recent growth of OCDM research activities. In this paper, first, fundamentals of OCDM concept are presented, highlighting optical encoding and optical time gate detection which realize time spreading/despreading. Next, current research activities of OCDM are reviewed by focusing particularly on the optical implementations and the proof-of-concept experiments. It is devoted to three categories; high bit rate point-to-point transmissions, gigabit multiple access, followed by optical path networks using optical code. Finally, future issues are briefly summarized.

In inter-vehicle communication (IVC) expectation for spread spectrum techniques is high. However, in a decentralized network environment, power control is difficult and until now perfect power control has been assumed. In this paper the use of sector antennas are proposed as a solution to the problems of power control in inter-vehicle communication. Results are shown for an IVC protocol in both no power control and imperfect power control environments in a realistic fading channel. Omni-directional, uniform sector antennas and non-uniform sector antennas are examined by computer simulation. Non-uniform sector antennas are shown to be the best solution and to have high packet reception rates even for no power control environments.

In this paper, a type of multi-carrier direct sequence code division multiple access (MC-DS-CDMA) system which uses frequency spread coding is proposed and investigated for the down-link. An MC-DS-CDMA system is a combined system of CDMA and multi-carrier modulation. This system is often categorized as a "serial to parallel (S/P) type" system because serial to parallel converted data symbols are transmitted. They use different sub-carriers which are narrow-band DS waveforms. In this system, benefits of path or frequency diversity can not be obtained because of the narrow-band transmission of each data symbol. In order to benefit from the diversity, we propose to adopt frequency spread coding in an MC-DS-CDMA system. The proposed system exploits frequency diversity without additional redundancy, i. e. , no frequency or time redundancy is required to improve the performance. Computer simulation is carried out in a frequency selective fading channel and the results show its effectiveness in terms of average bit error rate (BER). Furthermore, the proposed system is compared with a multi-carrier (MC-) CDMA system which is often categorized as a "copy type" system and a single-carrier (SC-) DS-CDMA system using a RAKE receiver.

One of the problems in multi-carrier modulation is nonlinear distortion due to nonlinearity of channels, such as in HPA (High Power Amplifier). This problem is also true of multi-carrier SS (Spread Spectrum) systems. In this paper, an inter-modulation compensation scheme for multi-carrier M-ary/SS system is proposed. We propose two methods to control the sequences transmitted in parallel to avoid the occurrence of severe inter-modulation distortion. One is the "package sequence selection" method, which requires slight redundancy. The other method is based on error correction code, which requires no additional frequency or power except the redundancy for error correction. We confirm the validity of our proposed scheme by computer simulation, and the BER (Bit Error Rate) performance in an AWGN (Additive White Gaussian Noise) channel is presented.

To improve frequency efficiency or user capacity in multi-path fading environments, we introduce and investigate an orthogonal multi-carrier frequency hopping-code division multiple access (FH-CDMA). These improvements are achieved by combining the orthogonal frequency division multiplexing (OFDM) and FH-CDMA schemes. The basic idea has been previously proposed by the authors. The aim of study in this paper is to evaluate the performance of this scheme in various environments. The theoretical analysis of bit error rate (BER) performance in this paper includes the effects of frequency selective fading in land mobile communications and of nonlinear amplification in satellite communications. A modified scheme of controlling transmission power to be controlled according to the number of simultaneously accessing users is also discussed. This modified scheme improves BER performance for frequency selective fading when the number of simultaneously accessing users in a cellular zone is small. Furthermore, an error-correcting code and its erasure decoding are applied in order to reduce errors due to hits in asynchronous FH/CDMA for reverse link as well as errors due to fading and noise.

The transmission quality in mobile wireless communications is affected by not only the thermal noise but also the multipass fading which changes drastically an amplitude and a phase of received signal. The paper proposes the theoretical and approximate methods for deriving an average bit error rate in DS-CDMA systems under the Rician fading environment on the assumption of the frequency non-selective fading, as parameters of the number of simultaneous access stations, the maximum Doppler frequency and so on. It is confirmed from the coincidence of theoretical and approximate results with simulation ones that the proposed approach is applicable to a variety of system parameters.

Design of wireless communications systems with a transmission rate of 6 and 10 Mbit/s is presented for the 2.4 GHz Japanese ISM band, in which a spread spectrum technique named "CFO-SS (Carrier Frequency Offset-Spread Spectrum)" scheme is employed. In the CFO-SS system, a single PN code is commonly assigned to all the synchronized multiplexed carriers, and the frequency separation between carriers is determined by the transmission rate of each carrier. To realize the CFO-SS system, a timing acquisition and tracking scheme, an important part of the design, is presented first. Next, bit and packet error performance is investigated under severe multipath environments with/without a RAKE receiver. Degradation by channel bandwidth limitations, frequency inaccuracy of the hardware and co-channel interference (CCI) is also investigated by computer simulation. Simulation results presented confirmed sufficient performance of the CFO-SS system for wireless LAN systems using the 2.4 GHz ISM band.

This paper describes the effects of traffic distributions on uplink and downlink communications qualities in CDMA cellular systems. Many researches have been done from the viewpoint of the system capacity under ideal conditions in both uplink and downlink. However, there are few studies regarding traffic distributions that concurrently affect the uplink and downlink quality. The characteristics in both links are different even in a spatially uniform traffic distribution because the system structures are not symmetric between both links. When non-uniform radio environments are assumed, both link qualities become very different from each other. It is therefore important to design systems in consideration of link-specific characteristics in whole service area. This paper clarifies the difference in both link characteristics in CDMA systems regarding traffic distributions.

This paper discusses the employment of adaptive array antennas at the base station of a Multi Processing Gain (MPG) CDMA system. It is shown that the adaptive array antenna with the weight control scheme based on the signal before despreading procedure does not increase but even decreases the performance than that with an omni-directional antenna, and the cause of this serious performance degradation is revealed. Then it is shown that the performance with the weight control scheme based on the signal after despreading procedure is always better than that with an omni-directional antenna. Furthermore, the possibilities of performance improvement by the combination of adaptive array antenna and interference cancellation techniques are mentioned.

While higher chip rate can provide better performance for Direct Sequence/Code Division Multiple Access (DS/CDMA) systems due to larger process gain, it may also induce spectrum emission to adjacent channels, i. e. , adjacent channel interference. Especially, if different operators use adjacent channels in the same area with uncoordinated power levels, such interference becomes large, and excessively higher chip rate will decrease the efficiency of a system. In this context, this paper evaluates the relation between chip rate and capacity in DS/CDMA cellular communication systems considering adjacent channel interference from other systems. First, the classification of adjacent channel interference between two independent DS/CDMA systems is described, and the concrete interference levels are calculated for several chip rates. Then, by using computer simulation, the system CDMA capacity is evaluated under adjacent channel interference. From these results, we can find that the excessively higher chip rate can not always provide the larger system CDMA capacity in spite of the larger process gain, and there exists the appropriate chip rate for a certain given bandwidth.

In this paper, system capacity of the reverse link of a wireless multimedia CDMA system with transmission power control is analysed for receivers with and without CCI cancellers. For N classes of users, system capacity is represented by a point in an N-dimensional space. It is shown that system capacity is improved considerably with CCI cancellers, that system capacity region is non-convex in general, and that its boundary is well approximated with a unique hyper plane when CCI cancellers are fully employed.

This paper proposes a novel DS/CDMA system with code-diversity techniques constituted by a simple system to suppress multiple access interference (MAI) without estimating the PN sequence of interference at the receiver. In the transmitter, the data signal is modulated with a sum of several PN sequences, and, two types of code- diversity reception are proposed, (1) maximal-ratio combining (MRC) code-diversity by autocorrelation, and (2) MRC code-diversity by anti-crosscorrelation. By computer simulations, it is shown that MRC code-diversity by anti-crosscorrelation is superior to the other one. It is also shown that MRC code-diversity by anti-crosscorrelation can improve BER more effectively for the interference which takes the phase to degrade BER at the worst. Next, to design the optimum number of branches for code-diversity, average BERs are calculated for several combinations of codes in code-diversity. As a result, the optimum number of branches varies for each combination of codes, however, it is decided from 3 to 7 branches. Finally, the effectivity of the proposed system in a near-far problem is presented.

In this paper, we propose the introduction of space diversity techniques to the code acquisition of a direct-sequence spread-spectrum signal. In this scheme, both a transmitter and a receiver have multiple antennas and the signals corresponding to all the combinations of the transmitter and receiver antennas are combined at the acquisition circuit of the receiver. The performance is evaluated for an indoor packet radio communication system from the viewpoints of the average time for acquisition, the probability of success of acquisition, and the necessary preamble length. As the result, we show great performance improvements by the proposed scheme under slow and flat Rayleigh fading environment.

This paper describes a digital delay-lock Loop (DLL) to which delta-sigma (Δ Σ) modulation technique is applied in order to reduce circuit elements. The DLL is evaluated in both transient and steady-state behavior by theoretical analysis, computer simulations and circuit experiments. Not deteriorated by the internally generated Δ Σ-modulation noise, the DLL shows good tracking performance in transient response and steady-state RMS jitter of phase error against additive white Gaussian noise. Using the proposed DLL most parts of receiving circuits are realized by digital integrated circuits. After realizing the circuit, power-line communication system with spread spectrum is possibly expected in a small size with low cost.

This paper proposes a novel spread spectrum (SS) modem for random access satellite communication systems that employs digital matched filters. The proposed modem employs a parallel structure to ensure detection of packet arrival. Code timing detection with a combination of a coarse detector and a fractional error detector reduces the sampling rate while maintaining the BER performance. An in-symbol pilot multiplexing scheme is also proposed for fast and stable carrier synchronization with a simple hardware. A performance evaluation shows that the proposed modem achieves the UW miss-detection probability of 10^-4 at the E_b/N_o of 0 dB. The overall BER performance achieved in experiments well agrees simulation.

Most of error correcting codes applying to DS/SS systems are such that information data is first (bit-by-bit) encoded and then spread by pseudo noise (PN) sequence. Thus, coding gain achieved by such systems are mainly due to the error correcting codes and the redundancy produced by the spreading codes shows no effect on the coding gain. In this paper, a chip-by-chip Turbo coding for DS/SS systems is proposed. The input information data is first spread by PN sequence and then fed into the Turbo-encoder which operates in chip timing. As the Turbo-encoder operates in chip timing, a large interleaving size would be obtained, which improves the performance. As results, superior performances with coding gain of more than 3.0 dB and 5.0 dB for AWGN and Rayleigh-fading channel, respectively, were found with short frame size of information data.

Frank and Zadaff had proposed a class of polyphase orthogonal sequences. Now the sequences are called "Frank sequences. " Suehiro has proposed a modulation method for Frank sequences. For example, the number of 4-phase orthogonal sequences of period 16 is 32 by Suehiro's method, on the other hand, the number of 4-phase Frank sequences of period 16 is 2. In this paper, the modulation method is expanded again and we prove that the sequences obtained by the proposed method are polyphase orthogonal sequences. Furhtermore, we prove that the number of N-phase orthogonal sequences of period N² obtained by the proposed method is N^N-2(N-1)!. As a result of the proposed expansion, the number of 4-phase orthogonal sequences of period 16 is 96.

This paper discusses factorization of bent function type complex Hadamard matrices of order pⁿ with a prime p. It is shown that any bent function type complex Hadamard matrix has symmetrical factorization, which can be expressed by the product of n matrices of order pⁿ with pⁿ⁺¹ non-zero elements, a matrix of order pⁿ with pⁿ non-zero ones, and the n matrices, at most. As its application, a correlator for M-ary spread spectrum communications is successfully given, which can be simply constructed by the same circuits with reduced multiplicators, before and behind.

This paper presents a quadriphase sequence pair, whose aperiodic auto-correlation functions for non-zero shifts and cross-one for any shift take pure imaginary values. Functions for pairs of length 2ⁿ are formulated, which map the vector space of order n over GF(2) to Z₄. It is shown that they are bent for any n, such that their Fourier transforms take all the unit magnitude.

The design of a liner decorrelating detector for multi-processing gain code-division multiple-access (MPG-CDMA) systems is proposed, and its performance is discussed. As the result, the performance improvement by this detector is confirmed. Also, it is found that that the degrees of the noise enhancement depend on the processing gains of the signals.

Recently portable computers and mobile communications have been developed rapidly. Along with the developments of mobile computing environment, the demands of transmitting images over wireless channels are growing. However, high efficiency image coding techniques are not designed for the wireless communication channels. The use of variable length codes (VLCs) in those coding techniques makes a receiver hard to synchronize with the codewords on the noisy channel. On the other hand, a variable length code transmission technique utilizing multicode CDM (Code Division Multiplex) has been proposed. In this paper, the multicode CDM technique is applied to wireless transmission of images compressed by the H. 263 algorithm. Especially, the proposed technique is employed for motion vector transmission. In consequence, the proposed transmission technique reduces the required E_b/N₀ as compared with the conventional transmission technique.

Non-stationary glint noise is often observed in a radar tracking system. The distribution of glint noise is non-Gaussian and heavy-tailed. Conventional recursive identification algorithms use the stochastic approximation (SA) method. However, the SA method converges slowly and is invalid for non-stationary noise. This paper proposes an adaptive algorithm, which uses the stochastic gradient descent (SGD) method, to overcome these problems. The SGD method retains the simple structure of the SA method and is suitable for real-world implementation. Convergence behavior of the SGD method is analyzed and closed-form expressions for sufficient step size bounds are derived. Since noise data are usually not available in practice, we then propose a noise extraction scheme. Combining the SGD method, we can perform on-line adaptive noise identification directly from radar measurements. Simulation results show that the performance of the SGD method is comparable to that of the maximum-likelihood (ML) method. Also, the noise extraction scheme is effective that the identification results from the radar measurements are close to those from pure glint noise data.

In this paper, the Linear Exponential Quadratic Gaussian with Loop Transfer Recovery (LEQG/LTR) methodology is employed for the design of high performance induction motor servo systems. In addition, we design a speed sensorless induction motor vector controlled driver with both the extended Kalman filter and the LEQG/LTR algorithm. The experimental realization of an induction servo system is given. Compared with the traditional PI and LQG/LTR methods, it can be seen that the system output sensitivity for parameter variations and the rising time for larger command input of the proposed method can be significantly reduced.

In this paper, we present a new approach which is spanning tree-based genetic algorithm for solving a multi-objective transportation problem. The transportation problem as a special type of the network optimization problems has the special data structure in solution characterized as a transportation graph. In encoding transportation problem, we introduce one of node encodings based on a spanning tree which is adopted as it is capable of equally and uniquely representing all possible basic solutions. The crossover and mutation were designed based on this encoding. Also we designed the criterion that chromosome has always feasibility converted to a transportation tree. In the evolutionary process, the mixed strategy with (µ+λ)-selection and roulette wheel selection is used. Numerical experiments show the effectiveness and efficiency of the proposed algorithm.

This paper proposes an associative memory neural network whose limiting state is the nearest point in a polyhedron from a given input. Two implementations of the proposed associative memory network are presented based on Dykstra's algorithm and a fixed point theorem for nonexpansive mappings. By these implementations, the set of all correctable errors by the network is characterized as a dual cone of the polyhedron at each pattern to be memorized, which leads to a simple amplifying technique to improve the error correction capability. It is shown by numerical examples that the proposed associative memory realizes much better error correction performance than the conventional one based on POCS at the expense of the increase of necessary number of iterations in the recalling stage.

This paper introduces an acoustic echo canceller system materialized with a 16-bit fixed point processing type DSP (Analog Devices, ADSP-2181). This experimental system uses the tri-quantized-x individually normalized least mean square (INLMS) algorithm little degrading the convergence property under the fixed point processing. The experimental system also applies a small step gain to the algorithm to prevent the double-talk from increasing the estimation error. Such a small step gain naturally reduces the convergence speed. The experimental system compensates the reduction by applying the block length adjustment technique to the algorithm. This technique enables to ceaselessly update the coefficients of the adaptive filter even when the reference signal power is low. The experimental system thus keeps the echo return loss enhancement (ERLE) high against the double-talk.

The explicit formula for the coefficients of maximally linear digital differentiators is derived by the use of Taylor series. A modification in the formula is proposed to extend the effective passband of the differentiator with the same number of coefficients.

This paper proposes a design method of STC for discrete unstructured systems based on D-L networks. The proposed scheme determines the control input composed of arbitrary system matrices of constructed D-L networks, and thus it makes it possible to design the STC for unstructured systems whose priori knowledges are not available. The simulation results show that the proposed scheme provides satisfactory performances both in MPS and NMPS for the STC design of unstructured systems.

This paper presents a new fuzzy dynamic output feedback controller design technique for the Takagi Sugeno (T-S) fuzzy model with unknown-but-bounded time-varying modeling error. It is shown that the quadratic stabilization problem of the T-S fuzzy modeled system can be converted into an H control problem of the scaled polytopic Linear Parameter Varying (LPV) system. Then, a controller satisfying a prescribed H performance is designed for the stabilization of the T-S fuzzy modeled system.

I would like to draw the attention of the editorial board of IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences and its readers to a recent paper, Tianruo Yang, "The integrated scheduling and allocation of high-level test synthesis," vol. E82-A, no. 1, January 1999, pp. 145-158. (Here we call this paper the Yang's paper. ) Yang did not give the correct information about the originality of the paper. I will point out that the writings (and the idea accordingly) of section 6 of Yang's paper came from papers [1] and [2].

As many research works are based on some previous results, my paper, namely The Integrated Scheduling and Allocation of High-Level Test Synthesis, makes use of some techniques by T. Kim. However, I did not state explicitly that some parts of my work are based on Kim's approach although I have referred to his paper. I would like to express my deep apology to Kim for not having emphasized Kim's contribution to my work. But my intention was not to steal Kim's ideas. I would like to emphasize the following difference.