This paper proposes a novel DS/CDMA system with code-diversity techniques constituted by a simple system to suppress multiple access interference (MAI) without estimating the PN sequence of interference at the receiver. In the transmitter, the data signal is modulated with a sum of several PN sequences, and, two types of code- diversity reception are proposed, (1) maximal-ratio combining (MRC) code-diversity by autocorrelation, and (2) MRC code-diversity by anti-crosscorrelation. By computer simulations, it is shown that MRC code-diversity by anti-crosscorrelation is superior to the other one. It is also shown that MRC code-diversity by anti-crosscorrelation can improve BER more effectively for the interference which takes the phase to degrade BER at the worst. Next, to design the optimum number of branches for code-diversity, average BERs are calculated for several combinations of codes in code-diversity. As a result, the optimum number of branches varies for each combination of codes, however, it is decided from 3 to 7 branches. Finally, the effectivity of the proposed system in a near-far problem is presented.

Optical code division multiplexing (OCDM) is the other class of multiplexing techniques than time division multiplexing (TDM), wavelength division multiplexing (WDM) and space division multiplexing (SDM). OCDM has been proposed in mid '70s. It has been long since OCDM remains outside the mainstream of research community of optical communications, however, possible scarcity of the wavelength resource in future photonic networks, the simple access protocol as well as versatility of optical codes motivate recent growth of OCDM research activities. In this paper, first, fundamentals of OCDM concept are presented, highlighting optical encoding and optical time gate detection which realize time spreading/despreading. Next, current research activities of OCDM are reviewed by focusing particularly on the optical implementations and the proof-of-concept experiments. It is devoted to three categories; high bit rate point-to-point transmissions, gigabit multiple access, followed by optical path networks using optical code. Finally, future issues are briefly summarized.

This paper proposes a new polling-based dynamic slot assignment (DSA) scheme. With the rapid progress of wireless access systems, wireless data communication will become more and more attractive. In wireless data communication, an efficient DSA scheme is required to enhance system throughput, since the capacity of radio links is often smaller than that of wired links. A polling-based DSA scheme is typically used in centralized slot assignment control systems. It, however, is difficult to assign the slots to the targeted mobile terminals in a fair-share manner if only a polling-based scheme is used, especially in unbalanced-traffic circumstances, as revealed later. To solve this problem, we propose the exponential decreasing and proportional increasing rate control as is employed in available bit rate (ABR) service in ATM so that fair slot assignment is achieved even in heavily-unbalanced-traffic circumstances. Moreover, so that an AP operating with a large number of MTs can avoid long transmission delays, a polling-based resource request scheme with random access is featured in a new algorithm. Simulations verify that the proposed scheme offers fair slot assignment for each user while maintaining high throughput and short delay performance.

To improve frequency efficiency or user capacity in multi-path fading environments, we introduce and investigate an orthogonal multi-carrier frequency hopping-code division multiple access (FH-CDMA). These improvements are achieved by combining the orthogonal frequency division multiplexing (OFDM) and FH-CDMA schemes. The basic idea has been previously proposed by the authors. The aim of study in this paper is to evaluate the performance of this scheme in various environments. The theoretical analysis of bit error rate (BER) performance in this paper includes the effects of frequency selective fading in land mobile communications and of nonlinear amplification in satellite communications. A modified scheme of controlling transmission power to be controlled according to the number of simultaneously accessing users is also discussed. This modified scheme improves BER performance for frequency selective fading when the number of simultaneously accessing users in a cellular zone is small. Furthermore, an error-correcting code and its erasure decoding are applied in order to reduce errors due to hits in asynchronous FH/CDMA for reverse link as well as errors due to fading and noise.

This paper outlines technical trend of multimedia mobile and broadband wireless access systems utilizing comprehensive concept and new categorization of wireless access. It is pointed out that mobile communications have much potential market in future high-speed data or IP (Internet Protocol)-based traffic. Categorization of three kinds of wireless access is presented based on the definition adopted by the Radiocommunication Sector in ITU. IMT (International Mobile Telecommunications)-2000 and broadband wireless LANs (Local Area Networks) each representing mobile and nomadic wireless access will bring about a turning point to a new era that wireless communications become dominant media in access networks. With this perspective technical subjects, spectrum consideration and deployment scenario for these systems are discussed.

In this paper, we propose a dynamic TDMA with priority-based request packet transmission scheme (D-TDMA/PRPTS) which applies priority-based request packet transmission scheme instead of slotted ALOHA (S-ALOHA). D-TDMA/PRPTS can avoid collisions between voice request packets and data request packets and transmit voice request packets preferentially. This makes D-TDMA/PRPTS enlarge the system capacity for voice users with SAD. We analyze voice packet dropping probability and channel utilization for voice traffic by using an appropriate Markov model. We also present simulation results to verify the analysis and to investigate data performances as well, with the voice-data integrated scenario.

Internet communication is increasingly becoming an important element in daily life. Keeping this network safe from malicious elements is an urgent task for network management. To maintain the security level networks are generally, monitored for indications of usage with ill-intentions. Such indications are events which need to be collated, correlated and analyzed in real-time to be effective. However, on an average medium to large size network the number of such events are very large. This makes it practically impossible to analyze the information in real-time and provide the necessary security measures. In this paper, we propose a mechanism that keeps the number of events, to be analyzed, low thereby making it possible to provide ample security measures. We discuss a real-time Intrusion Detection System (IDS) for detecting network attacks. The system looks out for TCP ACK/RST packets, which are generally caused by network scans. The system can extract the tendency of network flows in real-time, based on the newly developed time-based clustering and Dynamic Access Tree creation techniques. The algorithm, implemented and deployed on a medium size backbone network using RMON (Remote MONitoring) technology, successfully detected 195 intrusion attempts during a one month period. The results of the pilot deployment are discussed. In this paper, the proposal, implementation and evaluation will be described.

Newer off-chip DRAM families, including Synchronous DRAMs (SDRAMs) and RAMBUS DRAMs (RDRAMs), are becoming standard choices for the design of high-performance systems. Although previous work in High-Level Synthesis (HLS) has addressed exploiting features of page-mode DRAMs, techniques do not exist for exploiting the two key features of these newer DRAM families that boost memory performance and help overcome bandwidth limitations: (1) burst mode access, and (2) interleaved access through multiple banks. We address pre-synthesis optimizations on the input behavior that extract and exploit the burst mode and multiple bank interleaved access modes of these newer DRAM families, so that these features can be exploited fully during the HLS trajectory. Our experiments, run on a suite of memory-intensive benchmarks using a contemporary SDRAM library, demonstrate significant performance improvements of up to 62.5% over the naive approach, and improvements of up to 16.7% over the previous approach that considered only page-mode or extended-data-out (EDO) DRAMS.

The minimum mean-squared error (MMSE) linear detector has been proposed to successfully suppress the multiple access interference and mitigate the near-far problem in direct-sequence code-division multiple access communication systems. In the presence of unknown or time-varying channel parameters, the MMSE linear detector can be implemented by the blind Griffiths' algorithm, which uses the desired signal vector instead of a training sequence of symbols for initial adaptation. In this paper, a variable step-size (VSS) Griffiths' algorithm is proposed for accelerating the convergence speed, especially in the presence of strong interference. Numerical results show that the convergence properties of the VSS Griffiths' algorithm are robust against the wide eigenvalue-spread problem of the correlation matrix associated with the received signal vector compared to the Griffiths' algorithm using a fixed step-size.

A T-user uniquely decodable (UD) code {C1,C2,,CT} over an integer set {0,1,,k} with arbitrary code length is developed for a multiple-access adder channel (MAAC). Each of the T users is equipped with two codewords, one of which is zero vector. The T-user UD code is used to identify users through the MAAC. It is shown that a T(f+g+1)-user UD code with code length f+g+1 can be arranged from two given T(f)-user and T(g)-user UD codes. This idea makes it possible to construct recursively a T-user UD code for an arbitrary code length n and a positive integer k. The T-user UD code includes the Jevticode.

In order to lead an ongoing software project to success, it is important to flexibly control its dynamically-changing software process. However, it is generally impossible not only to exactly pre-define the production process but also to prescribe the process change process (meta-process). To solve the problem, we have focused on communication between the project staff through which process change requests presented by individuals can be immediately shared, designed, verified, validated and implemented. This paper proposes a communication model which can represent a wide variety of communication states between the project manager and developers discussing how to implement process change requests. The communication model has been derived by investigating the sort of process change requests and, based on the model, we have implemented a cooperative process planning system (called CooPs). CooPs is a communication environment designed for software projects and supports information sharing for discussing the process change requests. By using CooPs, the software project can flexibly deal with not only expected change requests but also unexpected ones. To evaluate the applicability of the communication model and the capabilities of CooPs, we have conducted an experiment which is an application of CooPs to the ISPW6 example problem. This paper describes the concepts of CooPs, the system implementation, and the experiment.

This paper proposes a novel universal line termination scheme for the ONUs (optical network units) of fiber-optic local access systems. Its main feature is that only low cost AD/DA converters for Hi-Fi audio are needed. Because audio AD/DA converters are insufficient for ISDN basic rate access (● 320kbaud) and cause waveform distortion, we develop a simple detection algorithm that does not use any equalizing filter. The algorithm can handle plural channels with one general purpose MPU (micro-processing unit). Based on this, a novel architecture for a fiber-optic local access system is presented that removes the MPUs from each optical network unit (ONU) and places them in the central office (CO). The proposed system yields a small, service-uniform ONU that supports a wide range of narrow-band services (POTS & ISDN) with no distinction. To realize this system at the lowest possible cost, a high-speed code division multiplexing (CDM) scheme with novel code word sets is developed.

Two classes of nonlinear feedforward logic (NLFFL) pseudonoise (PN) code generators based on the use of AND and majority logic (ML) gates are compared. Cross-correlation and code-division multiple-access (CDMA) properties of properly designed NLFFL sequences are found to be comparable with the properties of well-known linear PN codes. It is determined that code design employing ML gates with an odd number of inputs is easier compared with designing with AND gates. This is especially true when the degree of nonlinearity is large, since the nonbalance problem, e. g. , at the output of an AND gate, can be avoided. ML type sequences are less vulnerable to correlation attack and jamming by the m-sequence of an NLFFL generator

An access attempt overload control method is proposed for CDMA cellular system to provide good communication quality to mobile stations under the traffic overload condition. Under the normal traffic condition, the effect of access attempts on the interference is negligible. But under the traffic overload condition, the interference due to access attempts could be large and deteriorate the communication quality (or reduce the capacity of a reverse link) if access attempts are not controlled properly. The numerical and simulation results show that our access attempt overload control method can maintain the interference caused by access attempts very low and the capacity of the reverse link is not reduced under the traffic overload condition.

We propose a new bit timing recovery (BTR) scheme, called perturbed sampling BTR (PSBTR), that can operate near the symbol rate in high-bit-rate wireless systems. A peculiar sample clock, the duty factor of which is not 50%, is used in the PSBTR scheme. We call this type of clock a perturbed sample clock and use it for clock recovery. In PSBTR, there is no cycle slip of the sample clock, and the PSBTR circuit is mostly digital. We examine the performance of the PSBTR scheme under additive white Gaussian noise (AWGN) by computer simulation and experiment, and from these results, clarify the relationship between the performance and circuit parameters of the PSBTR circuit. The overall results indicate that the PSBTR scheme performs well and can be employed as a BTR scheme for high-bit-rate wireless systems.

Minimizing the guard time, Tguard, in the TCM-TDMA PDS scheme is essential in maximizing TCM transmission efficiency. As a replacement for the commonly adopted worst-case approach to TCM-TDMA PDS system estimation, this paper proposes a statistical approach. The level distributions of losses and n-th order Fresnel reflections are determined from published measurements. The proposed approach estimates the reflection of the optical access network.

This paper describes a multibeam antenna which uses switched parasitic and switched active elements to obtain multiple simultaneous directional beams that can be steered in azimuth. A 13 element monopole multibeam array has been optimised for gain and front to back ratio. Results from numerical simulation and measurements in an anechoic chamber are presented. The 13 element array can achieve up to three beams simultaneously with a minimum gain over 360 azimuthal coverage of 1.2 dB less than the maximum gain. Located on a ground plane with diameter of 2λ at 1.5 GHz, the maximum elevation angle was 20.2 with -3 dB vertical beamwidth of 88.

The Joint CDMA/PRMA (JCP) protocol, proposed by Brand and Aghvami, is modified to increase the capacity for mobile communication systems. To reduce multiuser interference, the modified JCP uses an access channel additionally to the traffic channels, on which each mobile terminal reserves a unique slot and code. Furthermore, the modified JCP employs receiver-based code scheme. In the case of voice-only traffic, the throughput increases by up to 15% compared to that of the conventional JCP, when Ploss is 0.01. Also, for the mixed traffic case, the throughput increases about 20%.

A variable partition duplex scheme on packet reservation multiple access protocol (VPD-PRMA) is analyzed in this paper. We assume a four-state speech model for a conversational pair and successfully obtain performance measures by approximate Markovian analysis. Analytical results show that they quite fit simulation results; and VPD-PRMA can get higher statistical multiplexing gain than fixed partition duplex (FPD)-PRMA, due to the trunking effect. We further investigate the effect of design parameters of permission probability and enlarged reservation duration on system performance by computer simulation. Simulation results shows that it exists appropriate values for these two design parameters so that the packet dropping probability can be minimized. The adjustment of permission probability can greatly improve the performance of uplink traffic with slight deterioration of the performance of downlink traffic; the provision of enlarged reservation duration scheme can enhance the system performance.

The cable TV (CATV) network is an ideal backbone for PCS due to its prevalent and broadcast nature. This significantly reduces the implementation costs and the handoff overheads. This paper proposes two architectures for the CATV-based PCS system. In the first architecture, each base station is equipped with multiple fixed receivers to provide fast and seamless handoffs for mobile users. Nevertheless, the hardware cost for this implementation is relative expensive. To overcome this, in the second architecture, each base station is equipped with only one tunable receiver. This simple and economic architecture suffers from the possibility of offset conflict when mobile terminals handoff between cells. Three offset assignment algorithms based on the concepts of random assignment, neighbor usage, and clustering, respectively are proposed to solve the offset conflict problem. Simulation results indicate the one with the concept of clustering performs much better than the other two schemes in terms of offset conflict probability.