We propose a multiuser MIMO precoding algorithm that combines the block Tomlinson-Harashima precoding and the vector perturbation (BTHP-VP). BTHP-VP supports multi-stream transmission without additional estimation of each user's effective channel and achieves full spatial diversity. Computer simulations show that BTHP-VP can achieve similar sum rate and improved BER performance compared to the BTHP with maximum likelihood receiver.

In this letter, we propose a novel approach using wireless sensor networks (WSNs) to enhance the safety and efficiency of four-way stop-sign-controlled (FWSC) intersections. The proposed algorithm provides right of way (RoW) and crash avoidance information by means of an intelligent WSN system. The system is composed of magnetic sensors, embedded in the center of a lane, with relay nodes and a base station placed on the side of the road. The experimental results show that the vehicle detection accuracy is over 99% and the sensor node battery life expectancy is over 3 years for traffic of 5,800 vehicles per day. For the traffic application we consider, a strong effect is observed as the projected conflict rate was reduced by 72% compared to an FWSC intersection operated with only driver perception.

In this letter, a Tomlinson-Harashima precoding (THP) scheme is proposed for the downlink of multiuser MIMO systems with multiple antennas at each receiver. Assuming single data stream communication for each user, joint transmitter and receiver design is done to maximize the signal to noise ratio (SNR) for each user. Furthermore, a heuristic user ordering algorithm is proposed to optimize the encoding order and improve the bit error rate (BER) performance. Simulation results have shown that the proposed approach is superior to some existing precoding schemes.

The performance of single-carrier (SC) transmission in a frequency-selective fading channel degrades due to a severe inter-symbol interference (ISI). Using frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can improve the bit error rate (BER) performance of SC transmission. However, the residual ISI after FDE limits the performance improvement. In this paper, we propose a joint use of Tomlinson-Harashima precoding (THP) and FDE to remove the residual ISI. An approximate conditional BER analysis is presented for the given channel condition. The achievable average BER performance is evaluated by Monte-Carlo numerical computation method using the derived conditional BER. The BER analysis is confirmed by computer simulation of the signal transmission.

In this letter, we discuss the problem of receive antenna selection in the downlink of multiuser multiple-input multiple-output (MIMO) systems with Tomlinson-Harashima precoding (THP), where the number of receivers is assumed equal to that of transmit antennas. Based on the criterion of maximum system sum-capacity, a per-layer receive antenna selection scheme is proposed. This scheme, which selects one receive antenna for each receiver, can well exploit the nonlinear and successive characteristics of THP. Two models are established for the proposed per-layer scheme and the conventional per-user scheme. Both the theoretical analysis and simulation results indicate that the proposed scheme can greatly improve the equivalent channel power gains and the system sum-capacity.

In this letter, we investigate the application of Tomlinson-Harashima precoding (THP) in the downlink of multiuser multiple-input multiple-output (MIMO) systems, where multiple antennas are located at all the transceivers. Based on the criterion of maximum system sum-capacity, a per-layer optimization scheme is proposed, in which the subchannel ordering and transceiver filters design are generated. In the proposed scheme, the successive character of THP can be fully exploited, so that both the minimum cost of interference suppression and the maximum power and diversity gains can be implemented, and hence, the system sum-capacity can be improved effectively.

Optical fiber communications require multiple-access schemes to access a shared channel among multiple users. The coherent ultrashort light pulse code-division multiple-access (CDMA) system is one such scheme, and it also offers asynchronous-access communication. This system usually employs 2-level, i.e., binary, m-sequences as signature codes because of their low correlation. If the number of active users is greater than the length of the m-sequence, i.e., code length, distinct m-sequences are used. However, the distinct 2-level m-sequences do not exhibit low correlation, resulting in performance degradation. We therefore propose a coherent ultrashort light pulse CDMA communication system with distinct 4-level, i.e., quaternary, m-sequences to improve system performance when the number of users is greater than the code length. We created the 4-level m-sequences from 2-level m-sequences, and assess the correlation of the 4-level m-sequences. We also theoretically derive the bit error rate (BER) of the proposed system taking into account multiple-access interference (MAI), beat noise, amplified spontaneous emission (ASE), shot noise, and thermal noise. The numerical results show that BER for distinct 4-level m-sequences is more than an order of magnitude smaller than that for distinct 2-level m-sequences. BER is limited by MAI and beat noise when the power of the received signal is high, otherwise BER is limited by ASE, shot noise, and thermal noise.

We theoretically analyze the performance of coherent ultrashort light pulse code-division multiple-access (CDMA) communication systems with a nonlinear optical thresholder. The coherent ultrashort light pulse CDMA is a promising system for an optical local area network (LAN) due to its advantages of asynchronous transmission, high information security, multiple access capability, and optical processing. The nonlinear optical thresholder is based on frequency chirping induced by self-phase modulation (SPM) in optical fiber, and discriminates an ultrashort pulse from multiple access interference (MAI) with picosecond duration. The numerical results show that the thermal noise caused in a photodetector dominates the bit error rate (BER). BER decreases as the fiber length in the nonlinear thresholder and the photocurrent difference in the photodetector increase. Using the nonlinear optical thresholder allows for the response time of the photodetector to be at least 100 times the duration of the ultrashort pulses. We also show that the optimum cut-off frequency at the nonlinear thresholder to achieve the minimum BER increases with fiber length, the total number of users, and the load resistance in the photodetector.

Δ-Timed Atomic Broadcast is the broadcast ensuring that all correct processes deliver the same messages in the same order, and that delivery latency of any message broadcast by any correct process is some predetermined time Δ or less. In this paper, we propose a Δ-timed atomic broadcast algorithm in a synchronous system where communication delay is bounded by a known constant d and processes suffer both crash faults and timing faults. The proposed algorithm can tolerate fc crash faults and ft timing faults as long as at least ft + 1 processes are correct, and its maximum delivery latency Δ is (2f' + 7)d where f' is the actual number of (crash or timing) faulty processes. That is, the algorithm attains the early-delivery in the sense that its delivery latency depends on the actual number of faults rather than the maximum number of faults that the algorithm can tolerate. Moreover, the algorithm has a distinct advantage of guaranteeing that timing-faulty processes also deliver the same messages in the same order as the correct processes (Uniformity). We also investigate the maximum number of faulty processes that can be tolerated. We show that no Δ-timed atomic broadcast algorithm can tolerate ft timing faults, if at most ft processes are correct. The impossibility result implies that the proposed algorithm achieves the maximum fault-resilience with respect to the number of faulty processes.

The oscillation characteristics of a 40 GHz, 1-3 ps regeneratively and harmonically mode-locked erbium-doped fiber laser have been investigated in detail with respect to stability, linewidth, and mode hopping. We show that because the Q value of the microwave filter in the feedback loop is limited to around 1000, which is almost the same as that in a 10 GHz laser, the cavity length should not be greatly increased as this would result in as much as a fourfold increase in the number of longitudinal beat signals. We undertook a detailed stability analysis by using three cavity lengths, 60, 80, and 230 m. The 80 m long cavity greatly improved the long-term stability of the laser because the supermode noise was suppressed and there were not too many longitudinal modes. We measured the linewidth of the longitudinal mode of the laser using a heterodyne method, and it was less than 1 kHz. We also point out that there is a longitudinal mode hopping effect with time that is induced by very small changes in temperature.

An all-optical switch based on a single photonic crystal defect with an air-bridge configuration and two-photon absorption was proposed, fabricated and characterized. In optical measurements, we obtained a sharp defect mode with a quality factor higher than 600 at 1.55 µm. More importantly, we observed its nonlinear response to the excitation of ultrashort pulses by utilizing two-photon absorption. Nonliner refractive index change of about -410-3 was achieved at a pumping power density of 3.6109 W/cm2.

Ultrafast all optical switching using pulse trapping by 100 fs ultrashort soliton pulse across zero dispersion wavelength is investigated. The characteristics of pulse trapping are analyzed both experimentally and numerically. Using the pulse trapping, 1 THz ultrafast all optical switching is demonstrated experimentally. Arbitral one pulse is picked off from pulse train. Pulse trapping for CW signal is also demonstrated and ultrashort pulse is generated by pulse trapping. From these investigation, it is shown that ultrafast all optical switching up to 2 THz can be demonstrated using pulse trapping.

We designed an FPGA-based parallel machine called "RASH"(Reconfigurable Architecture based on Scalable Hardware) for high speed and flexible signal/data processing. Cryptanalysis is one of the killer applications for FPGA-based machines because huge amounts of logical and/or simple arithmetic operations are required and FPGA is suitable for this. One of the well-known activities in cryptanalysis is the DES (Data Encryption Standard) cracking contest conducted by RSA Data Security. TMTO (Time-Memory Trade-Off) Cryptanalysis is a practical method to dramatically shorten the time for key search when plaintext is given in advance. A string of ASCII characters is used as the key much like a password. The ASCII character is 7-bit character and is changed to 96 kinds of value. The 56-bit DES key is given with a string of 8 ASCII characters. Although the DES key has 64 trillion(=256) possibilities, the key that is given with a string has only 6.4 trillion(=968) possibilities. Therefore, we improve TMTO cryptanalysis so that we search only the limited key by ASCII characters and reduce the quantity of computation. In this paper, we demonstrate how TMTO cryptanalysis for limited key is well suited to our FPGA-based RASH machine. By limiting the key to a string, DES key will be found at 80% probability within 45 minutes after ciphertext is given on 10 units of RASH. The precomputation before starting key search takes 3 weeks on the same RASH configuration.

Traditionally, UNIX has been weak in data sharing. By data sharing, we mean that multiple cooperative processes concurrently access and update the same set of data. As the degree of sharing (the number of cooperative processes) increases, the existing UNIX virtual memory systems run into page table thrashing, which causes a major performance bottleneck. Once page table thrashing occurs, UNIX performs miserably regardless of the hardware platforms it is running on. This is a critical problem because UNIX is increasingly used in environments such as banking that require intensive data sharing. We consider several alternatives to avoid page table thrashing, and propose a solution of which the main idea is to share page tables in virtual memory. Extensive experiments have been carried out with real workloads, and the results show that the shared page table solution avoids the page table thrashing and improves the performance of UNIX by an order of magnitude.

We investigate dispersion compensation using dispersion-compensating fibers (DCFs) for ultrashort light pulse code division multiple access (CDMA) communication systems in a multi-user environment. We employ fiber link that consists of a standard single-mode fiber (SMF) connected with two different types of DCFs. Fiber dispersion can be effectively decreased by adjusting the length ratios of DCFs to SMF appropriately. Some criteria for dispersion compensation are proposed and their performances are compared. We theoretically derive a bit error rate (BER) of ultrashort light pulse CDMA systems including the effects of the dispersion and multiple access interference (MAI). Moreover, we reveal the mutual relations among BER performance, fiber dispersion, MAI, the number of chips, a bandwidth of a signal, and a transmission distance for the first time. As a result, we show that our compensation strategy improves system performance drastically.

Very reliable mode-locked semiconductor lasers have been developed. These devices provide high signal-to-noise ratio optical clock pulses of a few picoseconds temporal width in the 1.5-micrometer wavelength region. Potential applications of these lasers for high-bit-rate optical communication systems operating at over 40 Gbps including all-optical signal processing, and for very high-speed measurement systems are described.

One major breakthrough on the communication society recently is the extension of networking from wired to wireless networks. This has made possible creating a mobile distributed computing environment and has brought us several new challenges in distributed protocol design. Obviously, wireless networks do have some fundamental differences from wired networks that need to be paid special attention of, such as lower communication bandwidth compared to wired networks, limited electrical power due to battery capacity, and mobility of processes. These new issues make traditional recovery algorithm unsuitable. In this paper, we propose an efficient algorithm with O(nr) message complexity where O(nr) is the total number of mobile hosts (MHs) related to the failed MH. In addition, these MHs only need to rollback once and can immediately resume its operation without waiting for any coordination message from other MHs. During normal operation, the application message needs O(1) additional information when it transmitted between MHs and mobile support stations (MSSs). Each MSS must keep an ntotal_h*n cell_h dependency matrix, where O(ntotal_h) is the total number of MHs in the system and ncell_h is the total number of MHs in its cell. Finally, one related issue of resending lost messages is also considered.

We propose two fault-tolerant broadcasting schemes in star graphs. One of the schemes can tolerate up to n2 faults of the crash type in the n-star graph. The other scheme can tolerate up to (n3d1)/2 faults of the Byzantine type in the n-star graph, where d is the smallest positive integer satisfying nd!. Each of the schemes is designed for the single-port mode, and it completes the broadcasting in O(n log n) time. These schemes are time optimal. For the former scheme we analyze the reliability in the case where faults of the crash type are randomly distributed. It can tolerate (n!)α faults randomly distributed in the n-star graph with a high probability, where α is any constant less than 1.

Optical frequency division multiplexing (FDM) technique has the advantage of fully orthogonal transmissions. However, FDM system permits only a small number of FDM channels despite of a great effort, such as frequency stabilization. On the other hand, frequency-domain encoding code-division multiple-access (FE-CDMA) has been widely studied as a type of optical CDMA. In this system, encoding is done in the frequency domain of an ultrashort light pulse spread by optically Fourier transform. However, FE-CDMA accommodates very limited number of simultaneous users, though this scheme uses a vast optical bandwidth. It is attractive to consider the combination of both advantages of FDM and FE-CDMA. We propose FE-CDMA enhancement of FDM (FDM/FE-CDMA). Since in FDM/FE-CDMA the total bandwidth is partitioned into M optical bands and each band is encoded by the code with code length of Nc, we expect nearly perfect orthogonal transmissions. In addition, since the creation of FDM bands is realized by a passive filter, the optical frequency is precisely controlled and the optical frequency allocation is flexible. We derive the bit error rate (BER) as a function of the number of simultaneous users, bit rate, and the utilization efficiency of total bandwidth. We compare the performance of FDM/FE-CDMA with that of the conventional FE-CDMA in terms of the number of simultaneous users on condition that each chip width is constant. As a result, we show that FDM/FE-CDMA can support the larger number of simultaneous users than the conventional FE-CDMA at a given bit error rate under the same total bandwidth.

The novel application potential of mode-locked laser diodes (MLLDs) in ultrafast optical signal processing in addition to coherent optical pulse generation is described. As the most fundamental function of MLLDs, we show that the generation of ultrashort (2 ps) coherent optical pulses with low timing jitter (<0. 5 ps) at precisely controlled wavelength and repetition frequency can be achieved by employing a rigid module configuration for an external-cavity MLLD. We then discuss new aspects of MLLDs which are functions of ultrafast all-optical signal processing such as optical clock extraction and optical gating. All-optical clock extraction is based on the timing synchronization of MLLD output to the injected optical data pulse. When the passive mode-locking frequency of an MLLD is very close to the fundamental clock pulse frequency of optical data, the former frequency is pulled into the latter frequency by optical data injection. We show that same-frequency and subharmonic-frequency optical clock pulses can successfully be extracted from optical data pulses at bit rates of up to 80 Gbit/s with very simple configurations and very low excess timing jitter (<0. 1 ps). On the other hand, optical gating is due to absorption saturation and the following picosecond absorption recovery in a saturable absorber (SA) in an MLLD structure incorporating optical gate-pulse amplification. Here, MLLDs are anti-reflection coated and used as traveling wave devices instead of laser oscillators, and small saturation energy (<1 pJ) and ultrafast recovery time (<8 ps) are demonstrated. By combining all these MLLD functions, we successfully demonstrated an experiment with 40- to 10-Gbit/s all-optical demultiplexing processing.