This paper starts by describing the advantages of cascaded modulation, i.e., using multiple concatenated external modulators to modulate CW (Continuous Wave) light. Next, the paper examines computer simulations of the resulting modulated light waveform shapes and intermodulation distortion values to elucidate the basic modulation characteristics of a cascaded modulation scheme. Examples of applying cascaded modulation to a multi-channel optical signal transmission system are shown, and the characteristics are clarified by optical transmission experiments. For example, the dependency of the signal quality on the modulation depth values of each external modulator is clarified. Moreover, experiments show that cascaded modulation permits the remote insertion of local broadcast programs into wide area broadcast programs. Last, the paper shows that cascaded modulation offers better modulation properties than the conventional single modulation approach.

OBS is a realistic solution to the mismatch of the capacity of optical fiber and electrical switching in backbone photonic networks. One of the critical issues in OBS networks is to avoid burst contention at transit nodes. This problem induces the rapid growth of burst-transmission delay time under heavy traffic loads. In this paper, we propose a low-delay burst transmission scheme using burst segmentation at source node to suppress the growth in burst-transmission delay. In our scheme, a burst is divided and burst-transfer time is determined by the multiple information about reservation of other bursts at all transit nodes. We analyzed capabilities of the proposed scheme and found that it more efficiently suppresses the growth of the burst-transmission delay time in heavy traffic loads compared with some conventional signaling schemes.

With the advances of micro-electronic and wireless communication technology, deploying a large number of low cost, small-sized sensor nodes over a vast area for environment monitoring is becoming more practical. Setting up gradients for delivering queries or data reports by flooding the sensor networks consumes a great deal of energy especially for large scale sensor networks. In this paper, we propose an energy conserving observer-initiated data dissemination protocol, called Grid Map Data Query Protocol (GMDQP), for multiple mobile sinks on a large scale sensor network. It conserves communication energy by employing a grid map data query technique to avoid full network events or queries flooding. A data source only announces the existence of data within a local area and a sink collects data by sending query message in a grid map. Nodes at cross point of grid check and query the data. It conserves energy by avoiding full network flooding in setting up data forwarding path. Simulations show that the proposed architecture is quite energy efficient.

After the popularization of sensor networks, the size of the monitoring region and the number of sensor nodes will grow to an enormous scale. In such large-scale sensor networks, multi-hop communications between sensor nodes will be necessary to cover the whole monitoring region. Moreover, sensor nodes should be grouped into clusters to enhance scalability and robustness. Therefore, we believe that multi-hop communication between clusters is preferable for large-scale sensor networks. To clarify the fundamental characteristics of this form of communication, we analytically derive the network's power consumption and compare it with other routing methods using TDMA communication and an interference-free transmission schedule. The results show multi-hop communication between clusters is preferable in large-scale sensor networks because it can alleviate heavy relaying loads near the sink node and it has a shorter data collection time compared with simple multi-hop communications without clusters. Knowing how much performance degradation arises when interference is unavoidable is essential for multi-hop communications in clustered sensor networks. Therefore, we compare interference-free TDMA communication with CSMA/CA communication which can cause interference in clustered sensor networks. Consequently, we show that although the data collection time is about 3.7 times longer when using CSMA/CA, the power consumption can be suppressed to 12%.

During these years we have been focusing on developing ultra high-data-rate wireless access systems. One of such kind of systems is called DPC-OF/TDMA [2]-[4] (dynamic parameter controlled orthogonal frequency and time division multiple access) which targets at data rates beyond 100 Mbps. In order to support higher data rates, e.g., several hundreds of mega bps or even giga bps, it is necessary to evolve DPC-OF/TDMA on MIMO-OFDM (multiple-input multiple-output orthogonal frequency division multiplexing) platform. In this paper, we propose two MIMO-OFDM evolution schemes for DPC-OF/TDMA: M1 scheme and M2 scheme. M1 scheme is based on the combination of V-BLAST (vertical Bell laboratories layered space-time architecture) and OFDM. It invests all transmit antennas on multiplexing while exploits no diversity in the transmitter. M2 scheme is based on multi-layer space-time block coded OFDM (multi-layer STBC OFDM). This scheme achieves a good compromise between multiplexing and diversity in the transmitter. We conduct exhaustive simulations for 44, 46, 48, 66, 68, and 88 systems. We are assured that both evolution schemes are very promising in supporting several hundreds of mega bps data rates. Moreover, we find that each evolution scheme has its own prevailing area. When the receive diversity order is limited, M2 scheme has better performance since it embeds transmit diversity; as the receive diversity order increases, the performance gap between the two schemes shrinks and finally M1 scheme prevails in performance. Therefore, the proper choice depends on the system configuration, i.e., how many transmit and receive antennas are used.

In this paper the coexistence issue between the DS-UWB systems and Wi-Max/IEEE-802.16 based Fixed Broadband Wireless Access (FBWA) systems has been investigated. The aim is to evaluate the impact that the interference resulting from a realistic UWB hot spot scenario may have on the performance of a FBWA receiver. A mathematical model is developed through which the interference power produced by the UWB hot spot can be calculated. The benchmarks against which the UWB interference is assessed are also developed using the specifications of Wi-Max/IEEE-802.16 systems. Several simulations have been done to both validate the analytical results and calculate the UWB interference. In doing so, the effects of parameters like victim receiver bandwidth, carrier frequency, activity factor, the number of users and the distance from victim receiver in a realistic hot spot scenario have been studied.

The use of frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion can significantly improve the downlink bit error rate (BER) performances of DS- and MC-CDMA in a frequency-selective fading channel. However, the uplink BER performance degrades due to a strong multi-user interference (MUI). In this paper, we propose frequency-interleaved spread spectrum (SS) using MMSE-FDE, in which the subcarrier components of each user's signal are interleaved onto a wider bandwidth. Then, the frequency-interleaved frequency-domain signal is transformed into a time-domain signal by the inverse fast Fourier transform (IFFT). Frequency-interleaving patterns assigned to different users are orthogonal to each other. The proposed scheme can avoid the MUI completely while achieving frequency diversity gain due to MMSE-FDE. It is shown by computer simulation that the use of frequency-interleaving can significantly improve the uplink performance in a frequency-selective Rayleigh fading channel.

We introduce an adaptive subchannel, bit, and power allocation (ASBPA) algorithm to maximize the bandwidth efficiency of the mobile communication system that use orthogonal frequency division multiplexing (OFDM). We propose a suboptimal rate adaptive ASBPA algorithm that guarantees fairness in resource allocation and overcomes inherent co-channel interference (CCI) in the cellular system. Furthermore, we evaluate the maximum possible bandwidth efficiency of the cellular OFDM system achieved by the ASBPA algorithm which is practical to implement. Our simulation results show that the proposed algorithm outperforms the existing ones and achieves the cellular bandwidth efficiency of up to 5 b/s/Hz/cell. We also investigate some of the conditions that govern the bandwidth efficiency of the cellular OFDM system using the proposed ASBPA algorithm.

This paper proposes a new channel estimation method and a new interference cancellation scheme for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems in the presence of intersymbol interference (ISI). The proposed channel estimation method uses special training sequences (TSs) to have a desirable crest-factor of the transmitted training signal, and to prevent the influence of ISI on the channel estimation performance. By using the recommended training sequences, the ill-conditioned problem of the least square (LS) filter integrated in the proposed channel estimator can be avoided. The proposed interference cancellation scheme uses the estimated channel coefficients and the channel state information (CSI) to reproduce the interference components, which are then iteratively cancelled from the received signals. To reduced the error-floor of the demodulated symbols using for the calculations of the interference components, the so-called remodulation technique is also included in the proposed interference cancellation scheme. Simulation results show that the proposed channel estimation method outperforms conventional channel estimation methods, especially in the presence of ISI and if the signal-to-noise ratio (SNR) is larger than 15 dB. The combination of the proposed method with a space-time block code (STBC) to combat the interference influences results in an excellent system performance in terms of symbol error ratio (SER). In comparison with a STBC MIMO-OFDM system with sufficient guard interval (GI), this combination gains 1.52 dB of SNR at the same SER of 1.1・10^-6 even after performing only one iteration of interference cancellation.

This paper presents a low cost and portable DOA (Direction Of Arrival) estimation system for surveillance using a modifed beamspace MUSIC (MUltiple Signal Classification) by a quasi-orthogonal multi-beam. This is instead of DFT processing and hardware system consisting of chip-sized phase shifters, a single ADC (Analogue to Digital Converter) and a single TR (TRanceiver) module for an antenna array. In the beamspace MUSIC, generated beampatterns have orthogonal properties. This proposed system cannot make such a beampattern due to the variable range limitation of phase shifter, then we use the quasi-orthogonal beam obtained by the calculation of correlation coefficient for beampattern. We demonstrate the proposed system using 4-element microstrip array antenna and chip-sized phase shifters. The DOA experiment in anechoic chamber confirms the proposed system performance.

Adaptive modulation is an efficient method to increase the spectral efficiency of OFDM based high-speed wireless data transmission systems in multipath channel. Blind modulation classification schemes play an important role in adaptive modulation systems, eliminating the need for transmitting modulation information, thereby increasing spectral efficiency. In this paper, a novel blind modulation classification algorithm is derived from the finite alphabet property of information symbols and the equivalent parallel model of OFDM systems. The performances of the proposed algorithm and M2M4P algorithm [1] are tested and compared using Monte-Carlo simulations. It is found that, the novel algorithm yields performance better than that of M2M4P algorithm and with much less complexity.

We analytically evaluated the effects of the analog components on a high-speed downlink packet access (HSDPA) system standardized by 3GPP. We considered the phase noise of synthesizers, the imbalance of demodulators between in-phase and quadrature channels, and the filters. The components are represented by the appropriate equations. We applied adaptive modulation and coding methods for HSDPA systems and base station transmission of adequate data rate signals complying with quality estimated by mobile stations (MSs). The quality represents a data rate indicating that MSs can receive the signals. We estimated the quality using a conventional signal-to-interference measurement of the common pilot channel (CPICH) and found that the phase noise creates a mismatch relationship between the quality and the data rate, while the demodulator imbalance and filters create a suitable relationship. We confirmed this using analytic methods and computer simulation.

This paper proposes an iterative maximum a posteriori probability (MAP) receiver for multiple-input-multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM) mobile communications. For exploiting the space, time, and frequency diversity, the low-density parity-check code (LDPC) is used as a channel coding with a built-in interleaver. The receiver employs the expectation maximization (EM) algorithm so as to perform the MAP symbol detection with reasonable computational complexity. The minimum mean square error (MMSE), recursive least squares (RLS), and least mean square (LMS) algorithms are theoretically derived for the channel estimation within this framework. Furthermore, the proposed receiver performs a new scheme called backward symbol detection (BSD), in which the signal detection uses the channel impulse response that is estimated one OFDM symbol later. The advantage of BSD, which is explained from the viewpoint of the message passing algorithm, is that BSD can exploit information on the both precedent and subsequent OFDM symbols, similarly to RLS with smoothing and removing (SR-RLS) [25]. In comparison with SR-RLS, BSD reduces the complexity at the cost of packet error rate (PER) performance. Computer simulations show that the receiver employing RLS for the channel estimation outperforms the ones employing MMSE or LMS, and that BSD can improve the PER performance of the ones employing RLS or LMS.

The crosstalk phenomenon, wich occurs between transmission lines, is caused by electromagnetic fields of currents flowing through the lines. Crosstalk between two bent lines is studied by using a set of solutions of modified telegrapher's equations. By expressing electromagnetic fields in terms of voltages and currents in the line ends, the resultant network function in the form of an ABCD matrix is obtained. Electromagnetic fields caused by currents flowing in risers at transmission line ends are taken into account in addition to those fields in line sections. The validity of the proposed approach was confirmed by comparing experimental results with computed results and those simulated by a commercial electromagnetic solver for some bent-line models.

Recently, Wu-Chieu proposed an improvement to their original scheme, in order to make the scheme withstand impersonation attacks. However, the improved scheme is susceptible to an off-line password guessing attack and is inefficiently designed. Accordingly, the current letter demonstrates the vulnerability of Wu-Chieu's modified scheme to an off-line password guessing attack and evaluates the efficiency of their schemes and related schemes.

This letter investigates a practical space time block coded (STBC) cooperative system to determine under what conditions a cooperative system outperforms a direct one, and then proposes two criteria for selecting proper relay based on outage probability and capacity, respectively. Performance analysis and numerical results indicate that the STBC cooperative system is superior in practical scenarios.

In this paper, it is shown that the bit erasure probability of turbo codes with iterative decoding in the waterfall region is nonlinearly scaled by the information blocklength. This result can be used to predict efficiently the bit erasure probability of the finite-length turbo codes over the binary erasure channel.

In this letter, we study the blocking probabilities in an asynchronous optical packet/burst switching system with full wavelength conversion. Most of the existing work use Poisson traffic models that is well-suited for an infinite population of users. In this letter, the optical packet traffic arriving at the switching system is modeled through a superposition of a finite number of identical on-off sources. We propose a block tridiagonal LU factorization algorithm to efficiently solve the two dimensional Markov chain that arises in the modeling of the switching system.

A new current reused quadrature voltage controlled oscillator (QVCO) is proposed and implemented using UMC 0.18 µm CMOS 1P6M process. The proposed circuit topology is made up two low voltage LC-tank VCOs, where the QVCO is obtained using the transformer coupling and current reuse technique. At 1.8 V supply voltage, the phase noise of the VCO is -117.13 dBc/Hz at 1 MHz offset frequency from the carrier frequency of 2.18 GHz, the core power consumption is 4.14 mW, the total power consumption is 6.48 mW and tuning range is about 160 MHz.

We suggest a new minimum credit method for the dynamic bandwidth allocation in EPON. In the suggested method, to eliminate the unused transmission time-slot, each ONU requests no more than a predetermined maximum. We analyze the upstream channel resource wastage when traffic is light. Based on the analysis, we derive a minimum credit that eliminate the upstream channel resource wastage. The OLT estimates a traffic load and grants a minimum credit when the request is smaller than the minimum credit and traffic is light. Using simulation, we show the minimum credit discipline is superior than the existing methods in the mean delay and the frame loss rate.

Ad hoc DMAC protocols have been proposed to improve spatial reuse, but directional transmissions have the problem of deafness. In the ToneDMAC protocol [9], an omnidirectional out-of-band tone after transmitting DATA or ACK mitigates deafness, but cannot prevent the interference packets caused by retransmissions to node in deafness. In this paper, we propose a dual-tone DMAC protocol with the out-of-band start-tone and stop-tone. In the proposed MAC protocol, a start-tone prevents retransmissions to node in deafness and decreases the packet collision probability. Throughput performance of the proposed MAC protocol is confirmed by simulations using Qualnet ver. 3.8 simulator.

Downlink multiuser MIMO system has attracted considerable attention recently for its potential to increase the system capacity. However, due to the limitation on the number of transmit antennas, when there are more users than can be supported simultaneously in a cell, other multiple access schemes, such as TDMA, must be applied in combination with multiuser MIMO. In this paper, we aim to design practical user scheduling algorithms to maximize the system capacity. Because the brute-force search for optimal user allocation is computationally prohibitive, we propose three low complexity suboptimal scheduling algorithms that offer both low complexity and high performance.

In this letter, we consider an Orthogonal Frequency Division Multiplexing (OFDM) system without cyclic prefix (CP), i.e. with full spectral efficiency and present an optimum frequency-domain filtering scheme for mitigating inter-symbol interference (ISI) and inter-carrier interference (ICI). Simulation results show the signal to interference and noise ratio (SINR) improvement.

This letter focuses on uplink transmission in OFDMA systems. A subcarrier and power allocation problem is formulated that maximizes the throughput of OFDMA uplink systems while satisfying each user's power constraints. A greedy algorithm known to be the most efficient algorithm for this problem can provide a high quality near-optimal solution, but has the disadvantage of incurring a long computation time. As this problem should be solved in a real-time environment, computation time is a very important performance measure of algorithms. In this letter, a computationally efficient algorithm that provides a nearly identical quality, near-optimal solution as the greedy algorithm but requires less than 10% of the computation time of the greedy algorithm is proposed.

In this letter, we propose two different joint transmit and receive antenna subset selection schemes for multiple-input multiple-output (MIMO) systems on the basis of capacity maximization criterion. We assume that perfect channel state information (CSI) is known at the receiver but unknown to the transmitter. As the selection signaling is perfectly fed back to the transmitter, we propose a flexible two-step selection algorithm (TSSA) in practical MIMO channel scenarios. Computer simulations show that TSSA can maximize the capacity at low computation cost in most scenarios. It performs well in terms of capacity, computational complexity and flexibility. Furthermore, we propose a simplified algorithm based on the correlation matrix when the channel correlation information (CCI) is known to the transmitter. Simulation results show that the proposed correlation matrix based selection algorithm is only slightly inferior to an optimal selection algorithm.

Iterative receivers, which perform MMSE detection and decoding iteratively, can provide significant performance improvement compared with noniterative method. However, due to the high computational cost and numerical instability, conventional MMSE detection using a priori information can not be implemented in hardware. In this letter, we propose a newly-built iterative receiver which is division-free and numerically stable, and then we analyze the results of a fixed-point simulation and present the hardware implementation architecture.

A sub-optimal practical computationally efficient transmit power allocation algorithm is proposed under the constraints of data rate and max bit error rate (BER) in OFDM systems. By exploiting the relation between the water-filling level and the data rate, the proposed algorithm derives an efficient adaptive coarse searching method for water-filling level without need of preset step and other initial parameters, then determines the required water-filling level by the dichotomy method. When the channel state changes, the proposed algorithm works out a small domain of required water-filling level based on the latest allocation and then tracks the channel quickly. Simulation results show that the proposed algorithm converges quickly and is suitable for time-varying channels.

A new method to approximate the receive minimum distance is presented. In the proposed approximation, the geometric mean of the singular values of the channel matrix is used instead of the conventional minimum singular value. Numerical experiments show that the proposed approximation has less mean squared error than the minimum singular value bound and outperforms the minimum singular value bound in terms of bit error rate when they are applied to the antenna subgroup selection system.

In this letter,we design a special preamble composed of two OFDM training blocks with different numbers of identical parts. Based on the designed preamble, we propose a method to estimate frequency offset utilizing initial estimates from the two OFDM training symbols. By elaborately selecting the numbers of identical parts for the two training blocks, the proposed estimator provides a much larger estimate range than conventional estimators using identical parts. Computer simulations show that the proposed estimator exhibits superior estimate performance, while maintaining low computational complexity.

We propose new nulling vectors for the subcarrier grouping based low-complexity detection scheme of V-BLAST (vertical Bell Laboratories layered space-time) coded MIMO-OFDM. In each subcarrier group, the center subcarrier uses the conventional ZF-DFE (zero-forcing decision-feedback- equalization) algorithm and the non-center subcarriers use the reduced-complexity ZF-DFE with the new nulling vectors. The subcarrier grouping based detection scheme, ZF-DFE-SG, can significantly reduce the detection complexity compared with ZF-DFE-EX which exhaustively applies the conventional ZF-DFE at each subcarrier independently. The performance loss is very small. We conduct the simulations for channel coded V-BLAST OFDM system under estimated channel frequency responses. It is shown the complexity can be reduced by 80.6% with only 1.0 dB performance loss for a 44 system.

In this letter, the system capacity of multiuser diversity combined with spatial multiplexing schemes is analyzed. An analytic expression is derived for the ergodic system capacity with multiuser scheduling and dual multi-input multi-output (MIMO) systems by using a tight lower bound of the link capacity. The proposed analytic approach is verified through comparisons between analytic and simulated results, and is shown to make fairly precise predictions of the ergodic system capacity and the scheduling gains even when the numbers of antennas and users are small.

In OFDM based mobile communication systems, channel variation during one symbol period introduces intercarrier interference (ICI). Conventional pilot-aided equalization mitigates the ICI at the price of band inefficiency. On the other hand, the blind or semi-blind equalization method, which utilizes the known statistic properties of the transmitted data, will raise system complexity. In this letter, without bandwidth-consuming pilots, a novel channel estimation and tracking method based on an iterative equalization technique (IET) is proposed. The proposed approach successfully achieves a good compromise between bandwidth efficiency and system complexity, and its validity is demonstrated by numerical simulations, especially for fast fading channel.

In this paper, we investigate an efficient user selection and sub-band allocation algorithm in which each user transmits two-step partial CQI to reduce the amount of feedback in multi-user downlink OFDMA systems. Simulation results show that we can greatly reduce the feedback rate at the expense of negligible performance degradation compared to the full CQI feedback schemes or that we can greatly improve the performance with slightly reduced feedback rate compared to conventional partial CQI feedback schemes.

The cooperative relaying technique enables a terminal to get space-diversity through the support of other terminals. However, existing cooperative relaying techniques for code division multiple access (CDMA) system decrease the total system transmission rate. In this letter, a new relaying technique is presented which supplies lossless transmission-rate using 2 spreading codes per terminal. We verify the performance of the proposed technique through a bit error rate (BER) simulation for a direct-sequence ultra wideband (DS-UWB) system. It is also shown that forward error correction (FEC) coding provides a better environment for the cooperative relaying.

ACK thinning refers to the technique to discard or reduce TCP acknowledgements (ACKs) for the purpose of diverting scarce bandwidth to TCP data traffic. It has been shown that under some circumstances the technique is effective to boost the TCP throughput on wireless links, in particular the IEEE 802.11 wireless LAN (WLAN). In this letter, however, we show that ACK thinning backfires under congestion due to its cross-layer impact on the 802.11 MAC dynamics. With the ACK filtering example, we demonstrate the phenomenon and analyze the cause. Based on the analysis, we show how the IEEE 802.11 contention window size control solves the problem.

This paper presents an admission control technique for multi-carrier systems with an FRF(frequency reuse factor) of 1. The FRF of 1 is very attrative for more improved channel throughput but the forward link capacity is rapidly decreased at the cell boundary region due to the increase in the ICI(InterCell Interference). By measuring a region-based channel capacity and deriving a closed form of blocking probability, a QoS(Quality of Service) maintenance technique and mobility model can be acquired. In the simulation, the proposed scheme demonstrates a blocking probability reduction of up to 40% compared to the cell-based link capacity scheme.

The Multiple Access Interference (MAI) and the Multipath Fading (MPF) restrict the performance of Code-Division Multiple-Access (CDMA) systems. The Multiuser Detection (MUD) based on Particle Swarm Optimization algorithm (PSO) with Rake processing is proposed in this paper to overcome these obstacles, followed by full details of how to apply the Binary PSO MUD (BPSO-MUD) on a CDMA system. Simulations show that the BPSO-MUD has significantly better performance than the Conventional Detection (CD).

In road-vehicle communication systems, the transmission rate between user terminals in the vehicle and the access points degrades due to changing path-loss and time-varying fading. In this paper, we used an inter-vehicle packet relay technique to improve channel quality in road-vehicle communication systems. We evaluated this method using numerical analysis to validate our method.

We present a novel algorithm that can suppress impulsive noise in video colour sequences. It uses order statistics, and directional and adaptive processing techniques.

This paper introduces an adaptive GOP structure (AGS), which adaptively defines the GOP structure according to the time-varying temporal properties of video sequences, and thus improves the coding efficiency of the MPEG & ITU-T's Joint Scalable Video Coding (JSVC) scheme, the method proposed in this paper, which adaptively modifies the size of GOP based on the image characteristics of video sequence, improves the coding efficiency up to 0.77 dB compared to the JSVC JSVM (Joint Scalable Video Model).