Ambulance transport delays are a growing challenge in medical services, especially in aging urban areas. In practice, transport time often depends not on traffic or route conditions but on whether destination hospitals accept emergency patients, and repeated refusals can significantly prolong response time. This study proposes a data-driven hospital recommendation system designed in collaboration with the Nagoya City Fire Department and Emergency Medical Technicians (EMTs). The system predicts hospitals with high acceptance likelihood using machine learning models trained on real ambulance operation records collected through a general-purpose Automatic Vehicle Monitoring (AVM) system. Because AVM is widely deployable, the approach can be applied to other regions with minimal modification. Evaluation with real operational data from Nagoya demonstrates that the proposed system improves decision consistency and delays by recommending hospitals that are more likely to accept incoming patients.

With the global proliferation of autonomous driving technology, the advancement of sensor technology is essential to ensure its safety. In particular, sensor fusion plays a crucial role in autonomous driving systems. However, equipping vehicles with multiple sensors leads to increased costs, making it necessary to have a single sensor that can perform multiple roles. This study focuses on event cameras, exploring their potential among various sensor technologies. Event cameras possess characteristics such as high dynamic range, low latency, and high temporal resolution, and they can also leverage visible light communication. This enables high visibility in low-light and backlit environments, as well as excellent performance in detecting pedestrian movements and acquiring traffic information between traffic lights and vehicles. These characteristics are particularly beneficial for autonomous driving systems. Additionally, if distance estimation functionality can be provided by the event camera, it allows a single sensor to perform multiple roles, offering significant advantages in terms of cost efficiency. In this study, we achieved distance estimation based on triangulation using an event camera and two points on an LED bar installed along a road. Furthermore, by employing the phase-only correlation method, we achieved sub-pixel precision in estimating the distance between two points on the LED bar, enabling even more accurate distance estimation. This approach performed monocular distance estimation in outdoor driving environments at distances ranging from 20 to 60 meters, achieving a success rate of over 90 % with errors of less than 0.5 meters. We are considering implementing position estimation in the future, with the current distance estimation technology forming the foundation for this. By achieving high-precision distance estimation, the vehicle's position relative to surrounding ITS smart poles can be accurately determined, enabling more precise position estimation. This will allow autonomous vehicles to know their exact position in real-time and select the optimal driving route based on surrounding traffic conditions and road conditions. Ultimately, we believe that this technology can contribute to the development of a smart transportation system in the city.

In the fields of Advanced Driver Assistance Systems (ADAS) and Autonomous Driving (AD), sensors that serve as the “eyes” for sensing the vehicle's surrounding environment are essential. Traditionally, image sensors and LiDAR have played this role. However, a new type of vision sensor, event cameras, has recently attracted attention. Event cameras respond to changes in the surrounding environment (e.g., motion), exhibit strong robustness against motion blur, and perform well in high dynamic range environments, which are desirable in robotics applications. Furthermore, the asynchronous and low-latency principles of data acquisition make event cameras suitable for optical communication. By adding communication functionality to event cameras, it becomes possible to utilize I2V communication to immediately share information about forward collisions, sudden braking, and road conditions, thereby contributing to hazard avoidance. Additionally, receiving information such as signal timing and traffic volume enables speed adjustment and optimal route selection, facilitating more efficient driving. In this study, we construct a vehicle visible light communication system where event cameras are receivers, and multiple LEDs are transmitters. In driving scenes, the system tracks the transmitter positions and separates densely packed LED light sources using pilot sequences based on Walsh-Hadamard codes. As a result, outdoor vehicle experiments demonstrate error-free communication under conditions where the transmitter-receiver distance was within 40 meters and the vehicle's driving speed was 30 km/h (8.3 m/s).

This study aims to enhance flexibility in the communication range of rolling shutter based visible light communication (RS-VLC). One of the main challenges in RS-VLC is the trade-off between the data rate and communication distance. To address this issue, we proposed a method that integrates superimposed iterative transmission with parallel delay transmission. Superimposed iterative transmission overlays two datasets, one optimized for short-range communication and the other for long-range. This strategy enhances communication over longer distances without significantly reducing the data rate at shorter ranges. Meanwhile, the parallel delay transmission sends signals with varying delays simultaneously across multiple LEDs, facilitating efficient transmission. By combining these two techniques, we aim to extend the communication range. Experimental results illustrate that the proposed approach enables data reception at long distances (up to 270cm) while significantly reducing data rate degradation at shorter distances. This effectively mitigates the trade-off between data rate and communication range. Additionally, over communication distances of 50 to 290cm, the average throughput of our method improved by approximately 73% compared to conventional methods.

In bit-interleaved coded modulation (BICM) systems, the conventional demodulation assumes equal a priori probabilities for all constellation points, inherently leading to performance degradation. To enhance the performance of BICM, BICM with iterative decoding (BICM-ID) was developed. We aim to apply neural networks to BICM to achieve joint modulation and decoding and overcome the performance degradation, with the goal of surpassing the performance of BICM-ID. In this paper, we propose a neural demodulator that incorporates an additional probability layer to mitigate performance degradation in BICM systems. Furthermore, we introduce Joint Model-1, which integrates this neural demodulator with a belief propagation (BP) decoder for 16-QAM polar-coded BICM. To further improve performance, we use Joint Model-1 as a pre-trained model and extend it by adding an additional dense layer, resulting in Joint Model-2. Experimental results show that Joint Model-2 surpasses both BICM and BICM-ID systems under Ungerboeck labeling.

Recently, physical key replications has become easier, highlighting the growing need for new authentication technologies. This study focused on optical physical unclonable functions (PUFs). Optical PUFs use physical irregularities to generate unique identification information for each device, thereby providing increased tampering resistance. Optical PUFs are highly compatible with optical wireless communication (OWC), and their combination contributes to enhanced communication security. However, conventional optical PUFs face challenges in terms of their implementation complexity and cost. In this paper, a simply implemented scratch-based optical PUF is proposed. In the proposed method, scratches are applied to a transparent optical medium to create unique light paths, thus enabling a simple and practical optical PUF that achieves both high resistance to forgery and ease of implementation. Authentication is performed by comparing light paths images captured by a camera with reference data. To evaluate the decision accuracy of the proposed method, a prototype was developed, and 1.4 × 106trials were conducted. Based on the results, the proposed method exhibited a false negative rate of 8.75 × 10−4and a false positive rate of zero within 300 significant digits, which exceeds the accuracy of typical biometric authentication systems.

This research addresses challenges in visible light communication (VLC) systems that use two orthogonally aligned rolling shutters (RS) image sensors as receivers and an LED array as a transmitter. The study focuses on overcoming burst signal loss caused by unsensed periods between frames in RS image sensors. To improve VLC performance, we propose and compare three different schemes. The first is a conventional approach that superimposes a Barker code synchronization signal on the transmission signal using pulse width modulation (PWM). The second introduces a new method of spatial synchronization by dedicating specific LEDs in an array for synchronization signals. The third scheme extends this spatial approach by incorporating a 4 -level PWM for information transmission to increase data rates. The study aims to evaluate and compare the error rate characteristics of these three schemes, assessing their effectiveness in mitigating burst errors and enhancing overall VLC system performance. This research has potential applications, including vehicle communication, Internet of Things devices, and smartphones.

This study aims to provide flexibility to changes in communication range in rolling shutter based visible light communication (RS-VLC). One of the issues in RS-VLC is the trade-off between the data rate and communication range. To resolve this issue, we proposed a superimposed iterative transmission in which two sets of data, namely, for short-and long-range data, are superimposed and then transmitted simultaneously. At short-ranges, the proposed method can decode both short- and long-range data, thus suppressing data rate degradation. As the communication range increases, the proposed method decodes long-range data (and half of the short-range data), allowing for data reception over longer ranges, albeit at a reduced data rate. The experimental results show that the proposed method achieved data reception at a longer range (up to 130 cm) with suppressing throughput degradation at short-ranges, i.e., it mitigated the trade-off between data rate and communication ranges.

This paper proposes a novel transmitter that rearranges the 16-QAM (Quadrature Amplitude Modulation) constellation for a 1-bit ADC (Analogue-to-Digital Converter) receiver. A 1-bit ADC receiver is more eco-friendly and lower-cost than receivers with a high-resolution ADC. However, in conventional 1-bit ADC receivers, severe degradation of SER (Symbol Error Rate) in high Es/N0poses a critical problem. This degradation is due to the lack of noise power in high Es/N0, which prevents the receiver from sufficiently benefiting from stochastic resonance. To solve this issue, we propose a constellation rearrangement scheme for 16-QAM based on the probability density function of noise. The constellation rearrangement scheme mitigates the nonlinearity of a 1-bit ADC, enabling reliable demodulation of multilevel signals in high Es/N0conditions. Numerical examples demonstrate that the proposed transmitter achieves superior SER performance across a wide range of Es/N0, even when a 1-bit ADC receiver is employed, compared to conventional transmitters.

We propose an estimation method of light emitting diode(LED) position with a cross filter (CF) for rolling-shutter (RS) camera. The CF is set in front of the camera lens so that incident light can be scattered into two straight lines extending ±π/4 from the horizontal row of the CMOS image sensor (IS). With the geometry of a right-angled isosceles triangle, our method makes it possible to estimate the coordinates of the LED in the IS plane when the scattered light is captured at two points in a scanline. Furthermore, our method makes it possible rows by rows when multiple LEDs are used or when they are moving. Without the CF, estimation is done only once per frame when the scan of the sequentially activated operation of the RS camera reaches a row that has just captured the whole image of the target LED, and the estimated coordinates are not updated in that frame. Experiments verify that our estimation method works with the estimation error less than 5pixels, and to extend the region of a frame in which estimated coordinates are output, even in a moving environment, while the conventional method (without CF) obtains the estimated coordinates only once when the scan reaches a row that captures entire LED.

This study demonstrates the communication performance of rolling shutter based visible light communication (RS-VLC) using dual camera receivers. In RS-VLC, the modulated signals emitted from the light source are captured as stripes based on the exposure timing difference for each line. Using this sequential exposure mechanism, a data rate higher than the frame rate can be achieved. To achieve higher data rates, we apply M-ary pulse amplitude modulation (M-PAM) to RS-VLC. The challenges for M-PAM in RS-VLC include limitations in the available grayscale range and non-uniform luminance distribution, both of which cause symbol errors. To solve this problem, we use two images in which the light emitting diodes (LEDs) are captured at different positions obtained by dual rolling shutter cameras. In this paper, we propose a method to combine two received signals, demodulate the signal transmitted by M-PAM in RS-VLC, and experimentally confirm the improvement in the symbol error rate (SER).

In recent years, neural network decoding of polar codes, such as neural belief propagation (BP), has been intro-duced. These methods use deep learning to transform the factor graph into a neural network model by unfolding the decoding iterations, thereby enhancing the accuracy of traditional decoding processes. However, current prevalent methodologies for loss function calculation only take into account the output of the final layer. In our analysis, we found that when calculating the loss function using only the output from the last layer, the convergence speed of the decoder significantly decreases, especially when the number of unfolded iterations is higher. In this paper, we incorporate the output of all iterations into the loss function in the original neural BP structure. Additionally, we optimize the loss function by assigning different weights to losses at different iterations. As a result, the weighted loss function not only provides a lower Bit Error Rate (BER) compared to the original neural BP decoder at lower SNR, but also accelerates convergence speed.

Using a rolling shutter camera as a receiver, visible light communication (VLC) can achieve a significantly higher sampling rate than the frame rate because of its sequential exposure mechanism. This study focuses on optical orthogonal frequency division multiplexing (OFDM) in a rolling shutter based VLC for further high-speed data rates. Because an image sensor is employed, conventional optical OFDM, such as DC-biased optical OFDM (DCO-OFDM), must fit in the grayscale range of 0–255 levels. Therefore, DCO-OFDM suffers from poor error performance owing to its high PAPR. To address this problem, we propose a polarity-separated transmission, in which the bipolar OFDM signal is separated into positive and negative parts and then transmitted by two LEDs simultaneously. Polarity-separated transmission is expected to double the grayscale range, i.e. 0–511, and improve error performance because it reconstructs the original bipolar signal by combining the received signals from two LEDs. We experimentally demonstrate and compare the symbol error rate (SER) performance of DCO-OFDM and polarity-separated transmissions. Consequently, polarity-separated transmission improved the SER performance by 10–2in the fundamental frequency range of 300–700 Hz when 16-PSK was adopted for subcarrier modulation.

This paper reports an extension of the private 5G service area to the outside of the licensed area by integrating visible light communication (VLC). As VLC is license-free, integrating the VLC function into the private 5G networks gives a more flexible system design. For example, it will be possible to provide services to mobile vehicles and robots working outside the service area of a private 5G network. Alternatively, private 5G networks in different locations can be connected. A challenge is integrating the VLC function into a 5G core network. We successfully designed a testbed and an experiment using open-source 5G projects to evaluate and compare methods to integrate VLC and private 5G, including the simulation of poor network environments. Experimental results demonstrate the availability of integrating private 5G with VLC, which are compared with image-based methods and show better latency performance.

This paper proposes a new estimation method of short-term power delay profile to reduce the noise power included in the short-term power delay profile. By reducing the noise, the multipath signal components buried in the noise can be utilized, thereby reducing the transmitted signal power and extending the cell range of a mobile communications. The proposed method iteratively creates the short-term power delay profile while lowering the threshold which distinguishes between signal and noise. The purpose of the threshold is to suppress the noise in each profile and to extract the effective multipath components. Furthermore, the method aligns and averages the phase of each instantaneous delay profile. In this way, only the noise component can be reduced while maintaining a high multipath power. As a result, we successfully reduced the noise power by 15 dB by analyzing the raw data of 200 instantaneous delay profiles measured in the 50 MHz bands at 3.35 GHz. Furthermore, the delay spread increased from 0.66 µs to 1.47 µs.

High-Altitude Platform Station (HAPS) has recently been attracting much attention as a new mobile communication platform for ultra-wide coverage areas and disaster-resilient networks since it can provide communication services from an altitude of approximately 20 km via a balloon, an Unmanned Aerial Vehicle (UAV) or other aircraft. In order to design efficient cell configurations for HAPS-based services, we need radio wave propagation models that consider various factors of vegetation, terrain, urban-suburban areas, and building entry loss. This paper focuses on propagation loss at high-elevation angles in urban and suburban areas. Recommendation ITU-R (International Telecommunication Union Radio communication sector) P.2108-1 describes the clutter loss model, which is the propagation loss caused by features such as buildings. The applicable frequency for ITU-R P.2108-1 is above 10 GHz; however, this model has no parameters for urban structure. Therefore, it needs to consider a correction for an urban structure to precisely evaluate clutter loss. We propose a new model that corrects Recommendation ITU-R P.2108 using shielding building heights as the urban structure. Since this new model was created based on measurements from 0.7 GHz to 5.7 GHz, the applicable frequencies are from 0.7 GHz to 5.7 GHz. Therefore, it is necessary to confirm whether the model applies to frequencies above 5.7 GHz. This paper compares the results of new propagation measurements at 29.3 GHz using a helicopter with the predicted results of the proposed model. The results show that the proposed model is applicable above 5.7 GHz.

This study focuses on visible light communication (VLC) using dual rolling shutter (RS) cameras. In VLC using an RS camera as a receiver, called rolling shutter based VLC (RS-VLC), the transmitted signal is projected as stripes on the image when the information is transmitted by an LED at a modulation speed higher than the frame rate. Therefore, the RS-VLC can improve the data rate. However, non-uniform luminance distribution on the image causes communication errors. Because LED light sources emit light radially, the luminance between the center and the edge of the LED in the captured image differs. Specifically, the received signal intensity is strong and good channel conditions can be obtained in the center of the LED, whereas the received signal intensity is weak at the edges, making it difficult to recognize changes in signal amplitude and prone to communication errors. To solve this problem, this study aims to enhance the communication error using dual camera images with misaligned LED positions and selecting the one with the better channel condition from the two received signals.

Luminance inversion (LI) is proposed to compensate for nonlinear characteristics between a liquid crystal display (LCD) and an image sensor (IS) camera of the parallel transmission (PT) visible light communication (VLC). In the PT system reported in [1], which incorporates N spatial frequencies with phase shift keying (PSK) through M-point inverse discrete Fourier transform (IDFT), data are transmitted from M different regions of pixels on an LCD spatially in parallel. Since the transfer characteristics between the LCD and IS camera is nonlinear, the received signal at IS camera receiver is distorted. The effect is severe because signals of the PT system are, in essence, the OFDM signal, thus suffering from high peak-to-average ratio (PAPR). To linearize it between the LCD and IS camera, our approach is to calculate a complement of the digital data fed to the driver of the pixel and then to feed the pair of the digital data and its complement to the driver during two successive frames. In the receiver, the difference of the signals during the two frames is calculated for each of the M detected regions on the IS plane, and then they input to the following M-point DFT in the PT system. By introducing this LI process, wider linear characteristics in the PT system is obtained than without LI. Experimental results show that the number of bits per frame that the PT system with LI can accommodate without any error is 315bits/frame and 100bits/frame for 8- and 16-PSK, which are much larger than 12bits/frame and 0bits/frame for the 8- and 16-PSK PT system without LI, respectively.

This study focuses on visible light communication (VLC) using a photodiode as a receiver. Accurate data transmission by this type of VLC is a challenge because the photodiode cannot detect weak light of subthreshold intensity owing to its insufficient light sensitivity. To overcome this problem, we employ a stochastic resonance, which is a nonlinear phenomenon in which the response characteristics of a system improve as its noise intensity increases, for the receiver of this VLC system. We also employ additive light-emitting diode light, which interferes in the photodiode as intentional noise. As an experimental result, the stochastic resonance was achieved by setting the parameters of the noise source appropriately, and we recovered the weak transmitting signal at the receiver.

Visible light communication based intelligent transportation systems (ITS-VLC) show great potential for future urban mobility. This study presents a performance evaluation of range estimation between vehicles and infrastructures in an ITS-VLC system. In the proposed ITS-VLC system, it is easy to simultaneously conduct communication and ranging using stereo cameras. However, the stereo camera calibration becomes a problem during simultaneous communication and ranging due to vehicle vibration. Using the data from LED transmitters and stereo cameras, it can obtain multiple measurements of distance. The monocular-stereo fusion algorithm is applied to visible light ranging in the proposed scheme using particle swarm optimization. We employed real data from the field trial experiment and achieved a ranging accuracy of 60 ± 1.0 m.

This paper presents an image sensor communication (ISC) using a digital micromirror device (DMD) projector as a transmitter. In particular, we focus on ISC for intelligent transport systems (ITSs) because DMD projectors are expected to be used in road traffic, such as vehicle headlights, street lights, and traffic signs. The DMD projector controls light patterns by switching tilt of micromirrors at high speed. Compared to the conventional LED array transmitter, DMD projector can design and alter the shape of the light pattern and data rate of transmission more easily. In the proposed system, the data rate can be easily increased by increasing the number of multiplexes. However, as the number of multiplexes is increased, the number of received pixels on the image sensor is decreased, and thus the performance of symbol detection deteriorates. Therefore, the purpose of this paper is to examine the relation between the number of received pixels per cell and communication performance for transmission pattern design. Hence, we experimentally clarify this relation using our prototype system via a DMD projector and a high-speed camera.

Multiple-input multiple-output (MIMO) communication systems using multiple low-Earth orbital (LEO) satellites achieve higher capacity than conventional LEO systems. However, in previous research, control signals are allocated to a different frequency band for each satellite signal in order to estimate its Doppler frequency. The increase in the number of satellites reduces unoccupied MIMO signal bandwidth and hence the overall capacity. This study aimed to prevent such capacity reduction by introducing the superimposition of control signals. Such control signals occupy a frequency bandwidth of the equivalent of only one control signal; therefore, they can prevent the reduction of the overall capacity. The main challenge is estimating the satellites' Doppler frequencies from the waveforms of Doppler-affected superimposed control signals. To overcome this challenge, we propose the adoption of a deep learning technique.

A nonlinear transform scheme is proposed to convert M discrete samples of parallel transmission (PT) system into the dynamic range between the transmitter and receiver for image-sensor-based (IS-based) visible light communication (VLC). In the PT system reported in [1], although the M discrete samples incorporating N spatial frequencies through M-point inverse discrete Fourier transform (IDFT) are transmitted by M spatially located transmitters. However, the signal of the PT system is, in essential, orthogonal frequency division multiplexing (OFMD) signal, thus suffering from the drawback of the multicarrier systems, i.e., high peak-to-average ratio (PAPR). With our scheme, the M discrete samples are transformed into the dynamic range between the liquid crystal display (LCD) transmitter and image-sensor (IS) camera receiver. Further, although the M discrete signals are expanded in transforming them, some of the expanded discrete samples are out of the dynamic range. Such out-of-the-range samples are truncated and replaced by the maximum or minimum value, i.e., 255 or 0. Our scheme is carried out for the PT system of the LCD transmitter and a 1000-fps high speed camera receiver which are at the distance of two meters. Experimental results show that our scheme enables the PT system to carry more spatial frequencies with achieving error-free transmission than without the scheme, thus increasing the number of bits per frame.

This paper focuses on high-speed stereo camera receivers for visible light communication (VLC) in intelligent transport systems (ITSs), contrary to conventional studies where a single (high-speed) camera is used. An advantage of high-speed stereo cameras as VLC receivers is that both front-facing ranging and wireless communication can be achieved in one device. In this paper, utilizing features of VLC, we consider stereo ranging with high accuracy. Moreover, it is known that an LED transmitter for VLC has specific features, high-luminance and high- speed blinking. This paper proposes stereo ranging by applying LED transmitter detection (utilizing these features) to disparity estimation. Additionally, we introduce subpixel estimation using equiangular line fitting to obtain more accurate disparity. Finally, we experimentally demonstrate that the proposed stereo ranging achieves an accuracy of 60 m ± 0.1 m.

This paper presents a capacity study of multi-input and multi-output (MIMO) low earth orbit (LEO) satellite communication systems. Because of their low propagation loss and low latency, LEO satellites are attracting the attention of researchers. MIMO transmission is possible for LEO terminals as they connect to many satellites, which offers higher capacities than current geostationary earth orbit (GEO) systems which offer connections to just single satellites. One difficulty is that LEO satellites experience Doppler frequency shift, which is a significant determiner of MIMO-LEO system performance. We take account of the Doppler frequency shift in deriving the capacity of MIMO-LEO satellite communication systems. Simulations show that the MIMO transmission by three satellites achieves the greatest improvement in system capacity of the conventional system if the bandwidth is limited to 10 MHz; a four-fold increase, on average, is seen for 12 GHz down link channels in the Ku band. Moreover, further capacity improvement can be achieved if the system bandwidth is sufficiently larger than the Doppler frequency.

In-vehicle cameras can be used as receivers for visible light communication in addition to their primary functions: for viewing assistance and object recognition. Most studies with this design perform tests via a single high-speed camera that is used as the receiver, but this study employs a receiver that uses a high- speed stereo camera. The reception method developed in this paper effectively utilizes left and right stereo images to improve communication performance. Although the selective diversity reception method proposed in previous work improved communication performance, it cannot deal with cases wherein both the left and right cameras introduce errors. To further improve the communication performance of this system, maximal ratio combining (MRC) diversity is applied to the high-speed stereo camera in this study. Communication performance is then evaluated comparing to the selective diversity reception and conventional single-camera reception by experimental tests conducted under static and driving conditions.

This paper proposes a new demodulation method using a one-bit analog-to-digital converter (ADC). With one bit resolution, a signal having a multilevel signal, such as a quadrature amplitude modulated (QAM) signal, is largely degraded by nonlinearity. To demodulate the signals, a high- resolution ADC is used to receive the signals with high linearity. This study sets out to demodulate the QAM signals using a one-bit ADC, whose resolution is the lowest (one-bit) and possesses outputs of binary (nonlinear) values. We propose to solve the issue by focusing on the statistical characteristics of the output signals of the one-bit ADC combined with noise. The proposed demodulation method is based on maximum a posteriori (MAP) criterion. The theoretical analysis presented illustrates that reliable demodulation is possible even with the one-bit ADC, provided the output probability of the ADC corresponds to each transmitted symbol is known.

The concept of visible light communications (VLC) originates from an Asian professor and adopts light emitting diodes (LEDs) as a transmitter. Three Asian born Nobel Prize winners developed blue LEDs, key devices that produce white lighting. Consequently, VLC and its other forms can be considered as an Asian born wireless technology. This paper overviews VLC with emphasis on image sensor communication (ISC) that uses an image sensor as a reception device for VLC signals. Some results from the author vis-a-vis ISC and its application to automobile are also introduced.

This paper proposes an N-stream superimposed system for image-sensor-based visible light communications (ISbased VLC). Based on the principle of m-point discrete Fourier transform (DFT), a phase shift keying (PSK) signal is transmitted over m consecutive time slots mT, where T is a slot duration. In practice, the symbol rate of this paper is R/(m + 1) because a guard slot is inserted every m slots. In the superimposed system, N data streams where the symbol rate of the nth data stream, n = {1, 2, . . . , N}, is 1/(m + 1)n-1times the symbol rate of the first data stream are combined to form a pulse-width modulated (PWM) wave with a period time T. An IS receiver with a camera operating at a frame rate of R/(m + 1)n-1fps demodulates data streams whose symbol rate is R/m + 1)n symbol/s or lower. Our experimental results validate that from a superimposed PWM wave of 500-symbol/s and 125-symbol/s data streams, a 2000-fps camera receiver demodulates both of the two data streams and the 500-fps camera receiver does the 125-symbol/s data stream successfully, without shortening individual error-free transmission ranges, compared with non-superimposed systems in which 500-symbol/s and 125-symbol/s data streams are independently transmitted by different light emitting diode (LED) transmitters and received by 2000-fps and 500-fps camera receivers, respectively.

Spatial modulation (SM) is introduced into layered space-time coding (L-STC) used in image sensor (IS)-based visible light communication (VLC) systems. STC was basically investigated for extending the communication range of the IS-based VLC link [1], although it is out of the range when IS receivers are at the long distance from the LED array of the transmitter where the number of pixels capturing the transmitter on the image plane is less than the number of LEDs of the array. Furthermore, L-STC was done in [2] for increasing the reception rate with improving the pixel resolution while the receiver was approaching the transmitter. In this paper, SM is combined into L-STC by mapping additional information bits on the location of the pair of STC bit matrices of each layer. Experimental results show that additional SM bits are extracted with no error, without deteriorating the reception quality of and shrinking the transmission range of the original L-STC.

Digital signage, which is advertising media using a liquid crystal display (LCD), is increasing in popularity. One possible application is wireless communications between digital signage and viewers. We focus on image-sensor-based visible light communication (VLC). Image-sensor-based VLC requires only an image sensor (IS), which is common equipment in smart phones. In the proposed method, an information signal is displayed on LCD. However, embedding the information signal into an advertisement may decrease the quality. We propose blue color difference modulation to make such signals imperceptible. We implemented a VLC system between digital signage and IS to demonstrate, the effectiveness of the proposed modulation in a real environment.

This paper demonstrates the feasibility of layered space-time coding (STC) in an outdoor image-sensor-based (IS-based) visible light communication (VLC) system. We examined that for low-resolution IS-based VLC channel where intensity modulated signals from two different light emitting diodes (LEDs) are detected at one pixel on the image plane, STC allows to decouple them, thus receiving data with no errors. Consequently, STC offers extending the transmission distance. In the layered STC presented here, additional bit streams are aligned in the 2n×2nLED array for increasing the transmission rate per symbol duration for the case where the spatial resolution is improved. A prototype of three layered STC is built with an 8×8LED array, where each of the LEDs is modulated at 1 kbps and a high-speed camera with IS operating at 1000 fps. Our experimental results validate that additional bit streams (layer-2 and -3), aligned in the layer-1 STC matrix pair, are extracted with no errors when the receiver comes within 155m and 55 m, respectively, from the LED array, without decreasing 210m of the transmission distance of layer-1 bit stream.

Channel modeling is critical for the design and performance evaluation of visible light communication (VLC). Although a considerable amount of research has focused on indoor VLC systems using single-element photodiodes, there remains a need for channel modeling of VLC systems for outdoor mobile environments. In this paper, we describe and provide results for modeling image sensor based VLC for automotive applications. In particular, we examine the channel model for mobile movements in the image plane as well as channel decay according to the distance between the transmitter and the receiver. Optical flow measurements were conducted for three VLC situations for automotive use: infrastructure to vehicle VLC (I2V-VLC); vehicle to infrastructure VLC (V2I-VLC); and vehicle to vehicle VLC (V2V-VLC). We describe vehicle motion by optical flow with subpixel accuracy using phase-only correlation (POC) analysis and show that a single-pinhole camera model successfully describes these three VLC cases. In addition, the luminance of the central pixel from the projected LED area versus the distance between the LED and the camera was measured. Our key findings are twofold. First, a single-pinhole camera model can be applied to vehicle motion modeling of a I2V-VLC, V2I-VLC, and V2V-VLC. Second, the DC gain at a pixel remains constant as long as the projected image of the transmitter LED occupies several pixels. In other words, if we choose a pixel with highest luminance among the projected image of transmitter LED, the value remains constant, and the signal-to-noise ratio does not change according to the distance.

The present paper proposes a novel multiple-LED-arrays acquisition for an infrastructure-to-vehicle visible light communication (I2V-VLC) using LED arrays (transmitter) and an in-vehicle high-speed image sensor (receiver). In order to achieve a robust detection of LED arrays, we employ the block matching algorithm, which is a way of finding a corresponding position between two successive frames. The proposed method divides a captured image into a number of small domains (blocks) and determines if the LED array is present or absent using the block matching. We perform I2V-VLC experiments with multiple-LED arrays and evaluate the acquisition capability of the proposed method.

This paper proposes Alamouti-type space time (ST) coding to visible light communication (VLC) using light emitting diodes (LEDs) and image sensor-based direct detection with a high-speed camera. Although multiple LEDs of an array are used as transmitters to communicate to individual pixel elements of an image sensor which act as multiple receive elements, the received signal is essentially the image of the transmitting element, whose size in pixel reduces with distance. When the receiver captures the LED array at a distance where images of two neighboring LEDs in the array are not separated at the image plane, such distortion depreciates the symbol decision quality. Our proposed scheme aims at expanding the communication range limited by the pixel resolution of image sensor. Our implemented prototype demonstrates that ST coding is feasible to achieve 48-meter error-free transmission range while the repetition transmission achieves 26-m error-free range.

The objective of this research is to reduce intersymbol interference (ISI) and intercarrier interference (ICI) in a highly mobile environment by optimizing the performance of Biorthogonal Frequency Division Multiplexing (BFDM). In time-frequency dispersive channels, reducing the signal interference depends on the shaping of the signal pulse. BFDM was invented to reduce the interference through a flexible design of the signal pulse shaping. However, the shape of the Gaussian pulses used in BFDM is not always optimized in every situation and as a result the interference is not always minimized. This research seeks to optimize the shape of the Gaussian pulse. In particular, we optimize the Gaussian pulse in BFDM by maximizing the ratio of the ambiguity function of the transmitter (Gaussian) and its receiver pulses. Numerical results obtained show that the proposed method is effective for the optimization of BFDM. Furthermore, we show that the optimized BFDM outperforms the conventional OFDM in time-frequency dispersive channels.

In this paper, we focus on a road-to-vehicle visible light communication (VLC) system using LED traffic lights. In this system, an LED traffic light consists of a 2-dimensional LED array (2D LED array), and cars are equipped with high-speed 2-dimensional cameras (2D image sensors). An important issue of this system is frame loss. Sometimes, 2D image sensor in a car fails to get a frame. So as to mitigate the influence of frame loss, we propose to apply erasure coding to the road-to-vehicle VLC system. In the proposed system, a data sequence is encoded by LDPC code whose length is much longer than the size of 2D LED array. In addition, code synchronization is required for the proposed system. We also propose a code synchronization scheme, which makes use of error detection capability of LDPC code. We evaluate the performance of our proposed system, and show that it can recover frame loss for high SNR when less than 8 frames among total 18 frames are lost. In addition, we show that our proposed code synchronization scheme can ignore its errors.

Our goal is the improvement of the node activity rate in solar-powered wireless sensor networks. We consider the application of a cooperative transmission scheme that uses a relay node when a node sends data to reduce the consumption of energy of the entire network. However, if we do not consider the variations in energy harvesting, the node activity rate declines during nighttime. In this paper, we smooth out the day-and-night node activity rate variations by introducing the new inactive node reactivation condition and achieve the required high node activity rate.

In this paper, we propose two dynamic bandwidth allocation (DBA) schemes, a game theory based DBA and a successive DBA, for satellite/terrestrial integrated mobile communication system (STICS). The game theory based DBA allocate the bandwidth according to a utility function based on a weighted demand bandwidth of both satellite and terrestrial system. The successive DBA recursively allocates an unit of bandwidth according to a weighted number of bandwidth unfilled cells of both satellite and terrestrial systems, until the total bandwidth is fulfilled. As results, both DBA outperform the conventional fixed bandwidth allocation scheme. Between two proposed DBA, the successive DBA shows better spectral improvement ratio than the game theory based DBA.

Ultra-Wideband is an attractive technology for short range positioning, especially indoors. However, for normal Time of Arrival (TOA) positioning, at least three receivers with unblocked direct path to the transmitter are required. A requirement that is not always met. In our previous work we presented a novel 3D TOA UWB indoor positioning method that uses only two receivers. This is possible by exploiting the knowledge of some of the indoor features, namely ceiling and walls. In this work, we verify the usability of the method using measurement results in three environments: a lecture room, a cluttered laboratory and a corridor. By comparing the results of the method for three environments, we draw conclusions about its strengths and weaknesses.

This paper considers a novel error-correcting scheme exploiting chaotic dynamics for noncoherent chaos communication. In our proposed system, two successive chaotic sequences are generated from the same chaotic map; the second sequence is generated with an initial value which is the last value of the first sequence. In this case, successive chaotic sequences having the same chaotic dynamics are created. This feature gives the receiver additional information to correctly recover the information data and thus improves the bit error performance of the receiver. Further, enhanced efficiency also comes from operating on successively modulated data; by involving less redundancy in the error correction system, it can be designed with high coding rate. In this paper, we analyze the scheme's capability, by examining computational times and accuracy rates of error correction, bounds on its capability.

Pulse-shaping OFDM, or more generally BFDM and BFDM/OQAM, systems are originally proposed to combat intercarrier interference (ICI) caused by time-variance of the channel. The main property that makes these signals robust against ICI is that they allow to employ time-frequency well localized pulses, particularly Gaussian pulse which has the best time-frequency localization. However, this property also suppresses the outband or inband spectral leakage of the BFDM and BFDM/OQAM signal, making them attractive physical layer modulation scheme for cognitive radio. In this paper we analyze the power density spectra of the BFDM and BFDM/OQAM signals and numerically evaluate the inband and outband spectra for truncated Gaussian pulses.

As one of ITS technique, a new visible light road-to-vehicle communication system at intersections is proposed. In this system, the communication between a vehicle and an LED traffic light is conducted using an LED traffic light as a transmitter, and an on-vehicle high-speed camera as a receiver. The LEDs in the transmitter emit light in high frequency and those emitting LEDs are captured by the high-speed camera for making communication. Here, the luminance value of LEDs in the transmitter should be captured in consecutive frames to achieve effective communication. For this purpose, first the transmitter should be found, then it should be tracked in consecutive frames by processing the images from the high-speed camera. In this paper, we propose new effective algorithms for finding and tracking the transmitter, which result in a increased communication speed, compared to the previous methods. Experiments using appropriate images showed the effectiveness of the proposals.

In this paper, we consider visible light communication systems using LED array as a transmitter and high-speed camera as a receiver for Intelligent Transport System (ITS). Previously, we have proposed the hierarchical coding scheme which allocates data to spatial frequency components of the image depending on the priority. This scheme is possible to receive information of the high-priority even if communication distance is long. However, we need to distinguish multi-valued data from the received image by using a hierarchical coding. In this paper, we propose two improved decoding methods, and demonstrate to distinguish multi-valued data more correctly in the experiment.

Robust channel estimation scheme is essential for pulse-shaping OFDM systems in the multipath mobile environment. This paper proposes three types of channel estimation schemes for the general class of pulse-shaping OFDM systems. The first two types are suboptimal low-complexity maximum likelihood estimators. The last type is adaptive Kalman filter channel estimator. We numerically evaluate the performance of each estimator using computer simulation.

This paper proposes the error-correcting scheme without redundancy sequences based on the chaotic dynamics for noncoherent chaos communications. We generate successive chaotic sequences from the identical chaotic map. And for the next sequence we set the initial value to the end value of the former sequence. By such way we can create the successive chaotic sequences having the same chaotic dynamics. This feature gives the receiver additional information to correctly recover the received noisy signal. Therefore, by analyzing the chaotic dynamics at the receiver, it is possible to improve the error performance without redundancy signal. As results of computer simulations, we confirm about 3 dB gain in BER performance as compared with the conventional suboptimal receiver when using the short chaotic sequence length per 1 bit.

This manuscript proposes a cognitive radio system that shares a frequency with an existing primary system. In the proposed method, a secondary transmitter relays the primary signal and piggybacks its own data on it. Symbol error probabilities of the system are derived analytically. The result shows that the proposed system can communicate at a relatively high speed on the same frequency of the primary system without harm to it.

In this paper, we propose a new SC/MMSE Turbo equalizer for MC-CDMA to use in inter-vehicle communication (IVC). We modify the weight of the MMSE filter of SC/MMSE Turbo equalizer so that it can combat with frequency-selective fading in highly mobile IVC environment and also can mitigate the effect of multiple access interferences (MAI). As results, we show that better throughput is obtained by the proposed SC/MMSE equalization than the conventional one. The average reception range can be up to 70 [m] and the signals from 16 surrounding cars can be received. While for the conventional scheme, only three signals from the nearest cars are achieved.

We propose a contention-free medium access control (MAC) protocol for inter-vehicle communication systems that share information for the improvement of safety drive on the highway. In the proposed system, contention-free medium access is achieved by periodical one-way token transmissions. The token contains the frame information about the transmission/reception phase. While the token is relaid by one-way, the vehicle which has the token can send a packet to both up- and down-link. The token transmission interval is set not to be interfered from other transmit vehicles at a receive vehicle. In addition, multiple channels are used and allocated dynamically for every token transmission to reduce the transmission waiting time. We also evaluate the maximum end-to-end transmission delay of the proposed protocol, and clarify that the proposed protocol can guarantee the transmission within allowable delay time.

In this manuscript, a decode-and-forward cooperative diversity scheme for wireless packet transmission network with large and undetermined set of relay terminals is considered. Correctly received signals at some relay terminals are retransmitted to the receiver with the STBC-based cooperative relaying scheme. For large and undetermined set of relay terminals, performance of the conventional STBC-based cooperative diversity is known to be degraded due to the loss of the diversity gain and increasement of decoding complexity. In this manuscript, we propose a signaling scheme to reduce the performance loss. Our proposed scheme is based on the linear transformation of the STBC matrix by a signature vector, which has more degrees of freedom for the assignment of signals compared to the conventional STBC-based cooperative diversity.

LEDs are expected as lighting sources for next generation, and data transmission system using LEDs attract attention. In this paper, we present hierarchical coding scheme using LED traffic lights and high-speed camera for intelligent transport systems (ITS) application. Further, if each of LEDs in traffic lights is individually modulated, parallel data transmissions are possible using a camera as a reception device. Such parallel LED-camera channel can be modeled as spatial low-pass filtered channel of which the cut-off frequency varies according to the distance. To overcome, we propose hierarchical coding scheme based on 2D fast Haar wavelet transform. As results, the proposed hierarchical transmission schemes outperform the conventional on-off keying and the reception of high priority data is guaranteed even LED-camera distance is further.

For safety applications, automotive radar networks show an interesting potential. Apart from long range radars, networked short range radar systems are under development. Additional applications can be accomplished, such includes precrash detection, parking aid and blind spot surveillance. We research the automotive radar networks using short range radars. The network is built with multiple radars. In this paper, we consider data processing using measured ranges provided by the radars in order to estimate multiple target positions with high accuracy. Firstly, we derive a method based on MAP estimation. Secondly, we propose a method which reduces the calculation complexity compared to the above MAP method. Finally, we introduce a method which can detect targets with low undetected targets. The case often happens when the measured ranges are lost because of miss-detections. We evaluate the performance with computer simulations.

In the multihop cellular networks, multiple routes from a user station to a base station can be established because of the flexibility of network topology. In order to reduce packet errors of wireless transmissions, we proposed the multi-route coding scheme, in which packets are encoded and divided into subpackets, and they are transmitted on several routes from the user station to the base station. In order to improve the performance of the multi-route coding, a route selection scheme is very important issue. In this paper, we propose the route selection scheme for the multi-route coding in multihop cellular networks. The proposed scheme is based on the bit error rate of wireless links. We also apply the proposed route selection scheme to a hybrid routing protocol and evaluate its performance.

In sensor networks, because of densely deployment of sensor nodes, sensing area of each sensor node is overlapped with each other and its observed data sequence is correlated. The joint decoding scheme that uses the correlation between the observed data sequences can reduce bit errors in the wireless channel. In this paper, we extend the joint decoding to the case of more than two-hop transmission. For such extension, selection of fusion scheme is very important issue. If the global fusion scheme is employed and the observed data sequence of each sensor node is transmitted on an independent route, route diversity effect could be obtained by using the correlation between observed data sequences. We compare the bit error rate performance of local and global fusion schemes and clarify the route diversity effect

Pulse-shaping OFDM is well-known that it performs well in a mobile environment comparing with conventional OFDM. However, in highly mobile environment intersymbol and intercarrier interferences (ISI/ICIs) increase and can no longer be neglected. These ISI/ICIs deteriorate the performance of the systems. Proper channel equalization is needed for further improvement of the systems. In this paper, more general case, namely biorthogonal frequency division system based on offset QAM (BFDM/OQAM) is considered. We propose a multi-input multi-output (MIMO) transversal filter to equalize the time- frequency dispersive channel. Tap weights are calculated using the zero-forcing (ZF) algorithm. We also propose maximum- likelihood channel estimator and its low-complexity version. The proposed system can significantly improve the performance of BFDM/OQAM systems in the highly mobile environment.

This paper examines multihop cellular networks. There are some issues in these networks, such as route loss due to topological change and packet errors due to wireless nodes relay. To overcome these issues, we use a multiple route coding scheme in which packets are transmitted on multiple routes from a user node to multiple base stations. This scheme can achieve a diversity gain by combining packets that are transmitted along multiple routes. Some challenges must be met to establish multiple routes for this scheme. In this paper, we propose a routing protocol for multiple route coding. The key idea of this proposed scheme is using a hybrid routing protocol. This can mitigate an increase in control packets and multiple disjoint routes can be constructed. We evaluate the performance of this proposed scheme

For reduction in bit errors on wireless channels, we proposed the multi-route coding scheme on multiple routes for wireless multi-hop networks. In this paper, we introduce ARQ to the multi-route coding. In the multi-route coding, the destination node combines and decodes sub packets which are encoded and divided by the source node. The intermediate node relays a sub packet, that is, only a part of a packet. Therefore, the intermediate node cannot detect packet errors, and only the destination node can do after combining and decoding sub packets. We propose the ARQ scheme between the source node and the destination node. We analyze the proposed ARQ scheme and evaluate the throughput performance

This manuscript introduces a mathematically tractable and accurate model of narrowband power line noise based on experimental measurements. In this paper, the noise is expressed as a Gaussian process whose instantaneous variance is a periodic time function. With this assumption and representation, the cyclostationary features of power line noise can be described in close form. The periodic function that represents the variance is then approximated with a small number of parameters. The noise waveform generated with this model shows good agreement with that of actually measured noise. Noise waveforms generated by different models are also compared with that of the proposed model.

In this manuscript, wireless communication system for reliable wireless control of industrial machines in indoor environment is considered. In indoor environment, transmitted signal fluctuates due to fading and shadowing. To combat the fading and shadowing, macro/micro spatial diversity can be used as an effective countermeasure. In this manuscript, macro/micro diversity is achieved by multiple wireless relay terminals, which is convenient in setting and moving the macro diversity branches. In the proposed scheme, channel between the transmitter and the receiver through relay terminals is considered as a MIMO channel and separation of the signals re-transmitted from the relay terminals is not required. This feature makes it possible to have higher order diversity without the cost of bandwidth inefficiency. In numerical examples, the performance of the proposed scheme is evaluated by average frame-error rate and outage probability of frame-error rate. With two relay terminals, the diversity gain of the proposed scheme is about 5-6 dB, which is 1 dB higher than that of conventional macroscopic diversity

In real-time packet communications, packet retransmission is generally not used because it may produce undesired delays. In this case, however, packet errors caused by a channel may lead directly to a loss of transmitted information. Then it is necessary to employ a technique which can reduce the influence of packet errors. We propose to employ a multiple-route packet combining scheme in realtime communications on wireless multihop networks. The scheme, which we proposed previously (Nakagawa, N. et al., IEICE Commun. Soc. Conf., vol.B-5-119, p.496, 2003), can achieve diversity gain by combining multiple copies of the same packet that are transmitted along different routes. We analyze two characteristics of the proposed system. One is the average packet error probability, which is obtained as a function of the elapsed time after generating copied packet transmission requests. The other is the required time for achieving a required packet error probability. The results are evaluated from a quality of service (QoS) point of view. From numerical results, it is shown that for a given tolerable delay, there exists a number of multiple routes that minimizes the achievable average packet error probability. Moreover, if the number of multiple routes is selected appropriately, the required time for achieving a required packet error probability can be minimized.

In multimedia wireless communications, various types of traffic are handled. Generally, each type of this traffic has a different stochastic property. An access control protocol for multimedia wireless communications, therefore, are required to adopt a stochastic property of multimedia traffic. Conventional access control protocols, however, do not consider such characteristics of multimedia traffic. In this paper, we propose an access control protocol for multimedia CDMA packet communications. The proposed protocol controls the data packet transmissions based on the estimation of the instantaneous number of simultaneously transmitted voice and video packets. This estimation exploits the stochastic property of multimedia traffic. To carry out such estimation, we employ an adaptive algorithm as an estimation method. We evaluate the performance of the proposed protocol by comparing it with the performance in the case where the stochastic property of traffic is not considered.

A CDMA unslotted ALOHA system is a connectionless-type of CDMA packet communication system. In this system, a user station can transmit a packet asynchronously and randomly, and so the packet birth/death event is one of the most important problems for multiuser detection. We have proposed the CDMA unslotted ALOHA system using an adaptive filter receiver based on minimum mean square error criterion of Okada, Grant, Band and Ogawa (see IEEE International Symposium on Personal, Indoor and Mobile Radio Communication (PIMRC), Sep. 2000), and shown the improvement in the throughput performance even considering the effect of birth/death event. The ensemble-averaged squared error, however, increases at the point of packet birth. In this paper, we employ interleaving and forward error correction (FEC) coding techniques to mitigate the momentary increase in ensemble-averaged squared error. The use of FEC, however, causes the increase in MAI due to redundancy bits of FEC. Moreover, signal power is reduced under the condition that the energy of an information bit is the same. In such cases, whether an adaptive filter can operate effectively or not interests us. We evaluate the system performance and show that the improvement in throughput is achieved with interleaving and FEC techniques.

In this paper, we propose a new packet recognition and signal acquisition scheme with an adaptive linear filter based on a least mean square (LMS) algorithm for code-division multiple access (CDMA) packet systems. In CDMA systems, conventional receivers with matched filtering suffer from the near-far problem due to multiple access interference (MAI). In general, closed-loop power control is used in order to reduce the influence of the near-far problem. However, closed-loop power control is unsuitable for packet systems. Hence, in order to eliminate the influence of the near-far problem, we focus on an adaptive receiver based on an LMS algorithm for packet recognition and signal acquisition in CDMA packet systems.

Packet detection is one of the most important problems in packet communication systems. In a CDMA Unslotted ALOHA system, which is one of the wireless packet communication systems, multiple access interference (MAI) makes the performance of the packet detection worse. To reduce the effect of MAI, we propose a new packet detection scheme in this paper. In this scheme, Successive Interference Cancellation(SIC) is applied. The packet's signal is detected after the cancellation of MAI using SIC. This proposed scheme gives good performance of the packet detection.

We compare CDMA and FDMA (or TDMA) schemes in multibeam non-geostationary satellite systems, taking the effects of antenna patterns on inter-cell interference into consideration. The maximum acceptable number of users per cell in the uplink is employed as a measure of the system capacity. We have found that in FDMA (or TDMA) systems the maximum acceptable number of users varies according to the altitude of the satellites. While the performance is insensitive to the altitude in CDMA systems. For this reason, the altitude of the satellites is found to be an important factor for selecting multiple access schemes in non-geostationary satellite systems.

In wireless communication systems, as one of the effective techniques for combating fading, transmit diversity has attracted much attention, especially when receive diversity is expensive or impractical due to the constraint of terminal size. In this study we consider an adaptive carrier-by-carrier basis antenna selection scheme for transmit diversity in OFDM systems under the assumption that a transmitter has priori knowledge of the channel state information (CSI). In the proposed schemes transmit antennas are selected adaptively for each subcarrier according to CSI.

A modified version of the SAGE algorithm applied for joint estimation of delays, incidence azimuths and attenuation parameters characterizing the channel is presented. The modifications are based on using different observation intervals in optimizing the parameters and using time correlation in estimating delays, for the purpose of a further reduction in SAGE's computational weight, and improving its accuracy in delay estimation. We show analytically that short observation intervals are sufficient for optimizing the delays and the azimuths, and verify through simulation the reliability of using time correlation. Two other estimators are proposed combining the modified SAGE with a sequential estimation of the attenuation parameters suitable for slowly varying channels.

In an attempt to improve the performance under a frequency selective fading environment, we propose an orthogonal frequency division multiple access (OFDM) system in which MTCM (multiple trellis coded modulation) is applied to a set of multiple subcarriers. The code is designed to enlarge the summation of the Euclidean distance and the product distance. We also propose a scheme for assigning the symbols to subcarriers adaptively based on the channel state information (CSI) fed back from the reception receive end, and evaluate the performance applying these two codes and the proposed scheme under a multipath Rayleigh fading environment.

This paper proposes a transmission power control scheme for downlinks in CDMA/shared-TDD cellular packet systems that experience inter-cell interference between uplinks and downlinks. The proposed scheme increases the downlink transmission power for re-transmission packets and thus improves the signal-to-interference ratio for the packets. Simulation results show that the proposed scheme improves the downlink throughput without sacrificing the uplink throughput.

This paper examines the inter-cell interference between the uplinks and downlinks when a shared-TDD scheme is applied to CDMA packet communication systems in cellular environments. The CDMA/shared-TDD cellular packet systems experience two additional types of interference: one is the interference between base stations and the other is between mobile stations. The effect of these interferences on the throughput performance is evaluated. Computer simulation demonstrated that interference between base stations rarely degrades uplink throughput, but interference between mobile stations substantially degrades downlink throughput.

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/receiver antennas are combined at the acquisition circuit of the receiver. The performance is evaluated for indoor packet radio communication systems from the viewpoints of average time for acquisition, probability of success of acquisition, and necessary preamble length. As the result, we show great performance improvements by the proposed scheme under slow and flat Rayleigh fading environment.

In this paper, we propose a CDMA slotted-ALOHA (CDMA S-ALOHA) system with low-bit rate and high-bit-rate classes of data users applying the multi code CDMA (MC-CDMA) scheme. Performance evaluation is obtained in the respect of the throughput. Contrary to one might expect, we show that the throughput of the class with higher bit rate can be improved by the MC-CDMA scheme. Moreover, we show that applying the modified channel load sensing protocol (MCLSP) to the system gives further improvement in the throughput of the higher bit rate class, as well as to the total throughput of the channel.

In this paper, we analyze the CDMA slotted ALOHA systems with finite buffer and finite population assumptions by expanding the linear approximate solution based on restricted occupancy urn models. We evaluate the system performance in terms of throughput, average delay, and rejection probability. Moreover, we clarify the effect of buffer capacity by changing the buffer size.

In this paper, we discuss the performance of a system in which multiprocessing gain (MPG) spread spectrum signals are transmitted to a base station within the same bandwidth. At the receiver of the base station, an adaptive array antenna is employed. We evaluate the system performance in terms of signal to interference-and-noise ratio (SINR) and also of outage probability of low-rate users. We find that the employment of the adaptive array antenna may result in serious degradation of the outage probability of low-rate users. We then show that the cause of this serious degradation is the inadequate guidelines of conventional adaptive array antennas.

We consider a receiver with satellite diversity by using DS/SS signals from different LEO satellites. Since the signals from the different LEO satellites may have different frequency offsets due to the Doppler effect, we propose an acquisition scheme which can estimate the code timings and carrier frequencies of these signals at once. We evaluate the characteristics of the proposed scheme by the mean acquisition time.

In multipath fading channels, the direct-sequence spread spectrum (DS/SS) technique is often employed because of the capability to resolve signals from different paths and realize path diversity at the receiver. This advantage of DS/SS, however, is only when the time difference of propagation delay between paths is greater than a chip duration. In indoor communications, the delay spread is often very small and the above condition on the delay and chip duration is no longer satisfied, and this performance improvement by the path diversity with conventional DS/SS systems cannot be expected. In this paper we propose a new diversity technique for indoor wireless communication systems where delay spread is smaller than a chip duration. In this system, the transmitter sends signals at the same time from multiple antennas with intentional time delays, which allow a receiver to distinguish and combine the signals from different antennas.

We evaluate the bit-error rate performance of direct-sequence spread spectrum (DS/SS) system using /spl pi//4-shift QPSK and /spl pi//2-shift BPSK and observe the effect of the nonlinearity of the high power amplifier (HPA) of transmitters which occurs when the HPA is operated near its saturation region. The hard limiter is chosen as the example of the nonlinear amplifier and the numerical results are reported by using coherent as well as differential detection at the receiving end. With the employment of either /spl pi//4-shift QPSK or /spl pi//2-shift BPSK, the DS/SS signal is not much influenced by the nonlinearity of the HPA as the two modulation schemes have the advantage of nearly constant envelope.

In this paper, we present the throughput performance of CDMA slotted ALOHA systems with modified channel load sensing protocol (MCLSP). In MCLSP, a hub station continuously senses channel load for a certain number of slots, and computes the probability with which each user station should transmit its packet, then broadcasts the computed value. As the access control is based on this probability, we can apply it to slotted system. We show that MCLSP can achieve significant improvement in throughput performance of CDMA slotted ALOHA systems even in large offered load. One of the issues in such a protocol is the access timing delay, which is the sum of process time and propagation time for the packet access control. We also show the effect of the access timing delay on the throughput performance.

A transmit permission control method for improving the throughput characteristics of a low Earth orbit (LEO) satellite communication system employing spread-slotted ALOHA multiple-access scheme is proposed. Both nonfading and fading satellite links are considered. The basic idea of the proposed scheme is to decrease the level of interference at each satellite and, hence, to increase the probability of packet success, by prohibiting the packet transmission from the users with relatively high propagation loss to their connecting satellites. It is shown that the method has the ability to improve the throughput performance in heavy traffic loads and the peak value of the throughput, significantly. It is also shown that the average delay performance of the system employing the proposed scheme is superior to that of the conventional system at heavy traffic loads.

Throughput analysis of direct-sequence spread spectrum multiple access (DS/SSMA) unslotted ALOHA with fixed packet length is presented. As the levels of multi-user interference fluctuate during the packet transmission, we calculate the packet error probability and the throughput by considering not only the number of overlapped packets but also the amount of time overlap. On the assumption that packet generation is Poisson, the system can be thought as the queueing system M/D//spl infin/. With Gaussian approximation of multi-user interference, we obtain the throughput as the function of the number of chips in a bit, the packet length, and the offered load of the system. We also analyze the channel load sensing protocol (CLSP), and obtain the optimum threshold of CLSP.

We analyze a system which simultaneously transmits spread-spectrum signals with different processing gains. The main purpose of this study is to give an analytical framework that describes the influence of the interfering signals with different processing gains. The results show that the power of the interference component after the despreading procedure, the power ratio of the desired to undesired component, and the bit error rate are not constant but vary symbol by symbol. We also show that the processing gain of the interfering signal affects the performance.

We propose a coordinate-by-coordinate interleaved trellis coded QPSK with the T-algorithm. A rate 1/2 convolutional code with constraint lengths of 7 and 15 are evaluated on Rayleigh fading channels in terms of the bit error rate (BER) by an analysis and a computer simulation. As the effective code length (ECL) is equal to the code constraint length for the case of a QPSK system, the proposed system achieves a satisfactory BER performance even in the situation where no channel state information (CSI) is available at the decoder. Further, in order to reduce the decoding complexity, we employ the T-algorithm. It is shown that as the T-algorithm can track the fluctuating gains introduced by the fading channel, we can achieve a satisfactory BER performance with 0.01% of the decoding complexity of the Viterbi algorithm.

We present a new direct-sequence spread-spectrum demodulator using the block signal processing in the presence of large carrier frequency offset such as Doppler shift in low Earth-orbital satellites systems. The proposed demodulator employs the initial acquisition scheme which can roughly estimate both the timing and the carrier frequency offset. This scheme realizes the maximum likelihood estimation with the help of the FFT. As the result of the analytical discussion, it is shown that the proposed scheme makes the initial acquisition possible even in the presence of a large frequency offset where the conventional demodulator cannot achieve the acquisition.

In some recent studies, the use of low Earth-orbit satellites in various applications is considered. In all of these studies, uniform distribution of traffic load is assumed. In this paper, the performance of a low Earth-orbit satellite communication system which is designed to service to two kinds of users; i.e., voice users and data users is estimated. The distribution of population of these users is assumed to be nonuniform. According to the simulation results, it is shown that the nonuniformity in traffic affects the performance of the system by decreasing the signal quality at the satellite which has to service to populated areas and increasing it superfluously at the satellite with not so populated service area, in a given period of time. By modeling the satellites during their movements, the change in signal quality while experiencing a peak of traffic load in their route is also determined. A modified power control method based on the amount of traffic load of each satellite is also examined and is shown that this method can make some performance improvements in signal quality, which is limited by special features of low Earth-orbit satellite systems.>

This paper investigates the bit error rate (BER) performance of hybrid direct-sequence/slow-frequency-hopping spread-spectrum multiple-access (DS/SFH-SSMA) operating in the environment with the near-far problem. Rate 1/2 convolutional code of constraint length 7 is applied with an appropriate interleaver to increase the diversity effect not only on time domain but also on frequency domain. As a result, we found that the BER performance of DS/SFH system was always satisfactory in the presence of near-far problem. On the other hand, poor BER performance of the direct-sequence system is found regardless of E/sub b//N/sub 0/ or number of users.

This paper presents a novel spread slotted Aloha system with channel load sensing protocol (CLSP). CLSP is an effective scheme to improve the throughput performance in spread unslotted Aloha systems. In spread slotted Aloha systems, however, it does not make sense to utilize CLSP because the slot size is usually the same as the packet size. The slot size of the proposed system is set less than the packet size, thereby enabling the authors to apply CLSP and improving the throughput performance. Another feature of the proposed system is that the system is not likely affected by the time difference between channel load sensing and timing of packet access, which they call the access timing delay. Throughput performance of the proposed system is evaluated in the presence of the access timing delay and a significant increase of the throughput is shown compared with that of spread unslotted Aloha with CLSP.>