A novel blind channel estimation scheme is proposed for OFDM systems employing PSK modulation. This scheme minimizes the number of possible channels by exploiting the constant modulus property, chooses a best fit over the possible channels by exploiting the finite alphabet property of information signals, and achieves competitive performance with low computational complexity. Results comparing the new scheme with the finite-alphabet based channel estimation are presented.

In orthogonal frequency-division multiplexing (OFDM)-based wireless local area networks (WLANs), phase noise (PHN) and residual frequency offset (RFO) can cause the common phase error (CPE) and the inter-carrier interferences (ICI), which seriously degrade the performance of systems. In this letter, we propose a combined pilot symbol assisted and decision-directed channel estimation scheme based on the least-squares (LS) and the maximum-likelihood (ML) algorithms. Simulation results present that the proposed scheme significantly improves the performance of OFDM-based WLANs.

The complexity of the optimal decoding for vector quantization (VQ) in code-division multiple access (CDMA) communications prohibits implementation. It was recently shown in [1] that a suboptimal scheme that combines a soft-output multiuser detector and individual VQ decoders provides a flexible tradeoff between decoder's complexity and performance. The work in [1], however, only considers an AWGN channel model. This paper extends the technique in [1] to a frequency-selective Rayleigh fading channel. Simulation results indicate that such a suboptimal decoder also performs very well over this type of channel.

We theoretically analyze the performance of coherent ultrashort light pulse code-division multiple-access (CDMA) communication systems with a nonlinear optical thresholder. The coherent ultrashort light pulse CDMA is a promising system for an optical local area network (LAN) due to its advantages of asynchronous transmission, high information security, multiple access capability, and optical processing. The nonlinear optical thresholder is based on frequency chirping induced by self-phase modulation (SPM) in optical fiber, and discriminates an ultrashort pulse from multiple access interference (MAI) with picosecond duration. The numerical results show that the thermal noise caused in a photodetector dominates the bit error rate (BER). BER decreases as the fiber length in the nonlinear thresholder and the photocurrent difference in the photodetector increase. Using the nonlinear optical thresholder allows for the response time of the photodetector to be at least 100 times the duration of the ultrashort pulses. We also show that the optimum cut-off frequency at the nonlinear thresholder to achieve the minimum BER increases with fiber length, the total number of users, and the load resistance in the photodetector.

This letter evaluates the performance of an uplink multicarrier-code division multiple access (MC-CDMA) system when the frequency offsets of all users are random variables and the frequency offset for the desired user is compensated. The analysis confirms that performance degradation due to frequency offset is negligible if the estimation error of normalized frequency offset for the desired user is less than 10-1.

When digital broadcasting services are provided through cable television (CATV) networks, viewers watching interactive programs such as quizzes or auctions may respond to the program within a short period. If these responses are transmitted in the upstream channel of the CATV networks using TCP/IP, they will result in burst traffic. The numerous TCP connections will trigger congestion in the upstream transmission facilities and will cause a significant delay in conventional Internet services such as web-browsing. The present paper proposes a new method of controlling the CATV upstream channel to avoid such congestion. We introduce class-based queues at each cable TV station, in which each service class is related to a type of interactive service. The status of the queue is relayed to the cable modems of subscribers using a CATV-specific MAC protocol. This queue-status information is used to suspend further initiation of TCP connections at cable modems. As a result, the TCP connections will be arbitrated in the CATV network, while the delay of the response transmission is traded for smoothing of the burst traffic. We numerically evaluate the effect of the proposed method using the time distribution of responses to an actual quiz program. The results show that the proposed method successfully suppresses interference of the burst traffic with conventional best-effort services.

We have proposed two space division multiple access (SDMA) approaches for OFDM signals: "Virtual Subcarrier Assignment (VISA)" and "Preamble Subcarrier Assignment (PASA)," both of which can enhance the system capacity without significant change of transmitter/receiver structures for already-existing OFDM-based standards such as IEEE802.11a. In order to investigate the performance of the proposed approaches in real wireless scenarios, we conducted a measurement campaign to obtain real channel state data at 5-GHz band in an indoor environment. Using the measured channel data, we can make the performance evaluation realistic. In this paper, after the brief overview of the two proposed SDMA approaches, we describe our measurement campaign in detail. Furthermore, we evaluate the performance of VISA-based system and PASA-based system by computer simulations using the measured channel state data and present a comparative study on the performance of the two proposed SDMA approaches in the realistic wireless environment.

In 3G CDMA mobile communication systems, high data rate services are essential for many key applications. When an MS approaches the cell border, link performance is degraded and more power should be allocated to maintain the link performance. Since the maximum available signal power is limited, the link adaptation mechanism may diminish the data rate to maintain link performance. This implies that the valid coverage shrinks when the data rate increases. The shrinking of valid coverage under a predetermined data rate will strongly impact on the reliability of high data rate services. In this work, the encoded bit error probabilities of 3G CDMA mobile communication systems, over large-scale and large-small-scale fading channels, were analyzed based on SGA and SIGA methods. Analytic methods were also proposed to investigate the issues of coverage shrinking and service data rate variations. Furthermore, the outage probability, cell coverage percentage and the staying probabilities of available data rates were well examined. The proposed analytic methods can be applied, as a preliminary research, to the design of cellular-system-related techniques, such as QoS control, available data rate prediction, power reservation, and service adaptation.

In this letter, we propose a partial minimum mean-squared error (MMSE) with successive interference cancellation (PMMSESIC) method in frequency domain to mitigate ICI caused by channel variation. Each detection, the proposed method detects the symbol with the largest received signal-to-interference-plus-noise ratio (SINR) among all the undetected symbols, using an MMSE detector that considers only the interference of several neithborhood subcarriers. Analysis and simulations show that it outperforms the MMSE method at relatively high Eb/N0 and its performance is close to the MMSE with successive detection (MMSESD) method in relatively low Doppler frequency region.

This paper presents a high-accuracy 3D (three-dimen-sional) measurement system using multi-camera passive stereo vision to reconstruct 3D surfaces of free form objects. The proposed system is based on an efficient stereo correspondence technique, which consists of (i) coarse-to-fine correspondence search, and (ii) outlier detection and correction, both employing phase-based image matching. The proposed sub-pixel correspondence search technique contributes to dense reconstruction of arbitrary-shaped 3D surfaces with high accuracy. The outlier detection and correction technique contributes to high reliability of reconstructed 3D points. Through a set of experiments, we show that the proposed system measures 3D surfaces of objects with sub-mm accuracy. Also, we demonstrate high-quality dense 3D reconstruction of a human face as a typical example of free form objects. The result suggests a potential possibility of our approach to be used in many computer vision applications.

Displaced subdivision surface representation [13] is a new form of representing a polygonal surface model, where a detailed surface model is defined as a scaler-valued displacement map over a smooth domain surface; it puts forth a number of attractive features for editing, geometry compression, animation, scalability, and adaptive rendering of polygonal models. The construction of the smooth domain surface is a challenging task in the conversion process of a detailed polygonal surface model into this representation. In this paper, we propose a new efficient method for defining the smooth domain surface based on -subdivision scheme. The proposed algorithm not only performs better in terms of the quality of the generated surfaces but is computationally more efficient and occupies less memory as compared to the original algorithm [13] and generates surfaces with more levels of detail due to the specific nature of -subdivision when the prescribed target complexity of the generated mesh must not be exceeded. To corroborate the efficiency and the quality of the new technique, the conversion results for several public domain models have been presented.

By applying the differential space-time block code (DSTBC) to wireless multicarrier transmission, Diggavi et al. were the first to propose the two-input-multiple-output (2IMO) differentially space-time-time block coded OFDM (TT-OFDM) system. In this paper, we propose three novel differentially transmit-diversity block coded OFDM (DTDBC-OFDM) systems, namely, the FT-, FF-, and TF-OFDM systems. For instance, the TF-OFDM stands for the differentially space-time-frequency block coded OFDM. Moreover, the noncoherent maximum-likelihood sequence detector (NSD), and its three special cases, namely, the noncoherent one-shot detector, the linearly predictive decision-feedback (DF) detector, and the linearly predictive Viterbi receiver are incorporated to the 2IMO DTDBC-OFDM systems. Furthermore, a simple closed-form BER expression for the systems utilizing the noncoherent one-shot detector in the time-varying multipath Rayleigh fading channels is given. Numerical results have revealed that 2IMO DTDBC-OFDM systems employing the noncoherent one-shot detector can obtain significant performance improvement. However, when few antennas are available, the implementation of the linearly predictive DF detector or the linearly predictive Viterbi receiver is necessary for achieving better performance.

This paper presents a simple, yet effective hybrid of the minimum mean square error (MMSE) multi-user detection (MUD) and successive interference cancellation (SIC) for direct-sequence code division multiple access (DS-CDMA) systems. The proposed hybrid MUD first divides the users into groups, with each group consisting of users with a close power level. The SIC is then used to distinguish users among different groups, while the MMSE MUD is used to detect signals within each group. To further improve the performance impaired by the propagation errors, an information reuse scheme is also addressed, which can be used in conjunction with the hybrid MMSE/SIC MUD to adequately cancel the multiple access interferences (MAIs) so as to attain more accurate detections. Furthermore, the asymptotic multiuser efficiency (AME), a measure to characterize the near-far resistance capability, is also conducted to provide further insights into the new detectors. Furnished simulations, in both additive white Gaussian noise (AWGN) channels and slow flat Rayleigh fading channels, show that the performances of the proposed hybrid MMSE/SIC detectors, with or without the decision aided scheme, are superior to that of the SIC and, especially, the one with decision aided is close to that of the MMSE MUD but with substantially lower computational complexity.

Multi-projector technology has been under consideration in recent years. This technology allows the generation of wide field of view and high-resolution images in a cost-effective manner. It is expected to be applied extensively to training simulators where vivid immersive sensations and precision are required. However, in many systems the viewing frustums cannot be automatically assigned for distributed rendering, and the required manual setup is complicated and difficult. This is because the camera should be coincide exactly with a desired eye point to avoid perspective distortions. For the actual applications, the camera is seldom able to be set up at the desired eye point because of physical constraints, e.g., a narrow cockpit with many instruments. To resolve this issue, we have developed a "virtual camera method" that yields high-precision calibration regardless of the camera position. This method takes advantage of the quadratic nature of the display surface. We developed a practical real-time multi-projector display system for applications such as training simulators, that require high-accuracy in geometry and rapid response time.

A two-dimensional network for motion detection constructed of simple analog circuits was proposed and designed based on the frog visual system. In the frog visual system, the two-dimensional motion of a moving object can be detected by performing simple information processing in the tectum and thalamus of the frog brain. The measured results of the test chip fabricated by a 1.2 µm complementary metal oxide semiconductor (CMOS) process confirmed the correct operation of the basic circuits in the network. The results obtained with the simulation program with integrated circuit emphasis (SPICE) showed that the proposed network can detect the motion direction and velocity of a moving object. Thus, a chip for two-dimensional motion detection was realized using the proposed network.

A network for the detection of an approaching object with simple-shape recognition is proposed based on lower animal vision. The locust can detect an approaching object through a simple process in the descending contralateral movement detector (DCMD) in the locust brain, by which the approach velocity and direction of the object is determined. The frog can recognize simple shapes through a simple process in the tectum and thalamus in the frog brain. The proposed network is constructed of simple analog complementary metal oxide semiconductor (CMOS) circuits. The integrated circuit of the proposed network is fabricated with the 1.2 µm CMOS process. Measured results for the proposed circuit indicate that the approach velocity and direction of an object can be detected by the output current of the analog circuit based on the DCMD response. The shape of moving objects having simple shapes, such as circles, squares, triangles and rectangles, was recognized using the proposed frog-visual-system-based circuit.

For alleviating the high peak-to-average power ratio (PAPR) problem of orthogonal frequency division multiplexing (OFDM), the OFDM combined with time division multiplexing (TDM) using frequency-domain equalization (FDE) was proposed. In this paper, the theoretical bit error rate (BER) analysis of the OFDM/TDM in a frequency-selective fading channel is presented. The conditional BER expression is derived, based on a Gaussian approximation of the inter-symbol interference (ISI) arising from channel frequency-selectivity, for the given set of channel gains. Various FDE techniques as in multi-carrier code division multiple access (MC-CDMA), i.e., zero forcing (ZF), maximum ratio combining (MRC) and minimum mean square error (MMSE) criteria are considered. The average BER performance is evaluated by Monte-Carlo numerical computation method using the derived conditional BER expression.

The performance of a feedback-type adaptive array antenna (AAA) system placed only at a base station (BS) in an FDD/DS-CDMA system remains insufficiently clear. We evaluate the performance of this system by considering the effect of a rake receiver, spacing distance between antennas, the maximum Doppler frequency (fd), and control delay time (Td) on BER performance. In this system, the mobile station (MS) determines optimum weights of antenna elements and sends them back to BS as feedback information. We assume that the optimum weights are not quantized. Thereby, we estimate the performance degradation of 3GPP transmit diversity system, where the feedback information is quantized using a few bits. Computer simulation results show that the rake receiver achieves better BER performance because of the time diversity effect with rake receiver. The AAA with a wide antenna spacing gives high diversity gain for the received signals. For a high value of fd Td, BER performance becomes worse because weighting factors cannot follow the changing speed of channel characteristics. The degradation in performance of a 3GPP system is clarified.

As digital television transmission is becoming ubiquitous, a method that can remotely monitor the quality of the final and intermediate pictures is urgently needed. In particular, the case where standards conversion is included in the transmission chain is a serious issue as the input and output cannot simply be compared. This letter proposes a novel method to solve this issue. The combination of skipping fields/pixels and the previously proposed SSSWHT-RR method, using the information of correlation coefficients and variance of the picture, achieves accurate detection of picture failure.

Space-frequency transmit diversity (SFTD) and space-code transmit diversity (SCTD), which are both based on space-time block codes (STBC), were applied to time-direction spreading and two-dimensional spreading orthogonal frequency-division multiplexing code-division multiplexing (OFDM-CDM) systems, and the transmission performances were compared by computer simulation. SFTD is designed for space and two adjacent subcarriers whereas SCTD is designed for space and two distinct Walsh-Hadamard (WH) codes. The simulation results show that SCTD applied to time-direction spreading OFDM-CDM was superior to SFTD because frequency selectivity distorted STBC's orthogonality between sub-carriers in SFTD. In contrast, when they were applied to two-dimensional spreading OFDM-CDM, SFTD was superior to SCTD when the number of WH codes belonging to the same mother-code group is small because the frequency diversity provided by SFTD surpassed that provided by SCTD. In addition, both SFTD and SCTD provide high tolerance to large Doppler spread. It can be therefore concluded that both SCTD and SFTD can be used in the same frame by code-multiplexing according to their suitability to physical channels. SCTD is suitable for transmitting high-rate data via time-direction spreading, whereas SFTD is suitable for transmitting control data via two-dimensional spreading.