The intermodulation distortion (IMD) due to laser diode (LD) nonlinearity of an asynchronous direct-sequence code division multiple access (DS/CDMA) system in optical transmission is analyzed. A third-order polynomial without memory is used to present LD nonlinearity. In DS/CDMA systems, only one harmonic of the third-order inter-modulation term falls on the signal frequency band and influences the system performance. The average distortion is derived with only the information of autocorrelation functions. The results are useful for CDMA system design and performance analysis. With LD nonlinearity it is necessary to select an optimal modulation index that provides a maximum signal-to-noise ratio (SNR). The analytical method is applicable to other general nonlinearities in CDMA systems.

This paper investigates the problem of finding the optimal access point placement in simultaneous broadcast system using orthogonal frequency division multiplexing (OFDM) for public access wireless LAN with micrometer or millimeter frequency band. We define our design criteria such that the quality of service is provided uniformly throughout a given service area. The optimal access point placement with a uniform quality of service was obtained by setting the cost function as the combination of a standard deviation of BER and the average of BER in a very fast simulated annealing algorithm. We applied the algorithm to the cases of fixed and mobile terminals, and obtained optimal access point placement results for both cases.

In this paper, we propose an adaptive RAKE receiver, which does not need to send the sounding signals and can track the fluctuations caused by fading. The channel estimation can be done by using a least squares method of the first and second equations suppressing additive noise and tracking the channel fluctuations. It is confirmed by computer simulations that the result has good agreement with theory and the performance is almost same as that of the conventional RAKE with the sounding signals.

In a typical indoor environment such as in a building, delay spread tends to be small, which causes frequency non-selective fading. Therefore resolvable paths at the RAKE receiver can not be obtained, and effective path diversity can not be achieved. This paper proposes an artificial path diversity system in which one or multiple sectors at the base station are pre-selected according to the channel conditions for transmitting data. Each sector's signal is delayed by several chips to create artificial paths which can then be combined by using a RAKE receiver at the mobile station creating a diversity effect for an indoor environment. Moreover, only pre-selected sector antennas transmit signals to reduce inefficient signal usage in the sectors whose paths are blocked by using all sectors, therefore the transmission power is used efficiently at the base station. As a result of sector selection, the better BER performance and the reduction of interference signals between different channels can be achieved by means of sector selection. The performance of the proposed system is analyzed and demonstrated by computer simulation in a Rayleigh and log-normal fading indoor environment.

An enhanced Ultra Wideband (UWB) signaling scheme that employs PSWF (Prolate Spheroidal Wave Functions)-based orthogonal chip pulses and ternary complementary code sets is proposed for Direct-Sequence (DS) UWB systems. Every information bit of each user is modulated and transmitted over a set of parallel sequences of PSWF-based orthogonal chip pulses and are further assigned to a ternary complementary code set with additional zero padding if necessary. Moreover, the ternary complementary code sets are generated to be mutually orthogonal and assigned to any pair of multiple users. Hence, the mitigation of multipath interference as well as multiple user interference (MUI) can be expected. Furthermore, the ternary code length can be greatly shortened by taking advantage of pulse and code orthogonality. Thus, the proposed transmission scheme is especially suitable for high data rate DS-UWB systems that offer very high flexibility.

To increase the transmission rate without bandwidth expansion, the multiple-input multiple-output (MIMO) technique has recently been attracting much attention. The MIMO channel capacity in a cellular system is affected by the interference from neighboring co-channel cells. In this paper, we introduce the cellular channel capacity and evaluate its outage capacity, taking into account the frequency-reuse factor, path loss exponent, standard deviation of shadowing loss, and transmission power of a base station (BS). Furthermore, we compare the cellular MIMO downlink channel capacity with those of other multi-antenna transmission techniques such as single-input multiple-output (SIMO) and space-time block coded multiple-input single-output (STBC-MISO). We show that the optimum frequency-reuse factor F that maximizes 10%-outage capacity is 3 and both 50%- and 90%-outage capacities is 1 irrespective of the type of multi-antenna transmission technique, where q%-outage capacity is defined as the channel capacity that gives an outage probability of q%. We also show that the cellular MIMO channel capacity is always higher than those of SIMO and STBC-MISO.

In this paper, we propose a new transmitter diversity. We propose a combined system with path diversity gain of the distributed antennas and frequency diversity gain of the multi-carrier. The proposed system transmits different data using several sub-carriers which are correlated, while, transmitting the same data using several sub-carriers which are decorrelated. It can achieve combined path and frequency diversity in a variable frequency selective fading channel. It provides high data rate services by transmitting the different data using each correlated carrier, and supports good quality by transmitting the same data on decorrelated carriers using multiple antennas. The proposed system is applicable to multimedia service and can achieve high quality according to channel condition. Thus, the proposed system is sufficiently flexible enough to very support a variety of video, image, voice and data services at a high level of quality.

In this paper, Pre-Rake scheme for pulse based Ultra Wideband impulse radio (UWB-IR) communications system is proposed considering a standardized UWB channel model from IEEE 802.15 SG3a, which is based on an extensive set of short-range indoor channel measurements. Two kinds of pulse waveforms are taken in account here, namely, Gaussian mono-pulse and newly designed Prolate Spheroidal Wave Functions (PSWF) pulses corresponding to FCC Spectrum Mask. The Pre-Rake scheme is shown to contribute to the low-power, cost-efficient UWB system designing as well as Rake combining gain. Instead of building a Rake receiver at the receiving side (e.g. portable unit), the transmitter (e.g. access point) can pre-combine the UWB signal before transmission in the forward link by estimating the channel impulse response from the reverse link. While the Pre-Raked signal is convolved with the estimated channel impulse response, the function of Rake combination at the receiver is automatically performed. Meanwhile, in order to defeat inter-pulse-interferences (IPI) caused by severe multipath fading conditions, adaptive guard-time scheme for consecutive pulses is proposed as well. Monte-Carlo simulations are carried out to compare the Pre-Rake with Rake results and show that Pre-Rake scheme is as good as Rake combining for both types of pulse waveforms. Then the mobile or portable unit with a conventional receiver can still achieve the diversity gain of Rake combination. Moreover, the effects of placing guard-time between pulses are also verified.

A transmission power control technique on a TDD-CDMA/TDMA system for wireless multimedia networks is proposed. The assumed network connects mobile terminals to a node of an ATM based high speed LAN through a radio central unit. Only human interface facilities are implemented into the terminal so that users access integrated services through the node of the network. The uplink (from a mobile terminal to a radio central unit) employs a CDMA scheme to transmit human interface signals (2.4kbit/s) and the downlink employs a TDMA scheme to transmit display interface signals (24 Mbit/s). Both the CDMA and the TDMA signals occupy the same frequency band. To mitigate bit error rate degradation due to the fading, the radio central unit estimates the impulse response of the channel from the received CDMA signals and controls the transmission power of the TDMA signals to compensate the fading attenuation. The bit error rate performance of the downlink with the proposed transmission power control is theoretically analyzed under several fading conditions. Numerical results using the Nakagami-m fading model and recent propagation measurements show that the proposed power control technique compensates the fading attenuation and improves the bit error rate performances. The bit error rate of the downlink is reduced from 10-2 to 10-5 at the symbol SNR of 20dB by employing the proposed transmission power control, which is less sensitive to the severity of the fading. Furthermore, the proposed transmission power control is implemented without increasing the terminal complexity because all the processing on the power control of the downlink is carried out only in the radio central unit.

This letter clarifies properties of system using squeezed state signal in photon channel with energy loss. It is found that error rate of squeezed state system is always superior to that of coherent state, and that reduction of optimum squeezing parameter is allowed by optimization taking into account loss in comparison with optimization of transmitter squeezed state.

In recent years, Pre-Rake combining technique has become a hot topic of research as it decreases the complexity of the portable mobile unit, while achieving the same multipath diversity effect of the Rake receiver. The technique is based on precoding of the transmitted signal relying on knowledge of the channel estimation response before transmission. This a priori channel state information is available in Time Division Duplexing (TDD) systems, since the same channel is used both in uplink and downlink. In practice, the error in channel estimation in Pre-Rake system occurs due to time variability in mobile radio channel. Most previous works on Pre-Rake in TDD CDMA have not taken into consideration the effect of imperfect channel estimation. In this paper, we present Pre-Rake performance under imperfect channel estimation due to time variability in TDD system, depending on Doppler Frequency and compare it with the ideal Pre-Rake system. Numerical analysis and computer simulations were carried out to obtain the system error probability.

Multi-Carrier Code Division Multiple Access (MC-CDMA) is one of candidates for the next generation wireless communication systems. In an uplink, the MC-CDMA system suffers from the different access timing (asynchronous transmission), the different fading, and the different frequency offsets of each active user. In this paper we analyze the effects of the frequency offset compensation with MMSE-MUD (minimum mean square error based multi-user detection) for MC-CDMA in a quasi-synchronous uplink. We consider the MC-CDMA system with two subcarrier mapping schemes, the continuous mapping scheme and the discrete mapping scheme. From our theoretical analysis and computer simulation, we show that the MMSE-MUD can compensate the different frequency offsets among users. We also show that the MMSE-MUD significantly improves the bit error rate (BER) for the MC-CDMA system with the continuous mapping scheme.

Optical Feedering between Base Stations and Control Station is an effective technique for future microcellular mobile communication systems. The use of Laser Diode (LD) in such a system leads to the generation of intermodulation products, which consequently affect a system performance and ultimately restrain the maximum number of users that the system can serve. The problem becomes further intensified in case of CDMA system which is a candidate for future cellular mobile and personal communication systems. In this paper LD's Nonlinearity compensation technique for Direct Sequence spread spectrum CDMA signals in optical communication system is presented. This technique involves the implementation of a nonlinear block herein after called Post Nonlinearity Recovery Block (PNRB). This block is designed to exhibit the characteristics inverse to those of LD. The block is designed theoretically by deriving the complete expressions for Transfer functions. Some useful results of theoretical investigation of a proposed scheme have been presented, which form the basis for the experimental test system. The work is novel because, (i) Compensation analysis has been carried out for DS-CDMA signals for the first time, and (ii) Compensation has been proposed on the control station instead of base station, which is different from the conventional techniques and offers several additional advantages. Performance of the system with and without PNRB is evaluated by Intermodulation Distortion (IMD) and SNR Analysis. The results show that LDs' nonlinearity distortion level can be compensated to a remarkable extent.

The recent progress in communication devices has reduced the cost of spread spectrum systems that have been mainly used in the military field, enabling the possibility of using spread spectrum systems for nonmilitary use. Nonmilitary communication networks are classified into two categories: public communication networks and consumer communication networks. At the present, the public communication network share is much larger than that of the consumer communication networks. Both of them will increase; however, the consumer communication network share will be comparable to the public one. The progress of consumer communications is due to inexpensiveness (without charge), user privacy, free design from regulations and advantages of local and small zone communications. To overcome fading, power restrictions, interference and interception is very important for its progress, One of the key technologies of consumer communications is the spread spectrum. Consumer communication networks are characterized by user possession of the network itself in addition to the possession of personal terminals. From the viewpoint of analogy between communication networks and transportation networks, "from public to individual" seems to be today's trend. Several spread spectrum applications for consumer communications in Japan have been introduced as follows: a home security system using power line communication as a wire spread spectrum communication system; a data carrier, a radio remote control, low-power radio systems and a clock rate modulation system as radio spread spectrum communication systems. In addition SAW device applications and digital signal processing applications are introduced.

Atmospheric optical communication (AOC) system using subcarrier PSK modulation is proposed and its superiority to OOK modulation in the presence of scintillation is discussed theoretically. An experimental AOC setup with a subcarrier modulated by 155.52(Mb/s) DPSK at light wave-length λ=0.83(µm) over an 1.8(km) outdoor path is employed to show the performance. Theoretical and experimental results are compared under scintillation in clear weather and a good agreement is observed. Finally, AOC systems using subcarrier M-ary PSK and multiple subcarriers are proposed and discussed.

In the future, it will be necessary that robot technology or environmental technology has an auditory function of recognizing sound expect for speech. In this letter, we propose a recognition system for the species of birds receiving birdcalls, based on network technology. We show the first step of a recognition system for the species of birds, as an application of a recognition system for environmental sound.

A time division duplex (TDD) direct sequence spread spectrum communication (DS-SS) system is proposed for operation in channels with Rayleigh fading characteristics. It is shown that using the TDD method is advantageous because the devices can be designed more simply, the method is more frequency efficient and as a result the systems will be less costly and less power consuming. It is also shown that an efficient power control method can be implemented for the TDD systems. In contrast to the traditional access techniques such as frequency division multiple access (FDMA) and time division multiple access (TDMA) that are mainly frequency limited, the code division multiple access (CDMA) method which uses the DS-SS technique is interference limited. This means that an efficient power control method can increase the capacity of the DS-SS communications system. Computer simulations are used to evaluate the performance of the TDD power control method. Performance improvement of order of 12 to 17dB at bit error rate (BER) of 10-3 can be obtained for different methods of power control. The advantages of the TDD technique for the future DS-SS systems operating in the Industrial, Scientific and Medical (ISM) band are explained in an appendix to this paper.

In this paper we present a new tracking scheme using two tracking modes which are based on the concept of Delay Lock Loop (DLL). Under the multipath fading channels, a conventional DLL has problems of jitter performance degradation, lock-off and delay offset. It is necessary to solve these problems, because mobile communications have increased drastically. We propose the combination of a coarse tracking mode and a fine tracking mode. The former mode is employed for reducing the possibility of losing lock, the latter mode is used for suppressing the jitter of delay error and the delay offset in the presence of multipath fading. The both modes utilize the power of delay paths shown in the auto-correlation function of the received signal at the DLL. Computer simulation results show that our proposed scheme is extremely useful comparing with a conventional scheme over the multipath fading channels.

A hybrid system in which mobile terminals (MTs) themselves select whether to communicate directly or via base stations (BSs) from communication conditions, "cellularad-hoc united communication system," has been proposed by the authors. It has been confirmed to have effective channel usage and battery consumption, especially for MTs which tend to communicate with partners which are close together, such as operational mobile robots. In this paper, the hybrid system is examined with respect to handoff rate. This hybrid system uses two modes, a cellular mode and an ad-hoc mode. The former mode is for communication using BSs and the latter is for direct communication. Thus, there will be two kinds of handoffs. One is the ordinary one between BSs and the other is between the two modes. Such a system may suffer from frequent handoffs. Thus the handoff rate of the system was examined. However, by the addition of a simple hysteresis characteristic, the handoff rate was held down closer to that of a pure cellular system in a multipath environment.

MC-CDMA has attracted significant attention as a downlink communication method for fourth generation mobile communication systems. However, MC-CDMA has a peak power problem, similarly to other nonspread multicarrier systems. In this paper, we propose a novel peak power reduction technique for MC-CDMA using code selection at the transmitter. In the proposed system, the transmitter selects the code shift pattern from plural pre-assigned code shift patterns in each cluster to minimize the output peak power of downlink signals. The proposed technique can achieve the blind code detection of transmitting codes at the receiver. However, the detection performance degrades if the number of users is extremely large or extremely small. Therefore, in this paper, a code detection error recovery technique is also considered.