This paper discusses far-end crosstalk (FEXT) cancellation methods for multicarrier transmission system. A system arrangement and its tap update method are proposed when FEXT cancelers and a frequency-domain equalizer (FEQ) are jointly adapted to combat channel intersymbol interference, FEXT, and other additive noise. We present mathematical formulation of minimum mean-square error (MSE) and the optimum tap coefficients for the FEXT cancelers and the FEQ when FEXT cancellation techniques are introduced for multiuser discrete multitone (DMT) based very high-speed digital subscriber line (VDSL) transmission. It is shown that FEXT cancellation enhances the achievable bit rate in FEXT-limited systems. Computer simulation and analytical results show that the performance of jointly adapted FEXT cancelers and an FEQ is better than that of separately adapted FEXT cancelers and an FEQ.

In this paper, we propose OOK-CDMA system with optical hard-limiters (OHLs) using schemes of transmitting optical pulses with two-level intensities. The proposed system can be applied to both cases with single OHL and double OHLs. In the proposed system, a frame corresponding to a bit time is divided into two frames, where the low intensity pulses are transmitted in the weighted positions at the former frame, and the high ones are transmitted in the weighted positions at the latter frame. At the receiver side, in each weighted position of low intensity level, the ability of Multiple Access Interferences (MAIs) cancellation by OHL is equal to that of the conventional systems. However, in each weighted position of high intensity level, the OHL with the higher-level threshold value can easily eliminate MAIs with low intensity, while the MAIs are not eliminated in the conventional system. As a result, the ability of interference cancellation by OHL is significantly increased. When using single OHL in the proposed system, the optimum combinations of the values of low-level weights and low-level intensity ratio depend on the received laser power. This means that deploying dynamic control of the values of low-level weights and low-level intensity ratio is an imperative requirement in the proposed system with single OHL. On the other hand, when using double OHLs, the optimum combinations of the values of low-level weights and low-level intensity ratio are always constant. We analyze the performance of the proposed system and show its advantage in terms of bit error probability compared to the conventional system in both cases with single OHL and double OHLs. We also show that the proposed system with double OHLs is more effective and practical than the one with single OHL depending on the received laser power.

This paper examines the Inter Carrier Interference (ICI) due to Doppler spread in OFDM mobile reception and proposes the use of Beam-Space Adaptive Array Antennas for moving receivers. In the proposed system, firstly we separate the multi-path signals into multi-beams according to their incident directions, then correct the frequency shift of each beam signal, considering the beam direction, and finally combine the corrected signals based on Maximal Ratio Combining (MRC). Further this paper clarifies the excellent performance of the proposed system in suppressing the influence of Doppler spread by carrying out computer simulation. Particularly, it was certified that it is possible to suppress the influence of the Doppler spread efficiently for all the receiving directions by using eight-element beam-space array antenna with element spacing of (3/8)λ, and referring three past symbol data when calculating the weight vector of MRC.

This paper presents a blind frequency offset estimation method for Polynomial Cancellation Coded Orthogonal Frequency Division Multiplexing (PCC-OFDM) systems. We have theoretically derived the frequency offset estimator. The estimation exploits the Subcarrier Pair Imbalance (SPI) which is presented in terms of the power difference between two demodulated subcarriers in a PCC-OFDM subcarrier pair. The estimator can be used for high order QAM modulation schemes. In all cases, the estimator has an approximately linear relationship with the frequency offset. The potential application of the estimator in conventional OFDM systems is also investigated in this paper.

This paper investigates the performance of an integrated voice/data wireless mobile network where a finite buffer is provided for voice calls since they can endure a tolerable time, or the reneging time, for service. Based on a given humanistic reneging time, we analyze the voice traffic blocking probability. The probability distribution of receiving service within the reneging time is obtained for each buffered voice call and based on this result, an appropriate amount of voice buffer is obtained. To alleviate the impact on data blocking probability caused by the voice buffer and to enhance the efficiency of data service, a dynamic multi-channel allocation scheme with channel de-allocation and guard channels is proposed for data traffic. Compared with the conventional method where the system adopts a single-channel allocation scheme without guard channel for data users, the proposed scheme shows significant improvement in data blocking probability, throughput and the mean service time. Furthermore, a system with an appropriate size of buffer for voice traffic can receive good improvement in voice blocking probability.

IEEE 802.11 standard and the enhanced amendments have defined several transmission rates for mobile stations to transmit and receive data frames. With the characteristic of modulation schemes, a higher network throughput can be expected by using higher level modulation scheme, but frame error probability will also become higher. Doubtlessly, it is an open issue of selecting a proper modulation scheme for a pair of mobile stations in time-varying indoor environment. This paper proposes a safe multiple access-rates transmission (SMART) scheme to enhance the reliability of data transmission in IEEE 802.11 multi-rate infrastructure wireless networks. SMART scheme provides reliable transmission by reserving a retransmission period which immediately follows the transmitted frame and is estimated by a lower transmission rate according to the transmitted frame. The performance of SMART scheme is evaluated by analytical model and simulation experiment, which show that the proposed scheme is significantly better than the IEEE 802.11 standard under the real environment with asymmetric traffic load. An enhanced SMART (ESMART) scheme is also proposed especially to improve bandwidth waste for long packet length at access point (AP). The performance measurements, such as goodput, throughput, average access delay, and service rate, are obtained and compared for the different schemes.

This paper describes feasibility of a proposed fixed wireless access system with CDMA technology. The system adopts a primary modulation of 16 QAM and the same frequency allocation in all cells to improve spectral efficiency. The system capacity is 1 Gbps per cell within 120 MHz bandwidth. The number of available orthogonal codes corresponds to the orthogonal code length in the system. All subscribers can attain an error free condition with output power control in the presence of inter-cell interference. The following two items are considered to examine the proposed system feasibility. 1) A test modem is fabricated, and a back-to-back modem BER performance is measured. An inter-symbol interference (ISI) level of the modem is estimated with the measured performance. 2) A computer simulation of down-link power control is carried out considering inter-cell interference and impairment factors of the power control such as intra-sector interference caused by the ISI and limited ranges of total and relative output power controls. The simulation results show that the proposed system would be feasible because the obtained power penalties caused by the above impairment factors are negligible.

Synchronous Gaussian code-division multiple access (CDMA) systems employing group-orthogonal signature waveforms are proposed and analyzed. All users in the system are divided into groups of users. The signature waveforms are constructed such that all the signature waveforms in one group are orthogonal to all the signature waveforms used in all other groups. This construction of signature waveforms ensures that there is no inter-group interference (i.e., among users in different groups), but at the expense of having intra-group interference (i.e., among users in the same group). However, by choosing a small size for each group, the intra-group interference can be effectively handled by a low-complexity, optimal (or suboptimal) multiuser detector. It is shown that a significant improvement in the system capacity can be achieved by the proposed technique over the conventional one that uses signature waveforms constructed from Welch-bound-equality (WBE) sequences. In particular, it is demonstrated that, while the conventional system's error performance is very sensitive to even small amount of overload, the proposed system with an appropriate design of signature waveforms can achieve a much higher overload (up to 300% as shown in the paper) with an excellent error performance.

Direct sequence code division multiple access (DS-CDMA) provides flexible data transmission in wide range of data rates by the use of orthogonal multicode multiplexing. In a multipath fading environment, the transmission performance of multicode DS-CDMA degrades as that of single code DS-CDMA does. Chip interleaving is known to improve the bit error rate (BER) performance of the single code transmission by altering the fading channel into severely time selective fading channel. However, this partially destroys orthogonality property among spreading codes and thus, significantly degrades the BER performance of multicode DS-CDMA. In this paper, we propose the joint use of chip interleaving and time-domain minimum mean square error combining (MMSEC) equalization to improve the multicode DS-CDMA transmission performance. It is confirmed by computer simulations that the joint use of chip interleaving and MMSEC equalization significantly improves the BER performance of multicode DS-CDMA and achieves better BER performance compared to the single code DS-CDMA using chip interleaving and maximal ratio combining (MRC).

In this paper, the average bit error probability of uplink and downlink Multicarrier Code Division Multiple Access (MC-CDMA) system using coherent Maximal-Ratio Combining (MRC) and Equal Gain Combining (EGC) receivers is evaluated for frequency selective Nakagami fading channels. The analysis assumes that different subcarriers experience independent fading channels, but not necessary identically distributed. The analysis is based on Gaussian approximation of the multiple access interference. Generalized bit error probability (BEP) expressions for both uplink and downlink with MRC and EGC receivers were derived. The analytical results are supported with simulation results. The effect of fading parameters, number of users, and number of subcarriers were presented. The BEP performance of the EGC receiver in the uplink is highly influenced by the fading parameter compared with the MRC receiver. The EGC receiver outperforms the MRC receiver in the downlink, but the MRC receiver gives almost the same performance as the EGC in the uplink.

In this paper, using the exact expression for the pairwise error probability derived in terms of the message symbol distance between two message vectors rather than the codeword symbol distance between two transmitted codeword matrices, the exact closed form expressions for the symbol error probability of any linear orthogonal space-time block codes in slow Rayleigh fading channel are derived for QPSK, 16-QAM, 64-QAM, and 2 56-QAM.

A mobile ad hoc network (MANET) is characterized by multi-hop wireless links in the absence of any cellular infrastructure as well as by frequent host mobility. This paper proposes a SMPQ: Spiral-Multi-Path QoS routing protocol in a MANET, while the MAC sub-layer adopts the CDMA-over-TDMA channel model. This work investigates the bandwidth reservation problem of on-demand QoS routing in a wireless mobile ad-hoc network. The proposed approach increases the ability of a route to identify a robust path, namely a spiral-multi-path, from source host to destination host, in a MANET to satisfy certain bandwidth requirements. Two important contributions of the proposed spiral-multi-path are: (1) the spiral-multi-path strengthens route-robustness and route-stability properties and (2) the spiral-multi-path increases the success rate of finding the QoS route. Performance analysis results demonstrate that our SMPQ protocol outperforms other protocols.

Quasi-millimeter-wave-band Fixed Wireless Access (FWA) systems have higher transmission rates than 2.4-GHz or 5-GHz systems, because the available frequency bandwidth for quasi-millimeter-wave-bands is broader than the 2.4-GHz and 5-GHz bands. However, quasi-millimeter-wave-band systems are unsuitable for long-span transmission because the attenuation caused by rain is large. We propose that the symbol rate be lowered for rainfall; i.e., when it rains, a low symbol rate is used. This means narrowing the equivalent noise bandwidth so that a margin for rain attenuation is obtained. We compared a method in which the symbol rate is either high or low with one in which the symbol rate is selectable over a range of values. We verified the beneficial effect of the two-rate method through calculations and simulations. A case study in the Tokyo metropolitan area showed that the service zone radius of this method is double that of conventional systems. Changing to a low symbol rate decreases the transmission rate, but periods of heavy rainfall comprise only about 1% of the amount of time in a year, and so the average decrease in the transmission rate is approximately zero.

Group multicasting is a generalization of multicasting whereby every member of a group is allowed to multicast messages to other members that belongs to the same group. The group multicast routing problem (GMRP) is that of finding a set of multicast trees with bandwidth requirements, each rooted at a member of the group, for multicasting messages to other members of the group. An optimal solution to GMRP is a set of trees, one for each member of the group, that incurs the least overall cost. This problem is known to be NP-complete and hence heuristic algorithms are likely to be the only viable approach for computing near optimal solutions in practice. In this paper, we derive a lower bound on the cost of an optimal solution to GMRP by using Lagrangean Relaxation and Subgradient Optimization. This lower bound is used to evaluate the two existing heuristic algorithms in terms of their ability to find close-to-optimal solutions.

This paper discusses research into the capacity dimensioning of Virtual Private Network (VPN) access links for elastic traffic, such as the Web or ftp. Assuming that the core-VPN network is provisioned with a sufficiently large capacity, managing the capacity of the VPN access link comes to sharing the bandwidth for the elastic traffic of the two bottlenecks, the ingress and egress access links. In the case of a single bottleneck with a limited capacity for access links, the processor-sharing model provides a simple formula for mean transfer time, but here, the value may be less than the actual transfer time because multiple flow may compete the bandwidth of both ingress and egress links. In contrast, max-min fair sharing provides an accurate sharing model which is similar to the TCP, but it is difficult to obtain a closed form for performance statistics. We propose a closed form approximation for a max-min fair sharing model, within a specific but realistic topology, through an investigation into the difference between the max-min and the processor sharing model. Using approximation, we calculate the capacity dimensioning of VPN access links.

The authors propose a Global Open Ethernet (GOE) architecture as a cost-effective scalable solution for next generation of VPNs over optical network infrastructure. Three main approaches have been proposed for a cost-effective VPN solution on metro area network (MAN): Resilient Packet Ring (RPR), Ethernet over MPLS (EoMPLS), Extended Ethernet via Stacked VLAN tagging (EESVLAN). None of these schemes can satisfy the following requirements at the same time: network topology flexibility, affordable network functionalities, low equipment cost, and low operational cost. We propose the GOE architecture as an affordable VPN solution that addresses all these requirements. GOE combines the functionality of MPLS VPN with low cost and easy operation of Ethernet-based solutions in a cost-effective scalable manner. In this paper, we evaluate the performance of EoMPLS, EESVLAN and GOE VPN solutions in terms of the cost and network utilization. We show that the cost of GOE solution is two-three times smaller than the cost of the other approaches. We also demonstrate that the network utilization provided by GOE is 22% higher than that of both EoMPLS and EESVLAN. Therefore, GOE can be shown to be a cost-effective simple VPN solution with clear advantages in functionality, management and performance.

A polarization switchable slot-coupled microstrip antenna using PIN diodes is proposed and studied. The microstrip feed line installed behind the ground plane is divided into two branches and each tip of the branches is connected to the ground plane through a PIN diode. One of the diodes is oriented from the tip to the ground plane and the other is oriented from the ground to the tip so that a slot in the ground can be selected to feed the patch by switching the dc bias between positive and negative. This selection contributes to switch the polarization between horizontal and vertical. In this paper, the authors investigate the polarization switching antenna theoretically and experimentally and confirmed sufficient differencce of antenna gain between horizontal and vertical polarization.

In mobile OFDM systems, sub-carriers orthogonality will be broken due to Doppler shift, and this results in inter-carrier interference (ICI). Many methods have been proposed to compensate for this, however, these methods won't be suitable for fast fading caused by high mobile speed. In this letter, we propose a novel sampling theorem based pilot symbol-aided technique which can not only estimate the channel fading envelope (CFE) accurately under high relative Doppler frequency (RDF) but also achieve lower BER than conventional methods. The validity of the proposed method is demonstrated by computer simulations.

A new stopping criterion of turbo decoding based on the maximum a posteriori (MAP) decoder is proposed and applied to the turbo processing system. The new criterion suggested as the combined parity check (CPC) counts the sign difference between combined parity bits and the re-encode parity bits determined from decoded information bits. The CPC requires decoded parity bits and a turbo encoder but it performs better in terms of the bit error rate and the average number of iterations in the turbo processing system.

We analytically investigate combinatorial effects of timer control and backoff algorithms on performance of bulk data transfer over two-state Markovian packet error channels. Numerical results for throughput, energy efficiency, and the probabilities of packet loss and loss of bulk data indicate that linear backoff algorithms outperform binary exponential ones as a whole when they are employed at the logical link sublayer with timer control.

We propose a simple method for the chromatic dispersion measurement of optical fibers by using bi-directional modulation of a Mach-Zehnder electro-optical modulator embodied in a fiber loop mirror. The detected output of the bi-directionally modulated light, with time difference, creates fading in the RF domain. Dispersion is found by measuring the period of fading at different wavelengths.

Software-Defined Radio (SDR) receiver has the ability of operating in a multi-mode environment and has wide applications. However, efficient recognition of the currently active modulation system in real-time is a major problem faced by many applications. In this paper, an efficient method for the recognition of modulation system in a SDR receiver is proposed. The method is a classical two-stage approach based on (i) decision feature extraction and (ii) modulation system classification. In the first stage, decision features are extracted by the use of digital quadrature polyphase filter. In the second stage, an efficient parallel decision algorithm is proposed to classify the active modulation type. This proposed algorithm is proof to be more efficient than the conventional type of decision-tree approach. The complete recognition system is implemented using MATLAB. Simulation result shows that the proposed method achieved good robustness even with the presence of band-limited Additive White Gaussian Noise (AWGN). The overall successful recognition rate of 98.5% can be achieved even at a low signal-to-noise ratio (SNR) of 8 dB.

In this letter, we address the performance evaluation of a space-time block coded (STBC) orthogonal frequency and code division multiplexing (OFCDM) system encountered with time-variant channels. For the performance evaluation, the average bit error rate (BER) impairment due to imperfect channel information is investigated taking into account the effect of time-varying channels. Derived results show that the loss of orthogonality due to the time variation of the channel incurs the channel estimation error, which causes performance degradation.

This paper describes the design, fabrication, and measurement of a multiple U-shaped slot antenna for Hiper-LAN. The prototype consists of a U-shaped slot and two inverted U-shaped slot. To obtain sufficient bandwidth, a foam layer is inserted between the ground plane and substrate. A measured bandwidth of approximately 7.6% (VSWR 1.5) and gain of 2.9-5.3 dBi are obtained. The experimental far-field patterns are shown to be stable across the pass band, with the 3 dB beam width in azimuth and elevation at 50and 62, respectively.

This paper proposes an efficient fair queuing algorithm, called Medium Starting Potential Fair Queuing (MSPFQ), which has O(1) complexity for the virtual time computation while it has delay and fairness properties similar to Starting-Potential Fair Queueing (SPFQ). The key idea of MSPFQ algorithm is that it recalibrates the system virtual time to the medium value of the minimum possible virtual start times of HOL packets in each backlogged session. We show that MSPFQ belongs to the class of Rate-Proportional Server (RPS). In addition, we analytically prove that our algorithm has good delay and fairness properties.

Recently, Hwang and Li proposed a smartcard-based remote user authentication scheme. Later, Chan and Cheng showed that Hwang and Li's scheme is insecure against a kind of impersonation attack where a legitimate user can create another valid pair of user identity and password without knowing the secret key of the remote system. However, an assumption under Chan and Cheng's attack is that the attacker must be a legal user. In this paper, we further present a more fundamental and efficient impersonation attack on Hwang and Li's scheme. Using our attack, any users (including legal and illegal users) can easily get a specific legal user's password, impersonate this specific user to login to the remote system, and pass the system authentication.

The authors have performed a simple computer simulation for a topography that models change in propagation characteristics due to change in traffic volume. The results of this simulation revealed that path loss for a traffic volume of about 2000 vehicles every 30 minutes in a typical urban environment exhibits a Rayleigh distribution. This result agrees well with that of actual measurements demonstrating that even a simple simulation can be a useful tool in system design.

This letter proposes a millimeter-wave band transformer to connect a standard waveguide to a very thin post-wall waveguide. The post-wall waveguide height is the same as a microstrip or coplanar line. A dielectric substrate with slits etched on both edges is inserted in the standard waveguide for matching. A 22 GHz transformer gives 3.6% bandwidth for a 0.5 mm-height post-wall waveguide. The effects of various mechanical misalignments upon the frequency characteristics of the reflection are also estimated by analysis and measurements.

Multiple antenna systems are promising architectures for overcoming the effects of multi-path interference and increasing the spectrum efficiency. In order to be able to investigate these systems, in this article, we derive generalized spatial correlation equations of a circular antenna array for two typical angular energy distributions: a Gaussian angle distribution and uniform angular distribution. The generalized spatial correlation equations are investigated carefully by exact and approximate analyses.

A cache replacement policy which takes the transcoding time into account in making replacement decisions, for the emerging transcoding proxy servers is proposed. Simulation results show the proposed policy outperforms the conventional LRU in both the cache hit rate and the average object transcoding time.