Space-frequency transmit diversity (SFTD) and space-code transmit diversity (SCTD), which are both based on space-time block codes (STBC), were applied to time-direction spreading and two-dimensional spreading orthogonal frequency-division multiplexing code-division multiplexing (OFDM-CDM) systems, and the transmission performances were compared by computer simulation. SFTD is designed for space and two adjacent subcarriers whereas SCTD is designed for space and two distinct Walsh-Hadamard (WH) codes. The simulation results show that SCTD applied to time-direction spreading OFDM-CDM was superior to SFTD because frequency selectivity distorted STBC's orthogonality between sub-carriers in SFTD. In contrast, when they were applied to two-dimensional spreading OFDM-CDM, SFTD was superior to SCTD when the number of WH codes belonging to the same mother-code group is small because the frequency diversity provided by SFTD surpassed that provided by SCTD. In addition, both SFTD and SCTD provide high tolerance to large Doppler spread. It can be therefore concluded that both SCTD and SFTD can be used in the same frame by code-multiplexing according to their suitability to physical channels. SCTD is suitable for transmitting high-rate data via time-direction spreading, whereas SFTD is suitable for transmitting control data via two-dimensional spreading.

In OFDM systems, employing a cyclic prefix (CP) as the guard interval is a simple way to combat the inter-symbol interference (ISI) and the inter-carrier interference (ICI), however it reduces the transmission efficiency of the system, especially for some channels with a very long delay spread. In this paper, we consider the OFDM system with insufficient CP, much more efficient than conventional OFDM systems. First, we present the system mathematical model and give the ISI and ICI analysis. Then the signal-to-interference power ratio (SIR) performance is presented. To reduce the ISI and ICI due to the insufficient CP, we develop a minimum-mean-square-error decision feedback equalizer (MMSE_DFE). Based on the MMSE criterion, the optimum feedforward and feedback filter coefficients are derived. For time-varying channel, to avoid brute force matrix inversion in conventional schemes, we propose an adaptive LMS based solution to update the filtering coefficients by tracing the channel variation. Since the high complexity of MMSE_DFE, a reduced complexity scheme, ordered successive partial interference cancellation DFE (OSPIC_DFE), is developed. From the performance comparison between the MMSE_DFE and the OSPIC_DFE, we see that the latter is very near to the former. Finally the simulation shows these proposed methods are highly effective in combating ISI and ICI with low complexity.

Although the multiuser detection scheme based on Kalman filtering (K-MUD) proposed by Zhang and Wei, is referred to as a "blind" algorithm, in fact it is not really blind because it is conditioned on perfect knowledge of system parameter, power of the desired user. This paper derives an algorithm to estimate the power of the user of interest, and proposes a completely blind multiuser detection. Computer simulations show that the proposed parameter estimation scheme obtains excellent effect, and that the new detection scheme has nearly the same performance as the K-MUD, there is only slight degradation at very low input signal-to-interference ratios (SIR).

A hardware algorithm for modular multiplication/division which performs modular division, Montgomery multiplication, and ordinary modular multiplication is proposed. The modular division in our algorithm is based on the extended Euclidean algorithm. We employ our newly proposed computation method that consists of processing the multiplier from the most significant digit first to calculate Montgomery multiplication. Finally, the ordinary modular multiplication is based on shift-and-add multiplication. Each of these three operations is carried out through the iteration of simple operations such as shifts and additions/subtractions. To avoid carry propagation in all additions and subtractions, the radix-2 signed-digit representation is employed. A modular multiplier/divider based on the algorithm has a linear array structure with a bit-slice feature and carries out n-bit modular multiplication/division in O(n) clock cycles, where the length of the clock cycle is constant and independent of n. This multiplier/divider can be implemented using a hardware amount only slightly larger than that of the modular divider.

We propose a multiple dwell serial search (MDSS) code acquisition for optical code-division multiple-access (O-CDMA) systems and theoretically analyze its performance. The search/lock strategy (SLS) is used as verification scheme for the multiple dwell detector. The operation of SLS is modeled by finite Markov chain to analyze the performance of the proposed system. Effect of system parameters, such as number of users, threshold and mean photon count per chip, on the performance of the proposed system is investigated. The theoretical result shows that the performance of the proposed system is less sensitive to parameter settings than the conventional single dwell serial search (SDSS) code acquisition system is. In addition, the proposed MDSS code acquisition system offers shorter mean acquisition time than that of conventional SDSS system.

This paper presents a processor architecture for high-speed and reliable stereo matching based on adaptive window-size control of SAD (Sum of Absolute Differences) computation. To reduce its computational complexity, SADs are computed using images divided into non-overlapping regions, and the matching result is iteratively refined by reducing a window size. Window-parallel-and-pixel-parallel architecture is also proposed to achieve to fully exploit the potential parallelism of the algorithm. The architecture also reduces the complexity of an interconnection network between memory and functional units based on the regularity of reference pixels. The stereo matching processor is implemented on an FPGA. Its performance is 80 times higher than that of a microprocessor (Pentium4@2 GHz), and is enough to generate a 3-D depth image at the video rate of 33 MHz.

Low-density parity-check (LDPC) codes are one of the most promising next-generation error-correcting codes. For practical use, efficient methods for encoding of LDPC codes are needed and have to be studied. However, it seems that no general encoding methods suitable for hardware implementation have been proposed so far and for randomly constructed LDPC codes there have been no other methods than the simple one using generator matrices. In this paper we show that some classes of quasi-cyclic LDPC codes based on circulant permutation matrices, specifically LDPC codes based on array codes and a special class of Sridhara-Fuja-Tanner codes and Fossorier codes can be encoded by division circuits as cyclic codes, which are very easy to implement. We also show some properties of these codes.

The shuffle-like network (SL-Net) is known as a logical topology for WDM-based multihop packet-switched networks. Even if we fix the logical topology to an SL-Net, we can still reposition nodes in the SL-Net by re-tuning wavelengths of transmitters and/or receivers. In conventional node placement algorithms, routes between nodes are assumed to be given. In this paper, we propose two heuristic node placement algorithms for the SL-Net to decrease the average end-to-end packet transmission delay under a given traffic matrix in the case that routes are design variables. The principal idea is to prevent too many traffic flows from overlapping on any link. To attain the idea, in one of the algorithms, a node is selected one by one in a decreasing order of the sums of sending and receiving traffic requirements in nodes, and its placement and routes between the node and all the nodes already placed are simultaneously decided so that the maximum of the amounts of traffic on links at the moment is minimum. In the other algorithm, a node is selected in the same way, and first it is placed so that the average distance between the node and all the nodes already placed is as large as possible, and then routes between the node and all the nodes already placed are decided so that the maximum of the amounts of traffic on links at the moment is minimum. Numerical results for four typical traffic matrices show that either of the proposed algorithms has better performance than conventional algorithms for each matrix, and show that the proposed algorithms, which are based on a jointed optimization approach of node placement and routing, are superior to algorithms which execute node placement and routing as two isolated phases.

This letter deals with our investigations into improving the performance of a wireless uplink system when an orthogonal frequency-division multiple access (OFDMA) is used as an access scheme. To do this, the OFDMA-based uplink system adopts a frequency diversity coupled with a cyclic time shift (CTS) at the transmitter, which is named as the FD-OFDMA system with CTS. It is found that the multi-user FD-OFDMA system equipping with CTS can decrease the probability of destroying the orthogonality among the users and provide the MAI-robust reception without decreasing the bandwidth efficiency of the system.

Time-Division-Multiplexing (TDM) is usually aimed at digital signals, while Continuous Wave Time-Division- Multiplexing (i.e. CWTDM) presented in this paper mainly addresses the problem of multiple continuous signals to share a channel. According to the idea in Ref.[1], this paper proposes a novel method for implementing CWTDM, which can make multiple band-limited continuous signals time-division-multiplexed into one continuous signal without significantly expanding the frequency band. The new method has several important applications. In particular, it can be used to implement an on-board FDMA-CWTDM conversion to develop a new system of satellite communications with more efficient performance.

In order to simultaneously combat both of the inter-carrier interferences (ICIs) and multiple access interferences (MAIs) to achieve reliable performance in multi-carrier code division multiple access (MC-CDMA) systems, this letter proposes a maximum likelihood based scheme for joint frequency offset estimation and multiuser symbol detection. To reduce the computational complexity called for by the joint decision statistic without extra mechanisms, the genetic algorithm (GA) is employed to solve the nonlinear optimization involved. Due to the robustness of the GA, the joint decision statistic can be efficiently solved, and, as shown by furnished simulation results, the proposed approach can offer satisfactory performance in various scenarios.

OFDM-based networks utilizing the frequency reuse factor of 1 may produce the severe ICI (intercell interference) at the cell boundary even though overall cell capacity is increased and network deployment is facilitated. In the forward-link, the ICI may rise above a QoS (quality of service) threshold beyond some distance from BSs (base stations). In this paper, we analyze the forward-link capacity of an MC-CDMA system as a function of the ICI according to the distance from a cell. To achieve this goal, a closed form of the outage probability is derived and utilized to obtain the accommodated number of users and system parameters.

In this paper, we propose a novel architecture for optical transport networks and its operation scheme guaranteeing the QoS requirements based on real-time traffic measurement. The key concept of the proposed architecture, which we call hybrid optical transport network (HOTNET), is to adopt both optical circuit switching and optical message switching in an optical network. To implement two different switching technologies in a single network, we modify the optical burst switching scheme and merge it into a TDM wavelength routed network. Then, we propose a control framework and an architecture of a switching node for this hybrid switching paradigm. We also discuss a real-time bandwidth provisioning scheme which utilizes the advantages of two respective switching schemes for traffic engineering. Finally, we evaluate the performance of the proposed scheme via computer simulation and the results show that it can guarantee the traffic QoS requirements while maintaining high channel utilization.

We studied 10 Gbit/s-based time-spreading and wave-length-hopping (TS-WH) optical code division multiplexing (OCDM) using fiber Bragg gratings (FBGs). To apply it to such the high bit rate system more than ten gigabit, two techniques are adopted. One is encoding with the maximum spreading time of 400 ps, which is four times as data bit duration, to encode without shortening chip duration. Another is encoder design. The apodized refractive index profile to the unit-gratings composing the encoder is designed to encode the pulses with 10-20 ps width at 10 Gbit/s rate. Using these techniques, 210 Gbit/s OCDM is demonstrated successfully. In this scheme, transmission distance is limited due to dispersion effect because the signal has wide bandwidth to assign a wavelength-hopping pattern. We use no additional devices to compensate the dispersion, in order to construct simple and cost-effective system. Novel FBG encoder is designed to incorporate both encoding and compensating of group delay among chip pulses within one device. We confirm the extension of transmission distance in the TS-WH OCDM from the demonstration over 40 km-long single mode fiber.

The purpose of this paper is to improve a feedback-type adaptive array antenna (AAA) with feedback information quantized by one bit which was presented recently on TDMA system by an author of this paper. The improvement is made by using adaptive, instead of constant, update size of adaptive antenna weights control. Computer simulation results show that the performance of this system is improved to be almost equivalent to the performance of a system without quantization of the feedback information for wide range of fading speed. The results include the effect of control delay time and the maximum Doppler frequency under flat fading and frequency-selective fading.

In this letter, we present a space division multiple access (SDMA) approach for IEEE802.11a-based system employing pre-fast Fourier transform (FFT) adaptive array antenna (AAA) at base station (BS). As the core idea, we propose a preamble subcarrier assignment method to generate different preambles for different users using the same signal burst structure defined by IEEE802.11a, by which BS can effectively distinguish each user from other users and accurately estimate the channel impulse response (CIR) for each user. In this way, SDMA can be easily realized with no significant change in IEEE802.11a-based system. The performance of the proposed SDMA system is evaluated by computer simulation using a realistic spatio-temporal indoor wireless channel model.

In this paper, we propose an agent architecture for a combination of speculative computation and abduction. Speculative computation is a tentative computation when complete information for performing computation is not obtained. We use a default value to complement such incomplete information. Unlike usual default reasoning, the real value for the information can be obtained during the computation and the computation can be revised on the fly. In the previous work, we applied this technique to handling distributed problem solving under incomplete communication environments in the context of multi-agent systems and proposed correct procedures in abductive logic programming in terms of perfect model semantics. In the previous work, however, we regarded assumptions as defaults and used these assumptions for speculative computation. Thus, we could not perform hypothetical reasoning, that is, the original usage of abduction. In this paper, we extend our framework so that speculative computation and abduction can be both performed. As a result, our procedure becomes an extension of the abductive procedure developed by Kakas and Mancarella augmented by dynamic belief revision mechanism about outside world.

A novel phase error correction scheme is proposed for the high rate OFDM-based wireless local area networks (WLANs). The proposed scheme makes the system capable of efficiently compensating the whole phase error due to the residual sampling clock offset and frequency offset estimation error after timing and frequency offset compensation, as well as the phase noise.

A new and simple image processing approach for the measurement of the length of pedestrian crossings with a view to develop a travel aid for the blind people is described. In a crossing, the usual black road surface is painted with constant width periodic white bands. The crossing length is estimated using vector geometry from the left- and the right-border lines, the first-, the second- and the end-edge lines of the crossing region. Image processing techniques are applied on the crossing image to find these lines. Experimental results using real road scenes with pedestrian crossing confirm the effectiveness of the proposed method.

The direct optical switching CDMA radio-on-fiber network is proposed as multiplexing method for radio base stations in microcell/picocell technologies. In this system, aliasing distortions degrade the received signal quality and decreases the number of connectable radio base stations (RBSs) when the encoding rate becomes lower than the twice of bandwidth of RF signal. This paper proposes the optical switching speed reduction method and clarifies that the chaotic spreading sequences can suppress the aliasing distortion without additional processing at the receiver even if the encoding rate becomes lower than the RF signal bandwidth. The effect in switching speed reduction is theoretically investigated and the effect in aliasing distortions suppression is experimentally investigated by using the proposed method.