This paper proposes an antenna directivity control method of adaptive array antenna for secure wireless transmission by dividing the transmitted information into pieces of divided-information and sending each pieces of information in a spatially distributed manner along multiple paths in a wireless space. Based on the DCMP weight control algorithm, the authors propose an antenna directivity control method not only to steer its main lobe to one targeted path but also to suppress other side lobes to suppress unnecessary signal radiation. Computer simulation confirms that by suppressing the side lobes of the antenna directivity, the transmitted signal power can be concentrated along the targeted path. It is effective to provide multiple transmission routes for signal delivery in the wireless space. Therefore better performance in regard with the secrecy of the transmitted information can be achieved in the wireless space by sending the transmitted information in a spatially distributed manner along the selected paths between the sender and the destination.

IEEE802.11DCF is widely used wireless LAN standard. But, packet delivery ratio of broadcast transmission is much lower than unicast transmission because the IEEE802.11DCF executes broadcast transmission without RTS/CTS exchange and ACK process. Hence, many protocols have been proposed to improve reliability of the broadcast transmission. Most of these protocols are proposed based on collision recovery policy using ACK signal or NACK signal instead of ACK frame. Then, transmission overhead is increased by retransmission under the condition that network contains some hidden terminals since ACK or NACK signal could not prevent transmission of hidden terminals. This paper proposes a new broadcast protocol based on collision prevention policy using modified RTS/CTS exchange, and clarifies that our proposed protocol achieved highly reliability with computer simulations.

This paper investigates the minimum number of antenna elements of an adaptive array antenna (AAA) required to suppress unnecessary radiation to a single location in an indoor ubiquitous communication environment. In the investigation the relationship between the number of antenna elements of an AAA and its digitally-created antenna null width is demonstrated using antenna directivity control technique that has been proposed by the authors. Also, the angular spread around a terminal to a single location in an indoor communication environment is demonstrated with propagation measurements. Based on the computer simulation results and the angular spread obtained by propagation measurements, the authors specify that at least 6 antenna elements are required to suppress radiation to a single location in an indoor communication environment up to -60 dB.

This paper propose an unnecessary radiation power suppressing technique to electromagnetically restricted areas in ubiquitous communication environments. An adaptive array antenna is employed in each terminal to control a directivity of transmit antenna not to radiate unnecessary power to the electromagnetically restricted areas while keeping sufficient gain to a target terminal. To identify angle of departures to the electromagnetically restricted areas, a radio power sensor network is assumed. Moreover, channel sounding based delay profile estimation is conducted to identify the strongest path direction for the desired signal. Based on the identified strongest path for the desired signal and the direction of the unwanted radiation detected using the radio power sensor network, artificial delay profile for a transmitted signal is created, and antenna directivity for the transmitted signal is created using a deep and wide null creation technique based on the artificial delay profile. Computer simulation confirms that the unnecessary radiation to the electromagnetically restricted areas is sufficiently suppressed while keeping throughput at a satisfactory level.

This paper proposes a channel estimation technique for multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM)/time division multiple access (TDMA) (MIMO-OFDM/TDMA) cellular systems based on the dynamic parameter controlled-OF/TDMA (DPC-OF/TDMA) systems. In the proposed scheme, impulse responses of all the transmit antennas are multiplexed in the time domain using a carrier interferometry (CI) signal, which is equivalent to an impulse signal, and each impulse response is discriminated using a time window with the length of the maximum delay time of delayed paths, in order to estimate a channel state information (CSI) using only 64 subcarriers. Computer simulation confirms that the proposed channel estimation scheme is effective even in the case of 64 subcarriers under MIMO and multi-cell conditions

This paper proposes a channel estimation technique for the dynamic parameter controlled - orthogonal frequency and time division multiple access (DPC-OF/TDMA) system. In the proposed scheme, channel impulse response that represents channel state information (CSI) is measured using a carrier interferometry (CI) which is equivalent to impulse transmission. Moreover, because the minimum unit (subchannel block) of subcarriers to be allocated to each terminal is 64 subcarriers, to achieve high accuracy of the estimated CSI with a spreading factor of 64, code-multiplexed pilot signal is applied to identify base station (BS) in the cell search process, and CSI between a terminal and each transmit antenna element in the BS is detected using a time-domain-multiplexed CI signal for ease of extension of the DPC-OF/TDMA to multi-input multi-output (IMO) systems. Furthermore, we will also propose a CSI estimation scheme flexible to the plural subchannel block assignment cases. Conmputer simulation confirms that the proposed scheme can estimate the CSI regardless of the number of subchannel blocks assigned to each terminal.

IEEE802.11 employs distributed coordination function (DCF) as a media access control method. In order to address the hidden terminal problem, the DCF equips the virtual carrier sensing function in addition to the traditional carrier sensing function, CSMA. The virtual carrier sensing is realized by the RTS/CTS short packets exchange and the NAV. The RTS/CTS exchange can keep terminals other than the sender and the receiver silent for ensuring a data packet transmitted correctly. The NAV is a timer that indicates the amount of time that the medium will be reserved. The first modification is to replace the NAV for RTS of DCF to the one used in the protocol MACA (the idea of RTS/CTS exchange is originally proposed in the MACA). Simulations show that this modification improves the throughput performance. On the other hand, the DCF is designed to use ACK packet for ensuring correct data transmission in the media access control layer, while the ACK did not used in the MACA. We secondly show that the use of the ACK reduces the throughput performance of DCF when the RTS/CTS exchange is in use, concluding that by omitting the ACK from the DCF, the performance can be improved. This paper not only shows that these two simple modifications can improve the performance of the DCF but also points out that the functions being implemented in the DCF do not always work well for realizing higher performance.

In order to improve spectral efficiency, CDMA overlay, which means co-existence of wideband and narrowband signals in the same frequency band, has been considered. However, signals from existing narrowband system are not necessarily transmitted with low enough power level not to give damage to DS-CDMA (direct sequence code division multiple access) signals, so an efficient rejection technique for narrowband interferers is required for successful CDMA overlay. Furthermore, it is necessary to cope with sudden changes of interference power levels associated with their appearance/disappearance. In this paper, we propose a canceller to reject such high power narrowband interference for the DS-CDMA system and show the rejection performance obtained from the computer simulation.

This paper proposes a spatial filtering technique to effectively receive narrow angle spread multipath signals incoming to the typical macro cell base station in DS/CDMA cellular systems. In the proposed system, a hypothetical received signal for each antenna element is generated in the receiver using the measured delay profiles and known QPSK symbol sequences, and this signal is employed for antenna weight calculation. At this stage to create high gain pointing toward all the desired multipath components in the spatial filter, phase and delay time of the measured delay profiles for the hypothetical received signal is adjusted to be equal to enhance the generated hypothetical received signal with inphase. Computer simulation confirms that the proposed algorithm can create an effective spatial filter for receiving narrow angle multipath and achieves better performance than the conventional algorithm.

This paper proposes an adaptive resolution control scheme for a RAKE receiver in direct sequence/code division multiple access (DS/CDMA) systems to effectively resolve and combine multipaths for high-bit-rate wireless multimedia services. In the proposed scheme, the receiver measures a delay profile for the received signal with its resolution equal to the highest chip rate, estimates delay profiles for lower chip rates using a delay profile conversion technique, and selects a chip rate that maximizes the received signal to noise plus interference power ratio (SNIR) under a condition that the number of RAKE fingers is limited by a certain number. Computer simulation confirms that the proposed scheme with a 3-finger RAKE receiver can reduce the average transmit power by about 5 dB without degrading the transmission quality under a 30-ray Rayleigh fading channel.

In order to improve spectral efficiency, CDMA overlay, which means co-existence of wideband and narrowband signals in the same frequency band, has been considered. However, signals from existing narrowband systems are not necessarily transmitted with a low enough power level not to damage DS-CDMA (direct sequence code division multiple access) signals, so an efficient rejection technique for narrowband interferers is required for successful CDMA overlay. In this paper, we propose a canceller to reject such high power narrowband interference for the DS-CDMA system and show the rejection performance obtained from the computer simulation.

This paper proposes an accurate signal to interference plus noise power ratio (SINR) measurement technique to correctly proceed transmit power control (TPC) in M-sequence-based M-ary/spreading spectrum (M-ary/SS) scheme to realize high bit rate transmission for M-ary/SS/code division multiple access (M-ary/SS/CDMA) systems. Because the accuracy of TPC depends largely on the accuracy of SINR measurement, to sufficiently proceed TPC, the proposed scheme measures power ratio of the correlation peak to the other correlations in the correlator for M-ary/SS employing a correlation peak detector that regenerates the spreading sequence from the Viterbi decoded output data. Computer simulation confirms that the M-ary/SS scheme with regenerating spreading sequence can achieve much better bit error rate (BER) performance with decrease of peak transmit power of 5 dB in comparison with the conventional direct sequence/spread spectrum (DS/SS) scheme under ITU-Vehicular B model.

This paper proposes a delay profile information based subcarrier power control combined with a partial non-power allocation technique for OFDM/FDD systems to achieve high bit-rate and high quality non-voice data transmission for wireless communication systems. To improve the bit error rate (BER) performances for the convolutionally encoded OFDM system, the proposed system controls the transmit power for each subcarrier in order to keep the received power of each subcarrier constant using a measured delay profile and noise level given by a delay profile information (DPI) channel in the return-link. To further reduce the transmit power needed to achieve high quality data transmission, a partial non-power allocation (PNPA) scheme combined with a punctured coding technique is further applied to the subcarrier power control. In this scheme, the deleted bits by the punctured coding are allocated to the nonreliable subcarriers and transmit power for these subcarriers are power-suppressed. Computer simulation confirms that the proposed system can achieve high quality data transmission and reduction of transmit power as compared with that of a conventional system in frequency selective fading environments.

This paper evaluates the effect of angle spread on the system capacity for a DS/CDMA system using a delay profile measurement based adaptive array antenna combined with beam pattern selection diversity and carrier-to-interference plus noise (CINR) based transmit power control (TPC) techniques. In the proposed system, beam directivity is controlled by the delay profiles for each terminal measured in each antenna element. Moreover, to reduce the number of computation for the beam directivity control, the proposed system calculates the optimum beam directivity for only a limited number of terminals that locates far from the base station, and signals from the rest of the terminals are demodulated by selecting one of the most reliable adaptive array combined signals. This technique is called beam pattern diversity. For the evaluation of impact of the angle spread of the multipath components on the capacity of DS/CDMA systems with an adaptive array antenna, computer simulation is conducted. The results show that the proposed scheme gives better performance than the sector antenna systems under practical angle spread conditions; i.e. less than 10 degree, with relatively a small number of computation.

Microwave oven interference seriously degrades the performance of digital radio communication systems, therefore, in order to obtain a good error performance under a microwave oven interference environment, the digital radio communication systems should be newly designed for this environment. In this paper, using the Middleton's (1977) canonical class-A impulsive noise model, we propose a statistical model of microwave oven interference and discuss the performance improvement achieved by optimum reception based on this statistical model. Numerical results conclude that the first order statistics of microwave oven interference can be modeled by class-A impulsive radio noise, and a great performance improvement is obtained by using the optimum receiver designed against microwave oven interference with the introduction of the sample interleave scheme.

This paper discusses intelligent and flexible radio access/transmission technologies necessary to smoothly upgrade supported services as well as to economically extend coverage and operational environments. After a brief discussion on classification of key technologies for intelligent and flexible radio access/transmission technologies, the paper discusses the adaptive transmission parameter (TP) control techniques for both time division multiple access (TDMA) based and code division multiple access (CDMA) based systems. Finally, this paper proposes a time division multiplex (TDM) based adaptive access control technique to further enhance system capacity and to introduce higher grade services for CDMA-based wireless multimedia communication systems.

In this paper, we discuss the co-existence of DS-CDMA and TDMA systems where both systems share the same frequency band to improve the spectral efficiency. We evaluate the capacity of a DS-CDMA/(frequency-multiplexed) TDMA frequency sharing system, and show that the system can provide a greater capacity than a single multiple access scheme-based system such as DS-CDMA or TDMA, when we employ an active narrowband interference rejection technique for the TDMA signals. The rejection of the narrowband TDMA signals can significantly improve the bit error rate performance of the CDMA system and total capacity of co-existence.

We investigate the effects of microwave ovens interference on the performance of digital radio communications systems (PDC, PHS, ISM-band (2.45 GHz) wireless LAN). At first, we experimentally measure the interference radiated from microwave ovens and discuss the statistical characteristics of interference. Then, by using the measurements results, the effects of interference on the performance of /spl pi//4-shift QDPSK and DS/CDMA systems are evaluated. Numerical results conclude that the performance of digital radio communication systems are significantly degraded by the effect of interference. Moreover, we propose a new TDMA channel assignment method for microwave oven interference and investigate the performance improvement achieved by proposed channel assignment.

This paper proposes an adaptive modulation system (AMS) with a discrete power control combined with a predistortion (PD)-type nonlinear distortion compensation technique for high quality and high power efficient high-bit-rate wireless communication systems. In the PD-type nonlinear distortion compensator, predistortion look-up table is updated using a teaching signal embedded in the guard time. In this process, nonlinear input signal vs. output distortion performance is discretely measured during the guard time period, and at the end of the guard time, a sequence of the measured distortion is interpolated using a cubic spline function to reduce memory size and the number of computation. More over, in the power control section, three levels of transmitter power attenuation, i.e., 0, 10 and 20 dB are prepared, and they are selected according to the expected carrier power to noise power spectral density (C/N/sub 0/) of the transmitter. Computer simulation confirms that the proposed method can sufficiently suppress the non-linear distortion in the adaptive modulation TDMA/TDD systems.

This paper proposes a direct sequence/code division multiple access (DS/CDMA) cellular system using channel activation and soft power control techniques to increase both the forward and reverse link capacities. To equivalently control the received signal to interference and noise power ratio (CINR) to be constant, the proposed system employs a channel activation technique in which higher channel coding gain and small processing gain are employed with a shorter burst length during higher CINR period, and a lower channel coding gain and larger processing gain are employed with longer burst during lower CINR. Moreover, the proposed system employs a soft power control technique to enhance the advantages of the channel activation technique. Computer simulation confirms that the proposed system can achieve a higher system capacity with a smaller soft hand-off margin and a smaller average transmitter power.

This paper proposes automatic repeat request (ARQ) protocols combined with an adaptive modulation system (AMS) (ARQ/AMS) to achieve high throughput data transmission for wireless multimedia communication services. First, it analyses the cause of the throughput limitation of the conventional fixed modulation ARQ system, and discusses basic strategies to improve the throughput performance by effectively combining ARQ and AMS. Next, it proposes a selective repeat ARQ (SR-ARQ) with AMS (SR-ARQ/AMS), and shows its throughput performance improvement effect. Finally, this paper proposes a type-II hybrid ARQ with a punctured convolutional coding combined with the AMS (type-II hybrid ARQ/AMS) to further improve the throughput performance. Computer simulation confirms that the proposed type-II hybrid ARQ/AMS can achieve higher throughput performance than the conventional selective repeat ARQ with QPSK under any channel conditions.

This paper reports laboratory experimental results of a time division multiple access/time division duplex (TDMA/TDD) based adaptive modulation system to verify feasibility and its usefulness for future wireless multimedia communication systems. The developed system employs a symbol rate and modulation level controlled adaptive modulation in which the modulation scheme is selected from full-rate 64-ary quadrature amplitude modulation (64 QAM), full-rate 16 QAM, full-rate quaternary phase shift keying (QPSK), 1/2-rate QPSK, 1/4-rate QPSK, 1/8-rate QPSK and no transmission according to the expected channel condition for the transmitted signal. The experimental results confirm that the developed system is very effective in achieving high quality and high bit rate transmission of around 1 Mbit/s without any special anti-frequency selective fading techniques even under frequency selective fading conditions with its d delay spread of less than 250 ns and f/sub d/ of less than 10 Hz.

For future high speed indoor wireless communication, a non-directed diffuse optical wireless communication system attracts a great attention. In order to obtain sufficient received signal power for non-directed diffuse optical wireless communication system, a multi-beam transmitter system is proposed and the performance improvement achieved by the proposed system is evaluated. Numerical results show that a high received signal power can be obtained by using a multi-beam transmitter and the performance of the proposed system is much better than that of the conventional non-directed diffuse system. Moreover we evaluate the effect of the delay spread on the performance of the proposed system and it is confirmed that the performance improvement is achieved by suppressing the delay spread.

This paper proposes a new joint symbol timing, symbol period and frequency offset estimation method for orthogonal multi-carrier modulation signal. The method is based on the maximum likelihood criterion making effective use of "cyclostationary property" of the orthogonal multi-carrier modulation signal, and requires no special signal such as pilot tones to assist the estimation. After a likelihood function for the estimation is derived, an optimum estimator which maximizes the likelihood function is proposed. In addition, in order to reduce the estimator complexity, a suboptimum estimator is also proposed. Computer simulation results show that the optimum estimator requires only about 10 observation symbols for the accurate estimation in a frequency selective slow Rayleigh fading channel as well as in an additive white Gaussian noise channel, and the suboptimum estimator is effective because of the low complexity, although it requires a longer observation period.

In this paper, we propose a new group demodulator based on a complex multirate filter bank for multiple-channel mobile communication systems, which can handle asynchronously frequency-multiplexed signals. In addition, we propose a fast channel searching method to find active channels suited for the group demodulator. Computer simulation results show that the proposed group demodulator can achieve a relatively low hardware complexity keeping a good bit error rate performance.

The paper proposes a novel automatic frequency control (AFC) method using the discrete wavelet transform (DWT) for low earth orbit satellite, mobile and personal communications systems. The DWT is a useful analysis tool for non-stationary signals which uses short windows at high frequencies and long windows at low frequencies. The DWT AFC has its own simple structure and an intelligent characteristic of "observation and control according to frequency error", which makes it possible to achieve both fast initial acquisition and accurate tracking. Furthermore, it can make the irreducible frequency error arbitrarily small.

The paper proposes a novel concatenated coded modulation scheme for reliable data transmission over a Rayleigh fading channel. The outer code is a Hamming code and the inner code is a block code based on a coded modulation, which is optimized by the nonlinear optimization programming technique. Furthermore, a soft decision decoding scheme in which the soft decision is performed on not only the inner block coded modulation but also the outer Hamming code is proposed. In this decision scheme, the output symbols of the inner decoder have the reliability information, which enables the soft decision decoding of the outer code. Computer simulation result clarifies the concatenated coded modulation scheme with the soft decision decoding drastically improves the bit error rate performance in a Rayleigh fading channel. The bit error rate of 10/sup -4/ is achieved with an average E/sub b//N/sub 0/ of about 8.5 dB, which shows the performance of the proposed scheme in a Rayleigh fading channel is as same as that of uncoded BPSK in an additive white Gaussian noise channel.>

Double symbol error rates ofM-ary differentially coherent phase shift keying (DPSK) systems in a satellite-aircraft multipath channel are evaluated. A joint probability density function (pdf) of two consecutive differential phase errors in the DPSK outputs is newly derived. Numerically calculated results are shown in figures for binary, quaternary, and octonary DPSK systems. It is further shown that the analytical values agree well with the data observed by ATS-5 experimental tests.

The average error probabilities and outage rates of error probability for theM-phase CPSK signal through the Nakagami'sm-distributed fading channel are exactly evaluated both for nondiversity reception and for diversity reception. The probability density functions of the composite phase of fading signal and noise and those of the diversity-combined signal envelopes are newly derived in this paper. The results are generally obtained including the digital phase modulation component, the fading figure as a measure of fading depth, the average carrier-to-noise power ratio, the power correlation coefficient between two diversity branches, etc. The diversity improvements are also verified. Additionally some useful approximate formulas are briefly shown. This study will add a newly widened view to the considerations of system performances and designs for digital radio communications via fading channels.

This paper provides a general analysis of the transmission characteristics of a coherent PSK signal, where anM-ary coherent signal is transmitted via a repeater system composed of a cascade ofNbandpass hard limiters. The probability density function of a composite phase of the signal and noise over the system is newly derived to obtain an error probability. In addition, the comparison with the repeater system composed of a cascade ofNlinear amplifiers is made to clarify the features of the transmission characteristics. It is found that the probability density function of the composite phase over the system is symmetric about each input carrier-to-noise power ratio (CNR), and it reduces to some well-known results. It is also shown that for a reasonably large input CNR in each stage, the presence of the bandpass hard limiter (BPHL) provides some improvement, but that for small input CNR it causes degradation in performance.

Signal-to-noise ratios (SNR's) at the output of the squarelaw detector are determined as the functions of modulation degree and bandwidth ratio of the modulating signal and the noise carrier, when the input is an amplitude-modulated noise carrier.

Signal interaction is analyzed when the composite signal of a simultaneously amplitude- and angle-modulated wave, a nonmodulated sinusoidal wave, and a noise is added to a half-wave linear mixer. If a certain mixing condition is satisfied, a half-wave linear mixer operates like an ideal bandpass limiter with frequency conversion function. The limiter action is also verified by experiment.

An analysis is presented on the output signal-to-noise ratios for the FM correlation systems having an FM detector in each input channel of a conventional correlator. The input consists of a frequency-modulated signal combined additively with stationary narrow-band Gaussian random noise. A general expression is derived for the output signal-to-noise ratio. A detailed calculation is made for the output signal-to-noise ratios when each input signal is a carrier frequency-modulated by a Gaussian random process and the integrating filter is of RC low pass. The dependence of the output signal-to-noise ratios on several parameters is discussed.

By the term "FM correlation system," we mean the analog correlation system that consists of two channels, each containing an FM detector. Expressions are derived for the output of the FM correlation system for the following cases. Each channel input to be correlated is a conventional FM signal plus Gaussian noise; and each input is an amplitude-limited or a nonlimited narrow-band Gaussian random signal plus Gaussian noise. The effect of the input carrier-to-noise power ratios is made clear. The features of the FM correlation system are shown in comparison with a conventional correlation system. In addition, a detailed analysis of the effect of the envelopes of the narrow-band Gaussian random signals on the output is presented.