Spread-spectrum (SS) sharing with comb spectrum structure in a microcell/macrocell cellular architecture in order to increase spectral efficiency is proposed. Such method employs a filter in the code division multiple access (CDMA) transmitter to feature comb spectrum structure, and suppress interference with a narrowband time division multiple access (TDMA) system in using together in SS sharing. The relationship between microcellular capacity and macrocellular capacity of the system is explored and compared to those of conventional SS sharing and orthogonal sharing. To be concrete, we investigate two cases, i.e., using no power control and ideal power control in the TDMA system. In both cases, the proposed SS sharing gives better capacity results than the conventional SS sharing and in the comparison when ideal power control is used in th. TDMA system, it even has the property to oppose the orthogonal sharing in ideal condition without interference.

An efficient ARQ scheme based on the packet combining technique is investigated for multi-carrier modulation systems. In multi-carrier modulation systems, several sub-carriers are used for high data rate transmission and their individual received signal quality becomes different from one sub-carrier to others in a frequency selective fading channel. Therefore by changing the assignment of data to the sub-carriers in the retransmission packets, the distortion between the previous transmitted packet and the newly retransmitted one will be different. This is the principle of the proposed adaptive data order rearrangement for a packet combining ARQ scheme, which can achieve more diversity gain in packet combining and improve the ARQ performance. From the results of the theoretical analysis and the computer simulation, it is confirmed that the proposed packet combining ARQ with the proposed operation can achieve the better performance in terms of the average packet transmission success probability. In addition, this proposed scheme is also compared with the conventional multi-carrier modulation ARQ scheme based on the partial retransmission of a packet. The computer simulation results demonstrate that the proposed scheme has also advantage against the latter one, and it is considered to be as a more efficient ARQ scheme for multi-carrier modulation systems.

The bit error rate (BER) performance of multicode multi-carrier code division multiple access (MC-CDMA) severely degrades due to the inter-code interference (ICI) in a strong frequency-selective channel. Recently a spreading code group construction method was proposed for MC-CDMA. The Walsh-Hadmard (WH) codes are divided into a number of code groups such that the code orthogonality can be maintained within each group even in a strong frequency-selective channel; any code pair taken from different groups is not orthogonal. The number of spreading codes in each group is determined by the maximum time delay difference of the channel. In this paper, we point out that the number of codes in each group is determined by the distribution of time delay differences among the propagation paths of the channel, not the maximum time delay difference. Based on that observation, we show that more orthogonal spreading codes can exist in each code group. The conditional BER is derived taking into account the interference from other code groups and the achievable downlink BER performance using the proposed spreading code group construction is numerically evaluated in a frequency-selective Rayleigh fading channel.

Future electric lights will be comprised of white LEDs (Light Emitting Diodes). White LEDs with a high power output are expected to serve in the next generation of lamps. In this paper, an indoor visible data transmission system utilizing white LED lights is proposed. In the proposed system, these devices are used not only for illuminating rooms but also for an optical wireless communication system. This system is suitable for private networks such as consumer communication networks. However, it remains necessary to investigate the properties of white LEDs when they are used as optical transmitters. Based on numerical analyses and computer simulations, it was confirmed that the proposed system could be used for indoor optical transmission.

OFDM modulation has attracted attention for fourth-generation mobile communication systems and high-speed wireless LANs. However, it has a very serious problem of large peak power. PTS (partial transmit sequences) has been proposed as one solution to this problem. In PTS, the OFDM subcarriers are divided into several clusters, and the phase of each cluster is rotated by a complex weight to minimize the PAPR (peak-to-average power ratio). However, the weight of the phase rotation must be sent to the mobile terminal by using a side information channel. In this paper, we propose two weight estimation methods at the receiver to avoid weight transmission in side information channels. The first method uses pilot signals, while the second is a blind estimation method that changes the weight pattern. We evaluate the performance of these methods by computer simulation.

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

Joint transmission (JT) in time-division-duplex code-division multiple-access (TDD-CDMA) systems can provide a low peak-to-average power ratio (PAPR) for single paths, but causing a high PAPR in multipath environments. To avoid the high PAPR, we propose a new approach to JT technique by selecting certain paths instead of all paths used in JT processing so that PAPR becomes lower. The path selection proposal involves two methods; path selection by taking certain paths from all paths and by taking paths having path gains above a certain threshold value. To enhance the effectiveness of the proposed techniques, we evaluate a combination of the proposed technique with the clipping technique. We evaluate both PAPR and bit error rate (BER) performance for the proposed techniques and its combination with the clipping technique. We compare the results of the proposed techniques with conventional JT technique and the combination techniques with clipping technique. From the results of computer simulation, we show that the proposed path selection techniques perform low PAPR and good BER performance compared to the conventional JT processing. We also show that the combination of proposed path selection technique and clipping performs low PAPR performance without severe BER degradation compared to the conventional clipping technique.

A quasi-synchronous (QS) code division multiple access (CDMA) system is proposed for mobile communications. In the proposed method, which uses the time division duplex (TDD) mode of transmission, a mobile receiver can measure propagation delay changes. It then accordingly adjusts its transmission time so its signal can arrive at base station synchronously with other mobile units. A simple control unit is used at the mobile unit in order to reduce any error due to the propagation delay changes. The system operates as follows. At the start of a call, a mobile unit is quasi-synchronised through feedback control from the base station. The mobile unit then maintains synchronous status without any further base station feedback. The degree of the quasi-synchronous accuracy is determined by a clock in mobile units. This paper shows performance results based on using a clock rate of ten times faster than the spreading rate. Orthogonal codes are used for spreading the signals. The results demonstrate that the reverse link CDMA multiuser interference is to a great degree removed.

A coherent communication system using squeezed light is one of candidates for a realization of super-reliable systems. In order to design such a system, it is essential to understand and to analyze modulators mathematically. However, quantum noise of squeezed light has a colored spectrum which changes with respect to phase of a local laser. Therefore the optimization of the relationship between signal and quantum noise spectrums is required at a modulator to obtain the ultimate performance of the communication system. In this paper, some ideas of modulators for squeezed light are proposed and their spectrum transformations are given. After the brief summary of squeezed quantum noise, a new concept which originates from the restriction of the local laser phase is applied to it. This concept makes a problem originated from a colored quantum noise spectrum more serious. It results in the optimization problem for the relationship between the quantum noise spectrum and signal power spectrum. The solution of this problem is also given under the restriction of local laser phase. As a result, a general design theory for coherent communication system using the squeezed light is given.

In this paper, we have investigated power control techniques for two different routing schemes in a multi-hop Code Division Multiple Access (CDMA) packet radio network. These techniques control the signal power based not only on the transmission distance, but also on the transmission direction. These techniques reduce the amount of interference from the other terminals, and improve the probability of packet success and the expected forward progress per hop. Our results show that the power control techniques increase the expected packet progress towards the final destination per hop by 16% as compared with that of the no power control case. This performance improvement is achieved without increasing system complexity or sacrificing system performance.

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

In this paper, we propose a combination system of point to multipoint (P-MP) time division multiple access (TDMA) broadband wireless access (BWA) system and indoor wireless local area network (WLAN). In order to realize a high speed wireless communication, a wide bandwidth is required for both access lines and local area networks. The proposed system shares the frequency between BWA and WLAN to achieve an efficient use of frequency resources. This is based on the idea that an uplink band of the P-MP TDMA BWA system will provide relatively small interference slots, which are not used by subscriber stations nearby. In other words, there are useful small interference slots for another system using same frequency according to its position. Then we use such small interference slots for WLAN. In addition, direct sequence code division multiple access (DS-CDMA) can suppress such TDMA interference by spreading it over wide bandwidth. Therefore in the proposed system, DS-CDMA is used for WLAN in the same band with the BWA uplink. We also discuss WLAN packet error rate reduction techniques for this system. To confirm the availability of the proposed system, we evaluate the system performance by numerical analysis and computer simulation.

In this paper, we propose to adopt the asymmetric error correcting code for photon communication systems. The asymmetric error correcting code is an binary code correcting only 10 type transition errors (asymmetric errors). We show the following advantages obtained by employing the asymmetric error correcting code:(i) the codeword error probability is smaller than that of the symmetric error correcting code. (ii) the information rate per photon is larger than that of the symmetric error correcting code. Moreover, for asymmetric error correcting codes, we obtain the lower bounds on the codeword error probability and the upper bounds on the information rate per photon. By using these bounds, we can show that some asymmetric error correcting codes are optimum for these criteria.

Optical Frequency Division Multiplexing (OFDM) is an attractive multiplexing approach for exploiting optical communication technology. Although considerable progress has been made in this approach, it still suffers from numerous potential impairments, stemming from several phenomena. (i.e., laser unstability, residual temperature variations, linear and nonlinear cross talk.). Conventional serial coding technique is not practical in lightwave systems, as it changes the system's bit rate that is not desirable. In this paper a new Parallel Coded Optical Multicarrier Frequency Division Multiplexing (PCOM-FDM) technique has been investigated. The strategy of multicarriers, together with Parallel Forward Error Control (PFEC) coding, is a potentially novel approach as in this approach we have, 1) Investigated optical multicarrier communication that is effective in combating dispersion and increasing throughput, 2) Proposed PFEC coding which is different from conventional serial coding in respect that it does not change the system bit rate per carrier and prevents the effects of channel wandering. It is highly desirable in lightwave systems and thus holds a vital importance in practical high speed optical communication systems. Theoretical treatment shows that the proposed approach is promising and practical.

Multi-carrier (MC) signal has a large peak to mean envelope power ratio, so that the MC signal suffers from a high level of inter-modulation distortion due to the nonlinearity of the power amplifier stage. For portable terminals, it is undesirable to use linear amplifiers because high power efficiency is needed. To solve this problem, we propose a time division duplex (TDD)-code division multiple access (CDMA) communication system which uses an asymmetric modulation scheme between the forward and reverse links. The system consists of multicarrier modulation for the forward link and single carrier modulation for the reverse link. A pre-equalization method for the forward link transmission is also presented in this paper. In frequency selective fading, the system achieves a path diversity effect without any channel estimation unit at the mobile station by using the pre-phase equalizer. From the simulation results, it it found that the proposed system achieves better BER performance than the conventional MC-CDMA system and the single carrier RAKE system equipped at the mobile unit since the proposed system has the ability to suppress other user interfering signals.

In this paper, we focus on an OFDM peak power reduction method that uses clipping and filtering. This method can reduce the peak power of OFDM via clipping, and can also reduce the out-band emission via filtering, even if a nonlinear amplifier is used. However, the filtering causes peak power regeneration. For purposes of reducing the effect of peak power regeneration, we propose an adaptive clipping level control method for OFDM peak power reduction, as part of a technique using clipping and filtering. In this method, the clipping level is optimized by checking the peak power regrowth which is caused by inserting a filter, by using a multi-stage filtering simulator. Thus the peak power is adjusted to the target PAPR. If the target PAPR is decided to be the saturation power of an amplifier, the bit error rate performance is improved without increasing the out-band emission. In order to confirm the effectiveness of the proposed system, we evaluate its performance by using computer simulation.

The advantage of nonstandard quantum states such as two-photon coherent state (or squeezed states) and photon number state as transmitter state is strongly degraded by transmission loss in quantum communications. To cope with such a problem, a new application of these states is proposed, and it is shown that its system has infinite capacity.