Irregular Low-Density Parity-Check (LDPC) codes generally achieve better performance than regular LDPC codes at low E_b/N₀ values. They have, however, higher error floors than regular LDPC codes. With respect to the construction of the irregular LDPC code, it can achieve the trade-off between the performance degradation of low E_b/N₀ region and lowering error floor. It is known that a decoding algorithm can achieve very good performance if it combines the Ordered Statistic Decoding (OSD) algorithm and the Log Likelihood Ratio-Belief Propagation (LLR-BP) decoding algorithm. Unfortunately, all the codewords obtained by the OSD algorithm satisfy the parity check equation of the LDPC code. While we can not use the parity check equation of the LDPC code to stop the decoding process, the wrong codeword that satisfies the parity check equation raises the error floor. Once a codeword that satisfies the parity check equation is generated by the LLR-BP decoding algorithm, we regard that codeword as the final estimate and halt decoding; the OSD algorithm is not performed. In this paper, we propose a new encoding/decoding scheme to lower the error floor created by irregular LDPC codes. The proposed encoding scheme encodes information bits by Cyclic Redundancy Check (CRC) and LDPC code. The proposed decoding scheme, which consists of the LLR-BP decoding, CRC check, and OSD decoding, detects errors in the codewords obtained by the LLR-BP decoding algorithm and the OSD decoding algorithm using the parity check equations of LDPC codes and CRC. Computer simulations show that the proposed encoding/decoding scheme can lower the error floor of irregular LDPC codes.

This paper presents a new blind symbol timing recovery technique for OFDM systems. The proposed technique is based on modifying the conventional early-late loop used for single carrier systems. The new topology exploits the interference introduced by the loss of orthogonality due to symbol timing offsets. The new structure is completely non data-aided and is independent of the OFDM symbol structure. The new technique can extract symbol timing with carrier frequency offset up to 94% of the OFDM spectrum.

Vertical stacking is a novel technique for creating nonblocking (crosstalk-free) optical multistage interconnection networks (MINs). In this paper, we propose a new class of optical MINs, the vertically stacked Benes (VSB) networks, for crosstalk-free realization of permutations in a single pass. An NN VSB network requires at most O(Nlog N) switching elements, which is the same as the Benes network, and much lower overall hardware cost than that of the existing optical MINs built on the combination of horizontal expansion and vertical stacking of banyan networks, to provide the same crosstalk-free permutation capability. Furthermore, the structure of VSB networks provides a more flexible way for constructing optical MINs because they give more choices in terms of the number of stages used in an optical MIN. We also present efficient algorithms to realize crosstalk-free permutations in an NN VSB network in time O(Nlog N), which matches the same bound as required by the reported schemes.

It has been a very important issue to evaluate the performance of transmission control protocol (TCP), and the importance is still growing up because TCP will be deployed more widely in future wireless as well as wireline networks. It is also the reason why there have been a lot of efforts to analyze TCP performance more accurately. Most of these works are focusing on overall TCP end-to-end throughput that is defined as the number of bytes transmitted for a given time period. Even though each TCP's fast recovery strategy should be considered in computation of the exact time period, it has not been considered sufficiently in the existing models. That is, for more detailed performance analysis of a TCP implementation, the fast recovery latency during which lost packets are retransmitted should be considered with its relevant strategy. In this paper, we extend the existing models in order to capture TCP's loss recovery behaviors in detail. On the basis of the model, the loss recovery latency of three TCP implementations can be derived with considering the number of retransmitted packets. In particular, the proposed model differentiates the loss recovery performance of TCP using selective acknowledgement (SACK) option from TCP NewReno. We also verify that the proposed model reflects the precise latency of each TCP's loss recovery by simulations.

In this paper, we discuss the efficiency of tunneling techniques which are expected to accelerate multicast deployment. Our motivation is that, despite the proposal of many tunneling techniques, no paper has studied their impact on multicast efficiency. Through detailed computer experiments, we find that there is a critical size of multicast island, above which the loads imposed on tunneling endpoints are suddenly diminished. In addition, multicast islands equaling or exceeding the critical size reduce the overhead of forwarding states on routers. We also find a scaling law between the critical size and group size. Based on these findings, we present simple guidelines on using tunneling when deploying multicast systems. A possible explanation for our findings is uncovered by a simple analysis. Our work is the first to evaluate the impact of tunneling and clarify conditions in which multicast deployment is well supported by tunneling.

In this paper, we propose a method of detecting TCP performance degradation using only bottleneck-link utilization statistics: mean and variance. The variance of link utilization normally increases as the mean link-utilization increases. However, because link-utilization has a maximum of 100%, as the mean approaches 100%, the possible range of fluctuation becomes narrow and the variance decreases to zero. In this paper, using the M/G/R processor sharing model, we relate this phenomenon to the behavior of flows. We also show that by using this relationship, we can detect TCP performance degradation using the mean and variance of link utilization. In particular, this method enables a network operator to determine whether or not the degradation originates from the congestion of his/her own network. Because our method requires us to measure only link utilization, the cost of performance management can be greatly decreased compared with the conventional method, which requires dedicated functions for directly measuring the TCP performance.

During devastating natural disasters, numerous people want to make calls to check on their families and friends in the stricken areas, but many call attempts on mobile cellular systems are blocked due to limited radio frequency resources. To reduce call blocking and enable as many people as possible to access mobile cellular systems, placing a limit on the holding time for each call has been studied [1],[2]. However, during a catastrophe, emergency calls, e.g., calls to fire, ambulance, or police services are also highly likely to increase and it is important that the holding time for these calls is not limited. A method of limiting call holding time to make provision for emergency calls while considering the needs of ordinary callers is proposed. In this method, called the HTL-E method, all calls are classified as emergency calls or other according to the numbers that are dialed or the terminal numbers that are given in advance to the particular terminals making emergency calls, and only the holding time of other calls is limited. The performance characteristics of the HTL-E method were evaluated using computer simulations. The results showed that it reduced the rates of blocking and forced call termination at handover considerably, without reducing the holding time for emergency calls. The blocking rate was almost equal for emergency and other calls. In addition, the HTL-E method handles fluctuations in the demand for emergency calls flexibly. A simple method of estimating the holding-time limit for other calls, which reduces the blocking rate for emergency and other calls to the normal rate for periods of increased call demand is also presented. The calculated results produced by this method agreed well with the simulation results.

Automatic discovery of physical topology plays a crucial role in enhancing the manageability of modern large Ethernet mesh networks. Despite the importance of the problem, earlier research and commercial network management tools have typically concentrated on either discovering active topology, or proprietary solutions targeting specific product families. Recent works [1]-[3] have demonstrated that physical topology can be determined using standard SNMP MIB, but these algorithms depend on Filtering Database and rely on the so-called spanning tree protocol (IEEE 802.1d) in order to break cycles, thereby avoiding the possibility of infinitely circulating packets and deadlocks. A previous work [1] requires that Filtering Database entries are completed; however it is a very critical assumption in a realistic Ethernet mesh network. In this paper, we have proposed a new topology discovery algorithm which works without the complete knowledge of Filtering Database. Our algorithm can discover complete physical topology including inactive interfaces eliminated by the spanning tree protocol in LEMNs. The effectiveness of the algorithm is demonstrated by an implementation.

In wireless ATM mobile communication environments, an abrupt increase in the number of mobile users in a specific base station (BS) will overload the BS and turn it into a so-called hot-spot cell, which causes an unbalanced traffic load condition if the traffic load in the neighboring BSs is light. In order that efficiently allevates to utilize a limited bandwidth, it is necessary to use a good mobile handoff strategy to alleviate unbalanced traffic load condition. This paper proposes a complete traffic shedding algorithm (CTSA) for soft handoff efficiently to alleviate the overloaded traffic in a hot-spot cell by adjusting handoff parameters, which virtually reduces the service coverage of the heavy loaded BS without adjusting the transmission power. Mobile terminals (MT) located in the outer area of the hot-spot cell will be handed off early to the neighboring BSs with light traffic load; and MTs in the neighboring BSs will not be handed off to the hot-spot cell, so that the traffic load of the hot-spot cell can be decreased. After analyzing the proposed algorithm and comparing the other methods, variable threshold soft handoff (VTSH) and enhanced soft handoff (ESH), by simulation, the performance of CTSA is better than the others. CTSA obviously increases system throughput and decreases soft handoff call dropping ratio, new call blocking ratio, and total call refusing ratio.

Conventional orthogonal frequency division multiplexing (OFDM) system utilizes cyclic prefix (CP) to remove the channel-induced inter-symbol interference (ISI) at the cost of lower spectral efficiency. In this paper, a generalized sidelobe canceller (GSC) based equalizer for ISI suppression is proposed for uplink multi-antenna OFDM systems without CP. Based on the block representation of the CP-free OFDM system, there is a natural formulation of the ISI suppression problem under the GSC framework. By further exploiting the signal and ISI signature matrix structures, a computationally efficient partially adaptive (PA) implementation of the GSC-based equalizer is proposed for complexity reduction. The proposed scheme can be extended for the design of a pre-equalizer, which pre-suppresses the ISI and realizes CP-free downlink transmission to ease the computational burden of the mobile unit (MU). Simulation results show that the proposed GSC-based solutions yield equalization performances almost identical to that obtained by the conventional CP-based OFDM systems and are highly resistant to the increase in channel delay spread.

With the growing demand for mobile communications, multicarrier (MC) schemes are receiving an increasing amount of attention, primarily because they handle frequency selective channels better than ordinary single-carrier schemes. However, despite offering several advantages, MC systems have certain weak points. One is their high sensitivity to interchannel interference (ICI). The influence of Doppler shift and ICI are the focus of this paper. Newly proposed B3G/4G systems are developed for data transmission rates higher than those of the IEEE 801.11. It is then necessary that the bandwidth of the subcarrier be small. Moreover, for a higher carrier frequency and mobile speed, the influence of the Doppler shift will be large; therefore, the influence of ICI becomes severer. Using a Markov chain approach, we synthesized a turbo equalizer (TE) that minimizes ICI when interference affects the arbitrary number M of adjacent subchannels. This approach shows the complexity of the proposed algorithm exhibits linear growth with respect to M and independence with respect to the total number of subchannels in the multicarrier system. The proposed ICI cancellation scheme can also be effective in the case of multiple Doppler frequency offsets. This makes the proposed approach attractive for practical implementations.

This paper addresses several relevant issues in detection process of a reverse link multicarrier code division multiple access system with frequency offset. Due to the presence of frequency offsets, power of a group of subcarriers is spread to all the available subcarriers. In order to collect all the scattered signal power, we propose a two stages receiver. First stage of the proposed receiver performs maximum ratio combining. Following from the first stage, multiple access interference is suppressed in the second stage. We propose various multiple access interference suppression techniques, having different complexity and performance. Simulation results demonstrate the advantage of the proposed receiver. We also investigate the performance of 2 major subcarrier assignment schemes, namely continuous subcarrier assignment scheme and interleaved subcarrier assignment scheme, in both ideal independent fading channel and practical correlated fading channel.

In this paper, we propose an angle of arrival (AOA)-based beamforming structure with strong interference resistant capability for OFDM systems. First, we present novel interference resistant angle of arrival (AOA) estimation schemes for each multipath without training sequences or symbols as the pre-processing of the proposed structure. The generalized sidelobe canceler (GSC) structure is thus applied with the AOA information to avoid calculating the beamforming weight vector individually on a per subcarrier basis, wherein the GSC structure with the reduced-rank multistage Wiener filter (MSWF) is adopted. We also propose a signal transformation scheme to improve performance before the signals are fed into the GSC receiver. The proposed receiver offers better performance than the GSC form of the constrained Wiener filter-based receiver due to the faster convergence property of reduced rank processing and the signal transformation scheme.

Multiple-input multiple-output (MIMO) eigenbeam space division multiplexing that uses adaptive modulations for substreams is a promising technology for improving transmission capacity. A fundamental drawback of this approach is that the modulation levels determined from the carrier-to-noise ratio at each substream are sometimes overly optimistic so the use of these modulation levels results in transmission errors and diminished transmission performance. A novel method of determining substream modulation levels is proposed that alleviates this degradation. In the proposed method, the expected bit error rates for possible modulations of each substream are calculated from delay profiles. Simulation results indicate that transmission capacity is improved by 30% using the new method compared with the conventional method.

The system under study is a convolutionally coded and orthogonally modulated DS-CDMA system over time-varying frequency-selective Rayleigh fading channels in multiuser environments. Iterative soft demodulation and decoding using the Turbo principle can be applied to such a system to increase the system capacity and performance. To combat multiple access interference (MAI), we incorporate the interference cancellation (IC) and decision-directed channel estimation (CE) in the demodulator. However, both IC and CE are subject to performance degradation due to incorrect decisions. In order to prevent error propagation from the decision feedback, soft interference cancellation and channel estimation assisted demodulation is proposed in this paper. The performance of this strategy is evaluated numerically and proved to be superior to the hard decision-directed approach with a minor increase in complexity.

In multi-carrier code division multiple access (MC-CDMA) uplink (mobile-to-base station), since different users' signals go through different frequency-selective fading channels, large multi-access interference (MAI) is produced. The use of frequency-domain equalization reception can only partially restore the orthogonality among different users' signals, resulting in a severe degradation in the bit error rate (BER) performance. Hence, frequency-domain pre-equalization transmission, which equalizes the MC-CDMA signal before transmission, is recently attracting attention. In this paper, we present a generalized minimum mean square error (GMMSE) frequency-domain pre-equalization transmission suitable for MC-CDMA/TDD uplink. The pre-equalization weight is derived based on the method of Lagrange multipliers. The theoretical analysis of BER performance using the GMMSE frequency-domain pre-equalization transmission in a frequency-selective Rayleigh fading channel is presented and the result is confirmed by computer simulation.

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.

Recently, the direct conversion scheme has been actively investigated for the purpose of cost miniaturization and low power consumption of wireless receivers. IQ imbalance is one of the problems for the direct conversion receiver. In the case of OFCDM modulations, this IQ imbalance causes intercarrier interference (ICI) in the demodulated signals. In this paper, the decision directed scheme for IQ imbalance compensation is proposed. In the proposed scheme, the combination of received symbols which satisfies orthogonality conditions is used for compensation of IQ imbalance. Therefore, in addition to the pilot symbols, the received symbols can be used in order to improve the accuracy of the compensation matrix and BER can be reduced.

This paper examines polarimetric scattering characteristics caused by a dihedral corner reflector of finite size. The dihedral corner reflector is a basic model of double-bounce structure in urban area. The detailed scattering information serves the interpretation of Polarimetric Synthetic Aperture Radar (POLSAR) data analysis. The Finite-Difference Time-Domain (FDTD) method is utilized for the scattering calculation because of its simplicity and flexibility in the target shape modeling. This paper points out that there exists a stable double-bounce squint angle region both for perfect electric conductor (PEC) and dielectric corner reflectors. Beyond this stable squint angular region, the scattering characteristics become completely different from the assumed response. A criterion on the double-bounce scattering is proposed based on the physical optics (PO) approximation. The detailed analyses on the polarimetric index (co-polarization ratio) with respect to squint angle and an experimental result measured in an anechoic chamber are shown.

This paper presents an improved gate drive circuit for high power GTO thyristors. The energy-storage/transfer characteristics of an air-core reactor and the fast switching characteristics of FET are employed to make a high gate current of sharp pulse form. The power loss in the gate drive circuit is reduced by using the low resistance and the hysteresis comparator to detect and control the steady on-gate current. The proposed gate drive circuit is analyzed and its usefulness is confirmed by experiments.

The network performance of a single joint multicasting capable optical cross-connect (jMC-OXC) integrating both space- and frequency-splitters is simulated. The results show that the jMC-OXC architecture with limited frequency-splitters can obtain a close performance to that with full frequency splitters. The improvement offered by jMC-OXCs on the performance of multicasting routing is also discussed.

It is well known that deploying a proxy at the boundary of wireless networks and the Internet is able to improve the performance of transmission control protocol (TCP) over wireless links. Snoop protocol, acting like a transport layer proxy, performs local retransmissions for packets corrupted by wireless channel errors. In this letter, an improvement for the Snoop protocol is proposed to shorten the time spent on local recovery by sending extra copies in every local retransmission attempt. This enables TCP to quickly return to normal, effectively eliminating several of the problems that may cause throughput degradation.

Recently, Chien et al. proposed an efficient timestamp-based remote user authentication scheme using smart cards. The main merits include: (1) user-independent server, i.e., there is no password or verification table kept in the server; (2) users can freely choose their passwords; (3) mutual authentication is provided between the user and the server; and (4) lower communication and computation cost. In this paper, we show that Chien et al.'s scheme is insecure against forgery attack because one adversary can easily pretend to be a legal user, pass the server's verification and login to the remote system successfully. An improved scheme is proposed that can overcome the security risk while still preserving all the above advantages.

A blind symbol synchronization scheme for MIMO and Multi-User OFDM systems is proposed, which utilizes short-time Fourier Transformation (STFT) to obtain 2D (time and frequency) timing information from the received signals. By analyzing the obtained 2D time-frequency amplitude spectrum, intervals where no inter-symbol interference (ISI) exists are checked out for symbol synchronization, and samples during these intervals are used to carry out carrier frequency offset estimation. Theoretical analysis and simulation results show that the proposed method is more robust and provides more accurate carrier frequency offset estimation than traditional schemes.

In this letter, a concept of virtual smart antenna (SA) is introduced for OFDM-based cellular systems with a frequency reuse factor equal to 1. A method of estimating intercell symbol timing offsets (STOs) from received OFDM signals impinging on virtual antenna is proposed for users at a cell boundary. Also, adaptive beamforming methods for virtual SA are proposed to reduce intercell interference (ICI) from adjacent base stations (BSs).

In this letter, a simple peak-to-average power ratio (PAPR) reduction method using subband permutation is proposed for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. The proposed method is shown to have a 2-3 dB PAPR reduction gain for 2 transmit antennas, compared with the conventional MIMO-OFDM, and a low computational complexity at the transmitter and receiver.

In this letter, we propose an OFDMA/CDM-based cellular system, which accommodates users in different frequency bands and multiplexes user data symbols with frequency domain spreading. The proposed system utilizes random codes to discriminate between cells and adopts the pre-equalization to enhance the performance. An efficient power allocation scheme is suggested for cellular applications with a transmit power constraint. In particular, the validity of the OFDMA/CDM-based cellular system is examined, by investigating its performance as a function of the number of multiplexed data symbols at different locations.

An effective access control scheme in CDMA networks is proposed to provide multimedia services. The proposed scheme controls the access of multimedia traffic using the delay of data traffic and the transmission rate control of video traffic. Numerical results show that throughput and delay are improved by allowing delay of data traffic under low and medium traffic conditions, and by decreasing the transmission rate of video traffic under heavy traffic conditions.

This letter describes four-branch open-loop transmit diversity schemes that are based on group-coherent codes using space-time block codes (STBC-GCC), for orthogonal frequency-division multiplexing and code-division multiplexing (OFDM-CDM) systems. Open-loop STBC-GCC is designed for transmitting control data by two-dimensional spreading, which is achieved by applying two distinct Walsh-Hadamard (WH) codes within the same mother-code group to two different groups of transmit antennas. The simulation results demonstrated that open-loop STBC-GCC applied to two-dimensional spreading provides space diversity gains and frequency diversity gains. It can therefore be concluded that open-loop STBC-GCC is suitable for transmitting common control data or broadcast data via two-dimensional spreading OFDM-CDM.

In this letter, a pilot structure and an efficient algorithm for downlink synchronization and cell searching in OFDM-based cellular systems are proposed. The pilots, randomly allocated in the frequency domain, allow us to minimize inter-cell interference (ICI) as well as to increase cell searching capability, estimation range of integer carrier frequency offset (CFO), and estimation accuracy of symbol timing offset (STO). The proposed low-complexity joint algorithm for integer CFO estimation, cell searching, and downlink detection is robust to ICI, multipath channel, STO and fine CFO.

In this letter, we propose a multi-user detection scheme based on a hidden training sequence for DS-UWB systems. The hidden training sequence, which uses a fraction of the informative sequence's transmitting power as training information, is utilized for the receiver filter adaptation and channel estimation. By using this, the proposed scheme offers increased bandwidth efficiency (no period dedicated for training) and also shows reasonably good performance and near-far resistance in single and multiple-access UWB indoor multipath channel environment.

In this letter, we focus on rearranged pilot patterns for channel estimation in a mobile communication system using Orthogonal Frequency Division Multiplexing (OFDM). The conventional pilot patterns for channel estimation in OFDM systems do not have robust characteristics in time-varying channels. In order to overcome this weakness of the conventional pilot patterns, we propose the pilot patterns with robust mobility for OFDM systems, which can achieve a good error performance in time-varying multi-path fading channels. Simulation results show that the bit error rate (BER) performances of the proposed pilot patterns are better than those of the conventional pilot patterns in fast time-varying fading channels under the same pilot density and data rate.

This paper investigates a modifying orthogonality factor for synchronous DS-CDMA uplink in dispersive Rician multipath fading channels, which reflects upon the effects of specular path power as well as decaying channel characteristics. Using this investigation, the orthogonal factors in indoor environments are evaluated and compared with the various parameters such as decaying factor, line-of-sight component, and the number of multipaths.

The newest video coding standard called H.264 provides considerable performance improvement over a wide range of bit rates and video resolutions compared to previous standards. However, these features result in an extraordinary increase in encoder complexity, mainly regarding to mode decision and multiple reference frame motion estimation (ME). This letter presents two algorithms to reduce the computational complexity caused by motion estimation. The adaptive search range decision method determines the search range size according to the motion vector predictor dynamically and the early termination scheme defines a criterion to early terminate the search processing for multiple reference frames. Experimental results show that the proposed algorithms provide significant improvement of coding speed with negligible objective quality degradation compared to the fast motion estimation algorithms adopted by reference software.

As digital television transmission is becoming ubiquitous, a method that can remotely monitor the quality of the final and intermediate pictures is urgently needed. In particular, the case where standards conversion is included in the transmission chain is a serious issue as the input and output cannot simply be compared. This letter proposes a novel method to solve this issue. The combination of skipping fields/pixels and the previously proposed SSSWHT-RR method, using the information of correlation coefficients and variance of the picture, achieves accurate detection of picture failure.